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654889ffc573f893f1e64546 | 19 | The spin states of 2 deserve a comment as several different results have been obtained in the literature: a somewhat noisy electron paramagnetic resonance (EPR) spectrum was recorded of 2 in TaAA9, suggesting that a triplet is energetically within reach. Meanwhile Jones et al. obtained an EPR silent tyrosyl (2) for Hj AA9, while a very recent study proposes a triplet spin state for 2 in LsAA9 (also based on EPR). Comparisons to highly accurate multiconfigurational wavefunction (CASPT2) calculations on LPMO intermediates have shown that discrepancies for spin-state splittings can occur. Since the spin-state splittings generally are small for 2, we can only expect qualitative results (this is also the reason we mostly will base our conclusions on results obtained from two different DFT functionals). |
654889ffc573f893f1e64546 | 20 | We find that for LsAA9, the singlet and triplet states are essentially degenerate with both functionals. Similarly, ref. reported that the splittings for LsAA9 are small (< 5.5 kJ/mol) for both functionals (the triplet state was found to be slightly more stable, independent of the functional used). Thus, for this LPMO our results are commensurate with the recent EPR results in ref. , i.e., the triplet state is energetically within reach. The results for TaAA9 are more ambiguous: the triplet state is most stable according to B3LYP (by 9 kJ/mol), whereas |
654889ffc573f893f1e64546 | 21 | An alternative to the reaction investigated in the previous subsection is the abstraction of a hydrogen from the histidine brace by the [CuO] + moiety in 1 (i.e., reaction II in Fig. ). Indeed, we recently investigated this for LsAA9 as the first step of the oxidative self-damage reaction. We compare the reaction profiles (barrier and reaction energy) of reaction II for TaAA9 and LsAA9 in Fig. (all calculated energies associated with reaction II are provided in Table ). As can be seen from Fig. , LsAA9 and TaAA9 overall have the same energy profile for the H-abstraction from the active site histidine (His1). The reaction barrier differs only by 5 kJ/mol (1 kJ/mol for TPSS) and the reaction energy differs by less than 3 kJ/mol for both functionals. Thus, the oxidative damage can proceed through this pathway, independent of the LPMO. |
654889ffc573f893f1e64546 | 22 | Structures for the reactants, transition states, and products, along with selected distances, are shown in Fig. (additional distances are provided in Table in the SI). As can be seen from this Figure, structural changes during reaction II are similar for TaAA9 ), which increases by 0.09-0.10 Å, consistent with the protonation of the oxyl. We also note that for TaAA9 we were able to obtain an isomer of 3, here denoted as 3 ′ (shown in Fig. ). This isomer differs from 3 in Fig. in that the OH-group of the tyrosine points towards the C ϵ1 in the imidazole ring of His1. In 3, the OH group instead forms a hydrogen bond with the oxygen in Gln173. Since the energy difference between the isomers is rather small (∆E = 12 -19 kJ/mol in their triplet state depending on the functional), it is likely that they both exist in solution. The structure of the 3 ′ conformer seems optimal for the transfer of the H-atom of the tyrosine OH-group to the histidyl, according to reaction III in Fig. as recently suggested. We investigate this possibility in the next subsection. |
654889ffc573f893f1e64546 | 23 | Based on the spin-densities, differences between the electronic changes during reaction II are also minor between the two LPMOs: the spin densities (Table ) decrease significantly on oxygen in the OH group of the [CuOH] + moiety in 3, compared to O ox in [CuO] + (1), while the spin density increases on His1. The main increase occurs on the de-protonated C ϵ1 , suggesting that 3 is indeed a histidyl radical, coupled to a [CuOH] + moiety. . |
654889ffc573f893f1e64546 | 24 | Based on HERFD-XAS and UV-vis spectroscopy, a histidyl intermediate was claimed to be characterized as an open-shell spin singlet, 41 although it is unclear how the spin state was determined. The histidyl intermediate was only characterized for LsAA9 and we find the spin-state splitting for 3 in LsAA9 to be small, but somewhat functional dependent: the splitting is only 5 kJ/mol with TPPS, the open-shell singlet being most stable. With the B3LYP functional we obtain a splitting of 18 kJ/mol with the triplet being most stable. For TaAA9 the open-shell singlet calculations converged into a closed-shell singlet for 3, while we obtain both triplet and open-shell singlet states for the conformer 3 ′ ; in this case the open-shell singlet and triplet are essentially degenerate (the triplet is only 3 kJ/mol more stable with both TPSS and B3LYP). In light of the results for the tyrosyl radical (see previous subsection), it is likely that small differences in the active site architectures between TaAA9 and LsAA9 lead to small differences in spin-state splittings. Since the splittings are small, this can also lead to differences in which spin states are most stable. However, with the present accuracy of the used functionals, the splittings are generally too small to clearly differentiate the spin states. |
654889ffc573f893f1e64546 | 25 | Before investigating reaction III, we note that we have here only investigated H-abstraction from His1. This was decided based on the structure in TaAA9, where H-abstraction from His1 appeared more plausible compared to His86, given the notably shorter distance between H ϵ1 and O Oxyl (2.48 Å) in contrast to the H ϵ1 of His86 (3.39 Å). This was confirmed by a test calculation (Table ) for the triplet potential energy surface (PES), showing that the reaction barrier is indeed significantly higher compared to the abstraction from His1 (27-28 kJ/mol depending on the functional). Moreover, the product is thermodynamically less stable by 19-21 kJ/mol. It is interesting to note that in LsAA9, the abstraction from His78 (equivalent to His86 in TaAA9) occurs with only minor changes in energy (<8 kJ/mol for the reaction barrier and energy for both functionals), compared to the H-abstraction from His1. The difference between the two LPMOs occurs since the distances of H ϵ1 on His78 and His1 to O Oxyl in 1 is much closer (2.31 Å and 2.70 Å35 ) in LsAA9. This underlines that small differences in the active site architecture can lead to mechanistic differences, and this will become more apparent in the discussion regarding reaction III in the next subsection. |
654889ffc573f893f1e64546 | 26 | Spurred by the recent proposal of a histidyl radical 41 (3) as the first intermediate in a protective hole-hopping pathway, we investigated whether the histidyl radical (3) can abstract a hydrogen from the tyrosine, thereby restoring histidine while forming a tyrosyl radical (2). This reaction is labeled III in Fig. . The reaction barrier and energies are shown in Fig. and Fig. . Interestingly, the two LPMOs are remarkably different: for TaAA9 the reaction is kinetically feasible with a barrier of 75 kJ/mol (59 kJ/mol for TPSS), and is predicted to be thermodynamically favorable with a reaction energy of -129 kJ/mol for both functionals. |
654889ffc573f893f1e64546 | 27 | In comparison, LsAA9 shows a high reaction barrier of 155 kJ/mol (126 kJ/mol for TPSS) with a reaction energy of -129 kJ/mol (-128 kJ/mol for TPSS). The reactant, transition Figure : Energy diagrams (in kJ/mol) for the reaction III for TaAA9 (left) and LsAA9 (right). The reactants 3 ′ and 3 for TaAA9 and LsAA9, respectively, were used as reference. Results were obtained for the triplet state with an extended QM region (see computational details). Results for the smaller QM region are provided in Table in the SI. state, and product structures are shown in Fig. (additional distances for chosen atoms are provided in Table in the SI). The transition states also display somewhat different bond-distances in the first coordination sphere to copper; the Cu-O Tyr distance varies from 2.9 in TaAA9 to 2.6 Å in LsAA9, highlighting that the otherwise quite similar active-site architectures may lead to different reactivity. As for reaction I, the de-protonation of tyrosine's OH-group leads to a decrease in the Cu-O Tyr175 distance of 0.2-0.3 Å in the product (2). We finally note that we decided to use 3 ′ for TaAA9 in Fig. and Fig. . This choice was made since we then have consistently small MM energies for the barrier (the reaction energy is less affected) as the QM region was enlarged for 3 ′ . We have not attempted to extend the QM region for 3, but based on the calculations with smaller QM region, the energy difference between 3 and 3 ′ for TaAA9 is sufficiently small so that none of the above conclusions change (see Table ). ). The structures were optimized using TPSS/def2-SV(P) while the energies given were obtained employing B3LYP/def2-TZVPP. Key distances are given in Å and energies in kJ/mol with reference to the reactant. Further bond distances for the different intermediates can be found in Table . Note that the structures were obtained with an extended QM region including Phe43 and Pro30 for TaAA9 and LsAA9, respectively. |
654889ffc573f893f1e64546 | 28 | This work represents the first comparison of oxidative damage and protective mechanisms in two different LPMOs: we have shown that it is energetically (and kinetically) feasible for TaAA9 and LsAA9 to form both tyrosyl (2) and histidyl (3) radicals. This commensurates with experiments for both TaAA9 and LsAA9, were a number of studies detected 2 for different LPMOs, including TaAA9 36 and LsAA9. The detection of a histidyl radical (3) has only been proposed recently for LsAA9, 41 but our results show that at least the formation of this radical is energetically similar for TaAA9 and LsAA9. In Fig. we compare reactions I-III for both LPMOs using the Cu(II)-oxyl (1) as reference. Note that we have included results involving His78 (LsAA9) and His86 (TaAA9) in the SI (Table and S5) but we will concentrate the discussion on His1 (this does not lead to any change of conclusions). |
654889ffc573f893f1e64546 | 29 | From Fig. , we can also see that the formation of the tyrosyl radical ( ) is generally preferable to the formation of the histidyl radical (3). We thus consider it more likely that 2 and 3 are formed in competing reactions: the overall barrier for forming 2 via 3 is 199 kJ/mol for LsAA9, using [CuO] + (1) as a reference (see Fig. ). The barrier is lower for TaAA9, but with 149 kJ/mol, it must still be considered too high to be feasible. The high overall barrier may be a result starting from the Cu(II)-oxyl (1) species; we cannot exclude that another oxidative intermediate such as [Cu-OH] 2+ or a free OH radical is responsible for initiating the oxidative damage, and we are currently investigating such alternatives. However, if we consider the barrier for conversion 3→2 alone (in Fig. ), we still find that for LsAA9 the barrier is rather high (155 kJ/mol for B3LYP). It is interesting to compare this barrier (and reaction energy) to the 49 kJ/mol (and -287 kJ/mol) calculated for the recombination reaction of LsAA9 in ref. 35: this reaction is part of the oxidative damage pathway, where the formation of 3 is followed by recombination of the OH group from [Cu-OH] + in 3 to the histidyl radical, forming a 2-hydroxy-histidine. This reaction is evidently much more favorable than forming 3 -this is not changed if we look at the corresponding numbers for the overall reaction with the Cu(II)-oxyl (1), which were calculated to be 101 and -234 kJ/mol, respectively. Clearly, this is still more favorable than the 199 kJ/mol and -44 kJ/mol in Fig. . In conclusion, the formation of 3 is more likely to lead to oxidative damage than for 3 to be part of the protective hole-hopping mechanism, and this result is independent of choosing 1 as a reference. |
654889ffc573f893f1e64546 | 30 | Our results provide a mechanistic explanation for that, similar to other oxoreductases, the LPMO active-site tyrosine is critical for initializing the protective hole-hopping mechanism. This tyrosine is widely conserved in most LPMO families, except for the majority of AA10 LPMOs, where it is replaced by phenylalanine. Our results can thus explain that it has recently been discovered that fungal AA9 LPMOs are less prone to oxidative damage than their bacterial (AA10) counterparts. In a previous paper 40 we compared calculated UV-vis spectra with the characteristic Shown are only the most feasible energetics for B3LYP/def2-TZVPP (structures were optimized with TPSS/def2-SV(P). Energies in grey are from ref. Note that the reaction barrier TS III was obtained with a bigger QM region. observed bands around 400-420 nm for 2, Considering that we carried out the TD-DFT calculations as vacuum calculations, the intense peak from the open-shell singlet at 386 nm (3.22 eV) is in reasonable correspondence with the experimental room temperature absorption spectrum for TaAA9 with the most intense peak at 420 nm (2.95 eV). The corresponding experimental value for LsAA9 is 414 nm (2.99 eV) 41other AA9 LPMOs show intense peaks in the same region (see refs. ). For the triplet state of TaAA9, we also tried to calculate the spectrum of 2 including electrostatics the enzyme as point-charges, but the effect of the point-charges on the position of the intense transition due to the tyrosyl is minimal (see Fig. in the SI). |
654889ffc573f893f1e64546 | 31 | Since we could qualitatively reproduce the UV-vis spectra of 2, we additionally calculated the UV-vis spectrum of 3 for both TaAA9 and LsAA9 (Figures S6 and S7 in the SI). Intermediate 3 has only been observed for LsAA9 and the peak that experimentally is assigned the histidyl intermediate 41 is obtained at 360 nm (3.44 eV). The TD-DFT calculations do predict intense transitions involving orbitals of histidyl character (further discussions are provided in the SI). However, these are at somewhat lower energies at 427 nm (2.91 eV) for the triplet and 406 nm (3.05 eV) for the open-shell singlet. The correspondence with the experimental values was clearly better for the tyrosyl radical (2). Intriguingly, the calculated spectrum for 3 in TaAA9 is broader with two intense transitions at 342 nm (3.63 eV) and 395 nm (3.15 eV). Particularly, the former corresponds well to the observed band in LsAA9, but we cannot presently explain why the calculated spectrum for TaAA9 fits better with the experimental spectrum of LsAA9. More benchmarks of the accuracy of TD-DFT -and preferably also investigations with multiconfigurational wave functions -will be required to confidently assign the spectrum of 3. Along these lines, Zhao et al. find that 17% exact exchange is optimal to reproduce relative intensities for HERFD-XAS spectra, but we have refrained from attempting to re-parameterize the functional at this point. |
654889ffc573f893f1e64546 | 32 | We have compared the initial of the oxidative damage and protective hole-hopping mechanisms of two LPMOs, namely TaAA9 and LsAA9. The two investigated LPMOs have very similar active site architectures. We find similarities as well as remarkable differences in the protective mechanisms, highlighting that investigations on LPMOs as far as possible should consider several LPMOs. Regarding the similarities, our calculations show that the [CuO] + moiety in the Cu(II)-oxyl intermediate (1) for both TaAA9 and LsAA9 is capable of oxidizing tyrosine to a tyrosyl radical. The electronic structures of the formed tyrosyl radicals are overall similar in the two LPMOs, based on their spin densities and their calculated UVvis spectra. The latter is commensurate with recent experimental investigations, with intense peaks around 400 nm. |
654889ffc573f893f1e64546 | 33 | We also find that the formation of a histidyl intermediate (3) has essentially the same reaction energy profile in the two LPMOs. Thus, we propose that this intermediate can be formed, as recently suggested by an experimental investigation. However, it is more favorable energetically for both LPMOs to form the tyrosyl radical (2) than the histidyl radical (3). |
654889ffc573f893f1e64546 | 34 | The histidyl radical (3) has been proposed to be inter-converted to the tyrosyl radical (2), making it part of the protective hole-hopping mechanism. The above comparison between barriers and reaction energies for the formation of 3 and 2, cannot be reconciled with this mechanism. In fact, we find that the formation of a tyrosyl (2) via a histidyl (3) intermediate is generally not feasible with a reaction starting from a Cu(II)-oxyl species (1). However, the LPMOs generally show quite different energetics regarding conversion between 3 and 2: For TaAA9, the conversion (the barrier between 3 and 2) is feasible, and cannot be entirely ruled out, although it is unlikely with the presently employed oxidizing species (1). this conversion for LsAA9 seems not to be possible, regardless of the oxidizing species, since conversion between 3 and 2 has a very high barrier. Another remarkable difference between the LPMOs is that the barrier for formation of the tyrosyl radical (3) is generally larger for LsAA9. A consequence of such differences may be that different LPMOs . For 1 in TaAA9, the triplet is most stable, but the states are (as expected) close in energy with splittings of either 17 and 16 kJ/mol, depending on the functional. This is similar to what most previous studies have found. For LsAA9, the triplet and singlet are essentially degenerate with TPSS (the triplet is less stable, but only by 2 kJ/mol). With B3LYP we obtain a larger splitting of 31 kJ/mol, where the triplet is most stable. Small splittings were also obtained in ref. with a splitting of 12-13 kJ/mol (the triplet being more stable), while ref. obtained the triplet as the ground state but again with small splittings of 5 kJ/mol (TPSS) or 16 kJ/mol (B3LYP). The difference to these previous studies is either that we here use a slightly larger QM region 40 or a different functional in the structure optimization. Notably, we also observe for LsAA9 that TPSS gives an intermediate between the closedshell and open-shell singlet, with a spin distribution similar to that of the B3LYP open-shell singlet but with 2-9 times lower magnitude and high spin contamination. Similar issues with spuriously low spin populations with TPSS were seen in some of our previous calculations, although it was less pronounced in the present calculations for 1 in LsAA9. |
654889ffc573f893f1e64546 | 35 | For 1, the spin-densities (Tab. S7 and S8) overall fits with the expected triplet or singlet coupling between the unpaired electron of d 9 Cu(II) and the unpaired electron of the oxyl (O • -), although we occasionally observe small differences in spin-state splittings to previous calculations. and distances of all states associated with reaction I: was not obtained for LsAA9). Since the energy of this intermediates is essentially the same as the transition state, we decided not to include it in Fig. with a highly delocalized nature of the transitions. |
654889ffc573f893f1e64546 | 36 | Intermediate 3 for TaAA9 also shows the intense feature around 400 nm seen for 3 in the LsAA9 spectrum. However, an additional transition is also seen at higher energy: the two most intense peaks in Fig. (green spectrum, left) are at 342 nm (3.63 eV) and 395 nm (3.14 eV). An analysis of the major orbital contributions of these peaks show a large involvement of the histidyl. The main peak at 395 nm mainly involves transitions from orbitals of hydroxyl lone pair character to an orbital of π-character on the histidyl His1. The less intense peak at higher energies (342 nm) occurs due to ligand-to-metal charge transfer transitions including orbitals on the histidyl. Similar transitions can also be observed for the isomer 3 ′ , albeit with the two most intense peaks red-shifted to 348 nm (3.57 eV) and 406 nm (3.05 eV) for the triplet state and 384 nm (3.23 eV) and 416 nm (2.98 eV) for the open-shell singlet. Overall, the spectra of 3 and 3 ′ are quite similar. |
61d817f76be4207ba824e05e | 0 | Advances in diagnostic imaging, as well as surgical technique and instrumentation have synergistically enabled rapid growth of minimally invasive procedures. These procedures have advantages over traditional surgical techniques, such as shortening the treatment time and reducing the risk of complications. These surgical approaches motivate further research into wide range of injectable materials that can enable tissue repair, drug delivery, cell therapy, sensing, imaging, etc. Of particular interest are materials that can be injected as a liquid and then crosslinked in situ, under different stimuli, such as light, temperature, or pH. While hydrogel based materials have been most popular, curable elastomeric materials can be well suited for specific tasks, such as heart muscle repair after infarct or hernia repair. Towards this aim, we have previously developed injectable and photocurable ester-urethane macromonomers based on fatty acid derivatives obtained from vegetable oils. Polyurethanes are already widely used in medical and pharmaceutical applications due to their favorable safety profiles and mechanical properties, as well as potential for biodegradability. In the case of our injectable macromonomers, once photocured, the obtained elastomeric materials exhibit similar mechanical properties to human soft tissue (i.e. abdominal wall), along with slow enzymatic and hydrolytic degradation. Overall, these materials displayed low toxicity and similar immune response to polylactic acid in a rabbit model, making them promising for minimally invasive surgical procedures. However, in our previous work the ester-urethane macromonomers were synthesized with use of organotin-based catalyst, dibutyltin dilaurate (DBTDL). Overall, tin-based catalysts are commonly used for the synthesis of polyesters and polyurethanes as elastomers and coatings. |
61d817f76be4207ba824e05e | 1 | They act as Lewis acids and possess high catalytic activity. However, tin-based compounds and organotin compounds, such as DBTDL in particular, exhibit cytotoxicity, are difficult to remove from polymers, and have adverse environmental effects. Therefore, their usage in medical applications is limited and there is growing interest in "greener" organotin-free reaction pathways. Towards this aim, alternative catalytic systems are being explored that replace tin with other metals, such as zinc, titanium, zirconium, manganese, etc. Metal catalysts based on bismuth and zinc are of particular interest, due to their role in the human metabolism and use in pharmaceuticals, cosmetics, etc. Recently, they were tested in the synthesis of biodegradable polyesters, such as poly(ε-caprolactone) or poly(lactide). Overall, the activity of zinc and bismuth catalysts was similar, but lower than that of the organotin catalyst DBTDL. |
61d817f76be4207ba824e05e | 2 | The aim of this work was to test two bismuth and one zinc-based catalysts as possible alternatives to organotin compounds for the synthesis of injectable and photocurable esterurethane macromonomers with telechelic methacrylic groups. Additionally, we also switched to a "green" solvent, ethyl acetate (EtOAc), instead of dichloromethane (DCM) used in our previous study. We examined the effect of catalyst concentration on the reaction kinetics and chemical structure of the synthesized macromonomers, as well as on the physico-chemical properties and cytotoxicity of final photocured elastomeric materials. |
61d817f76be4207ba824e05e | 3 | In the first step, 25 ml of EtOAc was introduced into a 250 ml round-bottom flask and degassed with three argon/pump cycles. Next, an appropriate amount of catalyst (2 or 4 mol% calculated relative to amount of polyester polyol) and 6.5 ml (0.052 mmol) of IPDI were added into the flask that was placed in an ice bath. At the same time, 25 g (0.013 mmol) of polyester polyol was dissolved in 25 ml of EtOAc. Next, the dissolved polyol was added dropwise into the ice-cold mixture. When the addition was completed, the flask was transferred to an oil bath and the reaction was continued at 70 °C. Progress of the reaction was monitored by tracking the ratio between FT-IR absorbance at 2262 cm -1 , which corresponds to N=C=O vibration in isocyanate groups of IPDI, and at 1526 cm -1 , which corresponds to N-H bending vibrations of the formed urethane bonds. The first step was considered completed when the ratio stabilized, typically at values between 3 and 5. |
61d817f76be4207ba824e05e | 4 | were introduced, while protecting the reaction from the light. After all of the isocyanate groups were converted, as determined by FT-IR by the absence of the band at 2262 cm -1 , the flask was removed from the oil bath and cooled down to room temperature. The product was then precipitated into four-fold excess of ice-cold methanol three times and any residual solvent was evaporated under reduced pressure at 50 °C. The obtained product was a transparent, highly viscous, sticky, yellowish liquid. The same procedure was repeated for all catalysts, for both 2 and 4 mol% concentrations. The series of macromonomers synthesized with different catalysts are abbreviated as follows: PrDBTDL, PrBiNDE, PrBiHex, PrZnAc. |
61d817f76be4207ba824e05e | 5 | Then, 1-mm-thick films were produced by pouring the final composition onto glass plate and spreading with a steel applicator. The composition was then irradiated with a DYMAX Bluewave LED Prime UVA (USA) light source, with a narrow spectral range and maximum intensity at a wavelength λ max of 385 nm. The intensity of the radiation was adjusted to 20 mW/cm 2 with the help of radiometer, AktiPrint (Technigraf GmbH). Photocrosslinking was carried out in air atmosphere, as well as under argon, in a glove box. The exposure time was 150 seconds for each spot (2.25 cm 2 ) and was carried out stepwise across the entire plate (10 cm x 20 cm). |
61d817f76be4207ba824e05e | 6 | Fourier transform infrared spectroscopy (FTIR) was performed by using BRUKER ALPHA Platinum apparatus (Germany) at room temperature in the range of 4000-600 cm -1 , at a resolution of 2 cm -1 and using 32 scans. Liquid (viscous) macromonomers were analyzed in transmission mode, after pouring samples between NaCl plates. Spectra of films after photocrosslinking were obtained using reflection mode and the ATR snap-in with diamond crystal. Spectra were analyzed using EZ OMNIC software. |
61d817f76be4207ba824e05e | 7 | Nuclear magnetic resonance (NMR) spectra of all obtained macromonomers were recorded using Bruker DPX HD-400 MHz. The instrument was equipped with a 5 mm Z-gradient broadband decoupling inverse probe. All experiments were conducted at 25 °C. Samples for NMR were prepared by dissolving approx. 20 mg of macromonomer in 0.7 ml of CDCl 3 -d. |
61d817f76be4207ba824e05e | 8 | Gel permeation chromatography (GPC) was used to determine the final molecular weights and molecular weight distributions of synthesized macromonomers. GPC measurements were In order to assess the gel fraction, the cross-linked samples were weighted (W initial ) and then refluxed for 6 hours in Soxhlet apparatus (Behr Labor-Technik, Germany) in EtOAc, which was found to be a suitable solvent for the macromonomers. The samples were dried under reduced pressure until a constant mass (W final ) was achieved. The gel fraction was calculated according to following equation (Equation ): |
61d817f76be4207ba824e05e | 9 | Dynamic viscosity tests of macromers were carried out using a BROOKFIELD AMETEK rotary rheometer (USA) with the following parameters: measuring head in the plate-plate system with a diameter of ϕ = 40 mm, distance between plates h = 1 mm, deformation of 30%, constant shear rate 𝛾̇ (1/s): 0.200, and at a temperature of 25 °C. |
61d817f76be4207ba824e05e | 10 | The mechanical tensile properties of photocured samples were assessed using an Instron 3366 (UK) testing system with a 500 N load cell, at crosshead speed of 25 mm/min. The crosshead speed was selected from the range of 10-50 mm/min, as typical for tensile testing of soft connective tissue. The samples were of rectangular: 10 mm in width, 60 mm in length, and 0.5 mm thick. The following parameters were determined: tensile strength (s br ), elongation at break (e br ), and modulus of elasticity (E). The modulus was calculated at 2%-3% and 5%-6% elongation, respectively. |
61d817f76be4207ba824e05e | 11 | Cytotoxicity of materials was tested via extract tests according to ISO10993-5 in similar fashion to our previous works. Briefly, strips of photocured materials (area: 3 cm 2 , thickness: 0.5 mm) were cut into smaller pieces, placed in a well of a 24-well plate, and incubated in 1 ml of complete growth medium (Dulbecco's Modified Eagle Medium (DMEM), containing 10% |
61d817f76be4207ba824e05e | 12 | Fetal Bovine Serum (FBS), 2 mM L-glutamine, 100 U/mL penicillin and 100 µg/mL streptomycin for 24 hours in a CO 2 incubator for cell culture at 37°C. The following controls were used: 1) negative control: commercial, non-toxic poly(e-caprolactone) (PCL, Capa™ 6430), 2) positive (toxic) control: nitrile glove (Mercator Nitrylex Classic, Kraków, Poland), 3) sham: media in an empty well. In parallel, 10×10 3 L929 cells (passage 10-25) were plated per well in a 96-well plate in complete growth medium. After incubation for 24 hours in a CO 2 incubator for cell culture at 37 °C, the medium in the plate with cells was aspirated and replaced with 100 µL of extract medium from a given sample (5-6 technical replicates, 2 samples per material). The plates were then returned to the cell culture incubator for another 24 hours of culture. Cell viability was then assessed using light microscopy (Delta Optical IB-100, Mińsk Mazowiecki, Poland) and the resazurin viability assay. Briefly, 20 μl of resazurin stock (0.15 mg/mL in phosphate-buffered saline) was added to each well (as well as to blank wells with media but no cells) and then incubated for 4 hours in the cell culture incubator at 37 °C. |
61d817f76be4207ba824e05e | 13 | Proper choice of non-toxic catalysts is the key to successful synthesis of polymers for biomedical applications. Here, we assessed the potential of two bismuth derivatives (BiNDE and BiHex) and a zinc (ZnAc) compound as alternatives to the organotin catalyst, DBTDL for the synthesis of photocurable macromonomers via two-step reaction (see scheme in Figure ). |
61d817f76be4207ba824e05e | 14 | The chemical structures of all four catalysts are presented in Figure . Overall, the reaction progress for all of the tested catalysts was similar. As a representative example, the FT-IR spectra used to monitor the progress of the synthesis with the use of BiHex 2 mol% are presented in Figure (the data for the other catalysts are available in the SI, see |
61d817f76be4207ba824e05e | 15 | Figures ). The kinetics of all the reactions (ratio of A 2262 to A 1526 as a function of time) are presented in Figure . A summary of the reaction conditions, times, and yields are presented in Table . As can be seen from Table , the reaction yields for all of the performed reactions were similar, ranging from 62 to 70%. All of the current reactions took less time, as compared to the total reaction time in our previous study (approx. 72 hours) where DCM was used as the solvent during the reaction. Further, clear differences between catalysts and concentrations were observed in the reaction times. In the case of using 2 mol% of the catalyst, it was observed that BiHex was the fastest, with a total reaction time of 7 hours, while the reactions with the other catalysts were similar-and considerably longer (19-25 hours). |
61d817f76be4207ba824e05e | 16 | When the catalyst amount per step was doubled to 4 mol%, the reaction times were reduced, as anticipated, for all catalysts, except BiHex. The increase in catalyst concentration had a particularly marked effect in the case of DBTDL, reducing the duration of II step to 1.5 hours and the total reaction time from 19 to 4.5 hours. Meanwhile, for both BiNDE and ZnAc the change in reaction time was modest (<10 hours), with total reaction times of approx. 20 hours. |
61d817f76be4207ba824e05e | 17 | These differences in the concentration dependence may be explained by differences in the catalytic mechanism between DBTDL and the remaining non-organotin catalysts (see Figure ). DBTDL catalyst acts as a Lewis acid that can interact with N=C=O groups-in this case coming from IPDI-and increase their electrophilicity (Figure ). Further, DBTDL is a Lewis acid with pKa 11 and therefore this catalyst can be expected to be more active at higher concentrations, resulting in reduced reaction times. In contrast, according to literature, the catalytic activity of both of the bismuth catalysts and the zinc one is based on an insertion mechanism (Figure ). In this case, the catalysts exchange their ligands with the alcohol-in this case the polyester polyol-or interact with hydrogen from the -OH in alcohol (here the polyester polyol). Therefore, steric effects may have a major effect on the reaction time. This can then account for the differences between BiHex and BiNDE; while both catalysts have 3 ligands, BiHex is less bulky (8 vs 10 carbons), explaining the faster reaction rate. The increase in total reaction time (from 7 to 26 hours) observed after increasing the concentration of BiHex catalyst to 4 mol%. Longer reaction time in II step for PrBiHex at 4 mol% can result from side reaction manifested by formation of allophanate crosslinks. The effect of formed allophanate crosslinks during the second step effected an increased reaction time. BiHex is used an allophanatization catalyst for polyurethane formation within the referred patent . Additionally, allophanatization reactions are reversible and dissociation of the obtained allophanates occurs quickly , therefore, we did not observe allophanates in the final structure (confirmed by 1 Hand C-NMR spectroscopy, see in Figure ). |
61d817f76be4207ba824e05e | 18 | The FT-IR measurements also permitted us to confirm that the chemical structures of the obtained macromonomers were consistent with the assumed reaction mechanism and with our previous work where DBTDL was used as catalytic system. Overall, all of the obtained spectra were similar, indicating that the macromonomers obtained using the new catalysts had the same chemical structures. A representative FT-IR spectrum of PrBiHex_2 macromonomer, as compared to the starting material, polyester polyol Priplast-1838, is presented in Figure . |
61d817f76be4207ba824e05e | 19 | The chemical structure of the obtained macromonomers was also confirmed by 1 H-NMR and C-NMR spectroscopy. The analysis of NMR spectra also confirmed that all of the obtained macromonomers had similar structures, consistent with our expectations based on our prior work. As a representative example, Figure presents 1 H-and C-NMR spectra of macromonomer obtained using BiHex at 2 mol% per step (PrBiHex_2). The remaining 1 H-NMR spectra for the materials synthesized with other catalysts are included in the SI (see Figure and S12). |
61d817f76be4207ba824e05e | 20 | The results of GPC analysis are presented in Figure and Table in the SI. For all of the macromonomers, an increase of molecular mass and decrease of dispersion was observed, as compared to the starting polyester polyol, Priplast 1838. For reactions conducted with 2 mol% of catalyst per step, the highest M w (~13000 g/mol) was obtained with BiHex as catalyst. |
61d817f76be4207ba824e05e | 21 | Meanwhile, for the case of reactions with 4 mol% of catalyst per step, the highest M w , also approx. 13000 g/mol, was obtained with use of ZnAc. The Ð of all obtained macromonomers was similar, approx. 1.75. Likewise, for all macromonomers, the shapes of the chromatograms are similar, with only small shifts observed. Importantly, the obtained macromonomers do not differ significantly in M w from those obtained in our previous work, despite the changes in reaction conditions (catalysts and solvent). The results of dynamic viscosity measurements are presented in Table and Figure . For all of the macromonomers a marked increase in viscosity was observed, as compared to Priplast, indicating successful synthesis. As expected, the data largely correlated with the M w results (see Figure in SI). For use in minimally invasive procedures, the rheological properties of an injectable materials play a key role. They must be viscous enough to be locally retained, but not so much that the material cannot be properly delivered. We conclude that the high viscosity of the obtained macromonomers should ensure that they are well retained at the desired site during the duration of the photocuring process. At the same time, with viscosities <1000 Pa s, the macromonomers show injectability with the use of 16 G needle (Figure and short video in Supplementary Information). While details of a clinical delivery device (syringe, catheter, etc.) are beyond the scope of this work, we conclude that these materials should be well-suited for minimally invasive surgical procedures that use relatively large gauges of instruments, including laparoscopic and catheter procedures, such as those that may be used for cardiac repair . |
61d817f76be4207ba824e05e | 22 | A difference in absorbance of the band at 1643 cm -1 , corresponding to stretching vibration of C=C, was observed. The carbon-carbon double bonds present in the macromonomer from the attached HEMA are converted into single carbon-carbon bonds during photopolymerization process. Thus, the higher absorbance of this band after photocuring in air indicates that oxygen inhibition had occurred. However, the gel fractions (see Table , in the SI) of all photocured elastomers were similar, approx. 93%-curing atmosphere did not affect gel fraction results. |
61d817f76be4207ba824e05e | 23 | A summary of the mechanical properties of all of the obtained materials is presented in Figure , while representative stress-strain curves can be found in the Supplementary Information Figures S16-17. Overall, the results were broadly similar, with the differences possibly a consequence of manual sample preparation (both photocuring and cutting). It can be also observed that for the case of the non-organotin catalysts, the photocuring atmosphere (air vs argon) did not have a marked effect on mechanical properties. Overall, the tensile strength was measured up to 3.5 MPa, elongation was up to 120%, moduli at 2-3% strain and 5-6% strain were in the range of 2-6 MPa. However, in the case of the materials synthesized with both concentrations of the organotin catalyst DBTDL, the photocuring atmosphere did have an effect on the tensile strength and elongation at break: the values of these parameters obtained from materials cured under argon were approx. double those of materials cured in air. This may be explained by the oxygen inhibition having a negative effect on the photocuring process, and in result, in final material properties. We conclude that the mechanical properties of materials obtained using non-organotin catalysts indicate their potential in medical applications. While the values are higher than those reported for living human abdominal wall tissue, as measured during laparoscopic procedures, the values fall in ranges considered typical of "soft tissue" in the biomechanics literature: Young's modulus in the 0.1-10 MPa range and ultimate tensile strength and ultimate tensile strains in the range of 0.3-100 MPa and 10-120%, respectively . Further, compared to other materials intended for use in soft tissue engineering, like hernia repair, reported in the literature, our materials have approx. 10-times lower modulus of elasticity values . . As anticipated, the use of tin-free catalysts resulted in higher cell viability for all tested conditions (catalyst concentration and atmosphere), with a trend towards higher viability in the case of materials synthesized with ZnAc catalyst. As an organotin catalyst, DBTDL is known to be highly toxic and-equally importantly-is very difficult to remove from polymers during purification. Based on the literature, DBTDL can be estimated to have an IC50 of approx. |
61d817f76be4207ba824e05e | 24 | 3 µg/mL, which is an order of magnitude more toxic than various zinc and bismuth salts. However, the comparisons are not perfect due to differences in experimental details. We estimate that the upper bound for residual catalyst content in our samples as prepared for the cell culture tests is in the range of 100-150 µg. Thus, given the similar gel fractions for all samples, the differences in cytotoxicity are likely the result of toxicity of residual DBTDL (see also Figure ). In order to assess the potential for oxygen inhibition , we photocured materials in both inert (argon) atmosphere as well as in open air, as may be expected to occur during a surgical procedure. We did not observe any effect of the atmosphere present during photocuring, indicating that the effect of oxygen inhibition is relatively modest. Likewise, FTIR analysis of all of the materials confirms the complete purification of residual HEMA (Figure ), which has a reported IC50 of approx. 1.3 mg/mL . We conclude that all of the obtained materials using non-organotin catalysts can be considered well-suited for photocuring in situ without specialized oxygen-free conditions. At the same time, this study and the cell culture experiments were intended to serve as a proof-of-concept and screening; additional animal studies will be needed to address in vivo outcomes such as inflammatory response and fibrosis. |
61d817f76be4207ba824e05e | 25 | We report here the synthesis of macromonomers containing ester-urethane linkage using three different non-organotin catalysts (BiNDE, BiHex, and ZnAc). FT-IR and NMR studies indicated that all of the tested catalysts resulted in the same structures for the ester-urethane macromonomers. Further, after photocuring, the elastomeric networks obtained from macromonomers synthesized with zinc and bismuth catalysts had suitable mechanical properties for soft tissue regenerative medicine and lower cytotoxicity, as compared to DBTDL-regardless of photocuring atmosphere. Collectively, our results indicate that it is possible to reduce the overall health and environmental safety impact of this macromonomer synthesis reaction by using less toxic catalysts and "green" solvent (ethyl acetate). This is very beneficial not only from the point of view of potential biomedical applications, but also from the safety of the process and overall life cycle of the materials. After considering of all measured parameters as well as the reaction times, we conclude that 2 mol% of BiHex catalysts may offer the best compromise between reaction time, mechanical properties, and cell viability. |
655677246e0ec7777f175636 | 0 | Homogeneous photocatalysis has cemented itself over the last 15 years as a fruitful strategy for the synthesis of organic compounds, including those of relevance to the pharmaceutical and agrochemical industries, allowing access to a wide range of reactivities that would otherwise be thermally inaccessible. During this renaissance of photocatalysis, enormous progress has been made in both reaction and photocatalyst (PC) development. |
655677246e0ec7777f175636 | 1 | However, a broad overview of the use of organic PCs 5 has revealed that the photophysical and electrochemical parameters are frequently taken from the prior art, which are obtained in conditions distinct from those used in the reaction. Thus, the effect of the medium is not considered when assessing the thermodynamic driving force for the required photochemistry. |
655677246e0ec7777f175636 | 2 | Photocatalysis reactions generally proceed through one of two distinct pathways: photoinduced electron transfer (PET), commonly termed photoredox catalysis, and photoinduced energy transfer (PEnT) (Figure ). Both scenarios are initiated by photoexcitation of the PC to generate PC*. Providing that the lifetime of its excited state is sufficiently long-lived (on the order of nanoseconds), diffusion of the substrate and PC* to form an encounter complex occurs competitively to other radiative or non-radiative decay pathways. The rate of the photocatalytic reaction is generally assumed to be dependent upon the concentration of PC*, which is itself correlated with the molar absorptivity e of the PC at the wavelength(s) of irradiation as well as its excited-state lifetime. |
655677246e0ec7777f175636 | 3 | transfer (DET) occurs through a simultaneous double electron exchange mechanism between the PC* and the substrate in its ground state. For this to be operational, the excited-state energies between the donor (the PC) and the acceptor (the substrate) must be close as there must be spectral overlap between the emission of the PC and the absorption of the substrate; the difference in triplet energies (ET) of the PC and substrate are commonly used as a surrogate for spectral overlap to predict whether a transformation is likely to occur via DET (Equation ). ∆𝐸 9 = 𝐸 9 (𝑃𝐶) -𝐸 9 (𝐴) (7) The matching of excited-state energies is encompassed in the spectral overlap requirement of DET, that is the overlap of the phosphorescence of the PC and the spin-forbidden absorption of the substrate. Practically, spectral overlap in these cases is challenging to quantify on account of the spin-forbidden nature of the transitions, especially the low absorptivity of the triplet absorption of organic substrates. Additionally, DET also requires orbital overlap between the two species involved in the encounter complex. The quantitative correlation of these prerequisites with the rate constant of DET, kDET, is described by Equation . |
655677246e0ec7777f175636 | 4 | where K defines specific orbital interactions between the donor (PC*) and the acceptor (substrate), J is the spectral overlap integral, RDA represent the donor-acceptor distance and L is the sum of the van der Waals radii of the donor and acceptor. A smaller ΔET (Equation ) will correlate with a greater degree of spectral overlap, and as a consequence, a faster kDET (Equation ). In addition, for the energy transfer to be exergonic, ΔET should be greater than zero. For the cases where ΔET < 0, the transition is endergonic. As spectral overlap remains a requirement of DET, the implication of ΔET < 0 is that the PC* must have vibrational or rotational states that are greater in energy than some of those of the acceptor in its electronic excited state in order for the DET to be thermodynamically feasible. To fully assess and understand the yields achieved by a particular PC in a photocatalytic reaction, the aforementioned optoelectronic parameters that govern PET or PEnT must be determined. For example, for PET an accurate measurement of both the ground and excited state redox potentials, the latter dependent on the excited-state (singlet or triplet, depending on the type of PC) optical gaps, are required. For PEnT, the ET of the PC should be evaluated. |
655677246e0ec7777f175636 | 5 | These parameters in many cases are solvent dependent and the values that are cited in the photocatalysis literature are frequently recorded in different media to that used in the photocatalysis reaction, thus obscuring the real thermodynamic driving force. To best identify a suitable PC for a specific reaction and to rationalize its performance, assuming that kPET/kPEnT governs reaction yield, optoelectronic data obtained in the same solvent need to be available. |
655677246e0ec7777f175636 | 6 | In terms of photophysical measurements, as long as the PC is sufficiently soluble in the solvent, a useful measurement can be taken; however, for electrochemical measurements that require a conductive solution, non-polar solvents are generally unsuitable. Gratifyingly, a survey of the photocatalysis literature indicates that polar aprotic solvents are most often used, particularly MeCN, DMSO and DMF. A library of these parameters would serve as a useful resource to aid in the decision making as to which PC should be used for a particular reaction class and substrate. Indeed, an accurate knowledge of the optoelectronic properties of the PC in concert with those of the substrate(s) are essential for an analysis of the thermodynamic driving force for a particular reaction. However, an assessment of the competing kinetics of both photophysical and photochemical processes are often neglected, as are the solubility and the stability of reaction intermediates. To address this identified issue and given the complex influences the solvent can have on a photocatalytic reaction, we selected a series of eight versatile and commonly used PCs for optoelectronic characterization in four solvents of varying polarity, to understand how variation of the photophysical properties of the PC with solvent polarity can be correlated to photocatalytic performance. Although we recognise that solvent polarity will also influence the optoelectronic properties of the substrates in the photocatalysis reaction, as a first step we have focused exclusively on the PC in this study. We then evaluated the efficiency of some of these PCs in representative photocatalysis reactions, encompassing reactions across a range of these solvents proceeding via either PET or PEnT mechanisms, and attempted to correlate yields to the thermodynamic parameters of the PCs. |
655677246e0ec7777f175636 | 7 | [Cu(dmp)(xantphos)]PF6, Eosin Y, 4CzIPN, 2CzPN and pDTCz-DPmS instead emit via thermally activated delayed fluorescence (TADF). After photoexcitation, the compound rapidly relaxes to its first singlet excited state (S1), after which fluorescence can occur, which in the case of TADF compounds is referred to as prompt fluorescence, with emission lifetimes on the order of nanoseconds. SOC is much less efficient in these compounds than in Ru and Ir complexes, however, the degree of state mixing is inversely proportional to the energy difference between the excited singlet and triplet states. These five compounds instead display a small energy gap, ΔEST, (i.e., < 0.2 eV) between the first singlet and triplet excited states (S1 and T1, respectively), thus enabling both ISC and reverse ISC (RISC) to occur. One of the manifestations of ISC/RISC in these compounds is the presence of delayed fluorescence, with emission lifetimes on the order of microseconds. A corollary is that both S1 and T1 states are populated. When ISC and RISC (kISC and kRISC, respectively) are fast relative to electron or energy transfer (kPET and kPEnT, respectively), a steady-state approximation may be assumed, and a Boltzmann distribution will govern the relative populations of the singlet and triplet excited states. This implies that the majority of excitons will exist in the triplet state owing to its lower energy. If kISC and kRISC are competitive or slower relative to kPET and kPEnT, then the relative populations of the excited states are determined by the relative magnitude of the rate constants. Regardless, as the energy gap between the S1 and T1 states is much smaller than the energy gap between these and the ground state of the substrates, then there should be a similar thermodynamic driving force for PET originating from either of these excited states. Most of the organic TADF PCs used for photocatalysis in the literature thus far |
655677246e0ec7777f175636 | 8 | The ground-state redox potentials of the PCs in the four solvents were determined using a combination of cyclic voltammetry (CV) and differential pulse voltammetry (DPV). While both MeCN and DCM are useful solvents for electrochemistry on account of their wide redox window, THF and DMF both have narrow electrochemical windows and so oxidation processes cannot necessarily be captured in these solvents (Figure ). As a result, the ground-state oxidation potential, Eox, could not be obtained for the majority of the PCs in these two solvents. |
655677246e0ec7777f175636 | 9 | Fortunately, the ground-state reduction potential, Ered, could be acquired in all four solvents when solubility allowed; indeed, all electrochemistry solutions were homogeneous, except for Eosin Y in THF, which was a suspension. The redox potentials versus SCE are collated in The Ered values were generally found to also shift cathodically with increasing solvent polarity (Table ), suggesting that polar media serve to render the PC given that the UV-Vis absorption spectrum is insensitive to solvent polarity (vide infra), which implies that all solvents contain the same structural form of Eosin Y. As solvent polarity increases, the energy gap between the ground state oxidation and reduction potentials, ΔE, was generally found to decrease (Table ). However, as stipulated previously, we can only cautiously assign this trend given that Eox could typically only be determined in two out of four of the solvents. In order to understand the origins of the effect of solvent on the electrochemical properties of the PC we conducted DFT studies to predict the redox properties of the PCs. As a first step, the IP and EA of each PC was calculated using DFT via the delta-SCF approach in each of the four solvents of interest (Figure , Table ). Each solvent was modelled implicitly, using the integral equation formalism polarizable continuum model (IEF-PCM) and the default dielectric constant for each solvent as implemented in Gaussian 16. For all the DFT results, the organometallic complexes were modelled as single species (i.e., without their outer-sphere counter anion), due to the difficulty of optimising the interaction with a loosely bound ligand. |
655677246e0ec7777f175636 | 10 | Such a methodology has been effectively used to accurately predict IP and EA in previous reports. Both the IP and EA are reported as electron binding energies (i.e., as negative values) to function as more accurate predictions of the PC's oxidation and reduction potentials than those that are estimated from HOMO and LUMO energies. |
655677246e0ec7777f175636 | 11 | The calculated IPs showed a strong dependence on the solvent (Table ), with an average range of 265 meV across the four solvents, and a greatest absolute difference of 541 meV for [Ru(bpy)3] 2+ in THF (-6.58 eV) versus in DMF (-7.12 eV). The calculated IPs for all the PCs were less negative in higher polarity solvents (equivalent to a cathodic shift of the Eox), except for [Ru(bpy)3] 2+ , which has a more negative IP in the more polar solvents. This may be due to the more shielded electron density on the metal in this complex than the other compounds. |
655677246e0ec7777f175636 | 12 | In relation to photocatalysis, this would make the PC +• a stronger ground-state oxidant (Figure ). Thus, in more polar solvents, only [Ru(bpy)3] 3+ is predicted to be a stronger ground-state oxidant, while the other seven PC +• are predicted to be weaker oxidants, with their oxidizing capacity increasing with decreasing solvent polarity. This computed trend for the PCs By contrast, the calculated EAs showed less variation with solvent than the IPs (Table ), |
655677246e0ec7777f175636 | 13 | The UV-Vis absorption spectra of the PCs across the four solvents are shown in Figures , with the absorption maxima and molar absorptivity collated in Table and reflected graphically in Figure . If there is a large change in the permanent dipole moments between ground and excited states, solvatochromism can be observed. For the majority of the PCs investigated, and particularly for the transition metal complexes, minimal changes in the absorption profile were detected. Molecules with a small permanent dipole moment in the ground state (close to zero) often display negligible absorption solvatochromism. CT states, and consequently, small variations in ε can be observed, as may be the case for the changes in spectra for 4CzIPN and 2CzPN (Figure and S25). In photocatalysis, the reaction rate is dependent on the number of photons absorbed by the PC, which is governed by e. Modern excitation sources, such as Kessil LEDs, emit light over a narrow range, with a spectrum that is Gaussian in nature. By contrast, compact fluorescence light (CFL) sources |
655677246e0ec7777f175636 | 14 | emit irregularly yet broadly over the visible light spectrum. Thus, one surrogate for an assessment of reaction rates of photocatalysis reactions using LED excitation sources would be to evaluate to relative magnitudes of ε of the different photocatalysts at wavelength of maximum intensity of the excitation source, noting that this provides only a crude estimate given that light is absorbed by the PC across the entire emission spectrum of the excitation source. |
655677246e0ec7777f175636 | 15 | Any subtle changes in the UV-Vis absorption spectra will ultimately have an impact on the ε value, meaning that aside from the thermodynamic driving force that may change with solvent (such as redox potentials), the kinetics of the reaction will also be impacted by the polarity of the solvent. To allow for a fair cross-comparison of the PCs, the same optical density of the excited PC should be present in the reaction mixture. This in practice is not done (and would not be something that most synthetic photochemists would take into account) and we can only comment on the variation of the ε values at the excitation wavelengths chosen for the photocatalysis reactions (456 nm and 390 nm) and assess how this may correlate with the yield of the reaction. |
655677246e0ec7777f175636 | 16 | If the excited-state dipole moment (μe) is greater in magnitude than the permanent dipole moment associated with the ground state (μg), positive emission solvatochromism is more pronounced than the absorption solvatochromism. For PCs whose lowest energy excited state is CT in nature, μe is expected to be large, thus leading to a strong positive solvatochromism (Figures ). Positive solvatochromism is observed in each of the organic PCs 4CzIPN, 2CzPN and pDTCz-DPmS (Figure , Table ), confirming the CT character of the emissive As illustrated in Figures , solvent can influence the energy and profile of the emission spectra of the PC and, as a result, in the optical gap, E0,0 (Table , Figure ). For the 4d and 5d transition metal PCs, the optical gap between the ground state and first triplet excited state, E0,0(T1), is most significant because, as previously discussed, PET occurs exclusively from the triplet excited state. E0,0(T1) can therefore be inferred from the onset of the phosphorescence spectrum. For the organic PCs and [Cu(dmp)(xantphos)]PF6 it is less clearly defined whether the SET will originate from the S1 or T1 states owing to their intrinsic TADF nature. However, as previously discussed, it is more likely that the SET will occur from the T1 state. Despite this, from the room temperature, steady-state emission measurements, only E0,0(S1) can be determined (from the intersection point between the normalized absorption and emission spectra). Therefore, this value was used for subsequent determination of excited-state redox potentials. |
655677246e0ec7777f175636 | 17 | Generally, E0,0 decreases with increasing solvent polarity (Figure ), reflecting the positive solvatochromism exhibited by many of the PCs in this study. For example, the E0,0 The ET measurements for TADF compounds are typically obtained from the gated emission spectra at 77 K; however, at this low temperature the solvent forms a glass. In the glass, there is no opportunity for solvent reorganization and thus, the measurements at 77 K do not capture any reorganization of the solvent dipoles that would be responsible for the stabilization of the CT states. We nonetheless measured the gated emission spectra of [Cu(dmp)(xantphos)]PF6 |
655677246e0ec7777f175636 | 18 | and 4CzIPN (Figures ), with the T1 values provided in Table . The unproductive for the determination of ET. We thus estimated the ET from the difference between the experimentally determined S1 energy at RT and the DEST obtained at 77K. The ΔEST of these PCs can be inferred from the difference in energy of the onsets of the steadystate and millisecond-gated emission spectra at 77 K (Table ). Since the S1 state of TADF compounds typically has greater CT character than the T1 state, the S1 state will be stabilized to a greater degree than the T1 state in polar solvents, meaning that ΔEST values will decrease in polar solvents compared to the value obtained at 77 K. Thus, the true value of ET will be lower in energy than the one estimated by us. Using our method, the ΔEST was determined at 77 K to be 0.03 eV for 4CzIPN, resulting in an estimated ET value of 2.62 eV in MeCN. The experimentally determined/estimated ET values of the PCs are collated in The excited-state redox potentials, E*ox and E*red, which are themselves dependent on the E0,0 |
655677246e0ec7777f175636 | 19 | After demonstrating that the optoelectronic properties of the eight PCs do vary significantly with solvent, we sought to establish whether there were any correlations between these values and reaction yields in a photocatalysis reaction. Since data collection was most comprehensive for Ered and E*red as opposed to Eox and E*ox, we initially focused on evaluating a photoredox reaction proceeding via a reductive quenching cycle. |
655677246e0ec7777f175636 | 20 | As such, the photocatalytic pinacol coupling reaction was chosen as a model reaction, using benzaldehyde as the substrate (Figure ). This reaction has previously been described to proceed through a reductive quenching cycle, whereby the excited PC is reduced by the sacrificial reductant N,N-diisopropylethylamine (DIPEA) [(E(DIPEA•+/DIPEA) = 0.65 V vs SCE in MeCN], which then is employed to reduce the aldehyde to its radical anion. Although reduction of benzaldehyde is challenging (Ered = -1.80 V vs SCE in DMF), the weak attraction of the protonated DIPEA with the aldehyde has been proposed to render this process less thermodynamically demanding. The oxidation potential of DIPEA was first measured in each of the solvents used (Figure and Table ). There is a progressive cathodic shift of the Eox of DIPEA with increasingly solvent polarity (Eox = 0.93 V in THF and 0.65 V in MeCN). Therefore, despite the PCs generally being weaker photoreductants in more polar solvents, this is somewhat offset by DIPEA being more easily oxidised in polar media. |
655677246e0ec7777f175636 | 21 | In the first instance, the conditions of Schmid et al., were followed. As documented in the literature, and confirmed in our set-up, [Ru(bpy)3](PF6)2 cannot perform this reaction in any of the four solvents investigated (Table ). In light of our recent work, where we showed that increasing the reaction time of the pinacol reaction from 2 h to 24 h can lead to substantial increases in reaction yield, we conducted the reactions using each of Ir(ppy)2 |
655677246e0ec7777f175636 | 22 | , Eosin Y, 4CzIPN and 2CzPN over 24 h (Table ). At this longer reaction time it was observed that, in general, with increasing solvent polarity the yield of the target product increases. This correlates with the trend observed in the electrochemistry (Figure ); as the solvent becomes more polar, the PC Aside from the redox potentials changing as a function of solvent, the molar absorptivity of the PCs is also affected, thereby affecting the concentration of the PC* and the probability for a formation of a productive encounter complex. The ε values of the PCs at 456 nm, the excitation wavelength, are provided in Table . There is, however, no correlation between these ε values and yields. The implication of a lack of correlation where one should exist, suggests that under the reaction conditions, the rate of PET between PC* and DIPEA does not govern the product yield. Rather, the second SET, involving the challenging reduction of benzaldehyde, is more likely to the rate-limiting step in the reaction. Under our reaction conditions, the mixture is a homogeneous solution, except for Eosin Y in THF and DCM, and |
655677246e0ec7777f175636 | 23 | Alongside redox potentials and molar absorptivity, it is important to acknowledge that other off-cycle processes within the reaction may be solvent dependent such as PC degradation and unproductive side reactions. These will affect the global yield achieved and the rates of these processes may also be solvent dependent. This therefore makes establishing correlations between solvent polarity and yield difficult to ascertain. |
655677246e0ec7777f175636 | 24 | a Yields determined by 1 H NMR analysis of the crude reaction mixture using 1,3,5-trimethoxybenzene as the internal standard. Yields provided are the sum of the meso:dl isomers and are the average yields of at least two independent runs with the standard deviation provided. b Reactions conducted for 2 h. c |
655677246e0ec7777f175636 | 25 | To better understand the variations in yield with solvent, Stern-Volmer (SV) quenching studies were performed with some of the PCs in the four solvents, using DIPEA as the quencher. We could observe no correlation between the SV quenching results and the solvent polarity (Figures S54-S59 and Table ). Since the Stern-Volmer quenching studies surprisingly provided no additional insight into the yields obtained, we next assessed the photostability of the PCs in each solvent under the pinacol coupling reaction conditions. UV-Vis absorption spectra were acquired before and after each 24 h reaction (see Figures for all UV-Vis absorption spectra). It was observed that the absorption spectra of all the PCs changes significantly after the reaction compared to before. |
655677246e0ec7777f175636 | 26 | For both iridium PCs, [Ir(ppy)2(dtbbpy)]PF6 and [Ir(dF(CF3)ppy)2(dtbbpy)]PF6, the MLCT/LLCT absorption band remains relatively unchanged after irradiation in THF, DMF and MeCN, but at wavelengths below 300 nm, the spectra are typically blue-shifted compared to those before irradiation. This implies that structural changes are likely occurring on the ancillary ligand during the pinacol coupling reaction, a conclusion that is consistent with the work of Bawden et al. who showed that 2,2'-bipyridine type ancillary ligands, when used in combination with hydrogen atom transfer (HAT) electron donors, are susceptible to reaction; |
655677246e0ec7777f175636 | 27 | for example, [Ir(ppy)2(dtbbpy)] + was shown to form [Ir(ppy)2(dtb-H3-bpy)] when irradiated with blue LEDs in the presence of DIPEA or triethylamine. In DCM, the absorption spectra of both iridium PCs show considerable changes also in the visible region, particularly the formation of a new band at 412 nm, as well as a band at 314 nm for [Ir(dF(CF3)ppy)2(dtbbpy)]PF6. This suggests that there is significantly more photodegradation in DCM relative to the other solvents, implying that the solvent itself may not be benign under the reaction conditions, which may be why the observed yields lowest in this solvent. Homolysis of the C-Cl bond in DCM can occur photochemically, subsequently decomposing this solvent, although very high energy excitation is typically required to induce this (typically less than 225 nm). It has however been documented that DCM can decompose in the presence of heterogeneous photocatalysts, such as TiO2, with UV irradiation above 300 nm. |
655677246e0ec7777f175636 | 28 | Similar to the iridium complexes, the UV region of the absorption spectrum of reactive oxygen species causing a decomposition of the PC. Moreover, the UV-vis absorption spectrum of Eosin Y has previously been shown to be sensitive to pH. In a photoborylation reaction of diazonium salts using Eosin Y as the PC, no band between 450 -550 nm was present in the UV-vis absorption spectrum, while after the addition of base, this band appeared. This is presumably due to the formation of the dianonic form of Eosin Y that is associated with this band. Therefore, under the present reaction conditions, the loss of this band after irradiation suggests that the dianionic form of Eosin Y is no longer present, either because it resides back in the neutral form (although this seems unlikely due to the large excess of basic amine present), or the PC has degraded. |
655677246e0ec7777f175636 | 29 | The low energy CT band in the absorption spectra of both 2CzPN and 4CzIPN is significantly blue-shifted after irradiation, consistent with photosubstitution studies previously reported by us and others. In particular, the work of Kwon et al. revealed that one of the cyano groups can be substituted by an alkyl group when the cyanoarene-based PCs are irradiated in the presence of DIPEA; in the case of 4CzIPN, the nitrile was photosubstituted by an ethyl group as the major product, and a methyl group as the minor product. Although 4CzIPN is observed to photodegrade similarly in all four solvents, the absorption spectra of 2CzPN are considerably less changed in THF relative to DCM, DMF, and MeCN. This observation is consistent with 2CzPN achieving a higher yield in THF of 31% compared to the 2-11% yield in the other solvents, implying that the photodegradation occurring in the other solvents inhibits the photocatalysis, likely due to the poorer spectral overlap with the 456 nm excitation source. |
655677246e0ec7777f175636 | 30 | In order to assess the photostability of the PCs when applied to reactions not involving sacrificial amine donors, we selected two other photoredox reactions: an atom transfer radical addition (ATRA) reaction of tosyl chloride with styrene (Figure ) and the Giese type addition of N-Cbz-Pro to diethyl maleate (Figure ). The 1 H NMR yields obtained in these reactions for the PCs in MeCN are provided in Tables and. For both reactions, literature yields could be replicated in our set-up; in the ATRA reaction [Ru(bpy)3](PF6)2 achieved 75% yield of the target product, which is comparable to the literature yield of 80%. Similarly, in the Giese type addition reaction, 4CzIPN yielded 77% of the functionalized product using our set-up, comparable to the 80% literature yield. The UV-Vis absorption profiles before and after irradiation for all PCs employed in both reactions are given in Figures . In general, PCs that did not promote the formation of the target product or did so only in very low yields largely retained their UV-Vis absorption profile. For example, 4CzIPN yielded only 3% of the target product in the ATRA reaction and the UV-Vis absorption spectrum is essentially unchanged after the reaction compared to that prior to irradiation (Figure ). In contrast, when PCs can photocatalyze the reaction, the UV-Vis absorption profile is significantly altered. This is exemplified for [Ru(bpy)3](PF6)2, which produces 75% of the product in the ATRA reaction and after irradiation, the characteristic MLCT absorption band at 450 nm is blue-shifted to 400 nm (Figure ). |
655677246e0ec7777f175636 | 31 | To check whether the PCs were degrading into the same species, irrespective of reaction conditions, the post-irradiation absorption spectra were overlayed for [Ir(ppy)2(dtbbpy)]PF6, [Ir(dF(CF3)ppy)2(dtbbpy)]PF6, [Cu(dmp)(xantphos)]PF6 and 4CzIPN (Figure ), since these PCs were shown to produce the desired product in at least two of the three photoredox reactions considered. For the metal complexes, no trend could be discerned; however, for 4CzIPN, the post-irradiation absorption spectra obtained in both the pinacol coupling reaction and the Giese type addition reaction are almost identical (Figure ), implying that for this PC, the photodegradation product is similar. This is very likely linked to the photosubstitution of the nitrile group of 4CzIPN for an alkyl group from DIPEA in the pinacol coupling, as shown by Kwon et al., and for an alkyl group from the decarboxylated N-Cbz-Pro, as shown by the work of König and co-workers. The photostability experiments from the ATRA and the Giese type addition reactions indicated that photodegradation of the PCs is not limited to reactions containing sacrificial amine donors, but instead is observed more widely in these other representative photoredox reactions. As such, it is clear that the photostability of the PC should be assessed as a required experiment by the photocatalysis community. Recently, a few reports have explored the complex issue of photostability, for example Wenger et al. noted that functionalised isoacridone dyes tended to decompose upon irradiation in solution; 63 the predominant photodecomposition pathway was proposed to involve the T1 state, which is sensitized in the presence of anthracene. The photodegradation of the PC may be a reason why some PCs perform poorer than expected in certain reactions, despite having suitable thermodynamic properties, which may explain why trends in PC yields cannot always be rationalized by factors such as redox potentials. The photodegradation of the PC means that it becomes difficult to ascertain whether the parent compound and/or its photodegraded version is/are the active PC(s) in the reaction. |
655677246e0ec7777f175636 | 32 | To understand the influence of solvent in a PEnT mechanism, the popular E/Z isomerisation of alkenes was chosen as a representative reaction. Four of the aforementioned 4CzIPN were investigated in the E/Z isomerisation of stilbene across all four solvents (Figure ). The E and Z isomers of stilbene have ET of 2.2 and 2.5 eV, respectively, determined from the absorption spectra in ethyl iodide. Therefore, to maximise the yield of the Z-isomer, simplistically the PC should have an ET within the range of 2.2-2.5 eV. This allows the PC to chemoselectively transfer energy to the E alkene, converting it to the Z isomer without also sensitizing the Z isomer back to the E isomer. |
655677246e0ec7777f175636 | 33 | The experimentally derived ET values of the PCs (Table ) can then be applied to the target E/Z isomerization of E-stilbene (Figure ). Given that the ET of the E and Z isomers of stilbene are determined from the onset of the absorption spectra in ethyl iodide and the ET of the PCs are determined from the onset of emission spectra (steady-state or gated emission at 77 K), the triplet energies are not strictly directly comparable since different methods and solvents have been used for their estimation. However, since triplet energies only serve to predict whether an ), with 89% of Z-stilbene remaining. This mirrors the results of the forward E/Z isomerisation reaction; 91% of Z-isomer is formed when irradiating E-stilbene, indicating that the same Z/E ratio is obtained whether irradiating E-stilbene or Z-stilbene for 24 h. |
655677246e0ec7777f175636 | 34 | Given that variation of up to 270 mV was observed in the redox potentials as a function of solvent, greater attention should be paid to this factor when assessing the thermodynamic feasibility of a PC to react with a particular substrate. To better predict the capacity of a PC to undergo specific SET with a substrate, optoelectronic properties should be measured in the same solvent used for the subsequent photochemistry, this will also facilitate the rationalisation of proposed reaction mechanisms. |
655677246e0ec7777f175636 | 35 | We investigated the impact of solvent in model electron transfer and energy transfer photocatalysis reactions. In the case of photoredox catalysis, the PCs that could successfully form the target product were found to photodegrade, and as shown in the pinacol coupling reaction, this occurred regardless of solvent choice. The implication of this observation is that photochemical reactions involving radical chemistry appear to alter the properties of the PC and in these cases, it would be more appropriate to call these reactions photosensitized rather than photocatalyzed. Significantly greater attention should be paid to the photostability of PC in photoredox catalysis as this may explain why some PCs may perform better than others. |
655677246e0ec7777f175636 | 36 | In relation to energy transfer reactions, the solvent choice had a more pronounced effect on the ET for PCs that have low-lying CT excited states. For both PEnT reactions, no photodegradation of the PC was observed; however, ET was found to be an unreliable indicator of PEnT efficiency. This study reveals that the long-held dogma used to identify the optimal PCs for both PET and PEnT reactions should be questioned. |
65fac53d66c138172962dc12 | 0 | Redox Flow batteries (RFBs) stand out among other energy storage technologies due to their modular design and long cycle life . Vanadium Redox Flow Batteries (VRFBs) are currently the market leaders, owing to the possible re-utilization and rebalancing of their electrolytes . However, they still need to boost their competitiveness, given their higher costs and lower overall performance compared to other energy storage technologies, such as lithium-ion batteries . To improve the performance and operational flexibility of VRFBs, a comprehensive understanding of the electrolyte flow within the electrochemical conversion cell and the storage tanks is imperative. In particular, operational variations of electrolyte density and viscosity are known to affect VRFB operation. |
65fac53d66c138172962dc12 | 1 | Density differences between the renewed and resident electrolyte give rise to buoyancy forces that affect the electrolyte fluid dynamics and mixing within the tanks, potentially leading to the formation of heterogeneous regions containing unreacted electrolyte, which reduces the VRFB capacity utilization . Similarly, viscosity plays a pivotal role within the electrochemical cell, affecting ion mass transport fluxes and, consequently, the overall electrochemical performance , as well as the pressure work. In summary, accurate density and viscosity data of vanadium electrolytes is necessary to improve mathematical models and optimize VRFB design. |
65fac53d66c138172962dc12 | 2 | Mousa reported density measurements for the discharged negative electrolyte for varying total vanadium concentration, sulphates concentration and temperature. Later, Rahman et al. measured the density of the positive electrolyte at 5 M vanadium concentration in 8 M sulphates concentration. Xu et al. investigated the density and thermal expansion coefficient of VOSO 4 . Skyllas et al. reported density data for both electrolytes, using 2 M vanadium concentrations in 5 M total sulphates for varying State of Charge (SoC) and temperature. However, they did not give any details regarding the experimental procedure nor provided error estimations. More recently, Ressel et al. measured the density of the negative electrolyte as a function of SoC obtaining results that were consistent with those of Skyllas et al. . Except for these two works, prior studies lack information regarding the variation of density with the SoC, a crucial parameter intrinsic to battery operation. Furthermore, a recent study by the authors has provided experimental evidence supporting previous density measurements of the negolyte, but suggesting potential overestimations in the density variations of the posolyte . |
65fac53d66c138172962dc12 | 3 | The viscosities of the posolyte and the negolyte are notably influenced by the total vanadium concentration and temperature . Li et al. compiled the most extensive database of viscosity values for varying SoC and temperature, using 1.6 M and 1.8 M vanadium concentration in 2.6 M and 2.7 M sulphuric acid solutions, respectively. However, this research did not include measurements for fully charged/discharged electrolytes, nor did it explore the combined influence of vanadium and sulphate concentrations. |
65fac53d66c138172962dc12 | 4 | In the referenced works, we hypothesize that a significant source of measurement error may come from the sample preparation methods. Previous research relied solely on electrochemical measurements and visual color cues to prepare solutions at specific SoCs, omitting more precise characterization techniques such as UV-Visible spectroscopy . To summarise, the gaps in the literature call for open access, precise and extensive data regarding the physical properties of vanadium electrolytes, especially those describing their variations with the SoC. This paper aims to provide a comprehensive and accurate database containing density and viscosity values for both positive and negative electrolytes, measured at well defined compositions, while varying the relevant parameters, i.e., the SoC, total vanadium and sulfates concentrations, and temperature. |
65fac53d66c138172962dc12 | 5 | The work is structured as follows. First, we describe the measuring equipment, the experimental procedures and the protocols employed to prepare the electrolyte samples and measure their physical properties. In a second part, we present the density and viscosity data as well as their changes with all the aforementioned parameters. We also provide multivariate regression fits that capture the variation of the electrolyte properties in all parameter space. This paper comes with Supplementary Information materials, mainly describing the quantification of the sulfate and vanadium concentrations. The Supplementary Information also contains additional density and viscosity plots and details about the empirical regression of viscosity. Finally, as part of our open science commitment, we provide the raw density and viscosity data in csv format. |
65fac53d66c138172962dc12 | 6 | The densimeter is a DMA 4500 M (Anton Paar) that uses the oscillating U-tube principle (accuracy and repeatability 5 • 10 -5 and 5 • 10 -6 g/cm 3 , respectively). The viscometer is a Lovis 2000 ME (Anton Paar) falling ball micro-viscometer with 0.5% accuracy and 0.1% repeatability. The capillary diameter was 1.59 mm and a gold coated ball (1.5 mm diameter) was used to prevent corrosion. The two systems, coupled in series in the same equipment, include ThermoBalance ™ temperature management, which allows to quickly perform accurate measurements at different temperatures, guaranteeing long-term stability for temperature scans. Upon injecting the sample, the liquid fills both the densimeter and the viscometer simultaneously. Approximately 2 mL is needed to fill the system. The samples were 4 mL, injected via a 6 mL syringe. Between each measurement, the cavities were purged injecting solvents in the following order: water-dryingacetone-drying. Deionized water was used to dilute and evacuate vanadium compounds whilst acetone was used to evacuate water for fast drying. We performed repeatability measurements to ensure that this cleaning protocol matched the values given by the manufacturer. Additionally, we regularly checked the densimeter and viscometer calibrations, ensuring that there was no remaining residue inside the system. |
65fac53d66c138172962dc12 | 7 | We generated the samples of desired composition via pipetting previously prepared reference solutions of vanadium electrolytes, sulfuric acid and water with phosphoric acid. For the negolyte, we mixed V 2+ and V 3+ (V II /V III ) and, for the posolyte, VO 2+ and VO + 2 (V IV /V V ). The pipettes were calibrated using deionized water and a scale, yielding 0.3% repeatability and 0.45% accuracy. We prepared the reference solutions via electrochemical charge/discharge from an initial equimolar V III /V IV commercial solution (Oxkem Limited, UK). The electrolyte data sheet was provided with, a vanadium, sulfuric acid and phosphoric acid concentration of, respectively, 1.8 M, 4.6 M, and 0.05 M. However, we deemed these values unreliable for our study since the commercial datasheet indicated a 10% error in the vanadium and sulfate concentrations. In particular, the density of 98% sulfuric acid is 1.83 g/cm 3 . Thus, we expect the concentration (ca. 22% in weight) of sulfuric acid to have a significant impact on the density measurements. For these reasons, we determined the sulfate, phosphate and vanadium concentration of our starting electrolyte, whose exact composition is listed in Table . We quantified the sulfate concentration using the procedure outlined in Oreiro et al. , based on the precipitation of sulfate ions and barium ions as well as precise density measurement. We measured the total vanadium concentration via spectrophotometric titration of VO 2+ 2 , oxidized by potassium permanganate (KMnO 4 ). The phosphoric acid concentration was measured by induction coupled plasma-optical emission spectroscopy (ICP-OES). Details regarding these procedures are given in the Supplementary Information. Additional information regarding the electrochemical preparation is described in our recent paper by Maurice et al. . It is worth noting that the reference V II solutions were degassed before pipetting, as during the charge of the negolyte some diluted hydrogen appears because of the electrolysis of water. |
65fac53d66c138172962dc12 | 8 | This section describes the data obtained with the procedure outlined above for measuring the electrolytes density and viscosity, while varying the State of Charge, SoC, total vanadium concentration, c V , and sulphates concentration, c S . We prepared four sample groups with c V = (1.830, 1.525, 1.220, 0.915) M while keeping c S = 4.07 M constant. In addition, we prepared three additional groups with c S = (3.40, 2.80, 2.20) M and c V = 0.915 M constant. For each sample group, we prepared six dilutions varying the SoC = (0, 0.2, 0.4, 0.6, 0.8, 1). The density and viscosity of these samples were measured at three temperatures T = (10, 20, 30) • C. Furthermore, we added 10 randomised samples with arbitrary values of the four parameters chosen within their respective evaluated range. The measurement points, with their respective parameter values and density and viscosity measurements, were collected in a separate csv format file that is freely available as indicated in the data availability section. It is important to note that in the main sample groups we varied only c V while keeping c S constant, or vice versa. This prevented the observation of the effect of simultaneous changes in both c V and c S . The random samples enabled us to capture the collective influence of all parameters at once, therefore improving the precision of the fit. Accounting for the posolyte and negolyte, the number of samples doubled to a total of 272 measurement points. |
65fac53d66c138172962dc12 | 9 | To enable the use of raw laboratory data in studies requiring the assessment of local electrolyte properties, the density and viscosity measurements were subjected to polynomial fitting using multivariable regression techniques. Below, regressions for the density, ρ j , and viscosity, ν j , of both electrolytes, j = {+, -}, are presented and discussed. The regressions are conveniently expressed as Taylor series centered around the reference values c V,0 = 0.915 M, c S,0 = 2.20 M, T 0 = 10 • C, and SoC = 0 facilitating the isolation of the effects of various parameters in distinct terms. During the derivation of the polynomial fits, terms with coefficients significantly smaller than others were neglected. |
65fac53d66c138172962dc12 | 10 | represents the density of the discharged electrolyte, ρ j 0 ≡ ρ j 0 (c V , c S , T 0 , SoC = 0), at the reference temperature T 0 and the specified total vanadium and sulphate concentrations. These expressions assume that the electrolyte density varies linearly with T and SoC, but quadratically with the total vanadium and sulphate concentrations. As a result, ρ j T = (∂ρ j /∂T ) SoC j and ρ j SoC = (∂ρ j /∂SoC) T j denote the partial derivatives of ρ j with respect to T and SoC, A j is the density at the reference state (c V,0 , c S,0 , T 0 , SoC = 0), and B j to E j represent the linear and quadratic fitting coefficients for the density variations with the total vanadium and sulphate concentrations. Density is thus expressed as the density ρ j 0 of the discharged electrolyte at a given temperature and composition (2) plus its variations with the SoC and temperature during VRFB operation . The multivariate regression coefficients obtained from the experimental campaign are listed in Table . The regression for ρ + yielded a root mean squared error (RMSE) of 7.80 • 10 -4 g/cm 3 , while that for ρ -gave a RMSE of 9.10 • 10 -4 g/cm 3 . |
65fac53d66c138172962dc12 | 11 | Figure shows the density of the posolyte (left) and negolyte (right) as a function of SoC for different temperatures and total vanadium concentrations. In all cases, the measurements (symbols) follow linear trends as highlighted by the empirical regressions (dotted lines). The density of the posolyte shows a slight increment with SoC, with ρ + SoC = 2.73 • 10 -3 g cm -3 > 0, independent from c V . The experimental results suggest a slight reduction in ρ + SoC with decreasing c V . But the reduction is so weak that it cannot be accurately captured thorugh regression fitting, as it falls within the same order of magnitude as the estimated error. Regarding the temperature dependence, the posolyte density is seen to decrease linearly with T , the value of ρ + T remaining virtually independent across all parameter space. In the context of VRFB operation, during charge the density of the posolyte should slightly increase due to the SoC increment, but, as the temperature tends to decrease , the overall density variation could be roughly cancelled. Since these effects are known to reverse during discharge, the posolyte should therefore show negligible density variations during the entire charge/discharge cycle. |
65fac53d66c138172962dc12 | 12 | By way of contrast, the density of the negolyte exhibits a notable reduction with SoC, leading to a considerably larger (absolute) value of ρ - SoC = -(1.30 + 1.46 (c V -c V,0 ))•10 -2 g cm -3 that grows with c V . These density variations are relevant for VRFB operation, as they have the potential of affecting the fluid dynamics of mixing in the negative tank . The resulting buoyancy induced flows cause the renewed electrolyte to either rise or sink upon discharge in the tank, with a direct impact on capacity utilization confirmed in recent work . Moreover, the value of ρ - T is similar to that of the posolyte, and it also exhibits negligible variations with electrolyte composition. The sulfate concentration does not interact with other parameters and only affects the term ρ j 0 reflecting the dependence on the composition of the discharged electrolyte. Figure shows contour plots of ρ + 0 (top) and ρ - 0 (bottom) in the range of vanadium and sulphates concentrations under study, highlighting the position of the measurement points in the (c V , c S ) plane. As seen in Table , the two electrolytes have very similar values of the coefficients A j -E j , so the maps of ρ j 0 are almost indistinguishable to the naked eye. In summary, the value of ρ j 0 is fundamentally determined by the parameters c V and c S , which are independent of each other, and for given values of these parameters it is practically independent of the electrolyte. |
65fac53d66c138172962dc12 | 13 | The region labeled as unexplored in Figure corresponds to values of vanadium and sulfate concentrations that have not been addressed in this study. Thus, while our empirical regression yields reasonably accurate values within the parametric range examined, its predictions will be increasingly less precise the further we go into the uncharted region. |
65fac53d66c138172962dc12 | 14 | Figure shows the viscosity of the posolyte (left) and the negolyte (right) plotted versus the SoC for various total vanadium concentrations at T = 20 • C and c S = 4.07 M. Hereafter all results will be presented in terms of kinematic viscosity. As seen in the figure, both electrolytes exhibit the same behavior: the viscosity increases with vanadium concentration but decreases with SoC. The negolyte exhibits higher viscosities (5.82, mm 2 /s) compared to the posolyte (4.55, mm 2 /s), along with a more pronounced interaction between vanadium concentration and SoC, characterized by a nonlinear dependence that decreases for increasing values of c V . . The temperature is seen to have a significant impact on the viscosity of both electrolytes, specially at higher total vanadium concentrations. At c V = 0.915 M, the viscosity behaves almost linearly with SoC. The rise in sulfates concentration also contributes to increase the viscosity, but to a lesser extend than the vanadium concentration and temperature. As a general trend, the negolyte exhibits higher viscosity and greater sensitivity to parameter variations than the posolyte. |
65fac53d66c138172962dc12 | 15 | with the coefficients F i,k,l,m being listed in Table . Note that the summations extend over all integral values of i, k, l, and m such that i + k + l + m < 4, hence the third-order multivariate polynomial fit. The regression for ν + has a root mean squared error (RMSE) of 1.48 • 10 -2 mm 2 /s while that for ν -has a RMSE of 3.16 • 10 -2 mm 2 /s. Figure shows contour maps of the fitted viscosity function for the posolyte (top) and the negolyte (bottom), for SoC = 0 (left), 0.5 (center), and 1 (right) at 20 • C. In all cases, the viscosity is seen to increase with the total vanadium concentration, and, more weakly, with the sulfates concentration. The Supplementary Information includes two additional figures showing viscosity contours for SoC = (0, 0.5, 1) at 10 • C and 30 • C (Figures and). Just as in the previous section, the region corresponding to values of vanadium and sulfate concentrations outside the parametric range of this study is labeled as unexplored. Across all cases, but particularly noticeable in the negolyte, we observe a shift in trend near the boundary of this unexplored region, suggesting a reduced influence of sulfates concentration on viscosity. This trend shift is stronger c S and as concentrations depart from the studied range. In fact, viscosity isocontours eventually reverse their direction deeper into the unexplored domain. This could simply be an artifact, as the empirical regression of ν -is not expected to accurately capture viscosity variations in this range due to the lack of experimental data. The posolyte viscosity ν + shows a similar behavior, but to a lesser extend and farther away from the studied parametric range. |
65fac53d66c138172962dc12 | 16 | An high-quality open-access database of density and viscosity measurements for the positive and negative electrolytes of vanadium redox flow batteries has been presented. The data contains 272 measuring points across a wide parameter space, including state of charge, total vanadium concentration, sulfate concentration, and temperature. The experimental data has been used to derive empirical regressions that provide the density of both electrolytes with a RMSE below 10 -3 g/cm 3 , and the viscosity with a RMSE of 1.48 • 10 -2 mm 2 /s for the posolyte and 3.16 • 10 -2 mm 2 /s for the negolyte. |
65fac53d66c138172962dc12 | 17 | The results reveal that the variations of density with SoC differs for the posolyte and the negolyte. Thus, while the posolyte density slightly increases during charge, that of the negolyte decreases by up to 2%. Compared to density, viscosity variations are more pronounced in relative terms. They are affected by all the studied parameters, the SoC being the most relevant (without considering extreme unrealistic variations in the other parameters). This effect is amplified with higher vanadium concentrations and lower temperatures. For instance, at 10 • C, a fully charged 1.83 M negolyte exhibits a viscosity that is 38% lower than when discharged. |
65fac53d66c138172962dc12 | 18 | To the best of our knowledge, this database stands out as the most extensive and highest quality available in the open literature. We attribute the quality of our data to two key factors: one the one hand, the spectrophotometric titrations used to prepare the solutions and, on the other hand, the preference for liquid weighing (whenever possible) to minimize errors associated with volume measurements. This commitment to high sample quality is reflected on the minimal statistical noise exhibited by the data points presented in section 3. We hope that these results can be of It is important to note that Li et al. utilized a Lovis 2000 M (Anton Paar) falling ball microviscometer that relies on a known density value to compute dynamic and kinematic viscosity. However, they did not specify the origin of this value, whether it was measured, computed, or obtained from the literature. This raises concerns about the accuracy of their results, as it is likely that they used the only densities available in the literature at the time, those by Skyllas et al. . As previously discussed, the lack of precise information limits our ability to further explain the discrepancies between the different datasets. |
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