Sulfoxide hemithioindigo tweezers – visible light addressable capture and release
Introducing responsive elements into supramolecular recognition systems offers great advantages for the control of intermolecular interactions and represents an important stepping stone towards multi-purpose and reprogrammable synthetic systems. Of particular interest is implementation of light-responsiveness because of the unique ease and precision of this signal. Here we present visible light responsive hemithioindigo-based molecular tweezers that bear a highly polar sulfoxide function as an additional recognition unit inside their binding site. Sulfur oxidation allows to simultaneously enhance all crucial properties of this receptor type i.e. photoswitching capability, thermal stability of individual switching states, binding affinity, and binding modulation upon switching. With a novel titration method the thermodynamic binding parameters were determined using reduced sample amounts. Employing these strongly enhanced molecular tweezers allowed to demonstrate photocontrol of intermolecular charge transfer in a reversible manner.
Introduction<!>Results and discussion<!>Conflicts of interest
<p>The control of molecular recognition processes by external stimuli is a current focus of supramolecular chemistry as an important milestone on the way to programmable synthetic behavior and multi-purpose synthetic systems. In this context reversible control is highly desirable and many switchable molecular processes have been devised that allow regulation of supramolecular association by different signals. [1][2][3][4][5][6][7][8] Of particular importance in this regard are capture and release systems with prospects in e.g. molecular machine building, 5,9 switchable catalysis, [10][11][12][13][14][15] drug delivery, 16 or responsive molecular tools for chemical biology. 17,18 A very promising signal is light irradiation because of its ease of application, high spatio-temporal resolution, waste-free nature, and straight-forward intensity regulation. [6][7][8]10,11,15 These advantages have been recognized early and consequentially light controlled hydrophobic cyclodextrin binding or metal ion capture and release represent some of the rst purely synthetic examples in the eld. [19][20][21][22][23][24] Despite a plethora of variants currently available (see ref. 25-49 for selected examples) challenges remain to this day even for fundamental light-controlled catch and release. Examples are the scarcity of visible light addressable systems, restrictions in the particular types of weak bonding interactions that can be invoked, unwanted thermal deterioration of switching states, or low ON/OFF modulation for the affinities upon switching. 6,7,38,39,50 More specically, for the majority of lightresponsive host-guest systems stilbenes, azobenzenes, diarylethenes, or spiropyranes are used as photochromic unit requiring UV light for at least one photoswitching direction. Currently many efforts are therefore directed at establishing visible light responsive behavior in such host-guest systems. [51][52][53] Typical Gibbs energies of activation for thermal cisto-trans isomerization of azobenzenes and spiropyranes are in the 22-26 kcal mol À1 range. This leads to half-lives of the metastable switching states in the min to hours time regimes at ambient temperatures and thus unwanted spontaneous backconversion. Additionally switching capacities are oentimes <80% and residual binding of low affinity forms can still be sizable leading to diminished overall responsiveness. As to the types of weak interactions that can be used to drive association, pure aromatic interactions, halogen bonding, or anion-aromatic interactions are currently strongly underrepresented in light-responsive supramolecular associations. We have addressed some of these issues with the development of hemithioindigo (HTI) based supramolecular receptors that are responsive to visible light and allow ON/OFF switching of either weak polar aromatic interactions with electron decient guest molecules (Fig. 1) 27,28 or hydrogen bonding driven recognition. 54 For example, we presented a bis-HTI receptor 1 that folds into a helix in its high affinity state in response to blue light irradiation and planarizes into a low affinity state aer heating. 27 Subsequently we devised two different molecular tweezers [55][56][57][58][59] motives 2 and 3 that respond reversibly to the same wavelengths of light with opposite affinity changes. 28 Tweezers 2 and 3 possess the same HTI core structure as switching unit making them responsive to visible light in both switching directions. The stilbene fragment (see Fig. 1 for assignment of the two fragments) is ring-fused to the photoisomerizable double bond to rigidify the structure and increase preorganization. The two electron rich biphenyl arms each bear two ortho-methyl substituents at their attachment points to the HTI core structure. The resulting sterical hindrance forces the biphenyls into a mutual parallel arrangement and thus preorganize them for the intended sandwich-type aromatic interactions. At the thioindigo fragment two different attachment points are used for tweezers 2 and 3 resulting in high affinity for the E conguration of 2 with both biaryl arms pointing to the same side of the molecule. In contrast the high affinity for tweezers 3 is obtained in the Z-conguration. In combination the tweezers 2 and 3 allowed to control supramolecular guest translocation by reduced signaling. Only one visible light signal orchestrates geometry changes, release from the rst tweezers, and capture by the second. A different wavelength of visible light reverses the translocation of the guest molecule in this system. Although both molecular tweezers 2 and 3 responded to the same wavelengths of light and both were able to regulate capture and release in an ON/OFF fashion, tweezers 3 showed nonoptimal performance in their maximum affinity as well as in their switching ability. For both molecular tweezers 2 and 3 thermal stability of the switching states was also not very high with associated Gibbs energy of activation in the 25 kcal mol À1 range. In this work we present a synthetically straight-forward approachone-step oxidation to the corresponding sulfoxide to signicantly improve all important characteristics of HTI tweezers (Fig. 1). With this approach it is possible to increase switching capabilities, affinity, as well as thermal stability of the switching statesall at the same time. In addition, we present a method enabling very convenient, precise, and repeatable assessment of association parameters for reversibly switchable supramolecular association phenomenaparticularly if light is used as stimulus.</p><!><p>In this work we introduce another type of photoswitchable molecular tweezers 4, which are based on the earlier reported tweezers 3 but point a highly polar sulfoxide moiety into their binding site (Fig. 1). Tweezers 4 incorporate a central HTI 60 moiety as photoresponsive element, which belongs to the class of indigoid chromophores 61 currently emerging as promising alternative photoswitch motives. 27,[62][63][64][65][66][67][68] Two electron rich biphenyl side arms are attached on each, the stilbene and the thioindigo, molecular fragment. The high affinity "closed" state of 4 possesses Z conguration of the photoisomerizable double bond and arranges the two biphenyl side-arms in a parallel fashion on the same side of the molecule. The arms spatial orientation and distance are ideally suited for binding of an electron decient planar aromatic guest molecule via polar aromatic interactions. Oxidation of the sulfur atom to the corresponding sulfoxide introduces a strongly negatively polarized group inside the binding cavity. In this way additional attractive interactions were projected to be available for binding of electron decient guest molecules. Thus, increased binding affinities as compared to the related unoxidized tweezers 3 were expected. Upon Z to E photoisomerization the two side arms are spatially separated and project outwards at opposite sides of the central HTI core. As a result, the binding site opens up, available attractive interactions towards the guest molecule are reduced, and the guest molecule can effectively be released. A computational study on the DFT B3LYP/6-311G(d,p) level of theory was conducted to elucidate the molecular geometries of both isomers as well as electronic and polarity characteristics (Fig. 2a and c). In addition, the supramolecular complexation with an electron decient aromatic guest was also studied theoretically to test especially interactions with the sulfoxide function. To be able to compare affinities with the previously reported HTI tweezers 3 the same aromatic guest molecule DTF ( 5) was used throughout this study. The optimized structures of Z-4 and E-4 are shown in Fig. 2a with the electrostatic potential surfaces (ESP) depicted as well. In the Z isomeric form the electron rich biphenyl arms reside at the same side of the sulfur atom atop of each other. The thioindigo-fragment and the stilbene-fragment are twisted with respect to each other by 40 , which leads to some displacement of the attached biphenyl arms. The distance between the two arms is 7.3 A as measured center-to-center between the parallel methyl-substituted aromatics directly attached to the HTI core. This distance is ideally suited for intercalation of aromatic guest molecules. As can clearly be seen by the ESP the sulfoxide moiety establishes high electron density at the center of the binding site. It offers additional positive interactions to a bound guest molecule e.g. as hydrogen bond acceptor. Together with the electron rich biphenyl arms a highly electron rich pocket is thus created that should be ideally suited for binding of electron decient aromatics via multiple noncovalent interactions. In the open E isomeric form the two biphenyl arms appear completely separated spatially and any synergistic chelating effects should thus be removed. When studying the binding of DTF guest 5 several minima structures were found for the associated complex by Monte Carlo conformer search on the MM3* level of theory. The main difference in these structures are the orientations of the DTF guest inside the tweezers cavity. The ve lowest energy structures were further optimized on the DFT B3LYP-GD3BJ/6-311(d,p) level of theory. The energetically most stable structure was found to be 2.9 kcal mol À1 lower in energy than the next minimum structure and consequentially should be populated almost exclusively. For this reason, it is discussed in the following in more detail (see the ESI † for the other minimum structures). In the complex the DTF guest is intercalated between the two Z-4 tweezers arms in a sandwich-type fashion (Fig. 2d). The distances between guest and biphenyl arms are found to be below the van der Waals radii of the constituting carbon atoms thus directly reporting on favorable polar aromatic interactions. Additional close interactions between the positively polarized hydrogen atoms of DTF and the sulfoxide oxygen atom are observed in the complex. In fact, as judging by the ESP of DTF the most positively polarized edge of the guest molecule is found pointing towards the sulfoxide oxygen, which emphasizes the importance of additional hydrogen-bond type interactions. As a result high binding affinities are expected in the closed Z-4 form but signicantly less affinities in the E-4 form. Aer this promising theoretical assessment of tweezers 4 binding capabilities, their synthesis and experimental characterization was conducted next.</p><p>Molecular tweezers 4 were synthesized by sulfur-oxidation of tweezers 3 to the corresponding sulfoxide as described in the ESI. † For the rst time crystals suitable for X-ray structural analysis could be obtained for HTI tweezers, fortunately for the closed tweezers form Z-4 (Fig. 2b). The molecular structure of Z-4 as obtained in the crystal was found to be remarkably similar to the DFT-optimized structure. This can be seen especially when overlaying the two structures as shown in Fig. 2c. There is almost perfect superposition between theoretical and crystal structure in the HTI part of Z-4. Deviations are seen mostly on the outer methoxy-substituted aromatics of the biphenyl arms. As these moieties can rotate without signicant barriers the overall agreement of the two structures is still surprisingly good. A subpopulation observed in the crystal possesses opposite stereo conguration at the sulfoxide but the rest of the structure remains exactly the same. This behavior emphasizes the effect of packing forces for the induced helicity about the isomerizable central double bond as well as for the biphenyl orientations. Because of the crystal structure the solution spectra of 4 could straightforwardly be assigned to the respective Z and E isomers facilitating study of their switching behavior and binding affinities.</p><p>Upon oxidation the (photo)physical and photochemical properties of 4 change signicantly in comparison to 3. In agreement with a recent report from our group, oxidation of the sulfur atom to the corresponding sulfoxide leads to a hypsochromic shi of the absorptionand thus to a blue-shi of the switching wavelengths. Photoswitching of 4 was tested in CDCl 3 solvent for a variety of wavelengths. Up to 88% of the E isomer could be accumulated in the photostationary state (pss) at 365 nm and up to 83% E isomer in the pss at 405 nm. Effective photoswitching can thus still be obtained with blue light, which renders these tweezers fully responsive to visible light. At 470 nm irradiation a pss containing 80% of the Z isomer was obtained, which could be increased to 83% at the same wavelength when using CD 2 Cl 2 as solvent. Compared to the unoxidized tweezers 3 (maxima of 63% E isomer at 435 nm and 84% Z isomer at 530 nm were obtained) the photoswitching was therefore also improved signicantly by oxidation of the sulfur atom albeit at the expense of hypsochromic absorption shis (Fig. 3).</p><p>For tweezers 4 the Z-isomeric state is the thermodynamically most stable state and spontaneous thermal relaxation of the metastable E isomer therefore needs to be investigated. Aer accumulation of E-4 via blue light irradiation the thermal E to Z isomerization was attempted to be followed by 1 H NMR spectroscopy in the dark. Toluene-d 8 was used as solvent to facilitate measuring the associated kinetics on manageable time scales at higher temperatures. However, heating the sample to 100 C for 27 h in the dark did not lead to a change in the isomer composition. It was thus possible to establish a lower limit for the associated Gibbs energy of activation DG ‡ by assuming conservatively that 5% isomer conversion was obtained during the heating but was not detectable by the 1 H NMR experiment. With this assumed conversion and applying rst order kinetics for a fully converting system, a lower limit of DG ‡ ¼ 33.0 kcal mol À1 was obtained (see ESI † for details). Accordingly in chlorinated solvents such as CDCl 3 or CD 2 Cl 2 no thermal conversions were observed at the limited higher temperatures (up to 50 C) available. With such high energy barrier truly bistable HTI tweezers are now available for the rst time.</p><p>We then investigated the affinity of tweezers 4 for electron decient aromatic guest molecule 5 with the aid of 1 H NMR spectroscopy (see Fig. 4). In the open E isomeric form no binding affinity is observed as proton signals of the guest and tweezers E-4 show no visible shis in the spectra. Likewise no signal shi of the DTF guest is observed in the presence of pure E-4. This behavior is similar to tweezers 3 in their low affinity open E isomeric form. The situation is quite different for the closed Z conformation of 4 where a binding constant K ¼ 1090 L mol À1 for a 1 : 1 stoichiometry was measured at 20 C by a newly developed and convenient in situ irradiation-titration method. For the latter an excess of tweezers 4 was added to solutions of the DTF guest molecule 5. Excess of guest could not be used because of its low solubility in CDCl 3 . By irradiation of the obtained solutions for varying duration times different ratios of binding Z-4 to guest 5 (with constant concentration) were obtained representing different titration points. The exact tweezers to guest ratios were determined by integration of suitable 1 H NMR signals. Because of the full reversibility of the photoisomerization of 4 it is possible to collect as many titration points as desired using the same single sample. Analysis of the chemical shi differences occurring during titration then allowed us to determine the binding affinity of Z-4 with high precision. Another advantage of this titration method becomes evident when measuring the temperature dependence of binding constants. Again the same sample can be used for measuring at different temperatures, each time collecting as many titration points as desired by simple stepwise irradiations. In this way we have determined the enthalpy and entropy contributions to the binding affinity of Z-4 with respect to DTF 5. Based on this assessment the binding between Z-4 and DTF is mainly enthalpy driven with an enthalpy contribution of DH ¼ À4.77 kcal mol À1 and an entropy of DS ¼ À2.4 cal mol À1 . When comparing the binding affinities of tweezers Z-4 (K ¼ 4100 L mol À1 at À20 C) with Z-3 (K ¼ 2300 L mol À1 at À20 C) for the same DTF guest a signicant improvement of the binding for the former is observed.</p><p>In order to test if the observed improvements of binding performance upon oxidation is a general property of HTI-based molecular tweezers we have measured the photoswitchable host-guest performance for three additional guest molecules 6, 7, and 8 comparing both molecular tweezers 3 and 4 (see Table 1 as well as Fig. 4e for a comparison of the binding parameters for all guests and tweezers combinations). All titration experiments were done in independent duplicates to obtain error estimates for the obtained binding constants K. Because of solubility issues the binding constants for guests 6 and 8 were measured in CD 2 Cl 2 solution instead of CDCl 3 . For all cases the same behavior was observed: high affinity for the closed Z-isomeric tweezers conguration and basically no affinity for the open Eisomeric form. Roughly a doubling of the binding constant for the oxidized tweezers 4 was found as compared to the unoxidized version 3 with the exception of guest 8, which was bound only 1.2 times better by 4. Additionally a remarkable selectivity for guest molecules with more extended p-surfaces was observed since the binding constants with guests 6 to 8 were signicantly weaker compared to guest 5 by factors of 31-47 (values are compared at the same temperature of 293 K).</p><p>Therefore, it can be concluded that the additional introduction of a strongly negatively polarized residue at the binding site is indeed very effective in increasing the binding affinity in HTI-based molecular tweezers for electron decient aromatic guests.</p><p>When binding of DTF (5) was studied, strongly visible changes in the color of the solutions were noticed. As shown in  Fig. 4c addition of excess 5 to a CHCl 3 solution of pure Z-4 results in a change from the initial yellow to a deep brown-red color. Such color change is indicative of a pronounced intermolecular charge transfer (CT) complex. [69][70][71][72] No signicant color change is observed for the mixture of 5 and pure E-4 except for a slight deepening of the yellow hue. Therefore, the strong ON/ OFF modulation of binding between the two isomeric states of 4 is evident by the naked eye. Upon photoirradiation of the respective solutions reversible photoisomerization of 4 takes place and an orange coloration is observed. The less obvious changes visible to the naked eye upon reversible switching between the PSS at 405 nm and 470 nm irradiation can be explained by the less than 100% switching conversion in both directions and the excess of 5 used in the experiments. This leads to still signicant color impression to the human eye even in the presence of only small amounts of remaining Z isomer. Nevertheless a deeper coloration is seen in Z-4 enriched solution as compared to the E-4 enriched one. In this way visible light photocontrol of intermolecular CT events was achieved by employing the herein reported molecular tweezers 4.</p><p>In summary we present a novel variant of HTI molecular tweezers bearing a highly polar sulfoxide within their binding site. As a result of the sulfur oxidation all properties of the tweezers are improved signicantly. Photoswitching capabilities are enhanced and fully reversible >80% enrichment of either isomer is now possible with visible light irradiation. Thermal stability of the metastable E isomer is heightened and the associated energy barrier reaches beyond 33 kcal mol À1 , which renders the oxidized tweezers truly bistable. Binding affinity of the Z isomer towards electron decient aromatic guests is almost doubled as compared to the unoxidized tweezers, while for the E isomer no discernible binding affinity is present. Because of this favorable behavior a combined irradiation-titration method could be developed that allows repeated and precise measuring of the association constant and elucidation of its temperature dependence using the same sample. Overall very high ON/OFF modulation of supramolecular aromatic binding is observed with this receptor allowing to bring intermolecular CT under the control of visible light. In the future, further applications will be explored with special emphasis on achieving water solubility and recognition of biological substrates.</p><!><p>There are no conicts to declare.</p>