Tunable solid-state multicolor emission in substitutional solid solutions of metal-organic frameworks
Realization of organic-based substitutional solid solutions will facilitate the preparation of solid-state materials with properties that arise from phenomena only displayed by molecules in solution. We demonstrate that multivariate metal-organic frameworks, MOFs, exhibit organicbased substitutional solid solution behavior by tuning their fluorescence, dictated exclusively by predetermined ratios of multivariate organic links. By combining non-fluorescent links with dilute mixtures of red, green, and blue fluorescent links we prepared zirconia-type MOFs that exhibit features of solution-like fluorescence. Our study found that MOFs with fluorophore link concentration of around 1 %mol exhibit fluorescence with decreased inner filtering demonstrated by changes in spectra profiles, quantum yields, and lifetime dynamics expected for excited state proton transfer emitters. Our findings enabled us to prepare organic-based substitutional solid solutions with tunable chromaticity. These materials emit multicolor and white light with high quantum yields, long shelf life, and superb hydrolytic stability at ambient conditions. Main TextIn solid-state inorganic chemistry, an effective strategy for the preparation of functional materials is to use substitutional solid-solutions (SSS). 1 SSS are crystalline materials formed when a solute (e.g., Cr 3+ , In 3+ , Figure 1a) is substituted into a uniform crystalline matrix (Al2O3, GaN), so that the solute atoms are dissolved within the matrix allowing for fine-tuned control of their properties for applications such as lasers 2 and blue LEDs. 3 A particularly useful property of SSS is that the materials often retain isostructural crystal types despite variation in the amount of solute that is added, 4 allowing a high degree of synthetic control over the concentration of the solute. This enables solute dependent properties to be tuned with a high degree of precision. The concept of preparing SSS, however, does not generally apply to organic materials for three
<!>Results and Discussion<!>Synthesis of Zr6O4(OH)4[NF]6 (NF MOF)<!>Synthesis of Zr6O4(OH)4[BxNF1-x]6 (BxNF1-x MOF)<!>Synthesis of Zr6O4(OH)4[(BxG1-2xRx)yNF1-y]6 ((BxG1-2xRx)yNF1-y MOF)<!>Photophysical Studies
<p>reasons: 1) unlike atoms, organic molecules are typically non-spherical, so matrix-dopant matching is a non-trivial task; [5][6] 2) they have low symmetry and unpredictable crystal-packing; 7 and 3) mixtures of organic compounds tend to phase-separate during crystallization. Examples of organic SSS are Shpol'skii matrices, where organic solutes are incorporated in crystalline hydrocarbons, but these are only stable at cryoscopic temperatures, limiting their potential as applied materials. 8 Overcoming these challenges would enable accessing materials with predetermined properties that can be easily fine-tuned following the same molecular design principles used for liquid solutions and that are also stable at ambient conditions. One notable example of a tunable, predetermined property is multicolor emission, where the bulk fluorescence of a single material is the result of the combined emission of two or more fluorophores. [9][10][11] Varying the ratios of red, green, and blue (RGB) emitters in a solid can be used to tune its emission colors, often producing white light in its many temperature variations.</p><p>Utilizing organic-based fluorophores would allow for a finer control of the photophysical properties, because of the synthetic ability to fine-tune the fluorescence of solutes and to incorporate earth abundant elements, decreasing dependence on lanthanides and other expensive elements. Here, we demonstrate that organic-based multicolor fluorescence in the solid state can be tuned in materials that are stable at room temperature, by utilizing multivariate metal-organic frameworks 12 (MOFs) as matrices for the preparation of MOF-based SSS. By diluting RGB florescent links in a non-fluorescent MOF matrix (Figure 1b), we are able to achieve tunable multicolor, bulk emission. The prepared materials retain their crystallinity at room temperature for months under normal laboratory humidity, even after direct exposure to water. We found that dilution of fluorescent links decreased their inner filtering and produced materials with high quantum yields, tunable emission colors, including white light, and other solution like behavior like excitation-emission wavelength independence, concentration dependent energy transfer, and excited-state proton transfer (ESPT). 13 Combining all these effects we were able to, for the first time, produce materials with predictable fluorescence.</p><p>Multivariate MOFs are optimal platforms for probing the SSS behavior of organic molecules, because of their high symmetry, uniform and predictable crystal packing, and their open structures that allow random distribution of links. 14 We chose the water stable, zirconium-based PPPP-PIZOF MOF (Figure S1 and 3D model in the Online Supplementary Materials) as the crystalline matrix (interweaved face centered cubic network, space group Fd-3m), 15 mixed with solute links containing emissive ESPT dyes. [16][17] Diaryl-phenanthroimidazole was chosen as the fluorophore template for its stability under MOF synthesis conditions, high emission quantum yield, and ease of synthetic variability. 15 In mixed fluorophore systems, the large apparent Stokes shift of these ESPT dyes is advantageous because it allows for single wavelength excitation of the high energy enol form of the fluorophore (Figure 1c) and emission from the low energy keto form with minimal spectral overlap. [18][19] Covalent attachment of diaryl-phenanthroimidazole to the links ensures that fluorophores will not diffuse or leach out of the solid. [20][21] The SSS were prepared using the multivariate links shown in Figure 1d, herein referred to as NF (nonfluorescent), R, G, and B. RGB were designed by modifying NF to contain diaryl phenanthro imidazole moieties functionalized at the 2-position to tune the keto emission to the desired wavelengths.</p><!><p>Crystalline single fluorophore MOFs of NF with R, G, or B produced solids samples with formulas Zr6O4(OH)4[RxNF1-x]6, Zr6O4(OH)4[GxNF1-x]6, and Zr6O4(OH)4[BxNF1-x]6, hereafter named RxNF1-x, GxNF1-x, and BxNF1-x, respectively for simplicity, with substitutional input x values between 0.001 and 0.10. All samples were prepared utilizing solvothermal MOF crystallization conditions 15 and exhibited the same powder X-ray diffraction (PXRD) pattern of the pure NF MOF (Figure 2a). Predictable and adjustable linker incorporation was demonstrated from the synthetic input/structural output plot shown in Figure 2b (Section S6).</p><p>Despite having similar absorption profiles (Figure S2), upon excitation at 365 nm, B0.10NF0.90, G0.10NF0.90, and R0.10NF0.90, exhibit solid-state emission maxima at 430, 510, and 630 nm (Figure 2c), respectively, with relatively high color purity (Figure 2d). The emission profiles of these MOFs are drastically different than the neat solid linkers (Figure S3). In contrast, the low energy keto emission in the MOFs closely resembles that of the ester forms of the RGB links solvated in toluene (Figure 2c, dashed lines), suggesting that the pore microenvironment of the MOF more closely resembles the toluene-solvated, rather than neat solids of the linker. The fluorophore mimics liquid-phase solution behavior in that the RGB links show concentration dependent changes in the emission profiles for all three fluorophore MOFs (Figure 5 2e-g). These changes are most notable in RxNF1-x and GxNF1-x where there is a gradual increase in the high energy enol emission with decreasing concentration and a shift in the CIE coordinates from the red and green regions towards the blue portion of the diagram (Figure S4). Spectral changes are accompanied by an emission lifetime and quantum yield increase from x = 0.10 to 0.01 (Figure 2e-g, Table S3). These observations are consistent with an energy transfer event 13 10 where spectral overlap between enol emission and enol absorption inhibits the higher energy light from exiting the solid. A similar effect can be observed in concentrated solutions but is presumably greatly enhanced by the high local concentration of the fluorophores in the MOF. 22 Regardless, the similarity in lifetime and quantum yield between B0.01NF0.99 and B0.001NF0.999 (Figure 2e) suggests that the inner filtering effect is minimal at concentrations of x = 0.01. This dilution can be interpreted in terms of molar concentration of the links in the MOF, or G to B as x varies from 0.9 to 0.5 to 0.1 (Figure 3a and 3b). The excitation-emission maps (Figure S5) as well as CIE coordinate diagrams with respect to λex (Figure S4) indicate that emission from the mixed fluorophore systems are excitation wavelength independent. The additive nature of the emission profiles and the wavelength independent CIE coordinates again indicates that there is minimal energy transfer in the y = 0.01 MOFs. Consequently, selective color generation along these axes could be achieved by tuning the fluorophore ratio. The three-fluorophore system with a composition of (RxG1-2xBx)yNF1-y, exhibited emission profiles that depend on the total fluorophore concentration y. As expected for y = 0.1, the emission is strongly dependent on the excitation wavelength (Figure S6-8) and generally dominated by yellow emission (R + G) with minimal high energy contribution (B) due to strong inner filter effects. For the (R0.33G0.33B0.33)0.1NF0.90 MOF, the excited state lifetimes for B (τ460 nm = 1.5 ns), G (τ560 nm = 1.8 ns), and R (τ660 nm = 1.9 ns) were similar to that of R0.10NF0.90 (τ660 nm = 1.5 ns) but notably shorter than for G0.10NF0.90 (τ560 nm = 3.1 ns) and B0.10NF0.90 (τ460 nm = 5.7 ns).</p><p>The similarity in lifetimes of the three peaks indicates an energy transfer equilibrium condition where the collective emission decay kinetics are dictated by the emitter with the fastest radiative decay rate (i.e., R) 23 . That is, B energy transfer to R and G, G enol energy transfer to R and B, and R enol energy transfer to G and B, with all these processes occurring in parallel. In contrast, for (R0.33G0.33B0.33)0.01NF0.99, lifetimes of 3.9, 2.6, and 1.6 ns for B, G, and R, respectively, more closely resemble the single component systems with no energy transfer. The excitation-emission maps (Figure S5) show a decrease in excitation wavelength dependence, as seen in other dilute samples. At these lower concentrations an increase in the enol emission is observed, most strongly for R, which results in deviations from the linear trend in the CIE coordinates (points L and M, Figure 3c). Once the concentration of R is sufficiently increased (ca. 40% of the total fluorophore concentration) the low energy keto emission is predominant (Figure 3c, point N). This sample exhibits the combined broadband emission from RGB, with CIE coordinates very close to the center of the white light region (CIE coordinates 0.31, 0.33) and a quantum yield of 4.3%. It is also worth noting that due to the blue enol emission, only a two component, R and G system (e.g., (R0.5G0.5)0.01NF0.99) is necessary to achieve white emission (CIE coordinates 0.29, 0.33, Figure S9). To our knowledge, this is the first example of crystalline bulk materials whose fluorescent properties were adjusted following the dilution strategies used in liquid phase fluorescence.</p><p>All of the MOFs synthesized are porous, stable to water, humidity, and up to 400 ºC (Figure S92), and also completely composed of earth-abundant elements. For example, B0.10NF0.90 and (R0.33G0.33B0.33)0.01NF0.99 have Brunauer-Emmett-Teller (BET) surface areas of SBET = 1619 and 1602 m 2 g -1 respectively(Table S5). Water stability in B0.10NF0.90 was demonstrated by following changes in the diffraction pattern (Figure S56) and BET surface area before and after exposure to water (Figure S56-57 and Table S5). Furthermore, the long-shelf life of the materials was demonstrated by the lack of changes in their PXRD even after 10 months of storage under laboratory conditions (Figure S58 for B0.10NF0.90). Utilizing this approach allows for circumvention of common challenges associated with emissive MOFs including: high inner filtering effects that cause complex emission, due to high fluorophore concentration, 24 and high synthetic costs due to inclusion of lanthanides. 11,25 This approach will enable the ability to study other similar organic-based molecular phenomena, and can provide MOF-based SSS materials for high precision applications such as in photonic devices, and high-definition displays.</p><!><p>Adapted from (14). NF linker (25 mg, 0.055 mmol, 1.0 eq) was added to a borosilicate glass tube, followed by ZrCl4 (14 mg, 0.061 mmol, 1.1 eq), anhydrous DMF (3.1 mL), and acetic acid (0.603 mL, 10.54 mmol, 190 eq). The tube was immersed in an ultra-sonication bath for 5 min.</p><p>The tube was then flash frozen in liquid nitrogen, evacuated to a pressure of 150 mTorr, flame sealed, and then placed in a 120 o C oven for 5 days. The tube was cooled to room temperature and immersed in an ultrasonication bath for a few seconds to release crystals from the walls. The tube was then cracked open and the solids were collected via filtration, rinsed with DMF and CH2Cl2, and immersed in DMF for 48 h, replacing the solvent after 24 h, then the DMF was decanted off and replaced with CH2Cl2 for 24 h. After solvent exchange, the solvent was decanted off and dried under reduced pressure (50 mtorr) at room temperature for 12 h resulting in a white crystalline solid.</p><!><p>A stock solution of B linker was prepared at 1.8 mM with anhydrous DMF. B linker (3.43 mL, 6.2 µmol, 0.1 eq) and NF linker (25 mg, 0.055 mmol, 0.9 eq) were added to a borosilicate glass tube. ZrCl4 (18 mg, 0.077 mmol, 1.1 eq) and anhydrous acetic acid (670 µL, 11.72 mmol, 190 eq) was added and the tube was flame sealed and mixed in a vortex mixer to insure homogeneity.</p><p>The tube was placed in a 120 o C oven for 5 days. The MOF was collected by vacuum filtration and rinsed with DMF and CH2Cl2. The MOF were submerged in DMF for 48 h, replacing the DMF halfway through the soaking process and then the DMF was replaced with CH2Cl2for 24 h.</p><p>The MOF was then exposed to ultrahigh vacuum, 25 mTorr.</p><!><p>Varied ratios of B, G, and R linker solutions were mixed with NF linker similar to previously mentioned procedures, see Table S4 for complete input/output values. Example x = 0.33 and y = 0.1, B linker (1.13 mL, 2.0 µmol, 0.033 eq), G linker (1.13 mL, 2.0 µmol, 0.033 eq), R linker (1.13 mL, 2.0 µmol, 0.033 eq), and NF linker (25 mg, 0.055 mmol, 0.9 eq) were added to a borosilicate glass tube. ZrCl4 (18 mg, 0.077 mmol, 1.1 eq) and anhydrous acetic acid (670 µL, 11.72 mmol, 190 eq) was added and the tube was flame sealed and mixed in a vortex mixer to insure homogeneity. The tube was placed in a 120 o C oven for 5 days. The MOF was collected by vacuum filtration and rinsed with DMF and CH2Cl2. The MOF were submerged in DMF for 48 h, replacing the DMF halfway through the soaking process and then the DMF was replaced with DCM for 24 h. The MOF was then exposed to ultrahigh vacuum, 25 mTorr.</p><!><p>Steady-state emission spectra was collected using an Edinburgh FLS980 fluorescence spectrometer at room temperature. A housed 450 W Xe lamp/single grating (1800 lines/mm, 250 nm blaze) Czerny-Turner monochromator was used as an output to excite the samples. Emission from the sample was passed through a single grating (1800 lines/mm, 500 nm blaze) Czerny-Turner monochromator and then detected by a Peltier-cooled Hamamatsu R928 photomultiplier tube. Solution state emission samples were contained in a 1 × 1 cm special optical glass cuvette.</p><p>Solid state emission samples were sandwiched between two 2 x 2 cm glass slides and placed at a ~45 degree angle relative to the incident excitation source.</p>