Facile Preparation of TICT@MOF Solids with Unprecedented PL Quantum Yields

Luminescent solids exhibit unique optical and electronic properties, and it is important to develop a simple experimental procedure to reduce the mass of weakly emissive hybrid inorganic–organic solids. The instant mixing of (1‐cyano‐2‐[α‐terthiophen‐2‐yl]‐vinyl)carboxylic acid (3TCC) and a metal–organic framework (MOF‐177; ZnO4(BTB)2; BTB = 1,3,5‐benzenetribenzoate) in an organic solvent, followed by evaporation of the solvent, produces solids with distinct photoluminescence (PL) properties, turning over the PL quantum yields (PLQY) of the generated 3TCC@MOF‐177 solids by 2‐ to 40‐fold compared to the 3TCC solution is demonstrated. The new solids are characterized by various methods and optical measurements. Contrarily to the trend in the photophysical results of 3TCC solutions, the anisotropic rotational times of 3TCC@MOF‐177 solids inversely correlate with the corresponding PLQY values depending on the protic solvent used in the initial preparation step in the following order: methanol>ethanol>butanol. This behavior is attributed to the complexation of 3TCC dye with metal clusters within the pores of MOF‐177 and hydrogen bonding of the CN group in the dye with the OH group in the linkers. These factors interplay with the kinetics of dye twisting. These observations reflect the potential of new solid luminescent architectures with remarkable PLQY that can be easily manufactured.


Introduction
Solid luminescent architectures based on organic, inorganic, and hybrid fluorophores have been developed for decades owing to their photonic applications. [1,2]These solids are characterized by a fast response and low detection limit, making them suitable for fabricating miniature photonic devices with unique properties such as noninvasiveness, accuracy, fast response, high spatial DOI: 10.1002/admi.202300889resolution, and applicability for electronic and electromagnetic measurements under dynamic conditions. [1,2]o discover solids with unique properties, researchers have focused on the infiltration of small synthetic organic chromophores in matrices such as metalorganic frameworks (MOFs), [3] which are synthetic cage systems bearing large voids.,15,18] Some attempts have been made to incorporate special chromophores with twisted intramolecular charge transfer (TICT) capabilities [21][22][23] as guest molecules within MOF matrices.[18,20] However, research in this field remains motivated by the highest photoluminesce nce quantum yield (PLQY) that can be achieved reasonably easily, and direct modifications to the matrix never exceed 25% for any infiltrated guest probes [1,[4][5][6]8,[11][12][13][14][15][16] including TICT molecules.6][27][28] An experimental synthesis of MOF-177 was first reported by Chae et al. [24] in 2004.

Preparation and Characterization of 3TCC@MOF-177 Solids
3TCC@MOF-177 solids were prepared by soaking 3TCC (≈0.01 mol for 1 wt%) to MOF-177 in 1 mL of solvent prior to evaporation.The weakly emissive off-white MOF-177 and nonemissive brown 3TCC (Figures S1, S2, Supporting Information) evolved into brightly emissive solids, with color depending upon the solvent (Figures S1, S2, Supporting Information).The FTIR spectra of the starting materials and modified MOF-177 solids are depicted in Figure 2A, and their peaks are listed in Table S1 (Supporting Information).
The peak at ≈3388 cm −1 can be attributed to the hydroxyl group (─OH) stretching vibration in MOF-177, whereas that at ≈3450 cm −1 corresponds to the same group in 3TCC.The former peak shifted to 3380 cm −1 following the introduction of 3TCC.The flow of 3TCC into the pores of MOF-177 also shifted the sharp CN peak from 2208 to 2223 cm −1 .Owing to the confinement of 3TCC in the pores of MOF-177, peaks corresponding to the symmetric and asymmetric vibrations of carboxyl groups (─COOH) and the double bond of benzene rings present in the organic ligands of MOF-177 shifted from 1612 and 1401 cm −1 (with a shoulder at 1542 cm −1 ) to 1603 (with a shoulder at 1536 cm −1 ) and 1404 cm −1 , respectively.These results are analogous to those observed for another dye incorporated in an MOF with a Zn cluster. [31]The same functional groups appeared at 1685, 1568, and 1408 cm −1 in 3TCC prior to incorporation.Moreover, the peak corresponding to the C─H in-plane bending vibration of the benzene ring of BTB in MOF-177 at 1015 cm −1 shifted to 1024 cm −1 in the final solid.Noticeably, the vibrational stretching mode of Zn─O shifted from 494 to 484 cm −1 upon incorporating 3TCC inside the pores of MOF-177.These results demonstrate that the flow of 3TCC into the pores of MOF-177 weakens the interactivity between the Zn node and organic linker moieties.The results also imply a possible hydrogen bonding interaction between the CN group in 3TCC and the hydroxyl group in MOF-177, and that 3TCC binds Zn through its carboxylic group.The formation of pure MOF-177 crystals is confirmed by power X-ray diffraction (PXRD), [8,9,24,25,27,28] as previously reported for the TGA of activated materials [26,27] (see Figure 2B,C).In Figure 2B, the texture properties changed and some fine peaks were broadened, presumably owing to the interaction of the anchoring group of 3TCC with the metal center in MOF-177, changing the orientation of the crystals.The framework of the new solids did not change, as the strongest peaks at 2 = 5.2 and 10.8 remained intact.However, the decreased crystallinity resulting from several days of atmospheric exposure [28] cannot be overlooked.Consequently, it is challenging to draw additional evidence for the entrapment of 3TCC by MOF-177.Yet, the EDX data confirm the inclusion of 3TCC in the pores of MOF-177, indicating the appearance of sulfur peaks from 3TCC in the final solid (Figure S3, Supporting Information).The TGA traces in Figure 2C provide information pertaining to the composition and thermal decomposition of the new solid.The mass reduction percentages were calculated from the TGA curves for all samples to estimate the concentration % of 3TCC in the final product (Table S2, Supporting Information).

Spectroscopic and Photophysical Properties of 3TCC@MOF-177 Solids
Results corresponding to the spectroscopic, photophysical, and chromatic properties of 3TCC in neat solvents and solvent mixtures (Figures S4-S11 and Tables S3, S4, Supporting Information) confirm previous reports [22,23] that changing the solvent modulates electronic-state mixing and coupling on the charge transfer states of 3TCC molecules, which form due to twisting of the single bond linking the thiophene donor and cyanoacetate acceptor moieties. [30]Following intramolecular twisting, the TICT state returns to the ground state through red-shifted emission or nonradiative relaxation.Because the emission properties potentially depend on the environment, TICT-based 3TCC fluorophores are applied as sensors for solvents, (micro)viscosity, and chemical species, as well as in organic optoelectronics, nonlinear optics, and solar energy conversion. [29]We measured DRS spectra for all new solids and MOF-177 to verify the effects of infiltration with the TICT probe on the band gap of MOF-177 at 298 K without changing the linkers or metal clusters (Figure 3; Figure S12, Supporting Information).
New emission peaks emerged in the PL spectra of 3TCC@MOF-177 (≈600 nm) and MOF-177 (≈380 nm), with excitation at ≈320 and 345 nm, respectively [8] (Table S5, Supporting Information).The excitation/emission spectra for the new composites unfolded different origins for the new emission band (515/598 nm in Figure 4).Both PL and PLE measurements confirmed the penetration of 3TCC through MOF-177.Moreover, the excited-state PL average lifetime values of all solid powders upon insertion of 3TCC into MOF-177 were lower than that of MOF-177 (Figure S14 and TablesS6, S7, Supporting Information). [8]When compared to those of MOF-177, the new 3TCC@MOF-177 powders exhibited distinct photophysical (photonic) properties.These observations correlate with previously reported results for infiltrated MOF-177 matrices. [8]e observed very high PLQY values for all 3TCC@MOF-177 solids prepared using different solvents (Figure S15, Supporting Information).The PLQY values do not depend on the wt% of the dye inside the MOF range from 0.5 to 2% (Table S5 and Figure S16, Supporting Information).We also observed that a figure of 1% produced the highest PL intensity compared to 10%, 40%, and 90% without a significant change in the PL/PLE maxima (Figure S17 and Table S5, Supporting Information).
We also measured correlation rotational times to determine how PLQY excited-state or PL average lifetime values depend on the selected solvent during preparation (Figure S18 and Table S5, Supporting Information).Contrary to the solution data (Figure S11, Supporting Information), a clear correlation was found between rotational times from anisotropy data and corresponding PLQY values (Figure 5B), following the order of methanol>ethanol>butanol.No correlation was found between for MOF-177 (345 nm) [8] and (515 nm) Figure S13 (Supporting Information) for 3TCC@MOF-177.S5, Supporting Information).

PLQY excited-state and PL average lifetime values (Table
The slight deviations pertinent to DMF (aprotic solvent) and glycerol-water mixture (solvent mixture) require further investigation with other solvents.In the selected solvents, data for modified solids exhibit an inverse trend compared to those for the 3TCC in solution, i.e., butanol>ethanol>methanol (points 1, 2, and 3 in Figure 5 for neat protic solvents).

Interpretation of QY Enhancement of TICT@MOF-177 Solids
Our experimental results collectively demonstrate that PLQY increases by up to 43-fold when the selected TICT fluorophores are relocated between the pure solvent and MOF pores.In a pure solvent, charge transfer can be expected to be promoted upon twisting the donor component of the TICT dye relevant to the acceptor component.The unprecedented turnover in absolute  S3 Supporting Information); 1) methanol, 2) ethanol, 3) butanol, 4) dimethylformamide, 5) 50% glycerol in water, and 6) 70% glycerol in water.B) Plot of PLQY versus rotational time for 3TCC@MOF-177 (taken from Table S5, Supporting Information) prepared with 1 wt% of 3TCC in MOF-177 using the same solvents: 1) methanol, 2) ethanol, 3) butanol, 4) dimethylformamide, 5) 20% glycerol in water, and 6) 70% glycerol in water.
PLQY when TICT probes are present inside the pores of MOF-177 can be attributed to an entirely different mechanism in the MOF pores.The daylight and emission colors of the new solids are affected by the type of solvent selected during preparation, which confirms the presence of solvents inside the pores (Figures S1, S2, S19, and Table S5, Supporting Information).In an accompanying work, we demonstrate the solvent coordination to the Zn center inside the pores 3TCC@MOF-177, leading to the trend observed in Figure 5.More details on the role of solvents are to be published elsewhere.

Conclusion
In this study, we demonstrated the entrapment of a TICT dye inside a MOF matrix by simply mixing the two components in a selected solvent and subsequently evaporating said solvent, with the final solids exhibiting distinct emission properties.The formation of 3TCC@MOF-177 is facilitated by the complexation of 3TCC to Zn and hydrogen bonding between 3TCC and BTB linker.The easily prepared luminescent solids can be utilized for sensing, optical memory, switching materials, and imaging technologies, enriching the photonics industry with new solid properties of MOF materials that can be easily fabricated, providing novel concepts in the development of optoelectronic devices.

Experimental Section
Samples: All solvents (purity >99.9%) -namely toluene, dichloromethane, THF, methanol, ethanol, butanol, acetone, acetonitrile, butyl nitrile, chlorobenzene, chloroform, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethyl acetate, and glycerol -were obtained from Sigma-Aldrich (St. Louis, MO) and used as received.All reagents and the deuterated solvent (DMSO-d 6 ) in the synthesis procedure were also purchased from Sigma-Aldrich and used without further purification (purity >99.9%).Details pertaining to the syntheses of 3TCC and MOF-177 following previously reported procedures are presented in the Supporting Information.
Preparation and Characterization of 3TCC@MOF-177: Solids of 3TCC@MOF-177 were prepared by soaking a given weight percentage of 3TCC (≈0.01 mol/1 wt%) to MOF-177 in 1 mL of a given solvent, with continuous stirring for 24 h.The solids were then thoroughly washed and repeatedly rinsed with the same solvent (except in the case of the glycerolwater mixture, wherein the final solid was washed with water alone) seven times per hour.The solvent washings were analyzed for 3TCC using UVvisible absorption spectroscopy, with no 3TCC detected.Following decantation, the residual solids were activated overnight at 80 °C in all solvents (except in DMF at 120 °C).
Thermogravimetric Analysis (TGA): TGA analyses were conducted using Shimadzu thermogravimetric analysis equipment.A 0.01 g sample was heated at a rate of 5°min −1 from 35 to 600 °C.

X-Ray Diffraction (XRD):
The XRD investigation was conducted using a Shimadzu-6100 powder XRD diffractometer with Cu-K radiation set to  = 1.542 A. Diffraction data were gathered in the range of 2-80°a rate of 1°min −1 .
Scanning Electron Microscopy (SEM): The internal morphology of the solid samples after gold coating was deduced from the SEM images, which were taken using an electron microscope (Inca Energy EDS System, Oxford, United Kingdom) equipped with an energy-dispersive X-ray (EDX) detector operating at a high vacuum and a 30 kV accelerating voltage.
Nuclear Magnetic Resonance (NMR) Spectroscopy: NMR spectra were recorded using a Varian-400 MHz spectrometer ( 1 H-NMR at 400 MHz and 13 C-NMR at 100 MHz; Agilent Technologies, Santa Clara, CA, USA) using dimethyl sulfoxide-d 6 (DMSO-d 6 ) as a solvent.Tetramethylsilane (TMS) was used as an internal reference, and chemical shifts were measured as part-per-million; (, ppm).
Fourier Transform Infrared (FTIR) Spectroscopy: Infrared spectra were measured using KBr pellets on a Thermo Nicolet model 470 FT-IR spectrophotometer and processed with Spectrum IR software.Solid samples were mixed with dry KBr (FT-IR grade Sigma-Aldrich, St.Louis, MO) in a ratio of 1:100 and compressed into pellets using a hydraulic press.The transmittance of the resulting pellets was recorded within a range of 4000-450 cm −1 with 32 scans.
Diffuse Reflectance Spectroscopy (DRS): Absorption spectra of the solid samples were obtained using the Kubelka-Munk conversion (K-M = (1 − R) 2 /2R) of the recorded diffusive-reflectance spectra at room temperature on an FS5 spectrometer (Edinburgh, UK) equipped with an SC-30 (integrating sphere) as the sample holder.A polytetrafluoroethylene (PTFE) polymer was used as a reference.The bandgap energies (Eg) of the solid samples from the DRS spectra were calculated using E g = 1240 eV nm l −1 , where l is the absorption edge (nm).
Photoluminescence (PL) and Photoluminescence Excitation (PLE) Measurements: PL spectra of the solid samples were recorded using an FS5 spectrofluorometer (Edinburgh instrument, Livingston, UK), with a xenon lamp to excite the samples.
Absolute PL Quantum Yield (QY) Measurements: Absolute QY for the solid samples was approximated on the FS5 spectrometer by utilizing an integrating sphere (SC-30) and comparing the measured direct and indirect emissions from the sample to those generated from the PTFE reference through direct excitation.The error was 2% of the estimated experimental value.
Excited-State PL Lifetime and Time-Resolved PL (TRPL) Measurements: PL decay measurements were collected using the time-correlated singlephoton counting (TCSPC)-based Edinburgh Instrument (LifeSpec II spectrometer, Livingston, UK).The source was a picosecond diode laser with a  ex of 375 nm for the liquid samples, and a laser diode of 320 nm for the solid samples, with instrument functions of ≈30 and 90 ps, respectively.A repetition rate of 20 MHz was used for both sources.A red-sensitive highspeed photomultiplier tube detector (Hamamatsu, H5773-04) with a total count rate of 10 000 s −1 was utilized, and the data were convoluted with an instrument response function (IRF) using the Levenberg-Marquardt algorithm to minimize  2 .Fluorescence decay was analyzed in terms of the multi-exponential model to calculate the average lifetime value as The contribution of each component to the steady-state intensity was obtained by where  i are lifetimes with amplitudes  i and ∑ i = 1.0.The denominator of Equation (2) represents the sum for all decay times and amplitudes.The estimated experimental error was 2% for <1 ns and 20% for a lifetime of ≈5 ns.Cell holders with front-face geometries maintained the emission polarizer at a magic angle of 54.7°to the vertically polarized excitation beam.In the solution experiments, the IRF was determined by measuring the scattering of a LUDOX solution.In all experiments, a desired temperature was maintained by a Peltier system with an accuracy of ±0.1 °C.Time-Resolved Fluorescence Anisotropy Measurements: Measurements were performed every 10 s using a LifespecII apparatus fitted with an automatic set of polarizers.PL decay was measured for parallel (I VV ) and perpendicular (I VH ) to measure the G factor.All cases of anisotropy decay were best-fit by single-exponential decay curves.Goodness-of-fit was approximated by reduced  2 values and the distribution of weighted residuals among the data channels.The estimated experimental error was 2%.