Design of High-Contrast Mechanochromic Materials Based on Aggregation-Induced Emissive Pyran Derivatives Guided by Polymorph Predictions

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Mar 2022Design of High-Contrast Mechanochromic Materials Based on Aggregation-Induced Emissive Pyran Derivatives Guided by Polymorph Predictions Wen Wang, Ruohan Li, Shuzhang Xiao, Qilin Xing, Xia Yan, Jiayu Zhang, Xinghong Zhang, Haichuang Lan and Tao Yi Wen Wang College of Biological and Pharmaceutical Sciences, China Three Gorges University, Hubei, Yichang 443002 , Ruohan Li Department of Chemistry, Fudan University, Shanghai 200438 , Shuzhang Xiao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Biological and Pharmaceutical Sciences, China Three Gorges University, Hubei, Yichang 443002 , Qilin Xing College of Biological and Pharmaceutical Sciences, China Three Gorges University, Hubei, Yichang 443002 , Xia Yan College of Biological and Pharmaceutical Sciences, China Three Gorges University, Hubei, Yichang 443002 , Jiayu Zhang College of Biological and Pharmaceutical Sciences, China Three Gorges University, Hubei, Yichang 443002 , Xinghong Zhang College of Biological and Pharmaceutical Sciences, China Three Gorges University, Hubei, Yichang 443002 , Haichuang Lan College of Biological and Pharmaceutical Sciences, China Three Gorges University, Hubei, Yichang 443002 and Tao Yi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Fudan University, Shanghai 200438 State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620 https://doi.org/10.31635/ccschem.021.202100885 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail High-contrast mechanochromic (MC) materials are prominent candidates for sensor, security, and memory applications; however, the development of materials with a large luminescence change (Δλem > 100 nm) under external stimuli is challenging. Considering that polymorphic molecules usually exhibit reversible mechanochromism, polymorph prediction is adopted for the first time to guide the design of high-contrast MC materials in this study. We designed and synthesized a series of donor–π–acceptor pyran derivatives bearing different electron donors and acceptors as model systems. The polymorph prediction indicated that 4-dicyanomethylene-2,6-distyryl-4H-pyran and 4H-pyran-4-one derivatives had the potential to crystallize in both monomer and π-dimer aggregates, while barbituric acid-based compound tended to pack tightly in all aggregated states. The experimental results agreed well with the prediction that the derivatives potentially possessing both monomer and π-dimer aggregate structures exhibit excellent MC behavior, whereas the fluorescence difference for the barbituric acid-based compounds is minimal. Moreover, a compound with an excellent fluorescence difference of three colors during reversible mechanochromism was chosen as the candidate for an optical recording material and security ink. This work proposes an effective method to guide the design of stimuli-responsive materials, which may open promising avenues for the development of high-contrast MC molecules. Download figure Download PowerPoint Introduction The development of stimuli-responsive organic fluorescent materials has received tremendous attention as a result of their fundamental research and promising applications in information storage,1,2 anti-counterfeiting paper,3,4 sensors,5–7 and bioimaging.8–10 In particular, mechanochromic (MC) materials that use mechanical force as a stimulus have been recognized as ideal candidates for preparing anti-counterfeiting labels and pressure-sensitive devices.11–13 Compared with inorganic MC materials, organic MC materials have a number of key advantages, such as abundant synthetic techniques, low preparation costs, and functional diversity. A large number of organic MC materials have been developed based on their conversion between different conformations or molecular packing styles. However, most of these materials exhibit a small emission difference upon grinding and low sensitivity to mechanical force, which limits their practical applications. Currently, the design of high-contrast organic MC materials with large emission wavelength differences remains a significant challenge. Theoretically, the luminescence change during mechanochromism is the result of stimuli-induced transitions between different conformations or molecular packings.14,15 Therefore, organic molecules with multiple polymorphs are usually MC because their molecular packing or conformations are interchangeable under external stimuli.16–20 For example, the phenothiazine unit in a D–π–A compound can exhibit both quasi-axial and -equatorial conformations that emit local excited emission (LE) or intramolecular charge-transfer (ICT) fluorescence, and these two conformations can be converted to each other by external stimuli.21,22 As for planar fluorescent molecules, cyano-substituted oligo(p-phenylenevinylene) derivatives can form monomers and excimers under different stimuli, therefore making them high-contrast multicolored MC materials.23,24 In some cases, both the conformation and molecular packing might change under various stimuli.25–27 Therefore, questions arise regarding which kind of molecule has the potential to aggregate in both monomer (namely without intermolecular π–π interaction) and π-dimer (with strong π–π interaction between neighboring molecules) and what kind of molecule can comprise conformations with a significant difference? If we can calculate the potential polymorphs of the designed molecules, we might be able to predict their MC behavior. In fact, crystal structure prediction has been proven successful in many cases, based on the minimization of the crystal lattice energy. With polymorphs being identified as local minima of relatively low energy, the general framework encompasses a number of crystal structure prediction methods, such as Accelrys Cerius2 modeling, GRACE, MOLPAK, UPACK, and so on.28,29 Using these prediction methods, we might be able to calculate the potential crystal structures of designed molecules and then identify their luminescence differences between different crystalline states. Herein, we propose the prediction of the potential molecular packings of organic D–π–A molecules to guide the design of MC molecules. Considering that intense emission in the solid state is important for a MC molecule, solid emissive materials with a planar π system are chosen as the candidates. Normally, planar chromophores favor strong intermolecular π–π stacking to quench the emission in the solid state30; fortunately, the fluorescence-quenching problem can be overcome by introducing aggregation-induced emission (AIE) groups to restrict intramolecular rotation in the aggregate state.31–34 Pyran derivatives bearing a strong electron acceptor, such as carbonyl,35 cyano,36–38 or indene-1,3-dione groups,39,40 are planar in chemical structure but AIE active. Therefore, we designed a series of pyran derivatives bearing different substituted electron donors and acceptors (Figure 1a). According to the polymorph prediction, 4-dicyanomethylene-2,6-distyryl-4H-pyran and 4H-pyran-4-one derivatives have the potential to crystallize in both monomer and π dimer aggregates, resulting in different emissions. In comparison, barbituric acid-based compounds tended to pack tightly in all aggregated states with intermolecular π–π stackings present in all predicted structures. Our experimental results revealed that different emissive aggregates with significant emission differences (Δλem > 75 nm) were obtained for all the 4-dicyanomethylene-2,6-distyryl-4H-pyran and 4H-pyran-4-one derivatives, whereas only a 20 nm wavelength shift was observed for the barbituric acid-based compounds, in agreement with the results from the polymorph prediction. As far as we know, this is the first report of computation-guided design of MC molecules, which may open promising avenues for the development of high-contrast MC molecules with remarkable MC luminescence. Figure 1 | (a) Chemical structures of designed molecules; (b–g) predicted molecular packing with the most and least intense intermolecular π–π interactions: (b) 1-Ph, (c) 2-Ph, (d) 3-Ph, (e) 3-PhF, (f) 3-PhCl, and (g) 3-Np (π-dimer: two molecules completely overlapped; monomer: almost no molecule overlapping; cross overlapped: two molecules overlap partially in different directions; half-overlapped: two molecules overlap along one side). Download figure Download PowerPoint Experimental Methods Measurement Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker 400 NMR spectrometer. Chemical shifts were reported in ppm with tetramethylsilane as reference. Mass data were recorded on an Applied Biosystems Voyager-DE STR mass spectrometer. UV–vis and fluorescence spectra measurements were carried out on a Shimadzu UV-2600 and a Hitachi F-4600 spectrometers, respectively. The fluorescence quantum yields and fluorescence lifetime measurements of the solid powder were performed on an Edinburgh FS5 luminescence spectrometer with 450 nm excitation source. Powder X-ray diffraction (PXRD) patterns were measured on a Bruker Advance D8 X-ray diffractometer in the 2θ range from 5° to 50°. Single-crystal X-ray diffraction data were collected on a Rigaku XtaLAB PRO single-crystal X-ray diffractometer equipped with a graphite monochromated Cu Kα radiation (λ = 1.54184 Å) at 298 K. The single crystals of 3-Ph and 3-PhCl were cultured from slow evaporation of organic solvents and their CCDC numbers are 2023435 and 2039964, respectively. Electrochemical studies were carried out with a conventional three-electrode system using an AUTOLAB electrochemical work station (PGSTAT12) in deoxygenated and anhydrous N,N-dimethylformamide at room temperature. The potentials were reported versus ferrocene as the internal standard with a scan rate of 100 mV s−1, using a glassy carbon working electrode, Pt flake counter electrode, and saturated calomel electrode (SCE) as the reference electrode. The sample solutions contained 5.0 × 10−3 M as the prepared compound and 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) as a supporting electrolyte. Nitrogen was bubbled for 10 min before each measurement. Sample preparation for MC study The synthesis and characterizations of designed compounds were depicted in ESI ( Supporting Information Figures S12–S21). The initial powdered sample was obtained by vacuum evaporation of the column eluents or recrystallization. The initial state of 1-Ph was obtained by recrystallization from ethanol; 2-Ph was obtained by precipitation from a hot dimethyl sulfoxide (DMSO) solution upon cooling; 3-Ph was obtained by recrystallization from chloroform; 3-PhF and 3-PhCl were obtained by recrystallization from tetrahydrofuran (THF); 3-Np was obtained by precipitation from a hot acetonitrile solution upon cooling. The ground samples were prepared by grinding the initial powders in a mortar with a pestle. The fumed samples were acquired by fuming the ground powders with organic solvents for 15 min, and the annealed samples were obtained by heating the ground powders in a hot-stage with an automatic temperature-control system at 120 °C for 10 min and subsequently cooled to room temperature. Computational methods Geometry-optimized molecular structures computed by density functional theory (DFT) are used as inputs to the polymorph predictions. In the DFT study, we applied B3LYP with 6-31G(d,p) basis sets for the designed compounds. The search for potential structures was initially restricted to the ten most prevalent space groups (P21/c, P-1, P212121, P21, C2/c, Pbca, C2, Pna21, Pbcn, and Cc), using the simulated annealing algorithm of Karfunkel and Gdanitz as implemented in the Accelrys Polymorph Predictor module of the Materials Studio software.41 The final energy minimizations were performed using the Dreiding force field. Molecular packing in the calculated structures was analyzed by Mercury. The molecular cluster with the most intense intermolecular π–π interactions was chosen as the input and calculated at the M06-2X/6-31G(d,p) level with dispersion correction (Empirical dispersion = GD3). Then intermolecular interactions were analyzed by Multiwfn.42 Results and Discussion The potential molecular packing of the designed compounds was restricted to the 10 most prevalent space groups, using the simulated annealing algorithm of Karfunkel and Gdanitz as implemented in the Accelrys Polymorph Predictor. For a planar molecule in the aggregate state, the intermolecular π–π interactions usually determine its fluorescence properties. Therefore, the molecular clusters with the most intense intermolecular π–π interactions were extracted from the predicted crystal structures and analyzed and visualized as a "green cloud." If there is no planar overlapping in the predicted crystal structure, the clusters with other supramolecular forces were extracted and analyzed, and the negligible "ground cloud" was identified as weak intermolecular interactions, such as hydrogen bonding. The predicted details, including space groups, optimized packing styles, and the energy of the designed molecules, are shown in Supporting Information Tables S1–S6. The predicted clusters with the most and least intense intermolecular π–π stacking were extracted, as shown in Figures 1b–g. According to the polymorph prediction, both clusters with intense or no intermolecular π–π stacking were found in the predicted crystal structures of the pyran-4-one and dicyanomethylene-4H-pyran derivatives (namely, 1-Ph, 3-Ph, 3-PhF, 3-PhCl, and 3-Np), indicating that all these five compounds have the potential to form both π-dimer and monomer forms in the aggregate states. Therefore, they should be able to crystallize in different polymorphs and emit different fluorescence. In particular, a half-overlapped dimer was found in the polymorphs of 3-PhCl, indicating that 3-PhCl might be able to provide multicolored states (e.g., monomer, half-dimer, and π-dimer). Thus, the different emissive states of these five compounds should be observed and converted into each other under external stimuli, exhibiting high-contrast chromic properties. In comparison, 2-Ph with 1,3-dimethylbarbituric acid as an electron acceptor only tended to aggregate as π-dimers or partial-overlapping clusters in all polymorphs, with no monomer state found. These results revealed that the fluorescence of 2-Ph should be less active with external stimuli, unlike the other five compounds, which can form aggregates with significant differences. The designed compounds were facilely synthesized through typical nucleophilic addition reactions. Nucleophilic addition between commercially available 2,6-dimethyl-γ-pyrone and benzaldehyde gave 1-Ph directly.35 Condensation of 2,6-dimethyl-γ-pyrone with electron-withdrawing groups (malononitrile or 1,3-dimethylbarbituric acid), followed by condensation with different aromatic aldehydes, provided other target products with yields from 40% to 78%.36–38 All synthesized compounds were well-characterized. To verify the predicted results, we cultivated single-crystal structures using a slow evaporation method with different organic solvents. Finally, single crystals of 3-Ph and 3-PhCl suitable for X-ray single-crystal diffraction were obtained from mixed solvents (dichloromethane/acetonitrile 1∶1 for 3-Ph and acetone/THF 1∶4 for 3-PhCl). The 3-Ph crystallizes in an orthorhombic structure with space group P212121, similar to the reported structure ( Supporting Information Table S9).36 The main conjugated system exhibited good planarity, except that one terminal phenyl ring was slightly twisted from the plane with a dihedral angle of 17.0° (Figure 2a). Due to the planar character of the molecule, planar overlapping with a neighboring molecule was facilitated, thereby providing a π–π stacked dimer. The π-distance between two molecules was measured to be 3.39 Å, manifested in strong face-to-face intermolecular π–π interactions. However, only one side of the molecule overlapped with a neighboring molecule and the intermolecular π–π interaction on the other side was limited. As a result, the crystal emitted a red aggregate emission centered at 599 nm (Figure 2b). Among the 10 predicted crystal structures, six adopted this kind of packing but with different overlapping degrees, indicating that it was the predominant molecular stacking mode for 3-Ph ( Supporting Information Table S3). Figure 2 | (a and c) Conformation and intermolecular π–π interactions 3-Ph and 3-PhCl, respectively; (b and d) fluorescent spectra and dark field images of single crystals of 3-Ph and 3-PhCl, respectively. Download figure Download PowerPoint In comparison, 3-PhCl crystallized as a triclinic crystal in space group P-1 with one THF molecule in the crystal that adopted a highly planar conformation (Figure 2c). Theoretically, this high planarity promotes intermolecular π–π stacking; however, only half of the molecule overlaps with a neighboring molecule in the crystal structure. The π distance between two molecules was measured to be 3.4 Å, almost the same as that in 3-Ph. Although 3-PhCl comprised higher planarity compared with 3-Ph, the crystal of 3-PhCl emitted a shorter wavelength fluorescence with a broader band (orange fluorescence centered at 595 nm) (Figure 2d), which could be ascribed to its reduced overlapping degree in the cluster. This molecular packing matched the half-overlapped one of the predicted crystal structure very well (Figure 1f). According to DFT calculations, all these pyran derivatives comprised typical D–π–A structures and a planar conformation ( Supporting Information Figure S1). Values of the electronic states (highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels) and energy gaps of the six compounds were calculated by both UV–vis spectra and cyclic voltammetry (CV) curves ( Supporting Information Figure S2 and Table S7). The data calculated by CV curves match well with the results from UV absorption spectra. With stronger electron-withdrawing groups (barbituric acid and malononitrile), obvious red shifts of the absorption wavelength and fine structures were found (Figures 3a and 3b). Since the naphthalene unit is a stronger electron donor compared with phenyl, 3-Np absorbed at a lower energy frequency than 3-Ph. However, the halogen effect was insignificant on their absorptions, and the absorption bands were almost independent of the solvent polarity ( Supporting Information Figures S3–S8). These pyran derivatives emitted poor fluorescence in dilute solutions, which could barely be observed by the naked eye under 365 nm light irradiation. Figure 3 | (a) Absorption and (b) normalized fluorescence spectra of pyran derivatives in acetonitrile (1.0 × 10−5 M), λex: 320 nm for 1-Ph; 400 nm for 3-Ph, 3-PhF, and 3-PhCl; 420 nm for 2-Ph and 3-Np. Download figure Download PowerPoint Although all pyran derivatives emitted faint fluorescence in dilute solutions, intense emissions could be observed from the solids obtained from column chromatography, indicating their AIE behavior. Thus, the AIE properties of these compounds were studied in detail by the addition of water to their acetonitrile solutions. All these pyran derivatives exhibited higher fluorescence intensity with red-shifted wavelengths when water was added to the acetonitrile solution to provide aggregated particles (Figure 4). The suspended aggregates in the mixed solvent were confirmed by the Tyndall effect ( Supporting Information Figure S9). Among the compounds with phenyl rings as electron donors, 3-Ph bearing malononitrile as an acceptor emitted the most intense fluorescence in the aggregate state (Figure 4). The introduction of halogen atoms or larger aromatic rings results in decreasing fluorescence intensity when a large amount of water was added, indicating that the increased π-conjugated system intensified the intermolecular π–π interaction to decrease the fluorescence efficiency in the aggregate state. Moreover, a red shift was also observed in the absorption of these pyran compounds during aggregation ( Supporting Information Figure S10). Figure 4 | Fluorescent spectra changes of the designed compounds in acetonitrile/H2O mixture with fw = 0–99%. The insets show the relative fluorescent intensity (I/I0) change and fluorescent images with different fw under 365 nm light irradiation (1.0 × 10−5 M). Download figure Download PowerPoint Because the designed compounds have multipolymorph potential and AIE properties, their solids may exhibit the expected MC properties. A previous study showed that the fluorescent wavelength of 1-Ph underwent almost no change during the MC process.35 However, according to the polymorph prediction, 1-Ph should be able to crystallize in both π-dimer and monomer forms, resulting in different emissive solids. To verify these predictions, the MC behavior of 1-Ph was reinvestigated. The sample crystallized from ethanol gave a faint emission with a predominant peak at 422 nm with a shoulder at 470 nm (Figure 5a). The fluorescence decay time of the 1-Ph powder was 0.30, 2.64 ns (88/12%) ( Supporting Information Table S8), ascribed to the monomer and π–π stacked species. Upon grinding, its emission was significantly enhanced and red shifted to 499 nm (green emission) with the fluorescence decay time increased to 0.98, 3.54 ns (40/60%), indicating that intermolecular π–π interaction got intensified. It is noteworthy that the grinding powder was still crystalline with a decrease in the diffraction intensity and the appearance of two new small peaks at 23.1° and 25.8° (Figure 6). These new peaks might represent the existence of intermolecular π–π interactions with d spaces of 3.44 and 3.84 Å, respectively, which induced red shift of the fluorescence and extension of the fluorescence decay time ( Supporting Information Table S8). Upon annealing or fuming with ethanol, the diffractions become sharp again, but the peaks ascribed to π–π stacking did not disappear, indicating that fuming and could not completely its initial state. Figure | Fluorescent spectra and images of the pyran derivatives under external (a) 1-Ph; (b) (c) (d) (e) 3-PhCl; (f) 3-Np. Download figure Download PowerPoint Among the potential aggregation states of 2-Ph, intermolecular π–π interactions are present according to the prediction. The crystallized solids of 2-Ph from organic solvents (e.g., and emitted red fluorescence with wavelengths from to nm ( Supporting Information Figure red shifted compared with their dilute solutions indicating the of intermolecular π–π interactions in the solids. In the of 2-Ph, a strong diffraction peak at 2θ of Å) (Figure strong intermolecular π–π Upon grinding, this peak is still but fuming or the diffractions become intermolecular π–π interactions were present in all the aggregate states of 2-Ph during the MC which agreed well with the polymorph prediction. Figure | patterns of all samples during the MC Download figure Download PowerPoint Although 3-Ph has been to be AIE of its stimuli-responsive properties have been In fact, the solid powder of 3-Ph obtained by evaporation of the eluents from column was in and no obvious fluorescence change was observed However, the crystalline sample by recrystallization from emitted fluorescence centered at Because the π dimer with in the single crystal of 3-Ph exhibited a emission band centered at 599 the emission at nm could be ascribed to the grinding, a new band at nm and was red shifted to nm through fuming by indicating the enhanced intermolecular π–π the gave an emissive sample with two emissive bands at 599 and nm These results that the aggregate states of 3-Ph comprised different molecular them with than one emission in with their patterns because multiple diffractions were found in all the The halogen effect has been proven for the supramolecular of organic fluorescent Although the introduction of halogen atoms did not the properties of the molecules in dilute solutions, the potential halogen might molecular packings and properties. The initial solid powder of 3-PhF obtained by recrystallization from THF exhibited a emission band centered at nm with a shoulder band at grinding in a mortar to the crystalline powder a red emission centered at nm was observed or force provided a sample with emissions at and the existence of both monomer and π-dimer emissions. In comparison, when the group was by a the initial sample of 3-PhCl obtained by recrystallization from THF emitted at nm slightly shifted from that of the single crystal with the cluster Because strong diffractions at and π–π stacking were observed in its the emission of 3-PhCl was ascribed to the Upon grinding, an obvious red shift was observed and the fluorescence to nm However, the emission could be converted into the of the monomer emission at nm with a Therefore, states with significant differences were obtained for 3-PhCl, which agreed well with the polymorph prediction. The effect also an important in the supramolecular because the electronic in different result in various intermolecular π–π naphthalene were to the of phenyl 3-Np was expected to form both monomers and π-dimers with different emissions according to the polymorph prediction. In fact, the MC of 3-Np has been reported but only a nm difference in its emission wavelength was observed during the MC which is with the polymorph prediction. Therefore, the MC was The initial powder obtained by recrystallization from acetonitrile at grinding in a mortar to become a significant red shift (Δλem = 100 nm) was observed Finally, a emissive powder ascribed to the intensified intermolecular π–π stacking was fuming with or new crystalline the same fluorescence as the initial state was The conversion between and fluorescence could be ascribed to the from monomer to π-dimer Among the five compounds with large fluorescence differences during mechanochromism, three colors with obvious differences were obtained for Therefore, 3-PhCl was chosen as the candidate for as an optical recording material and security ink. a of was in a THF solution of in an fluorescent was The fluorescence is ascribed to we on the red emission) on the emissive (Figures and The could be by fuming with and this is The with red can be into a

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