Very bright mechanoluminescence and remarkable mechanochromism using a tetraphenylethene derivative with aggregation-induced emission

Two photoluminescent polymorphs exhibit different mechanoluminescence activities and mechanochromic behaviors.


Introduction
The mechanoluminescence (ML) phenomenon was rst found by Francis Bacon in 1605. 1 Heretofore, research on exploiting advanced ML materials has not yet been a major focus. 2 Materials with brilliant ML are actually of great importance from both fundamental and practical viewpoints because they are promising for usage in displays, as well as light sources and sensors. 2,3 However, a comprehensive understanding of the crystal properties required for ML activity and the corresponding mechanisms is less well demonstrated. 4 This lack in understanding leads to feasible design principles for these emitters, particularly those with satisfactory ML brightness, being rarely found. 5 As reported previously, the performance of organic ML compounds can be related to both their molecular and molecular-assembly structures. 6 Therefore, controlling the molecular arrangements in the solid state and achieving a molecular-level understanding of the relationship between the molecular conformations and packing characteristics and the resulting optical properties are the essential issues in obtaining efficient ML materials.
Notably, non-covalent intermolecular interactions, such as p-p stacking and hydrogen bonding, are important in constructing the supramolecular systems. 7 These interactions are able to inuence the nal packing structure strongly, thereby making polymorphism with different ML activities more probable. 6a,8 Nevertheless, in most cases, typical p-p stacking interactions oen lead to aggregation-caused quenching, which poses signicant difficulties for development of high-performance ML materials. 4b,9 By contrast, a diametrically opposed effect was recently found to be operative in a class of chromophores with twisted conformations (e.g., tetraphenylethene derivatives), which exhibit enhanced emission in the solid state with respect to the uid solution. 10 The discovery of this abnormal phenomenon, known as aggregation-induced emission (AIE), has sparked a rapid expansion in the eld of photoluminescent sensors and electroluminescent devices. 11 AIE also provides new possibilities for designing highly mechanoluminescent materials.
This study presents a new polymorphic system that can be facilely and controllably constructed using a tetraphenylethene derivative [i.e., 5-(4-(1,2,2-triphenylvinyl)phenyl)thiophene-2carbaldehyde (P 4 TA)] as the building block (Fig. 1). The two crystalline polymorphs of P 4 TA show strong blue-and greencolored photoluminescence (PL). The blue-light crystals are signicantly ML inactive, whereas the green-light ones are highly mechanoluminescent because of their distinctly different molecular packing mode and unique AIE character. The existence of polymorphs from the same molecule with exactly opposite properties provides a unique prototype to investigate the crystalline structures required for ML activity and the effect of AIE properties on ML enhancement. The relationship between the ML and the mechanouorochromism of P 4 TA is also presented.
Results and discussion P 4 TA was straightforwardly prepared through a palladiumcatalyzed coupling reaction by introducing 2-thiophenaldehyde to the tetraphenylethene moiety (Scheme S1 †). The puried material was then characterized using nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography. The satisfactory data obtained fully conrmed its expected molecular structure (ESI †).
The UV-visible absorption spectrum of P 4 TA was measured in a dichloromethane (DCM) solution. Two absorption bands centered at 317 and 366 nm were observed. These two bands were associated with the p-p* transition and intramolecular charge transfer, respectively (Fig. S1 †). The large Stokes shi (68 027 cm À1 ) between the absorption and uorescence spectra (l em,max ¼ 513 nm) of P 4 TA in DCM was indicative of the structural difference between the ground and excited states. The resulting P 4 TA compound would probably also be AIE-active on considering that tetraphenylethene is the most frequently used AIE unit. To conrm this probability, a tetrahydrofuran (THF) solution of P 4 TA was titrated with water and the change in the uorescence emission was monitored. P 4 TA exhibited an extremely weak emission in the THF solution, where it was well dissolved (Fig. 2a). Hence, almost no PL signal was recorded. However, when 90% (v/v) water was present, a strong green emission that peaked at 499 nm was observed and the corresponding intensity ($705 a. u.) dramatically increased by up to $100 times compared to that at the 0% water fraction ($7 a. u.). Adding water to the THF solution of P 4 TA signicantly induced the formation of nanoparticles because P 4 TA molecules contain highly hydrophobic aromatic rings. In other words, the emission enhancement was caused by molecule aggregation which suggests that P 4 TA is AIE-active. 12 The prominent AIE character of P 4 TA motivated the application of the material in the solid state. Accordingly, block-like crystals (C g -form) were achieved through solvent evaporation of P 4 TA in a mixed solvent of n-hexane and DCM ( Fig. S2a †). Compared to the emissive nanoaggregates in THF-H 2 O, the asprepared sample C g exhibited an even stronger green-light emission, centered at 498 nm [F F,s ¼ 52%] (Fig. 2b). By grinding the C g crystals with a pestle or shearing them with a spatula, a very bright green light emission peaking at 517 nm was observed in the dark without UV irradiation ( Fig. 3a and c and Video S1 †). This experiment unambiguously illustrated that P 4 TA in the C g -form was ML-active. The strong ML of C g was indeed clearly seen even under daylight at room temperature and was maintained while the crystals were crushed ( Fig. 3c and Video S2 †). Certain organic materials, such as coumarin, phenanthrene, N-acetyl anthranilic acid, N-isopropyl carbazole and N-phenyl imides, have also been reported to show mechanoluminescence activities. However, none of these materials could emit a ML strong enough to be observed with the naked eye under daylight at room temperature. 5,13 The poor performance of the conventional organic ML materials should be attributed to their intrinsic ACQ property, which leads to the low emitting efficiency in the solid state. By contrast, the unique AIE feature of P 4 TA perfectly surmounted the ACQ effect and  exhibited a positive effect on luminescence enhancement, thereby giving the brilliant ML of C g . To further demonstrate the ML characteristics of C g , a simple device was made by sandwiching the sample between two pieces of pre-sculptured glass. The capital letters 'AITL' were clearly displayed when pressure was used as the driving force, which suggests the ML 'display capability' of C g (Fig. 3b). As such, the extraordinary AIE-active ML material P 4 TA will be a promising candidate for displays and optical recording.
Interestingly, another type of prism-like crystal (C b -form) was observed while exploring different processing conditions. The C b -form crystals, which showed an intense blue-light emission that peaked at 476 nm (F F,s ¼ 36%), could be obtained by adding ethanol into a P 4 TA/DCM solution under the action of ultrasound (Fig. S2b †). However, in contrast to the vivid C g phenomenon, the P 4 TA sample completely lost its ML activity when aggregated in the C b -form (Fig. 3a). The C b crystals showed a small melting endothermic shoulder peak at 191 C and a sharp peak at 198 C in the rst heating curve of differential scanning calorimetry (DSC) (Fig. 4a), indicating that C b was mainly composed of microcrystals that melt at 198 C. This result is different from that of C g , which melts at 206 C. Moreover, the powder X-ray diffraction (XRD) spectra also exhibited distinctly different patterns for the two samples (Fig. 4b). These results imply that the different ML activities of C g and C b could be attributed to their dissimilar molecular packing modes. A single crystal X-ray analysis was thus performed for the P 4 TA crystals to obtain more insight into this aspect. Single crystals of the two polymorphs (i.e., SC g and SC b ) suitable for X-ray structural analysis were isolated through slow solvent evaporation of P 4 TA in mixtures of ethanol and CHCl 3 of different concentrations.
The SC g and SC b samples emitted intense green or blue light peaked at 499 and 476 nm, respectively ( Fig. 2b and 5a and b). These light emissions are similar to those of the asprepared crystals of C g and C b . The main peaks of the simulated XRD patterns of SC g and SC b also agree well with those in the patterns obtained from P 4 TA in the C g and C b phases, which suggests that the initial powders were mainly composed of P 4 TA microcrystals in the polymorphs of SC g and SC b (Fig. 4b). Further systematic analysis revealed that both SC g and SC b belong to the non-centrosymmetric polar space group of P(2)1 (Table S1 †). Some previous reports have shown that dipolar structures and non-centrosymmetric molecular arrangements are favorable for obtaining piezoelectric properties, which were closely pertinent to the ML activities of the crystals. 14 In principle, the fracture of crystals with a strong piezoelectric effect will lead to electronic discharge at the crack surface, which would result in dye excitation and generation of ML for the crystals. 5,15 The molecular structure of P 4 TA and the crystalline symmetry of SC g and SC b also meet  the requirements for piezoelectric properties, thus making the as-prepared crystals of C g and C b more active and more likely to achieve the ML character. However, the dipole moments and the HOMO-LUMO band gaps (DE g ) of the molecules in the SC g and SC b polymorphs were different. These differences were caused by their distinct molecular conformations and packing characteristics. The asymmetric units in both polymorphs (i.e., SC g and SC b ) were composed of two crystallographically independent molecules (i.e., SC g1 and SC g2 for SC g , and SC b1 and SC b2 for SC b ). Each molecule showed the formation of a C-H/O intermolecular hydrogen bond ( Fig. 5a and b, S3 and S4 †). Compared with SC b , the most notable conformational difference of P 4 TA in the SC g polymorph was the dihedral angle q between the thiophene and the adjacent phenyl ring. While the conformations of P 4 TA were twisted (q ¼ 18.9 for SC g1 and 6.1 for SC g2 ) in polymorph SC g , the two aromatic rings were nearly coplanar in polymorph SC b (q ¼ 2.5 for SC b1 and 4.7 for SC b2 ) (Table S2 †). In the case of SC g , the two thiophene rings of SC g1 and SC g2 were almost perpendicular to each other, showing a dihedral angel of 85.8 . By contrast, an anti-parallel packing mode was observed between the thiophene rings of SC b1 and SC b2 (q ¼ 3.7 ) in SC b .
The most popular B3LYP density functional theory was then used to calculate the dipole moments and the DE g of P 4 TA in the four conformations at the 6-31G(d, p) level based on their ground state geometries in the single crystals. Fig. 5c presents the results. The dipole moments of SC g1 and SC g2 in the SC g polymorph were 5.24 and 4.96 debye (D), respectively. Both values were larger than those of SC b1 (4.64 D) and SC b2 (4.78 D) in SC b . The larger dipole moments of the molecules combining the non-centrosymmetric molecular arrangement may result in a larger net-dipole moment of the crystalline structure, and would subsequently lead to a stronger piezoelectric effect in the SC g polymorph when breaking the crystals. The theoretical calculation results also suggest that the molecules in both SC g and SC b have ICT characteristics: the electronic transitions (mainly from HOMO to LUMO for all the four conformations in SC g and SC b ) from the occupied orbitals delocalized over the TPE (donor) moiety to the thiophenaldehyde (acceptor) moiety made major contributions to the excited states ( Fig. 5c and Table S4 †). 16 SC g1 and SC g2 showed even smaller DE g (HOMO / LUMO) values of 3.38 and 3.40 eV, respectively, as compared to those of SC b1 (3.62 eV) and SC b2 (3.50 eV) in the SC b polymorph. The calculations showed good agreement with the solid state UV-visible spectra of C g and C b , which absorbed at 373 nm and 367 nm, respectively (Fig. S5 †). The preceding results thus indicate that the electrons of the molecules in SC g can be excited with a lower energy. Hence, the stronger piezoelectric effect and the lower electronic transition energy resulted in the excitation of P 4 TA molecules and the generation of ML in the SC g phase by breaking the crystals. Meanwhile, all the molecules adopted a highly twisted propeller-like conformation in the SC g polymorph, which prevented the formation of detrimental species, such as excimers or exciplexes, caused by p-p stacking interactions. Furthermore, numerous intermolecular interactions such as C-H/p and C-H/S might also exist in the crystals aside from the C-H/O hydrogen bonding ( Fig. 5b and Table S3 †). These multiple interactions had rigidied the molecular conformations and impeded intramolecular rotation, which largely reduced the energy loss via non-radiative relaxation channels, and subsequently resulted in a notable AIE effect and high F F,s value for P 4 TA. The preceding factors consequently made the ML of sample C g , which was mainly composed of P 4 TA microcrystals in the SC g polymorph, highly emissive under the stimulus of mechanical force. By contrast, the weaker piezoelectric effect in the SC b polymorph seemed not to reach the higher energy requirement for the electronic excitation although SC b also exhibited strong photoluminescence. Consequently, P 4 TA lost its ML activity when aggregated in the C b phase. These results also suggest a feasible design direction for the development of efficient ML materials through combining the prominent piezoelectric property for molecular excitation and the abnormal AIE character for emission.
The PL of C b was remarkably changed when the sample was exposed to DCM or acetone vapors for about 10 min, passing from an initial blue to green light at 499 nm (C bfform). The resulting spectrum of C bf was superimposable on that of C g (Fig. 2b). The coincidence of the PL emissions suggested that the fumed sample of C bf probably had the same molecular arrangement as that of the C g polymorph. Further evidence for this standpoint was provided by their similar XRD patterns and their overlapping DSC curves with the same melting point at 206 C ( Fig. 4a and b). As mentioned in the preceding discussion, sample C g was MLactive. And expectantly, the ML activity of C b could be tuned by simply altering the molecular packing mode upon fumigation. To verify this hypothesis, the ML spectrum of C b was collected aer exposure to DCM vapor (C bf ). As anticipated, C bf also exhibited a strong green light emission without UV irradiation using pressure, which revealed that the ML of C b could be facilely turned on with the aid of DCM vapor. The ML emission maximum of C bf was located at 520 nm, which is close to that of C g (Fig. 3a).
Noticeably, the ML maxima of C g and C bf were both signicantly red-shied (Dl em,max z 21 nm) as compared to their PL spectra. This result shows a special mechano-uorochromic effect. To gain an understanding of this, the inuence of applied pressure on the luminescence was investigated. The PL maximum of the pristine P 4 TA in the C gform shied from 498 nm to 521 nm (G g ) aer pressing or grinding (Fig. 2a), agreeing well with its ML emission (Fig. S6 †), thereby conrming that the bathochromic shi between the ML and PL of C g was caused by its intrinsic mechanochromic properties. The phase characteristics of the ground sample G g were determined by XRD to decipher further the relationship between the ML and the mechanochromism of C g . Most of the diffraction peaks were diffuse or even disappeared, although some resolvable peaks of G g were consistent with those of their original crystals (Fig. 6a). This revealed that the ground sample was partially in a metastable amorphous state. Accordingly, DSC was performed for the sample aer grinding (Fig. 6b). Compared with C g , an additional exothermal peak around 86 C was observed in the DSC thermogram of G g , which demonstrated that the C g crystals were partially destroyed and converted to an amorphous state by the grinding or pressing treatment. The P 4 TA molecules in the C g phase adopt twisted conformations in the crystalline state to t into the crystalline lattice, and the crystalline lattices may collapse when triggered with mechanical force. The dye molecules also then relaxed to a more planar conformation, thereby emitting redder ML and PL. In other words, the bathochromic shi of the ML for C g originates from the microcrystal amorphization and the extension of molecular conjugation, which were believed to be the main reasons for the C g mechanochromism. 16,17 Unlike other conventional stimuli-responsive materials, the C g -form of P 4 TA can show a luminescence response and luminescence color change simultaneously and respectively both without and with UV irradiation under a force stimulus. This new kind of forceresponsive material with AIE properties has not yet been achieved before, and would facilitate applications of ML materials in the eld of sensors. 11c In addition, uorescence spectroscopy was also performed to evaluate the mechanochromic behavior of P 4 TA in the C b phase, and the C b sample exhibited a more remarkable emission wavelength change of 47 nm upon grinding. Furthermore, the corresponding PL spectrum (G b ) with l em,max ¼ 523 nm tted well with that for the powder ground from C g (Fig. 2b), which indicated that P 4 TA could switch to the same emission under a force stimulus regardless of its initial state. Also the uorescence 'writability' of C b can be veried by writing on a piece of lter paper as shown in Fig. 3d. The mechanouorochromism of C b should occur by a similar mechanism to that proposed for C g .