Fluorescence emission properties in the single molecule, crystalline and amorphous state are key factors determining the application fields and scope of luminogens. However, the luminescence efficiency of luminogens is usually inconsistent in these three existing forms. To explore multi-state emission properties and the inherent mechanism of luminogens, and obtain rare mechanical force-induced luminescent enhancement and multicolor switching, three D–A–D (Donor–Acceptor–Donor)-type organic boron complexes containing tetraphenylethylene (TPE) and different aromatic amine units are designed and synthesized. TPEDKBF₂DMeA, TPEDKBF₂Ca and TPEDKBF₂DBeA respectively exhibit aggregation caused quenching (ACQ), AIE and dual-state emission (DSE) fluorescence characters. The orange crystals (Oc) of TPEDKBF₂Ca show a twisted molecular configuration and head-to-tail J-type stacking mode, while the red crystals (Rc) of TPEDKBF₂Ca form scarce super unit cells with the help of two face-to-face antiparallel dimers and the other four single molecules. TPEDKBF₂DBeA adopts an antiparallel head-to-head J-type stacking mode, leading to π–π stacking interactions. The faint fluorescence emission of TPEDKBF₂DMeA is attributed to parallel J-type stacking based on theoretical calculations. Additionally, TPEDKBF₂Ca, TPEDKBF₂DBeA and TPEDKBF₂DMeA in turn display reversible multi-color switching, blue-shifted fluorescence emission and remarkable luminescent enhancement, and negligible wavelength shift. Detailed single crystal analyses and theoretical calculations indicate that the molecular stacking mode is the critical factor affecting the optical and mechanochromic properties. Meanwhile, this work demonstrates that aryl amine substituents can tune the multistate emission properties of organic fluorophores, which provides a deep understanding for molecular design of multi-state emitters and mechanochromic materials.

Aggregation-induced emission (AIE) is a unique photo-physical phenomenon that has become an emerging and hot research area. It has a wide range of applications due to its excellent luminous properties. In this paper, five compounds and their corresponding light conversion films were prepared based on AIE effects and the thermally activated delayed fluorescence (TADF) phenomenon. Furthermore, ultraviolet conversion, dispersion and photo-physical properties such as UV-vis spectra, fluorescence spectra, and photo stability as well as the mechanical properties of the light conversion films were investigated in detail. The results reveal that triphenylacrylonitrile (TPA) exhibits excellent photo stability and ultraviolet light conversion properties. In addition, the fluorescence emission spectrum of TPA corresponds well with the absorption spectrum of plants in the blue-violet region. In particular, the light conversion film with added TPA also shows enhanced mechanical properties and slightly lower visible light transmittance (3.79%) compared to PVC blank film. Based on the photo stabilities of the five compounds, it can be concluded that the electron-withdrawing cyano group can increase the photo stability of TPA, while carbazole substituents are proved to have an uncertain effect on the rate of photo oxidation, which is attributed to the electron-donating properties of carbazole and increased molecular distortion or rigidity. Finally, it is worth mentioning that this is the first report utilizing AIE-active luminogens (AIEgens) in agricultural light conversion film.

Tetraphenylethylene (TPE) is known for its aggregation-induced emission enhancement (AIEE) effect due to the restriction of intramolecular rotation. Some halogen-substituted fluorophores show interesting luminescent phenomena via C–H⋯halogen interactions. By combining TPE units, a halogen with a difluoroboron β-diketonate fluorophore, three complexes, 2a, 2b and 2c, were designed and synthesized. All of them exhibited intramolecular charge transfer, high fluorescence quantum yields, AIEE characteristics and high-contrast mechanochromism. By either grinding-fuming or grinding–annealing cycles, the pristine state or the original emission color of the three complexes can be almost recovered reversibly. In particular, 2c exhibits spontaneous reversibility after grinding at room temperature. Owing to the effect of halogen (Cl, Br), 2b and 2c show polymorphism-dependent fluorescence (yellow plate-like crystals and green needlelike crystals) and higher contrast than 2a before/after grinding. The maximum fluorescence emission wavelength of 2b and 2c shifts bathochromically 70 nm and 59 nm after grinding, respectively. To reveal the mechanochromic mechanism, a series of tests including powder XRD, fluorescence lifetime, DSC, and solid state UV-visible absorbance were completed. The results indicate that mechanochromism of the complexes can be correlated with the transformation between the amorphous and the crystalline states and metastable state formation after grinding, rather than molecular planarization, excimer or exciplex formation. To the best of our knowledge, this is the first example of no alkyl- or alkoxy-substituted difluoroboron β-diketonate complexes that exhibit reversible mechanochromic luminescence.

Well-designed full-color emission, AIE, CIEE-active luminogens and high contrast mechanochromic behaviour.

The discovery of mechanochromic materials with multicolor conversion has mainly depended on accidents and good fortune until now, but manipulating molecular aggregation modes to achieve polymorphism is highly efficient. To obtain multicolor conversion materials and explore the intrinsic mechanism of polymorphism, TPEB2F and TPEB3F were designed and synthesized based on molecular isomerism and aggregation-induced emission (AIE) theory. Although they have similar solvatochromism and AIE properties, TPEB2F and TPEB3F show a high-contrast polymorphism and mechanochromism in terms of reversibility, fluorescence emission intensity and wavelength shift. Only crystal 2R was obtained for TPEB2F, but four different crystals 3Y, 3R, 3N and 3F exist for TPEB3F in the same solvent environment. Emission maxima of 2R show a 22 nm red shift, and its quantum yield (Φ_F) was significantly increased to 0.37 from 0.09 after hard grinding. 3Y provided a maximum redshift (40 nm) and slightly reduced Φ_F due to different intermolecular stacking and arrangement compared to the others, while 3N exhibited a remarkable blue shift (48 nm) and emission enhancement by gently grinding. X-ray single crystal analysis indicated that mechanochromism was not only related to intermolecular stacking and arrangement but also to the crystal system, space group and density. Theoretical calculations revealed that 2R, 3Y and 3R have an intramolecular charge transfer effect, and furthermore, the key to the formation of polymorphism mainly depends on the dipole moment of stacked molecular pairs and culture solvents rather than the minimum energy of stacked molecular pairs and the dipole moment of a single molecule. In addition, the by-product was purified and characterized in detail for the first time.