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A polymorph is an ideal crystal model for studying the structures affect the optical waveguide behaviors of organic cocrystals, resulting from their multiple crystal structures with the same compositions. However, the polymorphism-based optical waveguide properties of cocrystals are rarely reported. Here, two polymorphic cocrystals of 9-acetylanthracene-1,2,4,5-tetracyanobenzene in a novel sheet-like form α and a known bulk form β were prepared by the evaporation of a donor (D)/acceptor (A) solution mixed in a ratio of 1 : 1 in a mixed acetonitrile and tetrahydrofuran solution. The two polymorphs present different molecular stacking structures due to the diverse intermolecular hydrogen bond interactions, causing differences in their intermolecular charge transfer (CT) interactions and photophysical properties. Structural and spectroscopic analyses revealed stronger CT transition in the form β than in the form α. Polymorphism-based luminescence properties and morphology- and color-dependent optical waveguide behaviors were observed in the polymorphic CT cocrystals via photoluminescence (PL) micro-imaging. The propagation losses of the form α were 0.37 and 0.6 times less than those of the form β in parallel and perpendicular direction with respect to the growth direction, respectively. Three-dimensional (3D) stereoscopic mode optical waveguide behaviors can be observed in the form β. The study provides a good method for the preparation and detection of polymorphic materials. The optical waveguide properties of the cocrystals were tuned by the crystal phase, which is helpful for a deep understanding of the structure-optical functional relation in organic cocrystals. The study will open new potential applications of cocrystal materials in 3D photonic and optoelectronic devices.

Piezochromic luminescent materials with multi-color switching have recently attracted much attention owing to their appealing applications in the field of advanced photonics, such as optical recording, memory and sensors. Mechanical grinding and diamond anvil cell (DAC) devices are two common ways to investigate piezochromic materials. This review comprehensively summarizes the recent progress of piezochromic materials and their behaviours under hydrostatic pressure generated from DAC, in particular, different types of organic piezochromic materials. We describe their spectroscopic phenomena, discuss corresponding mechanisms, and intend to find the inherent clues hidden in the scattered literature.

Two newly synthesized molecules with a twisted donor–acceptor–donor (D–A–D) structure were achieved, noted as TPA-Ph-CN and TPA-Py-CN, and their solvatochromic effects and mechanochromic properties upon grinding and/or under hydrostatic pressure were investigated deeply. The comparison of their solvatochromic behaviors demonstrates that TPA-Py-CN shows a stronger intramolecular charge transfer (ICT) effect than TPA-Ph-CN due to the substitution of electron-deficient acceptor from Ph-CN to Py-CN. Photophysical characterizations together with the analysis of crystal structures indicate that TPA-Py-CN exhibits a distinct dimer structure with C–H⋯π interactions, but TPA-Ph-CN does not. As a result, TPA-Py-CN shows green emission with three bands and the peak at 538 nm is assigned to excimer, while TPA-Ph-CN presents blue emission with only one band. Upon grinding, TPA-Py-CN shows more sensitive responsiveness to pressure than TPA-Ph-CN and exhibits a size-dependent emission property. TPA-Ph-CN only displays continuous red-shifted and broadened emission under hydrostatic pressure. In contrast, high pressure experiments and theoretical calculations of TPA-Py-CN demonstrate that the excimer fluorescence wavelength is pressure-independent. This study is helpful for better understanding intrinsic essences of the molecular structure and the properties of mechano-responsive organic molecules.

In this study, we report a newly synthesized molecule 1via linking a benzothiazole to rhodamine, which could exhibit AIEE, unique acidichromism and solvatochromism in solution, fascinating photochromism in a polymer matrix, intriguing mechanochromism in solid state and remarkable piezochromism in the crystal. 1 is a rarely reported organic molecule, which is sensitive to multiple stimuli such as acid/base, force, pressure, light and solvent polarity due to its distinct intramolecular hydrogen bonding and spiro-lactam structure. 1 demonstrated AIEE property in a THF/H2O system because of the water-induced aggregation and aggregation-suppressed nonradiative relaxation of the excited states. In the solution, 1 afforded yellowish-green emission at 520 nm (keto emission) in a non-polar solvent and blue emission at 447 nm (enol emission) in a polar solvent. Surprisingly, a blue shift in the emission peak was observed in a DCM solution upon treatment with acid or base, which is quite different from that of traditional rhodamine. In the solid state, the emission color of 1 changed from yellowish-green to red upon grinding or applying hydrostatic pressure, owing to the force-induced ring-opening reaction. In the polymer matrix, 1 manifested reversible photochromic behaviour and could serve as a good photoprinting material. This study is expected to contribute to the development of multistimuli-responsive fluorescent switch with multiple functions in different states.

Realizing fexible and tunable emission conversion in organic solid-state luminescent materials is always fascinating. Herein, we successfully achieved fexible photochemical conversion under pressure and temperature on two types of 4-methyl-2-(5-[4-dimethylaminophenyl]-1H-pyrazole-3-yl)phenol (4MPP) single crystals with distinct aggregation form and geometry showing as green (GC) and blue-violet (BVC). These conversions were studied in detail by fluoresence and Raman spectroscopies, followed with Hirshfeld surface analysis, and this study provides a significant material system for understanding the mechanisms of structural conversion between normal emission (E*) and excited state intramolecular proton transfer (ESIPT, K*) comprehensively under pressure and temperature. Owing to having a proton transferred tautomer, BVC exhibits remarkable changes in its emission feature with pressure increasing to 4.79 GPa, along with the blue-shift and fluorescence enhancement according to a conversion from E* to K*. This compression-induced conversion is proved to be caused by the contraction of the intermolecular distances, the increase of molecular planarization degree, and the enhanced O–H⋯N intramolecular interaction, along with the reduced distance of H⋯N and the elongation of the bond length of O–H. Differently, GC shows rapidly weakened fluorescence and slight blue-shift at a relatively lower pressure (1.01 GPa) based on its more restricted structure. Moreover, GC can realize an excited state transition from K* to E* when lowering its temperature. BVC experiences an abrupt phase transformation to GC at a high temperature of 190 °C. This work discloses the photo-physical properties and stimuli-responsive behaviours of two 4MPP polymorphs, which provides deep insight into the relationship between molecular structure and photoluminescent nature, especially in the crystals with the ESIPT property.