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The phenomenon that different molecular packing modes in aggregates result in different optical properties has attracted intense attention, since it can provide useful information to establish the relationship between the micro- and macro-world. In this paper, DBTDO-DMAC was designed with 9,10-dihydro-9,9-dimethylacridine (DMAC) as electron donor. DBTDO-DPA and DBTDO-Cz were designed for comparison, which adopted diphenylamine (DPA) with twisted structure and carbazole (Cz) with planar structure as donors, respectively. As expected, two polymorphs (Crystal G and Crystal Y) of DBTDO-DMAC were obtained and exhibited distinct properties. Crystal G originating from planar conformation exhibited mechanochromism (MC) phenomenon and the emission color changed from green to yellow with a redshift of 35 nm after grinding. Nevertheless, Crystal Y with folded conformation displayed obvious room-temperature phosphorescence (RTP) with yellow afterglow. Careful single crystal analyses, powder X-ray diffraction and theoretical calculation reveal that the different emissive behaviors are highly related to the molecular conformation and packing modes. The successful adjustment of molecular conformation provides some guidance in the design of other MC and/or RTP luminogens, broadens the molecule family with the tunable molecular conformation and opens up a new avenue for exploring possible adjustment of molecular packing in aggregates.

The mixed π–π packing of the donor (D) and acceptor (A) molecules is the highlighting feature of the intermolecular interactions following charge transfer (CT) issues in organic cocrystal systems. There is an inverse relationship between the D–A interplanar distance and the intermolecular CT interaction. However, the D–A C–C surface close contact (relative areas) on the intermolecular CT interactions in organic cocrystal systems is rarely investigated. Herein, we designed and constructed a novel cocrystal and its solvate cocrystal. The structural and electrostatic potential analyses suggest that the solvation destroys the N–H···N hydrogen bond interaction between phenothiazine (PTZ) and 1,2,4,5-tetracyanobenzene (TCNB), which causes the TCNB molecules to have a 90° rotation along the normal axis of the PTZ plane. Thus, the D–A C–C surface close contact is enlarged, strengthening the intermolecular π–π stacking interactions and intermolecular CT interaction between PTZ and TCNB, which are further evidenced by the absorption and Raman spectroscopic analyses. This study provides rare evidence of the enlarged C–C surface close contact in the mixed packing between D and A that greatly contributes to the intermolecular CT interaction in a D–A cocrystal system. It also provides a deeper understanding of the role of solvation in the structure–property relationship of organic cocrystal materials.

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.

At present, there is still an urgent demand for novel smart materials that can achieve a diverse range of practical applications in synthetic material area. Herein, we developed a simple but versatile aggregation-induced emission luminogen (1) with a donor-acceptor structure and pronounced intramolecular charge transfer property. Compound 1 showed a remarkable color change in sensitive response to polarity change making it to serve as a promising imaging probe for detecting environmental polarity in cells and selective visualization of lipid droplets in live tissues. Additionally, this compound exhibited a wide range of thermoresponsive behavior with ratiometric luminescence change and noticeable fluorescence color switching. It also can respond to anisotropic shearing force and isotropic hydrostatic pressure with prominent but contrast luminescence conversion due to the distinct disturbance of the weak intermolecular interactions and charge transfer processes. Meanwhile, compound 1 was sensitive to external electric stimulus and displayed reversibly three-color switched electrochromism and on-to-off electroluminochromism. Such property allowed the fabrication of high-performance non-doped OLED with a high external quantum efficiency of 5.22%. The present results may offer an important guideline for multifunctional molecular design and provide an important step forward to expand the real-life applications of AIE-active smart materials.