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Abstract The iso -cis/trans-aminobenzopyranoxanthenes (iso -cis/trans-ABPXs), as one of popular leuco dyes, were investigated in solution or crystal phase. The introduction of benzo-pentacyclic-ester group in iso -cis/trans-ABPXs leads to remarkable multi-stimuli responsive behaviors, including protonation effect, photochromism and piezochromism with notable color changes. The iso -cis/trans-ABPXs show different in the responsive sensitivity and the iso -cis-ABPX is highly sensitive to environmental stimuli relative to the trans-form, which can be explained by their different C-H⋯O interaction and/or C-O bond length. Interestingly, as the ultraviolet light irradiation time increasing, the iso -cis/trans-ABPXs solution gradually becomes transparent and orange precipitates are achieved, which is identified as iso -ABPXs±. Multi-stimuli responsive behaviors on the iso -cis/trans-ABPXs provide a comprehensive insight into the chromogenic mechanisms of this type of materials, suggesting that they may be potential candidate materials for sensors and indicators. Graphical abstract Image 1 Highlights • The iso -cis/trans-ABPXs show remarkable color and spectral changes under different stimuli respectively. • The iso -cis -ABPXs is more sensitive to the stimuli due to the differences in structures. • The orange solid of iso -cis/trans-ABPXs± can be produced from iso -cis/trans-ABPXs solution under the UV light irradiation. [ABSTRACT FROM AUTHOR]

Abstract Piezochromic luminescence behaviours of 9, 10-distyrylanthracene (DSA) derivatives with different substituents based on their single crystal structures were investigated under hydrostatic pressure via a diamond anvil cell (DAC), compared with the anisotropic force produced from grinding. Upon grinding, three single crystals show identifiable blue-shifts in fluorescent spectra because of the disturbance for ordered crystal structure to amorphous state, but nearly sightless color changes. The high-pressure fluorescent experiments applied by DAC reveal three DSA crystals all show large red-shifts. High pressure absorption spectra display that a new charge transfer state appears when -CH 3 or -2CN is introduced to DSA and all of them show obvious red-shifts. Once the pressure was relieved gradually to the ambient pressure, the fluorescence and absorption spectra both entirely recovered, indicating this geometric change is reversible in a certain pressure region, which is further confirmed by Raman spectra. The relationships between molecular structures and spectroscopic behaviours were further discussed by their single crystal data. This work provides deep insights into the relationship between molecular structure and photo-physical properties of practical mechanochromic luminescent materials, and is significant in the potential applications of pressure transducers. Graphical abstract Image 1 Highlights • Photo-physical behaviors of three DSA crystals with different substituents are disclosed. • DSA crystals exhibit different responses to anisotropic/isotropic pressures. • Self-recovery of DSA crystals during compression. • Remarkably large Red-shift of emission band under pressure is observed. • Introduction of -CH 3 and –CN to DSA cause a new CT state. [ABSTRACT FROM AUTHOR]

Herein, we report a new dibenzo a,c phenazine-phenothiazine dyad (BNPTZ) bridged by a C–N single bond, which realizes AIEE, polymorphism, mechanochromsim and high sensitivity to hydrostatic pressure, due to the twisted D-A structure. BNPTZ exhibits AIEE activity in THF/H 2 O system, owing to aggregation-inhibited non-radiative relaxation channels of the excited states. Fortunately, two types of single crystals of BNPTZ with distinctive luminescence were cultivated and the effect of different conformation and complicated intermolecular interactions on luminescent property was analysed. BNPTZ demonstrates responses to both shearing force and hydrostatic pressure. Interestingly, two different crystalline phases are involved in the color-changing process upon shearing and fuming. Upon shearing, the crystal powder suffers the luminescence color change from orange-yellow (565 nm) to redness (604 nm), because of the phase transition from OY-phase to amorphous triggered by external force. When fuming, the amorphous phase would recover to YG-phase unexpectedly. When subjecting two single crystals to hydrostatic pressure, the OY-phase single crystal underwent a clear color switch from yellow to orange and eventually to red. The YG-phase single crystal displayed a different color alteration from yellow-green to orange and further to red. Both of the two single crystals exhibited relatively high sensitivity to pressure with a wide spectral shift over 100 nm. Up to now, BNPTZ is a rare dyad that achieves polymorphs with high sensitivity to pressure. Image 1 • A new dibenzo a,c phenazine-phenothiazine dyad (BNPTZ) bridged by a C-N single bond is reported, which realizes AIEE, polymorphism, mechanochromsim and high sensitivity to hydrostatic pressure, due to the twisted D-A structure. • BNPTZ demonstrates responses to both shearing force and hydrostatic pressure and two different crystalline phases are involved in the color-changing process upon shearing and fuming. • Both of the two single crystals exhibited relatively high sensitivity to pressure with a wide spectral shift over 100 nm. [ABSTRACT FROM AUTHOR]

A novel halogen bond-based co-crystal, 4,4′-trimethylenedipyridine/1,4-diiodotetrafluorobenzene (TMDP-DITFB), was achieved, which displays distinct piezochromic luminescence from none to a broad emission band (500–700 nm) assigned to the new intermolecular charge transfer (CT) state caused by the enhanced I⋯N interactions between TMDP and DITFB. Interestingly, once the pressure was released to the ambient condition, the emission of TMDP-DITFB could not be restored to initial state, but exhibited a new red maximum emission band at 668 nm, which is different from most piezochromic materials. Almost no change for the TMDP crystal was observed during the compression-decompression cycle. Comparative analysis between TMDP and TMDP-DITFB demonstrates that irreversible changes in fluorescence are highly related to intermolecular interactions between TMDP and co-formers modulated by pressure. Pressure-induced enhanced and irreversible emission of organic materials is rarely reported. This work enables deeper understanding on the structure-property relationship and put forward a new strategy to realize new properties of materials. Image 1 • A TMDP-DITFB co-crystal is obtained via intermolecular I⋯N interactions. • Intermolecular charge transfer brings TMDP-DITFB remarkable pressure-induced emission enhancement. • TMDP-DITFB exhibits irreversible emission during compression-decompression process. [ABSTRACT FROM AUTHOR]

Two polymorphs of 9,10-bis(phenylethynyl)anthracene (OC and RC) were obtained and they were characterized by multi spectroscopies including X-ray diffraction, absorption, fluorescence and Raman. The single-crystal structure analysis shows that RC has richer intermolecular interactions (C–H ... π and C–H⋯H–C) than OC, which accounts for different spectroscopic behaviors of RC from OC in high pressure studies. Under mechanical grinding, two polymorphs have similar spectral changes in their fluorescence as significant enhancement and slight blue-shifts of the shoulder emission bands at lower wavelength, except that RC shows more sensitive to grinding than OC. During the compression process applied by diamond anvil cell (DAC), OC and RC both demonstrate remarkable red-shifts with increasing pressure, while RC shows a smaller shifting rate than OC. These spectral phenomena reveal that the richer intermolecular interactions in RC can be easily destroyed upon grinding. This work provides the in-depth insight into the mechanism of mechanochromism and the structure-property relation, and proves again that the intermolecular interactions play the key role in modulating the photo-physical properties. Image 1 • Pressure induced enhancement of the shoulder emission intensity at lower wavelength of two polymorphs upon grinding. • OC and RC show different responses to pressure. • Intermolecular interactions answer for different photo-physical properties of two polymorphs. [ABSTRACT FROM AUTHOR]

Flexible crystals with plasticity and elasticity that exhibit phosphorescent emission are quite rare yet highly significant for their potential applications. In our study, we present an example of such flexible (both elastic and plastic) organic crystals based on 1,2-bis(3-bromophenyl)-ethane-1,2-dione (BBPED), which demonstrates a lifetime of 0.504 ms (517 nm) and a quantum efficiency of 26.12 % at room temperature. These crystals exhibit reversible elastic properties in both their original and twisted states, and possess optical waveguide properties with minimal optical loss coefficients in the range of 0.26–0.48 dB/mm. In addition, the above crystals exhibit phosphorescence to phosphorescence blueshift under hydrostatic pressure, whereas mechanical grinding produces a phosphorescence to phosphorescence redshift phenomenon. Theoretical calculations indicate that the alterations in molecular conformation and intermolecular interactions both contribute to the piezochromic phenomenon. In conclusion, this work provides a new approach to the design of multifunctional phosphorescent molecular crystals with flexible characteristics. • Flexible crystals with plasticity and elasticity. • Crystals with 1D to 3 D phosphorescent waveguide properties. • Crystals with piezochromism properties. [ABSTRACT FROM AUTHOR]

Abstract The three molecules of SP-4-An-2, SP-4-An-9 and SP-5-An-9 were synthesized and their fluorescent mechanochromism was studied in their pristine crystalline state and single crystal form, respectively. The molecules have small structure dissimilarity but show quite different responsive behavior to anisotropic grinding and isotropic hydrostatic pressure either in the pristine crystallites or in the single crystals. Manual grinding transfers the ordered pristine crystallites phase to amorphous state, however, it can hardly shift the emission to the longer wavelength, suggesting the ring-opening isomer is rare in the system. As a comparison, hydrostatic pressure compels the anthracene units in the single crystals to be closer with enhanced π-π interaction, and induces the isomerization of spiropyran to be a more flat and conjugated merocyanine (MC) form, resulting in the gradually red-shifted emission and red fluorescence. In particular, the single crystal of SP-4-An-2 achieves a high contrast multicolored change and the largest emission red-shift over 190 nm. These results show the dependency of mechanochromic property on the molecular structure, i.e. marked position of anthracene-to-anthracene, and anthracene-to-spiropyran frameworks based on in-situ observation with the diamond anvil cell (DAC), which is in sharp contrast to the powder crystals and ambient fluorescence measurements. Graphical abstract Three single crystals of SP-4-An-2 , SP-4-An-9 and SP-5-An-9 were simply synthesized either by linking a spiropyran moiety to α or γ position of an anthracene unit, or through connecting an anthracene unit to 5-position of a spiropyran moiety. The slight structure dissimilarity offers them with distinct mechanochromic behaviors and quite different responsiveness to the anisotropic grinding and isotropic pressure. Manual grinding turns the original crystalline phase to the amorphous state, but it is very hard to initiate the isomerization of spiropyran for the molecules. While hydrostatic pressure forces the anthracene units closer packed and induces the ring-opening reaction of spiropyran, leading to the gradually red-shifted emission and bright red fluorescence. In particular, hydrostatic pressure endows SP-4-An-2 with a high contrast multicolor switching and the largest red-shifted emission more than 190 nm. Image 1 Highlights • The three molecules of SP-4-An-2, SP-4-An-9 and SP-5-An-9 were synthesized and their fluorescent mechanochromism was studied in their pristine crystalline state and single crystal form, respectively. • The single crystal of SP-4-An-2 achieves a high contrast multicolored change and the largest emission red-shift over 190 nm. • These results show the dependency of mechanochromic property on the molecular structure, i.e. marked position of anthracene-to-anthracene, and anthracene-to-spiropyran frameworks based on in-situ observation with the diamond anvil cell (DAC). [ABSTRACT FROM AUTHOR]