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Three cyclohexane-fused quinoxaline based thermally activated delayed fluorescence emitters were prepared, showing highly efficient device performance in organic light emitting diode.

Three kinds of thermochromic materials (DC8, DC12, DC16) were synthesized by linking the rigid 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (bpeb) with different lengths of alkyl chains. They exhibit remarkable fluorescent color changes under the irradiation of 365 nm light with elevating temperature, which is supposed to be caused by the transition between the crystal state and the amorphous state. Interestingly, the DC16 solid also has a photochromic character. It should be noticed that the phase transition temperatures of three materials measured by differential scanning calorimetry are higher than those of the fluorescence color changes during the heating process. Thus, the allochroic effect is attributed to the synergistic effect of both heating and photo-inducement (365 nm). Ethanol can turn the heated powder into the initial crystal again which indicates that their thermochromic behavior is reversible and makes the fluorescence recover.

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.

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.

A luminescent cocrystal system is reported to undergo crystal-to-crystal phase transformation from yellow-emitting polymorph I to green-emitting polymorph II, triggered by THF fuming or heating, and the green emission can recover to the initial yellow emission by grinding. The established spectroscopic and crystallographic analyses demonstrate that the phase transition occurred and benefits from the combined effect of similar molecular arrange sequence and unique alteration of intermolecular interactions from halogen/hydrogen bonds in I to π-π stacking in II. Furthermore, I and II exhibit red-shift emission under hydrostatic pressure. The emission of I and II shows a red-shift and recovers towards the initial emission upon acid-base fuming. This is a rare example of reversible luminescent switching of cocrystal based upon crystal-to-crystal phase transition, and provides an alternative strategy to develop multi-stimuli responsive materials.

There is still an urgent demand for novel smart materials that can achieve a diverse range of practical applications in the synthetic material area. Herein, we developed a simple but versatile aggregation-induced emission luminogen (AIEgen, 1). Compound 1 was sensitive to an electric stimulus and displayed reversibly three-color switched electrochromism and on-to-off electroluminochromism. Such properties allowed the fabrication of high-performance non-doped OLEDs with a high external quantum efficiency of 5.22%. Due to its AIE property and remarkable sensitive color change in response to polarity change, it can serve as a unique imaging probe for detecting environmental polarity in cells and selective visualization of lipid droplets in live tissues. More impressively, compound 1 exhibited a wide range of thermoresponsive behaviors with a ratiometric luminescence change and noticeable fluorescence color switching. As another remarkable feature, it can respond to anisotropic shearing force and isotropic hydrostatic pressure with prominent and contrasting luminescence conversion due to the distinct disturbance of the weak intermolecular interactions and charge transfer process. 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 smart materials.

Stimuli-responsive luminescent materials, which show various luminescence alterations upon the introduction of external mechanical forces, have attracted extraordinary attention owing to their promising applications in smart materials. Amongst, the phenomena of self-recovered mechanochromic luminescence constitute only a small proportion, and relevant systematic explorations are extremely limited. Here, we developed a series of new Au(I) complexes (AuIB-Cn, n = 5–10) featuring aggregation-induced emission properties, and systematically studied their solid-state self-recovery properties. We showed that the self-recovery speed could be elaborately tuned by introducing alkyl chains of different lengths, or changing the temperature. Increasing the alkyl chain length could speed up the self-recovery due to much faster intermolecular rearrangements. Lowering the temperature allowed for the capture of the originally superfast and unobservable self-recoveries of C8–C10 due to the decreased molecular mobility. In addition, we also demonstrated the great potentials of these AIE-active complexes serving as self-erasing rewritable paper and anti-counterfeit carbonless copy paper in practical applications.

In this article, we design and synthesize a new carbazole-based molecule, Cz2CN, with a twisted D-A structure, using the carbazole group as the donor and a dicyanoethylene fragment as the acceptor. Such a twisted D-A structure endows Cz2CN with two characteristic emission bands, LE emission and ICT emission. Cz2CN exhibits AIEE, solvatochromism, and different responses to anisotropic shearing force and isotropic hydrostatic pressure, due to the sensitivity of ICT emission to aggregation, solvent polarity and mechanical force. Aggregation benefits the ICT emission, leading to the AIEE of Cz2CN in the THF/water system. Polar solvents can help stabilize the ICT-excited states and make the ICT emission red shift. The original crystalline powder is strongly fluorescent with a high quantum yield of 40.4%. A single crystal of Cz2CN is obtained and dimers without π-π interactions among carbazole groups contribute to the strong emission. Anisotropic shearing alters the emission of Cz2CN powder from sky blue (474 nm) to green (520 nm). The single crystal of Cz2CN undergoes a distinct multicolored variation from sky blue (476 nm) to green (510 nm) and further to orange (590 nm) upon isotropic hydrostatic pressure. The emission of the original powder/crystal of Cz2CN is dominated by LE (locally excited state) emission in HLCT (intercrossed hybridized local and charge transfer excited state). To the best of our knowledge, Cz2CN is an interesting carbazole-based molecule that shows simultaneous AIEE, solvatochromism and force-induced multicolored variation.

In recent years, semiconductor-based surface-enhanced Raman scattering (SERS) substrates have received considerable attention and led to a forefront in the SERS field. However, the lack of electromagnetic (EM) enhancement contributions highly precludes the development of semiconductive-substrate-based SERS. In this study, Ga-doped ZnO nanoparticles (NPs) were fabricated and employed as novel SERS substrates based on the EM enhancement contribution of surface plasmon resonance (SPR). The obtained Ga-doped ZnO NPs exhibited obvious SPR absorptions in the visible and near- and mid-infrared regions. SPR absorption can be readily tuned by changing the doping ratios of Ga3+ ions. The SERS spectra of Ga-doped ZnO/4-mercaptopyridine (MPy) were investigated at different excitation wavelengths of 488, 532, 633, and 785 nm. The spectral enhancement of Ga-doped ZnO substrates depended on the doping ratios, excitation wavelengths, and nearby SPR absorption. Ga-doped ZnO NPs with the highest free charge carrier density and the doping ratio of 5% showed the strongest SERS spectra. For the fixed doping ratio of 5%, the better is the match between excitation wavelengths and SPR absorption, the higher is the SERS spectral enhancement. This study showed the feasibility of EM contributions to SERS by using semiconductive substrates and can contribute to the development of the semiconductor-based EM mechanism.

Single-cell metabolomics could be used to discover the chemical strategies of cells for coping with chemical or environmental stress because metabolomics provides a more immediate and dynamic picture of cell functionality. However, these small-molecule metabolites are the most difficult to measure because they characterize rapid metabolic dynamics, structural diversity, and incapacity to achieve signal amplification or labeling. In order to solve above problems, we presented a surface-enhanced Raman scattering (SERS)-microfluidic droplet platform to realize the label-free simultaneous analysis of multiplexed metabolites at the single-cell level via a versatile magnetic SERS substrate composed by silver nanoparticles (AgNPs, 30 nm)-decorated 400 nm Fe

A two-dimensional (2D) co-crystal composed of 1, 3, 5-trifluoro-2, 4, 6-triiodobenzene and 4, 4′-trimethylenedipyridine via halogen bonding (I···N) was obtained on highly-oriented pyrolytic graphite, and was characterized by scanning tunnelling microscope supplemented by the density functional theory calculation. Its special structure at the interface was further compared with its three-dimensional (3D) co-crystal structure. No orderly structures were observed when Au (111) was applied. The 2D co-crystal grown on the interface is significantly different from the 3D co-crystal. This study is of vital significance in understanding the growth mechanisms of 2D functional co-crystals.

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.

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.

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.

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 -CH3 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.

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.

Herein, we report the unique photochromic properties of aminobenzopyrano-xanthene (named cis- and trans-RhRh) in solution and the crystal state. cis- and trans-RhRh show reversible photochromism in DCM, along with the emergence of a new emission band at 550 nm and new absorption bands at ∼497 and ∼527 nm. The initial emission peaks of cis-RhRh and trans-RhRh single crystals appear at 769 nm and 755 nm, respectively, close to the infrared region, which are due to the slip-stacked dimers in the crystal. Upon irradiation with 375 nm UV-light, the emission color of the cis-RhRh single crystal changes from blue to orange-red, accompanied by a decrease in the intensity of the emission peaks at 440 nm and 769 nm. Meanwhile, two broad emission bands at 573 nm and 647 nm emerge, which are attributed to the photo-induced isomerization of RhRh in the crystal state. Interestingly, trans-RhRh is more sensitive to UV light than the cis form, both in solution and the crystal state. This is a rarely reported example of organic molecules achieving intriguing photochromism with fascinating fluorescent behaviors in solution and the crystal state.

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.

In this article, we report a newly designed TPE-rhodamine dyad bridged by a Schiff base structure, which can be synthesized facilely and achieves AIEE, a distinct response to shearing and hydrostatic pressure and intriguing sequential multicolored acidichromism, owing to the imine group and spiro-lactam structure. RhBTPE displayed the AIEE property in a THF/H2O system, due to water-induced aggregation and aggregation-suppressed nonradiative relaxation of the excited states. The single crystal of RhBTPE was obtained and the complex intermolecular interactions were analyzed. RhBTPE showed a distinct response to anisotropic shearing force and isotropic hydrostatic pressure. Under shearing force, the crystal powder underwent an emission color switch from blue (440 nm) to green (475 nm) and further to red (580 nm), which were ascribed to the LE emission, HLCT emission and ring-open species of rhodamine lactam, respectively. When subjected to hydrostatic pressure, the single crystal displayed a color change from blue to green and to dark color, because of the pressure-induced separation of HLCT and further pressure-induced emission quenching. More interestingly, RhBTPE realized a high-contrast sequential color switch from blue to yellow and further to red by fuming with TFA. The protonation of imine and rhodamine lactam played the key role in the multicolored change. To the best of our knowledge, RhBTPE is a rare dyad that achieves a sequential multicolored switch by using an external force and fuming with an acid, respectively. This study may contribute to developing a multi-stimuli responsive fluorescent switch with multicolored changes.

Two tetraphenylethene-based tetracationic dicyclophanes 1 and 2 were synthesized via a one-step SN2 reaction. Based on the central TPE unit and the slight difference of the outer linkers, 1 and 2 exhibited a classic aggregation-induced emission but contrasting mechanochromic luminescence under grinding, vaporing, or hydrostatic pressure in the solid state, and photochemical reactions with various emitting colours induced by photoirradiation in the solution state.

Complexation of single-stranded DNA (ssDNA) with a chiral single-walled carbon nanotube (SWCNT) exhibits surprising efficacy in CNT dispersion and sorting, optical sensing, and nanoelectronic device design. Studying the wrapping/unwrapping mechanism is challenging because an in situ method at the single-molecule level is required. Here, we developed a method based on single-molecule force spectroscopy to monitor the unwrapping/wrapping of ssDNA from/around a SWCNT. Our results reveal that the wrapping/unwrapping processes are reversible in water, and these processes occur in an equilibrium manner driven mainly by π-π interactions between DNA bases and CNTs. In phosphate buffered saline, the unwrapping process is loading rate-dependent, and ssDNA wrapping around a CNT undergoes two distinct stages dominated by both π-π interactions and hydrogen bonding. In addition, our results show that salts could further stabilize ssDNA/CNT complexes by blocking the electrostatic interactions between adjacent DNA segments and by catalyzing the formation of hydrogen bonds between DNA bases. The stability of ssDNA/CNT is dependent on the DNA sequence and CNT chirality. These results deepen our understanding of ssDNA-CNT interactions and provide effective means to tune the binding mode and strength.

A series of two-component co-crystals driven by IN interactions based on the bipyridine (BIPY) chromophore with one among three different co-former building blocks, iodopentafluorobenzene (IPFB), 1,4-diiodotetrafluorobenzene (DITFB) and 1,3,5-trifluoro-2,4,6-triiodobenzene (IFB), were prepared and analysed via infared spectroscopy and single-crystal X-ray diffraction. By comparing the IN distances in the co-crystal structures, we found that the higher the -F ratio in the building blocks the closer the contact of the IN bond, enhancing the intermolecular interactions in these co-crystals as well. That is, the positive electrostatic potential on the iodine atom(s) in the co-formers was enhanced by the presence of strong electron-withdrawing groups. The distinct spectroscopic behaviours (fluorescence and Raman spectra) among the two-component BIPY co-crystal systems in response to hydrostatic pressure were also investigated. Interestingly, the fluorescence of BIPY-DITFB presented intriguing abnormal evolution from dark to bright, suggesting a new charge transfer state due to the decreased intermolecular distance and the enhanced IN interactions. Theoretical simulations by Materials Studio also showed the shortened IN distance and the increased angle of C-IN, evidencing the enhanced IN interactions. In contrast, BIPY-IFB showed only slightly enhanced fluorescence intensity at 550 nm consistent with BIPY-DITFB. Once the pressure was relieved, both the Raman and fluorescence spectra for BIPY co-crystal systems entirely self-recovered. Remarkable emission enhancement in a solid-state co-crystal has been rarely reported in previous publications and in fact, this study paves a unique way for designing and developing novel stimuli-responsive photo-functional materials.

Continuous tunable photoemission of a novel co-crystal (9-acetylanthracene-tetrafluoroterephthalonitrile, 9ACA-TFP) depending on intermolecular charge transfer (CT) is achieved in situ under high pressure via a diamond anvil cell. Compared with other methods for tuning the emission of organic co-crystals based on organic synthesis or doped organic co-crystals, this pressure-based tuning method is simple and effective. The high-pressure data show that this organic co-crystal (9ACA-TFP) exhibits continuous redshifts in its emission bands (from 490 to 620 nm) with the pressure increasing, and its colour and emission bands can recover to their initial state once the pressure is completely released. Raman and ultraviolet-visible absorption spectra with pressure were also measured. All Raman vibrational bands move to higher wavenumbers and exhibit decreased intensities with pressure, proving that the shortened bond lengths and donor–acceptor–donor (D–A–D) interplanar distance result in stronger intermolecular interactions. The in situ absorption spectra of the 9ACA-TFP co-crystal show a 134 nm redshift from 0 to 7.22 GPa, indicating that high pressure enhances the intermolecular CT transition between D and A. The co-crystal materials show on/off switching of photoemission upon cycling compression and decompression. This study is of sigificance in determining the intermolecular CT interaction –photoemission property relation of organic co-crystals.

The effects of the high pressure on two single crystals, pyrene and N,N-diphenyl-4-(pyren-1-yl)aniline (TPA-Py), were studied by in situ fluorescent and Raman spectroscopies. During the compression, the pyrene with one structureless excimer emission band showed a continuous bathochromic-shift. In contrast, with the pressure increasing to 10.36 GPa, TPA-Py previously dominated with the hybridized local and charge transfer (HLCT) excited state gradually exhibited a new band at longer wavelengths, which is assigned to a new excited state species with the intramolecular charge transfer (ICT) state, caused by the pressure-induced changes on its molecular configuration. Accompanied by the spectral changes, a sequential color variation from blue to cyan was observed, giving a change to yellow and then red. The significant broadening of the full-width half-maximum (FWHM) of the TPA-Py is observed due to the enhanced dipole-dipole interaction and the existence of pressure gradient. Both pyrene and TPA-Py showed the delayed recovery of the luminescence in the compression-decompression cycle, which results from the poor reversibility of electronic structure caused by the compression-induced piezochromic effect. Furthermore, the evolutions of the Raman spectra of pyrene and TPA-Py indicated that the pressure-induced reversible transformation is caused by the molecular conformational change. This study is a deeper understanding of the structure-property relation of the HLCT species and will be a helpful reference for the regulation of photoluminescence in these intramolecular electron donor-acceptor crystal materials.

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.

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/H2O 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.

In the present study we have applied in situ fluorescence spectroscopy, in combining with two-dimensional correlation spectroscopy (2DCOS), to investigate the dynamic fluorescence changes of adenosine 5′-monophosphate capped gold nanoclusters (AuNCs@AMP) versus pressure in solid. Several weak and overlapped spectral components, developed with pressure, are differentiated and assigned in detailed by 2DCOS analysis. In addition, the sequential spectral variations are also revealed, which take place as follows. The ligands-surface Au(I) (denoting as 450, 480 nm), surface Au(I)⋯Au(I) (as 515, 520 and 565 nm), and even the inner metal core Au–Au interactions (as 605 nm), take place at pressure range from low to high. Moreover, the pressure-induced luminescence performance is also compared between AuNCs@AMP being prepared at 60 and 80 °C, respectively. It implies that it is more stable for AuNCs prepared at 60 °C as which has better pressure tolerance. Finally, it is confirmed that the orientation of AMP at the gold surface plays a crucial role in the tuning of luminance performance, especially the emission intensity to compression.

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.

We herein report a newly synthesized simple molecule, named TPE[double bond, length as m-dash]C4, with twisted D-A structure. TPE[double bond, length as m-dash]C4 showed two intrinsic emission bands ascribed to the locally excited (LE) state and the intramolecular charge transfer (ICT) state, respectively. In the crystal state, the LE emission band is usually observed. However, by applying hydrostatic pressure to the powder sample and the single crystal sample of TPE[double bond, length as m-dash]C4, dual-fluorescence (445 nm and 532 nm) was emerged under high pressure, owing to the pressure-induced emission band separation of the hybridized local and charge transfer excited state (HLCT). It is found that the emission of TPE[double bond, length as m-dash]C4 is generally determined by the ratio of the LE state to the ICT state. The ICT emission band is much more sensitive to the external pressure than the LE emission band. The HLCT state leads to a sample with different responsiveness to grinding and hydrostatic pressure. This study is of significance in the molecular design of such D-A type molecules and in the control of photoluminescence features by molecular structure. Such results are expected to pave a new way to further understand the relationship between the D-A molecular structure and stimuli-responsive properties.

The luminescence and structural changes of 4, 4′-bipyridine in the crystal and powder forms under the effect of high pressure applied by a diamond anvil cell has been investigated through the fluorescence and Raman spectroscopies. In its single crystal structure, the 4, 4′-bipyridine molecules are paralleled arranged with the identifiable C H⋯N and π⋯π interactions among molecules. However, in the powder form, these intermolecular interactions nearly diminish. The 4, 4′-bipyridine crystal shows the obvious bathochromic-shifting of the emission band, which is different from the powder sample that displays a fixed luminescent band during compression. Additionally, the Raman bands of them both show shifts to higher wavenumbers as different degrees. The detailed peak assignments are performed based on the theoretical calculation through B3LYP method. Comparisons in spectral behaviors between the crystal and powder under compression show the crystal form exhibits a superior mechanochromic performance relative to the powder one, because the intermolecular interactions in the crystal form play dominating roles and they can be easily tuned along with pressure in such a highly ordered structure compared to the powder form. The relation investigation between property and supramolecular interactions not only makes deeper understanding in the mechanochromic mechanisms of 4, 4′-bipyridine, but also gives a helpful reference for the molecular designs of coordination polymers and co-crystals with 4, 4′-bipyridine involved.

A novel single crystal, PyB, is produced in a high yield by the simple method of connecting a pyrene unit and a rhodamine B moiety together. PyB shows multiple functions of aggregation-induced emission, low-loss optical waveguiding, and tricolored mechanochromism. The crucial point for fabricating such a multifunctional single crystal is selecting the C═N group as a spacer, which simplifies the synthetic procedure, confines the molecular conformation to develop single crystals, and allows one to dynamically observe the color variation in situ and quantitatively analyze the effect of applied pressures. Such a simple approach may be extended to other fluorophores, thus providing a new opportunity for the real world application of mechanochromic materials for mechanical sensors, optical encoding, and optoelectronic devices, etc.

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.