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A pair of 1,8-naphthalimides (NPIs) were designed and successfully synthesized through embellishing amino-containing NPI with 4-diethylaminosalicyladehyde and 4-diethylaminobenzaldehyde, respectively. Their structures were fully confirmed by 1H/13C NMR, HR-MS and FT-IR spectroscopic studies. Their photophysical properties were systematically investigated in different solvents of varied polarity, in THF/water mixtures with varying water fractions (fw), and in THF solvent with varying concentrations of NPIs. It inferred that the distinct differences in emission between two NPIs during self-assembled process could be ascribed that the hydroxyl-containing NPI allowed the excited-state intramolecular proton transfer process between -OH and CH=N units in the aggregation state. Interestingly, the solid of 4-diethylaminosalicyladehyde-functionalized NPI exhibited multi-stimuli-responsive fluorescence changes involving mechanofluorochromism and HCl/NH3 vapor stimulus-induced conversion. However, no remarkable change was observed in the photoluminescence (PL) spectra for the solid of 4-diethylaminobenzaldehyde-functionalized NPI under the stimuli of mechanical force and organic solvent.

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A new deep blue emissive organic fluorophore (N-cyclohexyl-N-(cyclohexylcarbamoyl)-4-(diphenylamino)benzamide (NCDPB)) was designed and synthesized, which showed strong fluorescence both in solution and solid-state. Solid-state structural analysis of NCDPB revealed non-planar twisted molecular conformation with extended hydrogen bonding between the amide functionalities. The propeller shaped triphenylamine (TPA) and non-planar cyclohexyl unit prevented close π…π stacking and produced strong deep blue emission in the solid state (λmax = 400 nm, quantum yield (Φf) = 12.6 %). NCDPB also exhibited strong solvent polarity dependent tunable emission in solution (λmax = 402-462 nm, Φf = 1.15 (compared to quinine sulphate)). NCDPB showed reversible fluorescence switching between two fluorescence states upon mechanical crushing and heating/solvent exposure. Mechanical crushing caused red shifting of fluorescence from 400 to 447 nm and heating/solvent exposure reversed the fluorescence. Further, NCDPB also displayed off-on reversible/self-reversible fluorescence switching upon exposure to trifluoracetic acid (TFA) and NH3. The repeated fluorescence switching cycles indicated high reversibility without any significant change of fluorescence intensity. The drastically different fluorescence of NCDPB in CH3OH and EtOH was utilized to distinguish them and monitor CH3OH contamination in ethanol and benzene. It showed limit of detection (LOD) of methanol up to 0.25 % and 7 % in benzene and ethanol, respectively. The water sensitive fluorescence modulation of NCDPB in organic solvents was used to sensing water contamination in common organic solvents. Thus, integration of twisted TPA with H-bonding urea produced dual state emitting organic fluorophore with multi-responsive fluorescence switching and solvent sensing.

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The excimer emission based on discrete π-stacked dimers of polycyclic π-systems has generated significant interest in the structure-luminescence relationship of excimers owing to their ultra-large Stokes shift. Herein, a series of excimer emissive luminogens were obtained by conjugating different polycyclic aromatic aldehydes (anthraldehyde, pyrenealdehyde and perylenealdehyde) with triphenylmethylamine. In crystalline states, all the molecules were arranged in the form of π-stacked arene dimers which were spatially isolated from each other by the bulky triphenylmethyl groups, and thus emitted bright excimer emission. The anthracene and pyrene derivatives showed fluorescence enhancement responses to grinding and the enhanced fluorescence could recover to the original state upon heating. The aggregation-induced emission (AIE) properties of them were dependent on the shapes and sizes of the polycyclic aromatic groups. The pyrene derivative showed the most excellent excimer-based AIE behavior among them. All of them were more apt to exhibit the excimer emission when formed nanoparticles with pluronic F-127 than that without pluronic F-127. Furthermore, PETP was utilized for bioimaging of living Hela cells and the high-resolution image was observed.

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Dual-state emission (DSE) is an emerging phenomenon wherein organic luminescent molecules display bright emissions in both molecularly isolated and packed states, addressing the challenge associated with the traditional paradigm of dyes with mono-state emission. This study presents the design and synthesis of two unsymmetrical triads, TPCA and TPCT, featuring a D-π-A-D' electronic structure by integrating phenothiazines, triphenylamines, and cyanostilbene. Photophysical assessments reveal that both molecules serve as robust DSEgens, exhibiting strong emissions in both solution and solid phases. TPCA displays ΦTHF 53.2% and Φsolids 43.2%, while TPCT exhibits ΦTHF 49.6% and Φsolids 37.5%. However, due to differences in molecular conformation and packing, they diverge in solid-state emission wavelengths and mechanofluorochromic behavior. In the solid state, TPCA emits strong red fluorescence, contrasting with TPCT, which emits orange fluorescence. Furthermore, TPCA demonstrates significant mechanofluorochromism (MFC), shifting from yellow to yellow-red upon mechanical grinding, while TPCT exhibits negligible MFC owing to conformational distinctions. As robust and low-toxic bioimaging agents, both TPCA and TPCT prove highly effective for lipid-droplet imaging studies. This research contributes valuable insights to the evolving field of DSE materials, elucidating the promising applications and mechanisms governing their versatile emission behaviors.

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Although remarkable progresses are achieved in the design and development of the mono-shift in photoluminescence for mechanofluorochromic materials, it is still a severe challenge to explore the opposite mechanofluorochromic materials with both blue- and red-shifted photoluminescence. Herein, two unprecedented 4,5-bis(TPE)-1H-imidazole fused pyridine or quinoline-based fluorophores X-1 and X-2 were designed and synthesized, and X-1 and X-2, exhibit completely opposite mechanofluorochromic behavior. Under UV lamp, the color of pristine X-1 changed from blue to green with reversible redshifted 27 nm in fluorescence emission spectra after ground, while the color of pristine X-2 changed from red to yellow with reversible blue-shifted 74 nm after ground. The detailed characterizations (including PXRD, SEM and DSC) confirmed that this opposite mechanofluorochromism was attributed to the transformation of order-crystalline and amorphous states. The crystal structure analysis and theoretical calculation further explain that opposite mechanofluorochromic behavior take into account different π-π stacking mode by induced π-extended systems. In addition, these TPE-based fluorophores (X-1 and X-2) exhibited excellent bio-compatibility and fluorescence properties for bio-imaging, writable data storage and anti-counterfeiting materials.

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Dual-state emission luminogens (DSEgens) are receiving research interest in the construction of multifunctional materials due to their inherent advantage of high emission efficiency in both the molecularly dispersed solution state and the solid state. However, it remains challenging in synthesizing DSEgens via a delicate manipulation of the molecular structures. This work presents an example of bright DSEgen synthesis by tuning the molecular electronic structures and conformations. Three coumarin-tetraphenylethylene (TPE) molecules with a donor-acceptor electronic structure and highly twisting conformations have been synthesized. While compound resulting from direct conjugation of coumarin with a TPE unit shows aggregation-induced emission, compound with an N,N-diaminoethyl modification on the 7-position of coumarin and compound with a further phenyl linker between coumarin and TPE units feature strong dual-state emission. Benefiting from their strong solid emission and twisting conformations, these fluorophores display reversible mechanofluorochromism. Finally, applications for rewritable information storage in the solid state and live-cell imaging in the solution state were demonstrated.

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The development of circularly polarized thermally activated delayed fluorescence (CP-TADF) luminogens with stimuli-response characteristics remains challenging. Herein, a pair of organic enantiomers, S-CzTA and R-CzTA, with aggregation-induced emission properties, have been successfully developed by introducing chiral 1,2,3,4-tetrahydronaphthalene and carbazole to phthalimide. They present CP-TADF properties in toluene solutions, giving dissymmetric factors of 0.84×10-3 and -1.03×10-3 , respectively. In the crystalline state, both S-CzTA and R-CzTA can emit intense blue TADF and produce very bright sky-blue mechanoluminescence (ML) and remarkable mechanofluorochromism (MFC) under the stimuli of mechanical force. Single-crystal analysis and theoretical calculation results suggest that their ML activities are probably associated with their chiral and polar molecular structures and unique non-centrosymmetric molecular packing modes. Furthermore, the MFC properties of the enantiomers likely originate from the destruction of crystal structure, leading to the planarization of molecular conformation. This work may provide helpful guidance for developing new CP-TADF materials with force-stimuli-responsive properties.

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The preparation of multifunctionalized luminophores with full-color emission based on an identical core skeleton is a significative but challenging research topic. In this work, eight donor-donor-acceptor (D-D-A)-type luminogens based on a central carbazole core bearing a C6 hydrocarbon chain were designed by using different kinds of donor and acceptor units on the left and right, and synthesized in good yields. These D-D-A carbazole derivatives display deep-blue, sky-blue, cyan, green, yellow-green, yellow, orange and red fluorescence in the solid state, achieving full-color emission covering the whole visible light range under UV light illumination. Notably, the dicyano-functionalized triphenylamine-containing carbazole derivative exhibits rare aggregation-induced near-infrared emission and red-to-near-infrared mechano-fluorochromism with high contrast beyond 100 nm. Furthermore, the red-emissive luminogen can serve as a potential candidate for cell imaging and photodynamic therapy (PDT). This work not only provides reference for the construction of full-color emissive systems but also opens a new avenue to the preparation of multifunctionalized luminophores capable of simultaneous application in near-Infrared mechanical-force sensors and PDT fields.

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Tetraphenylethylene (TPE) derivatives bearing a xanthene moiety are of interest because they have novel optical properties. 9,9-Bis[4-(N,N-diphenylamino)phenyl] and 9,9-bis[4-(9-carbazolyl)-phenyl]methylidene-xanthylidenes 3 and 4 were synthesized using Suzuki-Miyaura coupling of 9,9-dibromomethylidene-xanthylidene with the corresponding boronic acids. Diphenylamino derivative 3 exhibits mechanochromism and mechanofluorochromism (MC and MFC) reflected in absorption and fluorescence color changes. In contrast, carbazolyl derivative 4 displays thermo- and crystallo-chromism in addition to MC and MFC in the solid state. Powder X-ray diffraction and single crystal X-ray crystallographic analysis reveal that the solid state photophysical properties of these substances are governed by conformational changes rather by the creation of planar π-conjugation extended geometries.

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Water as a reaction media in chemical transformations has several advantages in terms of safety and non-toxicity. However, dehydrative substitution reaction in water is a highly challenging operation. In this paper, we have reported a sulfamic acid-mediated dehydrative substitution reaction of benzofuryl alcohols with several nucleophiles in water towards the scalable, easily isolable, synthesis of unsymmetrically substituted triarylmethanes (TRAMs) in good to excellent yields (up to 92 %). Moreover, we have demonstrated that rationally designed propellor-shaped triarylmethanes promote aggregation-induced emission (AIE) through restricted intramolecular rotation with irreversible mechanoflurochromic property.

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In recent years, organic mechanofluorochromism (MFC) materials have attracted wide attention in many fields. However, the exploration of MFC materials with high-contrast, high-sensitivity and high-responsiveness remains a challenge. Herein, a series of MFC materials with 2-biarylyl cinchoninic acid skeleton were successfully established, which are based on interconversion of classical/frustrated Brønsted pairs. These compounds have the mechanochromic shift of up to 115 nm, as well as the property of stunning sensitivity and multiple responses to external mechanical force stimuli. The luminescence properties can be easily tuned by changing the substituents.

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Fluoride ions are indispensable in biology and environmental science and hence the selective and sensitive detection of fluoride is important. This work reports the design and synthesis of a tripodal Schiff's base 1 through a simple condensation reaction between a commercially available aldehyde and an amine. Single crystal X-ray crystallography revealed that compound 1 is a planar entity with the three salicylidene derivatives on the three arms of the central phenyl moiety linked by imine groups. Compound 1 forms a molecular dimer that resembles a six-petal flower and is stabilized through multiple intermolecular interactions such as C-H.π and π.π interactions. Compound 1 exhibited moderately good emission in the solid state with aggregation induced emission and reversible mechanofluorochromic properties. Moreover, 1 was observed to selectively detect fluoride among various anions with a limit of detection of ∼9 ppm. Compound 1 was also capable of detecting fluoride under a variety of conditions such as in thin films and under cellular conditions.

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Mechanochromic luminophors with strong solid-state emission are promising candidates for high-contrast mechanochromic luminescence materials. Meanwhile, mechanically responsive luminogenic molecules with tricolor switching are highly desirable but are seldom reported. In this work, three anthracene-based donor-acceptor-π-donor (D-A-π-D) type benzothiadiazole derivatives were designed and synthesized. These luminogens showed remarkable aggregation-induced emission (AIE) or aggregation-induced emission enhancement (AIEE) effect. Furthermore, these luminogens exhibited bright and different solid state fluorescence involving yellow-green, yellow and orange colors, and the fluorescence of their solids could be effectively regulated by mechanical grinding. For luminogen 1, its solid displayed reversible two-color mechanofluorochromic property. As for luminogens 2 and 3, their solids displayed fluorescent colors change from yellow to yellow-green upon slight grinding, and the yellow-green light-emitting solids were converted into orange fluorescent solids after heavy grinding, demonstrating interesting three-color mechanofluorochromism features.

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A novel D-A-D chromophore TBQM based on triphenylamine, acylhydrazone and dimethylamino was synthesized, and the emission properties of TBQM was studied in solutions as well as in aggregated state. TBQM showed obvious solvatochromism in different solutions. In addition, the reversible multicolor fluorescence switching properties under the stimulation of external forces and acid were also obtained. The fluorescence of TBQM changed from blue to bright blue, and finally to blue-green as the degree of grinding increased, which would recover after being fumigated by CH2Cl2. Meanwhile, TBQM could undergo twice protonation after fumigated by TFA. The fluorescence color changed from blue to bright blue, blue-green, and yellow-green until the fluorescence was quenched with the increasing of TFA fumigation time, which can be restored automatically or by TEA fumigation. Moreover, TBQM could be used as an advanced fluorescent anti-counterfeiting ink.

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A comparative study of the luminescence properties and crystal structure of boron difluoride 1-(anthracen-9-yl)butane-1,3-dionate (1) and their nitrogen-containing analogs with hydrogen and methyl substituents (2 and 3) has been performed. For boron difluoride beta-diketonate (1) and beta-ketoiminate with a hydrogen substituent (2), which does not create significant steric difficulties, the luminescence of crystals is determined by that of excimers and aggregates based on them. For the compounds 1-3, mechanofluorochromism has been observed, the spectral manifestation of which is different: at grinding the crystals of 1 and 2, a hypsochromic shift of the excitation and luminescence bands occurs, whereas in the case of crystals of 3, a bathochromic shift takes place. Dual luminescence has been detected for the solutions of 1. During the relaxation in the excited state of a molecule of 1 (S1 â†’ S'1), TICT is formed. It was possible to register the short-wavelength luminescence from the LE state in the dilute solutions of 1. TICT or LE luminescence can be excited, in the case of 1, by varying the wavelength of the exciting light.

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The use of organic nanomaterials in biomedical and optical devices has been widely studied. The key to improving the performance and stability of these devices is to control the fabrication process, which determines the phase stability and photophysical properties. In this study, fluorescence changes were observed during the reprecipitation process of mechanofluorochromic molecules of dibenzoyl(methanato)boron difluoride. The cyan-emission phase (C-phase) was first identified. The time evolution of the resolved fluorescence spectra revealed that the green-emission phase (G-phase) was formed from the amorphous phase with yellow emission via the C-phase, in addition to the direct formation of the G-phase. Combined with the results of the investigation into the thermal properties, the fluorescence changes clearly indicate a two-step nucleation process and Ostwald's rule of stages for polymorph transition, which enables us to not only provide guidance for controlling the fabrication process but also propose the ripening process for organic nanoparticle formation.

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A series of acetyl pyrenes and pyrenyl ynones with and without tert-butyl groups showed distinct mechanofluorochromism (MFC). Four pairs of polymorphic solids were found out of six compounds and interestingly, each of them showed hypsochromic, bathochromic or off-to-on MFC. The MFC properties were rationalized by categorizing the packing schemes into herringbone, sandwich, beta and gamma motifs depending on the relative contributions of C⋅⋅⋅C (or π-π) against C⋅⋅⋅H contacts. The bulky tert-butyl and trimethylsilyl groups served not only to reduce the number of aggregation patterns but also to prohibit the complete back reactions in solid state. Our results suggest that the simple pyrene derivatives may be promising candidates for a novel group of mechanically-sensitive materials.

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A novel anthracene-based hydrazide derivative (3,4-ENS) was designed and synthesized, and 3,4-ENS can form stable organogel in dimethyl sulfoxide (DMSO) solvent. Firstly, 3,4-ENS xerogel from DMSO exhibits mechanofluorochromic property and its maximum emission shifts from 433 nm to 484 nm upon grinding. Secondly, DMSO xerogel exhibits thermofluorochromic behavior and its maximum emission shifts from 433 nm to 481 nm upon heating at 110 °C. Thirdly, DMSO xerogel exhibits acidofluorochromic behavior and its maximum emission shifts from 433 nm to 466 nm upon fuming by TFA vapors. The experimental results confirmed that the different luminescent property of 3,4-ENS attributed to the phase transition from well-ordered crystals to metastable disordered aggregation.

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In this work, we synthesized a pair of positional isomers by attaching a small electron-donating pyrrolidinyl group at ortho- and para-positions of a conjugated core. These isomers exhibited totally different fluorescent properties. PDB2 exhibited obvious aggregation-induced emission properties. In contrast, PDB4 showed the traditional aggregation-caused quenching effect. Their different fluorescent properties were investigated by absorption spectroscopy, fluorescence spectroscopy, density functional theory calculations and single-crystal structural analysis. These results indicated that the substituent position of the pyrrolidinyl groups affects the twisted degree of the isomers, which further induces different molecular packing modes, thus resulting in different fluorescent properties of these two isomers. This molecular design concept provided a new accurate strategy for designing new aggregation-induced emission luminogens.

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For a specific fluorescent molecule, the increase of molecular conformation distortion is beneficial to endow it with aggregation-induced emission (AIE) and mechanofluorochromic (MFC) properties. Herein, 3,5-diphenyl-4H-pyran derivative 5 and 4,5-diphenyl-2,7-naphthidine derivative 7 with highly twisted conformations were synthesized. For compound 5, although the introduction of phenyl rings with large steric hindrance at 3 and 5 positions of the 4H-pyran skeleton realized the transformation from aggregation-induced quenching (ACQ)-active molecule to AIE-active molecule, it only showed a low-contrast MFC activity. Compound 7 was accidentally obtained from compound 5 and n-butylamine via a ring-opening and subsequent intramolecular ring-closing mechanism. Compound 7 was confirmed to have a highly twisted molecular conformation by the crystal structural analysis and exhibited AIE activity originated from the restriction of intramolecular rotation. Furthermore, compound 7 exhibited reversible high-contrast MFC activity. Upon grinding, the change of solid-state fluorescence color from orange to yellow was confirmed to be due to the partial destruction of crystal structure. This work provides new ideas for the design and synthesis of novel AIE-active and MFC-active fluorescent molecules based on ACQ-active parent molecules.