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Two twisted π-conjugated cruciform luminophores MDCS-TPA and FMDCS-TPA containing two conjugated donor and acceptor groups intersecting at the central benzene core have been rationally designed and successfully prepared. The two compounds display unique intramolecular charge-transfer (ICT) process from electron-rich triphenylamine and electron-poor cyanostyryl units, which are evidenced by theoretical calculations and spectral analysis. MDCS-TPA and FMDCS-TPA exhibit AIE effects with high fluorescence quantum yields of 0.321 and 0.312 in the solid state. Interestingly, MDCS-TPA and FMDCS-TPA possess the distinct mechanofluorochromic (MFC) behavior. The as-prepared powders of the two luminophores could emit strong green (491 nm) and yellow-green fluorescence (529 nm), and the fluorescence color changed into yellow (542 nm) and orange (576 nm) after grinding, respectively, giving the red shifts of 51 nm and 47 nm. Such mechanochromism is reversible by repeating both the grinding-fuming and grinding-annealing processes. The MFC behavior of MDCS-TPA and FMDCS-TPA originates from the transition between the crystalline and amorphous states and the red-shifts induced by grinding is the result of the planarization of the molecular conformation and subsequent planar intramolecular charge transfer (PICT) under the external force stimuli. Image 1 1 Two twisted π-conjugated cruciform luminophores displayed a unique ICT process. 2 They exhibited AIE effects with high fluorescence quantum yields in the solid state. 3 They showed high contrast mechanofluorochromic behavior with good reversibility. [ABSTRACT FROM AUTHOR]

Two twisted donor–acceptor cruciform luminophores ( DHCS-TPE and DMCS-TPE ) that were composed of two π-conjugated segments connected via a central benzene core have been prepared. Such cruciforms showed unique intramolecular charge transfer (ICT) emission, obvious aggregation-induced emission (AIE) properties (α AIE  = 19 and 20, respectively) and high solid state efficiency (0.913 and 0.424, respectively). Especially, the two compounds exhibited substituent-dependent mechanofluorochromic (MFC) behavior. The as-prepared powders of the more twisted DMCS-TPE could emit strong blue-green light centered at 474 nm, and the fluorescence color changed into yellowish green (531 nm) after grinding, a red shift of 57 nm was observed. Such mechanochromism was reversible upon the treatment of grinding and fuming with DCM. The high contrast MFC behavior of DMCS-TPE originated from the planarization of the molecular conformation and subsequent planar intramolecular charge transfer (PICT) under external force. By sharp contrast, the less twisted DHCS-TPE exhibited no MFC behavior. The reason is that the less twisted molecular conformation of DHCS-TPE generates strong intermolecular forces and close packing. Thus compared to DMCS-TPE , the crystalline solid powders of DHCS-TPE possess higher lattice energy and better structural stability. The grinding of the crystalline solid powders of DHCS-TPE cannot lead to amorphization and mechanochromic luminescence. [ABSTRACT FROM AUTHOR]

Two new benzo[ d ]oxazole- and benzo[ d ]thiazole-fused β-ketoiminate boron complexes ( BF 2 -TBO and BF 2 -TBT ) modified by tetraphenylbutadiene have been designed and successfully prepared, and their intramolecular charge-transfer (ICT), aggregation-induced emission (AIE) and mechanofluorochromic (MFC) properties were investigated. The results showed that the two D-π-A type compounds exhibited typical ICT emission, evident AIE characteristics (α AIE = 89 and 33, respectively), and high state emission intensity (up to 0.445 and 0.367, respectively). More importantly, BF 2 -TBO and BF 2 -TBT showed the reversible MFC behavior with their emitting colors changing from yellowish green (located at 528 and 530 nm) to yellow and orange (located at 552 and 572 nm) upon grinding, respectively, with red-shifts of 24 and 42 nm were obtained. The X-ray diffraction analyses confirmed that the MFC properties of BF 2 -TBO and BF 2 -TBT should be derived from a phase transition between crystalline and amorphous states. It was noticed that benzo[ d ]thiazole-fused BF 2 -TBT showed more contrasting MFC behavior relative to benzo[ d ]oxazole-fused BF 2 -TBO . The reason is that the existence of sulfur atom in BF 2 -TBT could lead to a larger degree of ICT in the excited state than that of BF 2 -TBO , which endowed it with greater planar intramolecular charge transfer (PICT) after grinding, resulting in a larger red-shift in the PL spectrum. [ABSTRACT FROM AUTHOR]

Two tetraphenylethene functionalized β-diketonate boron complexes C2TPE-CAR and C16TPE-CAR that were bridged by carbazole bearing alkyl chains of different lengths, have been designed and successfully synthesized. The two donor–acceptor luminogens showed typical twisted intramolecular charge-transfer (TICT) emission. In particular, they exhibited alkyl-dependent aggregation-induced emission (AIE), self-assembly and mechanofluorochromic (MFC) properties. Results showed that C16TPE-CAR with a long hexadecyl chain formed gels in certain solvents and also exhibited gelation-induced emission enhancement. H-aggregates were formed driven by the synergistic effect of π-stacking interaction and van der Waals force in the gel phase. However, C2TPE-CAR , which possessed the short ethyl group, exhibited no self-assembly behavior. Simple mechanical force could change the emission color of C2TPE-CAR and C16TPE-CAR powders from initial bright yellow (571 and 558 nm) to final red (617 and 610 nm), thus high contrast mechanofluorochromism with large spectral shifts of up to 46 and 52 nm, respectively, was observed. The powder XRD studies showed that the MFC behavior might be switched reversibly under the stimulus of external force because of crystalline-amorphous phase transformation. [ABSTRACT FROM AUTHOR]

Two twisted π-conjugated cruciform luminophores DMTCS-CC6 and DBTCS-CC6 characterized by rigid X-shaped geometry with two conjugated donor and acceptor intersecting at the central aromatic core have been rationally designed and successfully synthesized. Spectral analysis and theoretical calculations studies indicated that the two cross-conjugated compounds showed unique intramolecular charge-transfer (ICT) process from the donor (carbazole) to the acceptor (dicyanodistyrylbenzene) axis in the excited state. Especially, DMTCS-CC6 and DBTCS-CC6 exhibited efficient aggregation-induced emission (AIE) and high contrast mechanofluorochromic (MFC) properties. By simple grinding, the as-prepared powders of DMTCS-CC6 and DBTCS-CC6 emitting strong yellowish-green (540 nm) and yellow light (560 nm) under UV irradiation could be transferred into powders emitting yellow (582 nm) and red (609 nm), respectively, the red shifts of 42 and 49 nm were observed. Such mechanochromism was reversible upon the treatment of grinding and fuming with DCM. The XRD patterns suggested that the MFC behavior originated from the transition between the crystalline and amorphous states. More interestingly, DBTCS-CC6 exhibited more obvious AIE and more contrast MFC behavior relative to DMTCS-CC6 . This is a result of the quite larger space hindrance induced by 3,5-bis(trifluoromethyl)phenyl with two trifluoromethyl groups bonded at meta positions could make DMTCS-CC6 exhibit more twisted spatial conformation, leading more efficient solid state fluorescence and better structural transformability. [ABSTRACT FROM AUTHOR]

Two D-A type cruciform luminophores FB-CTPAEB and DFB-CTPAEB were designed and prepared. The two compounds exhibited unique intramolecular charge-transfer (ICT) and solid-state fluorescence behavior. The results show that both compounds exhibit AIEE behavior and high solid-state luminescence efficiency (up to 0.693 and 0.442, respectively). Moreover, FB-CTPAEB and DFB-CTPAEB show remarkable mechanofluorochromic (MFC) properties. Once the as-prepared samples were subjected to grinding, the emission of the two cruciform luminophores was red shifted by 52 nm and 46 nm, correspondingly, their fluorescence colors changed from yellow and reddish orange to red, respectively. The fluorescence emission changes induced by grinding can be restored by solvent-fuming and regenerated by grinding. PXRD analysis revealed that the MFC behavior of FB-CTPAEB and DFB-CTPAEB came from the transformation between crystalline and amorphous states upon external stimuli. Meanwhile, the extension in molecular conjugation caused by planarization of molecular conformation and subsequent planar intramolecular charge transfer (PICT) process were responsible for the red-shifts in the PL spectra. Image 1 • Two twisted donor–acceptor π-conjugated cruciform luminophores showing unique ICT properties were prepared. • The two cruciform luminophores displayed aggregation-induced emission characteristics and high solid state efficiency. • They possessed high contrast mechanofluorochromic behavior with good reversibility. [ABSTRACT FROM AUTHOR]

Two D-A type cruciform luminophores MODCS-TPE and FMDCS-TPE were successfully prepared via a two-step route involving Suzuki cross-coupling and Knoevenagel condensation reactions, and their intramolecular charge-transfer (ICT) and solid-state fluorescence were studied in detail. The results show that MODCS-TPE and FMDCS-TPE exhibit AIE behavior with high solid state fluorescence efficiency of 0.321 and 0.312. Interestingly, MODCS-TPE and FMDCS-TPE exhibit both typical AIE behavior with the AIE factors of 138 and 51and high contrast MFC effects. Once grinding had been performed, the emission of the two cruciform luminophores was red shifted by 34 nm and 44 nm from the blue regions (λ max = 471 nm and 478 nm) to the green and yellowish green regions (λ max = 505 nm and 522 nm), respectively. Moreover, the mechanochromic switching behavior was reversible and repeated many times without any fatigue in the grinding-heating (for MODCS-TPE) or grinding-fuming (for MODCS-TPE and FMDCS-TPE) fluorochromic process. The mechanism of MFC behavior that the transformation between crystalline and amorphous states upon external stimuli is revealed by PXRD and DSC analysis, and the large red-shifts induced by grinding originate from extended conjugation and subsequent planar intramolecular charge transfer (PICT) after grinding. Image 1 • Two twisted π-conjugated cruciform luminophores MODCS-TPE and FMDCS-TPE showing unique ICT properties were synthesized. • MODCS-TPE and FMDCS-TPE exhibited AIE characteristics and high solid-state luminescence efficiency. • MODCS-TPE and FMDCS-TPE showed high contrast reversibable mechanofluorochromic properties. [ABSTRACT FROM AUTHOR]