Twisted donor–acceptor cruciform fluorophores exhibiting strong solid emission, efficient aggregation-induced emission and high contrast mechanofluorochromism

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.