Phenothiazine modified triphenylacrylonitrile derivates: AIE and mechanochromism tuned by molecular conformation

Abstract

D–π–A type phenothiazine modified triphenylacrylonitrile derivatives PVTPAN, P3TPAN and P10TPAN have been synthesized. The three compounds exhibited AIE behaviors, and the emission enhancement of PVTPAN and P3TPAN, in which 3-position in phenothiazine was functionalized, was more significant than P10TPAN (10-position of phenothiazine was modified) with more twisted conformation. The single crystal structures revealed that the multiple intermolecular interactions, including π–π interactions and hydrogen bonds of C–H⋯π and C–H⋯N, would lock the molecular conformation and reduce the energy loss via non-radiative channels, leading to enhanced emission. Interestingly, the as-synthesized crystals of PVTPAN, P3TPAN and P10TPAN emitting strong yellow, yellowish-orange and yellowish-green light under UV irradiation could be transferred into powders emitting red, orange-red and orange light, respectively, after grinding. Such mechanochromism was reversible upon the treatment of grinding and heating/fuming with DCM. The XRD patterns suggested that the mechanochromism originated from the transition between the crystalline and amorphous states. It should be noted that the recovery time of the emission of the ground powders to that of crystals for P10TPAN was much shorter than those for PVTPAN and P3TPAN at the same temperature. Extremely, the emission of the ground powders of P10TPAN could recover at room temperature, but that of PVTPAN and P3TPAN could not. We suggested that such different mechanochromic behaviors of P10TPAN from PVTPAN and P3TPAN were coming from its more twisted conformation, which could lead to the exothermic peak appearing at lower temperatures and the easier molecular rearrangement of the amorphous state into crystals. Therefore, we could prepare the dyes with stable or reversible mechanochromism by tuning the molecular conformation.