Mechanical force-induced luminescence enhancement and chromism of a nonplanar D–A phenothiazine derivative

Nonplanar D–A phenothiazine derivative (PBA), in which phenothiazine and formylphenyl groups act as donor and acceptor, respectively, was designed and synthesized. The PBA emitted weak fluorescence in a solution, and its crystals exhibited weak blue-green emission with a luminescence efficiency of 7.5% under UV light irradiation owing to the lowest singlet (S1) of n–π* and the presence of intersystem crossing from S1 to T1. A grinding crystal provided a powder with strong yellow-green photoluminescence, whose luminescence efficiency increased by a factor of more than 6 after a mechanical force produced an enhanced TADF and an S1 of π–π*.