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*.