Influence of Molecular Conformation of a Phenothiazine Derivative on Photophysical Properties and Response to Force Stimuli

A phenothiazine derivative containing cyano and ester groups (PCNM) was designed to research the impact of the molecular configuration on photophysical properties and the response to force stimuli in the solid state. In the solution state, PCNM adopted a quasi-equatorial (q-eq) conformation, showing a long emission wavelength. Nevertheless, they exhibited polymorphism and crystallized into two types of crystals with distinct luminescent properties in different solvents. Single-crystal structural analysis revealed that PCNM adopted a quasi-axial (q-ax) form in the colorless crystal and emitted blue fluorescence, while it had a q-eq conformation in the yellowish crystal and emitted blue-green fluorescence. Additionally, both crystals displayed reversible mechanofluorochromic behavior but with different spectral shifts. The colorless crystal showed a luminescent spectral shift of 104 nm under force stimuli, and the yellow crystal showed a small shift of 37 nm under the ground state. Spectral analysis indicated that the conformation conversion from q-ax to q-eq was likely responsible for the larger spectral shift of the colorless crystal under force stimuli.