Impact of Isomerization of Diphenothiazine Derivatives on Photophysical Properties, Crystal Structure, and Force-Stimulus Response

Two D-A-D diphenothiazine derivatives, 24DPTCN and 26DPTCN, were prepared to investigate the impact of molecular isomerization on the luminescent behaviors in solution and crystalline states and the response to the external force stimuli. It was found that the two compounds had two emission bands in solutions. The quantum chemical calculations suggest that the coexistence of two configurations results in two emission bands, the short-wavelength bands are from the axial–equatorial (ax-eq) form for 24DPTCN and 26DPTCN, and the long-wavelength emission bands are ascribed to those molecules in equatorial–equatorial (eq-eq) form. In the crystalline phase, 24DPTCN adopts an ax-eq form and emits very weak blue fluorescence, and 26DPTCN has an eq-eq conformation and emits green fluorescence. More importantly, they also possess distinct response to force stimulus. 24DPTCN had a large redshift of 85 nm in the emission band under mild force stimulus, accompanied by an enhancement in the emission intensity because of the configuration conversion from ax-eq to eq-eq. On the other hand, only a shift of 28 nm was observed, and the fluorescence weakened after 26DPTCN crystals were ground because a small number of eq-eq 26DPTCN transferred into ax-eq ones.