Conformationally controlled thermally activated delayed fluorescence in a nitro-substituted phenothiazine: a combined spectroscopic and kinetic study.

Thermally activated delayed fluorescence (TADF) can convert triplet excitations into emissive singlet states through reverse intersystem crossing (rISC), but nitro groups usually promote nonradiative decay. Here, the photophysics of 4'-nitro-10-phenylphenothiazine (NPh-PTZ) was investigated by steady-state and time-resolved spectroscopy, including nanosecond transient absorption and temperature-dependent photoluminescence, together with (TD)DFT calculations. In cyclohexane, NPh-PTZ shows a prompt and an oxygen-sensitive delayed emission component with overlapping spectra, consistent with TADF; its measured fluorescence quantum yield is 0.015. Calculations suggest distinct roles for the axial and equatorial conformations: the former has stronger calculated spin-orbit coupling, whereas the latter has a smaller calculated singlet-triplet gap. Delayed emission is suppressed in the more polar solvent 1,4-dioxane. These results establish the photophysical behavior of NPh-PTZ and motivate comparative studies of structurally related nitro-substituted phenothiazines.