Red Thermally Activated Delayed Fluorescence and the Intersystem Crossing Mechanisms in Compact Naphthalimide–Phenothiazine Electron Donor/Acceptor Dyads

Controlling the electronic coupling between electron donor and acceptor subunits in a dyad is pivotal for the development of novel organic materials, for instance, thermally activated delayed fluorescence (TADF) materials and triplet photosensitizers. Herein, we prepared two compact electron donor/acceptor dyads based on phenothiazine (PTZ) and naphthalimide (NI) with different conformation restrictions induced by the C–N (NI-N-PTZ) or C–C (NI-C-PTZ) linkers. The effect of electronic coupling (matrix elements, VDA) on the photophysical properties, especially the intersystem crossing (ISC) and the TADF, were investigated. NI-C-PTZ shows stronger ground-state electronic coupling (VDA = 2548 cm–1) compared to NI-N-PTZ (VDA = 870 cm–1). TADF was observed only for NI-N-PTZ due to its smaller electronic coupling. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy indicated the presence two triplet excited states and three ISC mechanisms in NI-N-PTZ with different electron spin polarizations (ESP): radical pair ISC (RP-ISC) and spin–orbital charge transfer ISC (SOCT-ISC) for one triplet state, and spin–orbital coupling ISC (SO-ISC) for another. Moreover, for the second one, an inversion of the electron spin polarization (ESP) was observed at 0.5–1.1 μs delay time. NI-N-PTZ represents a rare example for compact electron donor/acceptor dyad showing TADF emission in the red spectral region.