Electronic Coupling and Spin–Orbit Charge-Transfer Intersystem Crossing in Phenothiazine–Perylene Compact Electron Donor/Acceptor Dyads

We prepared perylene (Pery)-phenothiazine (PTZ) compact donor/acceptor dyads with connection at either N- or 2-C positions of the PTZ moiety to attain molecular conformation restriction and to study the relationship between mutual chromophore orientation and spin–orbit charge-transfer intersystem crossing (SOCT-ISC) efficiency. In Pery-N-PTZ, the linkage is at the N-position of the PTZ moiety, and the molecule adopts an orthogonal geometry (φ = 91.5°), whereas in Pery-C-PTZ, the connection is at the 2-C position, resulting in a more planar geometry (φ = 60.6°). A diphenylamino derivative (Pery-DPA) was also prepared in which a N atom is fully π-conjugated with the perylene moiety. Highly solvent polarity-dependent singlet oxygen production was observed for the dyads (ΦΔ = 3–60%), which is an indication of the SOCT-ISC mechanism. The potential energy curve of the torsion about the C–N/C–C linker indicated different energy landscapes for the dyads; interestingly, we found that nonorthogonal geometry also induces efficient SOCT-ISC, which is different from previous studies. The ultrafast charge separation process (<100 fs) and the ISC rate (0.27 ps) were observed by femtosecond transient absorption spectroscopy. Time-resolved electron paramagnetic resonance spectroscopy further confirmed the SOCT-ISC mechanism. With perylenebisimide as the triplet acceptor and the dyads as the triplet photosensitizer, the triplet–triplet annihilation-induced delayed fluorescence was observed, with the luminescence lifetime up to 71 μs.