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 (phi) = 91.5 degrees), whereas in Pery-C-PTZ, the connection is at the 2C position, resulting in a more planar geometry (phi = 60.6 degrees). A diphenylamino derivative (Pery-DPA) was also prepared in which a N atom is fully pi-conjugated with the perylene moiety. Highly solvent polarity-dependent singlet oxygen production was observed for the dyads (Phi(Delta) = 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 mu s.