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 (Φ Δ = 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.

Recently varieties of Bodipy derivatives showing intersystem crossing (ISC) have been reported as triplet photosensitizers, and the application of these compounds in photocatalysis, photodynamic therapy (PDT), and photon upconversion are promising. In this review we summarized the recent development in the area of Bodipy-derived triplet photosensitizers and discussed the molecular structural factors that enhance the ISC ability. The compounds are introduced based on their ISC mechanisms, which include the heavy atom effect, exciton coupling, charge recombination (CR)-induced ISC, using a spin converter and radical enhanced ISC. Some transition metal complexes containing Bodipy chromophores are also discussed. The applications of these new triplet photosensitizers in photodynamic therapy, photocatalysis, and photon upconversion are briefly commented on. We believe the study of new triplet photosensitizers and the application of these novel materials in the abovementioned areas will be blooming.

Efficient triplet state production (singlet oxygen quantum yield: 80%) by SOCT-ISC was observed for a perylene–naphthalimide compact electron donor/acceptor dyad.