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.