Thermodynamic equilibrium between locally excited and charge transfer states in perylene–phenothiazine dyads

We report the excited-state dynamics of π-orthogonal donor-acceptor dyads based on perylene (Pe) and phenothiazine (PTZ), in which triphenylamine (TPA) units and a phenyl spacer were introduced to modulate donor strength and spatial separation. Among the series, Pe-PTZ(TPA)₂ exhibits a distinct thermal equilibrium between the locally excited (LE) state of the PTZ moiety and the photoinduced charge-transfer (CT) state. Femtosecond to microsecond transient absorption spectroscopy reveals that this equilibrium is facilitated not simply by enhanced donor ability, but presumably by excited-state planarization of the PTZ moiety, which lowers the energy of the LE state of the PTZ moiety. In contrast, Pe-Ph-PTZ(TPA)₂, in which the donor-acceptor distance is increased by a phenyl spacer, does not show clear equilibrium behavior. These results underscore the crucial role of excited-state structural relaxation in tuning photoinduced charge separation, and demonstrate that precise electronic and geometric design can enable controllable excited-state behavior in orthogonal molecular systems.