Manipulating Charge-Transfer and Charge-Separated States Through Solvent Effects and Excited-State Vibrations in a Phenothiazine-Porphyrin-Naphthalenediimide Triad.

A phenothiazine (PTZ)-zincporphyrin (ZnP)-naphthalenediimide (NDI) triad was designed and synthesized to address the effects of electronic and vibrational excitations on photoinduced charge separation (CS) and charge recombination (CR). Upon photoexcitation, the singlet excited state of ZnP (1ZnP*) was generated and rapidly converted into a charge-transfer (CT) state with partial excited-state character, PTZ-(ZnPδ+-NDIδ-)*, in toluene, 1,2-dichloroethane, and benzonitrile. In toluene, a secondary CT process from PTZ to (ZnPδ+-NDIδ-)* produced a fully charge-separated state, PTZ•+-(ZnP-NDI)•-. This behavior arises from slower CR in nonpolar solvents, which proceeds in the Marcus inverted region due to weaker stabilization of the CT state and smaller reorganization energy. Additionally, enhancement and blueshift of the ethynyl stretching band (2100 cm-1) were observed in the excited states of PTZ-ZnP-NDI and ZnP-ref, indicating the intrinsic contribution of ethynyl groups in the excited-state of meso-ethynyl-substituted porphyrins. Simultaneous electronic and vibrational excitations did not influence the photoinduced CS and CR dynamics. These findings provide fundamental insights into manipulating charge-transfer and charge-separated states via external stimuli.