Non-adiabatic dynamics simulations reveal the reversal of photoinduced electron transfer in zinc phthalocyanine/endohedral metallofullerene donor–acceptor complexes via substitution and solvent effects

Herein, we investigated the photoinduced dynamics of a zinc phthalocyanine N-pyridyl-substituted Sc3N@Ih-C80 (ZnPc-C80) donor-acceptor complex and its sulfonyl-substituted derivative ZnPcS-C80 with static electronic structure calculations and non-adiabatic dynamics simulations. We found that despite the similar geometric structures, the introduction of substituents significantly changes their electronic structures and leads to distinct photoinduced dynamics. In particular, in ZnPc-C80, the energy of the charge transfer (CT) state from ZnPc to C80 is lower than that of the local excitation (LE) state of ZnPc, indicating that electrons tend to transfer from ZnPc to C80 following the LE of ZnPc. In contrast, in ZnPcS-C80, the lowest energy CT state corresponding to CT from C80 to ZnPcS is lower than that of the LE state of ZnPcS, suggesting that the excitation of ZnPcS preferentially induces hole transfer from ZnPcS to C80. Moreover, the inclusion of solvent effects is crucial for reordering the energies of the relevant states, highlighting the significant role of the solvent in the photoinduced dynamics. Nonadiabatic dynamics simulations further corroborate these findings: for ZnPc-C80, excitation at 633 nm results in energy transfer from ZnPc to C80 within the first 750 fs, followed by electron transfer, whereas for ZnPcS-C80, excitation at 663 nm leads to a concerted process of energy transfer and hole transfer from ZnPcS to C80. This work not only elucidates the mechanisms underlying experimental observations but also demonstrates that chemical modifications can modulate the roles of molecular fragments in organic donor-acceptor (D-A) structures, which could be helpful for the design of high-performance organic D-A structures in the future.