Spin-Vibronic Effect in Photoinduced Electron Transfer

The spin-vibronic effect (SVE) accelerates quantum-mechanically forbidden electronic transitions, but experimental manifestations of this phenomenon remain limited. In this contribution, the role of SVE in photoinduced electron transfer (PET) dynamics is probed by coherent vibrational wavepacket (CVWP) motions in Pt(II) dimer-naphthalene diimide donor-acceptor complexes. Metal-metal-to-ligand charge-transfer (MMLCT) excitation of the donor triggers ballistic PET, driving the molecule away from equilibrium. The SVE then directs the PET trajectory from the ¹MMLCT excited state to an intermediate ligand-centered triplet state, modulating the CVWP dynamics along the reaction coordinates. Herzberg-Teller type oscillations indicate that the Pt-Pt stretching vibration, arising from the formation of the excited triplet intermediate, serves as the reaction coordinate for charge separation. Our experimental findings advance the understanding of the interplay between electronic, vibrational, and spin degrees of freedom in ultrafast photoactivated processes.