Emergence of Spin-Vibronic Coherence in the Intermediate Spin-Orbit Coupling Regime Revealed by Mixed Quantum-Classical Dynamics.

Understanding and controlling the interplay between intersystem crossing, internal conversion, and vibrational motion is crucial for engineering photochemical reactivity in transition-metal complexes. In this work, we investigate spin-vibronic coherence along the Pt-Pt stretching normal mode in a series of five structurally tuned, double-bridged Pt(II) dimers using nonadiabatic dynamics incorporating both spin-orbit coupling and derivative nonadiabatic couplings. Across these systems, the variations in cyclometalating and bridging ligands modulate Pt-Pt distances, excited-state electronic structure, and spin-orbit coupling strengths that enable systematic probing of coherence mechanisms. We show that ligand-controlled spin-orbit coupling dictates early time intersystem crossing behavior. Fourier analysis of time-dependent populations reveals that spin-vibronic coherence arises when the Pt-Pt stretching frequency becomes near-resonant with the spin-orbit coupling between singlet and triplet states. If spin-orbit coupling is too weak, vibrational motion dominates; if spin-orbit coupling is too strong, intersystem crossing and vibration become decoupled. These results establish a clear mechanistic picture and provide molecular design guidelines for modulating spin-vibronic coherence in photoluminescent and photocatalytic Pt(II) systems.