Vibronic contributions to hyperfine-mediated spin kinetics

The hyperfine interaction is a cornerstone of spin-dependent processes, yet the kinetic mod- elling of this mechanism remains limited by the Born-Oppenheimer approximation, obscuring the observation of coherent pathways that underpin many photophysical systems. Here, we introduce a unified theoretical framework that rigorously incorporates vibronic contributions through a phase-consistent Herzberg-Teller expansion of the hyperfine Hamiltonian. Its ap- plication to the FMNH•-Cys• radical pair found in AsLOV2 demonstrates how second-order vibronic coupling increases the effective rates of hyperfine-mediated electronic transitions be- tween spin states by a factor of up to a hundred million; from∼ 10^3 s−1 in the Franck-Condon limit to∼ 10^11 s−1 for the S◦ →T◦ transition, and∼ 10^9 s−1 for the T◦ →S◦ in the Herzberg- Teller regime. This observed nanosecond timescale correlates well with the microsecond life- time photoadduct formation central to the function of the wild-type. This work underscores the importance of incorporating a vibronic description in hyperfine-driven dynamics and pro- vides a generalised and transferable methodology that captures essential non-Condon effects even at the single-structure level; thus serving as a complementary tool for systems where full ensemble sampling is impractical or not yet integrated into standard workflows.