Spin-Vibronic Mechanism of Light-Driven Coordination-Induced Spin-State Switching in a Nickel(II) Complex

In Ni(II) complexes, spin-state switching is governed by changes in the coordination number and the nature of the ligand field. We report the ultrafast photorelaxation of an axial-pyridine-coordinated octahedral Ni(II) complex with a phenazine fluorophore using multi-configurational electronic structure calculations and quantum wavepacket dynamics. Potential energy surfaces for 20 states of high-spin and low-spin metal-centered (MC), ligand-centered, and charge-transfer (CT) characters were computed using the CAS(12,9)/NEVPT2 method and incorporated into a spin-vibronic Hamiltonian that was propagated using the multiconfigurational time-dependent Hartree method. Axial ligation stabilizes a dense manifold of 3MC states and enhances intersystem crossing via strong spin-orbit coupling. Two Ni-ligand stretching modes drive ultrafast population transfer from bright intraligand CT states into a predissociative MC state. This promotes fluorescence quenching and predissociative axial pyridine bond weakening that can potentially facilitate a coordination-induced spin-state switch from an octahedral to a square-planar geometry.