Vibrational Spin-Orbit Coupling Contributions to Excited-State Decay of Ligand-to-Ligand Charge Transfer States

Controlling excited-state relaxation processes is important in a variety of photochemical and photophysical processes, including the generation of ground- and excited-state spin polarization for quantum information science applications. Here, we address how specific static distortions, based on vibrational spin-orbit active modes at C_2v symmetry determined by group theory, in a series of low-symmetry ligand-to-ligand charge transfer complexes enable direct spin-orbit coupling contributions to T₁ → S₀ excited-state decay. These results are used to address spin-vibronic coupling contributions to T₁ → S₀ decay in a high-symmetry (^tBu₂bpy)Pt(S,S) (^tBu₂bpy = 4,4'-di-tert-butyl-2,2'-bipyridine and S,S = benzene-1,2-dithiolate) ligand-to-ligand charge transfer complex with effective C_2v symmetry, where T₁ relaxation is both spin- and orbitally forbidden due to the direct spin-orbit coupling matrix element being zero by symmetry. Low-frequency vibrations that involve a pyridine-pyridine twisting motion within the bpy ligand generate large ∂⟨S₀|H_SO|T₁⟩/∂Q_i values that will contribute significantly to T₁ → S₀ relaxation. The work advances ligand design strategies for the generation of tailored T₁ → S₀ relaxation rates, which can be utilized to optimize the generation of electron spin polarization and excited state decay processes in radical-elaborated ligand-to-ligand charge transfer complexes.