How Structural Subtleties Modulate Excited‐State Lifetimes in Cyclometalated Cobalt( III ) Complexes: Nonadiabatic Molecular Dynamics Study

Ultrafast excited‐state dynamics of two Co(III) complexes, bearing different alkyl groups ( Co1 : Methyl, Co2 : Ethyl) at the nitrogen of the imidazole ligand, are investigated using trajectory surface hopping based on a linear vibronic coupling model. Despite nearly identical MLCT absorption spectra, experimental singlet lifetimes differ markedly (1.45 vs. 4.3 ps). Relaxation occurs via rapid internal conversion within a strongly vibronically coupled singlet manifold, accompanied by intersystem crossing to long‐lived states. The prolonged lifetime of Co2 originates from enhanced vibronic coupling and larger structural distortions, which trap population in the singlet manifold rather than from spin–orbit effects. Minimum‐energy crossing points reveal low barriers for nonradiative decay, rationalizing emission quenching. These results demonstrate how subtle structural differences, by modulating vibronic coupling, can substantially influence excited‐state lifetimes in these complexes.