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 3 MC 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.