Thermally Activated Fluxionality Accelerates Nonradiative Decay in Titania Nanoclusters

Photoactive nanoclusters are often assumed to exist as a single isomer corresponding to the lowest-energy local minimum structure. However, at finite temperatures, multiple configurations may be thermally accessible. Using ab initio molecular dynamics combined with time-dependent density functional theory, we identify a fluxional mode associated with low-energy structural rearrangement that can directly influence excited-state dynamics in a model titania nanocluster. This thermally driven mode reshapes the low-energy excited-state manifold, producing a broader, bimodal S₁ excitation-energy distribution. Nonadiabatic dynamics reveals two distinct consequences. Relaxation toward S₁ accelerates because fluxionality changes the energetic end point of the decay pathway, whereas ground-state recombination accelerates through enhanced sampling of strongly coupled S₀-S₁ configurations. These findings establish local coordination fluxionality as a structural mechanism for reshaping excited-state landscapes in oxide nanoclusters, with implications for tuning nonradiative decay and charge recombination pathways in photocatalytic metal oxides.