Revealing Recombination and Ultrafast Relaxation Mechanisms in Atomically Precise Titania Nanoclusters

Understanding dynamics of excited electronic states is critical for optimizing photoactive nanomaterials in numerous technological applications, including those employing semiconducting nanoscale transition-metal oxides. In this work, using a combination of state-of-the-art experimental and computational methods, we provide detailed insights into the recombination and ultrafast relaxation processes in atomically precise size-selected titania (TiO₂)_<i>n</i> (<i>n</i> = 1-8) nanoclusters. Femtosecond pump-probe spectroscopy reveals two distinct dynamical regimes: an ultrafast subpicosecond relaxation from the initial excited state down to the lowest excited state (S₁), followed by a significantly slower recombination to the ground state (S₀) on time scales of tens to hundreds of picoseconds. Ab initio nonadiabatic molecular dynamics simulations accurately reproduce these observed time scales providing a sound theoretical support to the interpretation of the experiments. We find that the nonmonotonic dependence of the corresponding excited-state relaxation and recombination time scales on the nanocluster size emerges from the interplay of electronic energy gaps, nonadiabatic couplings, and densities of states. In larger nanoclusters, larger nonadiabatic couplings between electronic states enhance coherent population transfer within the dense manifold of excited states, facilitating repopulation of higher excited states and, counterintuitively, slowing down the excitation energy relaxation. In contrast, recombination to S₀ depends nonmonotonically on the cluster size: the dynamics is dominated by energy gaps and slows down for larger gaps when <i>n</i> ≤ 4, meanwhile the relaxation accelerates in larger systems (<i>n</i> ≥ 4) due to increased nonadiabatic couplings. Our findings provide generic mechanistic insights into excited-state dynamics in photoactive materials as exemplified by the important class of size-selected metal oxide nanoparticles.