Hot Hole Hopping in a Polyoxotitanate Cluster Terminated with Catechol Electron Donors

Fast hole hopping in Ti17cat4, a 1 nm diameter molecular polyoxotitanate cluster bearing four catechol ligands (Ti₁₇(μ₄-O)₄(μ₃-O)₁₆(μ₂-O)₄(cat)₄(OPrⁱ)₁₆), was investigated by ultrafast spectroscopy and quantum dynamics simulations. The catechol moieties coupled to the TiO₂ core of the cluster give rise to a charge-transfer band, the excitation of which promotes an electron from the highest occupied molecular orbital of the ligand to the inorganic core, resulting in the formation of {cat^+•,Ti³⁺}, a vibrationally hot polaronic exciton. Dynamic depolarization measurements indicate that within less than 100 fs the Franck-Condon polaronic state formed at the interface evolves into a fully charge-separated state and the injected electron delocalizes over the quasi conduction band of the cluster. The positive charge (hole) resulting from the injection does not remain static either. The initial hole hopping between the catechol sites occurs with the rate of ∼5 × 10¹¹ s^-1 or more and competes with the intramolecular vibrational relaxation. Upon thermalization, the hopping slows and continues at a rate of ∼5 × 10¹⁰ s^-1. The experimentally observed rate of hole hopping agrees well with the results of quantum dynamics modeling of the wavepacket propagation.