Photoinduced dynamics of the hole-transport material H101 in organic solvents and on mesoporous AlO and TiO thin films.

We present a comprehensive steady-state and time-resolved UV-Vis-NIR absorption and fluorescence study of the hole-transport material H101 to characterise its photophysics in different organic solvents and on mesoporous AlO and TiO thin films. Photoexcitation of H101 at 400 nm in organic solvents populates the S state which shows intramolecular relaxation on a picosecond time scale. Branching from the relaxed S state leads to population of the T triplet state and the ground electronic state S. Triplet formation is induced by the internal heavy-atom effect of the sulphur atom, and the triplet yield decreases substantially with solvent polarity. On mesoporous AlO, intermolecular exciton-splitting is observed leading to the formation of a radical cation - radical anion pair (H101(S) + H101(S) → H101˙ + H101˙) followed by exciton recombination. On mesoporous TiO, efficient electron injection is observed in addition to exciton-splitting. Complementary spectroelectrochemistry experiments enable a full spectral characterisation of the cation species H101˙ and H101. Extensive DFT/TDDFT calculations successfully assign the spectral features of all experimentally observed species. Implications for the function of H101 in photovoltaic devices are discussed.