Photoinduced dynamics of the hole-transport material H101 in organic solvents and on mesoporous Al2O3 and TiO2 thin films

Abstract

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 Al₂O₃ and TiO₂ thin films. Photoexcitation of H101 at 400 nm in organic solvents populates the S₁ state which shows intramolecular relaxation on a picosecond time scale. Branching from the relaxed S₁ state leads to population of the T₁ triplet state and the ground electronic state S₀. 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 Al₂O₃, intermolecular exciton-splitting is observed leading to the formation of a radical cation – radical anion pair (H101(S₁) + H101(S₀) → H101˙⁺ + H101˙⁻) followed by exciton recombination. On mesoporous TiO₂, efficient electron injection is observed in addition to exciton-splitting. Complementary spectroelectrochemistry experiments enable a full spectral characterisation of the cation species H101˙⁺ and H101²⁺. 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.