Homomolecular non-coherent photon upconversion by triplet–triplet annihilation using a zinc porphyrin on wide bandgap semiconductors

Non-coherent upconversion, realized by homomolecular triplet–triplet annihilation of a zinc metalloporphyrin absorber, has been measured for the metalloporphyrin adsorbed on metal oxide films (ZrO2, TiO2 and ZrO2/TiO2 mixed oxides) in which the semiconductor conduction band energy lies between the metalloporphyrin’s upconverted and prompt S1 excited states. Upconverted emission was observed for pure and mixed metal oxide films, regardless of the relative energies of the states, indicating electron transfer from upconverted states to the semiconductor was not competitive. Upconversion was attributed to aggregation of the absorber, likely in defects on the films, leading to efficient homomolecular triplet–triplet annihilation.