Photochemical Upconversion in Solution: The Role of Oxygen and Magnetic Field Response

Upconversion processes effectively convert two or more low energy photons into one higher energy photon, and have diverse applications in photovoltaics and biomedicine. Upconversion is generally spin-selective, and its magnetic field response can be used to examine the interplay between two different mechanisms for photochemical upconversion in solution: triplet-triplet annihilation, and singlet-oxygen mediated energy transfer. A kinetic model is developed and applied to explain the different photoluminescence profiles of oxygenated versus deoxygenated systems. From the magnetic field response, the triplet-triplet annihilation rate constant is estimated. The conditions required to maximize upconversion photoluminescence intensity in oxygenated solution are determined.