Empirical DFT Model to Predict Triplet Quantum Yield Through Singlet Oxygen Yields

Triplet photosensitizers can be used for a variety of applications, including photocatalysis, OLEDs, and photodynamic therapy. Excited triplet states can be quenched by triplet oxygen to make singlet oxygen. Often the singlet oxygen quantum yield (Phi(Delta)) is used as a lower approximation for the triplet yield. Unpredictable effects of even minor structural changes can drastically alter the Phi(Delta) and complicate the design of new triplet photosensitizers. The most common strategy to increase Phi(Delta) is to incorporate heavy atoms, promoting the "heavy atom effect". However, the position and the identity of the heavy atom greatly influences the Phi(Delta). We have created a predictive model that correlates calculated natural atomic orbital composition of the heavy atom(s) contributing to the frontier molecule orbitals of a photosensitizer with the experimental Phi(Delta). The model, derived from several fluorescein derivatives, provides a calculated Phi(Delta) in agreement with the experimental values for a variety of well-known photosensitizers, including rhodamine dyes, fluorescein derivatives, and octahedral metal complexes.