Delayed fluorescence of meso-tetraphenylporphyrin in acetone and in dimethylsulphoxide

Photodynamic therapy is based on photosensitisation of singlet oxygen by porphyrin-like molecules. We have performed a systematic study of delayed fluorescence of tetraphenylporphyrin in acetone (used as a spectroscopic standard) and in dimethylsulphoxide (clinically used solvent) to obtain spectra, kinetics, and quantum yields, including their dependencies on tetraphenylporphyrin concentration. In dimethylsulphoxide the repopulation of excited singlets and subsequent delayed fluorescence is caused by triplet–triplet quenching with rate constant of (2.2±1.0)×109 l mol−1 s−1. However, repopulation of excited singlets in acetone is also caused by singlet oxygen reaction with triplet tetraphenylporphyrin causing monoexponential delayed fluorescence decay with the lifetime 0.3 μs. Due to much lower viscosity of acetone compared to dimethylsulphoxide, triplet–triplet quenching constant in acetone is much higher (1.7±0.7)×1010 l mol−1 s−1. The rate constant for the reaction of singlet oxygen with triplet of tetraphenylporphyrin is (2.0±0.8)×1010 l mol−1 s−1 in acetone.