Efficiency factors of singlet oxygen generation from core-modified expanded porphyrin: Tetrathiarubyrin in ethanol

The photophysical properties and the singlet oxygen generation efficiency of tetrathiarubyrin have been investigated to elucidate the possibility of its use as a photodynamic therapy (PDT) photosensitizer by steady-state and time-resolved spectroscopic methods. The observed photophysical properties were affected by various molecular aspects, such as extended π conjugation, structural distortion, and internal heavy atom. The steady-state electronic absorption spectrum was red-shifted due to the extended π-conjugation, and the spin orbital coupling was enhanced by the structural distortion and the internal heavy atom effect. As a result of the enhanced spin orbital coupling, the triplet quantum yield increased to 0.90 ± 0.10 and the triplet state lifetime was shortened to 7.0 ± 1.2 μs. Since the triplet state decays at a relatively faster rate, the efficiency of the oxygen quenching of the triplet state decreases. The singlet oxygen quantum yield was estimated to be 0.52 ± 0.02, which is somewhat lower than expected. On the other hand, the efficiency of singlet oxygen generation during the oxygen quenching of triplet state, fδ^T, is near unity. Such high efficiency of singlet oxygen generation can be explained by the following two possible factors: The hydrogen bonding of ethanol which impedes the deactivation pathway of the charge transfer complex with oxygen to the ground state, the less probability of the aggregation formation.