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, fΔT, 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