Development of water-soluble phenazine-2,3-diol-based photosensitizers for singlet oxygen generation

Phenazine-2,3-diol-based dyes, KY-1Na and KY-2Na bearing one and two carboxylic acid sodium salts, respectively, have been newly developed as water-soluble photosensitizers (PSs) possessing the ability to generate singlet oxygen (O). In order to evaluate the solubility of KY-1Na and KY-2Na in water, the hydrophobicity/hydrophilicity of the two PSs was investigated by experimental measurement of the logarithms (log ) of the 1-octanol/water partition coefficient () for the PS. The log  values of both KY-1Na and KY-2Na were determined to be -0.9, indicating that both the PSs are more hydrophilic than Rose Bengal (-0.6) and have hydrophilicity equivalent to methylene blue (-0.9). Both the PSs in water show a broad photoabsorption band in the range of 500 to 600 nm. Thus, we estimated the O quantum yields () of KY-1Na and KY-2Na in water by using 9,10-anthracenediyl-bis(methylene)dimalonic acid (ABDA) as a water-soluble O scavenger. It was found that in water the value (0.19) of KY-2Na is higher than that of KY-1Na (0.06). Density functional theory (DFT) calculations suggested that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distributions for the molecular structure of KY-2Na are adequately separated, leading to a decrease in the energy gap (Δ) between the singlet state (S) and the triplet state (T) that causes efficient intersystem crossing (ISC), compared to that for the molecular structure of KY-1Na. Indeed, time-dependent DFT (TD-DFT) calculations demonstrated that the Δ(S-T) value (0.82 eV) of KY-2Na is smaller than that (0.98 eV) of KY-1Na, resulting in a relatively high value of KY-2Na. Consequently, we demonstrate that phenazine-2,3-diol-based PSs bearing carboxylic acid salts possess high solubility and moderate O generation ability in water.