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 ( 1 O 2 ). 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  P o/w ) of the 1-octanol/water partition coefficient ( P o/w ) for the PS. The log  P o/w 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 1 O 2 quantum yields ( Φ Δ ) of KY-1Na and KY-2Na in water by using 9,10-anthracenediyl-bis(methylene)dimalonic acid (ABDA) as a water-soluble 1 O 2 scavenger. It was found that in water the Φ Δ 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 E ST ) between the singlet state (S 1 ) and the triplet state (T 1 ) 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 E ST (S 1 -T 1 ) value (0.82 eV) of KY-2Na is smaller than that (0.98 eV) of KY-1Na, resulting in a relatively high Φ Δ value of KY-2Na. Consequently, we demonstrate that phenazine-2,3-diol-based PSs bearing carboxylic acid salts possess high solubility and moderate 1 O 2 generation ability in water.