Photosensitization by anticancer agents 12. Perylene quinonoid pigments, a novel type of singlet oxygen sensitizer

Abstract

The marked photosensitizing properties of perylene quinonoid pigments, a promising class of agents for photodynamic therapy of human tumors, have been recognized, but quantitative information on their excited states is lacking. For the first time, some fundamental photophysical parameters of hypocrellins A and B; and cercosporin in benzene have been determined. For hypocrellin A, k F = 1.4 × 10 8 s −1, Φ F = 0.14, τ S = 0.98 ns, E S = 48.5 kcal mol −1, Φ ST = 0.86, k ST = 8.6 × 10 8 s −1 and E T = 41.0 kcal mol −. For hypocrellin B, k F = 9.1 × 10 7 s −1, k C = 2.7 × 10 9 s −1, k ST = 1.1 × 10 10 s −1, Φ F = 0.058, Φ IC = 0.18, τ S = 0.66 ns, E S = 47.7 kcal mol −1, Φ ST = 0.76 and E T = 40.2 kcal mol −1. For cercosporin, k F = 8.3 × 10 7 s −1, k IC = 1.7 × 10 9 s −1, k ST = 1.1 × 10 10 s −1, Φ F = 0.061, Φ IC = 0.13, τ S = 0.75 ns, E S = 48.0 kcal mol −1, Φ ST = 0.81 and E T = 40.5 kcal mol −1. The quantum yields of singlet oxygen formation by these agents were also determined: Φ O 2 (hypocrellin A) = 0.83, Φ 1 O 2 (hypocrellin B) = 0.76 and Φ 1 O 2 (cercosporin) = 0.81. Finally, these perylene quinonoid agents have been shown to act as excellent singlet oxygen sensitizers with several advantages over existing sensitizers, including large molar extinction coefficients, wide UV—visible absorption bands, high quantum yields of singlet oxygen generation, good stability, good solubility and small solvent and concentration effects.