Development of phenazine-2,3-diol-based photosensitizers: effect of formyl groups on singlet oxygen generation

Abstract

Phenazine-2,3-diol derivatives KO-0–3, which have zero to three formyl groups, respectively, have been developed as photosensitizers (PSs) possessing the ability to generate singlet oxygen (¹O₂). The photoabsorption bands of KO-0–3 are significantly red-shifted compared to those of phenazine-2,3-MOM (methoxymethyl) derivatives 5–8, whose hydroxy and formyl groups are protected, and have onsets at around 600–650 nm. Furthermore, the fluorescence quantum yields (Φ_fl) of KO-0–3 (Φ_fl = 0.024–0.097) are lower than those of 5–8 (Φ_fl = 0.34–0.46) in solution. To gain insight into the ¹O₂ generation properties of KO-0–3, we evaluated the ¹O₂ quantum yields (Φ_Δ) and rate constants (k_obs), and demonstrated that KO-1–3 possess a higher ability to generate ¹O₂ under visible light irradiation than those of 5–8. Moreover, it was found that the Φ_Δ values of KO-0–3 increase in the order of KO-0 (0.036) < KO-1 (0.22) < KO-2 (0.33) < KO-3 (0.41) with increasing number of formyl groups. This result indicates that formyl groups facilitate the intersystem crossing (ISC) from the lowest singlet excited states of the PSs (S₁) to the triplet excited states (T_n) according to El-Sayed's rule. Consequently, this work provides useful knowledge in molecular design of efficient phenazine-2,3-diol-based PSs for photodynamic therapy (PDT).