Synthesis and optical properties of phenazinone-based photosensitizers for singlet oxygen generation

Abstract

Phenazinone derivatives PZ1–3 where a carbonyl group is incorporated into the chromophore are designed and developed as halogen-atom-free-heteroanthracene-based photosensitizers possessing the ability to generate singlet oxygen (¹O₂). Compared to the phenazine-2,3-diol (PZ) without a carbonyl group, the molar absorption coefficients of PZ1–3 at around 480 nm increased by a factor of five to six. Furthermore, the values of fluorescence quantum yields (Φ_fl) of PZ1–3 were significantly low (0.03 for PZ1, 0.04 for PZ2, and 0.05 for PZ3). To evaluate the ¹O₂ generation ability of PZ1–3, the ¹O₂ quantum yields (Φ_Δ) of PZ1–3 were estimated and it was demonstrated that PZ1 exhibited a higher Φ_Δ value than rose bengal (RB) known as a reference photosensitizer to generate ¹O₂ (0.86 for PZ1, 0.54 for PZ2, 0.069 for PZ3, and 0.68 for RB). Moreover, the rate constants (k_r, k_nr, k_ISC, and k_IC) of PZ1–3 indicated that the reason for the low Φ_Δ of PZ2 and PZ3 comes from the vibrational relaxation caused by a flexible methoxymethoxy and 4-chlorobenzoate group, respectively. Density functional theory (DFT), time-dependent DFT (TD-DFT), and natural transition orbital (NTO) calculations suggested that for PZ1–3, the intersystem crossing (ISC) processes from the lowest excited singlet state (S₁) to the third triplet state (T₃) are thermodynamically feasible and facilitated based on El-Sayed's rule. Consequently, we propose that the phenazinone skeleton is one of the promising halogen-atom-free-heteroanthracene-based chromophore photosensitizers.