Development of a D–π–A pyrazinium photosensitizer possessing singlet oxygen generation

Abstract

(D–π–)₂A pyrazinium dyes (OEJ-1 and OEJ-2) bearing a counter anion (X⁻ = Br⁻ or I⁻) have been newly developed as a photosensitizer possessing singlet oxygen (¹O₂) generation. The two dyes show specific solvatochromism, leading to a large bathochromic shift of the photoabsorption band in halogenated solvents, compared to polar and non-polar solvents. The effects of the counter anion and solvents on the ¹O₂ generation efficiency such as Φ_Δ and the rate constant (K_obs) have been investigated. It was revealed that OEJ-2 (X⁻ = I⁻) exhibits a higher ¹O₂ quantum yield (Φ_Δ) than OEJ-1 (X⁻ = Br⁻). This result indicates that the (D–π–)₂A pyrazinium dyes possess the ability to generate ¹O₂ under visible light irradiation, due to the effective intersystem crossing (ISC) from the singlet excited state of the photosensitizer (¹S*) to the triplet excited state (³S*) by the superior heavy-atom effect of I⁻ ion as the counter anion. Moreover, it was found that THF and dichloromethane are favorable solvents for the (D–π–)₂A pyrazinium dyes to efficiently generate ¹O₂ compared with the polar solvents such as acetonitrile and DMSO. On the basis of the ¹O₂ quantum yield, the rate constant for ¹O₂ generation, the HOMO and LUMO energy levels of OEJ-1 and OEJ-2, and density functional theory (DFT) calculation, the photoabsorption and ¹O₂ generation properties of the D–π–A pyrazinium dyes are discussed.