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 ΦΔ and the rate constant (Kobs) have been investigated. It was revealed that OEJ-2 (X- = I-) exhibits a higher 1O2 quantum yield (ΦΔ) 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.