Synthesis and photophysical properties of pyrazinopsoralen

Abstract

An efficient synthesis of 6,8-dioxa-1,4-diazacyclopenta[b]phenanthren-5 -one (Pyrazino-psoralen) (1) has been carried out by the Suzuki coupling reaction as a key step starting from 5-bromo-6-methoxybenzofuran (6a) and methyl 2-iodo-3-pyrazinecarboxylate (8). Pyrazinopsoralen (PzPs) had a similar UV-Visible absorption spectrum with other psoralens and showed stronger long-UVA absorption than 8-MOP. The $λ_{max}$ of UV absorption of PzPs shifted to longer wavelength with increasing solvent polarity, indicating that these UV absorption was due to the π→$π^*$ transition. It shows a much lower fluorescence quantum yield (0.0008 in ethanol at room temperature) than other psoralens as expected due to the increased proximity effect of $^1(n, π^*)$ to $^1(π, π^*)$ state. The fluorescence lifetime was 1.05 ns in nonpolar methylcyclohexane with a single exponential decay behavior, while more than two components were observed in other solvents with major pre-exponential factor in the short-lived component (〉95%). The triplet state of PzPs was not observable by phosphorescence, laser flash excitation, and singlet oxygen formation. It is very likely that the proximity of the $^1(n, π^*)$ state to the $^1(π, π^*)$ state leads to rapid radiationless decay such as $S_1~〉S_0, S_1~〉T_1$ and $T_1~〉S_0$.