Photochemistry of 1-(2'-naphthyl)-2-pyrazinylethylene

Abstract

The specral properties and mechanism of trans$\to$cis photoisomerization of trans-1-(2``-naphthyl)-2-pyrazinylethylene (2-NPyE) have been studied in various conditions. The fluorescence quantum yield of 2-NPyE is very low compared to 2-styrylnaphthalene (2-StN) due to the presence of (n,$\pi^\star$) state which can lead to very rapid radiationless decay processes. According to azulene quenching effect on trans $\to$ cis photoisomerization, phtoisomerization undergoes through both the singlet and triplet manifold at room temperature, singlet mechanism being favoured in polar solvents due to the decrease of intersystem crossing efficiency because of the energy level inversion of the $^1(\pi,\;\pi^\ast)$ and $^3(n,\pi^\star)$ states with increasing solvent polarity. The decrease of intersystem crossing rate constant in polar solvents leads to a coupled enhancement of fluorescence quantum yield. The quantum yields for direct trans$\to$cis photoisomerization are more affected by solvent viscosity than the solvent polarity. The result supports photoisomerization mechanism in which product is formed from two excited states, singlet and triplet states.