Regioselective phenyl-substitution effects on the solvatochromism of 2-phenylimidazo[1,2-a]pyrazin-3(7H)-one derivatives: expansion of the color variation range of a visible indicator for the proton donor ability of solvents

Phenyl-substitution to the C8 position of an imidazopyrazinone ring resulted in the significant bathochromic shift of the lowest energy band, while the substitution to the C6 position does not produce such a shift. This provides evidence that a color variation range of the solvatochromism of 2-phenylimidazopyrazinone derivatives depends strongly on the position of phenyl-substitution. Results indicated that an imidazopyrazinone with the phenyl groups at the C2 and C8 positions acts as a potential indicator of the proton donor ability of solvents.