Substituent Effects on the Solvatochromism of 2-Phenylimidazopyrazinones: Effective Control of the Color Variation Range and Sensitivity toward an Indication of the Proton-donor Ability of Solvents by an Electron-withdrawing Group Substitution

A para-substitution on the phenyl group of 2-phenylimidazopyrazinone successively resulted in a change of the solvatochromic property, especially of the color-variation range and sensitivity to solvents, demonstrating that derivatives possessing an electronwithdrawing group are preferable as potential indicators of the proton-donor ability of solvents.