Substituent effects on the spectroscopic properties of solvatochromic 2-phenylimidazo[1,2-a]pyrazin-3(7H)-ones: an effective control for the colorimetric sensor properties

A para-substitution of R on the phenyl group of a solvatochromic 2-phenylimidazopyrazinone (1) successively resulted in modulations of the spectroscopic properties; the UV/visible absorption and the fluorescence of 1 and the electronic absorption of charge-transfer complexes of 1 with TCNE and metal-ion complexes of 1.