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

Substituent effects on the spectroscopic properties of a solvatochromic compound, 7-methyl-2-phenylimidazo[1,2-a]pyrazin-3(7H)-one (), using derivatives with a para-substituent R on the phenyl group, were investigated systematically. In the UV/visible absorption spectra, the solvatochromic property of originating from hydrogen-bonding interactions was effectively regulated by the substituent effects. In particular, the cyano derivative showed significant solvatochromism with a wide color variation range and a high sensitivity toward solvents. Similarly, the fluorescence of showed a redshift as the electron-withdrawing property of R increased. The substituent effects were evaluated by AM1-COSMO calculations, which also suggested that derivatives are good electron donors and that the electron-donating ability is regulated by R. This was confirmed by the observation of low oxidation potentials and the formation of charge-transfer complexes with tetracyanoethylene. Absorption-spectrum changes of and caused by metal–ion complexation were also compared, showing that the cyano derivative is a good colorimetric sensor for the Lewis acidity of the metal ions. From these observations, it was established that para-substitution of R on the phenyl group of caused successive modulations of the colorimetric sensor properties.