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

Abstract

Substituent effects on the spectroscopic properties of a solvatochromic compound, 7-methyl-2-phenylimidazo[1,2- a]pyrazin-3(7 H)-one ( 1c ), using derivatives 1 with a para-substituent R on the phenyl group, were investigated systematically. In the UV/visible absorption spectra, the solvatochromic property of 1 originating from hydrogen-bonding interactions was effectively regulated by the substituent effects. In particular, the cyano derivative 1e showed significant solvatochromism with a wide color variation range and a high sensitivity toward solvents. Similarly, the fluorescence of 1 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 1 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 1c and 1e caused by metal–ion complexation were also compared, showing that the cyano derivative 1e 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 1c caused successive modulations of the colorimetric sensor properties.