Chemiluminescence of 2-methyl-6-arylimidazo-[1,2-a]pyrazin-3(7H)-one in protic solvents: electron-donating substituent effect on the formation of the neutral singlet excited-state molecule.

2-Methyl-6-arylimidazo[1,2-a]pyrazin-3(7H)-ones with a substituent such as phenyl, 4-methoxyphenyl or 4-trifluoromethoxyphenyl at the 6-position of the imidazo[1,2-a]pyrazin-3(7H)-one ring system, produced chemiluminescence emission in mixtures of water and DMF and in several mixtures of MeOH and DMF under neutral conditions. Under these protic luminescence conditions, the respective light emissions were generated from neutral singlet excited-state molecules. The electron-donating effect of the 4-methoxy substituent on the phenyl group increased the efficiency of the neutral singlet excited state formation, whereas non-substitution and a 4-trifluoromethoxy group having no electron donating ability decreased the efficiency. The compound having the electron-donating methoxy group substituent showed two chemiluminescence emitters, which generated light at lambda(max) 410-420 nm and 460 nm. It was determined that the neutral molecules in the excited state generating light emission at the shorter wavelengths are neutral singlet excited-state molecules suitable for highly efficient singlet excited-state formation. A role of the electron-donating effect of the methoxy group is postulated to be generation of the special neutral singlet excited-state molecules.