An investigation of the chemiluminescent properties of 2,6-diphenylimidazo[1,2-a]pyrazin-3(7H)-one derivatives (1), having substituted phenyl groups, is described. Among the derivatives 1, the 6-[4-(dimethylamino)phenyl] derivatives (1a,d–f) gave a high quantum yield (Φ_CL ≥ 0.0025) in diglyme/acetate buffer, which is a model reaction condition for the Cypridina bioluminescence. Their efficient chemiluminescence is mainly caused by the electronic effect of the substituent at C6. In particular, the electron-donating 4-(dimethylamino)phenyl group at C6 of 1a,d–f plays an essential role in increasing the chemiexcitation efficiency (Φ_S) by the charge transfer-induced luminescence (CTIL) mechanism. The results provide useful information for designing new Cypridina luciferin analogues showing efficient chemiluminescence.