Synthesis and chemiluminescent properties of 6,8-diaryl-2-methylimidazo[1,2-a]pyrazin-3(7H)-ones: Systematic investigation of substituent effect at para-position of phenyl group at 8-position

6,8-Diphenylimidazopyrazinone derivatives having a substituent R (R = CF3, H, and OMe) at para position of the 8-phenyl group were synthesized and their chemiluminescent properties were investigated. The chemiluminescence maxima (CLmax) of these compounds were observed to be in the range of 513–553 nm with a bathochromic shift that increased with the electron-withdrawing character of R, contrary to the previously observed substituent effect at the 6-postion. The chemiluminescence efficiencies (ϕCL) of these imidazopyrazinones were improved by the introduction of a p-substituted phenyl group at the 8-position. The quantitative investigation of the three quantum efficiencies (ϕR, ϕS, and ϕFL) whose product gives us ϕCL revealed that the ϕCL gains made were largely because of the increase in the values of the fluorescence quantum yields of the corresponding light emitters (ϕFL). The yields of the singlet-excited emitters (ϕS) during the chemiluminescent reaction were found to be very small (0.015–0.019), suggesting that one cannot construct an efficient imidazopyrazinone–chemiluminescence system that is comparable to the aequorin bioluminescence system only by using the electronic effects of substituents.