R1 and R2 are an aromatic group selected from the group consisting of a substituted or unsubstituted phenyl group, naphthyl group, and polyphenyl group and R3 is selected from the group consisting of a substituted or unsubstituted aryl, and alkyl group having from 1 to 18 carbon atoms and a cycloaliphatic group having from 3 to 12 carbon atoms, said groups being free of electron withdrawing groups