about 75 percent by weight of one or more compounds having the general formula: ##STR1## wherein wherein 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 R4 is selected from the group consisting of a substituted or unsubstituted biphenyl group, diphenyl ether group, alkyl group having from