The electron accepting compound of the invention is preferably represented by the following formula (I) or (II). ##STR1## wherein Y is a hydrogen atom, an alkyl group, an aryl group, a halogen atom or an electron attracting group such as ##STR2## --COR', --CO2 R', --SO2 R' or --CN in which R', R and R' each is a hydrogen atom, an alkyl group, an aralkyl group or an aryl group, Ar is a phenyl