R4's and R5's are each independently a hydrogen atom, a deuterium atom, a halogen atom, or a group selected from the group consisting of an alkyl group, an aryl group, an alkoxyl group and an amino group which are substituted or unsubstituted; and R6 is a hydrogen atom, a deuterium atom, a halogen atom, or a group selected from the group consisting of an alkyl group and an alkoxyl group which are