provided that at least one of R6 to R8 is an alkyl group, a cycloalkyl group, a group represented by --COOR13, a group represented by --OCOR14, or an alkenyl group, and that when two of R6 to R8 are a hydrogen atom, then the remaining one represents an alkyl group having 6 carbon atoms or more, a cycloalkyl group having 5 carbon atoms or more, an alkenyl group having 6 carbon atoms or more, a grou