When at least one of R5 and R6 is a substituted or unsubstituted alkyl group, it is preferably an unsubstituted alkyl group, a substituted alkyl group due to which the compound of formula (II) is dibasic acid ester, an alkyl group having at least one halogen atom, an alkyl group having at least one cycloalkyl group or a substituted alkyl group due to which the compound of formula (II) is citric ac