A preferable compound represented by general formula (I) is a compound in which, when n is 1, R1 is a phenyl group each of which is unsubstituted or substituted at the para-position by an alkoxy group having a carbon number of 1-18, an alkylthio group having a carbon number of 1-18 or a di(alkyl having a carbon number of 1-4)-amino group; an alkylphenyl group which is substituted by 1-5 alkyl grou