wherein n is an integer of 2 to 8, and A represents 9-anthryl group, a substituted or unsubstituted N-substituted carbazolyl group, N-alkylphenothiazyl group, or ##STR5## wherein R1 represents halogen, a lower alkyl group or a lower alkoxy group, and R2 and R3 each represent a substituted or unsubstituted alkyl group, provided that R2 and R3 are not a methyl group at the same time when n is an int