In the above formulae (I) and (II), when X1 to X8 are an unsubstituted or substituted straight-chain or branched alkyl group, or when the substituted alkoxyl group represented by X1 to X8 has an alkyl group as a substituent, specific examples of the alkyl group are as follows, provided that the alkyl group may be not only a saturated alkyl group, but also an unsaturated alkyl group: