In the case where only one of R6 to R8 is a hydrogen atom or none of these is a hydrogen atom (e.g., in the case where R6 is a hydrogen atom and R7 and R8 each is an alkyl group or where R6 to R8 each is an alkyl group), then the alkyl groups represented by the remaining two or all of R6 to R8 each is preferably a linear or branched alkyl group having 1 to 16, more preferably 1 to 14 carbon atoms.