arbon atoms, specifically preferable is an n-octyl group, a tert-octyl group, a 2-ethylhexyl group, an n-decyl group or an n-dodecyl group, and most preferable is a 2-ethylhexyl group.When R3 and R8 each are a group other than a hydrogen atom, preferable as R21 is an alkyl group having a molecular weight of not less than 300 and having not more than 20 carbon atoms.A compound represented by Formul