In the general formula (i) or (ii), when R5 is a hydrogen atom, R6 is a branched alkyl with 3 carbon atoms, or a linear or branched alkyl with 4-21 carbon atoms, preferably a branched saturated alkyl with 3 carbon atoms or a linear or branched saturated alkyl with 4-12 carbon atoms, more preferably a branched saturated alkyl with 3-12 carbon atoms.