"The phenyl or naphthyl group of Ar1 may have one or more substituents selected from, for example, a carboxyl group or a salt thereof, a sulfonic acid group or the salt thereof, or a linear or branched linear alkyl group of 1 to 4 carbons, a halogen atom such as fluorine, chlorine and bromine, alkoxy group of 1 to 4 carbons and an aryloxy group such as a phenoxy group." . . . . . . .