US 8,298,748 B2  16  15  -continued  The aryl group is preferably an aryl group having 6 to 10  carbon atoms because it can be synthesized at a low cost.  Specific examples thereof include a phenyl group and a naph› thyl group. Of these, a phenyl group is particularly desirable.  As examples of the alkyl group, alkoxy group and halogen  atom with which the hydrogen atoms of the aryl group may be  substituted, the same alkyl group, alkoxy group and halogen  atom as those with which the arylene group for R 2  may be  substituted can be given.  The alkyl group for R 3  and R 4  is not particularly limited  and includes, for example, a linear, branched or cyclic alkyl  group having 1 to 10 carbon atoms. In terms of achieving  excellent resolution, the alkyl group preferably has 1 to 5  carbon atoms. Specific examples include a methyl group, an  ethyl group, ann-propyl group, anisopropyl group, ann-butyl  group, an isobutyl group, an n-pentyl group, a cyclopentyl  group, a hexyl group, a cyclohexyl group, a nonyl group and  a decyl group. Among these, a methyl group is preferable  because it is excellent in resolution and can be synthesized at  a low cost.  In general formula (a0-2’), R 3  and R 4  may be mutually  bonded to form a ring with the sulfur atom.  In such a case, the ring including the sulfur atom is prefer- ably a 3- to IO-membered ring, and more preferably a 5- to  7-membered ring.  The ring structure formed with the sulfur atom may include  a hetero atom such as a sulfur atom or an oxygen atom  (---0-, =0).  Specific examples of the ring formed include a benzene  ring, a naphthalene ring, an anthracene ring, a phenanthrene  ring, a fluorene ring, a triphenylene ring, a naphthacene ring,  a biphenyl ring, a pyrrole ring, a furan ring, a thiophene ring,  an imidazole ring, an oxazole ring, a thiazole ring, a pyridine  ring, a pyrazine ring, a pyrimidine ring, a pyridazine ring, an  indolizine ring, an indole ring, a benzofuran ring, a ben- zothiophene ring, an isobenzofuran ring, a quinolizidine ring,  a quinoline ring, a phthalazine ring, a naphthyridine ring, a  quinoxaline ring, a quinoxazoline ring, an isoquinoline ring,  a carbazole ring, a phenanthridine ring, an acridine ring, a  phenanthroline ring, a thianthrene ring, a chromene ring, a  xanthene ring, a phenoxathiin ring, a phenothiazine ring and  a phenazine ring.  In the present invention, with respect to the structural unit  (a0-2), examples of the cation moiety of the group repre- sented by general formula (a0-2’) include groups represented  by the formulas shown below.  [Chemical Formula 6] 