. . . . . . . . "(7) ##STR6## in which each of R1, R2 and R3 is a substituted or unsubstituted alkyl group, and R1, R2 and R3 may be the same or different from each other; X- is an anion such as a perhalogenate ion, RF6 -, a substituted or unsubstituted benzene sulfonate, perfluoroalkane sulfonate, or an alkylsulfate; n is an integer of 0 to 3; a halogen atom exists on at least one of the forth, fifth, sixth and s" .