A salt represented by formula (I): wherein R1 and R2 independently in each occurrence represent a hydrogen atom, a hydroxy group or a C1-C12 hydrocarbon group where a methylene group can be replaced by an oxygen atom or a carbonyl group; Ar1 and Ar2 each independently represent a C6-C36 aromatic hydrocarbon group which can have a substituent or a C6-C36 heteroaromatic hydrocarbon group which can have a substituent; A− and A′− each independently represent an organic anion; and “m” and “n” independently each represent an integer of 1 to 2.