Abstract: An indole compound represented by general formula (I): wherein R1 and R2 each represent a hydrogen atom, a hydrocarbon group having 1 to 30 carbon atoms or a substituent represented by general formula (II); R3 and R4 each represent an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms or an aryl group having 6 to 30 carbon atoms; R5, R6, R7, and R8 each represent a hydrogen atom, a hydrocarbon group having 1 to 30 carbon atoms, a halogen atom, a hydroxyl group, a nitro group, a cyano group, NHR or NR?R?; R, R?, and R? each represent an alkyl group having 1 to 10 carbon atoms, or R? and R? are taken together to form a ring; R5 and R6, R6 and R7, or R7 and R8 may be taken together to form a ring; Anq? represents a q-valent anion; q represents 1 or 2; and p represents a coefficient necessary to maintain charge neutrality.