t least one member selected from the group consisting of halogen, hydroxyl, alkoxy and nitro,R2 and R3 independently of one another represent a hydrogen atom or an alkyl group containing 1 to 4 carbon atoms, R4 is a hydrogen atom or an optionally aromatically substituted alkyl group containing 1 to 18 carbon atoms, X- is an anion selected from the group consisting of anions of halogen atoms, anion