A method as claimed in claim 1, wherein in the formula (I) of claim 1 not more than three of the radicals R1 to R4 are a phenyl group and at least one of the radicals R1 to R4 is a group from the series comprising C8 F17 -CH2 -CH2 - and Rf-C2 -CH2 - (Rf=C6 F13 to C12 F25), and X- is BF4 -, F-, Cl-, Br- and I-, PF6 -, tetraphenylborate, p-chlorotetraphenylborate, p-methyltetraphenylborate or tetran