A compound of general formula (I): in which: R is selected from the group consistingof (C-C)alkyl, (C-C)alkenyl,aryl, (C-C)aralkyl； n and m are each independently aninteger of two to eight； p is one or two； A is selected from the group consistingof -B-, -B-(CH)r-B-, -B-(CH)-B-(CH)-B-,wherein r and z are an integer from 2 to 8, B is a -NR- or -N(R)l/pQ group, in which Rl is selected from hydrogen, (C-C)alkyl,(C-C)acyl, tert-butoxycarbonyl, and R isselected from hydrogen and (C-C)alkyl； Qis an anion selected from chloride, bromide, iodide, nitrate, sulphate, hydrogensulphate, perchlorate, RCOO wherein R hasthe same meanings as R, independently from one another and R-O-SOwherein R is (C-C)alkyl with the provisothat, when Q is selected from chloride, bromide, iodide, nitrate,sulphate, hydrogen sulphate, perchlorate, R is not (C-C)alkyl.
