X??? is a counterion; X1 is ???O??? or ???NH???; X2 is ???O??? or ???NH???; L is a divalent linking group selected from a direct bond or, a lower alkylene having 1 to 8 carbon atoms, optionally terminated with or interrupted by one or more divalent connecting groups selected from sulfonamide (???SO2NH???), amide ???(C???O)N???, ester ???(C???O)???O???, ether ???O???, sulfide ???S???, sulfone (???S