Described are compounds that are capable of forming adlayers, the compounds comprising at least one headgroup A, an optional linker B, a polymer C, a terminal group D and an optional crosslinking group R, wherein the headgroup A is a compound of the following formula ##STR00001## wherein X is independently selected from an acidic group, Y is a group derived from a convenient coupling group, Z.sup.1, Z.sup.3, and Z.sup.4 are C or N.sup.+ Z.sup.2, Z.sup.5 and Z.sup.6 are C-L.sup.1 or N.sup.+-L, wherein L.sup.1 is H or an electron withdrawing group, L is C.sub.1-C.sub.6 alkyl, in particular C.sub.1-C.sub.4 alkyl L.sup.2 and L.sup.3 are independently selected from H or C.sub.1-C.sub.6 alkyl, or L.sup.2 may form together with L.sup.1 or L of Z.sup.6 a heterocycle, in particular a positively charged heterocycle, n is 0, or 1, or 2, or 3, m is 0 or 1.