Compounds of the formula ##STR1##

wherein M is either (1) a metal ion having a positive charge of +p wherein p is an integer which is at least 2, said metal ion being capable of forming a compound with at least two chromogen moieties, or (2) a metal-containing moiety capable of forming a compound with at least two chromogen moieties, z is an integer representing the number of chromogen moieties associated with the metal and is at least 2, and R₁, R₂, R₃, R₄, R₅, R₆, R₇, a, b, c, d, Y, Q^-, A, and CA are as defined herein.