The role of the ammonium ion in directing self-assembly has been investigated, and the effect on the structure of changing halogen substituents has been examined. Ammonium 3-chloro- and 3-bromocinnamates, as well as ammonium 3-chlorocinnamate/3-chlorocinnamic acid and ammonium 3-bromocinnamate/3-bromocinnamic acid, have been synthesized and structurally characterized. The structures are layered with hydrophilic regions and double layers of anions. The ammonium ion tethers neighboring anions in a head-to-head relationship ca. 4 angstrom apart, the ideal distance for photoreaction. Changing the hydrogen bond donor-to-acceptor ratio from 2:1 to 5:4 does not change the general characteristics of the structures. Replacement of the meta ring substituent results in different structures. Apart from ammonium 3-chlorocinnamate/cinnamic acid, in which the double bonds are not parallel, the anion/acid units adopt the ideal geometry (distance and orientation) for photodimerization in all the other structures.