Nature Chem.http://dx.doi.org/10.1038/nchem.1199 (2011)

Credit: © 2011 NPG

Although the arrangement of tilings of surface-bound supramolecular networks has been intensely explored — mainly as a route to achieving tailored surface functionality — the degree of randomness has been difficult to measure and control in experiments. Now, by using combinations of isophthalate tetracarboxylic acids and solvents, Andrew Stannard and colleagues show that supramolecular rhombus tilings with varying randomness are possible, and that they can be characterized by an experimentally measured order parameter, which is a function of the fraction of adjacent parallel tiles and that of tiles that bind with unequal orientations. Additionally, the researchers show that the degree of order depends on the energy difference between parallel and non-parallel tiles, and that energy minimization competes with the maximization of configurational entropy (except for purely random tilings). Although a detailed microscopic description of the interactions between the molecules is still beyond reach, the authors' numerical simulations show that the order parameter can be related to an orientational dependence of the intermolecular interactions.