Published: June 14, 2011 r2011 American Chemical Society 12908 dx.doi.org/10.1021/jp201119g | J. Phys. Chem. C 2011, 115, 12908–12919 ARTICLE pubs.acs.org/JPCC Engineering Homologous Molecular Organization in 2D and 3D. Cocrystallization of Pyridyl-Substituted Diaminotriazines with Alkanecarboxylic Acids Adam Duong, † Marc-Andr  e Dubois, † Thierry Maris, † Val  erie M etivaud, † Ji-Hyun Yi, ‡ Antonio Nanci, ‡ Alain Rochefort, § and James D. Wuest* ,† † D epartement de Chimie and ‡ Facult  e de M edecine Dentaire, Universit  e de Montr  eal, Montr  eal, Qu ebec H3C 3J7, Canada § D epartement de G enie Physique,  Ecole Polytechnique de Montr eal, Montr eal, Qu ebec H3A 3A7, Canada b S Supporting Information ’ INTRODUCTION The scope of molecular crystal engineering has broadened dramatically in recent years, and its early emphasis on under- standing and controlling order in three dimensions (3D) 1 has expanded to encompass a growing interest in the organization of molecules adsorbed on surfaces in two dimensions (2D). 2,3 As a result, the insights of crystal engineers are contributing to advances in all areas of science and technology where molecular organization must be controlled, both in bulk materials and in thin films. Despite the growing utility of crystal engineering, it is still a young field with major unsolved problems. In particular, accurate predictions of the 3D structure of molecular crystals remain notoriously difficult, 4 and the problems are compounded in 2D by the subtle effects of the underlying surface on molecular arrangement. Stiff challenges must therefore be met before molecular organization can be truly mastered in either 2D or 3D. The challenges of crystal engineering in 2D and 3D are intimately linked. This calls out for an integrated approach in which molecular organization in 3D, determined by X-ray diffraction (XRD) or other methods, is compared systematically with 2D organization on surfaces, as revealed by scanning probe microscopy (SPM). This integrated approach promises to yield insights unlikely to emerge from studies focused more narrowly on 2D or 3D organization alone. In particular, (1) 3D structures resolved unambiguously by XRD can provide sound models for interpreting images of 2D organization obtained by SPM; (2) SPM can probe fine details of crystallization that cannot readily be examined by XRD, including dynamic phenomena involving individual molecules, structural features at the bound- aries of domains, and the nature of defects within domains; and (3) systematic comparison of 2D and 3D structures can clarify the role and relative importance of diverse forces that control molecular organization. Surprisingly, however, there are few reports of integrated analyses of 2D and 3D structures in extended series of related compounds. 5 Devising compounds that predictably favor similar structures in 2D and 3D is a particularly severe test of our current ability to understand and control molecular organization. When intermolecular Received: February 2, 2011 Revised: May 15, 2011 ABSTRACT: Isomeric pyridyl-substituted diaminotriazines 2aÀc and elongated analogue 3 are designed to adopt flattened structures with features that favor adsorption on surfaces and participation in multiple intermolecular interactions. In particular, pyridyl and diaminotriazinyl groups have strong affinities for graphite, and both form coplanar hydrogen-bonded adducts with alkanecarboxylic acids according to reliable motifs. Together, these properties predispose compounds 2aÀc and 3 to be coad- sorbed with alkanecarboxylic acids on graphite and to cocrystallize as structures built from hydrogen-bonded sheets. Comparison of the 2D structures of the ordered adlayers (as determined by scanning tunneling microscopy) with the 3D structures of the cocrystals (as determined by X-ray diffraction) showed striking homology, typically with quantitatively similar structural parameters. Together, these results illustrate how a series of related compounds can be engineered to form ordered adlayers and crystalline solids with closely analogous 2D and 3D structures. Specifically, the molecular components should have an affinity for the underlying surface and should engage in coplanar interactions that are strong relative to the energy of adsorption, thereby ensuring that the components are positioned reliably in sheets despite the effect of the surface. In general, compounds with these features should favor similar organization in different states, including monolayers, thin films, and bulk materials, and they promise to be useful in applications requiring behavior that depends predictably on dimensions, such as in thin-film molecular devices.