Frustrated 2D Molecular Crystallization

Grafting isophthalic acid groups to linear connectors produces tetracarboxylic acids 2−4, which are extended analogues of trimesic acid (1). Normal pairwise association of -COOH groups induces trimesic acid to form a hexagonal network held together by six hydrogen bonds per molecule. In contrast, analogues 2−4 are designed to form two polymorphs, parallel network II and Kagomé network III, which are linked by eight hydrogen bonds per molecule. The particular connectivity of these networks allows a smooth transition from one to the other without introducing discontinuities in hydrogen bonding. DFT calculations suggest that subtle differences in hydrogen bonding favor parallel motif II for short tetraacid 2 and Kagomé motif III for long tetraacid 4, whereas the two motifs are closely similar in energy for intermediate tetraacid 3. These preferences were confirmed by using STM to image the adsorption of compounds 2−4 on graphite. 2D crystallization of tetraacid 3 is frustrated, presumably because the two motifs are matched in energy and can merge smoothly. Nevertheless, adsorption of compound 3 shows a high degree of order, and most molecules have specific orientations relative to their neighbors, as dictated by motifs II and III. Such assemblies reveal the structure of a locally ordered 2D molecular glass, and they offer guidelines for the design of new aperiodic molecular materials.