Self-Assembly of Patterned Monolayers with Nanometer Features:  Molecular Selection Based on Dipole Interactions and Chain Length

Patterned cocrystal monolayers self-assemble on HOPG in contact with solutions containing complementary pairs of 1,5-chain-substituted anthracene derivatives. Monolayer unit cells containing three or four molecules and spanning 9−11 nm are generated. The monolayers consist of alternating aromatic and aliphatic columns. The designs and dimensions of the cocrystal patterns (unit cells) are determined by (i) the preferred packing alignment of identical length side chains, (ii) the selectivity of each side chain for neighboring chains, (iii) the identities of the two side chains on each anthracene, and (iv) the 2D-chirality of 1,5-substituted anthracenes. The aliphatic columns form by interdigitation of identical length side chains arrayed in an antiparallel alignment, with the nth heavy atom of one side chain in registration with the (ω+2-n)th heavy atom of two adjacent chains ((ω ↔ 2) packing). Adjacent side chains are attached, alternately, to anthracenes in one of the two flanking aromatic columns. The preference for (ω ↔ 2) packing optimizes side-chain van der Waals interactions. The composition and fidelity of patterning in the cocrystal monolayers requires an additional source of “molecular recognition” in addition to side-chain length. Dipolar interactions, both attractive and repulsive, between ether groups in neighboring, (ω ↔ 2) packed side chains, constitute a second recognition element needed for cocrystal self-assembly.