Morphology Control and Monolayer Patterning with CF2 Groups: An STM Study

Difluoromethylene (CF2) groups introduce large dipole moments (2 D) into aliphatic chains with minimal perturbation of chain geometry and shape. Proper placements of CF2 and other dipolar groups within aliphatic chains generate dipole-dipole repulsions or attractions between neighboring chains of self-assembled monolayers. The ability of CF2 groups to direct packing, morphology, and patterning of self-assembled monolayers is tested using six, 1,5-(aliphatic side chain substituted)anthracene derivatives with one CF2 group per linear side chain. STM is used to characterize the monolayer morphology assembled on graphite from phenyl octane solution containing one or two of these anthracene compounds. Dipolar interactions between CF2 and ether groups {CF2/ether} within contacting, even length side chains drive assembly of a morphology not normally observed from anthracene monolayers (omega <-> 3 packed, AA tape, C-2-C). However, {CF2/ether} interactions are not sufficient to disrupt the normal, more stable monolayer morphology (omega <-> 2 packed, AA tape, C-2-C) assembled by anthracenes with odd length side chains. By contrast, the larger dipolar interactions generated between pairs of CF2 groups in contacting chains {CF2/CF2} overwhelm the normal packing of anthracenes with odd length side chains and drive assembly of a previously unobserved monolayer morphology (omega <-> 3 packed, AA' tape, C-2-C). The potent {CF2/CF2} dipolar interactions also drive assembly of patterned (AB) monolayers from solution mixtures of dipolar complements; two anthracenes (A and B) whose complementary dipolar group placements generate stabilizing electrostatic interactions when packed next to each other using the normal alignment and morphology of odd length side chains (omega <-> 2, AA tape, C-2-C). The favorable packing (omega <-> 2) and highly stabilizing dipolar interactions between CF2 side chain complements produce only patterned monolayers from 1:1 solution mixtures; no domains exhibiting the single component morphologies have been observed in STM studies. Molecular mechanics simulations of monolayer sections on graphene sheets confirm the critical role {CF2/CF2} dipolar interactions exert in driving the morphology and patterning of these anthracenes' monolayers.