My dissertation works focus on supramolecular patterning of physisorbed self-assembled monolayers. The goal is to develop reliable molecular recognition strategies that direct sets of molecules to assemble multi-component monolayers. In my research, scanning tunneling microscopy (STM) has been extensively exploited to investigate the structure and morphology of the monolayers. The systems we studied are mostly 1,5-[side chain]-disubstituted anthracene derivatives on HOPG substrate. Supramolecular interactions between neighboring molecules can be tuned by tailoring the compositions and structures of their side chains. Our strategy for multi-component monolayer patterning is to outfit different molecules with side chains that generate complementary interactions so that molecules recognize their "designed" neighbors. One of our molecular recognition strategies seeks to control morphology and patterning of self-assembled monolayers using strong dipolar interaction. Gem-difluoride (CF2) and ketone groups introduce large dipole moments (2.2D and 2.6D) into aliphatic chains. Proper placements of CF2 and ketone groups within aliphatic chains generate dipole - dipole repulsions or attractions between neighboring chains of self-assembled monolayers. The ability of CF2 and ketone groups to direct self-assembly of single- and two-component monolayers is tested using series of CF2 or ketone incorporated 1,5-[side chain]-disubstituted anthracene derivatives. Another molecular recognition strategy utilizes molecular shape. Conjugated diyne units introduce distinct “kinks” in the side chains of 1,5-substituted anthracene derivaties. Diyne kinks positioned near side chain center allow (nearly) optimal packing of identical alkadiyne side chains (shape self-complementary). By contrast, placement of the diyne kink far from side chain center inhibits packing of identical side chains (shape self-incommensurate). The self-assembly of complex, supramolecular structure in monolayers can be programmed by employing pairs of molecules outfitted with shape self-incommensurate, but pairwise shape-complementary side chains. In our research, compositionally patterned two- and four-component self-assembled monolayers were successfully prepared. Furthermore, we identified correlations between details of "kinked" side chain shape and defect densities within self-assembled monolayers. This has lead to molecular self-assembly of defect-free monolayer domains covering nearly 1 µm2.
Xue, Yi,
"Programming Multi-Component Self-Assembly Using Dipolar Interaction and Molecular Shape"
(2014).
Chemistry Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://doi.org/10.7301/Z0ZK5F1B
