Hydrogen bonds induced supramolecular self-assembly of azobenzene derivatives on the highly oriented pyrolytic graphite surface

The self-assembly of azobenzene derivatives (C_nAzCOOH) with various lengths of peripheral alkyl chains (with carbon number of n = 8, 10, 12, 14, 16) were observed by scanning tunneling microscopy on highly oriented pyrolytic graphite (HOPG) surface. The effect of van der Waals interactions and the intermolecular hydrogen bonding on the two-dimensional self-assembly was systematically studied. No alkyl-chain length effect was observed according to the STM images. All kinds of C_nAzCOOH adopting the same pattern self-assembled on the HOPG surface, suggesting the formation of the two-dimensional structures was dominated by the hydrogen bonding of the functional groups. It could be found that two C_nAzCOOH molecules formed a hydrogen-bonded dimer with “head-to-head” fashion as expected; however, the dimers organized themselves in the form of relative complex lamellae. Three dimers as a group arranged side by side and formed a well-defined stripe with periodic dislocations due to the registry mechanism of the alkyl chain with the underlying HOPG surface. The hydrogen bonds between the adjacent dimers in one lamella were formed and dominated the self-assembled pattern.