Molecular organization of azobenzene derivatives at the liquid/graphite interface observed with scanning tunneling microscopy

Physisorbed monolayers of azobenzene derivatives were studied with a scanning tunneling microscope at the liquid/graphite interface. Three different compounds, namely, 4,4'-di(dodecyloxy) azobenzene (C-12(AZO)C-12), 5-[omega-(4'-dodecyloxy-4-azobenzene-oxy)-dodecyloxy] isophthalic acid (C-12(AZO)C(12)ISA), and 4,4' -bis(omega-[3,5-bis(carboxylato) phenyl-1-oxyl dodecyloxy) azobenzene (ISAC(12)(AZO)C(12)ISA) have been investigated. In all cases monolayers could be observed with submolecular resolution at the liquid/graphite interface, allowing one to identify the azobenzene, as well as the other parts of the molecules. For each monolayer structure a molecular model could be composed with a good correspondence to the experimental data. Differences in the observed monolayer structures could be related to the chemical structure of the investigated compounds. The introduction of an isophthalic acid (ISA) headgroup has a profound influence on the monolayer structure because of its capability of hydrogen bond formation with other ISA groups or with solvent molecules. In addition, for the trans isomer of C-12(AZO)C(12)ISA solvent codeposition was observed. The compound C-12(AZO)C(12)ISA was used to investigate the effect of cis-trans isomerization on the molecular packing of the resulting monolayers. Under the influence of uv light, the azobenzene groups isomerize from the trans isomer to the thermodynamically less stable cis isomer. This isomerization process significantly changes the composition and structure of the observed monolayers. (C) 1997 American Vacuum Society.