Observation of anthraquinone compounds using low temperature scanning tunneling microscopy

Three derivatives of anthraquinone deposited on Cu(1 1 1) have been observed by STM at liquid nitrogen temperature. Adsorption of the three molecules is found to be strongly dependent on the end group (phenyl, thienyl and pyridyl) surrounding the anthraquinone central part. While adsorption of the bi-pyridyl compound yields single molecules on the copper surface, evaporation of the phenyl derivative leads to the formation of stable dimers. The bi-thienyl anthraquinone has an intermediate behaviour with dimer formation and single molecule adsorption. These findings are assumed to result from the polar character of the molecules and from the bond strength of the different substituents with the metallic surface.