Supramolecular staircase via self-assembly of disklike molecules at the solid-liquid interface

A series of soluble hexabenzocoronene (HBC) derivatives with pendant optica lly active (S)-3,7-dimethyloctanyl and (R,S)-3,7-dimethyloctanyl (mixture o f stereoisomers) hydrocarbon side chains with and without a phenylene space r were assembled into differently ordered arrays at the interface between a solution and the basal plane of highly oriented pyrolytic graphite (HOPG). Molecularly resolved scanning tunneling microscopy (STM) images revealed t hat all derivatives self-assemble into oriented crystals in quasi-two dimen sions. However, while for the alkyl-substituted HBCs (1,4) all of the singl e aromatic cores within a monolayer exhibit the same contrast in the STM, t he single aromatic cores with a phenylene group between the alkyl side chai ns and the aromatic core (2a,2b,3) exhibit different contrasts within a mon olayer. For the disks carrying racemic branched or n-alkyl side chains (2b, 3) a random distribution of the two different contrasts within the 2D-cryst al is observed, while the optically active phenylene-alkyl-substituted HBC (2a) exhibits a periodical distribution of three contrasts within the monol ayer. We attribute the different contrasts of the aromatic cores in the pre sence of the phenylene groups to a loss of the planarity of the whole molec ule and different conformations, which allow the conjugated disks to attain different equilibrium positions above the surface of HOPG. In the case of the optically active side chains a regular superstructure with three distin ctly different positions such as in a staircase is attained. The self-assem bly processes are governed by the interplay of intramolecular as well as in termolecular and interfacial interactions. In the present case, the interac tions may induce both the molecules to acquire well distinct positions alon g the z axis and to adopt different conformations. The reported results ope n new avenues of exploration. For instance, the different couplings of conj ugated molecules with the substrate at different separations can be investi gated by means of scanning tunneling spectroscopy (STS). Furthermore, exper iments on the STM tip-induced switching of single molecules embedded in a m onolayer appear feasible.