Controlled Self-assembly in Solution

Strong van der Waals interactions bettveen the HOPG substrate and graphitic discs as well as alkyl chains induce ordered monolayer or bilayers at the solid/liquid interface. 

Alkylated HBCs form regular monolayers on the HOPG surface in solution and asymmetric 7/V characteristics were measured by STS [34b]. This diode-like effect has been attributed to positional asymmetry of either the vertical placement of the disc in the gap or of the frontier molecular orbitals relative to the electrodes. The alkylphenyl substituted HBCs self-assemble into 2D crystals of the discs with variable vertical displacements from the substrate [93]. When the alkylphenyl chains contain chiral centers, a regular staircase superstructure results. A submolecular visualization of a covalently linked HBC dimer on the HOPG surface revealed a contrast, which reflects the structure of the aromatic parts of the molecule, with the aromatic moieties being oriented like graphene layers in graphite (Fig. 3.25) [94]. 

Processing of large vanishingly soluble discs such as parent HBC and C132 49 is very difficult. However, epitaxial layers of these unsubstituted discs with electron acceptors can be obtained by self-assembly from solution [95]. STM studies reveal that the interaction with the substrate induces a strong perturbation of the electronic structure of the pure donor in the first expitaxial monolayer. In the second epitaxial layer with a donor/acceptor stoichiometry 2 1 the molecules are unperturbed. 

Other electronic components can be attached onto the HBC core and the self-assembled side-by-side monolayers provide opportunities to fabricate single molecular devices at the solid/liquid interface. The covalently tethered HBC and pyrene units co-assemble to form stable nano-phase-separated arrays [96]. A significantly different packing of the adlayer of the HBC-pyrene dyad 87 on the basal plane of the HOPG surface has been obtained, as shown in Fig. 3.26. This STM current 

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