Samorí P, Fechtenkötter A, Jäckel F, Böhme T, Müllen K, Rabe J P
Department of Physics, Humboldt University Berlin, Invalidenstrasse 110, 10115 Berlin, Germany.
J Am Chem Soc. 2001 Nov 21;123(46):11462-7. doi: 10.1021/ja0111380.
A series of soluble hexabenzocoronene (HBC) derivatives with pendant optically active (S)-3,7-dimethyloctanyl and (R,S)-3,7-dimethyloctanyl (mixture of stereoisomers) hydrocarbon side chains with and without a phenylene spacer 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 that all derivatives self-assemble into oriented crystals in quasi-two dimensions. However, while for the alkyl-substituted HBCs (1,4) all of the single aromatic cores within a monolayer exhibit the same contrast in the STM, the single aromatic cores with a phenylene group between the alkyl side chains and the aromatic core (2a,2b,3) exhibit different contrasts within a monolayer. For the disks carrying racemic branched or n-alkyl side chains (2b,3) a random distribution of the two different contrasts within the 2D-crystal is observed, while the optically active phenylene-alkyl-substituted HBC (2a) exhibits a periodical distribution of three contrasts within the monolayer. We attribute the different contrasts of the aromatic cores in the presence of the phenylene groups to a loss of the planarity of the whole molecule 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 distinctly different positions such as in a staircase is attained. The self-assembly processes are governed by the interplay of intramolecular as well as intermolecular and interfacial interactions. In the present case, the interactions may induce both the molecules to acquire well distinct positions along the z axis and to adopt different conformations. The reported results open new avenues of exploration. For instance, the different couplings of conjugated molecules with the substrate at different separations can be investigated by means of scanning tunneling spectroscopy (STS). Furthermore, experiments on the STM tip-induced switching of single molecules embedded in a monolayer appear feasible.
一系列带有光学活性的（S）-3,7-二甲基辛基和（R,S）-3,7-二甲基辛基（立体异构体混合物）烃侧链且带有或不带有亚苯基间隔基的可溶性六苯并蔻（HBC）衍生物，在溶液与高度取向热解石墨（HOPG）基面之间的界面处组装成不同有序排列。分子分辨扫描隧道显微镜（STM）图像显示，所有衍生物均在准二维中自组装成取向晶体。然而，对于烷基取代的HBCs（1,4），单层内所有单个芳香核在STM中表现出相同的对比度，而在烷基侧链与芳香核之间带有亚苯基的单个芳香核（2a、2b、3）在单层内表现出不同的对比度。对于带有外消旋支链或正烷基侧链的盘状分子（2b、3），在二维晶体中观察到两种不同对比度的随机分布，而光学活性的亚苯基-烷基取代的HBC（2a）在单层内表现出三种对比度的周期性分布。我们将在有亚苯基存在时芳香核的不同对比度归因于整个分子平面性的丧失和不同的构象，这使得共轭盘状分子在HOPG表面上方达到不同的平衡位置。在光学活性侧链的情况下，会形成一种具有三个明显不同位置（如阶梯状）的规则超结构。自组装过程受分子内以及分子间和界面相互作用的相互影响。在当前情况下，这些相互作用可能促使分子沿z轴获得明显不同的位置并采取不同的构象。所报道的结果开辟了新的探索途径。例如，可以通过扫描隧道谱（STS）研究共轭分子在不同间距下与底物的不同耦合。此外，对嵌入单层中的单分子进行STM针尖诱导切换的实验似乎是可行的。
