Publications

The unfolding process and self-assembly of a foldable oligomer (foldamer 1) at the liquid/graphite interface were investigated by scanning tunnelling microscopy. At the level of molecular conformation, we identified several molecular conformations (A(z), B, C, D, E) that represent intermediate states during unfolding, which may help to elucidate the unfolding process at the liquid/graphite interface. Adsorption at the interface traps the intermediate states of the unfolding process, and STM has proved to be a powerful technique for investigating folding and unfolding of a foldamer at the molecular level, which are not accessible by other methods. The STM observations also revealed that varying the solvent and/or concentration results in different self-assemblies of foldamer 1 as a result of variations in molecular conformations. The solvent and concentration effects were attributed to the changes in existing states (extended or folded) of foldamers in solution, which in turn affect the distribution of adsorbed molecular conformations at the interface. This mechanism is quite different from other systems in which solvent and concentration effects were also observed.

A series of conjugated oligo(p-phenylene-ethynylene) (OPE) molecules with backbone conformations (that is, the relative orientations of the contained phenylene units) controlled by competitive intramolecular hydrogen bonds to be either co-planar or random were synthesised and studied. In these oligomers, carboxylate and amido substituents were attached to alternate phenylene units in the OPE backbone. These functional groups were able to form intramolecular hydrogen bonds between neighbouring phenylene units. Thereby, all phenylene units in the backbone were confined in a co-planar conformation. This planarised structure featured a more extended effective conjugation length than that of regular OPEs with phenylene units adopting random orientation due to a low rotational-energy barrier. However, if a tri(ethylene glycol) (Tg) side chain was appended to the amido group, it enabled another type of intramolecular hydrogen bond, formed by the Tg chain folding back and the contained ether oxygen atom competing with the ester carbonyl group as the hydrogen-bond acceptor. The outcome of this competition was proven to depend on the length of the alkylene linker joining the ether oxygen atom to the amido group. Specifically, if the Tg chain folded back to form a five-membered cyclic structure, this hydrogen-bonding motif was sufficiently robust to overrule the hydrogen bonds between adjacent phenylene units. Consequently, the oligomers assumed non-planar conformations. However, if the side chain formed a six-membered ring by hydrogen bonding with the amido NH group, such a motif was much less stable and yielded in the competition with the ester carbonyl group from the adjacent phenylene unit. Thus, the hydrogen bonds between the phenylene units remained, and the co-planar conformation was manifested. In our system, the hydrogen bonds formed by the back-folded Tg chain and amido NH group relied on a single oxygen atom as the hydrogen-bond acceptor. The additional oxygen atoms in the Tg chain made a negligible contribution. A bifurcated hydrogen-bond motif was unimportant. From our results, in combination with the results from an independent study by Meijer et al.,([13]) it is evident that intramolecular hydrogen bonds involving back-folded oligo(ethylene glycol) moieties may differ in their structural details. Absorption spectroscopy served as a convenient yet sensitive technique for analysing hydrogen-bonding motifs in our study.

We report the observation of a series of perylene bisimide derivatives containing two or three perylene moieties. Dramatically different assembling behavior can be observed under ambient conditions on highly oriented pyrolitic graphite (HOPG) surface with scanning tunneling microscopy (STM). The dihedral angles between the perylene moieties are suggested to be sensitively dependent on the linker units. The linear linker units could make the perylene moieties with spatial configuration be observed on the surface. The length of linear connecting parts would also influence the dihedral angles. The fundamental processes occurring at solid-liquid interfaces have been found, because of the extended observation time or elevated temperatures. All these results would help to comprehend the conformational polymorphism of carrier transport in the organic thin films.