The process of molecular self-assembly on solid surfaces is essentially one of crystallization in two dimensions, and the structures that result depend on the interplay between intermolecular forces and the interaction between adsorbates and the underlying substrate. Because a single hydrogen bond typically has an energy between 15 and 35 kilojoules per mole, hydrogen bonding can be a strong driver of molecular assembly; this is apparent from the dominant role of hydrogen bonding in nucleic-acid base pairing, as well as in the secondary structure of proteins. Carboxylic acid functional groups, which provide two hydrogen bonds, are particularly promising and reliable in creating and maintaining surface order, and self-assembled monolayers of benzoic acids produce structure that depends on the number and relative placement of carboxylic acid groups. Here we use scanning tunnelling microscopy to study self-assembled monolayers of ferrocenecarboxylic acid (FcCOOH), and find that, rather than producing dimeric or linear structures typical of carboxylic acids, FcCOOH forms highly unusual cyclic hydrogen-bonded pentamers, which combine with simultaneously formed FcCOOH dimers to form two-dimensional quasicrystallites that exhibit local five-fold symmetry and maintain translational and rotational order (without periodicity) for distances of more than 400 ångströms.

The preparation of 7-Fc + -8-Fc-7,8-nido-[C 2 B 9 H 10 ] À (Fc + FcC 2 B 9 À )d emonstrates the successful incorporation of acarborane cage as an internal counteranion bridging between ferrocene and ferrocenium units.T his neutral mixed-valence Fe II /Fe III complex overcomes the proximal electronic bias imposed by external counterions,apractical limitation in the use of molecular switches.Acombination of UV/Vis-NIR spectroscopic and TD-DFT computational studies indicate that electron transfer within Fc + FcC 2 B 9 À is achieved through ab ridge-mediated mechanism. This electronic framework therefore provides the possibility of an all-neutral null state, ak ey requirement for the implementation of quantum-dot cellular automata (QCA) molecular computing.The adhesion, ordering, and characterization of Fc + FcC 2 B 9 À on Au(111) has been observed by scanning tunneling microscopy.

A series of experiments and electronic structure calculations were performed to identify metastable 1,1′-ferrocenedicarboxylic acid supramolecular structures formed during solution deposition in a vacuum on a Au(111) substrate, as well as to observe their evolution into more stable species under mild annealing conditions. Electrospray ionization mass spectrometry measurments were performed to determine which species are likely to be present in the rapidly evaporating droplet, and these experiments found that a hexamer can exist in solution during deposition, albeit as a metastable species. The molecular clusters present after solution deposition were observed and analyzed using ultrahigh-vacuum scanning tunneling microscopy, and the initial monolayer contains four basic classes of structures: ordered dimer domains, tilted dimer rows, square tetramers, and rectangular chiral hexamers. Electronic structure calculations indicate that the chiral hexamers consist of a central dimer surrounded by four molecules oriented to form birfurcated hydrogen bonds with other carboxylic acid groups and weaker hydrogen bonds with hydrogens from the aromatic rings. The calculations also indicated that the tetramers are clusters held together by carboxylic acid dimer bonds on each ring oriented perpendicular to each other, and that this conformation is slightly more stable than two dimers for a cluster of four molecules. Annealing this surface at 50 °C for 1 h results in the formation of both isolated tetramers and ordered tetramer rows at the expense of the end-to-end dimer domains, with few chiral hexamers remaining. Further annealing at 50 °C, as well as annealing at 65 °C drives the system to form chiral dimer domains, as well as several other minor structures. Annealing at 75 °C resulted in a dramatic decrease in apparent surface coverage, and most ordered structures existed as large tilted dimer rows, whether isolated or in ordered domains. This drop in surface coverage is likely due to some combination of decomposition of the molecule, desorption, or the growth of three-dimensional crystal structures. The observed coexistence of many forms of ordered dimer structures after annealing indicates that the equilbrium conformation of 1,1′-ferrocenedicarboxylic acid is some array of ordered dimers, and the variety of supramolecular structures present after annealing is an indicator that this system evolves under kinetically controlled growth conditions.

Monolayers of indole-2-carboxylic acid and indole-3-carboxylic acid on gold are studied using ultrahigh-vacuum scanning tunneling microscopy. Both molecules form symmetric, cyclic, hydrogen-bonded pentamers, a structure that is stabilized by the presence of a weak hydrogen-bond donor (NH or CH) adjacent to the carboxylic acid on the five-membered ring. In addition to pentamers, indole-2-carboxylic acid forms hexamers and catemer chains, while indole-3-carboxylic acid monolayers are generally disordered. Density functional theory calculations show that pentamers and hexamers have stability comparable to dimers or short catemers. The coexistence of all of these structures likely arises from the nonequilibrium conditions present in solution during pulse deposition of the monolayer.

Low-temperature scanning tunneling microscopy is used to observe self-assembled structures of ferrocenedicarboxylic acid (Fc(COOH)2) on the Au(111) surface. The surface is prepared by pulse-deposition of Fc(COOH)2 dissolved in methanol, and the solvent is evaporated before imaging. While the rows of hydrogen-bonded dimers that are common for carboxylic acid species are observed, the majority of adsorbed Fc(COOH)2 is instead found in six-molecule clusters with a well-defined and chiral geometry. The coverage and distribution of these clusters are consistent with a random sequential adsorption model, showing that solution-phase species are determinative of adsorbate distribution for this system under these reaction conditions.