Resumen de Chiral expression and transfer at the nanoscale
The creation of a nanomaterial involves both the optimization of the functional units and the control of their supramolecular order. This is particularly true for the preparation of chiral nanomaterials which have a wide range of properties arising from their asymmetric characteristics. The emergence of chirality at surfaces is especially important for the preparation of functional thin films.
In order to control chiral systems, the way in which the chirality of molecules will influence the structure and properties of their aggregates must be understood and, in the long term, predicted. This thesis aims to contribute knowledge to reaching this goal. Enantiomers of a chiral amphiphilic derivative have been synthesized for the study of the expression of its chirality on graphite at a solid-liquid interface and in a Langmuir monolayer at an air-water interface, where the molecular orientation is completely distinct. Langmuir monolayers of the compounds in their enantiopure and racemic forms were prepared. The pressure-area isotherms and the visualization of the monolayers by Brewster Angle Microscope (BAM) from zero pressure to collapse pressure were registered. At low pH the homogeneity and the domain structure of the monolayers are similar for the two enantiomers and the racemic mixture whereas a difference in their π/A isotherms was detected. This difference can be attributed to the formation of racemic domains when both enantiomers are mixed and a heterochiral monolayer is formed.
The self-assembly of the same compounds on graphite was investigated by Scanning Tunneling Microscopy (STM). Highly ordered chiral monolayers of the enantiomers form at the 1-phenyloctane/ Highly Oriented Pyrolytic Graphite (HOPG) interface. In these experimental conditions a spontaneous resolution of the racemic mixture is observed. These results illustrate the key role that orientation and substrate have upon chiral separation processes.
With the same compound we tried to see if diastereomeric recognition was possible at a liquid-graphite interface. We chose a chiral amine which could interact with the amphiphilic compound. Diastereomeric interactions between each enantiomer of the amphiphilic compound and the relative amine happen at the interface and could be visualized by STM by formation of typical domains. When the two enantiomers are mixed with one of the two enantiomers of the amine, both domains of diastereomeric interactions and domains of the enantiomer alone could be visualized.
Manifestation of chirality was studied on metal surfaces in ultrahigh vacuum preparing the enantiomers of a diacid compound similar to the amphiphilic one mentioned above. By changing the coverage and the annealing temperature the good conditions for forming chiral ordered structures and having the enantiomeric resolution of the racemic mixture were found. Importantly, sub-molecular resolution allowed direct identification of the two enantiomers on the surface using STM.
The formation of self-ordered nanostructures at surfaces is more complicated than in solution as it requires the additional control over interactions of either molecules or their aggregates with the surface. In an attempt to shed light on the key factors influencing the aggregate morphology, we have varied the molecular composition and constitution of a porphyrin derivative in order to explore the origins of these effects.
Solutions of these compounds in different solvents show different hydrogen bonding capabilities. The morphology of the porphyrin aggregates at the air-substrate interface were studied by acoustic mode Atomic Force Microscopy (AFM) after depositing a drop of a solution of the compounds on a freshly cleaved piece of highly oriented pyrolytic graphite (HOPG) substrate and allowing the solvent to evaporate. Depending on the solvent used, composition and constitution of the compound, the structures formed are very different. Round shape aggregates and larger dewetting type patterns were formed from solutions of the compounds in methanol # in which all the molecules exhibit non-specific aggregation - and when chloroform and toluene are used fibril like structures appear, whose alignment with the graphite axes depends on the length of the alkyl chains attached to the amide moiety in the molecules.
We created chiral supramolecular assemblies of porphyrins to be able to study stereochemical effects on superstructure and charge transport. The objective is to try and form helical structures at a surface such that they could be probed electronically. To this end, chiral porphyrin derivatives which differ only in the number and position of the stereogenic centers were synthesized. Our interest is to see how the stereogenic centres influence the supramolecular assemblies studied under different conditions.
Scanning Tunneling Microscopy was carried out at the substrate-liquid interface which showed well ordered chiral organization of molecules. The chirality of the monolayers decreases with the number of stereogenic centres in the molecule. Gelation ability was found for all of the chiral porphyrins and the network of the associated fibers of the resulting organogels was visualized by AFM. Each compound shows a different critical gel concentration and the different aggregation forces of the various compounds were deduced. In the end, Circular Dichroism was used to obtain information about the precise self-assembly process into an aggregated structure. The organization of molecules in solution changes with the number of stereogenic centres and the aggregation forces decrease as the number of stereogenic centres increases. In both solid-liquid interface and solution the presence of only one chiral centre is enough in inducing chirality in the superstructure. The capacity of molecules with such interesting properties to form nanofibers is a promising approach to miniaturize optoelectronic devices.
