Supramolecular Strategies to Control the Assembly of Organic-Based Materials

Abstract

The focus of this thesis is the use of supramolecular chemistry to functionalize polymers to obtain organic-based materials with a control of their structure and properties.

The non-covalent functionalization of polymers via supramolecular chemistry is a potentially efficient route to introduce a particular property with which a new functional molecular material can be originated. This strategy has specially been explored to incorporate conjugated structures in polymeric constructs for new semiconducting materials. Although important advances have been made in this field, the incorporation of π-electron functional units to polymeric backbones with well-controlled arrangement between the donor and acceptor parts is still an area of huge interest for material science.

Following this idea, the supramolecular functionalization of the homopolymer poly(4-vinyl pyridine) (P4VP) with an electron donating unit based on an tetrathiafulvalene (TTF) has been explored. Tetrathiafulvalenes (TTFs) are excellent π-electron donor units that are widely studied in molecular electronic applications due to their p-type character and stable doped states. Through complementary hydrogen bonding components of a derivative TTF (TTFCOOH), uniform films incorporating the redox active components have been prepared. When the films were doped using different oxidising agents, Electrostatic Force Microscopy (EFM) studies indicated that a reorganisation at the surface of the films occurred and of charges in their surfaces appeared.

Once the application of the P4VP-TTFCOOH films as charge-carrier hybrid material was demonstrated, the next step was to use the block copolymer poly(styrene-b-4-vinyl pyridine) (PS-b-P4VP) due to its amphiphilic behavior. Therefore, different molar ratios of PS and P4VP blocks have been used to prepare new PS-b-P4VP-TTFCOOH thin films. The influence of the phase segregation of the PS-b-P4VP-TTFCOOH film to the charge transportation upon oxidation was performed correlating the Small-Angle X-ray Scattering (SAXS) and EFM results.

On the other hand, microfluidics is a microfabrication technique that relies on the manipulation of fluids, which can be used to control the final microstructure and self-assembly of compounds, by the simple adjusting of the values of the hydrodynamic flow focusing (confinement effect). Moreover, microfluidics offers respect to the bulk conditions important advantages such as larger relation surface-to-volume, lower consumption values and a better control of the concentration gradients. The improvement of the intermolecular contact of disk-shaped C3-symmetric tris(TTF) system has been studied using microfluidics. This molecule possesses three bonded derivative TTF units through a central aromatic ring that, under determined conditions, large coiled helical fibers could be obtained by their aggregation. A de-doping process of the C3-symmetric tris(TTF) molecule has been studied using microfluidics, towards the study of the influence of the technique on the organization of this compound. Afterwards, a new characterization methodology was suggested, based on bimodal-Atomic Force Microscopy (bimodal-AFM) in order to draw a distinction between electrostatic and mechanical contributions of the coiled fibers and, corroborating that the charge transportation phenomenon occurred.

Furthermore, hydrogels have been in the spotlight due to their biocompatibility in, for instance, tissue engineering applications. Then, the structural enhancement of an ionically-driven coacervated hydrogel obtained via microfluidics platform by tuning flow focusing parameters was studied. In a further step, the functionalization of this hydrogel with TTFCOOH molecules under bulk and microfluidic conditions and its charge-carrier character were also analyzed. Additionally, with the objective to immobilize the hydrogel on surfaces, micro-contact printing, technique was tested.

Finally, the organization and phase segregation of the PS-b-P4VP coordinated with a metalloporphyrin system has also been approached. Porphyrins, in general, possess exceptional inherent optical and electronic properties that make them suitable for application related with solar cells. Moreover, due to their electrochemical and photophysic characteristics they can easily be modified with other functional groups. Porphyrins can be found free or complexed with metals and in both states they have self-assembly properties. The coordination between PS-b-P4VP and chiral and an achiral zinc (II) metalloporphyrin has been explored in order to study the chirality transfer from the porphyrin to the hybrid materials. In the last instance, and in an exploratory attempt to use porphyrins as molecular rotors, different strategies of surface immobilization of the porphyrin-block copolymer system were also approached by micro-contact printing.