Preparazione elettrochimica e caratterizzazione di polialchiltertiofeni con regiochimica regolare di sostituzione

The research activity reported in the PhD thesis has involved the electrochemical synthesis and the characterization of conjugated polymers which can be employed in the realization of technologically advanced applications such as smart windows, displays, supercapacitors, sensors, artificial muscles and photovoltaic cells. This research is motivated by the remarkable feature of conjugated polymers to vary their electronic conductivity within a large range (10-18 - 102 S cm-1) by means of "doping" processes. In this framework "doping" processes are redox reactions between an electrically neutral polymer and an oxidizing/reducing agent. Despite of the many works on the synthesis and characterization of conjugated polymers, a study on the effect of spacers position upon the electric properties of the substituted conjugated polymer was still missing. This lack motivated the compared analysis of the electric properties of substituted polymers differing in the position of the substituting groups along the chain. For this purpose it has been accomplished the investigation of the properties of the systems derived from the polymerization of the starting monomers 3',4' didodecyl 2,2':5',2" terthiophene and 3,3" didodecyl 2,2':5',2" terthiophene. The polymerization of these monomers has been realized through electrochemical oxidative coupling. The electrochemical polymerization has been preferred to the chemical route. The choice of the dodecylic chain as substituents has been motivated by the great solubility of dodecyl-derivatized polymers in the most common organic solvents. The electrochemical polymerization of oligomeric species as 3',4' and 3,3" DDTT could be achieved in relatively milder conditions with respect of the polymerization of a single thiophene ring. The electrochemical polymerization of 3',4' DDTT and 3,3" DDTT has been accomplished in the potentiodynamic mode because the deposition yield of polythiophenes resulted higher when the oxidation process is followed by a cathodic wave. This has been confirmed by means of the electrochemical quartz crystal microbalance (EQCM) which measured the increases of the electrode mass in correspondence of the whole range of the applied potential. The polyterthiophenes electrodeposition was also analysed with the probe beam deflection (PBD) technique which detects the changes of refractive index of an electrolyte in correspondence of the diffusion layer in front of the electrode as determined by fluxes of dissolved species. The PBD analysis is made necessary by the fact that thick polymeric deposits can give artifacts in the response of EQCM if the electrodeposited polymers don't possess a rigid layer behavior. The reversibility of the optical , electrical, magnetic, morphological, mechanical and mass effects associated with the simultaneous exchange of ions and electrons occurring during the doping processes was checked with in-situ visible and IR spectroscopy, conductimetry, in-situ electron spin resonance (ESR), scanning electron microscopy (SEM), PBD and EQCM. The comparison of the results for the two isomeric polymers showed that the pattern of monomer substitution mainly affects the kinetics of the electrochemical growth. This led to the electrochemical synthesis of isomeric polymers with different physical properties such as the degree of polymerization, film morphology, the optical absorption and the electrical conductivity. In the case of one polymer, it has been accomplished the study of the electron transfer between poly 3,3" DDTT and a redox couple dissolved in the electrolyte through the scanning electrochemical microscope (SECM). The SECM study was motivated by the fact that in the field of conducting polymers only few works were addressed to the effect of polymer conductivity upon the rate of electron transfer between a conducting polymer and a redox couple (mediator). This is because of the complexity of the phenomena which involve simultaneously the polymer doping, the electronic transport through the polymeric film and the heterogeneous electron transfer at the polymer/electrolyte interface. The main critical point for the realization of SECM experiment with poly 3,3" DDTT has been the choice of the mediator redox couple whose potential range of stability would have fit with the potential range of electroactivity for the polymer substrate. Moreover it was determining the blocking of possible electrochemistry between the mediator redox couple and the underlying metal through the proper thickening of the electrodeposited polymer in order to analyse the electrochemical processes involving the sole polymer.