ELECTROCHEMISTRY FOR THE DEVELOPMENT OF INNOVATIVE THREE-DIMENSIONAL AND CHIRAL THIOPHENE-BASED ORGANIC SEMICONDUCTORS

Organic conducting polymers are efficient materials for a wide range of applications, ranging from energetics and electronics (bulk-heterojunction solar cells, dye-sensitized solar cells, organic light-emitting diodes, organic field effect transistors) to sensoristics, offering the advantage of being light-weight, flexible, low-cost compounds, thus providing a valid and interesting alternative to traditional inorganic semiconductors. Electrochemistry plays an important role in the study of these smart materials, being a powerful tool to determine the mechanisms of the electron-transfer processes occurring during the reduction/oxidation cycles of these molecules when, deposited as films on electrodic surfaces. Cyclic voltammetry is an essential tool for the experimental evaluation of the HOMO and LUMO levels and of the HOMO-LUMO gap, to be compared with spectroscopical and theoretical values, in order to better understand the relationships between structure and electronic properties and to evaluate the most suitable application for the new molecules. In addition, the combination of cyclic voltammetry with electrochemical quartz crystal microbalance EQCM technique and electrochemical impedance spectroscopy (EIS) affords further information about the mechanisms of the electrochemical coupling of polymerogenic units and,the resitance of the polymer films to mass and charge transfers. In the present PhD thesis, innovative thiophene-based organic semiconductors have been designed and characterized by cyclic voltammetry, in combination with many other electrochemical and spectroscopic techniques (EQCM, EIS, in situ UV-Vis-NIR, ESR and circular dichroism spectroscopy, AFM and SEM imaging) in order to obtain a complete insight into the new materials, to better understand their intrinsic properties for a more efficient target-oriented design. Particular attention was devoted to three-dimensional molecules (i.e., the so called “genetically-modified” spider-like oligothiophenes, derived from a previous work on branched all-thiophene molecules)[1,2] that fulfil the requirements of branching (which provides the polymers with a remarkable solubility in common organic apolar solvents, ensuring an easy processability), of a significative effective conjugation and of the possibility of fine tuning the HOMO and LUMO levels, by suitable structural modifications in the light of possible applications in organic photovoltaics. Another class of three-dimensional multithiophene compounds is that of the inherently chiral molecules. It would be a highly innovative result to find a material which would combine the potentialities of chirality (i.e., the ordered spontaneous chain assembling induced by chirality, the noncentrosymmetry associated to chiral materials, which is a prerequisite for second order nonlinear optical applications, the ability of chiral molecules to discriminate between antipodes, as required in sensors designed for the detection of chiral analytes, the possibility for a chiral semiconductor to be employed in asymmetric electrosynthesis) with the advantages typical of the conducting polymers (i.e., electrical conductivity, redox and pH switching capability, electrochromism, low cost, easy processability, light weight). According to the literature, the most common strategy to obtain chiral conducting polymers is to attach chiral pendants (i.e. natural sugar and aminoacids, or manmade designed for specific applications) to the conjugated electroactive backbone. The presence of carbon stereocenters invariably characterizes the chiral substituents. Only in few cases, however, significant chirality manifestations have been found in polymers designed according to this strategy, also because the experimental conditions (i.e., solvent, pH, temperature) strongly affect the chirality manifestation of the polymers. In the present case, instead, chirality is due to a tailored torsion internally produced along the conjugated backbone, that does not completely interrupt the conjugated sequence, thus ensuring the conductivity of the material in the doped state. The stereogenic element is an atropisomeric bithiophene or bipyrrole scaffold, introduced into the conjugated backbone of the monomer. The polymerization sites of the monomers are homotopic, thus granting the constitutional regularity of the polymers. This requirement is satisfied only by molecules belonging to the C2 point group which guarantees that all the products of the polymerization, from olygomers to polymers, are C2 symmetric as well. Finally, the monomers are properly and easily functionalized in different positions in order to tailor their properties to specific applications. In the present work, a complete characterization of the monomers (and of the oligomers derived from them by electroxidation) has been performed: particularly interesting is the circular dichroism behaviour of the enantiopure films electrodeposited on ITO electrodes, detected in situ during cyclic voltammetry scans. Upon polarization (p-doping), the maximum intensity in the CD signal decreases (while another band increases at higher wavelength, corresponding to the formation of the polaronic state) possibly because of a partial flattening of the atropisomeric structure (coherent with the enhanced conductivity in the doped state). This phenomenon is completely reversible, and chirality is fully recovered switching the potential back to the neutral state. In conclusion, this work on different families of molecules has given evidence of how electrochemistry is an essential tool in modern materials science, being a fast and reliable method to determine the crucial parameters for applications in the most advanced technological fields, affording a deeper understanding of the relationships between molecular structure and electronic properties and effectively assisting the target-oriented molecular design. In addition, the class of the inherently chiral monomers opens up a new path to the applications of chiral organic semiconductors in many different fields, both as racemates (active layers in bulk-heterojunction solar cells, on the basis of the preliminary knowledge of the HOMO and LUMO gaps and levels in these molecules; the cavities present in the polymers could be tailored to comfortably host bulky fullerene units for the preparation of donor-acceptor blends, for the construction of traditional or Molecularly Imprinted Polymers sensors) and as enantiopure materials (preparation of sensors for the recognition of biorelevant chiral analytes, preparation of chiral electrodes for performing electrochemical enantioselective oxo-reduction processes, exploitation of the inherent chirality and of the highly ordered solid-state structure in photoelectrochemical applications). [1] T. Benincori, M. Capaccio, F. De Angelis,L. Falciola, M. Muccini, P. Mussini, A. Ponti, S. Toffanin, P. Traldi, F. Sannicolò, Chem. Eur. J., 2008, 14, 459 – 471; [2] T. Benincori, V. Bonometti, F. De Angelis, L. Falciola, M. Muccini, P. R. Mussini, T. Pilati, G. Rampinini, S. Rizzo, S. Toffanin, F. Sannicolò, Chem. Eur. J., 2010, 16, 9086 – 9098