Síntese e caracterização de novos polímeros conjugados derivados de tiofeno-pirrol-tiofeno (SNS) ligados aos corantes azo alaranjado de metila e vermelho de metila

Abstract

The search for new technologies for optoelectronics such as electrochromic devices and displays has been gaining attention in among industrial and research fields due to the growing application of conjugated polymers as an optically active layer in these devices. The main properties of conjugated polymers are related to the possibility of manipulation in their electrical and optical characteristics through changes in their chemical structures. Such changes can be made through couplings in the monomeric structure to different functional groups, such as electron donor and/or electron withdrawing groups or even by the copolymerization process. Among the alternative methodologies reported in the literature, we highlight in particular the synthesis of thiophene-pyrrole-thiophene (SNS) monomers due to their 'trimeric' structure that guarantees intermediate oxidation potentials between their constituents (pyrrole and thiophene) and provides possibilities for functionalization through the nitrogen atom, making them candidates for use as electrochromic materials. In addition, it produces centrosimetric polymeric precursors that seek to minimize the steric effect of the central substituent through thiophene spacers. One of the forms of functionalization is the preparation of hybrid systems with the insertion of an organic dye in the monomeric unit of the polymer in order to modify the optical properties. In this work, two new SNS monomers linked to the azo dyes methyl orange (AM) (pKa = 3.46) and methyl red (VM) (pKa = 4.85) were synthesized by nucleophilic acyl substitution reaction (95% yield) and Steglich coupling reaction (92% yield), respectively. All compounds were characterized by 1H NMR, 13C NMR and FTIR. The films of Poli (SNS-AM) and Poli (SNS-VM) were obtained by electrochemical polymerization (cycly voltammetry technique) in a solution containing 0.01 M of the constituent monomer and 0.1 M of the supporting electrolyte (C4H9)4NBF4 in ACN using a scan rate of 20 mV.s-1 and potential range from 0.0 V to 0.7 V. The films were characterized by cyclic spectrovoltammetry in a monomer-free solution containing 0.1 M of (C4H9)4NBF4 in ACN and showed electrochromic behavior between yellow color in the undoped state to magenta in the doped state. With the double-step spectrochronamperometric experiments, was possible to obtain some respective electrochromic parameters: Eg = 2.23 eV and ΔT% = 23% (1010 nm) for Poli (SNS-AM) and Eg = 2.20 eV and ΔT% = 13% (1016 nm) for Poli (SNS-VM). In addition, a stability study of the doping / undoping process showed a considerable loss of stability of the material when subjected to 100 cycles. In order to investigate the influence of optical properties against acid and base vapors, the Poli (SNS-AM) and Poli (SNS-VM) films were exposed to HCl and ethylenediamine vapors. The films of Poli (SNS-AM) and Poli (SNS-VM) showed a magenta color when exposed to HCl vapor, returning to a yellow color when exposed to ethylene diamine vapor. The characteristics of the films make them promising materials for application as active layer in optoelectronic devices, since for our knowledge Poly (SNS) derivatives with magenta color are scarce, and for optical sensor of pH.