Preparation and spectroscopic and spectroelectrochemical characterization of copolymers of 3-alkylthiophenes and thiophene functionalized with an azo chromophore

New polythiophene derivatives containing a nonlinear optical chromophore were prepared via copolymerization of 3-alkylthiophene and 2-[N-ethyl-N-[4-[(4-nitrophenyl)azo]Phenyl]amino]ethyl 3-thienylacetate. The presence of chromophore groups in the copolymer chain was confirmed by ¹H NMR, ¹³C NMR, and FTIR spectroscopies as well as by elemental analysis. The introduction of azo dye as a side group to the conjugated polythiophene backbone significantly alters electrochemical and spectro-electrochemical behavior of these compounds as compared to poly(3-alkylthiophene) homopolymers. In particular two redox processes are observed by cyclic voltammetry. The first anodic peak at lower potentials corresponds to the oxidative doping of the polymer whereas the second one is associated with the oxidation of the chromophore side group to a radical cation. Electrochemical oxidation of the copolymers results in a gradual bleaching of the absorption peak ascribed to the π-π* transition in the polyconjugated system At the same time the absorption peak characteristic of the chromophore undergoes a bathochromic shift from 485 to 516 nm. Thus, in the spectra of the oxidatively doped copolymers an additional peak unobserved in poly(3-alkylthiophene) homopolymers is present in addition to the doping-induced absorption in the near-infrared part at the spectrum. This additional peak has an important influence on the Raman spectroelectrochemical behavior of the prepared copolymers as probed by the green (λ_max = 514 nm) excitation line. Electrochemical oxidation significantly worsens the resonance conditions for the vibrations of the conjugated backbone whereas it improves the resonance conditions for the vibrations associated with the chromophore side groups. As a result at lower potentials only the lines characteristic of the 2,5-disubstituted thienylene ring are registered, and at higher potentials, lines due to the chromophore dominate the spectrum.