Hans-Peter Welzel

Reactive groups on polymer coated electrodes, 7. New electrogenerated electroactive polythiophenes with different protected carboxyl groups

By the protection of the carboxyl group of 3-thiopheneacetic acid with differently substituted benzyl groups a series of new thiophene derivatives were synthesized. While 3-thiopheneacetic acid is not electropolymerizable, the new obtained monomers can be easily electrooxidized to form stable electroactive polymers. The electrochemical characterizations show that the substitution of the benzyl groups exerts little effect on the electropolymerization process and that the obtained polymers exhibit the typical properties of polythiophene derivatives, e. g., redox behavior and electrochromism. SEM studies show that all synthesized monomers possess a very good film formability and the resulting polymers exhibit a rather compact and homogeneous morphology on the Pt electrodes also containing scattered particles. Using p-nitrobenzyl-protected polymer as an illustrative example, it was demonstrated that the used protecting group can be easily split off in the solid state and the desired reactive carboxyl group can be produced on the polymer surface. Therefore, through the protection of the carboxyl group, the electropolymerization and the following removal of the protecting group, a new type of polymer matrix material with reactive carboxyl groups was simply prepared by using commercially available 3-thiopheneacetic acid monomer.

Reactive groups on polymer coated electrodes, 8. Novel conducting polymer interfaces produced by electrochemical copolymerization of functionalized thiophene activated esters with 3-methylthiophene

With the synthesis of two new functionalized thiophene activated esters and their electrocopolymerization with 3-methylthiophene, two types of redox active polymers have been prepared. FTIR studies of the resultant polymers reveal that both types of activated ester groups withstand the applied electrooxidative conditions and are correctly integrated into the corresponding polymers. The electrocopolymerization experiments show that the composition of the obtained polymers strongly depends on the ratio of the components in the reaction medium. With the increase of the ratios of pentafluorophenyl thiophene-3-acetate/3-methylthiophene or succinimido thiophene-3-acetate/3-methylthiophene, a higher concentration of functionalized thiophene units is incorporated into the polymer chains. The measurement of the conductivity on these polymeric films gave a value in the range of 10 -3 to 10 -2 S.cm -1 , which is comparable to that of the unsubstituted polythiophene and consistent with the conjugation grade suggested by electrochemical and UV-vis spectroscopic data. As expected, the pendant reactive ester groups on the electrode surfaces react rapidly with different amino compounds without loss of the electroactivity of the polymers. Therefore, these novel polymeric materials can be used as electrode interfaces for further functionalizations, especially for the immobilization of amines, peptides, and enzymes.

Reactive groups on polymer covered electrodes—I. Electrochemical copolymerization of thiophene-3-acetic acid with 3-methylthiophene

Cyclic voltammetry (cv) experiments at Pt electrodes were carried out to produce copoly(3-methyl-2,5-thiophenediyl-2, 5-thiophenediyl-3-acetic acid) (3a) from the monomers 3-methylthiophene (1) and thiophene-3-acetic acid (2a), and to study the mechanism of electrocopolymerization. It was found by means of esteriflcation and amidation and further chemical reactions of the copolymers thus obtained that the COOH groups are covalently bound to the copolymer structure. The charge/discharge reversibility and the overoxidation (irreversible redox process occurring when the covered electrode is scanned beyond the positive potential limit) of 3a and some of its derivatives are demonstrated by representative cyclic voltammograms. In a series of copolymerization experiments the positive potential limit (Epos) was lowered step by step from 2.20 to 1.70V (vs. s AgAgCl) below the potential for the oxidation of 2a (Emox = 2.11 V vs. s AgAgCl) and, yet, the copolymer 3a was obtained. The results of the examination are discussed with the aid of data from cv, MS, FTIR-GIR (Fourier-Transform-Infrared-Grazing Incidence Infrared) spectroscopy, 1H NMR13C NMR spectroscopy, and elementary analyses.

Reactive groups on polymer covered electrodes 6-Copolymerization of 2,2′-bithiophene with methyl thiophene-3-acetate and 3-methylthiophene

Abstract Polythiophene films at the surface of electrodes containing ester groups are interesting potential carrier materials for reagents. Methyl thiophene-3-acetate (2) can be copolymerized with 3-methylthiophene (1) by means of cyclic voltammetry at potentials of 0–2.2 V. Higher potentials (0–2.4 V) lead to overoxidation of the copolymers. The ester groups were proved by FTIR spectra. Electrochemical investigations of 2,2′-bithiophene (4) and 2 at equimolar ratios shows no successful copolymerization at potentials of 0–1.3 V. If the copolymerization experiments of 4 with 1 or 2 were carried out at molar ratios of 1:50 at 1.3 V 4 with its low oxidation potential can be polymerized without copolymerization of the other monomers. However, if the oxidation potential is increased stepwise arising from 1.3 V the oxidation of 1 or 2 takes part forming copolymers containing both the monomer components. HPLC investigations of solutions containing mixtures of 4 and 2 and also 4 and 1 in acetonitrile/TEABF4 shows after exhaustive oxidation at a potential of 1.3 V the complete absence of 4. 1 and the ester 2 were not oxidized and copolymerized at this potentials. From the results of the copolymerization experiments as well as the HPLC investigations, it can be concluded that the dominant mechanism of the electrochemical polymerization is the radical cation dimerization.

Synthesis and electropolymerization of new p-nitrophenyl-functionalized thiophene derivatives

Electrooxidative polymerization of three new p-nitrophenyl-functionalized thiophene derivatives leads to materials which show the electroactivities of both nitrobenzene and polythiophene. However, other than the reversible redox transition in solution, the nitro groups exhibit a complex nature of the cyclic voltammetric curve in the obtained solid polymeric films. It was found that the electrochemical behavior of the nitro groups has a dramatic effect on the conjugated π-electron systems of the substituted polythiophenes. Upon electrochemical addressing of the nitro groups the electroactivities of the corresponding polymers decrease rapidly compared to their analogous polymers without nitro groups. FTIR studies of these polymers revealed that after the reduction/oxidation process of the nitro groups still a large amount of electrolyte salt remains in the polymer films. As typical of functionalized polythiophenes. however, these new polymers can be switched under controlled conditions between their oxidized and neutral forms and exhibit the typical properties of electroactive polythiophenes, e.g., electrochromism.

Reactive groups on polymer covered electrodes-3. Electrochemical copolymerisation of nitrofunctionalized 3-substituted thiophene derivatives

Abstract Electrochemical copolymerization of 3-(2,4-dinitrostyryl) thiophene (4) and 3-(2,4,6-trinitrostyryl)-thiophene (5) with 3-methylthiophene (6) produced interesting new functionalized polythiophene layers at the surfaces of electrodes suitable for further reactions and which have potential NLO properties.

EPR investigations of β-deuteriated 5,5″-dimethyl-2,2′:5′,2″-terthiophenes

The unambiguous assignment of the β-hyperfine coupling constants of the radical cations of 5,5″-dimethyl-2,2′ : 5′,2″-terthiophene (11) has been achieved by EPR spectroscopic investigations of the deuteriated compounds 12, 13 and 14. The syntheses of these substances were achieved by cross-coupling following the Negishi and Kumada reaction starting from deuteriated derivatives of chloromethylated thiophene derivatives. The generation of the radical cations was carried out by electrochemical oxidation at a constant potential in the EPR spectrometer. In the presence of acidic compounds a D–H exchange at the β-positions of oligothiophenes was observed. The experimentally determined spin density distribution was rationalized by means of semiempirical quantum mechanical calculations (RHF-INDO-SP). Theoretical and experimental assignments are in good agreement.

Studies on β-methylated end-capped bithiophenes

The reactivity of a variety of differently β-methylated oxidized α,α′-blocked (end-capped) bithiophenes has been studied. Electroanalytical methods such as cyclic voltammetry and studies with a rotating-ring disc electrode (RRDE) are employed, revealing two oxidation peaks. While the first electron transfer yields radical cations of increased stability as the number of β-methyl groups is increased, the second electron transfer is irreversible. For the first time, a dependence of the stability of radical cations (first oxidation step) on the degree of β-methylation is reported. Compounds with methylated 3-β-positions form more stable radical cations than 4-β-methylated ones. The stability of the oxidized end-capped bithiophene 6 can be explained by a reversible dimerization. The presence of β-substitution also exerts an influence on the reaction products of the chemical oxidation with FeCl3·6H2O, which have been isolated and purified by chromatography. In the case of bithiophene derivatives with methylated 3,3′-positions the main products are bithiophene-5-carbaldehydes; bithiophene derivatives with free 3-positions form bis(bithiophene)methanes. The reactive centers are α-methyl groups, not, as one might expect, free β-ring positions. In neither case has a β,β′-linkage between two thiophene rings been observed.