Anna Berlin

Monolayers and multilayers of conjugated polymers as nanosized electronic components.

Conjugated polymers (CPs) are interesting materials for preparing devices based on nanoscopic molecular architectures because they exhibit electrical, electronic, magnetic, and optical properties similar to those of metals or semiconductors while maintaining the flexibility and ease of processing of polymers. The production of well-defined mono- and multilayers of CPs on electrodes with nanometer-scale, one-dimensional resolution remains, however, an important challenge. In this Account, we describe the preparation and conductive properties of nanometer-sized CP molecular structures formed on electrode surfaces--namely, self-assembled monolayer (SAM), brush-type, and self-assembled multilayer CPs--and in combination with gold nanoparticles (AuNPs). We have electrochemically polymerized SAMs of carboxyalkyl-functionalized terthiophenes aligned either perpendicular or parallel to the electrode surface. Anodic coupling of various pyrrole- and thiophene-based monomers in solution with the oligothiophene-based SAMs produced brush-like films. Microcontact printing of these SAMs produced patterns that, after heterocoupling, exhibited large height enhancements, as measured using atomic force microscopy (AFM). We have employed layer-by-layer self-assembly of water-soluble polythiophene-based polyelectrolytes to form self-assembled multilayers. The combination of isostructural polycationic and polyanionic polythiophenes produced layers of chains aligned parallel to the substrate plane. These stable, robust, and dense layers formed with high regularity on the preformed monolayers, with minimal interchain penetration. Infrared reflection/adsorption spectroscopy and X-ray diffraction analyses revealed unprecedented degrees of order. Deposition of soluble polypyrroles produced molecular layers that, when analyzed using a gold-coated AFM tip, formed gold-polymer-gold junctions that were either ohmic or rectifying, depending of the layer sequence. We also describe the electronic conduction of model alpha,omega-capped sexithiophenes featuring a range of electron donor/acceptor units and lengths of additional conjugation. The sexithiophene cores exhibit redox-type conductivity, developing at the neutral/cation and cation/dication levels with values depending the nature of the substitution and the redox system. Extending the conjugation beyond the sexithiophene frame introduces further oxidation processes displaying enhanced conductivity. Finally, we discuss the ability of CP-based monolayers to coordinate AuNPs. Although thiophene- and pyrrole-based oligomers aggregate toluene-soluble AuNPs, alkyl substitution inhibits the aggregation process through steric restraint. Consequently, we investigated the interactions between AuNPs and polypyrrole or polythiophene monolayers, including those formed from star-shaped molecules. The hindered aggregation provided by alkyl substituents allowed us to adsorb thiol-functionalized oligothiophenes and oligopyrroles directly onto preformed AuNPs. Novel materials incorporating AuNPs of the same size but bearing different conjugated ends or bridges have great promise for applications in electrocatalysis, electroanalysis, and organic electronics.

Electrochemical synthesis and characterization of polyconjugated polyfuran

Polyconjugated polyfuran (PF) was produced by electroreduction of 2,5-dibromofuran in acetonitrile using Ni(bipy)32+ as catalyst. Polymerization occurs only in the presence of excess Ni(bipy)32+ at potentials in the range −1.7 to −2.0 V versus Ag/Ag+ via binuclear organonickel complex. IR spectroscopy, elemental analysis and mass spectroscopy confirm that PF is constituted by 2,5-linked furan units and indicate a degree of polymerization > 100. Red PF films (λmax = 420 nm, Eg = 2.2 eV) are reversibly oxidized at 0.45 V and reduced at −2.5 V. Doping with AsF5 gives PF a conductivity of 10−3 S/cm. Polymers produced by anodic coupling of furan were also investigated for comparison. IR spectroscopy and elemental analysis indicate that they display a lower degree of conjugation due to the concomitant acidcatalyzed polymerization.

Electrochemical preparation and electrochromic characteristics of dithienopyrrole-dithienothiophene, dithienopyrrole-thionaphtheneindole and dithienothiophene-thionaphtheneindole copolymers

Abstract Poly(dithienopyrrole-dithienothiophene) copolymers can be obtained as good conducting electrode films by electrolytic oxidation of acetonitrile solutions of monomer mixtures (electrolyte: tetrabutylammonium perchlorate). They show good electrochromic characteristics. Films of poly(dithienopyrrole-thionaphtheneindole) and poly(dithienothiophene-thionaphtheneindole) copolymers can be obtained in the same way as the previous ones. They are adherent to the electrodes but are insulating and non-electrochromic; the reason for this behaviour is proposed to be the break of conjugation in the polymeric chain due to the thionaphtheneindole moiety.

Mixed-valence conduction in redox-substituted polythiophenes. Enhancement of redox electron-exchange rate by polyconjugated chains

Abstract The in situ conductivity of some poly(cyclopentadithiophenes) modified at the 4 position with reducible ( p -nitrophenyl and p-N -methylpyridyl) or oxidizable (ferrocene) redox groups was investigated in acetonitrile. In the nitrophenyl- and pyridyl-functionalized polymers the reduction, which is confined to the pendant redox group, makes the polymer mixed-valence conductive. The conductivity ((3–6) × 10 −3 S cm −1 ), which is the highest reported for a redox polymer, remains within the limits expected for a direct intersite electron hopping. In the ferrocene-functionalized polymers the oxidation involves both the redox group and the backbone. Redox conduction at the ferrocene group is enhanced by hopping through the conductive polymer backbone. The hopping rate is increased by a decrease of the ferrocene-backbone distance and by conjugation of ferrocene with the backbone itself making conductivity increase up to 1 S cm −1 .

Electrochemistry of end-capped oligothienyls. New insights into the polymerization mechanism and the charge storage, conduction and capacitive properties of polythiophene

Abstract The kinetics of anodic coupling to dimers of thiophene oligomers ( n = 3–5), methyl protected at one α-terminal position, is second order in oligomer concentration and evidences high activation enthalpies and negative activation entropies. Activation free energies are linearly related to the inverse of the oligomer length n (the dimerization rate decreases as n is increased). Thin films of methyl end-capped thiophene oligomers ( n = 4, 6, 8 and 10) display reversible oxidations from a single one-electron step (tetramer) to a single two-electron step (octamer and decamer) through two separate one-electron steps (hexamer). ESR indicates strong magnetic dimerization for the one-electron-oxidized hexamer. The close resemblance of the electrochemical and ESR behaviour of the hexamer with that of polythiophene suggests that oxidation of the latter occurs via hexameric spin-dimerized polarons. The conductive and capacitive properties of the end-capped oligomers ( n = 6, 8 and 10) were investigated by in situ conductivity and chronopotentiometry. While conductivity of octamer and decamer is displayed at the two-electron (bipolaron) state, the hexamer, insulating at this state, is conducting at the mixed-valence polaron-bipolaron state; capacitive responses are evidenced at the bipolaron state for the octamer and decamer only. The difference of conductive and capacitive behaviour between the hexamer and the higher oligomers is explained by charge localization in hexameric segments.

Polythiophene‐ and Polypyrrole‐based Mono‐ and Multilayers

Nanosized conducting polymers are particularly interesting for molecular electronics. The production, on different surfaces, of well‐defined mono‐ and multilayers of these polymers is of high importance given their nanometer‐size one‐dimensional resolution. In this paper we review the work carried out by our group on the obtainment of novel thiophene‐ and pyrrole based nanometer‐size molecular structures on different surfaces with new and promising properties. The results on the obtainment of self‐assembled monolayers, brush structures, and electrostatic self‐assembled multilayers of pyrrole‐ and thiophene‐ based polymers are described. Finally recent results on the obtainment of hybrid materials constituted of oligo‐ or polythiophenes and gold nanoparticles are reported.

The role of water in the electrochemical polymerization of pyrroles

Abstract The influence of water in the anodic coupling of two pyrrole monomers (3-methylthiopyrrole (1) and dihydrobenzo-[1,2-b:4,5-b′]-dipyrrole (2)) to polypyrrole in acetonitrile was investigated by cyclic voltammetry and controlled potential electrolysis. Oxidation requires 1 e− mol−1 in anhydrous conditions and (2+x) e− mol−1 (x=0.25 and 0.5 for (1) and (2) respectively) in the presence of 1% H2O. The results are explained by competitive proton scavenging action of pyrrole and water.

Electrooxidation products of methylindoles: Mechanisms and structures

Electrochemical oxidation of 1-methylindole, 2-methylindole and 3-methylindole on a platinum anode in acetonitrile containing NaClO4 has been studied. In any case no polymeric deposit on the working electrode was obtained. The identification of the obtained soluble products is described and the mechanisms of formation are discussed. Analogously to the case of unsubstituted indole, electrooxidized 1–3 react through positions 2 and 3. The non polymerization of 1 is explained with the formation of a tetramer which is oxidized to a stable radical cation and dication without undergoing further coupling reactions.

Expeditious synthesis of dihydrobenzo-[2,1-b:3,4-b′]-,[1,2-b:5,4-b′], and [1,2-b : 4,5-b′]-dipyrroles

A simple, high-yield, two-step synthesis of the title compounds (1)–(3) is described, based on the catalytic reduction of the bis-enamines (4)–(6), in turn obtained by condensation of dimethylformamide diethyl acetal with the appropriate dinitro-xylene.

3,4-Ethylenedioxy-substituted bithiophene-alt-thiophene-S,S-dioxide regular copolymers. Synthesis and conductive, magnetic and luminescence properties.

Polyconjugated regular bithiophene-alt-thiophene-S,S-dioxide copolymers were produced by anodic coupling of variously 3,4-ethylenedioxy-substituted 2,5-bis(2-thienyl)thiophene-S,S-dioxide. The polymers were characterized by cyclic voltammetry, FTIR reflection-absorption and UV-vis spectroscopy, MALDI-TOF mass spectroscopy, electrochemical quartz crystal microbalance, in situ ESR and in situ conductivity techniques, photo- and electro-luminescence measurements. The regular alternation of electron-rich and -poor thiophene rings in the polymer chain operated by the ethylenedioxy and S,S-dioxide moieties produces a finite window of conductivity. Alkyl-protection of the β-positions of the thiophene-S,S-dioxide ring gave low-defect and soluble oligomers which were investigated in single-layer organic light-emitting devices (OLEDs). Photoluminescence quantum efficiency of ca. 1% and external electroluminescence quantum efficiencies of 0.01% photon/electron at a luminance of 100 cd m−2 were obtained.