David L. Officer

Towards functionalized poly(terthiophenes): regioselective synthesis of oligoether-substituted bis(styryl)sexithiophenes

A variety of new bis(oligo(oxyethylene)styryl)sexithiophenes have been prepared by chemical oxidation of ether-substituted styrylterthiophenes with FeCl3. In all cases dimers are formed in high yields, rather than the expected polymers. In addition, although three different regioisomers can potentially be formed from such an oxidation, the isolable products are shown to consist of only the head-to-head regioisomer. Theoretical calculations on alkoxystyrylterthiophenes show that this can be understood in terms of an uneven electron spin density distribution at the two alpha-positions available for polymerization. Electron density calculations on the resulting head-to-head alkoxystyrylsexithiophenes show that the spin density is concentrated in the core of the molecule rather than at the alpha-positions, a result that explains the absence of significant amounts of higher oligomers.

Photovoltaic devices based on poly(bis-terthiophenes) and substituted poly(bisterthiophene)

Polymers and copolymers from terthiophenes and substituted terthiophenes are of interest in the field of photovoltaic devices. This paper presents work carried out using novel polymers of bis-terthiophene as light harvesters, Homopolymers and copolymers were investigated for their performance in electrochemical cells. In addition, the effect of polymer growth conditions on the photovoltaic responses obtained was also investigated. The best device generated V oc = 212 mV, I sc = 475 μA cm -2 , Fill Factor = 0.346, ECE = 0.0692% under a halogen white (solar simulated) light intensity of 500 W m -2 . And the monomer mole ratio during growth had a big influence on the photovoltaic response.

The effect of oxidation on the structure of styryl-substituted sexithiophenes: A resonance Raman spectroscopy and density functional theory study

The structures and vibrational properties of a series of styryl-substituted sexithiophenes and their charged species have been examined using resonance Raman spectroscopy in conjunction with density functional theory calculations. The calculated geometries of the radical cations and dications indicate that the quinoidal charged defects are more strongly localized in the center of the thiophene backbone than is observed in other sexithiophenes. This defect confinement, induced by the positions of the styryl substituents, is particularly evident in the dication species. However, the defect confinement weakens when alkoxy groups are added onto the phenyl rings by causing the extension of the charged defect into the styryl groups. The Raman spectra of the neutral styryl sexithiophenes are dominated by intense thiophene symmetrical stretching modes in both the measured and predicted spectra. Oxidation generates radical cations and dications, both of which can be observed in the solution state resonance Raman spectra. Unlike other sexithiophenes, which generally show a downshift of the intense thiophene stretching mode from the radical cation to the dication, a small upshift is observed for the styryl-substituted sexithiophenes. The theoretical spectra predict an insignificant change during this transition and the eigenvector for this mode reveals that it is localized over the same area occupied by the confined defect. In contrast, the solid state resonance Raman spectra of electrochemically oxidized films reveal evidence of solely radical cations and there is an appreciable downshift of the intense thiophene stretching mode compared with the corresponding mode in the solution spectra. This implies that the increase in the effective conjugation length from the solution to the solid state is greater for the radical cations than for the neutral species. It therefore appears that the radical cations form pi stacks in the solid film and the resulting intermolecular interactions effectively allow a further extension of the electron delocalization.

Glassy Carbon Based Sensors

Sensing moiteies may be covalently attached to glassy carbon by reduction of functionalised diazonium salts [1,2]. The attachment of glucose oxidase with a cinnamic acid linker provides an example of simple tethering using this technique. An electrode formed in this way exhibits little susceptibility to interference from ascorbic acid and 4-acetamidophenol. A pH-reponsive (56.18 mV decade -1 ) glassy carbon electrode is formed by grafting quinone groups to the electrode with stilbene linkers.

Synthesis and polymerisation of fully conjugated polyether-substituted terthiophenes

A range of 3-styryl polyether-functionalised terthiophene monomers were synthesised via Horner-Emmons reactions, and electrochemical and chemical polymerizations were carried out on some of these compounds. Analysis of the products by UV/VIS, MALDI-MS and NMR revealed that they exclusively form head-to-head dimers. This is an excellent synthetic method for producing regioregular polyether-substituted sexithiophenes.

2,5-Bis(2-cyano-2-thienylvinyl)thiophene

In the crystal structure of the title compound, C18H10N2S3, the terminal thioxadphene rings are disordered. This disorder is of a flip-type and has been observed in some other thioxadphene derivatives. The molxadecule as a whole is approximately planar; the dihedral angle between the terminal five-membered rings is 6.0u2005(2)°. The crystal packing is determined by van der Waals interxadactions, π–π stair-like stacking between the thioxadphene rings, and some weak C—H⋯S and C—H⋯π short directional contacts.

Vibrational Spectra and Calculations on Substituted Terthiophenes

Conjugated polymers based on β-substituted terthiophenes are a very interesting class of materials with applications in solar cells and in electroluminescent screen technologies. In order to better understand the properties of these systems we have studied the terthiophenes using ub initio calculations. These calculations reveal a good correspondence with predicted and experimental electronic and vibrational spectral data. Analysis of the normal modes suggests localized terthiophene, phenyl and ethene modes are present.

CHARACTERISATION OF CONDUCTING POLYMERS USING ION BEAM ANALYSIS

Complex bis terthiophene porphyrin conducting polymers were used to investigate the uptake of zinc into the freebase porphyrin unit after polymerisation by acquiring depth profiles using Rutherford Backscatting Spectrometry (RBS) analysis. Issues such as the influence of the support material and extent of polymer oxidation on the dispersion of the counter ions through the polymer films are of importance. Gaining knowledge of the dispersion of counter ions provides new insights into the redox mechanisms for conductive polymers. The results were compared to those obtained from a sample where zinc was coordinated to the porphyrin prior to the polymerisation process. Unexpectedly high concentration of both nitrogen and oxygen were found which could be due to evidence of the trapped cations from the electrolyte ((Bu)4N+), and trapped water molecules, within the polymer films. The chlorine concentrations help to understand the dispersion of the perchlorate counter ion throughout the polymer films, and the combination of both RBS and Proton Induced X-ray Emission (PIXE) show that trace element impurities can be detected using ion beam analysis which other analytical techniques are unable to do.

Modelling oligothiophene polaron structures using ab initio calculations

Conjugated polymers offer a number of potential opportunities for utilisation in applications such as OLEDs and plastic photovoltaic cells.[1] Further enhancements in the efficiency of such devices will require elucidation of the precise mechanism of conduction in conjugated polymers. The key to understanding the conduction mechanism of conjugated polymers is the structure of the charge-carrying species. For p-doped non-degenerate polymers, such as polythiophene, these are positive polarons and bipolarons. The structures of these species may be modelled using quantum chemistry and the efficacy of such methods can be tested by experimental measurements. Ab initio calculations in conjunction with Raman spectroscopy have been used to model the behaviour of the substituted terthiophene 3′-[1E-2-phenyl ethenyl]-2,2′:5′,2′′-terthiophene (H-pet, depicted in Figure 1), its corresponding sexithiophene, and the polaron (radical cation) of the latter. Theoretical Raman spectra were calculated for each species using the B3LYP/6-31G(d) method and compared to experimental Raman data. The predicted Raman spectrum of the neutral H-pet monomer, for instance, was calculated and correlates very well with the measured Raman spectrum (Figure 2a). Calculations were also carried out on the neutral σ-dimer, (H-pet)2, also resulting in the successful prediction of the vibrational modes (Figure 2b). Both these results indicate that in each case the calculated structure is a reliable model of the true structure. This conclusion allowed the extension of this method to the expected oxidation products of H-pet; the radical cation in particular. Dimeric products were observed upon oxidation: reduction of the oxidised species with aqueous hydrazine resulted in neutral (H-pet)2 formation. The Raman spectrum of (H-pet)2 was therefore calculated and compared to the experimental spectrum (Figure 2c). Both spectra show a single intense band around 1400 cm. Although the position of this peak is not well reproduced by the calculation, the relative intensities and frequency downshifts relative to the neutral species are.