Theoretical investigation of the nature of the ground state in the low-bandgap conjugated polymer, poly(3,4-ethylenedioxythiophene)

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Abstract

Quantum-chemical calculations are performed to study the geometric and electronic structures of poly(3,4-ethylenedioxythiophene)—PEDOT. The relative stability of the two possible structures for PEDOT (aromatic-like and quinoid-like) has been evaluated on oligomers of increasing size. The results obtained on PEDOT are compared to those collected on polythiophene and polyisothianaphthene, i.e., two parent conjugated polymers that are known to possess an aromatic and quinoid ground state, respectively. The vibrational spectra of both forms of PEDOT have also been calculated and compared with recent experimental data. The calculations indicate that the ground state of neutral PEDOT is aromatic-like.

Introduction

One key feature of organic semiconductors, including π-conjugated polymers, is the possibility to tuning their electronic properties via chemical substitution along the backbone. This approach has been widely exploited to modify the ionization (oxidation) potential, the electron affinity (reduction potential), or the bandgap (hence, the energy of optical absorption and emission) in polyphenylenes, polythiophenes, and polyphenylene vinylenes [1], [2].

A very interesting example of such `molecular engineering' has been the design of low-bandgap conjugated polymers [3]. Upon doping, the valence to conduction band transition is strongly depleted in favor of lower energy transitions, which basically makes these materials transparent in the visible part of the spectrum (and electrically conductive). The search for low-bandgap polymers was pioneered in the mid-1980's by Wudl et al. [4] with the synthesis of polyisothianaphthene (PITN) (Scheme 5). The rationale behind the grafting of a benzene ring onto the thiophene ring was to bring some quinoid character to the conjugated backbone. The electronic pattern of the HOMO of polythiophene is aromatic (bonding interactions between α and β sites and antibonding interactions between β sites and between α sites) while the LUMO is quinoid (antibonding interactions between α and β sites and bonding interactions between α sites and between β sites) [5]. Therefore, driving the backbone towards a quinoid structure would destabilize the HOMO and stabilize the LUMO, i.e., reduce the bandgap. It turns out that the effect of benzene grafting is so strong that the ground-state geometric and electronic structure display in fact a quinoid character (the HOMO and LUMO levels having exchanged their bonding–antibonding pattern with respect to polythiophene). This was first proposed on the basis of theoretical calculations [6], [7] and later confirmed with a joint experimental and theoretical investigation of the polymer and model oligomers [8].

For practical applications, another transparent, conductive polymer has been recently very much exploited: poly(3,4-ethylenedioxythiophene), PEDOT (Scheme 5). The most interesting aspects related to the synthesis and the characterization of that polymer have been reviewed recently [9]. The accessability of the monomer and, especially the possibility of producing the conducting material as a water dispersion by using a water-soluble polymeric counterion, have made PEDOT a successful commercial product. It is extensively used as an antistatic coating on photographic films and as an electrode layer in flexible displays and organic light-emitting devices.

At first sight, PEDOT appears to be a simply substituted polythiophene; the reduction in bandgap relative to polythiophene being thought to originate from the influence of the electron-donor ethylene dioxy groups on the energies of the frontier levels of the π system [10]. Therefore, the ground-state geometric structure of neutral PEDOT is expected to be aromatic. However, in a recent study, Lapkowski and Pron [11] have proposed, from spectroscopic data, that the ground state of PEDOT is quinoid. This hypothesis is based on the observation that the Raman peak corresponding to the stretching of the Cα–Cβ bonds shifts to higher wavenumbers upon doping; this suggests that the electron density increases, hence that the character of the bond evolves from single to double. Similarly, the peak assigned to the Cα–Cα bond connecting the EDOT units shifts to lower wavenumbers. This raises the question of the nature of ground state of neutral PEDOT, as a low-bandgap conjugated polymer.

To address this question, we have carried out a theoretical investigation along two lines. On one hand, we have calculated the relative stability of the aromatic and quinoid forms of PEDOT (Scheme 5) and compared the results to those obtained on unsubstituted polythiophene (PTH) (as a reference for an aromatic system) and PITN (as a reference for a quinoid system). This approach has been successfully used previously [8] to determine the nature of the ground state in PITN. It is based on the evaluation of the energy per repeat unit (Epru) of the two forms, from quantum-chemical calculations on a series of oligomers of increasing size. On the other hand, the vibrational spectra of the two forms have been calculated and compared to the experimental spectra recorded by Kvarnström et al. [12], [13].

Section snippets

Theoretical methodology

The ground-state geometric structures of PTH, PITN and PEDOT oligomers (where the terminal α-carbons carry a single hydrogen atom Scheme 6, left), ranging in size from 1 to 10 repeat units (1–6 for PITN), were calculated at the restricted Hartree–Fock (RHF) and a hybrid density functional theory (DFT)/B3LYP levels. In the DFT calculations, we adopted Becke's three-parameter exchange functional and the correlation functional of Lee, Yang and Parr (B3LYP) [14], [15]; such a choice of functionals

Results and discussion

In the first set of calculations, conjugated chains with all carbon atoms being sp2-hybridized are considered. To force the quinoid structure, chain-end defects, under the form of substitution of the external carbon atoms with two hydrogen atoms (Scheme 6), are introduced in a second set of calculations.

Fig. 1 shows the evolution along the conjugated chain of the degree of bond-length alternation (BLA), defined as the difference between alternating carbon–carbon single and double bonds, as

Conclusions

By applying density functional theory and ab initio Hartree–Fock methods to model oligomers for PEDOT, we have explored the nature of the electronic ground state in the corresponding neutral polymer. Energetic considerations, based on the energy per repeat unit as obtained by both DFT and RHF techniques, lead us to conclude that PEDOT has an aromatic character in the ground state, as is the case for unsubstituted PTH but in contrast to PITN. These conclusions are fully supported by the

Acknowledgements

The work in Mons is partly supported by the IWT `DOTCON' project and the Belgian Federal Government Interuniversity Attraction Pole Program (PAI V/3: Chimie supramoléculaire et Catalyse Supramoléculaire). The work at Arizona is partly supported by the National Science Foundation. D.B. and R.L. are Senior Research Fellows of the Belgian National Fund for Scientific Research (FNRS).

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    Present address: Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium. Also corresponding author. Fax: +1-520-621-8407.

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