EDOT oligomers

Nevertheless the sequential process is widely applied in industry. The main reason is that premixed reactive solutions of EDOT and oxidizer are not sufficiently stable. Moreover typically a better electrical performance of the capacitor is achieved by the excess of EDOT, which favors the formation of nonconjugated EDOT oligomers as intermediates (see Chapter 8). 

These oligomers can, in spite of their low solubility and in contrast to the monomeric PheDOT, be readily electropolymerized. Theoretical calculations show a low reactivity of the PheDOT+ cation radical. The extension of the conjugated chain leads to a distribution of the singly occupied molecular orbital (SOMO) more similar to that of EDOT oligomers. 

A series of EDOT oligomers, blocked by phenyl end groups (Figure 14.5 EDOT1-EDOT4), has been synthesized to check EDOT-typical optical and redox properties without further oxidative polymerization. The p, P - unsubstituted thiophene analogs were also described (T1-T4) and compared to the EDOT derivatives. 

Phenyl-capped EDOT oligomers. (Data from J. J. Apperloo, L. Groenendaal, H. Verheyen, M. Jayakannan, R. A. J. Janssen, A. Dkhissi, D. Beljonne, R. Lazzaroni, and J.-L. Bredas, 2002, Chem Eur/8(10) 2384-2396.)... 

Turbiez, M., Frere, P., Blanchard, P., and Roncali, J., Mixed 7t-conjugated oligomers of thiophene and 3,4-ethylenedioxythiophene (EDOT), Tetrahedron Lett., 41, 5521, 2000. 

EDOT-based systems. These interactions and replacement of the sp bridging carbons by an olefrnic linkage lead to a perfectly coplanar structure (Figure 13.4). BVDOT and poly(BVDOT) show higher oxidation potential than BEDOT and poly(BEDOT) suggesting better environmental stability for the neutral form of the derived polymers and oligomers. 

A lot of work has been devoted to the elaboration of more and more purposely tailored monomers, followed by electropolymerization, in order to improve a given property of the resulting polymer, e.g., a low bandgap. Approaches have also been made to synthesize oligomers, in order to prepare a better-defined polymer with improved properties. However, little work has been published outside the three main families, namely thiophene (especially EDOT), pyrrole, and aniline derivatives. In this section, we will only recall which kind of approach in monomer tailoring has been followed, and focus on some examples where the electrochemical behavior of the monomers has been found different from what could be expected. 

About 15 years ago was published the unequivocal demonstration of the fact that the first steps of the polymerization of pyrrole and several pyrrole derivatives in dry acetonitrile were the coupling of cation-radicals [6,133]. Further studies by Hapiot and Audebert have shown that this could be extended to several types of thiophene [134] and pyrrole oligomers [98,135], from four to six units length. It has been assumed since then (and even before) that this was the same for all heterocycles, although the study of thiophene itself is impossible (the cation-radical is far too much unstable), as well as bithiophene and terthiophene, whose cation radicals are also too much reactive. This is also unfortunately the case of EDOT and its pyrrole analog EDOP, although the lifetime of the cation radical of EDOP could be recently estimated in particular conditions [136]. 

Similar to the previously mentioned polymers, attempts have been made toward polymerizable EDOT-based oligomers that have lower oxidation potentials than their respective monomers and longer conjugation lengths. BiEDOT (38) has an oxidation potential of 1.4 V lower than that of thiophene and 0.6 V lower than that of EDOT [139]. The dimer easily polymerizes, is air stable, and when polymerized, the resultant polymer has identical electrochromic properties to PEDOT. TerEDOT has also been synthesized with a slightly lower oxidation potential than BiEDOT. Although it is somewhat imstable and can be difficult to handle and store, it electropolymerizes to afford an electrochromic PEDOT. 

Terthiophene (2.179, n = 2) and EDOT (2.181) derivatives, in which 18-crown-6 moieties were attached directly to the 3,4-position of the central thiophene ring, were prepared by Zotti s group (Chart 1.37) [275]. These oligomers showed maximum absorption at 365 and 366 nm, respectively, which were significantly blue shifted (AA, = 20-30 nm) after the addition of 0.1 MNa" " or K+ ions. The polymer films prepared by electropolymerization of the oligomers were little influenced by the addition of alkali metal ions. Furthermore, electrochemical quartz crystal microbalance analysis of the alkali metal coordination ability of the polymer films in acetonitrile solution revealed a lower degree of coordination, which was attributed to the loss of degrees of freedom in the crown-ether moiety. 

Co-oligomers of EDOT and thienopyrazine 3.24 were synthesized by Berlin et al. (Chart 1.47) [337, 338]. Similarly to other thienopyrazine-based polymers, electrochemically generated polymers of 3.24 exhibited ambipolar electroactivity. P3.24a had an in situ conductivity of 0.5 Scm in the oxidized state and 0.01 S cm in the reduced state and corresponding values for alkylated derivative P3.24b were 15 and 0.03 Scm Both polymers had broad absorption bands peaking at 950 nm, suggesting small bandgaps [337]. 

This planar arrangement arises from intramolecular S- O interactions. The noncovalent contacts (2.807 and 2.846 A) are much shorter than the sum of the van der Waals radii for sulfur and oxygen (S + O = 3.35 A), which demonstrates the strength of these two-electron, three-centered nonclassical bonds. Similar short S- O contacts are observed in oligomers of EDOT and it is widely accepted that such interactions significantly influence the electronic properties of PEDOT. Notably, the bandgaps of poly(3,6-dimethoxythieno[3,2-fe]thiophene) and PEDOT vary by only 0.05 eV. 

Table 4.8 Molecular structure, oxidation and reduction peak potentials and maximum absorption of thienyl-S,S-dioxide-EDOT co-oligomers oxidation and reduction redox potentials, electrochemical energy gap, maximum absorption, optical energy gap and p-conductivity of the corresponding polymers obtained by anodic polymerization...

Hexyl end-capped hybrid oligothiophenes based on various combinations of thiophene and EDOT have been reported by Roncali and co-workers [118]. A synthetic route to oligomers alternating EDOT and thiophene up to the heptamer 48 was developed (Scheme 9.27). Electrochemical studies and theoretical calculations show that the positions of the EDOT units within the conjugated chain control the potential difference AEp between the first and second oxidation processes. 

Turbiez et al. synthesized a series of co-oligomers of thiophene and 3,4-ethylenedioxythiophene (EDOT) ranging from four to seven rings [72a]. They found that alternating thiophene and EDOT units leads to... 

The differences in the final properties of the copolymer according to the monomer s feed ratio were also observed for P(DHQT-co-DNBP) (12) films electrodeposited onto ITO/glass. DHQT is a fluorescent thiophene oligomer usually applied as active layer for assemble of organic field effect transistors (OFETs)/ photovoltaic cells and electrochromic devices/ while the polymer PDNBP and its copolymer with EDOT have... 

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