Reductive doping

SbFg, CF3(CF2)nS03 (n - 0,3,7), it appears that the maximum accessible oxidation levels are largely dictated by packing limitations (anion size). Evidence is also presented for reversible reductive doping of [Si(Pc)0]n in THF/(n-Bu)4N+BF4. 

Figure 12. Micro-scale electrochemical voltage spectroscopy showing oxidative as well as reductive doping of tetragonal Si(Pc)0]n in THF/(n-Bu)4N+BF4.

Fulleride anions are often more soluble, especially in more polar solvents, than the parent fullerenes. For example, in bulk electrolysis experiments with tetra-n-butylammonium perchlorate (TBACIO4) as supporting electrolyte, carried out in acetonitrile where Cjq is completely insoluble, fairly concentrated, dark red-brown solutions of 50 can be obtained [81]. Upon reoxidation, a quantitative deposition of a neutral Cjq film on the surface of a gold/quartz crystal working electrode takes place. This Cjq film can be stepwise reductively doped with TBA, leading to (Cjo )... 

These quarternized polymers can be viewed as self-doped but exhibit relatively low intrinsic conductivities. The polymers can be oxidatively doped with iodine, or reductively doped with TTF, to give highly conducting polyacetylenes with conductivities of 10"4 and 10 1 S/cm, respectively. One additional attractive feature of this system is that, unlike PA, these quarternized PAs are very stable in air. 

The addition of an electron-accepting material, such as iodine, to a polymer is known as oxidative doping because electrons are lost from the polymer. Reductive doping is the process by which an electron-donating material, such as sodium, is added to a polymer. In this case, the polymer gains an electron and becomes negatively charged. 

The end result is the same with both oxidative and reductive doping, however, because it is not the sodium or iodide ion formed that is mobile, but the deformation of the polymer chain itself that results in a flow of current through the molecule. 

Figure 5.3. The reduction (doping) of trans-polyacetylene. In the ideal case, the mobility of the charged states allows the bipolaron to collapse into a pair...

The electronic structures of poiy(fluoroacetylene) and poly(difluoroacetylene) have been investigated previously using the ab initio Hartree-Fock crystal orbital method with a minimum basis set (42). Only the cis and trans isomers with assumed, planar geometries were studied. The trans isomer was calculated to be more stable in both cases, and the trans compounds were predicted to be better intrinsic semiconductors and more conductive upon reductive doping than trans polyacetylene. However, our results show that head-to-tail poly(fluoroacetylene) prefers the cis structure and that the trans structure for poly(difluoroacetylene) will not be stable. Thus the conclusions reached previously need to be re-evaluated based on our new structural information. Furthermore, as noted above, addition of electrons to these polymers may lead to structural deformations that could significantly change the conductive nature of the materials. 

Pyrrole, thiophene and their derivatives can be oxidatively polymerised, either electrochemically or chemically, for example using iron(lll) chloride, to give mainly 2,5-coupled polymers. The initial neutral polymers are non-conducting, but on further oxidation are converted partially into cation radicals or dications, with incorporation of counter ions from the reaction medium - a process known as doping - giving conducting materials. Reductive doping is also possible in other systems. 

Grafts. Polyacetylene films were synthesized at -78°C using techniques similar to those developed by Shirakawa and coworkers (11). Reductive doping was carried out in a dry box by immersion of (CH)X films in 1 M sodium naphthalide/THF solutions for 2 minutes. The films were then washed several times with dry, 02-free THF and allowed to stand in fresh THF for approximately 1 hour. The conductivities and compositions of the films were in the range 5-50 S/cm and [CHNaQ O-0.25]x respectively. 

Fig. 36.12. Charged solitons without spin produced from oxidative or reductive doping.

The electrochemical behavior of poly(RCOT)s has also been examined [40]. As expected from the electrochemical properties of unsubstituted polyacetylene, films of poly(RCOT)s coated on an electrode and immersed in an acetonitrile electrolyte solution (in which the polymers are not soluble) are found to undergo reversible oxidative and reductive doping. Unlike unsubstituted polyacetylene, these films may be prepared readily by casting from solution, or, in the case of poly(scc-butylCOT), by electrodecomposition from a THF solution. In contrast to the voltammetry of polymer films, cyclic voltammograms of methylene chloride... 

The conducting and physical properties can be modified by the use of 3- and/or 4-substitutents, or A -substituents in the case of pyrrole. The counterions can be incorporated into a side-chain (self-doping) as in the polymer of 3-(thien-3-yl)propanesulfonic acid. Variation in the size of side-chains allows control of solubility. Mixed polymers with, for example, thiophenes and pyridines, are capable of both oxidative and reductive doping. 

PThs are different from PPy and PAn in that they can be both oxidatively or reductively doped in a proper solvent. The reason why they can be reduced and thus n-doped is perhaps because they have sulphur atoms. 

Table 1.1 shows a list of dopant ions and their source electrolytes which are currently being used in the electrochemical synthesis of conducting polymers. All of these dopant ions with the exception of the last two (marked by ) are anions and are associated with electrochemical oxidation of the polymers at the anode. Aizawa et al. [104] reported the first example of the reductive doping of an electrochemically synthesized polythioenylene film with cations like tetraethyl ammonium (Et4N" )... 

As mentioned above, La(2 x)BaxCu04 is superconducting below 35 K. The mother compound La2Cu04 is a cuprate and an insulator at T = 0 like CuO and all cuprates where copper has the well-defined oxidation state -1-2. CuO, the black, semiconducting oxide of copper, is formally not a cuprate (since a cuprate has two different metal ions). Cuprates, in contradistinction to CuO, can be made superconducting by oxidative or reductive doping. Cuprates are different from CuO by the presence of the Cu02 plane in the structure. 

It should be noted that the preparation of n-type (reduced) polyacetylene using strong organic bases (e.g., alkyl lithium compounds) or more commonly electron transfer reagents (e.g., sodium naphthalide radical anion) employs the two major classes of initiators used in anionic polymerization of monomers such as styrene and butadiene. Reductive doping can also be accomplished by deprotonation of, for example, acetylene/butadiene copolymers and related phenylenepentadienylenes." ... 

Electrochemical properties of three polymers were explored by cyclic voltammetry measurement (Fig. 4.lid). After introducing fluorine atoms, the reductive currents of both fluorinated polymers increase obviously and the reductive doping processes appear to be more reversible than those of BDPPV. Both HOMO and LUMO levels of FBDOPV-1 and FBDOPV-2 are lowered, but clearly the LUMO levels are more easily affected (Table 4.2). The LUMO levels of FBDOPV-1 and FBDOPV-2 reach -4.26 and -4.30, 0.16 and 0.20 eV lower than that of BDPPV. Computational results reveal that both fluorinated polymers exhibit almost planar conjugated backbones and their HOMOs and LUMOs are well delocalized along polymer backbones (Fig. 4.1 le). This contrasts with many donor-acceptor polymers, in which the LUMOs are mostly localized on the electron-deficient units of polymer backbones [27, 63]. 

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