Planar electron transport

The planarity of the thiopyran-4-ylidene moiety has proved crucial in the development of substituted analogues for use in materials, for example, synthetic metals, organic light emitting diodes and conducting polymers, where good electron transport properties are required. 

The planarity requirement for the donors and the acceptors is caused by the need for reasonable overlap between adjacent centers to facilitate larger Mulliken transfer integrals t [Eq. (8.6.12)], and therefore good electron transport. However, compact molecules with small intermolecular van der Waals distances and large overlaps may have moderate nonplanarity. 

Figure 3. Schematic diagram of an apparatus for measuring transmembrane oxidation-reduction in a planar bilayer membrane. The mechanism described is simple carrier-mediated electron transport. D = aqueous electron donor A = aqueous electron acceptor ...

Net reaction between hydrophilic oxidants and reductants separated by bilayer membranes has been convincingly demonstrated in numerous asymmetrically organized vesicle and planar bilayer systems [2a-c]. Particularly compelling examples are those in which electrogenic electron transport is accompanied by net translocation of lipophilic cations or anions, as is required to maintain electroneutrality. For example, MV + occluded within the inner aqueous phase of DHP vesicles can be completely reduced to the MV+ radical cation by 8204 ion in the bulk phase, but only if an equal or greater amount of viologen is initially present in the external environment [40]. Neither MV-+ nor 8204 is membrane permeable. The reaction is biphasic, with rapid bimolecular reduction of external being followed by... 

The quite remarkable chemistry of iron proteins is reflected by the fact that the square planar tetradentate porphyrin (XXXXVI) is coordinated to Fe in many different proteins but these molecules show such diverse behaviour as oxygen transport, electron transport and catalytic reactivity in metalloenzymes. The protein must be responsible, by providing the other ligand(s) and by general environmental (cf. solvent) effects, for the differences in reduction potential and chemical reactivity of complexes of the various iron porphyrin systems. 

The coupling of the electronic structure of the conjugated polythiophene to the conformation can account for chromic phenomena as well as for aspects of electronic transport of the materials in their undoped or doped states. The order-disorder transition of the polymer chains, between a planar and a non-planar state, is governed by molecular mechanisms related to the nature and regularity of side-chain substituents. 

For good intramolecular electron transport to be achieved, the molecule must be as planar as possible. The more extended the delocalized 7c-electron system, the better will be the conductivity. The specific conductivity thus increases sharply in the series coronene, ovalene, circumanthracene, graphite (Table 14-3). The activation energy El of the electronic conduction, which can be calculated using... 

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