Charge transfer chain

A series of electron donors and acceptors together can form a charge transfer chain (CTC). Examples of these are found in some electron transfer proteins, Electron transport in ID CTCs has been studied using the spin-boson model with multiple tight-binding electronic states. The nature of the transport depends on the strength of the dissipation of the environment. 

The highly conductive class of soHds based on TTF—TCNQ have less than complete charge transfer (- 0.6 electrons/unit for TTF—TCNQ) and display metallic behavior above a certain temperature. However, these soHds undergo a metal-to-insulator transition and behave as organic semiconductors at lower temperatures. The change from a metallic to semiconducting state in these chain-like one-dimensional (ID) systems is a result of a Peieds instabihty. Although for tme one-dimensional systems this transition should take place at 0 Kelvin, interchain interactions lead to effective non-ID behavior and inhibit the onset of the transition (6). 

Charge-transfer compounds can be isolated ia the crystalline state, although low temperatures are often required. The bromine—dioxane compound, for example, has a chain stmeture (42). 

Figure 4-8. 1NDO/SCI simulation of the wavcfunclion y/(x,xi, = 16, chain I) of the lowest charge transfer-excited stale in a cofacial dimer formed by two five-ring PPV oligomers separated by 4A. Ili/(x,x/, - 16, chain 1) represents the probability amplitude in finding an electron on a given site xt. assuming the hole is centered on site 16 of chain I. The site labeling is the same as that reported on top of Figure 4-7.

These treatments of periodic parts of the dipole moment operator are supported by several studies which show that, for large oligomeric chains, the perturbed electronic density exhibits a periodic potential in the middle of the chain whereas the chain end effects are related to the charge transfer through the chain [20-21]. Obviously, approaches based on truncated dipole moment operators still need to demonstrate that the global polarization effects are accounted for. In other words, one has to ensure that the polymeric value corresponds to the asymptotic limit of the oligomeric results obtained with the full operator. 

A C60 derivative with an attached fluorinated chain gave a limiting area of 0.78 nm molecule [266]. It was reported that this film was so mechanically rigid that it pushed the Wilhehny plate out of the water at 11 14 mN m The monolayer spreading of this compound arises from the even greater hydrophobicity of the fluorocarbon chains and their orientation away from the water surface. The LB films with a fluorinated tetrathiafulvalene derivative did not show evidence of charge transfer in their UV spectra. 

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