Hopping dendrimers

Electron-hopping is the main charge-transport mechanism in ECHB materials. There is precedence in the photoconductivity Held for improved charge transport by incorporating a number of redox sites into the same molecule. A number of attempts to adapt this approach for ECHB materials have been documented. Many use the oxadiazole core as the electron-transport moiety and examples include radialene 40 and dendrimer 41. However, these newer systems do not offer significant improvements in electron injection over the parent PBD. 

The cavities in dendrimers are not permanent, but can be redistrubuted as the branches, which can possess considerable degrees of flexibility, move about. The inclusion of guest molecules within a dendrimer may occur as a result of movements in the branches, which allow temporary inclusion of the guest molecule within the dendrimer structure. In solution, it is assumed that molecules of solvent are able to move with ease through the branches of dendrimers, hopping between such temporary cavities with little or no hindrance. When the solvent is removed, the dendrimer may collapse to a distinctly reduced volume. 

Energy hopping among the peryleneimide chromophores, revealed by anisotropy decay times [30], occurs with a rate constant of 4.6x10 s E When three peryleneimide and one terryleneimide chromophores are attached to the dendrimer rim, energy transfer from the former to the latter units takes place with... 

In the last section, electron transfer between dendrimers in films was mentioned. This type of hopping is important in thin-film organic devices (see above). How do dendrimers behave in solution Are the paradigms the same Actually, they are not. 

Energy hopping is a Forster-type process that is present in the multichromo-phoric dendrimer such as p-ClP4 and can be related to the spectral overlap as... 

Maus M, MitraS, LorM, Hofkens J, Weil T, Herrmann A, Mullen K, De Schryver FC (2001) Intramolecular energy hopping in polyphenylene dendrimers with an increasing number of peryleneimide chromophores. J Phys Chem A 105 3961... 

G = 3 PAMAM dendrimers surface modified with cationically substituted naphthalene diimides (Figure 44(a) and (b)). In all cases, the eonductivity was electronic and isotropic. Near infrared spectra showed the formation of extensive tt-stacking, which presumably favored electron hopping via a three-dimensional network. 

The mean diffusion layer thickness in this case is almost exactly the diameter of the dendrimer molecule. It therefore follows that the oxidation of aU redox sites will not be complete because a diffusion layer of oxidation achieved by successive electron hopping across the dendrimer surface is not yet complete. 

Multiple identical noninteracting redox-active centers in a large globular structure like dendrimer cannot be simultaneously in contact with or in close proximity with the electrode surface. Thus the electron transfer must involve their rotation and/or electron hopping mechanism within the measurement time... 

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