Donor-acceptor arrangements intramolecular charge transfer

The crystallographic results for the polymorphs and complexes of TTP are summarised in Table 5 and 6. The structure of a-TTP is shown in Fig. 15. The dominant directional interaction is of the n. .. a type, between O and Cl, as shown by d(0. .. Cl) = 2.98 A, while the antiparallel arrangement of adjacent molecules in the [001] direction suggests that dipole-dipole interactions are also important. The mean plane of the p-tolyl moiety is normal to the mean plane of the rest of the molecule, which lies in a crystallographic mirror plane. This perpendicular disposition of the mean planes of donor and acceptor parts of the molecule means that any intramolecular charge transfer absorption will be at high energies and the crystals are consequently colourless. 

Spectroscopic methods can be used to specify the position of donors and acceptors before photoexcitation [50]. This spatial arrangement can obviously influence the equilibrium eomplexation in charge transfer complexes, and hence, the optical transitions accessible to such species [51]. This ordered environment also allows for effective separation of a sensitizing dye from the location of subsequent chemical reactions [52], For example, the efficiency of cis-trans isomerization of A -methyl-4-(p-styryl)pyridinium halides via electron transfer sensitization by Ru(bpy) + was markedly enhanced in the presence of anionic surfactants (about 100-fold) [53], The authors postulate the operation of an electron-relay chain on the anionic surface for the sensitization of ions attached electrostatically. High adsorptivity of the salt on the anionic micelle could also be adduced from salt effects [53, 54]. The micellar order also influenced the attainable electron transfer rates for intramolecular and intermolecular reactions of analogous molecules (pyrene-viologen and pyrene-ferrocene) solubilized within a cationic micelle because the difference in location of the solubilized substances affects the effective distance separating the units [55]. 

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