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Circular dichroism of intra- and intermolecular charge-transfer complexes. Enhancement of anisotropy factors by dimer formation and by confinement.

The dynamic behavior of new CT-dyads (CT = charge transfer) has been studied by means of UV-vis, fluorescence, and NMR spectroscopies under a variety of conditions. It was found that the CT-dyads exhibit conformational variations, such as extended and folded monomers and an antiparallel dimer complex, depending on the conditions. The CT interaction was found in the folded conformation at ambient temperature, while the contribution of the dimeric species became evident at lower temperatures. Most interestingly, close examinations of the circular dichroism spectra of these CT-dyads reveal that the anisotropy (g) factors of the dimers are significantly enhanced by a factor of approximately 30 in the CT transition region. Such enhancement is rationalized in terms of the stronger CT interactions in the dimer through the double electronic coupling element, which imposes stronger restrictions on the rotation of alkyl group(s). Confinement of the CT-dyads in cyclodextrin (CD) and cucurbituril cavities afforded further insights into the chiroptical properties of the CT-dyad. The effects of confinement are clearly size-dependent, exhibiting a substantial enhancement of the g factors by a factor of 5-10 upon inclusion by beta-CD and also by cucurbit[8]uril, but with no appreciable changes upon complexation with the other CDs. These results indicate that the conformational fixation of CT-dyads, for example by dimer formation or by confinement in size/shape-matched cavities, is a conventional, yet powerful, tool for manipulating (mostly enhancing) the chiroptical properties of the CT transition, which should be applicable in general to a variety of molecular and supramolecular CT systems.