Multichromophoric cyclodextrins .4. Light conversion by antenna effect

A water soluble beta`-cyclodextrin (CD-NA) bearing seven naphthoyl chromophores forms ver stable 1:1 complexes with a merocyanine laser dye DCM-OH (4-dicyanomethylene)-2-methyl-6-(p-(bis(hydroxyethyl)amino)-styryyethyll)-4H-pyran): The antenna effect, i.e. energy transfer from the naphthoyl antenna chromophores to the encased dye,is shown to occur with 100% efficiency. The stability of the complexes is very high (K-s approximate to 10(5)) owing to the contribution of the naphthoate residues. The structural features of the complexes have been examined in detail: circular dichroism experiments confirm the expected axial orientation of DCM-OH in the cavity of CD-NA, and fluorescence anisotropy measurements together with C-13-NMR longitudinal relaxation time measurements show that the complex formed between CD-NA and DCM-OH is tight. The mechanisms of homotransfer (i.e., between naphthoate chromophores) and heterotransfer (i.e., from naphthoate chromophores to DCM-OH included in the cavity) are discussed in light of existing theories. In both cases, the major contribution arises more likely from Coulombic interaction than from short-range interactions. Such multichromophoric cyclodextrins are good model for mimicry of the antenna function in photosynthesis and show great promise as photochemical microreactors.