LINKAGE LENGTH DEPENDENCE OF INTRAMOLECULAR PHOTOINDUCED ELECTRON-TRANSFER REACTIONS IN AROMATIC DONOR-VIOLOGEN ACCEPTOR MOLECULES LINKED BY POLYMETHYLENE BRIDGES

The intramolecualr charge transfer (CT) complex formation and photoind uced electron-transfer reactions in aromatic donor-viologen acceptor d yad systems linked by polymethylene linkage (-(CH2)(n)-) have been stu died. The donors and the numbers of methylene unit in the linkages are 1-naphthoxyl with n = 3, 6, 8, and 10, 2-naphthoxyl with n = 3-10 and 12, and 2-dibenzofuranoyl with n = 3, 6, 8, and 10. The formation con stants of the intramolecular CT complexes (K-int) were determined from the absorbance of CT absorption by using the absorptivities of the co mplexes determined from the intermolecular complexation between the mo del donor compounds, the 1-aryloxy-3-aminopropanes, and dimethyl violo gen. The K-int,, values depend little on the length of the linkage and are about 0.2 for 1-naphthol and 2-naphthol derivatives, and 0.6 for dibenzofuranoyl derivatives. The addition of P-CD disrupts the formati on of the intramolecular CT complexes. The 1:1 association constants o f the dyad molecules with beta-CD (K-CD) were estimated from the depen dence of the CT absorption on the concentration of P-CD. Complexation of the dyad molecules with beta-CD or methylated beta-CD (Me-beta-CD) also enhances the fluorescence intensity of the excited-state aromatic donors. The 1:1 complexes further associate with CD molecules resulti ng further enhancement of fluorescence intensity. This was attributed to the extension of the dyad molecules in the CD complexes. The electr on-transfer quenching rate constants in the CD complexes formed in the presence of 150 mM Me-beta-CD were calculated from fluorescence lifet ime data and found to vary exponentially with the length of the linkag e. The apparent beta value is 0.86 Angstrom(-1) (1.09/C-C bond), regar dless of the nature of donor moieties. The distance dependence of reor ganization energies (lambda) of the CD complexes was evaluated. Compar ing the lambda value with Delta G degrees of the reaction, it appears that the reactions stay near the top of the Marcus curve. Comparison o f the effects of Me-beta-CD on steady-state fluorescence intensity and excited-state lifetime indicated that through-space/through-solvent e lectron transfer is the predominant quenching pathway in the molecules having the linkage shorter than heptamethylene chain and the quenchin g rate is fast enough to show a static-like behavior.