Linkage length dependence of intramolecular photoinduced electron transfer reactions in aromatic donor-viologen acceptor molecules linked by polymethylene bridges

Abstract

The intramolecualr charge transfer (CT) complex formation and photoinduced electron-transfer reactions in aromatic donor-viologen acceptor dyad systems linked by polymethylene linkage (-(CH2)n-) have been studied. 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 constants of the intramolecular CT complexes (Kint) were determined from the absorbance of CT absorption by using the absorptivities of the complexes determined from the intermolecular complexation between the model donor compounds, the 1-aryloxy-3-aminopropanes, and dimethyl viologen. The Kint 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 β-CD disrupts the formation of the intramolecular CT complexes. The 1:1 association constants of the dyad molecules with β-CD (KCD) were estimated from the dependence of the CT absorption on the concentration of β-CD. Complexation of the dyad molecules with β-CD or methylated β-CD (Me-β-CD) also enhances the fluorescence intensity of the excited-state aromatic donors. The 1:1 complexes further associate with CD molecules resulting further enhancement of fluorescence intensity. This was attributed to the extension of the dyad molecules in the CD complexes. The electron-transfer quenching rate constants in the CD complexes formed in the presence of 150 mM Me-β-CD were calculated from fluorescence lifetime data and found to vary exponentially with the length of the linkage. The apparent β value is 0.86 Å-1 (1.09/C-C bond), regardless of the nature of donor moieties. The distance dependence of reorganization energies (λ) of the CD complexes was evaluated. Comparing the λ value with ΔG° of the reaction, it appears that the reactions stay near the top of the Marcus curve. Comparison of the effects of Me-β-CD on steady-state fluorescence intensity and excited-state lifetime indicated that through-space/through-solvent electron transfer is the predominant quenching pathway in the molecules having the linkage shorter than heptamethylene chain and the quenching rate is fast enough to show a static-like behavior.