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 (K int) 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 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 β-CD disrupts the formation of the intramolecular CT complexes. The 1:1 association constants of the dyad molecules with β-CD (K CD) 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.

Strong ion pair charge transfer interaction of 1,8-naphthalimide–bipyridinium conjugates with basic anions – towards the development of a new type of turn-on fluorescent anion sensors

“…The k ET values in the 10 9 to 10 11 s À1 range were reported for the conjugates of viologens with various other electron acceptors. [34][35][36][37][38] Theoretical calculations were performed to study the PET in the case of the simpler dyad, NIV. In the ground-state optimized structure, the dihedral angle between the two pyridinium rings was found to be 411.…”

Photoinduced electron transfer reaction tuned by donor–acceptor pairs via the rigid, linear spacer heptacyclo[6.6.0.02,6.03,13.04,11.05,9.010,14]tetradecane

“…The quenching efficiency, indicated by the apparent F 1 fluorescence quantum yield (Table 1), is more effective in series 1a-c than those in 2a-c. The difference between these two categories can be rationalized qualitatively by the higher energy of LUMO for cetachol (5) than that of naphthanlen-diol (7), resulting in a faster electron transfer rate for 1a-c (vide infra).…”

Interplay of Hole Transfer and Host–Guest Interaction in a Molecular Dyad and Triad: Ensemble and Single‐Molecule Spectroscopy and Sensing Applications

“…Similar rapid fluorescence quenching that does not involve the formation of complexes may also be found in dyad 2 . Previous works have reported quasi‐static quenching for flexibly linked fullerene–coumarin dyads28 and aromatic electron donor–viologen acceptor dimers in the presence of cyclodextrin 29…”

“…Previous works have reported quasi-static quenching for flexibly linked fullerene-coumarin dyads [28] and aromatic electron donor-viologen acceptor dimers in the presence of cyclodextrin. [29] Solvent effect (i.e., changing solvent from water to DMF) on the overall quenching process in triad 1 is discussed next. Unlike in water, the emission from triad 1 in DMF is only slightly enhanced when compared with dyad 2 (Figure 1 (C)).…”