Long-Range Photoinduced Through-Bond Electron Transfer and Radiative Recombination via Rigid Nonconjugated Bridges: Distance and Solvent Dependence

Article abstract of DOI:10.1021/ja00245a014

A series of molecules 1 was synthesized containing a 1,4-dimethoxynaphthalene donor (D) and a 1,1-dicyanoethylene acceptor (A) interconnected by five different, rigid, nonconjugated bridges.The length of bridges varies with increments of two ?-bonds from four in 1(4) to 12 ?-bonds in 1(12), to provide donor-acceptor center-to-center separations (R_c) ranging from 7.0-14.9 Angstroem.In solvents of medium and high polarity, excitation of the donor D is followed by rapid intramolecular electron transfer.The rate constant (k_et) shows only small dependence upon the solvent polarity (a factor of 2-3 between benzene and acetonitrile, for example) but decreases with increasing separation ranging from >10¹¹ s^-1 for a four-bond separation to ca.4 x 10⁸ s^-1 for a 12-bond separation.In saturated hydrocarbon solvents photoinduced electron transfer is not observed for 10- and 12-bond separations, while it is not significantly decreased for the shorter homologues.Therefore the absence of electron transfer at 10- and 12-bond separations in saturated hydrocarbon solvents is attributed to a thermodynamic rather than to a kinetic effect.In solvents where electron transfer is thermodinamically feasible, its rate is considerably greater than that found from various other experimental studies where either different bridges were used or intermolecular electron transfer was studied.Through-bond interaction involving ?/? interaction between the bridge and the donor-acceptor pair is proposed to explain the very high electron transfer rates observed in 1; this is qualitatively correlated with independent information about this coupling derived from both theory and experiment (photoelectron spectroscopy).The observation of intramolecular charge-transfer absorption and emission for 1(4), 1(6), and 1(8) confirms the operation of such through-bond interaction.Rigid system like 1 can therefore not only provide more insight into the thermodynamics of electro transfer and its solvent dependence but especially also into the role of the nature of the coupling of donor and acceptor.The latter is of crucial importance for a better understanding of the factors governing the rates of electron transfer between redox centers in, e.g., biologically important redox proteins involved in photosynthesis and in the respiratory cycle.It is shown that the near-independence of the photoassisted charge separation dynamics on solvent polarity and the overall free-energy change (even for calculated variation of ca. 0.9 eV) is consistent with predictions of electron-transfer theory on the assumption that the solvent is a continuous dielectric.It is also shown how the parameters entering into the theoretical expressions (in particular, the intramolecular reorganization energy) may be correlated with those obtained from radiative transitions (e.g., charge recombination fluorescence).The dependence of the effective electron interaction element (J), which couples donor and acceptor, on the bridge length is discussed.