Photoinduced Electron Transfer: Synthetic Models of the Primary Processes in Photosynthesis

A general synthetic method is presented for the preparation of a series of meso-phenyloctamethylporphyrin-linker-quinone compounds for investigation of intramolecular photoinduced electron transfer rates by picosecond fluorescence spectroscopy. Distance effects were investigated through the incorporation of zero, one, or two bicyclo[2.2.2]octyl linker units separating the porphyrin and quinone. Addition of one bicyclo[2.2.2]octyl linker decreases kET by at least a full order of magnitude. The addition of a second bicyclo[2.2.2]octyl linker unit decreases the electron transfer rate by 500 to ≥1700. Investigation of solvent effects on the electron transfer rate, as obtained from the picosecond fluorescence lifetimes of the compounds, indicate weak solvent dependencies as expected for electron transfer from a neutral initial state. Conversely, dramatic solvent dependencies are expected for the back transfer rates in these compounds. Investigation of temperature effects on the electron transfer rate revealed a relatively temperature insensitive electron transfer rate (nuclear tunneling). This is the first synthetic porphyrin-quinone compound to date to exhibit electron transfer quenching at low temperatures. The nonexponential emission decays of samples at 77K in frozen solvent matrices are proposed to arise from an ensemble of rotational conformations between the porphyrin donor and the benzoquinone acceptor, which is well described by an angle-modulated decay analysis. The dependence of kET on the precise geometric orientation of the donor and acceptor reinforces the nonadiabatic nature of these transfers. Exothermicity effects for a structurally homologous series of porphyrin-benzoquinones prepared by the general synthetic method in four solvents of varying polarity indicate large changes in the electron transfer rate with small changes in the driving force at low exothermicities, followed by a relatively ΔG° insensitive region. At the highest exothermicity case studied, a modest decrease in the electron transfer rate was observed. No dramatic evidence for the inverted region was observed for the photoinduced electron transfer from the porphyrin excited singlet transfer to the quinone acceptor over the range of exothermicity studied (0.5 ≤ ΔG°rel ≤ 1.1 eV (benzene)). A comparison of the results with classical and semiclassical theories is presented.