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A new class of donor-{saturated hydrocarbon bridge}-acceptor (D–B–A) dyads were synthesized and utilized on a systematic approach to evaluate the corresponding photoinduced electron transfer (ET) process. Among these dyads heptacyclo[6.6.0.02,6.03,13.04,11. 05,9.010,14]tetradecane (HCTD) was used as a unique spacer, which possesses a geometry of high symmetry (D2d), rigidity and linearity. The spectroscopy and dynamics of excited-state ET as functions of donor/acceptor electronic states, orientation as well as solvent properties were analyzed with the aid of theoretical computations. It was observed that the quenching of donor fluorescence (the F1 band) correlated with the appearance of a broad charge-transfer (CT) emission. Both wavelength and intensity of the CT band varied with solvent-polarity, whereas its rise dynamics complied well with the decay of the F1 band. In acetonitrile, the CT state decays much faster than the rate of ET (∼63 ps−1) so that the corresponding steady-state emission cannot be resolved. An intriguing effect was observed in the case of benzene-1,2-dithioketals ( 3a and 3b ) where the D and A π-chromophores were aligned in different orientations. The estimated ET rate of 3a (3.9×1010 s−1) was substantially faster than that of 3b (7×108 s−1). The experimental data were tentatively fitted by a semi-log plot of ET rate constants (ket) against free energy (ΔG0), yielding a value of ∼17.3 cm−1 for the electron-coupling matrix (Hel). |
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