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

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[6.6.0.0(2.6).0(3.13).0(4.11). 0(5.9).0(10.14)]tetradecane (HCTD) was used as a unique spacer, which possesses a geometry of high symmetry (D-2d), 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 F-1 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 F-1 band. In acetonitrile, the CT state decays much faster than the rate of ET (similar to63 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 pi-chromophores were aligned in different orientations. The estimated ET rate of 3a (3.9 X 10(10) s(-1)) was substantially faster than that of 3b (7 X 10(8) s(-1)). The experimental data were tentatively fitted by a semi-log plot of ET rate constants (k(et)) against free energy (DeltaG(0)), yielding a value of similar to17.3 cm(-1) for the electron-coupling matrix (H-el). (C) 2003 Elsevier Ltd. All rights reserved.