Electron transfer in oligothiophene-bridged bisporphyrins

Oligothiophene-bridged zinc-tin bisporphyrinates were synthesized. Their absorption spectra have been analyzed in terms of exciton interaction and porphyrin-bridge coupling by through-bond interaction and the steady-state fluorescence spectra in terms of differential Stokes shifts for the electron-donating zinc and the electron-accepting tin porphyrinates. Strong quenching of the fluorescence intensity and acceleration of the fluorescence decay as compared to porphyrinate monomers (ZnTPP, SnTPP) were observed. Both phenomena were traced back to light-induced electron transfer by the occurrence of ion pair absorption bands in picosecond transient absorption spectra. Similar absorption spectra of both chromophores caused always simultaneous excitation and, consequently, two concurrent photoreactions. Combined evaluation of the time-dependent absorption and fluorescence data allowed the estimation of rates for the electron transfer reactions. The found dependence on the separation distance was much smaller than for donor-acceptor systems with saturated spacers. A damping factor of 0.05 was calculated for the charge separation proceeding from the excited state of the zinc porphyrin. The polarity of the solvent had a profound influence on the transfer rates. The charge recombination was 300 times faster in polar tetrahydrofuran than in nonpolar toluene.