Journal of the American Chemical Society, Vol.139, No.48, 17474-17483, 2017
Mediating Reductive Charge Shift Reactions in Electron Transport Chains
We report the synthesis of a full-fledged family of covalent electron donor-acceptor(1)-acceptor(2) conjugates and their charge-transfer characterization by means of advanced photophysical assays. By virtue of variable excited state energies and electron donor strengths, either Zn(II)Porphyrins or Zn(II)Phthalocyanines were linked to different electron-transport chains featuring pairs of electron accepting fullerenes, that is, C-60 and C-70. In this way, a fine-tuned redox gradient is established to power a unidirectional, long-range charge transport from the excited-state electron donor via a transient C-60(center dot-) toward C-70(center dot-). This strategy helps minimize energy losses in the reductive, short-range charge shift from C-60 to C-70. At the forefront of our investigations are excited-state dynamics deduced from femtosecond transient absorption spectroscopic measurements and subsequent computational deconvolution of the transient absorption spectra. These provide evidence for cascades of short-range charge-transfer processes, including reductive charge shift reactions between the two electron-accepting fullerenes, and for kinetics that are influenced by the nature and length of the respective spacer. Of key importance is the postulate of a mediating state in the charge-shift reaction at weak electronic couplings. Our results point to an intimate relationship between triplet-triplet energy transfer and charge transfer.