Interplay between Barrier Width and Height in Electron Tunneling: Photoinduced Electron Transfer in Porphyrin-Based Donor-Bridge-Acceptor Systems

Journal article, 2006

The rate of electron tunneling in mol. donor-bridge-acceptor (D-B-A) systems is detd. both by the tunneling barrier width and height, i.e., both by the distance between the donor and acceptor as well as by the energy gap between the donor and bridge moieties. These factors are therefore important to control when designing functional electron transfer systems, such as constructs for photovoltaics, artificial photosynthesis, and mol. scale electronics. The authors have studied a set of D-B-A systems in which the distance and the energy difference between the donor and bridge states (DEDB) are systematically varied. Zinc(II) and gold(III) porphyrins were chosen as electron donor and acceptor because of their suitable driving force for photoinduced electron transfer (-0.9 eV in butyronitrile) and well-characterized photophysics. The authors have previously shown, in accordance with the superexchange mechanism for electron transfer, that the electron transfer rate is proportional to the inverse of DEDB in zinc/gold porphyrin D-B-A systems with bridges of const. edge to edge distance (19.6 .ANG.) and varying DEDB (3900-17,600 cm-1). Here, the authors use the same donor and acceptor but the bridge is shortened or extended giving a set of oligo-p-phenyleneethynylene bridges (OPE) with four different edge to edge distances ranging from 12.7 to 33.4 .ANG.. These two sets of D-B-A systems-ZnP-RB-AuP+ and ZnP-nB-AuP+-have one bridge in common, and hence, for the first time both the distance and DEDB dependence of electron transfer can be studied simultaneously in a systematic way. [on SciFinder (R)]