Rigid Alkane-Bridged Donor-Acceptor Systems as Tools for the Investigation of Solvent-, Distance-, and Conformation-Effects in Electron Transfer Processes

Abstract

Several series of D(onor)-bridge-A(cceptor) systems have been developed in which a rigid alkane-bridge maintains a well defined distance and relative orientation of the electron donor-acceptor pair attached. For a given length of the bridge the effect of the solvating power of the medium on the rate of photoinduced intramolecular charge separation was studied as a function of structural variations at the D and/or A sites. The results of these studies are discussed in the context of current models for the thermodynamics and kinetics of electron transfer processes. It is shown that general and simple design criteria can be fonnulated, which define the properties of a D/A pair that is tuned to realize optimally fast electron transfer, in any solvent. across a bridge of a given length. The influence of the length and configuration of the alkane bridges on the rate of photoinduced charge separation as well as thennal recombination was investigated. Independently the ability of the bridges to mediate electronic coupling was studied in an experimental (PES, ETS) and theoretical investigation of bridged dienes. For the rate of photoinduced charge separation a very strong correlation with the structure dependence of the electronic coupling was observed. This demonstrates the importance of through-sigmabond interaction as a coupling mechanism for electron transfer, thereby stressing the decisive role of the intervening medium in mediating long-range electron transfer processes. Interestingly, charge recombination appeared to be equally responsive to the length but less responsive to the configuration of the bridge, and not at all responsive to temperature. The latter is attributed to a dominant contribution of nuclear tunneling. typical for electron transfer in the ′inverted region′.