Conformational Effects on Long-Range Electron Transfer: Comparison of Oligo-p-phenylene and Oligo-p-xylene Bridges

Phototriggered intramolecular electron transfer across variable-length ohgo-p-phenylene and oligo-p-xylene bridges was investigated in seven molecules. For both types of bridges, charge transfer rates decrease exponentially with increasing number of spacer units. The distance decay parameter is 0.21 angstrom(-1) for the phenylenes and 0.77 angstrom(-1) for the xylenes. A simple analysis based on superexchange theory indicates that this difference is due to unequal electronic coupling between adjacent bridging units. On the basis of the experimental data, phenyl-phenyl coupling is found to be roughly 7 times stronger than xylyl-xylyl coupling. This difference in electronic coupling strengths can be explained satisfactorily on the sole basis of conformational effects. It is consistent with equilibrium torsion angles of 35-40 degrees between two phenyls and 65-70 degrees between two xylyls. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)