Electron Tunneling through Oligo-p-xylene Bridges

A series of rigid rodlike molecules having a phenothiazine donor, oligo- p-xylene bridges, and a rhenium(I) tricarbonyl phenanthroline acceptor were synthesized and studied in the context of long-range electron transfer. By optical absorption spectroscopy, the p-xylene bridges are found to have essentially length-independent HOMO-LUMO energy gaps, which is in clear contrast to oligo- p-phenylene spacers. Nanosecond time-resolved luminescence spectroscopy reveals an exponential decrease of electron transfer rates with increasing donor-acceptor distance; the attenuation factor beta is 0.52 A (-1) for the xylene bridges, which is strikingly close to beta values reported previously for unsubstituted phenylene spacers.