Unexpectedly similar charge transfer rates through benzo-annulated bicyclo[2.2.2]octanes

  • Posted on: 10 July 2014
  • By: admin
TitleUnexpectedly similar charge transfer rates through benzo-annulated bicyclo[2.2.2]octanes
Publication TypeJournal Article
Year of Publication2008
AuthorsGoldsmith R.H, Vura-Weis J., Scott A.M, Borkar S., Sen A., Ratner M.A, Wasielewski M.R
JournalJournal of the American Chemical Society
Volume130
Pagination7659-7669
Date PublishedJun 18
ISBN Number0002-7863
Accession NumberISI:000256671400037
Keywordsbridge-acceptor systems, electron-transfer reactions, energy-transfer, free-radicals, model, molecular-wire, photosynthetic reaction centers, spin transport, through-bond, tunneling paths
Abstract

A 4-(pyrrolidin-1-yl)phenyl electron donor and 10-cyanoanthracen-9-yl electron acceptor are attached via alkyne linkages to the bridgehead carbon atoms of bicyclo[2.2.2]octane and all three benzo-annulated bicyclo[2.2.2]octanes. The a-system of bicyclo[2.2.2]octane provides a scaffold having nearly constant bridge geometry on which to append multiple, weakly interacting benzo T-bridges, so that the effect of incrementally increasing numbers of 7-bridges on electron transfer rates can be studied. Surprisingly, photoinduced charge transfer rates measured by transient absorption spectroscopy in toluene show no benefit from increasing the number of bridge pi-systems, suggesting dominant transport through the sigma-system. Even more surprisingly, the significant changes in hybridization undergone by the a-system as a result of benzo-annulation also appear to have no effect on the charge transfer rates. Natural Bond Orbital analysis is applied to both sigma- and pi-communication pathways. The transient absorption spectra obtained in 2-methyltetrahydrofuran(MTHF) show small differences between the benzo-annulated molecules that are attributed to changes in solvation. All charge transfer rates increase significantly upon cooling the MTHF solutions to their glassy state. This behavior is rationalized using combined molecular dynamics/electronic structure trajectories.

Alternate JournalJ Am Chem Soc