Electron transfer rates in bridged molecular systems 2. A steady-state analysis of coherent tunneling and thermal transitions

Dvira Segal, Abraham Nitzan, William B. Davis, Michael R Wasielewski, Mark A Ratner

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Abstract

The effect of dephasing and relaxation on electron transfer in bridged molecular systems is investigated using a simple molecular model. The interaction between the molecular system and the thermal environment is described on the level of the Redfield theory, modified when needed for the description of steady-state situations. Noting that transient as well as steady-state measurements are possible in such system, we discuss the relationship between the rates obtained from these different types of experiments and, in particular, the conditions under which these rates are the same. Also, a formal relation between the steady-state rate for electron transfer across a molecular bridge and the conductance of this bridge when placed between two metal contacts is established. The effect of dephasing and relaxation on the electron transfer is investigated, and new observations are made with regard to the transition from the superexchange to the thermal (hopping through bridge) regime of the transfer process. In particular, the rate is temperature-independent in the superexchange regime, and its dependence on the bridge length (N) is exponential, exp(-βN). The rate behaves like (α1 + α2N)-1 exp(-ΔE/kBT) beyond a crossover value of N, where ΔE is the energy gap between the donor/acceptor and the bridge levels, and where α1 and α2 are characteristic times for activation onto the bridge and diffusion in the bridge, respectively. We find that, in typical cases, α1 ≫ α2, and therefore, a region of very weak N dependence is expected before the Ohmic behavior, N-1, is established for large enough N. In addition, a relatively weak exponential dependence, exp(-αN), is expected for long bridges if competing processes capture electrons away from the bridge sites. Finally, we consider ways to distinguish experimentally between the thermal and the tunneling routes.

Original languageEnglish
Pages (from-to)3817-3829
Number of pages13
JournalJournal of Physical Chemistry B
Volume104
Issue number16
Publication statusPublished - Apr 27 2000

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Electron transitions
electron transfer
Electrons
Hot Temperature
thermal environments
electron capture
electric contacts
crossovers
Energy gap
Metals
Chemical activation
routes
activation
metals

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Engineering(all)

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Electron transfer rates in bridged molecular systems 2. A steady-state analysis of coherent tunneling and thermal transitions. / Segal, Dvira; Nitzan, Abraham; Davis, William B.; Wasielewski, Michael R; Ratner, Mark A.

In: Journal of Physical Chemistry B, Vol. 104, No. 16, 27.04.2000, p. 3817-3829.

Research output: Contribution to journalArticle

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