Superexchange and Sequential Mechanisms in Charge Transfer with a Mediating State between the Donor and Acceptor
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- 作者：Basil Pavlatos Paulson ; John R. Miller ; Wei-Xing Gan ; and Gerhard Closs
- 刊名：Journal of the American Chemical Society
- 出版年：2005
- 出版时间：April 6, 2005
- 年：2005
- 卷：127
- 期：13
- 页码：4860 - 4868
- 全文大小：155K
- 年卷期：v.127,no.13(April 6, 2005)
- ISSN：1520-5126
文摘
The rate of intramolecular charge transfer from biphenyl to naphthalene was determined for theradical anions and radical cations of molecules with the general structure: (2-naphthyl)-(steroid spacer)-(4-biphenylyl). Varied degrees of unsaturation (one double bond, NSenB; two double bonds, NSen2B; andthe b-ring completely aromatized, NSarB) were incorporated into the steroid spacer to examine the effectit would have on the charge transfer rate. The charge transfer rate, as inferred from the decay of thebiphenyl radical ion absorption, increased in all cases relative to the completely saturated 3-(2-naphthyl)-16-(4-biphenylyl)-5
-androstane (NSB) reference molecule. For the anion charge transfer, the decay ratesincreased by factors of 1.4, 4.2, and 5.1, respectively, and for the cation, the decay rates increased byfactors of 5, 276, and 470. To explain the results, the charge-transfer process was viewed as a combinationof two independent mechanisms: a single-step, superexchange mechanism, and a two-step, sequentialcharge transfer. Using a low level of theory, simple models of the superexchange and two-step mechanismswere developed to elucidate the nature and differences between the two mechanisms. The critical variablefor this analysis is the free energy of formation (
GI
) of the intermediate state: (2-naphthyl)-[spacer]1±-(4-biphenylyl). The conclusion from this treatment is that superexchange is the dominant mechanism whenGI is large, but at small GI, the sequential mechanism will dominate. This is because the superexchangerate is shown to have a weak dependence on GI, changing 10-fold for a change in GI of 2 eV, comparedto the sequential mechanism in which the rate can change over 103 for 0.5 V.
-androstane (NSB) reference molecule. For the anion charge transfer, the decay ratesincreased by factors of 1.4, 4.2, and 5.1, respectively, and for the cation, the decay rates increased byfactors of 5, 276, and 470. To explain the results, the charge-transfer process was viewed as a combinationof two independent mechanisms: a single-step, superexchange mechanism, and a two-step, sequentialcharge transfer. Using a low level of theory, simple models of the superexchange and two-step mechanismswere developed to elucidate the nature and differences between the two mechanisms. The critical variablefor this analysis is the free energy of formation (GI) of the intermediate state: (2-naphthyl)-[spacer]1±-(4-biphenylyl). The conclusion from this treatment is that superexchange is the dominant mechanism whenGI is large, but at small GI, the sequential mechanism will dominate. This is because the superexchangerate is shown to have a weak dependence on GI, changing 10-fold for a change in GI of 2 eV, comparedto the sequential mechanism in which the rate can change over 103 for 0.5 V.