Sequential or superexchange mechanism in bridged electron transfer distinguished by dynamics at a bridging molecule.
Sequential or superexchange mechanism in bridged electron transfer distinguished by dynamics at a bridging molecule.
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DOI：
10.1039/b822206a
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发表时间：
2009-06
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在DMA体系中，供体D和受体A通过桥连分子M以近距离固定的情况下，电子转移（ET）的两种机制是公知的。当中间态的自由能Δ G（m）|m远大于D（-）MA初始态以上的热能k（B）T，ET以超交换（SX）机制从D到A单步发生，通过|m作为量子力学虚态。然而，当DeltaG（m）变得比k（B）T高不了多少时，普通顺序（OS）ET可以显现其自身，其中，从|m到DMA（-）的终态发生在声子热化之后，|M.最近，随着DeltaG（m）的降低，SX机制如何切换到OS机制引起了很多兴趣。这一主题通常被描述为SX机制和OS机制的速率常数的总和。然而，这样的惯例是不合理的，因为这些机制是在相互相反的限制中实现的，这些限制与桥接分子介导ET的动力学有关，因此它们不能在单个系统中同时实现。Paulson、米勒、Gan和Closs（J. Am. 2005，127，4860）提供了该特征的一个很好的例子。在一个统一的框架下描述他们的观测结果，结果表明，开关发生在Δ G（m）与-0.5eV一致，这比他们报道的0.3eV低得多，其中热序ET揭示了自己，发生在声子的热化过程中，|M.
Two kinds of mechanisms are well known for electron transfer (ET) in the system DMA where a donor D and an acceptor A are fixed in a close distance by a bridging molecule M. When the free energy DeltaG(m) of the intermediate state |m of DM(-)A is much higher than the thermal energy k(B)T above the initial state of D(-)MA, the ET occurs unistep from D to A by the superexchange (SX) mechanism, passing |m as a quantum-mechanical virtual state. However, when DeltaG(m) becomes not much higher than k(B)T, the ordinary sequential (OS) ET may manifest itself, where the second ET from |m to the final state of DMA(-) takes place after thermalization of phonons in |m. Recently, much interest has been aroused in how the SX mechanism switches to the OS one as DeltaG(m) is lowered. This subject has often been described conventionally by summation of the rate constant for the SX mechanism and that for the OS one. However, such convention cannot be justified, since these mechanisms are realized in mutually opposite limits concerned with dynamics in mediation of ET by a bridging molecule, hence they cannot both be realized simultaneously in a single system. An observation of such a bridged ET by Paulson, Miller, Gan and Closs (J. Am. Chem. Soc. 2005, 127, 4860) provides a good example of this feature. Describing their observation in a unified framework for the ET, it is shown that the switch occurs at DeltaG(m) congruent with-0.5 eV, which is much lower than 0.3 eV reported by them, where the hot-sequential ET reveals itself, taking place during thermalization of phonons in |m.