Time-resolved EPR spectra of spin-correlated radical pairs: spectral and kinetic modulation resulting from electron-nuclear hyperfine interactions.
Time-resolved EPR spectra of spin-correlated radical pairs: spectral and kinetic modulation resulting from electron-nuclear hyperfine interactions.
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自旋相关自由基对的时间分辨 EPR 光谱：电子-核超精细相互作用产生的光谱和动力学调制。
DOI：
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发表时间：
2010
影响因子：
2.9
通讯作者：
M. Wasielewski
中科院分区：
文献类型：
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作者：
Q. Mi;M. Ratner;M. Wasielewski
本文对已建立的自旋相关自由基对(SCRP)的四态模型进行了扩展，使之包含了真实世界系统中普遍存在的局域核自旋，这是自由基对系统间交叉(RP-ISC)机制所必需的。这些核通过超精细相互作用耦合到未配对的电子自旋，并分裂出它们的电子顺磁共振(EPR)谱线。不是列举所有可能的核态，而是设计了一个算法来分类净超精细偏移量2q，它与电子自旋-自旋耦合2j一起表征了SCRP的行为。利用该算法，只需2J和单个自由基的EPR谱作为输入，就可以有效地模拟任意自旋的SCRP的EPR谱。具体说明性的是由光致电子转移产生的SCRP的情况，该SCRP由具有小的超精细分裂的光谱窄的阴离子自由基信号和具有许多大的超精细分裂的宽的阳离子自由基信号和高斯宽度西格玛组成，其中阴离子自由基的EPR峰表现出2(1/2)J(2)/西格玛的有效分裂。对于具有单重态和三重态电荷复合路径的SCRP，通过将衰变速率常数k(S)和k(T)作为虚能，并遵循已有的四态模型的推导，简明地得到了它们的动力学行为。这些模型被用来解释在光生SCRP的实验EPR谱中观察到的各种光谱和动力学调制模式，并提取反映施主-受主电子耦合的2J值。在光激发后的最初几百纳秒内，测得的时间分辨EPR谱的光谱和时间域特性体现了测不准原理的结果，而这两个域中的调制模式都是未配对电子和核自旋之间的超精细分裂的结果。
This paper expands the established four-state model of spin-correlated radical pairs (SCRPs) to include local nuclear spins which are ubiquitous in real-world systems and essential for the radical pair intersystem crossing (RP-ISC) mechanism. These nuclei are coupled to the unpaired electron spins by hyperfine interaction and split their electron paramagnetic resonance (EPR) lines. Rather than enumerating all possible nuclear states, an algorithm is devised to sort out the net hyperfine offset 2Q, which, along with the electron spin-spin coupling 2J, characterizes the behavior of SCRPs. Using this algorithm, the EPR spectra of SCRPs coupled to arbitrary nuclear spins can be efficiently simulated with only 2J and the EPR spectra of individual radicals as the inputs. Particularly illustrative is the case of a SCRP resulting from photoinduced electron transfer comprised of a spectrally narrow anion radical signal having small hyperfine splittings and a broad cation radical signal having many large hyperfine splittings and a Gaussian width sigma, where the EPR peak of the anion radical exhibits an effective splitting of 2(1/2)J(2)/sigma. For SCRPs having singlet and triplet pathways for charge recombination, their kinetic behavior is obtained concisely by considering the decay rate constants k(S) and k(T) as imaginary energies, while adhering to the existing derivation of the four-state model. These models are employed to interpret the diverse array of spectral and kinetic modulation patterns observed in the experimental EPR spectra of photogenerated SCRPs and to extract the 2J value, which reflects the donor-acceptor electronic coupling. During the first several hundred nanoseconds following photoexcitation, the spectral and time domain characteristics of the measured time-resolved EPR spectra manifest the consequences of the Uncertainty Principle, and the modulation patterns in either domain result from hyperfine splittings between the unpaired electron and the nuclear spins.