Penfold Thomas J, Gindensperger Etienne, Daniel Chantal, Marian Christel M
Chemistry - School of Natural and Environmental Sciences , Newcastle University , Newcastle Upon-Tyne NE1 7RU , United Kingdom.
Laboratoire de Chimie Quantique, Institut de Chimie UMR-7177 , CNRS - Université de Strasbourg , 1 Rue Blaise Pascal 67008 Strasbourg , France.
Chem Rev. 2018 Aug 8;118(15):6975-7025. doi: 10.1021/acs.chemrev.7b00617. Epub 2018 Mar 20.
Intersystem crossing (ISC), formally forbidden within nonrelativistic quantum theory, is the mechanism by which a molecule changes its spin state. It plays an important role in the excited state decay dynamics of many molecular systems and not just those containing heavy elements. In the simplest case, ISC is driven by direct spin-orbit coupling between two states of different multiplicities. This coupling is usually assumed to remain unchanged by vibrational motion. It is also often presumed that spin-allowed radiationless transitions, i.e. internal conversion, and the nonadiabatic coupling that drives them, can be considered separately from ISC and spin-orbit coupling owing to the vastly different time scales upon which these processes are assumed to occur. However, these assumptions are too restrictive. Indeed, the strong mixing brought about by the simultaneous presence of nonadiabatic and spin-orbit coupling means that often the spin, electronic, and vibrational dynamics cannot be described independently. Instead of considering a simple ladder of states, as depicted in a Jablonski diagram, one must consider the more complicated spin-vibronic levels. Despite the basic ideas being outlined in the 1960s, it is only with the advent of high-level theory and femtosecond spectroscopy that the importance of the spin-vibronic mechanism for ISC in both fundamental as well as applied research fields has been revealed with significant impact across chemistry, physics, and biology. In this review article, we present the theory and fundamental principles of the spin-vibronic mechanism for ISC. This is followed by empirical rules to estimate the rate of ISC within this regime. The most recent developments in experimental techniques, theoretical methods, and models for the spin-vibronic mechanism are discussed. These concepts are subsequently illustrated with examples, including the ISC mechanisms in transition metal complexes, small organic molecules, and organic chromophores.
系间窜越（ISC）在非相对论量子理论中是形式上被禁止的，它是分子改变其自旋态的机制。它在许多分子系统的激发态衰变动力学中起着重要作用，而不仅仅是在那些含有重元素的分子系统中。在最简单的情况下，ISC是由不同多重性的两个状态之间的直接自旋 - 轨道耦合驱动的。通常假定这种耦合不会因振动运动而改变。人们还常常假定，自旋允许的无辐射跃迁，即内转换，以及驱动它们的非绝热耦合，由于假定这些过程发生的时间尺度差异极大，所以可以与ISC和自旋 - 轨道耦合分开考虑。然而，这些假设过于严格。实际上，非绝热和自旋 - 轨道耦合同时存在所带来的强混合意味着自旋、电子和振动动力学常常不能独立描述。与雅布隆斯基图中描绘的简单能级阶梯不同，人们必须考虑更复杂的自旋 - 振动态能级。尽管其基本思想在20世纪60年代就已被概述，但直到高级理论和飞秒光谱学出现，自旋 - 振动机理在ISC中的重要性才在基础研究和应用研究领域中得以揭示，并在化学、物理和生物学领域产生了重大影响。在这篇综述文章中，我们阐述了ISC的自旋 - 振动机理的理论和基本原理。随后给出了估计该机制下ISC速率的经验规则。讨论了自旋 - 振动机理在实验技术、理论方法和模型方面的最新进展。这些概念随后通过实例进行说明，包括过渡金属配合物、小分子有机化合物和有机发色团中的ISC机制。
