Van 't Hoff Institute for Molecular Sciences (HIMS), University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.
Chemistry Department, University of Siena, Via Aldo Moro n. 2, 53100 Siena, Italy.
Chem Soc Rev. 2023 Apr 24;52(8):2643-2687. doi: 10.1039/d2cs00719c.
Over the last few decades, conical intersections (CoIns) have grown from theoretical curiosities into common mechanistic features of photochemical reactions, whose function is to funnel electronically excited molecules back to their ground state in regions where the potential energy surfaces (PESs) of two electronic states become degenerate. Analogous to transition states in thermal chemistry, CoIns appear as transient structures providing a kinetic bottleneck along a reaction coordinate. However, such a bottleneck is not associated with the probability of crossing an energy barrier but rather with an excited state decay probability along a full "line" of transient structures connected by non-reactive modes, the intersection space (IS). This article will review our understanding of the factors controlling CoIn mediated ultrafast photochemical reactions, taking a physical organic chemist approach by discussing a number of case studies for small organic molecules and photoactive proteins. Such discussion will be carried out by first introducing the "standard" one-mode model based on Landau-Zener (LZ) theory to describe a reactive excited state decay event intercepting, locally, a single CoIn along a single direction, and then by providing a modern perspective based on the effects of the phase matching of multiple modes on the same local event, thus redefining and expanding the description of the excited state reaction coordinate. The direct proportionality between the slope (or velocity) along one mode and decay probability at a single CoIn is a widely applied fundamental principle that follows from the LZ model, yet it fails to provide a complete understanding of photochemical reactions whose local reaction coordinate changes along the IS. We show that in these situations, in particular by focussing on rhodopsin double bond photoisomerization, it is mandatory to consider additional molecular modes and their phase relationship approaching the IS, hence providing a key mechanistic principle of ultrafast photochemistry based on the phase matching of those modes. We anticipate that this qualitative mechanistic principle should be considered in the rational design of any ultrafast excited state process, impacting various fields of research ranging from photobiology to light-driven molecular devices.
在过去的几十年里，锥形交叉点（CoIns）已从理论上的好奇心发展成为光化学反应的常见机制特征，其功能是将电子激发的分子引导回其基态，在两个电子态的势能面（PESs）变得简并的区域。类似于热化学中的过渡态，CoIns 作为瞬态结构出现，为沿着反应坐标提供了动力学瓶颈。然而，这种瓶颈与跨越能垒的概率无关，而是与沿着由非反应模式连接的瞬态结构的整个“线”的激发态衰减概率有关，即交叉空间（IS）。本文将通过讨论一些小有机分子和光活性蛋白的案例研究，从物理有机化学家的角度回顾我们对控制 CoIns 介导的超快光化学反应的因素的理解。这种讨论将首先通过引入基于 Landau-Zener（LZ）理论的“标准”单模模型来描述沿单个方向局部拦截单个 CoIns 的反应性激发态衰减事件，然后通过提供基于多个模式对同一局部事件的相位匹配的现代观点，从而重新定义和扩展对激发态反应坐标的描述。单模上的斜率（或速度）与单个 CoIns 处的衰减概率之间的直接比例关系是 LZ 模型广泛应用的基本原则，但它不能提供对沿 IS 变化的局部反应坐标的光化学反应的完整理解。我们表明，在这些情况下，特别是在关注视黄醛双键光异构化时，必须考虑接近 IS 的附加分子模式及其相位关系，从而提供基于这些模式的相位匹配的超快光化学的关键机制原则。我们预计，这个定性的机械原理应该在任何超快激发态过程的合理设计中考虑，这将影响从光生物学到光驱动分子器件的各个研究领域。
