For more than 30 years, nonlinear optical (NLO) properties of molecular systems have been actively studied both theoretically and experimentally due to their potential applications in photonics and optoelectronics. Most of the NLO molecular systems are closed-shell species, while recently open-shell molecular species have been theoretically proposed as a new class of NLO systems, which exhibit larger NLO properties than the traditional closed-shell NLO systems. In particular, the third-order NLO property, the second hyperpolarizability γ, was found to be strongly correlated to the diradical character y, which is a quantum-chemically defined index of effective bond weakness or of electron correlation: the γ values are enhanced in the intermediate y region as compared to the closed-shell (y = 0) and pure open-shell (y = 1) domains. This principle has been exemplified by accurate quantum-chemical calculations for polycyclic hydrocarbons including graphene nanoflakes, multinuclear transition-metal complexes, main group compounds, and so on. Subsequently, some of these predictions have been substantiated by experiments, including two-photon absorption. The fundamental mechanism of the y-γ correlation has been explained by using a simple two-site model and the valence configuration interaction method. On the basis of this y-γ principle, several molecular design guidelines for controlling γ have been proposed. They consist in tuning the diradical characters through chemical modifications of realistic open-shell singlet molecules. These results open a new path toward understanding the structure-NLO property relationships and toward realizing a new class of highly efficient NLO materials. WIREs Comput Mol Sci 2016, 6:198-210. doi: 10.1002/wcms.1242.

中文翻译：

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开壳分子系统中的非线性光学性质。


30多年来，分子系统的非线性光学（NLO）特性因其在光子学和光电子学中的潜在应用而在理论和实验上得到了积极的研究。大多数非线性光学分子系统都是闭壳层物种，而最近开壳层分子物种在理论上被提出作为一类新的非线性光学系统，其表现出比传统闭壳层非线性光学系统更大的非线性光学性质。特别是，三阶 NLO 性质，即第二超极化率 γ，被发现与双自由基特征 y 密切相关，双自由基特征 y 是有效键弱度或电子相关性的量子化学定义指数：γ 值在中间 y 区域与闭壳层 (y = 0) 和纯开壳层 (y = 1) 域相比。这一原理已通过对多环烃（包括石墨烯纳米片、多核过渡金属配合物、主族化合物等）的精确量子化学计算得到例证。随后，其中一些预测已被实验证实，包括双光子吸收。通过使用简单的二位点模型和价态构型相互作用方法解释了 y-γ 相关性的基本机制。基于这种 y-γ 原理，已经提出了几种控制 γ 的分子设计指南。它们包括通过对真实的开壳单线态分子进行化学修饰来调整双自由基特征。这些结果为理解结构-非线性光学性质关系和实现新型高效非线性光学材料开辟了新途径。 WIREs 计算分子科学 2016 年，6：198-210。 doi：10.1002/wcms.1242。