Theoretical Study on Diradical Characters and Nonlinear Optical Properties of 1,3-Diradical Compounds
Theoretical Study on Diradical Characters and Nonlinear Optical Properties of 1,3-Diradical Compounds
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DOI：
10.1021/jp508657s
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发表时间：
2014-11-13
影响因子：
2.9
通讯作者：
Nakano, Masayoshi
中科院分区：
文献类型：
--
作者：
Kishi, Ryohei;Murata, Yusuke;Nakano, Masayoshi
利用对称性破缺密度泛函理论方法研究了开壳层1，3-双自由基化合物的双自由基性质（y）与非线性光学性质（NLO）之间的关系.阐明了几种1，3-二苯基环戊烷-1，3-二基衍生物的2，2-取代基效应对构效关系的影响，这些衍生物被认为是具有弱或中等p-单键特征的体系。母体1，3-二苯基环戊烷-1，3-二基（1a：X = H）由于具有最高占据分子轨道（HOMO）和最低未占分子轨道（LUMO）而几乎是纯的双自由基（y近似于1）。能隙由通过空间耦合与通过键耦合效应的平衡决定。另一方面，在环戊烷-1，3-二基化合物（1b：X = OH，1c：X = F）的C2位引入吸电子取代基X，由于额外的通过键相互作用，y值降低。结果，发现1b和1c具有中间体y。静态第二超极化率（γ）的1b和1c被发现分别提高了4.5和6.4倍，与纯的单线态双自由基1a和三线态1a-1c相比。通过对电子密度三阶响应的分析，发现2，2-取代基的引入增强了整个体系的场致三阶极化。我们还研究了苯环对位的不对称给体/受体取代对响应特性的影响。虽然不对称供体/受体取代在本系统中对y没有显著影响，但发现它们提供了来自相应的未取代类似物的γ的增加。结果表明，在真实的1，3-双自由基化合物中，p-键特征（双自由基特征）与三阶非线性光学性质之间存在着很强的相关性。基于理论预测的真实的体系中的相关性，我们推测非线性光学测量可以作为研究定域双自由基化合物中独特化学键性质的新探针，这是化学中的基础课题之一。
We investigate the relationships between the diradical character (y) and nonlinear optical (NLO) properties of open-shell 1,3-diradical compounds using the broken-symmetry density functional theory method. The 2,2-substituent effects on the structureproperty relationship are clarified for several 1,3-diphenylcyclopentane-1,3-diyl derivatives, which are known as the systems with weak or intermediate p-single-bonding characters. The parent 1,3-diphenylcyclopentane-1,3-diyl (1a: X = H) is found to be almost pure diradical (y similar to 1) owing to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The energy gap is determined by the balance of the through-space coupling with the through bond coupling effect. On the other hand, the introduction of the electron-withdrawing substituents X at the C2 position of cyclopentane-1,3-diyls (1b: X = OH, 1c: X = F) is found to decrease the y-value owing to the effects of additional through-bond interactions. As a result, 1b and 1c are found to have intermediate y. Static second hyperpolarizabilities (gamma) of 1b and 1c are found to be enhanced by a factor of similar to 4.5 and similar to 6.4, respectively, compared with those of the pure singlet diradical 1a and those of the triplet 1a-1c. From the analysis of the third-order responses of electron density, the introduction of the 2,2-substituents is found to enhance the field-induced third-order polarizations over the whole system. We also investigate the effects of asymmetric donor/acceptor substitutions at the para positions of phenyl rings on the response properties. Although the asymmetric donor/acceptor substitutions have no significant impact on y in the present systems, they are found to provide the increase of gamma from the corresponding nonsubstituted analogues. The present results have revealed strong correlation between the p-bonding character (diradical character) and third-order NLO properties in the real 1,3-diradical compounds. On the basis of the theoretically predicted correlation in the real systems, NLO measurements are speculated to be utilized as a new probe of the unique chemical bonding nature in such localized diradical compounds, which is one of the fundamental subjects in chemistry.