Synthesis, intermolecular interaction, and semiconductive behavior of a delocalized singlet biradical hydrocarbon
Synthesis, intermolecular interaction, and semiconductive behavior of a delocalized singlet biradical hydrocarbon
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DOI：
10.1002/anie.200502303
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发表时间：
2005-01-01
影响因子：
16.6
通讯作者：
Nakasuji, K
中科院分区：
文献类型：
--
作者：
Kubo, T;Shimizu, A;Nakasuji, K
大多数KekulØ碳氢化合物，其特点是一个封闭的壳层电子构型，容纳π电子只在成键轨道，因此相当稳定。一般来说，这种稳定的KekulØ化合物具有较宽的HOMO-LUMO间隙，并且它们在基态的电子结构可以很好地用单电子组态来描述1Φ0。相反，一些KekulØ化合物，如醌类烃，具有相对较小的HOMO-LUMO间隙，这导致双激发构型的混合物1ΦH， H！L， L变成基态1Φ0中基态的描述。[1a]由于这种组态混合，考虑了两个电子之间的排斥，并且允许具有反平行自旋的两个电子在不同的空间中关联。这种电子相关降低了键共价，增加了体系的双基性。quinoidal KekulØ碳氢化合物具有高活性，在常温下容易二聚或聚合这种行为很容易让人联想到自由基反应。有关的有趣反应性已被观察到，即使在110 K下，通过两步环加成或对称禁止的[4s+ 4s]协同过程，它也很容易二聚化后一过程预计与1ΦH、H！L构型。类双基反应性意味着在基态有很强的分子间相互作用，这是单线态双基化合物最显著的特征。然而，由于分子间相互作用的不稳定性，在估计分子间相互作用时遇到了很大的困难。虽然动力学稳定的单线态双自由基，如Chichibabin碳氢化合物已经被分离和表征，但空间拥挤的结构掩盖了分子间有效的π -π相互作用为了估计分子间的相互作用，需要单线态双基的热力学稳定而不是动力学稳定。热力学稳定最有效的方法之一是使未配对电子离域。在这里，我们把重点放在苯烯基自由基作为一个高度自旋定域的分子上（方案1）。自由基是已知的
Most KekulØ hydrocarbons, which are characterized by a closed-shell electron configuration, accommodate their π electrons only in bonding orbitals and consequently are quite stable. In general, such stable KekulØ compounds have wide HOMO–LUMO gaps, and their electronic structures in the ground state can be well described by a single electron configuration 1Φ0. In contrast, some KekulØ compounds, such as quinoid hydrocarbons, have a relatively small HOMO–LUMO gap, which leads to an admixture of a doubly excited configuration 1ΦH, H! L, L into the ground configuration 1Φ0 in the description of the ground state.[1a] As a result of this configuration mixing, the repulsion between the two electrons is taken into consideration and the two electrons with antiparallel spins are permitted to correlate in separate spaces. This electron correlation diminishes the bond covalency and increases the biradical character of the system.The quinoidal KekulØ hydrocarbons are highly reactive and easily dimerize or polymerize at normal temperature.[2] Such behavior is strongly reminiscent of radical reactivity. Related interesting reactivity has been observed for pleiadene, which dimerizes readily even at 110 K through a twostep cycloaddition or a symmetry-forbidden [4s+ 4s] concerted process.[1] The latter process is predicted to be related to the 1ΦH, H! L, L configuration. The biradical-like reactivity implies strong intermolecular interactions in the ground state, which would be a most remarkable feature for singlet biradical compounds. However, great difficulties are encountered in estimating the intermolecular interaction due to the instability. Although kinetically stabilized singlet biradicals such as Chichibabin s hydrocarbon have been isolated and characterized, sterically crowded structures mask the effective intermolecular π–π interactions.[3] To estimate the intermolecular interactions, thermodynamic stabilization of the singlet biradicals would be required instead of kinetic stabilization. One of the most effective means of thermodynamic stabilization is delocalization of unpaired electrons. Herein, we focus on phenalenyl radical as a highly spindelocalized molecule (Scheme 1). The radical is known to be