Localization of Spin and Charge in Phenalenyl-Based Neutral Radical Conductors

We report the development of an experimentally based structural analysis to examine the degree of localization of the spin and charge in the phenalenyl-based neutral radical molecular conductorsthe results motivate a reinterpretation of the electronic structure of a number of the radicals that we have reported over the past 10 years. The analysis is based on the well-known relationship between bond order and bond length and makes use of the experimental bond distance deviations between the molecular structure of the radical and its corresponding cation. We determined the single crystal X-ray structure of the ethyl radical (1) at 11 temperatures between 90 K and room temperature so that we could follow the evolution of the structure and the electron density distribution through the magnetic phase transition that occurs in the vicinity of 140K. We show that the enhanced conductivity in the dimeric ethyl (1) and butyl (3) radicals at the magnetic phase transition results from the development of a complex, but highly delocalized electronic structure and not to the formation of a diamagnetic π-dimer. We find that the monomeric radicals 4, 12, and 13 have an asymmetric electron density distribution in the crystal lattice whereas radical 11 is the only monomeric radical which remains fully delocalized. The π-chain radicals (7, 8, 14, and 15) retain the strongly delocalized electronic structures expected for a resonating valence bond ground-state structure.

本研究报道了一种基于实验的结构分析方法，用于探究基于菲烯基（phenalenyl）的中性自由基分子导体中自旋与电荷的局域化程度；该研究结果促使我们重新诠释过去十年间报道的多种自由基的电子结构。该分析基于键级与键长之间的经典关联，并利用了自由基分子结构与其对应阳离子之间的实验键长偏差。我们在90 K至室温的11个温度点下，测定了乙基自由基(1)的单晶X射线结构，以此追踪其结构与电子密度分布在140 K附近发生的磁相变过程中的演化规律。研究表明，二聚体乙基(1)与丁基(3)自由基在磁相变阶段的导电性增强，源于复杂但高度离域化的电子结构形成，而非抗磁性π二聚体的生成。我们发现，单体自由基4、12与13在晶格中呈现非对称电子密度分布，而自由基11是唯一保持完全离域化的单体自由基。π链自由基(7、8、14与15)则保留了共振价键基态结构所预期的强离域化电子结构。