Department of Chemistry, University of California, Riverside, California 92521-0403, USA.
J Am Chem Soc. 2010 Dec 8;132(48):17258-64. doi: 10.1021/ja107201d. Epub 2010 Nov 16.
We have investigated the solid-state electronic structure and properties of a phenalenyl-based butyl-substituted neutral radical, 3, that shows a hysteretic phase transition just above room temperature. We quantitatively analyzed the electron density distribution of this radical throughout both branches of the hysteretic phase transition using solid-state X-ray structures and found two distinct electronic states in the hysteresis loop that accompanies the phase transition. The bistability of the two electronic states was observed through a number of measurements, including IR transmittance spectra of single crystals in the vicinity of the phase transition. By comparing the changes in the crystal structures of 3 and the related ethyl-substituted radical 1 (which exhibits no hysteresis) at various temperatures, we show that the change in the interplanar π-π distance within dimers is the most important structural parameter in determining the physical properties of the radicals. The large change in the C-H···π interaction in 3 occurs in concert with the spin redistribution during the phase transition, but these factors are not responsible for the hysteresis effect. We suggest that the presence of a high-temperature state inside the hysteretic loop during the cooling cycle is due to thermodynamic stability, while the existence of the low-temperature state during the heating cycle is due to the presence of a large energy barrier between the two states (estimated to be greater than 100 kJ/mol) that results from the large-amplitude motion of the phenalenyl rings and the associated lattice reorganization energy that is required at the phase transition.
我们研究了基于菲咯啉的丁基取代中性自由基 3 的固态电子结构和性质，3 在室温以上表现出滞后相转变。我们使用固态 X 射线结构定量分析了该自由基在滞后相转变的两个分支中的电子密度分布，并在伴随相转变的滞后环中发现了两个不同的电子态。通过包括在相变附近的单晶的红外透射光谱在内的多项测量，观察到两个电子态的双稳性。通过比较 3 和相关的乙基取代自由基 1（没有滞后）在不同温度下的晶体结构变化，我们表明，二聚体中面间π-π 距离的变化是决定自由基物理性质的最重要结构参数。在相变过程中，3 中 C-H···π 相互作用的巨大变化与自旋重新分布同时发生，但这些因素不是滞后效应的原因。我们认为，在冷却循环中滞后环内存在高温态是由于热力学稳定性，而在加热循环中存在低温态是由于两个态之间存在大的能垒（估计大于 100 kJ/mol）所致，这是由于菲咯啉环的大振幅运动和相变所需的相关晶格重排能引起的。
