Evidence of σ- and π‑Dimerization in a Series of Phenalenyls

Phenalenyl
and a wide variety of its derivatives form stable radicals,
which often associate in various aggregates with interesting properties
that include magnetism and high electrical conductivity. The two main
modes of aggregation involve π-stacking pancake multicenter
bond formation and σ-bond formation. We explore the energetics
of the various σ- and π-dimers for six phenalenyl derivatives
with both computational and experimental methods. A modern density
functional theory (M05‑2X) is used to survey the potential
energy surface revealing the mechanism of the aggregation. In order
to enrich experimental data, the triphenyl and trimethyl derivatives
are newly prepared and their aggregation behaviors are investigated
by various analytical methods including ESR, 1H NMR, UV–vis,
and single-crystal X-ray diffraction. The agreement between computations
and experiments are very good forming the basis of describing trends
in this series. We find that π-dimer formation can proceed via
an asynchronous concerted path from the monomers or in a stepwise
process via σ-dimers. The strength of the π-stacking pancake
interaction depends strongly on substituents and covers a wide range
both in terms of binding energies and contact distances. The spin
densities in the π-stacking dimers reflect these trends and
display a wide range of diradicaloid characters. Many σ-dimer
configurations compete some of which are separated by small barriers
leading to fluxional structures between σ-bonded configurations
or σ- and π-bonded configurations.

苊萘基（phenalenyl）及其多种衍生物均可生成稳定自由基，这类自由基常通过聚集形成多种聚集体，展现出磁性与高电导率等诸多有趣特性。该类自由基的主要聚集模式分为两类：一是形成π堆叠煎饼型多中心键，二是形成σ键。本研究结合计算与实验手段，针对六种苊萘基衍生物的各类σ型与π型二聚体的能量特性展开了系统探究。我们采用现代密度泛函理论（M05‑2X）对势能面进行扫描，以此揭示聚集过程的反应机理。为丰富实验数据集，我们全新合成了三苯基与三甲基取代的苊萘基衍生物，并通过电子自旋共振（ESR）、核磁共振氢谱（¹H NMR）、紫外-可见吸收光谱（UV–vis）以及单晶X射线衍射等多种分析手段，对其聚集行为开展了表征与研究。计算结果与实验结果吻合度极佳，为本系列衍生物的性能趋势分析提供了坚实依据。研究表明，π型二聚体的形成既可通过单体发生异步协同反应路径完成，也可经由σ型二聚体以分步过程实现。π堆叠煎饼型相互作用的强度极大程度上依赖于取代基种类，其结合能与接触距离均覆盖了宽泛的数值范围。π堆叠二聚体中的自旋密度分布也印证了上述趋势，并展现出跨度较大的双自由基特征。众多σ型二聚体构型之间存在竞争关系，其中部分构型之间仅需跨越较低的能垒，即可在σ键合构型之间，或是σ键合与π键合构型之间实现动态互变。