Resonating Valence Bond and σ-Charge Density Wave Phases in a Benzannulated Phenalenyl Radical

We report the preparation of the first benzannulated phenalenyl neutral radical conductor (18), and we show that the compound displays unprecedented solid state behavior: the structure is dominated by two sets of intermolecular interactions: (1) a π-chain structure with superimposed π-overlap of the benzannulated phenalenyls along [0 0 1], and (2) an interchain overlap involving a pair of carbon atoms (C4) along [0 1 0]. The π-chain-type stacking motif is reminiscent of previously reported phenalenyl radicals and the room temperature structure (space group P2/c) together with the conductivity of σRT = 0.03 S/cm and the Pauli-like magnetic susceptibility are best described by the resonating valence bond (RVB) model. The interchain interaction is unstable with respect to the formation of a σ-charge density wave (σ-CDW) involving pairs of C4 carbon atoms between adjacent radicals and this phase is characterized by the P21/c space group which involves a doubling of the unit cell along the [0 1 0] direction. The RVB and CDW phases compete for structural occupancy throughout the whole temperature range (15−293 K) with the RVB phase predominating at 15 and 293 K and the σ-CDW phase achieving a maximum structural occupancy of about 60% at 150 K where it produces clearly discernible effects on the magnetism and conductivity.

本研究报道了首例苯并稠合苊烯基中性自由基导体（18）的制备，并发现该化合物展现出前所未有的固态行为：其晶体结构由两类分子间相互作用主导：(1) 沿[0 0 1]方向的苯并稠合苊烯基之间形成叠加π重叠的π链结构；(2) 沿[0 1 0]方向涉及一对C4碳原子的链间重叠作用。π链型堆叠基元与此前报道的苊烯自由基相似；而该化合物的室温晶体结构（空间群space group P2/c）、室温电导率σ_RT=0.03 S/cm以及泡利型磁化率，均可通过共振价键（resonating valence bond, RVB）模型得到最佳阐释。该链间相互作用对于相邻自由基间由C4碳原子对介导的σ电荷密度波（σ-charge density wave, σ-CDW）的形成具有不稳定性；该物相以空间群P21/c为特征，其晶胞（unit cell）沿[0 1 0]方向发生加倍。在15~293 K的整个温度区间内，RVB物相与σ-CDW物相竞争结构占比：RVB物相在15 K与293 K时占主导地位，而σ-CDW物相在150 K时达到约60%的最大结构占比，此时其对材料的磁性与电导率产生了可清晰观测的影响。