Molecule Isomerism Modulates the Diradical Properties of Stable Singlet Diradicaloids

Inclusion of quinoidal
cores in conjugated hydrocarbons is a common
strategy to modulate the properties of diradicaloids formed by aromaticity
recovery within the quinoidal unit. Here we describe an alternative
approach of tuning of diradical properties in indenoindeno­dibenzo­thiophenes
upon anti → syn isomerism
of the benzothiophene motif. This alters the relationship of the S
atom with the radical center from linear to cross conjugation yet
retains the same 2,6-naphtho conjugation pattern of the rearomatized
core. We conduct a full comparison between the anti and syn derivatives based on structural, spectroscopic,
theoretical, and magnetic measurements, showing that these systems
are stable open-shell singlet diradicaloids that only access their
triplet state at elevated temperatures.

将醌式核心引入共轭烃类，是调控醌式单元内经芳香性恢复生成的双自由基体（diradicaloids）性质的常用策略。本文报道了一种调控茚并茚并二苯并噻吩（indenoindeno­dibenzo­thiophenes）类双自由基性质的替代策略：通过苯并噻吩基元（benzothiophene motif）的反式→顺式异构化实现。该异构化过程使硫原子与自由基中心的共轭作用模式从线性共轭转变为交叉共轭，同时保留了再芳构化核心的2,6-萘型共轭模式不变。我们基于结构表征、光谱学分析、理论计算与磁性测量，对反式和顺式衍生物开展了全面对比，结果显示这类体系均为稳定的开壳层单线态双自由基体，仅在升温条件下才会跃迁至三重态。