Luminescent radicals have been intensively studied as a new class of materials exhibiting novel photofunctions unique to open-shell systems. When luminescent radicals are assembled, intriguing spin-correlated luminescence phenomena emerge, including excimer-like emission and magnetic-field effects on luminescence (i.e., magnetoluminescence, MagLum). However, the underlying mechanisms of these phenomena arising from spin multiplicity and spin-dependent excited-state dynamics are poorly understood due to the limited number of luminescent polyradical systems available for study. In particular, the correlation between stronger intramolecular exchange interactions (|2J/k_B| > ∼10 K, where J and k_B are the intramolecular exchange coupling constant and the Boltzmann constant, respectively) and luminescence properties has not been fully explained. In this study, a novel carbazole-containing diradical emitter (1) and the corresponding monoradical (2) were prepared for the in-depth study of spin-correlated luminescence properties, with luminescence measurements under magnetic fields of up to 18 T. Diradical 1 has a negative 2J/k_B value of several tens of kelvin and exhibits a single-molecule MagLum and thermally activated luminescence, whereas 2 does not. Detailed quantitative analyses revealed that both the spin-correlated luminescence properties of 1 are strongly dominated by ground-state spin statistics based on the Boltzmann distribution (i.e., 2J/k_B values). Furthermore, diradical 1 exhibits external heavy-atom effects in heavy-atom-containing solvents such as iodobenzene, whereas monoradical 2 does not. This is the first experimental verification of external heavy-atom effects in polyradical emitters. This work demonstrates that polyradical emitters can be designed based on spin degrees of freedom in both ground and excited states.

中文翻译：

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含咔唑双自由基发射体的自旋相关发光：单分子磁致发光和热激活发射


发光自由基作为一类新型材料已得到深入研究，表现出开壳系统特有的新颖光功能。当发光自由基组装时，会出现有趣的自旋相关发光现象，包括类准分子发射和磁场对发光的影响（即磁致发光，MagLum）。然而，由于可用于研究的发光多自由基系统的数量有限，人们对自旋多重性和自旋依赖性激发态动力学引起的这些现象的基本机制知之甚少。特别是，较强的分子内交换相互作用（|2 J / k _B | > ∼10 K，其中J和k _B分别是分子内交换耦合常数和玻尔兹曼常数）与发光性质之间的相关性尚未得到充分解释。本研究制备了一种新型含咔唑双自由基发射体 ( 1 ) 和相应的单自由基发射体 ( 2 )，用于深入研究自旋相关发光特性，并在高达 18 T 的磁场下进行了发光测量。 双自由基1具有几十开尔文的负 2 J / k _B值，并表现出单分子 MagLum 和热激活发光，而2则没有。详细的定量分析表明， 1的自旋相关发光特性很大程度上受基于玻尔兹曼分布（即 2 J / k _B值）的基态自旋统计的支配。 此外，双自由基1在含重原子的溶剂（例如碘苯）中表现出外部重原子效应，而单自由基2则不然。这是多自由基发射体中外部重原子效应的首次实验验证。这项工作表明，可以根据基态和激发态的自旋自由度来设计多自由基发射器。