Spin-Correlated Luminescence of a Carbazole-Containing Diradical Emitter: Single-Molecule Magnetoluminescence and Thermally Activated Emission.

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 (|2/| > ∼10 K, where and 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 () and the corresponding monoradical () were prepared for the in-depth study of spin-correlated luminescence properties, with luminescence measurements under magnetic fields of up to 18 T. Diradical has a negative 2/ value of several tens of kelvin and exhibits a single-molecule MagLum and thermally activated luminescence, whereas does not. Detailed quantitative analyses revealed that both the spin-correlated luminescence properties of are strongly dominated by ground-state spin statistics based on the Boltzmann distribution (i.e., 2/ values). Furthermore, diradical exhibits external heavy-atom effects in heavy-atom-containing solvents such as iodobenzene, whereas monoradical 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.