Anti‐Kasha Emission in Organic Radicals: Mechanistic Insights and Experimental Caveats

Organic radicals generally adhere to Kasha's rule, where luminescence originates from the lowest excited state. However, certain radical systems deviate from this rule, exhibiting anti-Kasha emission from higher-energy excited states. This review examines the photophysical properties of these radicals, addressing key challenges in the field, to encourage rigorous scientific investigation and improve the reliability of future findings.

Organic radicals have recently emerged as promising candidates for luminescent materials, often exhibiting emissions that challenge the traditional Kasha's rule. While a growing number of reports claim anti-Kasha emission from higher excited states in radicals, the photophysical origins of such emissions remain ambiguous. In this Concept, we critically assess the mechanistic underpinnings of anti-Kasha behavior in radical systems and highlight the experimental caveats that may lead to misinterpretation—including emission from closed-shell impurities, photodegradation byproducts, or incomplete radical formation. By revisiting key examples and providing a framework for rigorous spectral validation, this article aims to recalibrate the standards of evidence in radical photophysics and guide the development of reliable design strategies for high-energy emitters.

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