Upper Excited Triplet State-Mediated Intersystem Crossing for Anti-Kasha’s Fluorescence: Potential Application in Deep-Ultraviolet Sensing

Owing to the Kasha rule, only the lowest excited state S₁ contributes to the photoemission or other photoinduced processes in general, causing a waste of photoenergy and the limitation of application scenarios. The anti-Kasha effect offers the possibility of utilizing high-energy excited states Sₙ to develop novel functions and applications. Here, an anti-Kasha fluorescence has been experimentally found in a pure organic molecule and investigated in detail to reveal the underlying mechanism. The experimental lines of evidence of the excitation mapping spectrum and time-resolved photoluminescence spectrum suggest that the anti-Kasha emission is governed by an upper excited triplet state Tₙ. Intersystem crossing (ISC) from S₅ to Tₙ can successfully compete with internal conversion, followed by reverse ISC from Tₙ to S₂ to form upper-state emission. Further, utilizing this effect for the deep-ultraviolet light sensor is discussed. These findings provide keen insights into manipulating the excited-state evolution, while offering the possibility for utilizing the high-energy excited states to explore novel functions and applications of organic molecules.