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 S-1 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 S-n 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 T-n. Intersystem crossing (ISC) from S-5 to T-n can successfully compete with internal conversion, followed by reverse ISC from T-n to S-2 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.