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.