Anti‐Kasha Emission from Organic Aggregates

For most compounds in nature, emission is mainly radiative transition from the lowest excited multiplet state to the ground state and is independent of excitation wavelength. However, there are some unusual emissions from upper excited states, known as anti-Kasha emissions. Numerous examples of such emissions have been reported, but the phenomenon is rather rare for organic aggregate luminogen systems, which is mainly limited by harsh prerequisites such as fast radiative process of upper excited states, limited internal conversion caused by energy gaps between upper excited state and the lowest excited state, decreasing nonradiative depopulation and so on. Herein, we will review some examples of anti-Kasha emission in organic aggregate systems, carefully and rigorously classify them into several potential molecular structures, including aromatic carbonyl systems, azulene derivatives, and other unconventional aromatic compounds and detailedly analyze several key factors in realizing anti-Kasha emission. These results facilitate the understanding of constructing the structure-property relationship between anti-Kasha emission and molecular structure, and provide theoretical guidance and structural model for molecular design for future researchers.