Excitation‐Dependent Multicolour Luminescence of Organic Materials: Internal Mechanism and Potential Applications

Excitation‐dependent emission (Ex‐de) materials have been of considerable academic interest and have potential applications in real life. Such multicolour luminescence is a characteristic exception to the ubiquitously accepted Kasha's rule. This phenomenon has been increasingly presented in some studies on different luminescence systems; however, a systematic overview of the mechanisms underlying this phenomenon is currently absent. Herein, we resolve this issue by classifying multicolour luminescence from single chromophores and dual/ternary chromophores, as well as multiple emitting species. The underlying processes are described based on electronic and/or geometrical conditions under which the phenomenon occurs. Before we present it in categories, related photophysical and photochemical foundations are introduced. This systematic overview will provide a clear approach to designing multicolour luminescence materials for special applications. According to Kasha's rule for luminescent materials, the emitting electronic level of a given multiplicity is the lowest excited level of that multiplicity. A characteristic exception, excitation‐dependent multicolour emission, is presented systemically, with the aim of enriching fundamental photophysics and potential applications. This Minireview will serve as a timely summary of the scattered reports of multicolour phenomena and their underlying mechanisms.