Dual Emission: Classes, Mechanisms, and Conditions.

There has been much interest in dual-emission materials in the past few years for materials and life science applications; however, a systematic overview of the underlying processes is so-far missing. We resolve this issue herein by classifying dual-emission (DE) phenomena as relying on one emitter with two emitting states (DE1), two independent emitters (DE2), or two correlated emitters (DE3). Relevant DE mechanisms for materials science are then briefly described together with the electronic and/or geometrical conditions under which they occur. For further reading, references are given that offer detailed insight into the complex mechanistic aspects of the various DE processes or provide overviews on materials families or their applications. By avoiding ambiguities and misinterpretations, this systematic, insightful Review might inspire future targeted designs of DE materials.