Color-Pure Organic Luminophores: Characteristics, Definitions, Physical Basis and Fundamental Design Principles

Color-pure all-organic emitters, i.e., with narrow spectral characteristics, are intensively studied for high-definition organic LEDs (mainly based on multi-resonance structures), as well as for multi-color bioimaging (mainly based on polymethine-like structures). To guide targeted materials design beyond tedious trial-and-error procedures, this educative review discusses the spectral characteristics (including units and unambiguous definition of the spectral width, and the occurrence of artifacts), the physical basis of the spectral broadening (vibronic coupling, environmental effects, and a basal understanding of geometries in the ground and excited states). Subsequently, the structural factors are distilled, which govern reduced geometry change between ground and excited state (GS, ES)-quantified as the change in bond-length alternation (Δ_BLA) upon excitation. On this basis, three complementary design strategies emerge, that is: (a) full and (b) partial bond length equalization, yielding intrinsically small- and mixed-BLA emitters, and (c) MO topology inversion, with large BLA in GS and ES; secondary factors include, (d) rigidification and (e) the extension of the conjugated path. Finally, we address challenges and solutions when targeting not only color-pure but also bright emitters. In all, our insights allow for an intuitive, chemical approach towards efficient color-pure emitters beyond sole generation of structures and numbers.