The replacement of inorganic semiconductors with molecule-based compounds for applications in current-to-light conversion has led to a significant increase in interdisciplinary collaborations worldwide, affording new improved organic-light emitting diodes (OLEDs) ripe for commercial applications, as well as light-emitting electrochemical cells (LECs) that have recently started to head to the market. This review highlights the role that transition metal coordination complexes (TMCs) have played in advancing the field of molecular electronics, from early conception to the advanced development of several polypyridyl complexes currently pursued for both OLED and LEC concepts. In this context, the design and synthesis of Ir(III), Pt(II), Cu(I) and Ag(I) complexes as the emissive components of OLEDs and LECs are thoughtfully presented. We discuss how molecular design is pivotal for fine-tuning color and optimizing power efficiencies, highlighting the key roles of the metal, cyclometalate, and ancillary polypyridyl ligands. We provide insight into the strategies exploited for the development of new, improved emitters and their fabrication into OLEDs/LECs with high external quantum efficiencies and stabilities. In addition, we have surveyed the remarkable photophysical properties of third generation TMCs capable of undergoing thermally activated delayed fluorescence (TADF). Since previous reviews of TADF materials are strongly biased towards organic-based systems, this overview compliments other synopses of light emitting TADF materials. Finally, we shed light onto the conceptual challenges that still need to be overcome to advance the rational design of TMC-based TADF emitters with tunable ligands and the subsequent fabrication of OLEDs/LECs, which are tailor-made for each specific application.
