Diverse emission properties of transition metal complexes beyond exclusive single phosphorescence and their wide applications

Luminescence properties and excited state reactivities of photoactive transition metal complexes have been harnessed for various applications in the fields of electroluminescent devices, luminescent sensing and bio-imaging, and solar energy conversion as well as photo-redox catalysis in organic transformation. While, beyond the conventional phosphorescence from triplet excited sates that are populated through heavy metal atom induced intersystem crossing (ISC), the past decade has witnessed blooming reports on intriguing luminescence behaviors such as sole fluorescence, dual fluorescence/phosphorescence, thermally activated delayed fluorescence (TADF), and dual phosphorescence. All these findings reveal that the possible photophysical mechanisms of transition metal complexes remain relatively underexplored. On one hand, the intersystem crossing (ISC) rate that is crucial to the depopulation of the singlet excited state and thus the population efficiency of the triplet excited state has proven to be not simply dependent on the spin-orbit coupling constant of the metal atom. On the other hand, the decay paths for triplet excited state(s) through reverse ISC (T1 → S1) and direct radiation from higher triplet excited state have been opened by appropriate ligand design. Moreover, these novel emission properties of transition metal complexes have expanded their applications into the fields of ratiometric sensing/imaging, noble-metal-free high efficiency emitters and photocatalysts and beyond. This review is aimed to discuss the unconventional emission properties and applications of transition metal complexes that were reported mainly in the past decade by focusing on the structural factors that are at play in realizing the novel excited state properties. It is conceived to provide a better understanding of the excited state properties tuning and to direct the design of new emissive metal complexes with tailored functions.