Recent development of the transition metal complexes showing strong absorption of visible light and long-lived triplet excited state: From molecular structure design to photophysical properties and applications

Transition metal complexes have been studied for decades due to their interesting photophysical processes and their applications in photocatalysis, photodynamic therapy (PDT) and photovoltaics. The photophysical properties, especially the strong visible light-harvesting ability (e.g. molar absorption coefficients epsilon > 50000 M-1 cm(-1)) and long triplet state lifetimes (e.g. tau(T) > 10 mu s), are crucial for the performance of these complexes in their applications. In this review article, the reasons for the weak absorption of visible light and short triplet state lifetime of the conventional transition metal complexes were discussed. Then we summarized the recent development of the transition metal complexes showing strong absorption capability in the visible region and long-lived triplet states, some representative complexes were introduced. We focused on the discussions of the strategies to design transition metal complexes showing strong visible light absorption and long-lived triplet state. We pointed out that matching of the excited state energy of the ligands and the coordination framework and effective intersystem crossing (ISC) have to be considered. In order to prolong the triplet state lifetime, excited state equilibrium, population of the ligand-localized triplet state or 'ping-pong' singlet/triplet energy transfer system can be used. The applications of the complexes in phosphorescence oxygen sensing, PDT and photocatalytic hydrogen (H-2) evolution were briefly introduced as well. (C) 2020 Elsevier B.V. All rights reserved.