Recent Advances in Metal Triplet Emitters with d6, d8, and d10 Electronic Configurations

Recent studies have shown that Au(III), Pd(II), and W(VI) complexes could exhibit strong phosphorescence or delayed fluorescence at room temperature with high quantum yield. The introduction of amino substituents on the ligand of Au(III) and W(VI) complexes could switch the emission from phosphorescence to TADF. One common feature of these complexes is that their triplet ligand-centered excited states have a small kr of 102–103 s–1 due to the lack of metal character. In addition to binuclear Pd(II) complexes with short intramolecular Pd–Pd distance, mononuclear Pd(II) complexes can display emissive 1/3MMLCT excited states upon aggregation with high emission quantum yield and large kr. The major driving force of aggregation is considered to be the cation–cation dispersive interaction, where the ligand–ligand dispersion plays an important role. Basic research on the photophysical and photochemical properties of metal triplet emitters has been fueled by their practical applications in diverse areas, including bioimaging, photocatalysis, and organic light-emitting diodes (OLEDs). In addition to the extensively investigated Ru(II), Ir(III), and Pt(II) complexes, a wide range of luminescent Pd(II), Au(III)/Au(I), Re(I), Cu(I), W(VI)/W(0), and Mo(0) complexes have recently been revealed via the judicious choice of ligands to exhibit unique emissive excited states with a large range of radiative and nonradiative decay rate constants. Here, we provide an account of the design strategies of and recent advances on metal triplet emitters. Questions regarding future research in this field are presented. Basic research on the photophysical and photochemical properties of metal triplet emitters has been fueled by their practical applications in diverse areas, including bioimaging, photocatalysis, and organic light-emitting diodes (OLEDs). In addition to the extensively investigated Ru(II), Ir(III), and Pt(II) complexes, a wide range of luminescent Pd(II), Au(III)/Au(I), Re(I), Cu(I), W(VI)/W(0), and Mo(0) complexes have recently been revealed via the judicious choice of ligands to exhibit unique emissive excited states with a large range of radiative and nonradiative decay rate constants. Here, we provide an account of the design strategies of and recent advances on metal triplet emitters. Questions regarding future research in this field are presented. an excited molecular complex of definite stoichiometry of the same molecular components but which is dissociated in its ground state. a radiationless electronic transition process that involves a change in spin multiplicity. an electronic transition from an occupied orbital that involves overlap of two metal orbitals upon forming a close metal–metal contact to a ligand-based unoccupied orbital. a radiative electronic transition from an excited state to ground state that involves a change in spin multiplicity. the interaction between the spin angular momentum and the orbital angular momentum. transitions between states of different spin multiplicities are spin-forbidden and not allowed. a process that an excited molecule in triplet manifolds absorbs the thermal energy to undergo reverse ISC from the triplet excited state to a higher-lying singlet excited state, followed by fluorescence. the transition dipole moment for a transition between states |i> and |f> is defined as μif = 〈i|μ|f〉 where μ = er is the electric dipole operator.