Understanding luminescence of metal-containing thermally activated delayed fluorescence (TADF) luminophores

Abstract

Metal TADF (thermally activated delayed fluorescence) emitters, here understood as those containing a d-block metal, are an important and ever-growing group of luminophores. Although they often seem to belong more in the transition metal complex world, they are rightful contenders of not only phosphorescent transition metal compounds, but also conventional, metal-free TADF emitters. Their unique properties include extremely short TADF lifetimes, often in the range 0.1–10 µs, fast intersystem crossing (ISC) and reverse ISC (RISC), minimal prompt fluorescence or lack thereof, small Stokes shifts, and temperature-dependent behaviour, including dual TADF/phosphorescence emission – but not every metal TADF emitter displays all of them at once! In this review, we discuss the general photophysical properties of metal TADF emitters and the relevant photophysical approaches applicable to studies of them. We make a brief overview of the most recent examples of computational works on metal TADF luminophores that shed some light on the up-conversion mechanism. Finally, we review some recent examples of Cu(I), Ag(I), Au(I)/Au(III) as well as Zn(II) TADF emitters, and discuss possibly all relevant works on Pd(II), Pt(II), Ir(III), and Zr(IV) TADF complexes. We identify that metal TADF complexes form two principal groups: (I) those analogous to donor–acceptor or charge-transfer TADF emitters and (II) those analogous to multiresonance TADF emitters.