Have we been wrong all along about the luminescence of iridium(III) complexes?

We demonstrate that the photoluminescence of certain types of iridium(III) complexes displays a substantial contribution from thermally activated delayed fluorescence (TADF), which enhances the efficiency of emission. The careful assessment of a range of literature data allowed TADF-emitting complexes to be identified, having a singlet-triplet energy gap ΔEST in the range 14 to 100 meV. The TADF contribution at room temperature (RT) is assessed from experimental data and computational modelling. While typically characterised by a relatively fast metal-facilitated phosphorescence, Ir(III) complexes may additionally benefit from the TADF mechanism, leading to even larger radiative rate constants kr. The typical features of such complexes that emerge are (i) dinuclearity, (ii) significant overlap between the absorption and emission spectra, and (iii) radiative decay rates kr > 106 s–1 at RT. Our findings suggest that, due to the presence of 1,3MLCT states the ΔEST of many Ir(III) complexes is sufficiently small to allow the re-population of their S1 states at RT. Nevertheless, if the f(S1->S0) has low singlet oscillator strength, the effect of TADF on the luminescence may be insignificant. Hence, whilst TADF might be widespread among Ir(III) complexes, its effect on photoluminescence may often be negligible. As luminescent iridium(III) complexes are common in various areas of study, this work may disrupt research on a broader scale beyond the immediate realm of photophysics.