Evidence for thermally activated delayed fluorescence in iridium(iii) complexes

Abstract

In this work we report the second ever example of a fully experimentally confirmed thermally activated delayed fluorescence (TADF) in a dinuclear Ir(III) complex. The said complex displays a singlet–triplet gap ΔE_ST = 28 ± 5 meV, in agreement with the computational prediction of 31.1 meV – a value smaller than the previous TADF Ir(III) complex. We also demonstrate a proof-of-concept, solution-processed OLED featuring this complex as the luminescent dopant in the emissive layer, achieving external quantum efficiency of up to ∼10% and maximum luminance of 18 000 cd m⁻² – values significantly exceeding those reported earlier for Ir(III) TADF. These findings are preceded by a detailed consideration of spectral signs of TADF in the already known Ir(III) complexes. The spectral overlap of photoluminescence (PL) with strong (i.e., spin-allowed) absorption bands is unusual for phosphorescent metal complexes, because the PL originates from the triplet state, which is normally significantly lower in energy than the lowest-lying singlet. In this study, we have scrutinized literature data on iridium(III) complexes that likewise show significant overlap between absorption and PL, and we conclude that a small singlet–triplet energy gap ΔE_ST in these complexes results in a TADF contribution to their emission. Such a mechanism has hitherto been overlooked in the large body of iridium(III) chemistry. We use computations to clarify the nature of the excited states in these complexes, demonstrating that the distinctive S₁ and T₁ character of states can be identified as well as confirming that ΔE_ST is small enough for TADF to occur at room temperature.