Thermally activated delayed fluorescence of a Ir(iii) complex: absorption and emission properties, nonradiative rates, and mechanism

Abstract

One recent experimental study reported a Ir(III) complex with thermally activated delayed fluorescence (TADF) phenomenon in solution, but its luminescent mechanism is elusive. In this work, we combined density functional theory (DFT), time-dependent DFT (TDDFT) and multi-state complete active space second-order perturbation theory (MS-CASPT2) methods to investigate excited-state properties, photophysics, and emission mechanism of this Ir(III) complex. Two main absorption bands observed in experiments can be attributed to the electronic transition from the S₀ state to the S₁ and S₂ states; while, the fluorescence and phosphorescence are generated from the S₁ and T₁ states, respectively. Both the S₁ and T₁ states have clear metal-to-ligand charge transfer (MLCT) character. The present computational results reveal a three-state model including the S₀, S₁ and T₁ states to rationalize the TADF behavior. The small energy gap between the S₁ and T₁ states benefits the forward and reverse intersystem crossing (ISC and rISC) processes. At 300 K, the rISC rate is five orders of magnitude larger than the phosphorescence rate therefore enabling TADF. At 77 K, the rISC rate is sharply decreased but remains close to the phosphorescence rate; therefore, in addition to the phosphorescence, the delayed fluorescence could also contribute to the experimental emission. The estimated TADF lifetime agrees well with experiments, 9.80 vs. 6.67 μs, which further verifies this three-state model.