Anomalous Phosphorescence from an Organometallic White-Light Phosphor

Abstract

We report theoretical results on the possible violation of Kasha's rule in the phosphorescence process of (acetylacetonato)bis(1-methyl-2-phenylimidazole)iridium(III) and show that the anomalous emission from both the T and T states is key to its white-light phosphorescence. This analysis is supported by the calculated Boltzmann-averaged phosphorescence lifetime of 2.21 μs, estimated including both radiative and nonradiative processes and in excellent agreement with the experimentally reported value of 1.96 ± 0.1 μs. The T state is found to be of metal-to-ligand charge-transfer character (dπ → nπ), and the d orbital contribution comes from 5d and 5d, whereas the S and T states both have dπ-pπ character with significant 5d orbital contribution, allowing for efficient intersystem crossing from the S to the T state and, in turn, phosphorescence from the T state. Our results open new opportunities for tailoring the phosphorescence wavelength and thus the design of molecules with improved photovoltaic properties.

Unveiling the Dual Emission Photo‐Deactivation Mechanism of a Neutral Heteroleptic Iridium (III) Complex

“…At first glance, the conversion between 3 ES(2) and 3 MC excited state is more likely, but at the low temperature, it is not beneficial for activating the system and going over the energy barrier. By contrast, at high temperature, the 3 ES(2) and 3 ES(1) undergo conversion and the energy barriers between the 3 ES(1), 3 ES(2) and 3 MC excited state become more activated.…”

“…However, compared to the 3 ES(1) emission, the 3 ES(2) emission has greater 3 MLCT character (MLCT = metal to ligand charge transfer). For the emission of transition metal complexes, the more direct participation of the metal d orbital or the greater 3 MLCT character can effectively enhance the SOC, leading to a decrease in the radiative lifetime and avoidance of the nonradiative process. Thus, the large 3 MLCT contribution is beneficial for obtaining a high phosphorescence quantum yield.…”

“…Additionally, based on the data presented in Table S4, the total Huang-Rhys factors for 3 ES(2) are 5.83, illustrating that the structural Table 4. Transition dipole moments m(S n ) (in Debye) for S 0 -S n transitions, singlet-triplet splitting energies DE(S n À 3 ES) (in eV) and the SOC matrix elementsh 3 ES j H SOC j S n i(cm À1 ) of Ir-1 at its respective optimized 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 distortion between the 3 ES(2) and the ground state is smaller compared to that between the 3 ES(1) and the ground state, which is in line with the values of RMSD shown in Figure 1. To summarize, compared to 3 ES(2), the structural distortion between 3 ES(1) and the ground state is larger and the energy gap between the 3 ES(1) and the ground state is smaller, so that the temperature-independent non-radiative decay rate of 3 ES(1)!S 0 transition may be higher than that of 3 ES(2)!S 0 transition.…”

“…Furthermore, the TS1, TS3 and MECP exhibit distinct metal-character-based spin densities as shown in Figure S3. Based on the geometries and energies of 3 ES, 3 MC, TS and MECP, the mechanisms of thermal deactivation pathways were constructed and are shown in Figure 3, where a relative energy referenced to the ground state is used. According to the calculated results collected in Figure 3, the populations of 3 MC excited state from 3 ES(1) and 3 ES(2) would need to overcome the energies of 22.47 and 11.15 kcal/mol, respectively.…”

“…The non-radiative decay process consists of the temperature-independent nonradiative decay and thermal deactivation decay process, that is, T 1 !TS [T 1 À 3 MC] (transition state)! 3 MC (metal center d-d excited state)!MECP (minimal energy crossing point between 3 MC and the ground state) conversion. Thus, the exploration of the geometries of the corresponding excited states is important for the elucidation of the photo-deactivation mechanism.…”