On the Importance of Ligand-Centered Excited States in the Emission of Cyclometalated Ir(III) Complexes

Abstract

The photophysical behavior of the cyclometalating Ir(III) complexes [Ir(ppy) 2 (bpy)] + , where Hppy is 2-phenylpyridine and bpy is 2,2′-bipyridine (complex 1 ), and [Ir(diFppy) 2 (dtb-bpy)] + , where diFppy is 2-(2,4-difluorophenyl)pyridine and dtb-bpy is 4,4′-di- tert -butyl-2,2′-bipyridine (complex 2 ), has been theoretically investigated by performing density functional theory calculations. The two complexes share the same molecular skeleton, complex 2 being derived from complex 1 through the addition of fluoro and tert -butyl substituents, but present notable differences in their photophysical properties. The remarkable difference in their emission quantum yields (0.196 for complex 1 in dichloromethane and 0.71 for complex 2 in acetonitrile) has been evaluated by characterizing both radiative and nonradiative decay paths. It has emerged that the probability of decaying through the nonradiative triplet metal-centered state, normally associated with the loss of the emission quantum yield, does not appear to be the reason behind the reported substantially different emission efficiency. A more critical factor appears to be the ability of complex 2 to emit from both the usual metal-to-ligand charge-transfer state and from two additional ligand-centered states, as supported by the fact that the respective minima belong to the potential energy surface of the lowest triplet T 1 state and that their phosphorescence lifetimes are in the same order of magnitude. In contrast, the emission of complex 1 can be originated only from the metal-to-ligand charge-transfer state, being the only emissive T 1 minimum. The results constitute a significant case in which the emission from ligand-centered states is the key for determining the high emission quantum yield of a complex.

Cationic Ir(III) Complexes Featuring Phenylimidazole-type Cyclometalated Ligands: Fluorine-Free Blue Phosphorescent Emitters for Light-Emitting Devices

“…As revealed by photophysical experiments, complexes 2–4 show a similar deep blue emission, but their PLQYs in solution largely differ because of their largely different k nr values (Table ). Recent research work has revealed that the difference in the character of emissive triplet states can lead to largely different PLQYs for cationic Ir­(III) complexes with similar strucures . Here, complexes 1–4 all show an emission originating from 3 π–π* (C ∧ N-centered)/ 3 MLCT (Ir → C ∧ N) states, and therefore, the difference in the character of emissive triplet states is unlikely to cause the largely different PLQYs in the solution.…”

“…Recent research work has revealed that the difference in the character of emissive triplet states can lead to largely different PLQYs for cationic Ir(III) complexes with similar strucures. 57 Here, complexes 1−4 all show an emission originating from 3 π−π* (C ∧ N-centered)/ 3 MLCT (Ir → C ∧ N) states, and therefore, the difference in the character of emissive triplet states is unlikely to cause the largely different PLQYs in the solution. It has been shown that the "dark" 3 MC states in blue emissive Ir(III) complexes cause notable nonradiative decay losses.…”

Tetradentate Pt(II) Complexes with Peripheral Hindrances for Highly Efficient Solution-Processed Blue Phosphorescent OLEDs

“…Noticeably, the electron distribution on T 1 locates throughout the chelating core of the ligand part. This result enables the complex to emit light under multiple transitional characters with numerous local excited states ( 3 MLCT py2→Pt , 3 MLCT pz→Pt , 3 LC pzpy , and 3 LC cz ), which appear to have high PLQYs. , …”

“…This result enables the complex to emit light under multiple transitional characters with numerous local excited states ( 3 MLCT py2→Pt , 3 MLCT pz→Pt , 3 LC pzpy , and 3 LC cz ), which appear to have high PLQYs. 28,32 Electroluminescent Properties. The peripheral groups with alkyl substitutions also endow the complexes with good solubility; therefore, Pt(pzpyOczpy-B1) and Pt(pzpyOczpy-B2) are suitable for solution-processed devices to evaluate the EL properties.…”

Cyclometalated Rhodium and Iridium Complexes Containing Masked Catecholates: Synthesis, Structure, Electrochemistry, and Luminescence Properties

“…The iridium complex [(F2ppy) 2 Ir(η-Cat)] (3) exhibits rather a broad emission band at λ max = 475 nm with ϕ = 4.3 × 10 −3 and τ = 38.75 ns, which might originate from 3 LC-3 MCLT transitions (Figure 7). 46 For comparison purposes with respect to the previous work on related [(MEppy) 2 M(η-Cat)] (ME = methylester; M = Rh, Ir) 30 but containing an ester group at the pyridyl unit instead of the phenyl ring, we studied the luminescence properties of both compounds 2 and 3 in a MeOH/EtOH (1/4) mixture at room temperature and at 77 K. Unlike the iridium ester compound, which emits in the red region (λ max = 600 nm), our current compounds displayed emissions at higher energy (vide infra). It should be mentioned that the rhodium ester compound is not emissive.…”