Elucidating the Nonradiative Deactivation Pathways in a Cationic Iridium Complex with 2,4-di(1H-pyrazol-1-yl)Pyridine as the Ancillary Ligand

Abstract

Deep insight into the nonradiative deactivation pathways in phosphorescent cationic iridium complexes is critically important for developing efficient blue-emitting complexes toward advanced applications. Here, we report the synthesis and photophysical and electrochemical characterizations of a blue-green-emitting cationic iridium complex [Ir­(ppy)2­(bipzpy)]­PF6 (Hppy is 2-phenylpyridine, and bipzpy is 2,4-di­(1H-pyrazol-1-yl)­pyridine). The nonradiative deactivation pathways in [Ir­(ppy)2­(bipzpy)]­PF6 have been elucidated through extensive density functional theory calculations. The calculations reveal that the higher-lying charge-transfer (CT) state in [Ir­(ppy)2­(bipzpy)]­PF6, which arises from Ir/ppy → bipzpy transitions, favors nonradiative deactivation because of its large structural distortion compared to the ground state. Both the CT state and the dark metal-centered (3MC) state can be thermally accessed by the lowest-lying emitting triplet state at room temperature, with the former being much more easily accessible, which causes additional nonradiative deactivations for the emitting triplet state. The active roles of the CT and 3MC states in the nonradiative deactivation pathways are, for the first time, confirmed in such blue-emitting complexes with pzpy-type ancillary ligands (pzpy is 2-(1H-pyrazol-1-yl)­pyridine).

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

“…However, to figure out the extent of nonradiative decays associated with 3 MC states, one needs to fully consider the transition states between nonradiative 3 MC and emissive triplet states as well as the minimum-energy crossing points (MECPs) between the S 0 and 3 MC states. 61,62 The present work indicates that nonradiative decays of emissive triplet states involve both 3 MC 1 and 3 MC 2 states and that a destabilization of the 3 MC states largely suppresses the nonradiative deactivations. Here, PEDOT:PSS, 26DCzPPy, and TmPyPB serve as the hole-injection layer, the host, and the electron-transporting/hole-blocking layer, respectively.…”

“…As shown in Figure , the fully relaxed 3 MC 2 states are located close in energy (<0.62 eV) to the S 0 states, which suggests that the 3 MC 2 states play a role in nonradiative decays of fluorine-free blue emissive complexes. ,, By destabilizing the 3 MC 2 states, a large suppression of nonradiative deactivations can be achieved for the emissive triplet states. However, to figure out the extent of nonradiative decays associated with 3 MC states, one needs to fully consider the transition states between nonradiative 3 MC and emissive triplet states as well as the minimum-energy crossing points (MECPs) between the S 0 and 3 MC states. , The present work indicates that nonradiative decays of emissive triplet states involve both 3 MC 1 and 3 MC 2 states and that a destabilization of the 3 MC states largely suppresses the nonradiative deactivations.…”

“…The phosphorescence of Ir(III) complexes can be quenched by the thermal population from an emissive triplet to nonemissive 3 MC states. 44,54,58,61 As shown in Figure 5, the 3 MC 1 and 3 MC 2 states are located nearby the emissive T 1 states in energy (within 0.27 eV), which should cause nonradiative decays of T 1 states. Complexes 1 and 2 show similar 3 MC 1 or 3 MC 2 states.…”

Cationic Iridium Complexes with 5-Phenyl-1H-1,2,4-triazole Type Cyclometalating Ligands: Toward Blue-Shifted Emission

“…26,66 In general, to fully evaluate the possibility of nonradiative deactivations caused by 3 MC states, the transition state between the emitting and 3 MC states and the minimum energy crossing point between the 3 MC and S 0 states should be comprehensively considered. 67 Nevertheless, for complexes 2−4, the rather weak emission in the solution strongly indicates that the low-lying 3 MC states play an active role in the nonradiative deactivation. 43,57,67 It is noted that other blue or blue-green emitting cationic iridium complexes (complexes R6 and R7 in Chart 1) with phenyl-1,2,3-triazole or phenyl-tetrazole C^N ligands also show low luminescent efficiencies in the solution, which has been attributed to the severe nonradiative deactivations caused by the 3 MC states.…”

“…67 Nevertheless, for complexes 2−4, the rather weak emission in the solution strongly indicates that the low-lying 3 MC states play an active role in the nonradiative deactivation. 43,57,67 It is noted that other blue or blue-green emitting cationic iridium complexes (complexes R6 and R7 in Chart 1) with phenyl-1,2,3-triazole or phenyl-tetrazole C^N ligands also show low luminescent efficiencies in the solution, which has been attributed to the severe nonradiative deactivations caused by the 3 MC states. 35,38,55,56 Electrochemical Properties.…”