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

Iván Soriano-Díaz,Enrique Ortí,Angelo Giussani

doi:10.1021/acs.inorgchem.1c01604

Iván Soriano-Díaz, Enrique Ortí + Show 1 more

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https://doi.org/10.1021/acs.inorgchem.1c01604

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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 T1 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 T1 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.

Highlights

Light-emitting electrochemical cells (LECs) based on ionic transition-metal complexes represent a promising alternative in the development of highly efficient electroluminescent devices.[1,2] LECs are attractive in lighting applications because, in contrast to organic lightemitting diodes (OLEDs), they have the advantage of a much simpler structure that does not require rigorous encapsulation, which in turn drastically reduces the manufacturing costs.[3]
Through the analysis of the density functional theory (DFT) orbitals and by the examination of the dominant monoexcitations obtained out of a time-dependent DFT (TDDFT) computation at (S0)min, it is possible to conclude that such a triplet state has an metal-to-ligand charge-transfer (MLCT) electronic nature, mixed with some ligand-to-ligand charge-transfer (LLCT) character
The state is described by one main electronic monoexcitation from the HOMO to the LUMO (Table 2), the former being localized on the phenyl rings of both ppy ligands and on the Ir atom, whereas the latter is mostly localized on the bpy ligand

Summary

Introduction

Light-emitting electrochemical cells (LECs) based on ionic transition-metal complexes (iTMCs) represent a promising alternative in the development of highly efficient electroluminescent devices.[1,2] LECs are attractive in lighting applications because, in contrast to organic lightemitting diodes (OLEDs), they have the advantage of a much simpler structure that does not require rigorous encapsulation, which in turn drastically reduces the manufacturing costs.[3] research on iTMCs for LEC applications is still facing different problems, mostly related to the need of obtaining complexes capable of emitting in a wide range of colors and having a high phosphorescence quantum yield.[4−11] The last property, of crucial importance for the efficiency of a device, is the result of the competition between phosphorescence emission and all other possible relaxation mechanisms operating in a complex It is well-known in chemistry that the properties of a molecule can be significantly changed by introducing electronically active substituents.[12] iTMCs for electroluminescent applications are not an exception, and a large part of the research conducted in the field has been attempted to fine-tune the photophysical properties of reference electroluminescent complexes. One of the most successful modifications was achieved through the introduction of fluoro and tert-butyl groups, giving rise to the [Ir(diFppy)2(dtb-bpy)]+ complex, where diFppy is 2-(2,4-

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