Advancing the a Posteriori Quest for Deep-Blue Phosphorescence: Quantifying Excitation-Induced Metal-to-Ligand Charge Transfer as a Guiding Indicator

Abstract

To advance the a posteriori quest for deep-blue phosphorescence, we quantify the degree of metal-to-ligand charge transfer (MLCT) upon excitation using density functional theory calculations. We also comprehensively determine how the MLCT nature of a state is correlated with the wavelength of potential phosphorescence through comparing the results of two typical ligand structures, i.e., 2-phenylpyridine and 1-phenyl-3-methylimidazolin-2-ylidene, with those of their derivatives. The MLCT nature can be defined as the difference in the electron density over the central metal ion that contributes to the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively). This study focuses on how the MLCT nature can be characterized upon excitation depending on the admixtures of molecular orbitals and how a stronger MLCT nature is preferable for efficient phosphorescence, while the MLCT nature exhibits a trade-off relationship with the wavelengths of phosphorescence in the blue region. Moreover, we elucidate how iridium­(III) complexes are suitable for efficient phosphorescence and discuss a conceptual grand design for transition-metal complexes with deep-blue phosphorescence using the ligand field theory. We examine why strong MLCT nature upon excitation does not necessarily ensure highly efficient phosphorescence, taking into account possible relative energy gaps between the singlet MLCT, triplet locally excited, and d–d transition states in a transition-metal complex formed after excitation. Nevertheless, the initial MLCT serves as a necessary condition that induces corollary energy-state configurations as well as decay from the excited states. Therefore, quantifying the nature of that MLCT, which is the first process upon excitation, with the possible wavelength of phosphorescence is a useful parameter for a conceptual grand design for candidate complexes that can efficiently emit deep-blue phosphorescence. This reverse method of analyzing candidate complexes before experimentally formulating and studying them can accelerate the a posteriori quest for deep-blue-emitting materials.

Insights into the Luminescence Quantum Yields of Cyclometalated Iridium(III) Complexes: A Density Functional Theory and Machine Learning Approach

“…This property underscores the potential of altering the emission color of the complex via ligand modification. For instance, by introducing electron-withdrawing groups into the ligands, blue-emissive Ir­(III) complexes have been crafted, effectively lowering the HOMO energy levels …”

“…For instance, by introducing electron-withdrawing groups into the ligands, blue-emissive Ir(III) complexes have been crafted, effectively lowering the HOMO energy levels. 20 To be applied as emitters in OLEDs, a strong emission intensity and an appropriate emission wavelength are required. The emission intensity, i.e., the luminescence quantum yield (LQY), heavily depends on ligands.…”

High Intrinsic Phosphorescence Efficiency and Density Functional Theory Modeling of Ru(II)-Bipyridine Complexes with π-Aromatic-Rich Cyclometalated Ligands: Attributions of Spin–Orbit Coupling Perturbation and Efficient Configurational Mixing of Singlet Excited States

“…Cyclometalated transition metal–polypyridine complexes have been the focus of interesting studies on photophysics and excited-state relaxation, ,− and these types of complexes have recently been applied in areas of solar energy conversion, − OLEDs, ,− and photodynamic therapy. , In previous work, the 77 K phosphorescence parameters of Ru-bpy chromophores for the cyclometalated ruthenium­(II)-(2,2′-bipyridine) complexes ([Ru­(bpy) 2 (CM)] + ) with a simply constructed cyclometalated (sc-CM) coordination ligand (sc-CM = 2-phenylpyridine (ppy) and benzo­[ h ]­quinoline (bhq)) showed unusually higher ι em(p) values compared to those of typical [Ru­(bpy) 3– n (Am) 2 n ] 2+ complexes. In addition, in the case of the observed variations in ι em(p) of the emitting 3 MLCT states of the Ru-bpy chromophores for [Ru­(bpy) 2 (sc-CM)] + and typical [Ru­(bpy) 3– n (Am) 2 n ] 2+ complexes, a good linear relation with the intensity component values of ∑SOCM-IS was found, as seen in eq .…”

“…Cyclometalated transition metal–polypyridine complexes have been the focus of interesting studies on photophysics and excited-state relaxation, 14 , 77 − 85 and these types of complexes have recently been applied in areas of solar energy conversion, 86 − 88 OLEDs, 14 , 89 − 91 and photodynamic therapy. 92 , 93 In previous work, the 77 K phosphorescence parameters of Ru-bpy chromophores for the cyclometalated ruthenium(II)-(2,2′-bipyridine) complexes ([Ru(bpy) 2 (CM)] + ) with a simply constructed cyclometalated (sc-CM) coordination ligand (sc-CM = 2-phenylpyridine (ppy) and benzo[ h ]quinoline (bhq)) showed unusually higher ι em(p) values compared to those of typical [Ru(bpy) 3– n (Am) 2 n ] 2+ complexes.…”