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

A series of π-aromatic-rich cyclometalated ruthenium(II)-(2,2'-bipyridine) complexes ([Ru(bpy)₂(π_Ar-CM)]⁺) in which π_Ar-CM is diphenylpyrazine or 1-phenylisoquinoline were prepared. The [Ru(bpy)₂(π_Ar-CM)]⁺ complexes had remarkably high phosphorescence rate constants, k _RAD(p), and the intrinsic phosphorescence efficiencies (ι_em(p) = k _RAD(p)/(ν_em(p))³) of these complexes were found to be twice the magnitudes of simply constructed cyclometalated ruthenium(II) complexes ([Ru(bpy)₂(sc-CM)]⁺), where ν_em(p) is the phosphorescence frequency and sc-CM is 2-phenylpyridine, benzo[h]quinoline, or 2-phenylpyrimidine. Density functional theory (DFT) modeling of the [Ru(bpy)₂(CM)]⁺ complexes indicated numerous singlet metal-to-ligand charge transfers for ¹MLCT-(Ru-bpy) and ¹MLCT-(Ru-CM), excited states in the low-energy absorption band and ¹ππ*-(aromatic ligand) (¹ππ*-L_Ar) excited states in the high-energy band. DFT modeling of these complexes also indicated phosphorescence-emitting state (T_e) configurations with primary MLCT-(Ru-bpy) characteristics. The variation in ι_em(p) for the spin-forbidden T_e (³MLCT-(Ru-bpy)) excited state of the complex system that was examined in this study can be understood through the spin-orbit coupling (SOC)-mediated sum of intensity stealing (∑SOCM-IS) contribution from the primary intensity of the low-energy ¹MLCT states and second-order intensity perturbation from the significant configuration between the low-energy ¹MLCT and high-energy intense ¹ππ*-L_Ar states. In addition, the observation of unusually high ι_em(p) magnitudes for these [Ru(bpy)₂(π_Ar-CM)]⁺ complexes can be attributed to the values for both intensity factors in the ∑SOCM-IS formalism being individually greater than those for [Ru(bpy)₂(sc-CM)]⁺ ions.

Theoretical Design of Blue-Color Phosphorescent Complexes for Organic Light-Emitting Diodes: Emission Intensities and Nonradiative Transition Rate Constants in Ir(ppy)2(acac) Derivatives

Theoretical calculations of phosphorescent spectra and nonradiative transition (NRT) rate constants for S₁ ⇝ T₁, T₁ ⇝ S₀, and S₁ ⇝ S₀ were carried out to determine the best candidate for a blue-color phosphorescent complex among several derivatives of bis(2-phenylpyridine)(acetylacetonate)iridium(III). The geometries of the ground state (S₀), the lowest triplet state (T₁), and the lowest excited singlet state (S₁) were optimized at the levels of density functional theory, in which B3LYP functionals and SBKJC+p basis sets were used. The NRT rate constants were derived by using a generating function method within the displaced harmonic oscillator model. The results of the calculation for phosphorescence showed that the introduction of F and/or CN substituents at the 4'/6'-th and 5'-th sites in 2-phenylpyridinate (ppy) ligands, respectively, causes a blue shift of the emission spectra. They also suggest that Ir(5-CN,6-F-ppy)₂(acac), denoted 3(56) in the text, is a good candidate for a blue-color phosphorescent complex because a blue shift of emission spectra and a moderate intensity are obtained for phosphorescence and, furthermore, this complex is calculated to have a large rate constant for S₁ ⇝ T₁ and relatively smaller rate constants for T₁ ⇝ S₀ and S₁ ⇝ S₀ based on the calculations of spin-orbit coupling and nonadiabatic coupling constants.

Role of Spin-Orbit Coupling in Long Range Energy Transfer in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are emerging as a promising platform for solar energy conversion applications. Their potential utilization as efficient chromophores in artificial photosynthesis is closely related to the understanding of light-harvesting and energy transfer processes that occur within these molecular scaffolds. Herein, we present the photophysical investigation of Ru(II), Ir(III), and Os(II) polypyridyl complexes incorporated into the backbone of UiO-67. In this work, we systematically study the effect of spin-orbit coupling on dipole-dipole energy transfer in MOFs using steady-state and time-resolved spectroscopic techniques. The results of our work indicate successful triplet-to-singlet energy transfer and a sizable increase in the transfer kinetics and critical distance, as direct consequences of strong spin-orbit couplings. Remarkably, the reported R0 value for OsDCBPY (R0 = 88 ± 10 Å) represents one of the largest Förster distances observed in an MOF. Collectively, this work contributes to the general knowledge of energy transfer in materials and provides groundwork for efficient utilization in artificial photosynthetic assemblies.

Quantum chemical investigation on the Ir(iii) complexes with an isomeric triazine-based imidazolium carbene ligand for efficient blue OLEDs

Ir(iii) complexes of isomeric triazine-based imidazolium carbene with phenylpyridine or bipyridine as an ancillary ligand show blue phosphorescence with high quantum efficiency.