Phosphorescent Dinuclear Pd-Pd Emitters in OLED Applications

Four phosphorescent dinuclear Pd-Pd complexes were synthesized by using the clamping ligand N,N'-diphenylformamidine and primary ligands of C∧N, which both have high ligand field strength to enhance the luminescence of the complexes. The single-crystal data confirmed their clamshell structures and revealed Pd-Pd distances between 2.829 and 2.864 Å. In addition, time-dependent density functional theory calculations revealed the phosphorescent nature of the metal-metal-to-ligand charge transfer and ligand-to-ligand charge transfer in these complexes. Moreover, complexes 3 and 4 were used to fabricate an OLED by a vapor phase deposition technique. At an optimal doping concentration of 5%, the 3-based device showed a maximum external quantum efficiency (EQEmax) of 8.79% for CIE coordinates of (0.49, 0.48) and the 4-based device showed an EQEmax of 13.10% for CIE coordinates of (0.61, 0.37).

Photoluminescence of Ni(II), Pd(II), and Pt(II) Complexes [M(Me2dpb)Cl] Obtained from C‒H Activation of 1,5-Di(2-pyridyl)-2,4-dimethylbenzene (Me2dpbH)

The three complexes [M(Me₂dpb)Cl] (M = Ni, Pd, Pt) containing the tridentate N,C,N-cyclometalating 3,5-dimethyl-1,5-dipyridyl-phenide ligand (Me₂dpb^-) were synthesised using a base-assisted C‒H activation method. Oxidation potentials from cyclic voltammetry increased along the series Pt < Ni < Pd from 0.15 to 0.74 V. DFT calculations confirmed the essentially ligand-centred π*-type character of the lowest unoccupied molecular orbital (LUMO) for all three complexes in agreement with the invariant reduction processes. For the highest occupied molecular orbitals (HOMO), contributions from metal d_yz, phenyl C4, C2, C1, and C6, and Cl p_z orbitals were found. As expected, the d_z² (HOMO-1 for Ni) is stabilised for the Pd and Pt derivatives, while the antibonding d_x²_-y² orbital is de-stabilised for Pt and Pd compared with Ni. The long-wavelength UV-vis absorption band energies increase along the series Ni < Pt < Pd. The lowest-energy TD-DFT-calculated state for the Ni complex has a pronounced d_z²-type contribution to the overall metal-to-ligand charge transfer (MLCT) character. For Pt and Pd, the d_z² orbital is energetically not available and a strongly mixed Cl-to-π*/phenyl-to-π*/M(d_yz)-to-π* (XLCT/ILCT/MLCT) character is found. The complex [Pd(Me₂dpb)Cl] showed a structured emission band in a frozen glassy matrix at 77 K, peaking at 468 nm with a quantum yield of almost unity as observed for the previously reported Pt derivative. No emission was observed from the Ni complex at 77 or 298 K. The TD-DFT-calculated states using the TPSSh functional were in excellent agreement with the observed absorption energies and also clearly assessed the nature of the so-called "dark", i.e., d‒d*, excited configurations to lie low for the Ni complex (≥3.18 eV), promoting rapid radiationless relaxation. For the Pd(II) and Pt(II) derivatives, the "dark" states are markedly higher in energy with ≥4.41 eV (Pd) and ≥4.86 eV (Pt), which is in perfect agreement with the similar photophysical behaviour of the two complexes at low temperatures.

Highly phosphorescent organopalladium(ii) complexes with metal–metal-to-ligand charge-transfer excited states in fluid solutions

Making Pd(ii) brightly shining: mononuclear analogues are non-emissive, but folded dinuclear palladium(ii) diacetylide complexes phosphoresce from MMLCT excited states with quantum yields up to 48%.

Palladium(ii) N-heterocyclic allenylidene complexes with extended intercationic Pd⋯Pd interactions and MMLCT phosphorescence

Pallas's shine: extended intercationic Pd⋯Pd contacts of 3.30 Å show distinct MMLCT transitions and low-energy emissions.