A series of phenylpyridine (ppy)-based 6/5/5 N*C^N^O and biphenyl (bp)-based 6/5/6 N*C^C*N Pt­(II) complexes employing tetradentate ligands with nitrogen or oxygen atoms as bridging groups have been developed. Ligand structural modifications have great influences on the electrochemical, photophysical, and excited-state properties, as well as photostabilities of the Pt­(II) complexes, which were systematically studied by experimental and theoretical investigations. The time-dependent density functional theory calculations and natural transition orbital analyses reveal that Pt­(bp-6), Pt­(bp-7), and Pt­(bp-8) have dominant ligand-centered (3LC) mixed with small metal-to-ligand charge-transfer (3MLCT) characters in T1 states, resulting in relatively low quantum efficiencies (ΦPL) of 5–33% and 12–32% in dichloromethane solution and PMMA film, respectively. By contrast, Pt­(ppy-1) possesses much more 3MLCT character in the T1 state, enabling a high ΦPL of 95% in dichloromethane and 90% in DPEPO film, and large radiative decay rates. The strength of the Pt–N1 coordination bond plays a critical role in the photostability. Pt­(ppy-1)- and Pt­(bp-6)-doped polystyrene films demonstrate long photostability lifetimes of 150 min for LT97 and LT98.5, respectively. A Pt­(ppy-1)-based green OLED using 26mCPy as host realized a peak EQE of 18.5%, which still maintained an EQE of 10.4% at 1000 cd/m2, and an L max of over 40 000 cd/m2 was achieved. This study should provide a valuable reference for the further development of efficient and stable phosphorescent Pt­(II) complexes.

A series of N-heterocyclic carbene (NHC)-based tetradentate Pd­(II) complexes employing phenylcarbene (Ph-NHC)-, benzocarbene (Ph/NHC)-, and pyridinocarbene (Py/NHC)-containing ligands were designed and synthesized. The NHC-based Pd­(II) complexes could be prepared by a metalation of the corresponding hexafluorophosphate ligand with Pd­(OAc)2 using K2CO3 as base in dioxane at 110 °C in 36–66% yields. All the Pd­(II) complexes are air-stable and not sensitive to moisture, and PdON5N-tt exhibits a thermal decomposition temperature (T d) of up to 416 °C. The electrochemical and photophysical properties of the Pd­(II) complexes are systematically investigated through experimental research and theoretical calculation. Their reduction potentials, frontier orbitals, and excited-state properties can be efficiently tuned through the ligand modification of the NHC moieties and also perturbed by the alkyl substituents on the pyridine and phenyl rings. Differential pulse voltammetry curves show two obvious reduction peaks for all the Pd­(II) complexes involvement with the electron-deficient alkyl pyridine and NHC moieties. Time-dependent density functional theory and natural transition orbital calculations reveal that the T1 excited-state properties are strong admixed metal-to-ligand charge-transfer (3MLCT)/ligand-centered (3LC) with some intraligand charge-transfer (3ILCT) characters for PdON5 analogues, admixed 3MLCT/3LC characters for PdON7p analogues, and admixed 3MLCT/3ILCT for PdON5N-tt. The Pd­(II) complexes emit deep-blue light in various matrixes and exhibit narrow emission spectra with a dominant peak at 437–439 nm and FWHM of 34–48 nm in dichloromethane solution and PMMA film. The Pd­(II) complexes show a PLQY and excited-state lifetime of 1–11% and 1.1–40 μs in dichloromethane, and 5–25% and 2.6–51 μs in PMMA film.

A series of novel tetradentate Pt­(II) and Pd­(II) complexes containing fused 6/6/6 or 6/6/5 metallocycles employing azacarbazolylcarbazole (ACzCz)-based ligands was developed. Systematic experimental and theoretical studies suggest that both the ligand structures and the central metal ions have great influences on the electrochemical and photophysical properties of the complexes. The time-dependent density functional theory (TD-DFT) calculations and natural transition orbital (NTO) analyses reveal that the Pt­(II) complexes possess 10.8–15.2% metal-to-ligand charge transfer (3MLCT) mixed with ligand-centered (3LC) characters, by contrast, the Pd­(II) complexes exhibit significantly decreased 4.2–7.1% 3MLCT characters and enhanced 3LC compositions. All of the Pt­(II) and Pd­(II) complexes possess various channels for the intersystem crossing (ISC) on the basis of small energy gaps ΔE S1‑Tn and matching transition orbital compositions; moreover, Pd­(ACzCz-1) and Pd­(ACzCz-2) also possess efficient reverse intersystem crossing (RISC) to show both delayed fluorescence (DF) and phosphorescence in PMMA films at room temperature (RT). Pt­(ACzCz-3) has ΦPL values of 57% with a τ of 5.1 μs in dichloromethane at RT and 50% with 3.9 μs in PMMA at RT. Notably, Pd­(ACzCz-1) exhibits ultralong low-temperature phosphorescence with a τ of 1307 μs. Pt­(ACzCz-2)-based green OLED employing 26mCPy as the host demonstrated a peak EQE of 8.2% and a L max of 24065 cd/m2.

A series of tetradentate Pd(II) and Pt(II) complexes containing fused 5/6/6 metallocycles with phenyl N-heteroaromatic benzo[d]imidazole (pbiz), benzo[d]oxazole (pboz) or benzo [d]thiazole (pbthz)-containing ligands was developed. Systematic studies by experiments and theoretical calculations reveal that both the central metal and the benzannulated N-heteroaromatic ring have significant influence on the electrochemical, photophysical and excited-state properties of the Pd(II) and Pt(II) complexes. In identical condition, compared to pbiz-based Pd(II) and Pt(II) complexes, the corresponding metal complexes with pboz and pbthz-containing ligand show significant red-shift emission spectra. Pd(II) complexes exhibit blue-shift emission spectra in comparison with the corresponding Pt(II) complexes. The Pt(II) complexes possess more metal-to-ligand charge transfer ( 3MLCT) character in their T 1 states, which enables the Pt(II) complexes to have much higher quantum efficiencies (Φ PL ) and shorter excited-state lifetimes (τ), resulting in large radiative decay rates (k r ). Especially, Pt(pbiz) has a Φ PL of 94% with a τ of 4.0 µs in 5 wt% doped PMMA film at room temperature. Pt(pbiz)-doped green organic light-emitting diode (OLED) using 26mCPy as host demonstrated a peak external quantum efficiency (EQE) of 21.6% and a maximum brightness (L max ) of 55481 cd/m 2 , which still maintained an EQE of 16.5% and 10.8% at 1 000 and 10 000 cd/m 2 , respectively.

The manganese porphyrin-catalyzed C−H bond hydroxylation and amidation of equilenin acetate developed by Breslow and his co-worker have been investigated with density functional theory (DFT) calculations. The hydroxylation of C(sp 2 )−H bond of equilenin acetate leading to the 6-hydroxylated product is more favorable than the hydroxylation of C(sp 3 )−H bond of equilenin acetate, leading to the 11β-hydroxylation product. The computational results suggest that the C(sp 2 )−H bond hydroxylation of equilenin acetate undergoes an oxygenatom-transfer mechanism, which is more favorable than the C(sp 3 )−H bond hydroxylation undergoing the hydrogen-atomabstraction/oxygen-rebound (HAA/OR) mechanism by 1.6 kcal/ mol. That is why, the 6-hydroxylated product is the major product and the 11β-hydroxylated product is the minor product. In contrast, the 11β-amidated product is the only observed product in manganese porphyrin-catalyzed amidation reaction. The benzylic amidation undergoes a hydrogen-atom-abstraction/nitrogen-rebound (HAA/NR) mechanism, in which hydrogen atom abstraction is followed by nitrogen rebound, leading to the 11β-amidated product. The benzylic C(sp 3 )−H bond amidation at the C-11 position is more favorable than aromatic amidation at the C-6 position by 4.9 kcal/mol. Therefore, the DFT computational results are consistent with the experiments that manganese porphyrin-catalyzed C−H bond hydroxylation and amidation of equilenin acetate have different regioselectivities.