Phosphorescent Tetradentate Platinum(II) Complexes Containing Fused 6/5/5 or 6/5/6 Metallocycles

Abstract

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.

Tetradentate Platinum(II) and Palladium(II) Complexes Containing Fused 6/6/6 or 6/6/5 Metallocycles with Azacarbazolylcarbazole-Based Ligands

“…By contrast, the dihedral angle of 6/6/5 Pt­(ACzCz-3) is much smaller, which is only 48.03° due to the small steric hindrance between the azacarbazole and pyridine moieties (Tables and S2). The coordination bonds M–N of the metal complexes are in a region of 2.191–2.228 Å, which are longer than those of M–C bonds with 1.984–2.015 Å (Table S2); this is in good agreement with the previously reported tetradentate Pt­(II) and Pd­(II) complexes. ,,,, …”

“…Besides the matching transition orbital composition and small Δ E S1‑T n , the lower-lying T n compared to S 1 can further facilitate the ISC; thus, Pt­(ACzCz-1) and Pt­(ACzCz-2) possess various efficient channels for the ISC because of the small energy gaps of Δ E S1‑T1 (0.048 and 0.084 eV, respectively) and Δ E S1‑T2 (0.018 and 0.030 eV, respectively); meanwhile, the S 1 /T 1 (HOMO → LUMO) and S 1 /T 2 (HOMO → LUMO + 1) have the same transition configurations, both of which can increase the rates of the ISC and are also favorable for the T 1 → S 0 transitions to generate phosphorescence. The energy gaps of Δ E S1‑T1 and Δ E S1‑T2 are much smaller than those of the previously reported tetradentate Pt­(II) complexes. , By contrast, the Δ E S1‑T1 (0.185 eV) of Pt­(ACzCz-3) is much larger compared to the other two Pt­(II) complexes discussed above. From the transition configurations, the frontier orbitals and NTO analyses of the Pt­(II) complexes, besides the d Pt , both the π ACz and the π Py orbitals also make great contributions to their ISC transitions involving S 1 , T 1 , and T 2 states (Figures and S4).…”

“…On the basis of the TD-DFT calculations , (Tables and S3–S7), the S 0 → S 1 and S 0 → T 1 transitions of the metal complexes predominantly involve the frontier orbitals of HOMO, LUMO, and LUMO + 1, which are the transition configurations of HOMO → LUMO and HOMO → LUMO + 1. Notably, the main orbital contributions of Pt­(ACzCz-2) and Pd­(ACzCz-2) for S 0 → T 1 are HOMO → LUMO + 1, which are much different from the other complexes (Table ).…”

“…Figures , S5, and S6 illustrate the comparison of the emission spectra of the ACzCz-based Pt­(II) and Pd­(II) complexes in various conditions, and Table shows the data of the photophysical properties of the complexes. All of the Pt­(II) and Pd­(II) complexes exhibit vibronically well-resolved emission spectra at 77 K in 2-methyltetrahydrofuran (2-MeTHF), indicating the dominant ligand-centered ( 3 LC) mixed with some 3 MLCT characters in the T 1 states. ,,,, With an increase of the conjugation degree of the PhOPy, CzPy, and PhPy, Pt­(ACzCz-1), Pt­(ACzCz-2), and Pt­(ACzCz-3) show a significant red shift with dominant emission peaks at 451, 461, and 512 nm, respectively. A similar result is also observed for the Pd­(II) complexes (Figures S5 and S6).…”

“…Carbazolylpyridine (CzPy) skeleton has been widely employed in the design of the tetradentate ligands to chelate with metal ions to form a six-membered ring metallocycle (Figure ). ,,,− ,,− CzPy connects with phenyl heteroaromatics, carbazolyl heteroaromatics, or acridinyl pyrazole by oxygen or nitrogen to develop various complexes containing fused 5/6/6 metallocycles, whose photophysical properties can be efficiently manipulated through modifying the heteroaromatics or the substituents on the pyridine ring with color across the whole visible region, such as PtONx, ,,,,, PtNxN, , PtN′1N, and their corresponding Pd­(II) complexes, , and the monomer/excimer emissions could be also tuned through intramolecular hydrogen bond design . Two CzPy moieties are linked by an oxygen atom to form symmetrical 6/6/6-type metal complexes, ,,− and PtNON can act as an ideal phosphorescent emitter for efficient and stable blue OLEDs. ,, CzPy moieties directly connect with the pyridinyl or phenyl group to develop a series of 6/5/6-type Pt­(II) complexes with extended conjugation systems, which are good candidates for green to red emitters, , such as Y-Pt, Pt­( bp -7), and Pt­( bp -8) …”