Highly Luminescent Tetradentate Bis-Cyclometalated Platinum Complexes: Design, Synthesis, Structure, Photophysics, and Electroluminescence Application

Abstract

N,N-Di(6-phenylpyridin-2-yl)aniline (L1), N,N-di(6-(2,4-difluorophenyl)pyridin-2-yl)aniline (L2), N,N-di(3-(pyridin-2-yl)phenyl)aniline (L3), N,N-di(3-(1H-pyrazol-1-yl)phenyl)aniline (L4), N,N-di(3-(3-methyl-1H-pyrazol-1-yl)phenyl)aniline (L5), and N,N-di(3-(4-methyl-1H-pyrazol-1-yl)phenyl)aniline (L6) undergo cyclometalation to produce two types of tetradentate bis-cyclometalated platinum(II) complexes: C--N*N(wedge)C platinum complexes 1 and 2 and N--C*C--N platinum complexes 3-6, respectively, where an "X--Y" (X, Y = C or N) denotes a bidentate coordination to the platinum to form a five-membered metallacycle and "X*Y" denotes a coordination to form a six-membered metallacycle. The crystal structures of 1, 3, and 5 were determined by the single-crystal X-ray diffraction analysis, showing distorted square-planar geometry, that is, two C--N coordination moieties are twisted. Complex 5 showed much greater distortion with largest deviation of 0.193 A from the mean NCCNPt coordination plane, which is attributed to the steric interaction between the two 3-methyl groups on the pyrazolyl rings. Density functional theory (DFT) calculations were carried out on the ground states of 1 and 3-6. The optimized geometries are consistent with the crystal structures. The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the molecules displayed a localized characteristic with the contribution (18-45%) of the platinum metal to the HOMOs. All complexes are emissive at ambient temperature in fluid with quantum yields of 0.14 to 0.76 in 2-methyltetrahydrofuran. The emission of the complexes covers from blue to red region with lambda(max) ranging from 474 to 613 nm. Excimer emission was observed for 1 and 2 at high concentration of the complexes. The emission lifetime at infinite dilution for 1 and 2 was determined to be 7.8 and 11.4 micros, respectively. Concentration quenching was observed for 3 and 4, but the excimer emission was not observed. The life times for 3-6 were determined to be in the range of micro seconds, but those of 4-6 (3.4-5.7 micros) were somewhat shorter than that of 3 (7.6 micros). The highly structured emission spectra, long life times, and DFT calculations suggested that the emissive state is primarily a (3)LC state with metal-to-ligand charge-transfer (MLCT) admixture. The ZFS of 23 cm(-1) for the emissive triplet state was observed directly by high resolution spectroscopy for 1 in a Shpol'skii matrix, which also suggested an emission from a triplet ligand centered ((3)LC) state with admixture of MLCT character. Complex 1 was incorporated into an organic light-emitting diode (OLED) device as an emitter at 4 wt % in the mixed host of 4,4',4''-tris(N-carbazolyl)triphenylamine (TCTA) and 2,2',2''-(1,3,5-benzenetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBI) and demonstrated excellent performance with maximum external quantum efficiency of 14.7% at the current density of 0.01 mA/cm(-1).

Recent advances in phosphorescent platinum complexes for organic light-emitting diodes

“…Tetradentate ligands have attracted an increased attention due to the even higher rigidity of the chromophoric scaffold that helps to suppress nonradiative decay pathways induced by large distortions around the metal atom [76–77]. …”

“…20. Such complexes, whose structure is derived from the parental symmetric systems previously reported by Huo and co-workers [91], bear a 3-methylindole, a carboxylic and a dangling phenoxy moiety, complex 47 , 48 and 49 , respectively, resulting in a general ligand structure with general formula being either C^N^N^C or C^N^N^O.…”

Cytotoxicity of cyclometalated platinum complexes based on tridentate NCN and CNN-coordinating ligands: Remarkable coordination dependence

“…Over last several years, our group has developed a number of luminescent platinum complexes based on cyclometalating ligands with diverse coordination patterns and geometries [29-32]. Luminescent platinum complexes have been investigated as imaging and labelling materials in biological applications [33-36].…”

Recent advances in white organic light-emitting diodes employing a single-emissive material

“…13,[72][73][74][75][76] The rigid molecular structure of tetradentate complexes is likely to achieve high efficiencies by suppressing nonradiative pathways through molecular distortion. Furthermore, removing the need for a monoanionic ligand as the fourth coordination to the Pt metal, as in the case in tridentate Pt complexes, may eliminate a potential degradation pathway to achieve Fig.…”