Fine tuning phosphorescent properties of platinum complexes via different N -heterocyclic-based CˆNˆN ligands

Photoluminescence of Platinum(II) Complexes with Diazine-Based Ligands

In the last past twenty years, research on luminescent platinum (II) complexes has been intensively developed for useful application such as organic light emitting diodes (OLEDs). More recently, new photoluminescent complexes based on diazine ligands (pyrimidine, pyrazine, pyridazine, quinazoline and quinoxaline) have been developed in this context. This review will summarize the photophysical properties of most of the phosphorescent diazine Pt(II) complexes described in the literature and compare the results to pyridine analogues whenever possible. Based on the emission color, and the photoluminescence quantum yield (PLQY) values, the relationship between structure modification, and photophysical properties are highlighted. Tuning of emission color, quantum yields in solution and solid state and, for some complexes, aggregation induced emission (AIE) or thermally activated delayed fluorescence (TADF) properties are described. When emitting OLEDs have been built from diazine Pt(II) complexes, the external quantum efficiency (EQE) values and luminance for different emission wavelengths and in some cases, chromaticity coordinates obtained from devices, are given. Finally, this review highlights the growing interest in studies of new luminescent diazine Pt(II) complexes for OLED applications.

Luminescent cyclometalated alkynylplatinum(II) complexes with 1,3-di(pyrimidin-2-yl)benzene ligands: synthesis, electrochemistry, photophysics and computational studies

In this article, we report on a series of cyclometalated chloro- and alkynyl-platinum(II) complexes bearing various tridentate N^C^N-cyclometalated ligands derived from 1,3-bis(pyrimidin-2-yl)benzene. The X-ray crystal structures of two alkynyl-platinum(II) complexes were determined and other structures were DFT-calculated. Electrochemical and DFT-computational studies suggest a ligand-centred reduction on the R¹-substituted N^C^N ligand, whereas oxidation likely occurs either on the Pt-phenylacetylide moiety and/or the cyclometalated ligand. In CH₂Cl₂ solution at room temperature, the complexes show phosphorescent emissions ranging from green to orange, depending on the R¹ and R² substituents on the ligands. In KBr solid state matrix, excluding complexes bearing a trifluoromethyl substituted ligand, all compounds exhibit red emission. The presence of an alkynyl ancillary ligand has limited influence on absorption and emission spectra except in the case of the complex with the strongly electron-donating diphenylamino R² substituent on the alkynyl ligand, for which a significant red-shift was observed. The alkynyl Pt(II) complex with OMe groups as both R¹ and R² substituents shows the best emission quantum yield (0.81 in CH₂Cl₂ solution) in this series. The full series of DFT calculated band gaps correlated generally well with the electrochemical and absorption data and reasonably model the impact of the substituents on the electronics of these complexes.

A New Pt(II) Complex with Anionic s-Triazine Based NNO-Donor Ligand: Synthesis, X-ray Structure, Hirshfeld Analysis and DFT Studies

The reaction of PtCl₂ with s-triazine-type ligand (HTriaz) (1:1) in acetone under heating afforded a new [Pt(Triaz)Cl] complex. Single-crystal X-ray diffraction analysis showed that the ligand (HTriaz) is an NNO tridentate chelate via two N-atoms from the s-triazine and hydrazone moieties and one oxygen from the deprotonated phenolic OH. The coordination environment of the Pt(II) is completed by one Cl⁻¹ ion trans to the Pt-N_(hydrazone). Hirshfeld surface analysis showed that the most dominant interactions are the H···H, H···C and O···H intermolecular contacts. These interactions contributed by 60.9, 11.2 and 8.3% from the whole fingerprint area, respectively. Other minor contributions from the Cl···H, C···N, N···H and C···C contacts were also detected. Among these interactions, the most significant contacts are the O···H, H···C and H···H interactions. The amounts of the electron transfer from the ligand groups to Pt(II) metal center were predicted using NBO calculations. Additionally, the electronic spectra were assigned based on the TD-DFT calculations.

Formation of Hetero-binuclear Pt(II)-M(II) Complexes Based on (2-(1 H-Tetrazol-5-yl)phenyl)diphenylphosphine Oxide for Superior Phosphorescence of Monomers

Novel hetero-binuclear platinum complexes (HBC-Pt(II)-M(II), M = Ca(II), Mg(II), Zn(II), and Cd(II)) have been synthesized by the reaction of the corresponding precursors [Pt(ppy)(μ-Cl)]₂ with (2-(1 H-tetrazol-5-yl)phenyl)diphenylphosphine oxide (TTPPO). The X-ray structures of the complexes show that two ancillary ligands TTPPO in the square-planar Pt(II) moiety act as a quadridentate chelating agent for the other metal center, eventually forming a distorted octahedral configuration. There are no significant π-π interactions and Pt-M metallophilic interactions in the crystal lattice, due to the steric hindrances associated with the rigid octahedral structure together with the bulky TTPPO. Consequently, HBC-Pt-M complexes show monomer emission characteristics with quantum yields up to 59% in powder, suggesting their great potential for practical applications. DFT and TD-DFT calculations on HBC-Pt-Zn reveal that the phosphorescence can be ascribed to intraligand charge transfer (³ILCT) combined with some metal-to-ligand charge transfer (³MLCT) in the Pt(ppy) moiety, which is consistent with the observations from the photophysical investigations.