Enhanced Circularly Polarized Luminescence Activity in Chiral Platinum(II) Complexes With Bis- or Triphenylphosphine Ligands

Distinct circularly polarized luminescence (CPL) activity was observed in chiral (C∧N∧N)Pt(II) [(C∧N∧N) = 4,5-pinene-6′-phenyl-2,2′-bipyridine] complexes with bis- or triphenylphosphine ligands. Compared to the pseudo-square-planar geometry of chiral (C∧N∧N)Pt(II) complexes with chloride, phenylacetylene (PPV) and 2,6-dimethylphenyl isocyanide (Dmpi) ligands, the coordination configuration around the Pt(II) nucleus of chiral (C∧N∧N)Pt(II) complexes with bulk phosphine ligands is far more distorted. The geometry is straightforwardly confirmed by X-ray crystallography. The phosphines' participation enhanced the CPL signal of Pt(II) complexes profoundly, with the dissymmetry factor (glum) up to 10−3. The distorted structures and enhanced chiroptical signals were further confirmed by time-dependent density functional theory (TD-DFT) calculations.


Synthesis
The precursors mononuclear complexes (-)-(C ∧ N ∧ N)PtCl and (+)-(C ∧ N ∧ N)PtCl were prepared according to previous procedures (Zhang et al., 2014(Zhang et al., , 2015b. The target dinuclear and mononuclear Pt(II) complexes have been facilely synthesized though the coordination reaction between different phosphine ligands and precursors in the proper proportion at room temperature (Scheme 1). The new obtained complexes were fully characterized by NMR and MS spectra. The preparation of their enantiomers was done using the same procedure.

Absorption and Emission Properties
As shown in Figure 2 and Figures S2-S4, all of the chiral dinuclear Pt(II) complexes show characteristic absorption bands (ε > 10 4 L mol −1 cm −1 ) in the UV region similar to those of bis-(diphenylphosphino)alkane bridged dinuclear Pt(II) complexes. The mononuclear Pt(II) complex (-)-7 also exhibits a similar intense absorption below 400 nm. According to previous studies, the intense bands (<400 nm) are attributed to intraligand π-π * transitions. In addition, weak absorptions in the region of 400-450 nm are designated as a mixture of metal-to-ligand charge transfer ( 1 MLCT) and ligand-to-ligand charge transfer ( 1 LLCT) transitions (Lu et al., 2002(Lu et al., , 2004Shao and Sun, 2007;Zhang et al., 2018). From the crystal structures of (-)-3-OTf, (-)-4, and (-)-7, it can be found that effective intramolecular/intermolecular Pt···Pt interactions are absent and that two [(C ∧ N ∧ N)Pt] + units manifest like two separated moieties (Sun et al., 2006). Correspondingly, the absorptions of all the complexes only extend to ∼470 nm, which agrees well with the spectrum of (-)-2, demonstrating the nonexistence of metal-metal-to-ligand charge transfer transition ( 1 MMLCT) . All of the chiral dinuclear and mononuclear Pt(II) complexes are highly emissive in solution. For all the dinuclear Pt(II) complexes, a broad and structureless emission band at 546 nm is seen, which resembles that of the mononuclear relative (-)-7 ( Figure S5). Similar to the absorption spectra, the emission energy of all the complexes also reflects the absence of effective intramolecular/intermolecular Pt···Pt interactions. The emission of all the complexes can be ascribed to a triplet metal-to-ligand charge transfer ( 3 MLCT) excited state (Lu et al., 2002(Lu et al., , 2004Shao and Sun, 2007;Zhang et al., 2018). At 77 K, the emissions are significantly blue-shifted and evolve to be more structured (Figure S5), a characteristic nature for 3 MLCT excited states.
An intense emission peak and a shoulder are observed at 515 and 550 nm, respectively, and the spacing of about 1,100 cm −1 correlates to the characteristic skeletal stretching of the free C ∧ N ∧ N ligand.

TD-DFT Calculation
Time-dependent density functional theory (TD-DFT) calculations were carried out, shedding light on the differences in the structural parameters and frontier molecular orbitals of optimized configurations. The optimized configurations of all the chiral dinuclear and mononuclear Pt(II) complexes are shown in Figure S6. Also, the calculated results of the reference mononuclear compounds (-)-(C ∧ N ∧ N)PtCl, (-)-(C ∧ N ∧ N)PtPPV, and (-)-(C ∧ N ∧ N)PtDmpi have been provided. The bond angles around the metal nucleus of chiral Pt(II) complexes coordinated with bis-or triphenylphosphine ligands are further away from linearity than those of the reference mononuclear compounds (Table 2), which is consistent with the results of the crystal structures. In optimized configurations with phosphine ligands, the angles of C1-Pt1-N2 and C2-Pt2-N4 are in the range of 157.10-158.08 • , and the angles of N1-Pt1-P1 and N3-Pt2-P2 range from 170.97 • to 176.82 • . It is further confirmed that the Pt(II) nucleus in bulk bis-or triphenylphosphine systems adopts a more distorted coordination geometry.

CONCLUSION
In summary, we introduced bulky bis-or triphenylphosphine ligands into the phosphorescent pinene-containing (C ∧ N ∧ N)Pt(II) complexes and their structures were determined by single-crystal X-ray analysis. The geometries around the Pt(II) nucleus upon coordinating with bis-or triphenylphosphine were more distorted than those in chloride, phenylacetylene, and 2,6-dimethylphenyl isocyanide systems, which was further verified by DFT calculations. Enhanced CPL activity was observed, with g lum up to 10 −3 order. This study may pave a new way for the preparation of CPL-active phosphorescent metal complexes by introducing bulky ligands.

DATA AVAILABILITY STATEMENT
The datasets generated for this study can be found in the Cambridge Crystallographic Data Centre (https://www.ccdc. cam.ac.uk/structures/) under the identifiers 1984372-1984374.

AUTHOR CONTRIBUTIONS
The preparation and characterization of all the complexes were done mainly by Q-YY, X-LH, and J-LW. The spectra measurement was done mainly by Q-YY, H-HZ, and YC. The TD-DFT calculation was done mainly by S-DW, L-ZH, and Z-FS. The manuscript was written by Q-YY with the guidance of X-PZ.