Neutral and Cationic NCN Pincer Platinum(II) Complexes with 1,3-Bis(benzimidazol-2′-yl)benzene Ligands: Synthesis, Structures, and Their Photophysical Properties

Competition between N,C,N-Pincer and N,N-Chelate Ligands in Platinum(II)

Replacement of the chloride ligand of PtCl{κ3-N,C,N-[py-C6HR2-py]} (R = H (1), Me (2)) and PtCl{κ3-N,C,N-[py-O-C6H3-O-py]} (3) by hydroxido gives Pt(OH){κ3-N,C,N-[py-C6HR2-py]} (R = H (4), Me (5)) and Pt(OH){κ3-N,C,N-[py-O-C6H3-O-py]} (6). These compounds promote deprotonation of 3-(2-pyridyl)pyrazole, 3-(2-pyridyl)-5-methylpyrazole, 3-(2-pyridyl)-5-trifluoromethylpyrazole, and 2-(2-pyridyl)-3,5-bis(trifluoromethyl)pyrrole. The coordination of the anions generates square-planar derivatives, which in solution exist as a unique species or equilibria between isomers. Reactions of 4 and 5 with 3-(2-pyridyl)pyrazole and 3-(2-pyridyl)-5-methylpyrazole provide Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[R'pz-py]} (R = H; R' = H (7), Me (8). R = Me; R' = H (9), Me (10)), displaying κ1-N1-pyridylpyrazolate coordination. A 5-trifluoromethyl substituent causes N1-to-N2 slide. Thus, 3-(2-pyridyl)-5-trifluoromethylpyrazole affords equilibria between Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[CF3pz-py]} (R = H (11a), Me (12a)) and Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N2-[CF3pz-py]} (R = H (11b), Me (12b)). 1,3-Bis(2-pyridyloxy)phenyl allows the chelating coordination of the incoming anions. Deprotonations of 3-(2-pyridyl)pyrazole and its substituted 5-methyl counterpart promoted by 6 lead to equilibria between Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[R'pz-py]} (R' = H (13a), Me (14a)) with a κ-N1-pyridylpyrazolate anion, keeping the pincer coordination of the di(pyridyloxy)aryl ligand, and Pt{κ2-N,C-[pyO-C6H3(Opy)]}{κ2-N,N-[R'pz-py]} (R' = H (13c), Me (14c)) with two chelates. Under the same conditions, 3-(2-pyridyl)-5-trifluoromethylpyrazole generates the three possible isomers: Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[CF3pz-py]} (15a), Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N2-[CF3pz-py]} (15b), and Pt{κ2-N,C-[pyO-C6H3(Opy)]}{κ2-N,N-[CF3pz-py]} (15c). The N1-pyrazolate atom produces a remote stabilizing effect on the chelating form, pyridylpyrazolates being better chelate ligands than pyridylpyrrolates. Accordingly, reactions of 4-6 with 2-(2-pyridyl)-3,5-bis(trifluoromethyl)pyrrole yield Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[(CF3)2C4(py)HN]} (R = H (16), Me (17)) or Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[(CF3)2C4(py)HN]} (18), displaying κ1-N1-pyrrolate coordination. Complexes 7-10 are efficient green phosphorescent emitters (488-576 nm). In poly(methyl methacrylate) (PMMA) films and in dichloromethane, they experience self-quenching, due to molecular stacking. Aggregation occurs through aromatic π-π interactions, reinforced by weak platinum-platinum interactions.

Twist to Boost: Circumventing Quantum Yield and Dissymmetry Factor Trade-Off in Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) enables promising applications in asymmetric photonics. However, the performances of CPL molecules do not yet meet the requirements of these applications. The shortcoming originates from the trade-off in CPL between the photoluminescence quantum yield (PLQY) and the photoluminescence dissymmetry factor (g_PL). In this study, we developed a molecular strategy to circumvent this trade-off. Our approach takes advantage of the strong propensity of [Pt(N^C^N)Cl], where the N^C^N ligand is 1-(2-oxazoline)-3-(2-pyridyl)phenylate, to form face-to-face stacks. We introduced chiral substituents, including (S)-methyl, (R)- and (S)-isopropyl, and (S)-indanyl groups, into the ligand framework. This asymmetric control induces torsional displacements that give homohelical stacks of the Pt(II) complexes. X-ray single-crystal structure analyses for the (S)-isopropyl Pt(II) complex reveal the formation of a homohelical dimer with a Pt···Pt distance of 3.48 Å, which is less than the sum of the van der Waals radii of Pt. This helical stack elicits the metal-metal-to-ligand charge-transfer (MMLCT) transition that exhibits strong chiroptical activity due to the electric transition moment making an acute angle to the magnetic transition moment. The PLQY and g_PL values of the MMLCT phosphorescence emission of the (S)-isopropyl Pt(II) complex are 0.49 and 8.4 × 10^-4, which are improved by factors of ca. 6 and 4, respectively, relative to the values of the unimolecular emission (PLQY, 0.078; g_PL, 2.4 × 10^-4). Our photophysical measurements for the systematically controlled Pt(II) complexes reveal that the CPL amplifications depend on the chiral substituent. Our investigations also indicate that excimers are not responsible for the enhanced chiroptical activity. To demonstrate the effectiveness of our approach, organic electroluminescence devices were fabricated. The MMLCT emission devices were found to exhibit simultaneous enhancements in the external quantum efficiency (EQE, 9.7%) and the electroluminescence dissymmetry factor (g_EL, 1.2 × 10^-4) over the unimolecular emission devices (EQE, 5.8%; g_EL, 0.3 × 10^-4). These results demonstrate the usefulness of using the chiroptically active MMLCT emission for achieving an amplified CPL.

Synthesis and crystal structures of N-H, N-phenyl and N-benzyl-2-(4-hexyl-oxyphen-yl)benzimidazoles

The title compounds, 2-(4-hexyl-oxyphen-yl)-1H-benzimidazole (C19H22N2O; 1), 2-(4-hexyl-oxyphen-yl)-1-phenyl-1H-benzimidazole (C25H26N2O; 2) and 1-benzyl-2-(4-hexyl-oxyphen-yl)-1H-benzimidazole (C26H28N2O; 3) were synthesized and their structures were determined by single-crystal X-ray analysis. The N-substituent at the imidazole moiety slightly affects the inter-planar angle between the 4-hexyl-oxyphenyl ring and the benzimidazole system. The unsubstituted benzimidazole (1) forms inter-molecular N-H⋯N bonds while in the crystal structures of 2 and 3, the mol-ecules are assembled only through π-π and C-H⋯π inter-actions.

White light emission from a green cyclometalated platinum(ii) terpyridylphenylacetylide upon titration with Zn(ii) and Eu(iii )

The cyclometalated Pt(ii) acetylide derivative with a 1,3-bis(N-octyl-benzimidazol-2-yl)benzene (N^C^N) ligand and a free terpyridine (TPY) receptor has been successfully synthesized and characterized. X-ray crystallography shows its inefficient conjugation degree between the [(N^C^N)Pt] and TPY planes. This bifunctional complex shows an enhanced 1MLCT/LLCT absorption band (ε = 3.30 × 104 dm3 mol-1 cm-1) centered at λmax = 365 nm, and the well-resolved vibronic-structured 3MLCT/LLCT emission bands (Φ = 0.08, τ = 3.43 μs) in the range of ca. 475-700 nm. Consecutive titrations show that added Zn2+ and Eu(HFA)3 bond to its free TPY receptor with 1 : 2 and 1 : 1 stoichiometry to form the heterotrinuclear Pt-Zn-Pt (Ka = 3.48 × 104 mol-1 dm3) and heterodinuclear Pt-Eu (Ka = 1.73 × 104 mol-1 dm3) complexes, respectively. A sensitizing effect of Zn2+ on the TPY unit, and the incomplete d → f energy transfer from the [(N^C^N)Pt(ii)] antenna donor to the Eu(iii) center with maximum efficiency of 51.8% are observed. Using an in situ mixed titration strategy, the R/G/B emission triads consisted of red [(TPY)Eu(HFA)3] and green [(N^C^N)Pt(ii)] dual phosphorescence and blue [(TPY)Zn(TPY)] fluorescence, which can be well balanced to realize the white-light-emission with CIE coordinates (x = 0.36, y = 0.36) by precisely controlling the molar ratio (9 : 1 : 2) of the parent complexes, Eu(HFA)3 and Zn(ClO4)2.

Synthesis and structure of an aryl-selenenium(II) cation, [C34H41N4Se+]2[Hg(SeCN)4]2-, based on a 5-tert-butyl-1,3-bis-(1-pentyl-1H-benzimidazol-2-yl)benzene scaffold

In the title salt, bis-{[5-tert-butyl-1,3-bis-(1-pentyl-1H-benzimidazol-2-yl)benzene]selenium} tetra-kis-(seleno-cyanato)-mercury, (C34H41N4Se)2[Hg(SeCN)4], the aryl-selenenium cations, [C34H41N4Se]+, are linked through [Hg(SeCN)4]2- anions by C-H⋯N hydrogen bonds. In the cation, the geometry around the Se atom in the 5-tert-butyl-1,3-bis-(1-pentyl-1H-benzimidazol-2-yl)benzene scaffold is T-shaped, resulting from the coordination of Se by the C atom of the central aromatic ring and the N atoms of both of the benzimidazole moieties. The trans Se-N bond lengths are almost equal [2.087 (3) and 2.099 (3) Å] and the Se-C bond length is 1.886 (3) Å. The N-Se-N angle is 159.29 (11)°. The geometry around the HgII atom in the [Hg(SeCN)4]2- anion is distorted tetra-hedral, with Se-Hg-Se angles ranging from 88.78 (3) to 126.64 (2)°. In [Hg(SeCN)4]2-, the Hg-Se bonds are unsymmetrical [2.5972 (4) and 2.7242 (5) Å]. One of the pentyl substituents is disordered over two equivalent conformations, with occupancies of 0.852 (8) and 0.148 (8).

Our Expedition in Linear Neutral Platinum-Acetylide Complexes: The Preparation of Micro/nanostructure Materials, Complicated Topologies, and Dye-Sensitized Solar Cells

During the past few decades, the construction of various kinds of platinum-acetylide complexes has attracted considerable attention, because of their wide applications in photovoltaic cells, non-linear optics, and bio-imaging materials. Among these platinum-acetylide complexes, the linear neutral platinum-acetylide complexes, due to their attractive properties, such as well-defined linear geometry, synthetic accessibility, and intriguing photoproperties, have emerged as a rising star in this field. In this personal account, we will discuss how we entered the field of linear neutral platinum-acetylide chemistry and what we found in this field. The preparation of various types of linear neutral platinum-acetylide complexes and their applications in the areas of micro/nanostructure materials, complicated topologies, and dye-sensitized solar cells will be summarized in this account.