Synthesis, Mesomorphism, Photophysics, and Device Properties of Liquid-Crystalline Pincer Complexes of Gold(III) Containing Semiperfluorinated Chains

Abstract

Gold(III) complexes of C ∧ N ∧ C -coordinating 2,6-diphenylpyridine pincer ligands with arylacetylide co-ligands are known triplet emitters at room temperature. We have reported previously that by functionalizing both the pincer ligand and the phenylacetylene with alkoxy chains, liquid crystallinity may be induced, with the complexes showing columnar mesophases. We now report new derivatives in which the phenylacetylene incorporates one, two, or three 1 H ,1 H ,2 H ,2 H -perfluoroalkyl chains. In terms of intermolecular interactions, solution 1 H NMR experiments suggest that the semiperfluoroalkyl chains promote a parallel, head-to-head arrangement of neighboring molecules relative to one another, rather than the anti-parallel, head-to-tail orientation found for the all-hydrocarbon materials. In terms of the liquid crystal properties, the complexes show columnar phases, with the addition of the more rigid fluorocarbon chains leading to a stabilization of both the crystal and liquid crystal mesophases. Mesophase temperature ranges were also wider. Interestingly, the amphiphilic nature of these complexes is evident through the observation of a frustrated columnar nematic phase between a Col r and a Col h phase, an observation recently reported in detail for one compound ( Liq. Cryst. , 2022, doi: 10.1080/02678292.2021.1991017 ). While calculation shows that, despite the “electronic insulation” provided by the dimethylene spacer group in the semiperfluoroalkyl chains, a small hypsochromic shift in one component of the absorption band is anticipated, experimentally this effect is not observed in the overall absorption envelope. Complexes with substituents in the 3,3′,4,4′-positions of the phenyl rings of the pincer ligand once more show higher-luminescence quantum yields than the analogues with substituents in the 4,4′-positions only, associated with the lower-energy-emissive state in the former. However, in contrast to the observations with all-hydrocarbon analogues, the luminescence quantum yield of the complexes with 3,3′,4,4′-substitution on the pincer increases as the number of semiperfluoroalkyl chains on the phenylacetylide increases, from 20% (one chain) to 34% (three chains). External quantum efficiencies in fabricated OLED devices are, however, low, attributed to the poor dispersion in the host materials on account of the fluorinated chains.

On the mercuration, palladation, transmetalation and direct auration of a C^N^C pincer ligand

“…The compound is prepared by reaction of 2,6-dibromopyridine with 2,3-dimethoxyphenylboronic acid using a Suzuki-Miyaura protocol (89% yield), with the 2,2',3,3'tetraalkyloxy-substituted ligands (butyl and dodecyl) resulting from demethylation using pyridinium chloride (90% yield) followed by re-alkylation under Williamson ether conditions using 1-bromoalkane (53% yield C4, 65% yield C12). Thus, the dodecyloxy-substituted ligands have been those of greatest interest in relation to generating liquid-crystalline materials, [8][9][10] however these prove challenging to crystallise (as do methoxy derivatives on account of low solubility) and so we have typically deployed butyloxy-substituted ligands to obtain single crystals for study by diffraction. The steric demands of these materials mean that in the free ligand, the phenyl and pyridyl rings cannot be co-planar (Fig.…”

“…However, for the Hg-Cl distances there is a more complex story, which is described in greater detail in the Supplementary Information. This dimercury complex can then be transmetalated to give the related gold(III) complex (6)(7)(8)(9)(10)(11)(12) in an isolated yield of 24%, which gives an 18% overall yield from the ligand (Scheme) In comparison, formation of 6-4 from 5-4 proceeds in 47% yield, which gives an overall conversion of ca 26%. These represent improvements over yields from the impure mercury intermediates described above, 8 but still requires the intermediacy of mercury.…”

“…Figure 3 (a) Structure of the 2,6-bis(2,2',3,3'-tetraalkoxy)pyridine ligands; (b) 1 H NMR spectrum of the pyridyl hydrogens of 2,6-bis(2,2',3,3'-tetramethoxy)pyridine (3-1)showing the two AB spin systems; (c) 1 H NMR spectrum of the pyridyl hydrogens of the chlorogold(III) complex of 2,6-bis(2,2',3,3'-tetra-dodecyloxy)pyridine(6)(7)(8)(9)(10)(11)(12) showing the AX 2 spin system.…”

“…2). 9,10 Briefly, the vast majority of the complexes are liquid crystalline, showing almost exclusively columnar phases with the complexes having four chains on the pincer ligand having photoluminescent quantum yields (PLQY) of up to 36%, while those with only two chains had PLQY values of <4%. Encouragingly, OLED devices fabricated from the all-hydrocarbon complexes showed external quantum efficiencies of >7%.…”

Gold(III), Mercury(II), and Palladium(II) Complexes of a Series of Isomeric Bis(mono- and dialkoxyphenyl)pyridines: Introduction of Gold through Transmetalation and Catalysis

“…Our own contributions in this area have been to modify C,N,C-pincer ligands and attendant phenylacetylide coligands to confer liquid-crystal properties on them and we have reported on the preparation, mesomorphism, photophysics, and device behavior. , In particular, one complex, which had hydrocarbon chains on the pincer ligand and semiperfluorocarbon chains on the phenylacetylide, showed rather remarkable liquid-crystal properties. Thus, a fluid nematic phase, assigned as columnar nematic (N Col ), was found between two much more ordered phases, namely, a columnar hexagonal (Col h ) phase to higher temperature and a columnar rectangular phase (Col r ) to lower temperature.…”