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 1H,1H,2H,2H-perfluoroalkyl chains. In terms of intermolecular interactions, solution ¹H 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.

Figure 1

Luminescent gold(III) complexes described by Yam and co-workers. Typically R′ = H and R = 4-C₆H₄–X.

Figure 2

Luminescent, all-hydrocarbon gold(III) liquid crystals reported previously.

Figure 3

Synthesis of the alkoxy-substituted phenylacetylenes and their reaction to form the target gold(III) complexes. Conditions: (i) Tf₂O, pyridine (in CH₂Cl₂/dioxane), 0 °C, 1 h, N₂; (ii) TfO(CH₂)₂C_mF_2m+1, K₂CO₃, acetonitrile, 16 h, r.t.; (iii) CBr₄, PPh₃, Et₃N, CH₂Cl_2, 30 min, 0 °C, N₂; (iv) EtMgBr, THF, 1 h, r.t., N₂; and (v) Ar–C≡C–H/CuI/Et₃N/CH₂Cl₂. Compound 14: 4,4′-didodecyloxy-substituted CNC ligand; compound 15: 3,3′,4,4′-tetradodecyloxy-substituted CNC ligand (Figure S1).

Figure 4

Structure of 21a (R = C₆F₁₃CH₂−) and 21b (R = C₈F₁₇CH₂−), showing the hydrogen identification system used.

Figure 5

¹H NMR spectra of 21a at the concentrations indicated, showing the downfield shift of the aromatic hydrogens on the pincer ligand and on the phenylethynyl ligand with dilution.

Figure 6

¹H NMR spectra of 21a at the concentrations indicated, showing the downfield shift of the O–CH₂ protons of the alkyl chains.

Figure 7

Proposed aggregate of 21b in concentrated solution showing the relative disposition of the complexes. Different colors are used for the two complex molecules for clarity, but in both of these, the fluorinated chains are shown in green.

Figure 8

Transition temperatures and phases for 16–21. For reasons of diagrammatic clarity, the monotropic Col_r³ phase of 20c is not shown.

Figure 9

(a) Photomicrograph of the Col_r phase of 17b at 134.4 °C on cooling from the isotropic liquid and (b) corresponding SAXS pattern at 140.0 °C on cooling from the isotropic liquid.

Figure 10

Photomicrographs on cooling from the isotropic liquid of (a) 19a at 177.5 °C showing focal conics and (b) 19b at 81.6 °C showing a mosaic texture.

Figure 11

Photomicrographs of 20c in the nematic phase on cooling at 156.4 °C.

Figure 12

Absorption spectrum (black), excitation spectrum (dashed green), and emission spectra at 298 K (red) and 77 K (blue) for 16a (left) and 19a (right).

Figure 13

Structures of the complexes used in the calculations. X = H or F.

Figure 14

Selected molecular orbitals for compound 17F-Me at the pbe0/def2-TZVPP level. Isosurfaces set to the 0.02 level.

Figure 15

(a) Device structure and energy level scheme; (b) molecular structures of the host and charge transport materials; (c) EQE–voltage curves (inset: EL spectra); and (d) J–V–L curves of the devices.