Observation of a frustrated nematic phase in amphiphilic, disc-like complexes of gold(III) containing hydrocarbon and semiperfluorocarbon terminal chains

The mesomorphism is reported of a discotic complex of gold(III) that is amphiphilic on account of the presence of both hydrocarbon and fluorocarbon chains. The all-hydrocarbon analogue of the complex shows only a columnar hexagonal phase between the melting and clearing points, whereas this new, amphiphilic example shows a nematic phase between a columnar rectangular phase and a columnar hexagonal phase. The nature and formation of the nematic phase are discussed and it is proposed to be a columnar nematic formed as a result of frustration driven by the amphiphilic nature of the complex

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

A series of isomeric bis-2,6-(monoalkoxyphenyl)pyridine and bis-2,6-(dialkoxyphenyl)pyridine ligands were synthesized and characterized. In order to prepare their chlorogold(III) complexes, intermediate chloromercury(II) complexes were first prepared, but unlike observations from previous studies where they were obtained impure and at best in moderate yield, here pure complexes were synthesized, many in rather high yields. Depending on the substitution pattern of the alkoxy chains on the ligands, mono- and/or dimercurated complexes were obtained, characterized by 1H, 13C{1H}, and 199Hg NMR spectroscopy as well as, in several cases, by X-ray crystallography. Factors that may explain this unusual reactivity are discussed. In most cases, transmetalation to the related chlorogold(III) complex proceeded smoothly, although lower yields were obtained when starting from doubly mercurated precursors. Prompted by the propensity of these ligands to mercurate, attempts were made to effect direct auration, but none was successful. However, dimeric, orthometalated complexes of palladium(II) could be prepared and were also amenable to transmetalation to the chlorogold(III) complex, providing for a mercury-free synthesis

Halogen-bonded liquid-crystalline complexes formed from 4-alkoxyphenylpyridines with iodine and with interhalogens

Strongly halogen-bonded complexes are formed between 4-alkoxyphenyl-4-pyridines and iodine as well as the interhalogen compounds ICl and IBr, and examples of each are characterised by single crystal X-ray crystallography. On heating, all but one of the complexes display a liquid-crystalline smectic A phase, although there is evidence of decomposition as the materials are heated through the mesophase. For such short molecules, the mesophases are rather stable and small-angle X-ray scattering shows that the complexes form a type of antiparallel, head-to-head dimeric arrangement in the mesophase. Quantum chemical calculations at the DFT (M06-2X) and MP2 levels of theory show the complexes to have very high dipole moments (between ≈ 9-12 D) and the mesophase stability of the complexes is rationalised in terms of antiparallel correlations induced by the strong molecular dipoles

High-efficiency circularly polarized emission from liquid-crystalline platinum complexes

Realizing both a high emission efficiency and luminescence dissymmetry factor (glum) in circularly polarized solution processable organic light-emitting diodes (CP-OLEDs) remains a significant challenge. In this contribution, two chiral phosphorescent liquid crystals based on cyclometalated platinum complexes are prepared, in which the chiral s-2-methyl-1‑butyl group is introduced into the cyclometalating ligand and the mesogenic fragment is attached to the periphery of the ancillary ligand. The platinum complexes exhibit both smectic and chiral nematic phases as evidenced by polarized optical microscopy, differential scanning calorimetry and small-angle X-ray diffraction. Remarkably, a high photoluminescent quantum efficiency of over 78% and clear circularly polarized luminescent signal with gPL of about 10–2 are observed for the complexes. Further, solution-processed CP-OLEDs show maximum external quantum efficiencies (EQE) of over 15% and strong circularly polarized electroluminescent signals with a gEL ≈ 10–2. This research demonstrates that both liquid crystallinity and the number of chiral centers play key roles in improving the chiroptical property, paving the way for a new approach for the design of high-efficiency CPL emitters

Liquid-Crystalline Thermally Activated Delayed Fluorescence : Design, Synthesis, and Application in Solution-Processed Organic Light-Emitting Diodes

Realizing both high efficiency and liquid crystallinity in one molecule remains a challenge in thermally activated delayed fluorescence (TADF) emission. Herein, two isomeric compounds- m-DPSAc-LC and p-DPSAc-LC with different connection positions between donor and acceptor moieties- were synthesized and characterized. Diphenylsulfone (DPS) was used as the acceptor, acridine (Ac) was used as the donor, and biphenyl derivatives (LC) were employed as the mesogenic group. Both compounds showed a smectic mesophase evidenced by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and temperature-dependent small-angle X-ray scattering (SAXS). The compound p-DPSAc-LC clearly exhibited thermally activated delayed fluorescence due to the much more distorted geometry, whereas m-DPSAc-LC showed simple fluorescence. Compared to the parent TADF molecules without appended mesogenic groups (DPS-Ac), these liquid-crystalline emitters possessed higher hole mobilities and improved device performance. The OLEDs fabricated via solution processing using the liquid-crystalline compound p-DPSAc showed a maximum external quantum efficiency of ∼15% and as such is the first example of a liquid-crystalline TADF material in an OLED device

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

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 Colr and a Colh 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