Duncan W. Bruce

Phosphorescent, Terdentate, Liquid‐Crystalline Complexes of Platinum(II): Stimulus‐Dependent Emission

Liquid crystals shining bright. A highly efficient platinum(II) luminophore is rendered liquid crystalline using a simple and flexible synthetic approach. Ordering in the liquid-crystalline state allows monomer emission when the characteristic for the material is exciplex-like emission. More than that, emission characteristics are subject to tribological control, with the initial state re-obtained by thermal cycling.

Homogeneous catalysts based on water-soluble phosphines

Aryl- and alkyl-substituted tertiary phosphines are among the most common ligands used in homogeneous catalysts, but they are hydrophobic, as in general are the products of the catalysis. Therefore, there are two basic problems in homogeneous catalysis, namely the separation and subsequent recycling of the catalyst. These problems can be elegantly solved by using two-phase catalysis where, for example, the catalyst can be in a hydrophilic phase in which the organic products are insoluble. In order to transform homogeneous catalysts based on tertiary phosphines into water-soluble moieties, one approach is to make the phosphines water-soluble. Thus, when the catalytic reaction is complete, the products are in an organic phase and the catalyst is in the aqueous phase. Then, a simple phase separation enables the continuous re-use of the catalyst. However, if we compare biphasic reactions with their monophasic equivalents, we find that rates are lower in two-phase systems. This is mainly due to the fact that when the catalyst is in one phase and the substrates are in another, the interaction between the catalyst and the substrates is lower than in a monophasic system, thus reducing the rate of the reactions. A way to remove disadvantages of both monophasic and biphasic catalysis is to introduce ligands that confer a thermoregulated phase-transfer function to the catalyst. The strategy of the thermoregulated phase-transfer catalysis (TRPTC) is that before the reaction, the catalyst resides in the aqueous phase and the substrates in the organic phase as in regular biphasic catalysis, but at higher temperature the catalyst can transfer into the organic phase to catalyse the reaction, as in a homogeneous system, and then return to the aqueous phase to be separated from the product at lower temperature.

Iron(III) salen-type catalysts for the cross-coupling of aryl Grignards with alkyl halides bearing β-hydrogens

Iron(III) salen and related complexes are active catalysts for the coupling, under mild and simple reaction conditions, of aryl Grignard reagents with primary and secondary alkyl halide substrates bearing beta-hydrogens.

Intercalated liquid-crystalline phases formed by symmetric dimers with an α,ω-diiminoalkylene spacer

Four series of symmetric, dimeric molecules with flexible α,ω-diiminoalkylene spacers have been synthesised and characterised by polarising optical microscopy, differential scanning calorimetry, and X-ray diffraction. The effects on the mesomorphism of the various alkylene spacer (number of methylene units m = 5 to 8) as well as the terminal alkoxy chain length (n = 4 to 14) on the mesomorphic properties have been studied. The compounds with an even-numbered spacer display, in addition to a nematic phase, various smectic phases whose nature depends on the ratio between the length of the terminal chain and the length of the spacer (n/m). A modulated SmA phase was found for the ratio n/m = 1 to 1.6 and a SmC/crystal G polymorphism emerges at n/m > 1.7. In contrast, all liquid-crystalline dimers with odd-numbered spacers display an intercalated B6 phase regardless of the chain length. The preference for intercalated structures is discussed based on dipolar considerations following electronic structure calculations using a model molecule.

Fluorinated liquid crystals formed by halogen bonding

New, halogen-bonded fluorinated mesogens are reported; the expected microphase separation associated with perfluoroalkyl chains is surprisingly absent in the mesophase.

Structure-function relationships in liquid-crystalline halogen-bonded complexes.

New liquid-crystalline materials were prepared by self-assembly driven by halogen bonding between a range of 4-alkoxystilbazoles, 4-alkyl-, and 4-alkoxy-substituted pyridines as halogen-bonding acceptors, and substituted derivatives of 4-iodotetrafluorophenyl as halogen-bonding donors. Despite the fact that the starting materials are not mesomorphic, the dimeric, halogen-bonded complexes obtained exhibited nematic and SmA phases, depending on the length of the alkyl chains present on the components. The modularity of this approach also led to new chiral mesogens starting from non-mesomorphic chiral compounds.

Mesogenic, trimeric, halogen-bonded complexes from alkoxystilbazoles and 1,4-diiodotetrafluorobenzene

New, halogen-bonded mesogens are formed as trimeric complexes of two molecules of alkoxystilbazole and one of 1,4-diiodotetrafluorobenzene. The pure complexes show only monotropic nematic phases, while mixtures show a enantiotropic nematic with a range of up to 11 °C. A possible correlation between nematic phase stability and halogen bond strength is suggested.

Liquid-crystalline ionic liquids

Low-melting point salts, the basis of industrially relevant ionic liquids, exhibit smectic A mesophases over extended temperature ranges.

Metallomesogens by ligand design

In this Perspective, some of the basic principles of ligand design will be described as they apply to the synthesis of metal complexes with liquid crystal properties. Liquid crystal properties of metal complexes are a delicate function of both ligand and metal and examples will be chosen to illustrate this statement. In particular, the effect of the metal complex moiety on the mesomorphism of a series of imine and diimine liquid crystals is described.

Bending and shaping: cubics, calamitics and columnars

The mesomorphism of a series of complexes of Pd(II), Pt(II) and Ag(I) is discussed and systematic structural variations are highlighted which lead to an appreciation of important factors determining the liquid-crystalline polymorphism of these complexes. Models are proposed for cubic phase formation and the occurrence of an unusual lamellar phase in-between a SmC and a columnar phase is discussed.