A new form of discotic metallomesogens: the synthesis of metal-bridged triphenylene discotic dimers

Discotic metallomesogens are becoming increasingly important due to their electronic and optoelectronic properties. Preliminary results on the synthesis and characterization of a new form of discotic metallomesogens in which a Hg atom is covalently linked with two substituted triphenylene moieties are presented.     

Novel monofunctionalized electron-deficient anthraquinone-based discotic liquid crystals

A series of electron-deficient 1-hydroxy-2,3,5,6,7-pentaalkoxyanthra-9,10-quinones has been synthesized. All nine members of the series were found to be liquid crystalline, forming columnar mesophases over a broad temperature range. Such supramolecular building blocks can be used for the preparation of novel discotic dimers, oligomers, polymers and metallomesogens.     

Novel monofunctionalized electron-deficient anthraquinone-based discotic liquid crystals

A series of electron-deficient 1-hydroxy-2,3,5,6,7-pentaalkoxyanthra-9,10-quinones has been synthesized. All nine members of the series were found to be liquid crystalline, forming columnar mesophases over a broad temperature range. Such supramolecular building blocks can be used for the preparation of novel discotic dimers, oligomers, polymers and metallomesogens.     

An example of an unsymmetrically substituted,monofunctionalized anthraquinone-based discotic liquid crystal

We report here the synthesis and characterization of an unsymmetrically substituted, monofunctionalized anthraquinone-based discotic liquid crystal, namely 5-hydroxy-1-(4-nitrobenzyloxy)2,3,6,7-tetrapentyloxy-9,10-anthraquinone. Interestingly, this compound, which has only four alkyloxy chains attached to the anthraquinone core, shows a hexagonal columnar phase (Col_h) of much wider temperature range compared with analogues having six alkyloxy chains. Incidentally, among the anthraquinone-based disc-like molecules exhibiting columnar phases known so far, this is the first molecule, which does not possess any C₂-axis of symmetry. Besides its unsymmetrical structure and interesting mesomorphic behaviour, monofunctionalized monomers of this kind, unlike the available C₂-symmetric difunctionalized tetraethers, can serve as potential synthons for obtaining dimers in a unique way. In addition, various anthraquinone-based oligomers, metallomesogens, and side group liquid crystalline polymers can be obtained from these monomers.     

An example of an unsymmetrically substituted, monofunctionalized anthraquinone-based discotic liquid crystal

We report here the synthesis and characterization of an unsymmetrically substituted, monofunctionalized anthraquinone-based discotic liquid crystal, namely 5-hydroxy-1-(4-nitrobenzyloxy)2,3,6,7-tetrapentyloxy-9,10-anthraquinone. Interestingly, this compound, which has only four alkyloxy chains attached to the anthraquinone core, shows a hexagonal columnar phase (Col_h) of much wider temperature range compared with analogues having six alkyloxy chains. Incidentally, among the anthraquinone-based disc-like molecules exhibiting columnar phases known so far, this is the first molecule, which does not possess any C₂-axis of symmetry. Besides its unsymmetrical structure and interesting mesomorphic behaviour, monofunctionalized monomers of this kind, unlike the available C₂-symmetric difunctionalized tetraethers, can serve as potential synthons for obtaining dimers in a unique way. In addition, various anthraquinone-based oligomers, metallomesogens, and side group liquid crystalline polymers can be obtained from these monomers.     

Synthesis of two naphthyl-containing homologous series of mesogenic ligands and the related metallomesogens containing Cu(II)

Two new naphthyl-containing homologous series of mesogenic ligands, the 4-n-alkoxy-2-hydroxybenzylidene-2'-naphthylamines (series I) and 4(4'-n-alkoxybenzoyloxy)-2-hydroxybenzylidene-2'-naphthylamines (series II), as well as the related metallomesogens of higher homologues containing a Cu(II) atom, have been synthesized. All the ligands and complexes were characterized by a combination of elemental analysis and standard spectroscopic methods. In series I, n-heptyloxy and n-octyloxy derivatives are non-mesogenic whereas the remaining higher members synthesized exhibit a monotropic nematic mesophase. In series II, all the members synthesized exhibit an enantiotropic nematic mesophase. All the metallomesogens of series I synthesized exhibit a monotropic smectic A mesophase, except for the n-octyloxy derivative, which is non-mesogenic, whereas metallomesogens of series II exhibit enantiotropic nematic mesophases up to the n-tetradecyloxy derivatives; the n-tetradecyloxy and n-hexadecyloxy derivatives also exhibit smectic C mesophases. All the members of series II and their metallomesogens exhibit mesophases with wide temperature ranges and greater thermal stability as compared to series I and their metallomesogens, respectively. The mesomorphic properties of both the present series and their metallomesogens are compared with each other and with other structurally related series to evaluate the effect of the naphthalene moiety on mesomorphism.     

The Contribution of Ruthenium in the Development of Metallomesogens

Ruthenium remains one of the most popular metals in inorganic and organometallic chemistry. The multiple oxidation state that it possesses and the versatility in its coordination mode have contributed to this success. However, despite the facility to synthesize all kind of ruthenium-based complexes, the insertion of ruthenium in metallomesogens remains scarce. This review presents all metallomesogens incorporating ruthenium over the last 35 years, suggesting that the contribution of ruthenium in the development of metallomesogens has just begun.     

My 20 years of research in the chemistry of metal containing liquid crystals

Twenty years ago, in 1977, I began, with important exterior collaborations, original research on metal containing liquid crystals, also known as metallomesogens. I wished to synthesize in this paper my 20 years of research in this field. These compounds combine the known properties of organic liquid crystals with those introduced by the presence of one or more metals (colour, magnetism, polarizability, multiple localized charges) and provide new geometrical shapes including square planar, octahedral, square pyramidal, lantern structures which are unobtainable in purely organic compounds. We have synthesized both rod-like and disc-like metallomesogens, and observed examples of almost all the main mesophase types. In 1977 was the beginning of a systematic research into metallomesogens, the phenomenal growth occurred in early 1980, when many laboratories entered the field.     

Thermotropic mesomorphism in penta- and hepta-coordinated metal complexes

New Ni(II) and UO₂(II) metallomesogens obtained from mesomorphic N,N'-salicyliden(3,3'-diamine-N-methyldipropylamine) ligands containing a pentadentate N₃O₂ chelating cavity and bearing two or four lateral substituents, are isolated and fully characterized. Some of the synthesized nickel and dioxouranium complexes show a SmC mesophase. To the best of our knowledge, these species are the first pentacoordinated Ni(II) and heptacoordinated UO₂(II) metallomesogens to be reported.     

Polynuclear Silver(I) Triazole Complexes: Ion Conduction and Nanowire Formation in the Mesophase

Examples of polynuclear metallomesogens are few. Herein,1,2,4‐triazole ligands were used to prepare mono‐ and polynuclear silver(I) triazole metallomesogens. Besides showing an SmA phase in the mesophase, two interesting properties were observed. First, higher ion conductivity is always found for the polynuclear complexes than for the mononuclear complexes with the same anion, an observation contrary to the knowledge that migration of a monomeric cation should be faster than that of a polymeric cation. Second, thermolysis of the polynuclear silver(I) triazole complexes in the assembled mesophase yielded Ag nanowires, in an excellent demonstration of the assembled nature of the polynuclear silver(I) ions in the thermolysis process.     

Metallomesogens: Metal Complexes in Organized Fluid Phases

Metallomesogens, metal complexes of organic ligands which exhibit liquid crystalline (mesomorphic) character, combine the variety and range of metal-based coordination chemistry with the extraordinary physical properties exhibited by liquid crystals. Thermotropic metallomesogens have been made incorporating many metals, including representatives of s-, p-, d-and even f-block elements. Both rodlike (calamitic) and disklike (discotic) thermotropic metallomesogens are known, and examples of all the main mesophase types are found. Many different varieties of ligand can be used: monodentate (4-substituted pyridines), bidentate (β-diketonates, dithiolenes, carboxylates, cyclometalated aromatic amines), or polydentate (phthalocyanines, porphyrins). As with organic mesogens, molecular shape and intermolecular forces play an important role, i.e. the ligands are important in determining mesophase character. The chief requirement for a metallomesogen is a rigid core, usually unsaturated and either rod- or disklike in shape, bearing several long hydrocarbon tails. The metal atom is usually at or near the center of gravity of the molecule. In some cases the ligands are themselves mesogenic, but this is not a requirement. The presence of one or more metals opens many exciting possibilities: new shapes, not easily generated by organic compounds, and hence new properties are then accessible. The incorporation of d-block metals brings with it features such as color and paramagnetism. Profound effects arise from the large and polarizable concentration of electron density that every metal atom possesses, since the molecular polarizability is a key factor in determining whether a molecule will form liquid crystals. Enhanced physical properties (e.g. high birefringencies), as well as new and unexpected ones, will result. A major requirement for metallomesogens to find applications in new device technology is that the metal–ligand bonds are strong and inert and the complexes stable; this can be accomplished with, for example, chelating ligands and the 5d metals.     

Supramolecular metallomesogens. Self-organization of an H-bonded tetrameric assembly into columnar phase from single component

The first example of supramolecular metallomesogens derived from palladium complexes exhibiting columnar phase is reported, and the formation of the mesophase is induced by H-bonding in a tetrameric assembly.     

Listening to Lanthanide Complexes: Determination of the Intrinsic Luminescence Quantum Yield by Nonradiative Relaxation

The photophysical properties of lanthanide complexes have been studied extensively; however, fundamental parameters such as the intrinsic quantum yield as well as radiative and nonradiative decay rates are difficult or even impossible to measure experimentally. Herein, a photoacoustic (PA) method is proposed to determine the intrinsic quantum yield of lanthanide complexes with lifetimes in the order of milliseconds. This method is used to determine the intrinsic quantum yields for europium(III)‐containing metallomesogens as well as terbium(III) complexes. The results show that the PA signal is sensitive to both the lifetime and the ratio of the fast‐to‐slow heat component of the samples. It is found that there is an efficient ligand sensitization and a moderate intrinsic quantum yield for the complexes. The intrinsic quantum yield of Eu³⁺ in the metallomesogens exhibits an obvious increase upon the isotropic liquid to smectic A transition. The proposed PA method is quite simple, and can contribute to a clearer understanding of the photophysical processes in luminescent lanthanide complexes.     

Luminescent lanthanide complexes with liquid crystalline properties

Lewis-base adducts of tris(β -diketonato)lanthanide(III) complexes were prepared, where the β -diketone is para -alkoxy-substituted 1,3-diphenyl-1,3-propanedione. These compounds are the first examples of liquid crystalline lanthanide complexes in which the mesomorphism is introduced via a β -diketonate ligand. Depending on the type of the Lewis base, the metallomesogens exhibit a monotropic smectic A or a monotropic highly ordered smectic phase. Intense photoluminescence was observed for the europium(III) complexes at room temperature.     

One-dimensional iron(II) compounds exhibiting spin crossover and liquid crystalline properties in the room temperature region

A novel series of 1D Fe(II) metallomesogens have been synthesized using the ligand 5-bis(alkoxy)- N-(4 H-1,2,4-triazol-4-yl)benzamide (C n -tba) and the Fe(X) 2. sH 2O salts. The polymers obey the general formula [Fe(C n -tba) 3](X) 2. sH 2O [X = CF 3SO 3 (-), BF 4 (-); n = 4, 6, 8, 10, 12]. The derivatives with n = 4, 6 exhibit spin transition behavior like in crystalline compounds, whereas those with n = 8, 10, 12 present a spin transition coexisting with the mesomorphic behavior in the room-temperature region. A columnar mesophase has been found for the majority of the metallomesogens, but also a columnar lamellar mesophase was observed for other derivatives. [Fe(C 12-tba) 3](CF 3SO 3) 2 represents a new example of a system where the phase transition directly influences the spin transition of the Fe(II) ions but is not the driving energy of the spin crossover phenomenon. The compounds display drastic changes of color from violet (low-spin state, LS) to white (high-spin state, HS). The compounds are fluid, and it is possible to prepare thin films from them.     

New metallomesogens with enaminoketonato ligands

Several metallomesogens based on enaminoketonato ligands have been prepared. 1-Amino-1-(4'-butoxy-4-biphenyl)oct-1-en-3-one was synthesized by heterocycle hydrogenolysis in 3-(4'-butoxy-4-biphenyl)-5-amyl-isoxazole. Liquid crystalline complexes were obtained by reaction of this enaminoketone with nickel(II) or copper(II) acetates. Hydrogenolysis of the heterocycle in 3-(4-hydroxyphenyl)-5-amylisoxazole led to 1-amino-1-(4-hydroxyphenyl)oct-1-en-3-one. The corresponding copper(II) enaminoketonate was then obtained. The subsequent reaction of this compound with 4-alkoxy-benzoic acid chlorides gave liquid crystalline complexes with ester bridges in the core. A nematic phase was observed for all the synthesized metallomesogens. The bridging group influence on melting points, mesophase ranges and thermal stability of copper(II)complexes is discussed.     

Luminescent lanthanide complexes with liquid crystalline properties

Lewis-base adducts of tris( β-diketonato)lanthanide(III) complexes were prepared, where the β-diketone is para -alkoxy-substituted 1,3-diphenyl-1,3-propanedione. These compounds are the first examples of liquid crystalline lanthanide complexes in which the mesomorphism is introduced via a β-diketonate ligand. Depending on the type of the Lewis base, the metallomesogens exhibit a monotropic smectic A or a monotropic highly ordered smectic phase. Intense photoluminescence was observed for the europium(III) complexes at room temperature.     

Columnar Phases from Covalent and Hydrogen-Bonded Liquid-Crystalline Ferrocene Derivatives

The synthesis and liquid-crystalline properties of tetracatenar covalent and H-bonded bis-ferrocene derivatives 1 and 2 , respectively, are reported. Both compounds gave rise to enantiotropic columnar liquid-crystalline behavior with a hexagonal molecular organization. To explore the possibility to obtain also calamitic liquid-crystalline phases from H-bonded ferrocene-containing liquid crystals, a rod-shaped ferrocene mesogen 3 was synthesized, which gave rise to enantiotropic smectic C and smectic A phases. For the first time, a rational synthetic design at the ferrocene level led to ferrocene-based liquid-crystals with columnar behavior and to H-bonded metallomesogens.     

Mesomorphic properties and magnetic behaviour of di-(5-substituted-salicylidene)ethylenediaminato cobalt(II) complexes

The result of optical X-ray, DSC and magnetic susceptibility studies of new Co(II) metallomesogens are reported. The di-(5-substituted-salicylidene)ethylenediaminato cobalt(II) complexes exhibit broad range S_a phases. A comparison with corresponding copper(II) and nickel(II) complexes leads to conclusions about how the properties are influenced by the metal atom. Cobalt(II) complexes of this kind form different modifications which are either active or inactive against oxygen. The conversion process (inactive-active) is monitored by time dependent X-ray diffractometric measurements.     

4,5-Dihydroisoxazoles in the synthesis of new metallomesogens

A new approach to the synthesis of metallomesogens containing β-enaminoketone fragments as chelating moieties was developed. The corresponding enaminoketone was synthesized by the oxidation of 3-(4′-butoxybiphenyl-4-yl)-5-pentyl-4,5-dihydroisoxazole to 3-(4′-butoxybiphenyl-4-yl)-5-pentyl-4,5-isoxazole, followed by opening of the isoxazole ring. The reaction of the enaminoketone with copper(II) and nickel(II) acetates gave the target mesogenic metal complexes.