Banana-shaped mesogens: effect of lateral substituents on seven-ring esters containing a biphenyl moiety

The synthesis and characterization of five homologous series of symmetrical compounds composed of banana-shaped molecules containing a biphenyl moiety are reported. All these compounds are non-Schiff's bases and are esters. The effects of lateral substituents such as fluoro, methyl and ethyl in the side arms of these molecules are examined. These substituents have a strong influence in reducing the clearing temperatures. Banana phases such as B 1 , B 2 and B 6 were observed in the above series of compounds. The mesophases were characterized by a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optic studies.     

Carbon-13 chemical shifts of monosubstituted cyclohexanes

The carbon-13 chemical shifts of monosubstituted cyclohexane derivatives are compared with those of aliphatic compounds. The polar substituents exert a similar influence on the α-, β- and γ-carbons in both series of compounds. The δ-effect is shown to be characteristic mainly to the cyclic compounds, however. The appearance of a δ-effect is discussed as a possible consequence of electron delocalization in alicyclic molecules. A correlation of these δ-effects with inductive parameters of the substituents is presented.     

Structural factors controlling the self-assembly of columnar liquid crystals

A series of disc-shaped molecules were prepared by the condensation of 1,2-diamines with 2,3,6,7-tetrakis(hexyloxy)phenanthrene-9,10-dione to investigate the relationship between changes in molecular structure and the self-assembly of columnar liquid crystalline phases. A comparison of compounds with different core sizes indicated that molecules with larger aromatic cores had a greater propensity to form columnar phases, as did compounds substituted with electron-withdrawing groups. In contrast, molecules with electron-donating substituents were nonmesogenic. The clearing temperature of columnar phases increased linearly with the electron-withdrawing ability of the substituents, as quantified by Hammett sigma-values. The observed trends can be rationalized in terms of the strength of pi-pi interactions between aromatic cores in the liquid crystalline phases and suggest that both electrostatic interactions and dispersion forces play important roles in the self-assembly of these materials.     

Rotation in biphenyls with a single ortho-substituent

Barriers to rotation in a range up to 15.4 kcal mol(-1) were determined by dynamic NMR spectroscopy for a series of biphenyl compounds 1a-1h and 2a-2d with a single ortho-substituent. Ab initio calculations reproduce these barriers satisfactorily and indicate the ground-state conformation of these molecules. Results are discussed in terms of the contribution of individual substituents to the barrier and of the buttressing of adjacent positions in a benzene ring by substituents.     

Electronic properties of tricoordinated phosphorus in hexagonal phosphininium compounds and molecular aromaticity

Calculations with B3LYP within quantum chemical density functional theory have been carried out for 1-H-phosphininium cation and a series of 1-R-phosphininium molecules (R=cyclopentadiene, alpha and beta pyrroles, alpha and beta phosphole, C5BH5-- and CH2--). The negative nuclear-independent chemical shift values and the positive aromatic stabilization energies confirm that they are aromatic compounds. In particular, the 1-H-phosphininium cation even exhibits stronger aromatic character than the well-known aromatic phosphinine. The aromatic substituents have strong capability to attract electrons. It is the conjugation and aromaticity that keeps the stability and conformations of the molecules investigated. Owing to the perturbation of the aromatic substituted groups, the predicted large T values and the enlarged HOMO-LUMO gap of the phosphininium cation indicate that these compounds are expected in experiment.     

Electronic Tuning of Mixed Quinoidal‐Aromatic Conjugated Polyelectrolytes: Direct Ionic Substitution on Polymer Main‐Chains

The synthesis of conjugated polymers with ionic substituents directly bound to their main chain repeat units is a strategy for generating strongly electron‐accepting conjugated polyelectrolytes, as demonstrated through the synthesis of a series of ionic azaquinodimethane (iAQM) compounds. The introduction of cationic substituents onto the quinoidal para‐azaquinodimethane (AQM) core gives rise to a strongly electron‐accepting building block, which can be employed in the synthesis of ionic small molecules and conjugated polyelectrolytes (CPEs). Electrochemical measurements alongside theoretical calculations indicate notably low‐lying LUMO values for the iAQMs. The optical band gaps measured for these compounds are highly tunable based on structure, ranging from 2.30 eV in small molecules down to 1.22 eV in polymers. The iAQM small molecules and CPEs showcase the band gap reduction effects of combining the donor‐acceptor strategy with the bond‐length alternation reduction strategy. As a demonstration of their utility, the iAQM CPEs so generated were used as active agents in photothermal therapy.     

Effects of symmetrically 2,5-disubstituted 1,3,4-oxadiazoles on the temperature range of liquid crystalline blue phases: a systematic study

Four series of symmetrically 2,5-disubstituted 1,3,4-oxadiazoles with different lateral substituents and terminal alkoxy chain length were synthesised and characterised. Then, all these bent-shaped compounds were separately doped into the blue phase (BP) liquid crystal host and their effects on the BP range of the host were investigated, which shows that the BP ranges varied greatly with the difference in the structure of 2,5-disubstituted 1,3,4-oxadiazoles and their doped concentration. Moreover, in order to explore the influence of the inherent molecular geometry and properties of the bent-shaped molecules on the stability of BPs, the molecular structures of all the compounds were studied by using the theory of density functional theory, and the optimised structural parameters of the molecules were calculated at the B3LYP/6–31G* level.     

Orthogonal hexatic smectic phase-rare or common?

The hexatic smectic B (Hex B) phase is commonly encountered among hydrogen-bonded liquid crystal materials. Among enaminoketone and Schiff's base compounds, twenty homologous series (c. 10² compounds) exhibiting orthogonal mono-or bi-layer hexatic phases were identified by combined microscopy and DSC. Phase transitions from crystal B (CrB) of S_f to Hex B as well as from Hex B to S_f, S_a or S_c phases were observed within the range 50 to 190°C. Temperature ranges of the Hex B phase detectable by DSC varied from 0.2 to 20°C. Within two groups of three-ring enaminoketones, the formation of the Hex B phase was found to be controlled by both inductive and mesomeric components of the Hammett constants of the terminal substituents of the molecules.     

Observation of filamentary growth patterns at the isotropic–SmCPA phase transition in new unsymmetrical compounds composed of bent-core molecules

The synthesis and mesomorphic properties of several compounds composed of bent-core molecules, unsymmetrically substituted about the central phenyl ring, are discussed. These compounds contain substituents such as fluoro, chloro and trifluoromethyl at the terminal position of one of the arms of the bent-core molecules. Many of the compounds containing a chloro terminal substituent show filamentary growth patterns at the transition from isotropic to polar antiferroelectric smectic C phase. The liquid crystalline phase exhibited by these compounds has been characterized using a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical studies.     

Observation of filamentary growth patterns at the isotropic-SmCPA phase transition in new unsymmetrical compounds composed of bent-core molecules

The synthesis and mesomorphic properties of several compounds composed of bent-core molecules, unsymmetrically substituted about the central phenyl ring, are discussed. These compounds contain substituents such as fluoro, chloro and trifluoromethyl at the terminal position of one of the arms of the bent-core molecules. Many of the compounds containing a chloro terminal substituent show filamentary growth patterns at the transition from isotropic to polar antiferroelectric smectic C phase. The liquid crystalline phase exhibited by these compounds has been characterized using a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical studies.     

Infrared and NMR study of some 2-styrylindolium dyes

A series of 18 molecules of the 2-styrylindolium dyes group is examined by means of infrared and ¹H NMR spectroscopy. The effect of electron-withdrawing and electron-releasing substituents on the spectral behavior of the compounds is assessed and the most appropriate structure for these molecules is suggested.     

Structure effects on the photobehaviour of 2,2-diphenyl(2H)chromenes

The photochromic behaviour of a series of 2,2-diphenyl(2H)chromenes, bearing different substituents (methyl, thienyl, B(OH)₂ and nitro groups), has been investigated. The photochromism of these molecules is described in terms of absorption spectra, photocolouration and photobleaching quantum yields, kinetics of thermal bleaching and competition between photochemical and photophysical relaxation. The quantum yield of UV photocolouration is generally fairly high (0.7–1.0), but decreases for compounds with thienyl and nitro substituents. The ring-open coloured form thermally rearranges to the closed uncoloured form, whereas visible photobleaching is substantially negligible (≤10⁻³). The thermal bleaching process is markedly affected by the substituents and, in the case of the nitro-derivatives, is characterised by a drastic decrease of activation entropy and enthalpy.     

Efficient Syntheses of Novel Fluoro‐Substituted Pentacenes and Azapentacenes: Molecular and Solid‐State Properties

Non‐symmetrical 6,13‐disubstituted pentacenes bearing trifluoromethyl and aryl substituents have been synthesized starting from pentacenequinone. Diazapentacenes with a variety of fluorine substituents were prepared either via a Hartwig–Buchwald aryl amination route or by a S_NAr strategy. As a result of a non‐symmetric substitution pattern containing electron‐donating substituents in combination with electron‐accepting fluorine substituents, the synthesized compounds feature distinct molecular dipoles. All compounds are analyzed regarding their optoelectronic properties in solution with special focus on the frontier orbital energies as well as their molecular packing in the crystal structures. The analyses of isolated molecules are complemented by thin‐film studies to examine their solid‐state properties. A precise comparison between these and the molecular properties gave detailed insights into the exciton binding energies of these compounds, which are explained by means of a simple model considering the molecular packing and polarizabilities.     

Thermal analysis of binary liquid crystalline mixtures

Two series of binary mixtures composed of bent shaped and rod like molecules are reported. The first star shaped bent core molecules were synthesized and used as a component of binary mixtures. The chiral rod like compounds having commensurable length with the arms of the bent core compounds have been chosen for these mixtures. The resulted compositions show various thermotropic liquid crystalline phases that are characteristic to both types of liquid crystalline materials. In case of mixing the rod like molecules to the bent core compound the B2, B7 and induced B1 phases have been observed. While using the star-shaped bent core and chiral rod like compounds in mixture, the paraelectric smectic A, ferroelectric smectic C* and orthogonal hexatic smectic B phases were preferred. The appearing mesophases were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction methods.     

Design of TTF-based phosphazenes combining a good electron-donor capacity and possible inclusion adduct formation

The physical properties of porous material can be modulated by intercalation of small molecules, whose size, in the case of tris-o-phenylenedioxy)cyclotriphosphazene (TPP)-based materials, vary with the different side groups. Starting from the TPP structure, a series of new derivatives were constructed through the core ring [(NP)(3)] substitution by [(CNH)(3)], [(CO)(3)], and [(CS)(3)] or/and the side group substitution by tetrathiafulvalene and a series of related fragments including bis(ethylenedithio)tetrathiafulvalene, 2-methylene-1, 3-dithiole, and 2-methylene-5,6-dihydro[1,3]dithiolo[4,5-b][1,4]dithiine. In the side fragment, such a substitution corresponds to the replacement of a ring heteroatom, an addition of substituents, or both. With use of theoretical methodologies based on DFT-B3LYP/6-31G* and HF/6-31G*//B3LYP/6-31G* approaches, molecular geometries and electronic properties including the LUMO and HOMO energies, the HOMO-LUMO gap, as well as the ionization potential (IP) were calculated. In comparison with the commonly used organic superconductors, most of the molecules investigated were predicted to show comparable or better electron-donor strength. Interestingly, a number of cyclophosphazene [(NP)(3)]-containing compounds were predicted to show the "paddle wheel" shape responsible for inclusion adducts formation, making these compounds to be potential candidates for organic superconductors with the ease of modulating their conducting properties by intercalation of suitable acceptors.     

Disk-shaped Symmetric Hexa-substituted Triphenylene Derivatives: Synthesis,Physical Properties and Self-assembly

A series of triphenylene derivatives with six symmetric substituents was synthesized from hexabromotriphenylene. The synthesis was conducted by six-fold palladium-catalyzed Hagihara-Sonogashira cross- coupling reactions to yield the hexa-alkynyl substituted triphenylene derivatives of HTP1, HTP2, HTP3 and HTP4. The six symmetric substituents can not only endow the triphenylene the longer π-conjugated range, but also increase the solubility of the compounds. Their photophysical, electrochemical, thermal properties were investigated respectively. With the comparison of their properties, the structure-property relationships were established which demonstrated the influences of different substituents on the electronic nature and the mesomorphic phase of these disk-shaped molecules. In addition, with the scanning electron microscopy(SEM) and polarized optical microscopy(POM) characterization, the self-assembly behaviors of the compounds were also investigated.     

Influence of steric and electronic effects of substituents on the molecular structures and conformational flexibility of 1,8-naphthalenedicarboximides

A series of 1,8-naphthalenedicarboximide derivatives containing substituents of different steric and electronic nature were studied by X-ray diffraction analysis.Ab initio quantumchemical calculations in the HF/3–21G approximation demonstrated the high conformational flexibility of the imide tetrahydro ring in the molecules of these compounds. The electronic nature of the substituents has no effect on the geometry and conformational flexibility of naphthalenedicarboximides due to weak conjugation between the imide and naphthalene fragments in the molecules. However, the steric effects of the bulky substituents noticeably affect the equilibrium geometry of the imide ring by increasing its conformational flexibility. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 68–73, January, 2000.     

Influence of steric and electronic effects of substituents on the molecular structures and conformational flexibility of 1,8-naphthalenedicarboximides

A series of 1,8-naphthalenedicarboximide derivatives containing substituents of different steric and electronic nature were studied by X-ray diffraction analysis.Ab initio quantumchemical calculations in the HF/3–21G approximation demonstrated the high conformational flexibility of the imide tetrahydro ring in the molecules of these compounds. The electronic nature of the substituents has no effect on the geometry and conformational flexibility of naphthalenedicarboximides due to weak conjugation between the imide and naphthalene fragments in the molecules. However, the steric effects of the bulky substituents noticeably affect the equilibrium geometry of the imide ring by increasing its conformational flexibility. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 68–73, January, 2000.     

Characterisation of sulphoxides by atmospheric pressure ionisation mass spectrometry

An observation that a series of proprietary compounds containing a methyl thiophenyl group all underwent metabolic S-oxidation, and that the product ion spectra of the resulting S-oxides showed methyl radical loss under low-energy atmospheric pressure ionisation tandem mass spectrometry (API-MS/MS) conditions, has led to an investigation of the fragmentation of commercially available sulphoxides. The phenyl methyl sulphoxides studied do lose methyl radicals under MS/MS conditions on triple quadrupole mass spectrometers. In addition, the phenyl sulphoxides, with simple substituents other than a methyl group, also showed a tendency to lose the substituent as a radical. It was concluded that radical loss from these simple sulphoxides was characteristic of S-oxidation of these molecules. Radical losses, such as those reported here, are used in-house to distinguish S-oxidation from N- and C-oxidation in metabolism studies.     

New liquid crystalline di- and tetra-acrylates for network formation

New liquid crystalline diacrylates and tetra-acrylates containing four to six aromatic rings were synthesized and characterized, and their mesophase behaviour was investigated. They are designed to be used in combination with chiral molecules to form cholesteric mesophases which can be crosslinked by photopolymerization. The acrylates presented exhibit broad mesophase ranges since mesogenic moieties longer than three are employed. Most diacrylates show no isotropization, due to premature thermal polymerization above 180°C. Additionally, liquid crystalline dipropionates were synthesized as reference compounds which cannot be crosslinked, and selected examples of these exhibit isotropization temperatures as high as 238°C prior to thermal degradation. Substituents at the mesogenic moiety have a great influence on the mesophase characteristics. Bulky substituents such as the tert-butyl group, induce a nematic mesophase, whereas compounds with small substituents (e.g. OCH3) or unsubstituted molecules also exhibit smectic phases. Tetra-acrylates with unsubstituted and substituted mesogenic units feature nematic mesophases only as a result of the additional spacers attached. Here isotropization was observed without polymerization at temperatures around 120-160°C.