Effect of molecular structure on mesomorphism XX. Unsymmetrical dimeric mesogens

A series of unsymmetrical dimeric mesogens was prepared, members of which were composed of a 4'-pentyloxybiphenyl-4-carboxyl core and a 4'-substituted-benzylidene-4-oxyaniline core joined via an alklyene spacer. The structural variants were cyano, nitro and trifluoromethyl. Comparison of mesophase behaviour of the unsymmetrical dimers with that of corresponding monomeric 'halves' and also with that of corresponding symmetrical dimers indicated a tendency for the unsymmetrical dimers to form smectic phases although not as highly ordered as those of some of the monomers corresponding to a relevant core group.     

Unsymmetrical dimers possessing a cholesteryl ester moiety and a difluoro-substituted biphenyl core: synthesis and mesomorphic behaviour

Some new unsymmetrical dimers consisting of a cholesteryl ester moiety, covalently linked to either a 4′-(1,3-difluoro-4-n-octyloxy) biphenyloxy or a 4′-(2,3-difluoro-4-n-decyloxy) biphenyloxy through odd-even parity paraffinic central spacers, have been synthesized and investigated for their mesomorphic behaviour. Except for one, all the dimers exhibit enantiotropic smectic A, twist grain boundary (TGB) and chiral nematic mesophases. Five of the eight unsymmetrical dimers synthesized show a chiral smectic C (SmC*) phase. Interestingly in some of the compounds the SmC* exists over a wide temperature range including room temperature. Among the eight compounds, a dimer having a C₇ central paraffinic spacer and a C₈ alkoxy terminal chain shows an enantiotropic twist grain boundary with SmC* blocks (TGB_C*) phase. It appears that the variation in the length of the spacer has a remarkable influence on the phase transition temperatures as well as on the mesomorphic behaviour.     

Azobenzene-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of two new sets of non-symmetric liquid crystal dimers is reported, the 1-(4-substitutedazobenzene-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl)hexanes (CB6OABX) and 1-(4-substitutedazobenzene-4′-yloxy)-6-(4-cyanobiphenyl-4′-yloxy)pentanes (CBO5OABX). The terminal substituents are methyl, methoxy, butyl, butyloxy, nitrile and nitro. All the CB6OABX dimers exhibit twist-bend nematic (N_TB) and nematic (N) phases. The CBO5OABX dimers also all show an N phase but only the butyl and butyloxy homologues exhibit the N_TB phase. The transitional behaviour of the non-symmetric dimers is compared to that of the corresponding symmetric dimers, the 1,5-bis(4-substitutedazobenzene-4′-yloxy)pentanes (XABO5OABX) and either 1,7-bis(4-cyanobiphenyl-4′-yl)heptane or 1,5-bis(4-cyanobiphenyl-4′-yloxy)pentane. The XABO5OABX dimers all show a nematic phase and in addition, the butyl homologue exhibits a smectic A phase. The difference in transitional behaviour between the CB6OABX and CBO5OABX dimers is attributed to the difference in their molecular shapes arising from different bond angles between the para axis of the cyanobiphenyl unit and the first bond in the spacer. Specifically, the all-trans conformation of a CBO5OABX dimer is more linear than that of the corresponding CB6OABX dimer. Differences within each set of dimers are attributed to changes in the average molecular shape and the strength of the mixed mesogen interaction on varying the terminal group. Crystal structures are reported for CB6OABOMe, CBO5OABNO₂ and MeOABO5OABOMe.     

Organic Selenium Compounds,Part IV: Synthesis and Applications of Some New Diaryl Selenides Containing Azomethine and Oxazole Moieties

3,3′-Diacetyloxy (2), 3,3′-dihydroxy (3), 4,4′-diamino (4), and 4-amino (5) diphenylselenide derivatives were prepared as new precursors for the title studies. Compound 6 was obtained by condensation of 4 with an appropriate aromatic aldehyde. Unsymmetrical diphenylselenides 7 and 8 were obtained by condensation of 4 and/or 5 with an aromatic aldehyde. Compound 7 undergoes facile condensation with the same aldehyde present in its arylidene moiety to yield 6, while condensation with another different aromatic aldehyde yielded unsymmetrical 4-arylideneamino diphenylselenide derivative 9. Oxidation of 6, 8, and 9 using lead tetra acetate and/or N-bromosuccinimide yielded symmetrical bis-(2-aryl benzoxazol-6-yl) (10), unsymmetrical 3′-hydroxy, 2-aryl benzoxazol-6-yl selenides (11), and 2-aryl benzoxazol-6-yl, 2′-aryl′ benzoxazol-6′-yl selenide derivatives (12), respectively. Compound 10 was prepared in one-pot unequivocal synthesis by fusion of 4 with the appropriate aromatic aldehyde, while 12 was prepared by fusion of 4 with two different aromatic aldehydes. In certain cases, 6 and 9 were heated on a direct flame until complete homogeneity afforded the corresponding 10 and 12. The structures of the synthesized compounds are based on physical data, IR, ¹H NMR, ¹³C NMR, chemical means, and mass spectral data. Some of the synthesized compounds were biologically tested.

Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.     

Oligomeric liquid crystals: From monomers to trimers

A homologous series of linear liquid crystal trimers, the 4,4′-bis[ω-(4-methoxyazobenzene-4′-yloxy)alkoxy]azobenzenes, has been synthesized and characterized. The transitional properties of the trimers are compared with those of the corresponding series of dimers, the α,ω-bis(4-methoxyazobenzene-4′-oxy)alkanes, and monomers, the 4-methoxy-4′-alkoxyazobenzenes. Characteristically pronounced odd-even effects were seen for the transitional properties of both dimers and trimers on varying the spacer lengths. The clearing temperatures of the trimers were higher than those of the corresponding dimers, but as the length of the flexible spacers was increased this difference became rather small. The ratios of T _NI, and ΔS _NI/R for monomer:dimer and dimer:trimer are discussed. These are very similar to reported values for similar materials, suggesting that there may be a rather general relationship between the transitional properties of liquid crystal oligomers as the number of mesogenic units is increased.     

Structure–property relationships in twist-bend nematogens: the influence of terminal groups

ABSTRACT

The synthesis and characterisation of a range of non-symmetric liquid crystal dimers designed to exhibit the twist-bend nematic phase is reported. Beginning with 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl) hexane, each nitrile group is exchanged systematically for a methoxy group. The behaviour of these dimers is interpreted in terms of their bent shape being the predominant driving force for the formation of the twist-bend nematic phase, and the small differences between the twist-bend nematic–nematic transition temperatures are attributed to the differences between the interaction strength parameters of the mesogenic units. The 4-alkyloxyphenyl 4-[6-(4′-cyanobiphenyl-4-yl)hexyloxy]benzoates with ethyl, butyl, hexyl and octyl chains show the twist-bend nematic phase, whereas the corresponding 4-alkyloxyphenyl 4-[5-(4′-cyanobiphenyl-4-yloxy)pentyloxy]benzoates do not. This difference in behaviour is attributed to the more bent structure of the former. Increasing the terminal chain length initially decreases the twist-bend nematic–nematic transition temperature and this suggests that the chain disrupts the interactions between the mesogenic units. Subsequent increases in chain length have a smaller effect suggesting that the chain can be accommodated within an intercalated arrangement. The transitional behaviour of 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-butyloxybiphenyl-4′-yl) hexane is compared to that of 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-((S)-2-methyl)butyloxybiphenyl-4′-yl) hexane, and it is shown that chain branching strongly destabilises the twist-bend nematic phase. This is attributed to difficulties associated with packing the dimers.     

Novel oxazepinedione-derived symmetric dimers: synthesis and mesophase characterisation of seven-membered heterocyclic compounds

The synthesis and characterisation of three sets of symmetric dimeric compounds composed of seven-membered oxazepinedione heterocyclic rings were carried out. All the dimers possess the tetradecyl- (n = 14) alkyl side chain attached to the nitrogen atom of the oxazepinedione core. The oxazepinedione core in turn was connected with varied connecting spacers (n = 4, 6, 8, 10 and 12). The dimers were spectroscopically characterised by FT-IR, ¹H-NMR, ¹³C-NMR and elemental analysis techniques. The compounds were investigated for liquid crystalline properties using differential scanning calorimetry and polarising optical microscopy with heating assembly. The precursor imines 2a–e itself started exhibiting liquid crystalline SmA/tilted hexatic mesophase. Further fusion of 2a–e with maleic anhydride, succinic anhydride and phthalic anhydride gave the novel oxazepinedione-derived symmetric dimers 3a–e, 4a–e and 5a–e respectively. The dimers 3a–e and 4a–e did not exhibit any liquid crystal (LC) properties. However, the phthalic anhydride-fused oxazepinediones 5a–e show monotropic nematic liquid crystalline phase. The results indicate that the formation of mesophase is dependent on the type of fused oxazepinedione ring.     

Synthesis and Photochemical Properties of Covalently-Linked Dimers of Ruthenium(II) Tris (Bipyridine) Complex: Comparison with Ru(II)(bpy)2 (Poly-4-Methyl-4′-Vinylbipyridine)

Covalently linked dimers of Ru(bpy)₃ ²⁺ (3 and 4) connected by two or three carbon atoms were synthesized as models for Ru(bpy)₃ ²⁺-containing vinyl polymer (1). The luminescence properties of 3 and 4 were compared with those of its component monomer, 4,4′-dimethyl-2,2′-bipyridine-bis(2,2′-bipyridine)ruthenium(II) complex (2). In excited 3 and 4, intramolecular interaction leading to enhanced quenching was not observed. Electron-transfer quenching of the excited dimer with methylviologen (MV²⁺) and the zwitterionic viologen, 1,1′-bis(3-sulfopropyl)4,4′-bipyridinium (SPV), was studied and compared with that of 1 and 2. The low quenching efficiency in 1, 3, and 4 systems can be ascribed to the steric hindrance of unexcited ruthenium complex. Energy migration between ruthenium complexes in the excited 1, 3, and 4 can be ruled out from the kinetic evidence.     

Unsymmetrical N and/or O-bridged calixarene derivatives: synthesis,structure and encapsulation of solvent molecules in the solid state

Eight unsymmetrical N and/or O-bridged calixarene derivatives were obtained by 1 (naphthalene-2,7-diol), 2 (bis(4-hydroxyphenyl)methanone), 3 (4,4′-methylenedianiline), 4 (3,3′-methylenedianiline), 5 (4,4′-oxydianiline) and 6 (4,4′-(perfluoropropane-2,2-diyl)dianiline) reacting with fragment a (4,4′-bis(dichloro-s-triazinyloxy)propane-2,2-diyldibenzene) and b (N,N′-bis(dichloro-s-triazinyl)-4,4′-methylenedianiline) under very mild reaction conditions via efficient fragment coupling strategy. We also obtained the crystal structure of 1a (tetraoxocalix[2](propane-2,2-diyldibenzene,naphthalene)[2]triazine) which can form a molecular capsule by two dimers with C–H?N and C–H?O quadruple hydrogen bonds, and it has the encapsulation ability toward solvent molecules.     

Synthesis of Novel Bithiophene-Substituted Heterocycles Bearing Carbonitrile Groups

Symmetrical and unsymmetrical bithiophene-substituted heterocycles bearing carbonitriles including imidazo[1,2-a]pyridine, benzimidazole, and pyridine derivatives have been synthesized via different synthetic protocols. The bithiophene bis-imidazo[1,2-a]pyridine derivatives 3a,b were achieved in three steps starting from 2-acetyl-5-bromothiophene. Suzuki coupling reaction of 2a with 5-formylthiophen-2-ylboronic acid forms the formyl derivative 5, which by condensation with 3,4-diaminobenzonitrile in the presence of sodium bisulfite furnishes the unsymmetrical bithiophene derivative6. The bis-benzimidazole derivative 8 was obtained via hexabutylditin-mediated homocoupling of 5-bromothiophene-2-carboxaldehyde, while the benzimidazole derivatives 12a,b were prepared via the formyl derivatives 11a,b, a product of Velsmier formylation reaction of 10a,b. Two synthetic protocols for the aryl/hetaryl-2,2′-bithiophene derivative 14 have also been presented. In addition, the guanyl hydrazones of bithiophenes, 16 and 17, were prepared from bis(tri-n-butylstannyl)-2,2′-bithiophene through a Stille coupling reaction followed by a condensation step.     

Liquid crystal dimers and the twist-bend nematic phase. The preparation and characterisation of the α,ω-bis(4-cyanobiphenyl-4′-yl) alkanedioates

Eleven members of the homologous series of liquid crystal dimers, the α,ω-bis(4-cyanobiphenyl-4′-yl) alkanedioates, have been synthesised and their transitional properties characterised. These dimers consist of two cyanobiphenyl units connected by an alkyl spacer attached via ester linkages. All eleven members exhibit exclusively nematic behaviour. The nematic–isotropic transition temperatures, T_NI, and associated entropy changes, ?S_NI/R, exhibit pronounced alternations as the length and parity of the spacer is varied; this is characteristic behaviour of liquid crystal dimers. The transitional properties of the ester-linked dimers are compared with the corresponding materials having either ether, methylene or carbonate linkages between the spacer and mesogenic units. For short spacer lengths and both odd- and even-membered dimers, the ester-linked materials show the highest values of T_NI and the methylene-linked the lowest. For longer spacer lengths, T_NI of the carbonate-linked dimers fall between those of the corresponding ester- and ether-linked dimers. The ether-linked materials show the largest alternation in ?S_NI/R on varying spacer length and the carbonate-linked dimers the lowest. This behaviour is interpreted in terms of the molecular geometry and it is suggested that the ether- and ester-linked odd-membered dimers have rather similar shapes. A phase diagram constructed using binary mixtures of the pentyl member of this ester-linked series and the known twist-bend nematogen, 1,7-bis(4-cyanobiphenyl-4′-yl)heptane (CB7CB), is presented. The twist-bend nematic–nematic transition temperature of the mixtures shows a striking convex curvature as the concentration of CB7CB is decreased, and so it is not possible to estimate a virtual twist-bend nematic–nematic transition temperature for the ester-linked material.     

Structure of Dimers and the Nature of Color in 4′-Nitro-4-dimethylaminoazobenzene Solutions

A mechanism is proposed for the formation of stable dimers of 4′-nitro-4-dimethylaminoazobenzene dye (4′-nitro-DAB)₂ in solution, the existence of which in a vapor phase was established in 1965. It is shown that (4′-nitro-DAB)₂ dimers are stable also in neutral and moderately acidic solutions. Their UV–Vis spectra are similar to the spectra of the same solutions of 4-dimethylaminoazobenzene (DAB) containing DAB₂ dimers as the ground state. Like DAB₂, (4′-nitro-DAB)₂ dimers are reversibly converted to di- and triprotonated dimers under conditions of low and moderate acidity. In the process, the yellow color of the initial solution changes to red. It is concluded that the great stability of the dimers in the liquid and vapor phases is due to their inter-monomer bonds being formed with the participation of electrons promoted from the sp²- orbitals of N-atoms in azo-groups N=N to the Rydberg 3s-orbitals of the N=N-group (Rydberg bonds). It is established that at high acidities, the triprotonated dimers (the red solution) decompose reversibly into diprotonated monomers (the yellow solution). Structural formulas are proposed for the protonated species.     

Catalytic Oxidative Domino Degradation of Alkyl Phenols Towards 2- and 3-Substituted Muconolactones

The catalytic oxidative domino degradation of phenols was investigated. Hydrogen peroxide (30% aq.) was used as an oxidant and 2,2′-dinitro-4,4′-ditrifluoromethyldiphenyl diselenide 4e as a catalyst. The products were muconic acid 5, and muconolactones muconolactones—5-carboxymethylfuran-2(5H)-ones 7 and 9. Phenols with alkyl groups at 2 or 4 positions of the benzene ring were converted regioselectively to corresponding muconolactones substituted at alkenylene ring carbon atoms. The reaction mechanism is proposed.     

Studies on Organophosphorus Compounds VII1,2: Transformation of Steroidal Ketones with Lawesson's Reagent into Thioxo and Heterofused Steroids. Results of Antimicrobial and Antifungal Activity

The reactivity of Lawesson's reagent (LR) toward some steroidal hormones was studied, 4-androsten-3,17-dione 2 reacted with LR to produce the corresponding thioxosteroids 3 and 4. Epi-androsterone 5 showed a great activity to LR and produced 3β-mercaptospiro-(androstan-17,4′dithiaphosphetan)thione 8 and the sulfide derivative 9. Also progesterone 10 reacted with LR to yield the thiaphospholo[3′, 4′:16,17]androsten-3-one 13 and the sulfide product 16. The new modified steroidal derivatives thieno[2′,3′:2,3]cholestan 18 and thieno[2′,3′:2,3]-androstan 20 were synthesized and examined against LR, the corresponding thiazaphosphorinothieno steroidal derivatives 23 and 24 were isolated respectively. The in vitro biological activity of some newly synthesized compounds against bacteria, yeast, and fungi was studied.     

A NON-EMPIRICAL LCAO MO SCF INVESTIGATION OF SOME ASPECTS OF STRUCTURE AND BONDING IN THE THIATHIOPHTHEN RING SYSTEM

Abstract

Non-empirical LCAO MO SCF calculations have been performed on prototype symmetrical and unsymmetrical thiathiophthen ring systems to investigate structure and bonding as a function of change in geometry. Substituent effects have been investigated and it is shown that within the theoretical limitations the energy differences between symmetrical and unsymmetrical structures are quite small. Comparison has been made with UPS and ESCA data pertaining to valence and core levels respectively and some consideration given to the thiathiophthen radical anion and dianion ring systems.     

Synthesis of New β,β′-Diketodithioethers via Swern Oxidation and Study of Their Hydrazone Formation Rate

The synthesis of β,β′-diketodithioethers 4b–j from corresponding β,β′-dihydroxydithioethers 3b–j was carried out by a Swern oxidation using DMSO-oxalyl chloride as oxidizing agent. β,β′-Dihydroxydithioethers 3b–j were prepared by the reaction of two molar equivalents of epoxides 1b–j with dimercaptoethane 2 in the presence of a saturated aqueous solution of potassium carbonate. The reactivity behavior of imine formation of the β,β′-diketodithioethers 4b–j by 2,4-dinitrophenylhydrazine was also investigated, and a mechanism was proposed by using molecular orbital (MO) calculations. To confirm the proposed mechanism, the role of the thia function to activate hydrazone formation by measuring HOMO-LUMO energy levels was also demonstrated.     

Cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of several members of the 1,ω-bis(4-cyanobiphenyl-4′-yl) alkane (CBnCB) and the 1-(4-cyanobiphenyl-4′-yloxy)-ω-(4-cyanobiphenyl-4′-yl) alkane (CBnOCB) homologous series are reported. The new odd members described CB5CB, CB13CB, CB4OCB, CB8OCB and CB10OCB all exhibit twist-bend nematic and nematic phases. The members of these series already reported in literature, CB7CB, CB9CB, CB11CB and CB6OCB, were also prepared in order to allow for a direct comparison of their transitional properties. The properties of these dimers are also compared to those of the corresponding members of the 1,ω-bis(4-cyanobiphenyl-4,-yloxy) alkanes (CBOnOCB). For any given total spacer length, for odd members of these series, the nematic–isotropic transition temperatures and associated entropy changes are greatest for the CBOnOCB dimer and lowest for the CBnCB dimer. These trends are understood in terms of molecular shape. For short spacer lengths, the twist-bend nematic–nematic transition temperature (T_NTBN) is higher for the CBnOCB series than for the CBnCB series but this is reversed as the spacer length increases. Of the CBOnOCB dimers, a virtual value of T_NTBN was estimated for CBO3OCB and T_NTBN was measured for CBO5OCB. These values are considerably lower than those observed for the corresponding members of the CBnCB or CBnOCB series. The dependence of T_NTBN on molecular structure is discussed not only in terms of the molecular curvature but also in the ability of the molecules to pack efficiently. As the temperature range of the preceding nematic phase increases, so the twist-bend nematic–nematic transition entropy change decreases and the transition approaches second order for the longer spacers. For comparative purposes, the transitional behaviour of the even-membered dimers CB6CB, CB5OCB and CBO4OCB is reported and differences accounted for in terms of molecular shape.     

Synthesis of Zn-, Mn-, and Fe-Containing Mono- and Heterometallated Ethanediyl-Bridged Porphyrin Dimers

New ethanediyl-bridged unsymmetrical mono- and heterometallated dimers of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin (H₂oep) containing transition-metal ions (Mn and Fe) were synthesized by a facile stepwise metallation/demetallation process under mild conditions. The novel metallation strategy initially involved the predominant insertion of Zn into one of the two porphyrin rings of the free-base dimer, followed by the incorporation of Mn or Fe into the other porphyrin ring under exceptionally mild conditions, giving corresponding heterometallic dimers; the subsequent removal of Zn yielded mono-transition-metal dimers. The emission spectrum of the monozinc dimer predominantly exhibited fluorescence bands of the free-base porphyrin component, indicating a very efficient energy-transfer process. Conversely, emission of the free-base or Zn porphyrin component of transition metal containing dimers was strongly quenched due to photoinduced electron transfer.     

A Convenient Synthesis of Unsymmetrical N,N′-Disubstitutedα,ω-Diaminoalkanes

A general procedure is described for regiospecific construction of unsymmetrical N-alkyl (or aralkyl)-N′-aryl-α,ω-diaminoalkanes 3 (n=2,3,4) by reduction of N-(ω -arylaminoalkyl)amides 2 with borane. Compounds 2 are readily obtained by condensation of N-(ω-haloalkyl)amides 1 with aromatic amines.     

Hydrogen bond-induced ferroelectric liquid crystals

Abstract

Hydrogen-bonded dimers of 4-alkoxy-4′-stilbazole homologues (C _n OSB, proton acceptors) and the mono-(2-methylbutyl)ester of terephthalic acid (MBTA, proton donor) behaved like conventional thermotropic mesogens. For chiral MBTA mixtures, chiral nematic and blue phases were observed with n = 1 and 2, and chiral smectic C phases were observed with n ≥ 5. Achiral phases were observed for mixtures with racemic MBTA as proton donor. Transition temperatures and enthalpies are similar for the chiral and achiral systems.