Alignment of liquid crystal and dichroic dye molecules in mixed Langmuir and Langmuir-Blodgett films

A study of dichroic dye-liquid crystal mixtures (guest-host systems) in monolayers formed at a gas-liquid interface (Langmuir films) and at a solid surface (Langmuir-Blodgett films) has been made. As a host 4- n -octyl-4'-cyanobiphenyl (8CB) or 4- n -pentyl-4"-cyano- p -terphenyl (5CT) were chosen, while three dichroic azo dyes with various molecular structures were used as guest species. The dyes were added to the liquid crystal matrices at a concentration corresponding to the whole range of molar fractions and the surface pressure-mean molecular area isotherms for Langmuir films were recorded. On the basis of the isotherms, conclusions about the molecular organization and the miscibility of the components in the ultrathin films were drawn. The Langmuir films were transferred onto the quartz plates at surface pressures below the collapse point. The polarized absorption spectra of the Langmuir-Blodgett films were recorded and information about the alignment and intermolecular interactions in the mixtures of the non-amphiphilic dichroic dyes and the liquid crystals with strongly polar terminal groups were obtained.     

Alignment of liquid crystal and dichroic dye molecules in mixed Langmuir and Langmuir-Blodgett films

A study of dichroic dye-liquid crystal mixtures (guest-host systems) in monolayers formed at a gas-liquid interface (Langmuir films) and at a solid surface (Langmuir-Blodgett films) has been made. As a host 4- n -octyl-4′-cyanobiphenyl (8CB) or 4- n -pentyl-4″-cyano- p -terphenyl (5CT) were chosen, while three dichroic azo dyes with various molecular structures were used as guest species. The dyes were added to the liquid crystal matrices at a concentration corresponding to the whole range of molar fractions and the surface pressure-mean molecular area isotherms for Langmuir films were recorded. On the basis of the isotherms, conclusions about the molecular organization and the miscibility of the components in the ultrathin films were drawn. The Langmuir films were transferred onto the quartz plates at surface pressures below the collapse point. The polarized absorption spectra of the Langmuir-Blodgett films were recorded and information about the alignment and intermolecular interactions in the mixtures of the non-amphiphilic dichroic dyes and the liquid crystals with strongly polar terminal groups were obtained.     

Influence of substituent length in dichroic dye molecules on the orientational properties of guest-host liquid crystal mixtures

The polarized absorption spectra of eight dichroic dyes, alkyl derivatives of 4-amino-(N-ethylnaphthalimide) dissolved in the nematic liquid crystals 5CB and 6CHBT have been measured as a function of temperature. On the basis of these spectra, the guest order parameter has been evaluated. The influence of the alkyl chain length of the dye molecule on the molecular orientation has been examined. Moreover, the nematic-isotropic transition temperatures for the dye-liquid crystal mixtures have been determined. The experimental results have been compared in some detail with calculations made on the basis of the mean field theory for binary mixtures.     

Synthesis and spectroscopic investigation of some arylazo derivatives of thiazolidin-4-one

The reaction of 2-phenylimino- and 2-(p-tolylimino)thiazolidin-4-ones with benzene-, p-nitrobenzene, p-sulfamoylbenzene-, and p-toluenediazonium chlorides in glacial acetic acid in the presence of anhydrous sodium acetate (pH of the medium 4.5–5.0) has been studied. A spectroscopic investigation in the IR and UV regions has shown that the 5-(p-nitrophenylazo) derivatives of thiazolidin-4-one exist in the azo tautomeric form; the 5-phenylazo and p-tolylazo derivatives of thiazolidin-4-one are mixtures of the azo and hydrazone tautomers.     

Differential dichroic spectroscopy in liquid crystals using unpolarized radiation

An unpolarized, differential technique is described for obtaining the dichroic spectra of uniaxial liquid crystals from a single spectral scan. Using this method, we investigate the near infrared dichoic spectrum of n-heptyl cyanobiphenyl (7CB). The temperature dependence of the orientational order parameter has been derived close to the nematic-isotropic transition and this is in good agreement with earlier Raman data. The differential method is also advantageous for dichroic studies on oriented guest molecules in liquid crystals. Illustrative results are presented for p-nitrotoluene dissolved in a nematic medium.     

Guest–host systems containing anthraquinone dyes with multiple visible transitions giving positive and negative dichroic order parameters: an assessment of principal molecular axes and computational methods

ABSTRACT

Three 1,4-disubstituted anthraquinone dyes with bis(4-n-butylphenyl) substituents connected via amine or amide linking groups have been studied as guest molecules dissolved in the nematic host E7. UV-visible absorption spectroscopy has shown each of the dyes to exhibit multiple absorption bands in the visible region, and dichroic order parameters obtained from polarised spectra of aligned guest–host samples were shown to differ significantly between the bands for each dye, and between the dyes. Time-dependent density functional theory calculations indicated that each dye exhibits several transitions, giving transition dipole moment vectors with a range of orientations, and fully atomistic molecular dynamics simulations of the guest–host mixtures showed differences in the calculated molecular alignments of the dyes. Combining the results from these two sets of calculations enabled a comparison of molecular alignment models based on the moments of inertia and the surface tensors of the dyes. The match between calculated and experimental values was improved significantly when using the surface tensor rather than the moment of inertia model, indicating that the shapes of the molecular surfaces of these dyes are crucial to their alignment. A novel method of calculating polarised UV-visible absorption spectra of dyes in liquid crystal hosts is also presented.     

Conformation of Poly-(L-lysine) Containing Some Azoaromatic Side Chains

The pH dependence of the conformation of poly-(L-lysine) with 4.6 mol% of its side chain amino groups attached to an azo dye, 4′ -dimethyl aminoazobenzene-4-carboxylic acid, has been studied. Circular dichroic spectra showed that in acidic as well as in neutral media the polymer exists in the random coil conformation, like that of poly-(L-lysine). In basic medium the polypeptide acquires a β-structure, unlike poly-(L-lysine) which exists in an α-helical conformation.     

Analysis of host-assisted guest protonation exemplified for p-sulfonatocalix[4]arene--towards enzyme-mimetic pKa shifts

The pD dependence of the complexation of p-sulfonatocalix[4]arene (CX4) with the azoalkanes 2,3-diazabicyclo[2.2.1]hept-2-ene (1), 2,3-diazabicyclo[2.2.2]oct-2-ene (2), 2,3-diazabicyclo[2.2.3]non-2-ene (3), and 1-methyl-4-isopropyl-2,3-diazabicyclo[2.2.2]oct-2-ene (4) in D(2)O has been studied. The pD-dependent binding constants, determined by (1)H NMR spectroscopy, were analyzed according to a seven-state model, which included the CX4 tetra- and penta-anions, the protonated and unprotonated forms of the azoalkanes, the corresponding complexes, as well as the complex formed between CX4 and the deuteriated hydronium ion. The variation of the UV absorption spectra, namely the hypsochromic shift in the near-UV band of the azo chromophore upon protonation, was analyzed according to a four-state model. Measurements by independent methods demonstrated that complexation by CX4 shifts the pK(a) values of the guest molecules by around 2 units, thereby establishing a case of host-assisted guest protonation. The pK(a) shift can be translated into improved binding (factor of 100) of the protonated guest relative to its unprotonated form as a result of the cation-receptor properties of CX4. The results are discussed in the context of supramolecular catalytic activity and the pK(a) shifts induced by different types of macrocyclic hosts are compared.     

Comparative Study of Some Methods for the Determination of the Analgesic Anti-Inflammatory Compound “IS4”: 4-(Anisylideneamino) Antipyrine

A comparative study of some quantitative methods has been undertaken. Chemical and colorimetric procedures have been developed for the determination of IS₄: 4-(anisylideneamino) antipyrine. The non-aqueous titration and the nitritometric techniques were used. The formation of colored azo dye derivatives through diazotization and coupling with phenolic and amine reagents has been the basis of the colorimetric determination. Statistical analysis of the results has shown that a good agreement of the results has been found on applying the proposed methods to 4-aminoantipyrine.     

Synthesis and characterisation of liquid crystalline anthraquinone dyes with excellent dichroism and solubility

A novel series of dichroic anthraquinone dyes have been successfully synthesised with conventional Sonogashira reaction. These synthesised anthraquinone dyes were found to have high dichroic ratios, good solubilities, excellent hues and to be potential candidates for guest–host liquid crystal displays. The dichroic ratio is as high as 13.26, which is in the top level of the anthraquinone dyes reported so far. In addition, this paper provides an effective method to modulate the optic properties of anthraquinone dyes from the molecular level via the introduction of different radicals on the anthraquinone cores.     

Birefringence and dichroism of oriented epoxy thermoset films

A series of oriented liquid–crystalline epoxy thin films were prepared by the in situ polymerization of a liquid–crystalline diepoxide, 1,4‐phenylene bis[4‐(2,3‐epoxypropoxy)benzoate], with an aromatic diamine, 4,4′‐diaminobiphenyl, in a 7.0‐T magnetic field. The birefringent measurements of the oriented films were made from 543.5 to 830 nm. In this range, the values of birefringence (Δn) range from 0.155 to 0.130. When they are extrapolated to the microwave region, Δn = 0.105. The dichroism of a guest azo dye, 4‐(4‐nitrophenylazo)‐3‐hexyloxyaniline, in the oriented thin films was examined in the visible region. From the results, the order parameter of the polymer was calculated to 0.65 by extrapolating the concentration of the guest azo dye to zero. The guest azo dye compound does not affect the birefringence. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 915–919, 2001     

Effect of temperature on the photoalignment of azo dyes in thin films

The temperature dependences of the induced dichroic ratios (DRs) of azo dyes after their photoalignment in thin films 80 to 200 nm thick are studied. It is found that the DR values of layers containing dyes of the benzeneazodiphenyl series fall from 6.0 to 1.6 as the temperature rises from 60 to 130°C, respectively. A reduction in induced DR as the temperature rises (from 20 to 100°C) is also observed for the thin films of the dyes of benzeneazo-5,5’-dioxodibenzothiophene group. The absence of induced DR after irradiation with polarized light at 100°C indicates there is no alignment of molecules at this temperature.     

Miscibility studies of the smectic phases of trans,trans-4'-alkylbicyclohexyl-4-carbonitrile (CCH) liquid crystals

Complete temperature-composition phase diagrams for binary mixtures of 4'-ethyl-, 4'-propyl-, and 4'-butylbicyclohexyl-4-carbonitrile (CCH-2, CCH-3, and CCH-4, respectively) in each of the three possible combinations have been constructed from differential scanning calorimetry and thermal microscopy data. The highest temperature smectic phases of each of the pure mesogens are immiscible with one another, even though CCH-3 and CCH-4 have both been previously assigned the bilayer crystal-B structure on the basis of X-ray diffraction studies. The present studies indicate that the enantiotropic smectic phase of CCH-4 is slightly higher-ordered than is the monotropic smectic phase of CCH-3. The smectic phase of CCH-2, which previously has been found to be characterized by rhombohedral packing on the basis of X-ray diffraction data, is miscible with the second, previously uncharacterized (monotropic) smectic phase (S₂) of CCH-3. Photographs illustrating the subtle differences in the microscopic textures of these smectic phases are also presented.     

A Host‐Guest‐Recognition‐Based Electrochemical Sensor for Sequence‐Specific DNA Detection

We herein constructed a sensor that converts target DNA hybridization‐induced conformational transformation of the probe DNA to electrochemical response based on host‐guest recognition and nanoparticle label. In the sensor, the hairpin DNA terminal‐labeled with 4‐((4‐(dimethylamino)phenyl)azo)benzoic acid (dabcyl) and thiol group was immobilized on Au electrode surface as the probe DNA by Au‐S bond, and the CdS nanoparticles surface‐modified with β‐cyclodextrins (CdS‐CDs) were employed as electrochemical signal provider and host‐guest recognition element. Initially, the probe DNA immobilized on electrode kept the stem‐loop configuration, which shielded dabcyl from docking with the CdS‐CDs in solution due to the steric effect. After target hybridization, the probe DNA underwent a significant conformational change, which forced dabcyl away from the electrode. As a result, formerly‐shielded dabcyl became accessible to host‐guest recognition between β‐cyclodextrin (β‐CD) and dabcyl, thus the target hybridization event could be sensitively transduced to electrochemical signal provided by CdS‐CDs. This host‐guest recognition‐based electrochemical sensor has been able to detect as low as picomolar DNA target with excellent differentiation ability for even single mismatch.     

Co-conformational variability of cyclodextrin complexes studied by induced circular dichroism of azoalkanes.

The solution structures of the beta-cyclodextrin complexes between 2,3-diazabicyclo[2.2.2]oct-2-ene (1) and its 1-isopropyl-4-methyl derivative 2 have been investigated by means of induced circular dichroism (ICD) and MM3-92 force-field calculations, which considered the effect of solvation within a continuum approximation. Of primary interest was the so-called co-conformation of the host-guest complex, i.e., the relative orientation of the guest within the host. A pool of low-energy complex structures, which were located by means of a Monte Carlo simulated annealing routine, was generated to evaluate the dynamic co-conformational variability of the complexes. The ICD effects were calculated for the computed low-energy structures by applying a semiempirical method. The experimental and theoretical ICD as well as the calculated low-energy complex geometries suggest solution co-conformations in which the parent compound 1 adapts a lateral arrangement with the ethano bridge of the guest penetrating deepest into the cavity and the azo group aligning parallel to the plane of the upper rim. In contrast, the alkyl derivative 2 prefers a frontal co-conformation with the isopropyl group penetrating deepest into the cavity and the azo group aligning perpendicular to the plane of the upper rim. The validity of the predictions of the Harata rule regarding the sign and the intensity of the ICD signals for the n(-)pi, n(+)pi, and pipi transition of the azo chromophore in dependence on the complex co-conformation are discussed. With respect to the co-conformational variability of the complexes of the two azoalkanes, it was observed that the nearly spherical guest 1 forms a geometrically better defined complex than the sterically biased, alkyl-substituted derivative 2. This dichotomy is attributed to the largely different modes of binding for azoalkanes 1 and 2. It is concluded that the goodness-of-fit in a host-guest complex cannot be directly related to the "tightness-of-fit".     

The big squeeze: guest exchange in an M4L6 supramolecular host

Guest exchange in an M4L6 supramolecular host has been evaluated to determine whether host rupture is required for guest ingress and egress. Two mechanistic models were evaluated: one requiring partial dissociation of the host structure to create a portal for guest passage and one necessitating deformation of the host structure to create a dilated aperture for guest passage without host rupture. Three related lines of inquiry support the nondissociative guest exchange mechanism. (a) Equally facile guest exchange is observed in labile ([Ga4L6]12-) and inert ([Ti4L6]8- and [Ge4L6]8-) hosts. (b) Molecular mechanics calculations demonstrate that the structural deformations required for enlargement of an M4L6 aperture in a nonrupture or nondissociative guest exchange mechanism are plausible. (c) As predicted by the calculations, CoCp*2+, a sterically demanding guest, significantly inhibits guest exchange. These results bring new insight to the application of the M4L6 supramolecular host for encapsulated reaction chemistry for which there are now several examples.     

Spherical shape complementarity as an overriding motif in the molecular recognition of noncharged organic guests by p-sulfonatocalix[4]arene: complexation of bicyclic azoalkanes

[reaction: see text] The complexation of p-sulfonatocalix[4]arene (CX4) with 2,3-diazabicyclo[2.2.1]hept-2-ene (1), 2,3-diazabicyclo[2.2.2]oct-2-ene (2), 2,3-diazabicyclo[3.2.2]non-2-ene (3), 1-methyl-4-isopropyl-2,3-diazabicyclo[2.2.2]oct-2-ene (4), and 1-phenyl-2,3-diazabicyclo[2.2.2]oct-2-ene (5) was studied in D2O at pD 7.4 by 1H NMR spectroscopy. The formation of deep inclusion complexes was indicated by large upfield 1H NMR shifts of the guest protons (up to 2.6 ppm), which were also used to assign, in combination with 2D ROESY spectra, a preferential inclusion of the isopropyl group of 4 and a dominant inclusion of the azo bicyclic residue for 5. The bicyclic azoalkanes 1-3 showed exceptionally high binding constants on the order of 1000 M(-1), 1-2 orders of magnitude larger than for previously investigated noncharged organic guest molecules. The strong binding was attributed to the spherical shape complementarity between the guest and the conical cavity offered by CX4. Interestingly, although the derivatives 4 and 5 are more hydrophobic, they showed a 2-3 times weaker binding, which was again attributed to the deviation from spherical shape in these bridgehead-substituted derivatives. The preferential inclusion of the hydrophilic but spherical bicyclic residue of 5 rather than the hydrophobic aromatic phenyl group provides a unique observation in aqueous host-guest chemistry and corroborates the pronounced spherical shape affinity of CX4.     

Vtbrational spectra of the triclinic K3[ReO2(CN)4] single crystal

Infrared spectra of K₃[ReO₂(CN)₄] have been recorded using polarized radiation and oriented single crystals. The dichroic behaviour of the bands is analyzed and discussed on the basis of molecular geometry and the oriented molecular model. Experimentally determined dichroic ratios are used to obtain a full set of vibrational frequencies. By combining these data with those obtained from Raman spectra, the fundamental internal and lattice vibrations are assigned.     

Synthesis and study of N,N-disubstituted 4-aminophenylazobenzaldehydes

A series of ^N,^N-disubstituted 4-aminophenylazobenzaldehydes (azo dyes) were synthesized by the reaction of phenylpiperazine derivatives with 4-formylbenzenediazonium tetrafluoroborate. The salt was easily prepared by reacting poly(aminobenzaldehyde) with sodium nitrite in the presence of tetrafluoroboric acid. A representative sample was studied by crystallography, and the charge distribution of the molecule was calculated with the aim of understanding the form of molecular stacking on the basis of crystal data. The mesogenic behaviour of the azo dyes was also studied.