Ewa Chrzumnicka

Molecular Orientation in Liquid Crystalline Side Chain Polymers Doped with Dichroic Dye as Studied by Optical Spectroscopy Methods

Abstract The polarized absorption and emission spectra of fluorescent stilbene dye dissolved in two side chain liquid crystalline polysiloxanes have been recorded as a function of temperature in the mesomorphic and glassy phases. From these spectra the order parameters and as well as the orientational distribution function of the side mesogenic groups have been estimated. The results have been compared with the data obtained for low molecular weight liquid crystal by using the same optical methods.

Electrochemical cell with bacteriochlorophyll c and chlorophylls a and b in nematic liquid crystal

Abstract The photocurrent generation and current-voltage (I(V)) characteristics of the electrochemical cells consisting of a layer of chlorophyll a, chlorophyll b or bacteriochlorophyll c solution in a nematic liquid crystal sandwiched between two semiconducting (In2O3) electrodes covered by an orienting layer SiOx were investigated. Bacteriochlorophyll c was introduced into the nematic liquid crystal in monomeric and in aggregated predominantly tetrameric forms. The photoelectrical properties of bacteriochlorophyll c in monomeric and aggregated forms are different. All investigated samples exhibit cathodic photocurrent related to the extraction of electrons from the conduction band of the illuminated semiconducting electrode. Amplitudes of generated photocurrents and the shapes of the current-voltage voltammogramic loops, at the same conditions of illumination, are for various chlorophylls different.

Photoelectric properties of chlorophyll and carotene solutions in nematic liquid crystal located between semiconducting electrodes.

The photopotential and photocurrent generation for chlorophyll a, beta-carotene and a mixture of these pigments dissolved in nematic liquid crystal and located between transparent semiconducting electrodes were measured. Both pigments exhibit photopotential and photocurrent generation. From the photocurrent amplitudes it follows that the efficiency of electron transfer to a semiconducting electrode from beta-carotene is higher than from chlorophyll alpha. The photocurrent amplitude of the pigment mixture is slightly lower than that calculated as a sum of amplitudes of pigments located in separated cells. This difference can be explained by secondary effects, such as competition between carotene and chlorophyll molecules in a process of adsorption on a semiconducting electrode. Therefore it seems that no charge transfer complexes of chlorophyll and carotene are formed in the investigated model system.

Orientational order in nematic, smectic A, and smectic C phases of doped with dichroic dye§

The long-range orientational order of mesogenic 4-n-pentylphenylthio-4′-n-octyloxybenzoate ( ) doped with a dichroic dye was studied by means of electronic absorption and fluorescence measurements. The order parameters ⟨P2 ⟩ and ⟨P4 ⟩ as well as the orientational distribution function were determined on the basis of absorption and emission spectra of linearly polarized light recorded as a function of temperature in the nematic, smectic A and smectic C phases. An influence of the dye molecular structure on the orientational order of the dye/liquid crystal mixture was observed. A correlation between the order parameter values and the phase transitions temperature shift due to the dye addition was found. § This article is dedicated to Prof. Jerzy A. Janik on his 80th birthday.

Orientational properties of perylene tetracarboxylic diimide molecules in liquid–crystalline matrices

Molecular orientational order of perylene tetracarboxylic diimide, N,N’-di(n-hexyl)-1,6,7,12-tetrachloroperylene-3,4,9,10-carboxylic acid imide (Cl4-PTCDI-6C), dissolved in homologous series of liquid crystals, i.e. 4-(trans-4′-n-alkylcyclohexyl)-isothiocyanato-benzene (nCHBT) and 4-n-alkyl-4′-cyanobiphenyl (nCB), is investigated by using UV–V is absorption and fluorescence spectroscopies. The orientational order parameter , determined experimentally for Cl4-PTCDI-6C molecules in nCHBT and nCB, is much lower than that for some other derivatives of perylene dissolved in the same liquid crystal hosts. The order parameter , obtained for Cl4-PTCDI-6C in nCHBT and nCB, is also smaller than obtained previously from Raman scattering in the liquid crystal hosts. The orientational order parameter for the Cl4-PTCDI-6C probe takes extremely low negative values, which indicates the importance of higher rank order parameters in the description of the dye distribution in the liquid–crystalline matrices. The bulky chlorine atoms, substituted in the perylene tetracarboxylic diimide molecular core, seem to be primarily responsible for the small orientational order parameters of the dye molecules. Finally, the odd–even effect, manifesting itself in an alternating increase and decrease of and for Cl4-PTCDI-6C along with increasing number of carbon atoms in the alkyl chains in nCHBT homologous series, is discussed.

Orientational order of tetra n-hexylesters of perylene and tetrachloroperylene tetracarboxylic acids in low-molar-mass liquid crystal investigated using absorption and fluorescence methods

The orientational order parameters and of perylene-3,4,9,10-tetra-(n-hexylester) (PTHE) and 1,6,7,12-tetrachloroperylene-3,4,9,10-tetra-(n-hexylester) (4Cl-PTHE) dye molecules dissolved in low-molar-mass liquid crystal, 4-(trans-4-n-hexylylcyclohexyl)-isothiocyanobenzene, are calculated from the experimental values of polarised absorption and fluorescence anisotropies. The experimental data show that the substitution of chlorine atoms instead of hydrogen in the perylene core reduces the value of parameter. Moreover, the and values for molecules of the perylene derivatives, playing the role of the guest in the liquid crystal matrix, are much less than these values for molecules of the liquid crystal host. The extremely low negative values of for the perylene dyes are explained as the result of the preferable oblique orientation of the main molecular symmetry axis of the probe with respect to the direction of the macroscopic orientation of the liquid crystal host molecules.

Spectral properties of naphthalimide dyes mixed with 4-heptyl-4'-cyanobiphenyl (7CB) in Langmuir-Blodgett films

Langmuir-Blodgett (LB) films formed of some fluorescent dyes, namely derivatives of 4-aminonaphthalimide, and their binary mixtures with the liquid crystal 4-heptyl-4′-cyanobiphenyl (7CB) have been studied. The electronic absorption and fluorescence spectra were recorded. On the basis of these spectra, the spectral properties of the dyes and the intermolecular interactions in ultrathin ordered films were determined. The conclusions about the tendency to the creation of aggregates by dye molecules in LB films have been drawn. The measurements of absorption by using linearly polarised light have allowed us to determine the arrangement of the dye and liquid crystal molecules on the air-solid substrate interface.

Molecular Orientation and Odd-Even Effect in Nematogenic Homologous Series of 4-Cyanophenyl-4′-N-Alkylbenzoate

The orientational behavior of liquid crystals from homologous series of 4-cyanophenyl-4′-n-alkylbenzoate (CnH2n+1PhCOOPhCN) doped with dichroic dyes was investigated by means of optical spectroscopy methods: electronic absorption and fluorescence. The absorption and emission spectra of linearly polarized light were recorded as a function of temperature in whole range of the mesophase. The second- and fourth-rank order parameters (, ) and molecular distribution function were determined.The odd-even effect for the order parameter, explained as the alternation of interaction between alkyl chains, was observed. An influence of the dye molecular structure on the orientational order of the liquid crystal/dye mixture was reported.

Studies of Orientational Order of Some Nematogens by means of Raman Scattering Spectroscopy

The orientational behaviour of some liquid crystals with various molecular structures was studied by means of the Raman scattering depolarization method. The Raman scattering spectra of linearly polarized light were recorded as a function of temperature in the nematic phase. On the basis of these spectra the order parameters 〈P2〉 and 〈P4〉 as well as the molecular distribution function were determined. The obtained data were compared with those estimated on the basis of polarized light absorption and emission measurements. The influence of the molecular structure on the orientational order of liquid crystals was discussed.

Fluorescent discotic liquid crystals in Langmuir-Blodgett films for application in organic light emitting diodes

Langmuir-Blodgett (LB) films formed of some discotic liquid crystals, namely 3,4,9,10-tetra-(n-alkoxy-carbonyl)- perylenes, have been studied. The absorption and fluorescence spectra were recorded. The results obtained have led to conclusions about formation of dimers both in ground and excited states at the air-solid substrate interface. Moreover, information about the molecular arrangement in LB films has been obtained.