Cone motion viscosity and optical second harmonic generation of ferroelectric liquid crystalline dendrimers

We report second harmonic generation in a ferroelectric liquid crystalline trimer and ferroelectric liquid crystalline dendrimers of first, second and third generation. Thin cells were filled with the compounds by capillary forces at elevated temperature, and cooled from the surface stabilized ferroelectric state to below the glass transition temperature, while kept in an electric field. The cone motion viscosity and the threshold electric field for unwinding of the helix axis of the chiral tilted smectic mesophases were studied separately at elevated temperature, and these data were used to optimize the preparation of the films. The measured response time was between 0.3 and 3ms, which corresponds to a cone motion viscosity between 0.5 and 50 Pa s. Second harmonic generation was studied both at elevated temperature with an electric field and at room temperature with and without electric field. The first generation dendrimer exhibited a strong increase in the second order non-linear optical response with time at room temperature. The d ₂₃-coefficient of this dendrimer was approximately four times larger than for the other macromolecules and was 0.045 pm V^-1. The relatively large d-coefficient of the first generation dendrimer is ascribed to crystallization, which improved the orientation of the molecular dipoles.     

Optical second harmonic generation in ferroelectric liquid crystals under oblique incidence

The second harmonic generation (SHG) in ferroelectric liquid crystals under oblique incidence was studied by using a numerical band matrix method that uses boundary conditions directly. No approximations were used except for the nondepleted fundamental wave. By matching the edge of the full-pitch band with that of the half-pitch band, the SH intensity increased significantly. The thickness (L) dependence of the SH intensity showed L ^6.93–L ^7.09 dependence without the limitation of the thickness.     

Second-order nonlinear optical properties of side-chain liquid crystalline polymers studied by second harmonic generation from thin films

The two independent elements of the second-order nonlinear optical susceptibility tensor of a range of contact poled, donor–acceptor substitued side-chain polymers are reported. The susceptibilities were measured by second harmonic generation from thin films, typically less than 0.5 μm thick, at a fundamental wavelength of 1064 nm. The largest value was χ = 2.64 pm/V which is three times greater than the χ value of KDP and was measured in a nitrobenzylidene side chain, polyhydroxystyrene polymer with an eleven unit alkyl chain spacer attaching the side group to the backbone. Typical susceptibility values obtained were χ～0.3 pm/V and X⁽²⁾₃₃～1 pm/V. The coherence lengths of the materials, which lay in the range 4–12 μm, were measured at 1064 nm by the maker fringe technique using thick, wedge-shaped samples.     

Polarization and viscosity measurements of ferroelectric liquid crystalline monomer polymer mixtures

Measurements of spontaneous polarization, tilt angle and response time on mixtures of a ferroelectric side chain polyacrylate with a low molecular mass liquid crystal of a very similar structure are reported. From the obtained values the rotational viscosity for the ferroelectric switching process is calculated. All mixtures exhibit an Arrhenius-like temperature dependence of the rotational viscosity, the values for the polymer being three orders of magnitude higher than for the low molecular mass compound.     

Second harmonic generation in ferroelectric liquid crystals and their mixtures

The second harmonic generation (SHG) in the ferroelectric liquid crystal (FLC) state has been studied as functions of electric field strength, rotating angle, temperature and molecular structure. It has been confirmed that a sharp angularly phase-matching curve of the SHG controlled by an electric field is observed even in the liquid crystal. The temperature dependences of the phase-matched SHG and Maker fringe in the ferroelectric phase have also been studied, and temperature dependences of non-linear optical coefficients obtained. The SHG in several kinds of FLC and dye doped FLC have also been measured, and the enhancement of SHG realized by means of doping the FLC with several kinds of dye.     

Second harmonic generation in ferroelectric liquid crystals and their mixtures

Abstract

The second harmonic generation (SHG) in the ferroelectric liquid crystal (FLC) state has been studied as functions of electric field strength, rotating angle, temperature and molecular structure. It has been confirmed that a sharp angularly phase-matching curve of the SHG controlled by an electric field is observed even in the liquid crystal. The temperature dependences of the phase-matched SHG and Maker fringe in the ferroelectric phase have also been studied, and temperature dependences of non-linear optical coefficients obtained. The SHG in several kinds of FLC and dye doped FLC have also been measured, and the enhancement of SHG realized by means of doping the FLC with several kinds of dye.     

Second harmonic generation in planar samples of ferroelectric liquid crystals

Non-linear optical properties of ferroelectric liquid crystals are often studied through second harmonic generation measurements on homeotropic cells. However, it is not always possible to prepare well aligned unwound samples in the homeotropic geometry, especially when dealing with photopolymers or tightly twisted SmC* materials. Here we analyse the problem of extracting the second order susceptibility coefficients dij from second harmonic generation measurements performed on planar samples. Several experimental configurations of polarizer, sample and analyser dispositions are proposed to deduce the complete dij tensor together with the dispersion of the refractive indices and the birefringence both at the fundamental and second harmonic frequencies. Measurements carried out on planar and homeotropic cells for a classical non-linear optical material (W314) are presented and the resulting dij coefficients are compared.     

Invited Lecture. Studies of liquid crystal monolayers and films by optical second harmonic generation

It is shown that optical second harmonic generation can be used to probe a number of important properties of liquid crystals: second order non-linearity of liquid crystal molecules, polar ordering and orientation of liquid crystal monolayers at various interfaces, and bulk alignment of molecules in supported and freely suspended films. Surface effects on bulk alignment can also be investigated.     

Electro‐optical behavior of ferroelectric liquid crystalline polyphenylene derivatives

Liquid crystalline (LC) polyphenylene derivatives, such as poly(para‐phenylene) (PPP), poly(meta‐phenylene) (PMP), poly(meta‐biphenylene) (PBP), and poly (meta‐terphenylene) (PTP) derivatives, were synthesized through substitution of fluorine‐containing chiral LC groups into side chains, with an aim to develop ferroelectric LC (FLC) conjugated polymers. All the polymers, except PTP, showed enantiotropic liquid crystallinities, where several types of mesophases were observed in both heating and cooling processes. Among them, PPP and PMP derivatives showed chiral smectic C (S_C*) phases responsible for ferroelectricity. In fact, they exhibited quick response to electric field, in spite of high viscosities inherent to polymers, giving rise to switching times of less than 1 s between two S_C states with reversely directed alignment. Hysteresis loops between the polarization and electric field were also observed for PPP and PMP. The spontaneous polarization (P_S) of PMP remained unchanged even after the electric field became zero, affording the residual polarization (P_R) whose value was the same as that of P_S. This indicates that PMP has a prospective memory function based on FLC nature. The present study is the first report for realizing a quick switching in macroscopic alignment using electric field and also for generating a potential memory function in π‐conjugated polymers. It is elucidated that unusual polymer main chains such as polyphenylenes can be used to prepare new ferroelectric polymers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3591–3610, 2008     

Enhancement of second harmonic generation in helical Sc liquid crystals

In a ferroelectric liquid crystal, a special type of phase-matching for optical second harmonic generation (SHG) is possible, where two counter-propagating fundamental waves create second harmonic waves at the edge of the selective reflection band. We compute the SHG intensity in such a situation and show that, at slight detuning from exact phase-matching, useful resonance enhancement can be obtained. A considerable amount of SHG also appears when the second harmonic frequency is in the reflection band, where the SHG wave is non-propagating.     

Functional Janus dendrimers containing carbazole with liquid crystalline,optical and electrochemical properties

ABSTRACT

Novel liquid crystalline Janus dendrimers that combine a mesogenic block and an electroactive block have been synthesised. The mesogenic block is based on two third-generation Percec-type dendrons bearing six or eight terminal dodecyloxy alkyl chains, whereas the electroactive blocks are formed by one or two carbazole units. The liquid crystal behaviour was investigated by polarised-light optical microscopy, differential scanning microscopy and X-ray diffraction. The Janus dendrimers with one electroactive unit exhibited cubic or columnar liquid crystal phases, whereas the Janus dendrimers with two electroactive units did not show liquid crystalline behaviour. The UV-vis absorption and emission properties of the Janus dendrimers were investigated. The spectra suggested the existence of π-π stacking and the formation of aggregates in the solid state. Electrodeposition of the carbazole-containing dendrimers afforded semi-globular particles in which the number of electropolymerizable units and the flexible or rigid character of the linker have a decisive influence in the particle size.     

Polymer films on metals investigated by optical second harmonic generation

In two different types of experiments, polymer films on gold substrates were investigated by optical second harmonic generation (SHG). Thickness dependent ex situ measurements on Langmuir-Blodgett films of the octadecylammonium salt of polyamic acid (PACS) show a preferential orientation of the polymer molecules. In situ SHG experiments were performed to monitor the growth of polyamic acid films deposited from the gas phase. Pyromellitic dianhydride (PMDA), 4,4-oxydianiline (ODA), and 4,4-diaminodiphenyl disulfide (DAPS) served as monomers. Similar to the Langmuir-Blodgett films, an oriented growth is observed. The thickness dependence of the SHG signal is strongly dependent on the interfacial chemistry which is very different for the two amines used as monomers. Based on a comprehensive three layer model, the relation between the structure of the films and the SHG signal is discussed.     

Fluorescent ferroelectric liquid crystalline copolyacrylates

Fluorescent liquid crystalline side chain polymers were synthesized by copolymerization of a ferroelectric monomer and 5 per cent of various blue fluorescent naphthalic imide dye comonomers. Those copolymers were characterized by DSC, X-ray, GPC and optical microscopy. In favourable cases, fast switching fluorescent ferroelectric polymers resulted, exhibiting high tilt angles (up to ∼ 34°) and spontaneous polarization values (up to ∼ 115 nC cm^-2) in the S*_c phase. One fluorescent copolymer shows orthogonal smectic phases exclusively due to the structure of the incorporated fluorescent comonomer. In this case a strong electroclinic effect and high induced tilt angles (12° 10 V μm^-1) have been observed in the S_a phase. Order parameters, S, of the dye moieties up to 0.64 were measured in the room temperature S_b phase for the copolymers     

Molecular mechanics of the ferroelectric to paraelectric phase transition in LiNbO3 via optical second harmonic generation

Temperature dependent optical second harmonic generation (SHG) results are used to monitor the 7° trigonal deformation of the NbO₆ octahedron in ferroelectric LiNbO₃. The SHG structural results compare favorably with those obtained from conventional (X-ray) techniques. The temperature dependence of the spontaneous polarization P_s, calculated via the same trigonal deformation model is consistent with the experimental observations.     

Quantitative measurement and interpretation of optical second harmonic generation from molecular interfaces

Second harmonic generation (SHG) has been proven a uniquely effective technique in the investigation of molecular structure and conformations, as well as dynamics of molecular interfaces. The ability to apply SHG to molecular interface studies depends on the ability to abstract quantitative information from the measurable quantities in the actual SHG experiments. In this review, we try to assess recent developments in the SHG experimental methodologies towards quantitative analysis of the nonlinear optical properties of the achiral molecular interfaces with rotational isotropy along the interface normal. These developments include the methodology for orientational analysis of the SHG experimental data, the experimental approaches for more accurate SHG measurements, and a novel treatment of the symmetry properties of the molecular polarizability tensors in association with the experimentally measurable quantities. In the end, the recent developments on the problem of surface versus bulk contribution in SHG surface studies is discussed. These developments can put SHG on a more solid foundation for molecular interface studies, and to pave the way for better understanding and application of SHG surface studies in general.     

Convergent synthesis of second generation CCK-functional dendrimers

This paper describes the facile synthesis and characterization of novel dendrimers bearing 9 and 12 CCK pepfide fragments on the surface by a convergent method. The structures of dendrimers are characterized by NMR and MS spectrum.     

Nanohybrids from liquid crystalline extended amphiphilic dendrimers

A novel extended amphiphilic dendrimer with linear poly(ethylene oxide) (PEO) attached to a PEO-like dendritic core as hydrophilic fraction and eight docosyl chain branches as hydrophobic fraction has been prepared for the use as structure-directing agent for silica-type materials. The extended dendrimer exhibits a hexagonal columnar liquid crystalline phase in the melt. Organically modified inorganic precursors and the extended dendrimer co-assemble into nanostructured hybrids. Hybrids with 0.44 weight fraction (fw) of aluminosilicate show a lamellar morphology, while hybrids with 0.21 fw exhibit a cylindrical structure. Nanostructures were characterized by a combination of small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The results suggest that dendrimer-based amphiphiles may provide an exciting platform for the formation of multifunctional organic-inorganic nanostructured hybrid materials with unique structural characteristics.     

The second and third order non-linear optical properties of liquid crystalline polymers

Abstract

The second and third order non-linear optical susceptibilities of several donoracceptor substituted side chain polymers, some of which exhibit mesophases, will be reported. The susceptibilities were measured by harmonic generation from thin films, typically 0·5 μm thick, at a fundamental wavelength of 1064 nm and 1579 nm. For second harmonic generation, the spin coated films were contact poled on a glass substrate with an interdigited electrode pattern. Typical values obtained were χ⁽²⁾ ₃₁ = 1 × 10^?9 esu and χ⁽²⁾ ₃₃ = 3 × 10^?9 esu, the largest value was χ⁽²⁾ ₃₃ = 6·3 × 10^?9 esu. The difficulty in deriving reliable second order structure-property relations because of the large effect which the contact poling process has on the non-linear optical susceptibility of the films will be highlighted. The highest χ⁽³⁾ value of 5·8 × 10^?12 esu obtained from an amorphous film of a polymer with an asymmetrically substituted azo side group is comparable with the susceptibility of more typical third order non-linear materials such as the main chain polymers polyphenylacetylene and poly-p-phenylenebenzobisthiazole.