Marek Szablewski

Langmuir–Blodgett alignment of zwitterionic optically non-linear D–π–A materials

The synthesis, Langmuir–Blodgett (LB) deposition and charge-transfer spectra of Z-β-(1-alkyl-4-quinolinium)-α-cyano-4-styryldicyanomethanide (R-Q3CNQ where R = C6H13 to C20H41) and four substituted analogues are described. The deposition is Z-type and the properties are dependent upon the hydrophobic chain length. C6H13-Q3CNQ to C14H29-Q3CNQ form green LB films (λmax= 614 ± 4 nm; HWHM = 37 ± 2 nm) and occupy 28–34 A2 per molecule at 25 mN m–1. In contrast, C15H31-Q3CNQ to C20H41-Q3CNQ occupy 40–50 A2 per molecule; their films are purple with λmax= 565 ± 4 nm and HWHM = 22 ± 1 nm. The shift is attributed to a change in molecular tilt which causes the transition to alter from an intermolecular to an intramolecular process. Multilayer films of C16H33-Q3CNQ exhibit strong second-harmonic generation (SHG) and the intensity increases quadratically with the number of LB layers.

Rectifying characteristics of Mg|(C16H33-Q3CNQ LB film)|Pt structures

The Mg|(LB monolayer)|Pt structures of Z-β-(1-hexadecyl-4-quinolinium)-α-cyano-4-styryldicyanomethanide (C16H33–Q3CNQ) show asymmetric current–voltage characteristics; the behaviour is attributed to the organic monolayer although whether it is due to the presence of the permanent dipole moment or molecular rectification is unclear.

ORIENTATION AND SOLVATOCHROMISM OF DYES IN LIQUID CRYSTALS

The orientation and solvatochromism of some dye molecules in a liquid crystal have been investigated. Interactions with the host and the structure of the dye molecule affect the macroscopic alignment of dichroic dye molecules in a liquid crystal. It was observed that some dye molecules show a large bathochromic shift of their absorption maxima in the liquid crystal host relative to the situation in isotropic solvents. It is suggested that this is due to the occurrence of a much weaker reaction field in the anisotropic, rigid host. These dye molecules show little or no apparent order in the anisotropic host despite the observation of a reduction in the electro optic switching time when the dye is present. The highest degree of macroscopic alignment was observed for a merocyanine compound, which showed the smallest solvatochromic shift in the liquid crystal host. These results are discussed in terms of the steric, dipolar and hydrogen bond interactions between the guest and the host.

Current–voltage characteristics of an LB monolayer of didecylammonium tricyanoquinodimethanide measured between macroscopic gold electrodes

Didecylammonium tricyanoquinodimethanide, a zwitterionic molecule of the type D+–π–A− (where D+ = ammonium, π = ethylene “bridge”, and A− = tricyanoquinodimethanide), forms a Pockels–Langmuir monolayer film at the air–water interface. The molecules can be transferred quantitatively as a Langmuir–Blodgett (LB) monolayer onto a gold surface. Using the “cold-gold” evaporation technique, a set of several macroscopic gold electrode pads were evaporated atop the LB monolayer. Pads which had macroscopic defects were electrical short circuits, but across many pads finite electrical currents were observed as a function of applied bias; however, no obvious electrical rectification was found. The monolayer thickness on a gold substrate was measured by XPS to be ∼22 A. Reflection-absorption infrared spectroscopy (RAIRS) indicates that the molecules have a preferred orientation in the film, but are not well ordered. Angle-resolved X-ray photoelectron spectroscopy (XPS) suggests that many of the molecules are oriented anti-parallel to each other, possibly because dipole–dipole forces favor such an anti-parallel orientation. This latter observation helps to explain why no electrical rectification was found.

Local field and aggregation dependence of the micro- and macroscopic optical non-linearity of zwitterionic molecules

When a dipolar-conjugated molecule can lower its energy through gaining aromaticity, there can be a quite dramatic evolution of its polarisation properties dependent on the local reaction field generated in its environment. We have studied conjugated dipolar derivatives of tetracyano-p-quinodimethane (TCNQ) that would have quinoidal structure were it not for even the modest reaction fields produced in any solvent medium or polymer host environment. Measurements of dipole moment, μ, and of the first hyperpolarisability, β, have been undertaken in solvent media of varying polarity. We discuss the evolution of these properties as evidenced by solvatochromism and hyper Rayleigh scattering. We also discuss one major consequence of the planarity and high polarity in these molecules, association between them, which can seriously undermine practical implementation. Nevertheless, we refer to work demonstrating that this common problem can be overcome by suitable steric control of molecular shape and show that the prospects for achieving unprecedented electro-optic coefficients from polymer media are good.

Temperature Dependence of Electrical Transport in a Pressure-Sensitive Nanocomposite

Printed nanocomposites are of significant application potential in numerous technologies, such as touch-sensitive sensors and surfaces. Here, temperature dependent electrical transport measurements were undertaken on a recently developed screen-printed, multicomponent, nanocomposite ink to develop a detailed understanding of the electrical transport mechanisms. A theoretical model combining contributions from linear percolative conduction and nonlinear conduction attributed to field-assisted quantum tunneling successfully describes the temperature dependent conduction observed.

4-t-butyl-CuPc-PODT molecular composite material for an effective gas sensor

The sensing properties of molecular composites, tetratertbutyl copper phthalocyanine (TBCPC) and polyoctadecyl thiophene (PODT), were investigated by optical and conductance measurements. A standard substrate for gas sensing was made, consisting of a set of eight interdigitated gold electrodes, 230 /spl mu/m wide and spaced, and 0.10 /spl mu/m thick, photo-lithographically fixed on an SiO/sub 2/ coated silicon wafer. LB films of different thickness and composite compositions were fabricated at different surface pressures. The gas sensing behavior of the films, on exposure to NO/sub 2/ and NH/sub 3/ gases, depends on the mutual ratio of components of the composite. The film thickness determines the response speed on exposure to gases and their reversal. It was also found that the films deposited at lower surface pressures show larger and faster response and better recovery kinetics. The electrical properties of gold contacts to these Langmuir-Blodgett (LB) layers were evaluated by current-voltage characteristics, which are linear over the whole measuring range. Very short response times, reasonably good reproducibility, and fair composition dependent sensitivity at room temperature make this molecular composite a very promising candidate for neuron network sensing elements.

Structural Studies of a Series of Organic Non-Linear Optical Materials

The crystal structures of {4-[bis(diethylamino)methylium]phenyl}dicyanomethanide (I), {4-[cyano(4-methylpiperidyl)methylium]phenyl } dicyanomethanide (II), {4-[cyano(1-pyrrolidinyl)methylium]phenyl}dicyanomethanide (IV) and (Z)-{4-[1,3-dicyano-3-(diethylamino)-2-propenylium]phenyl} dicyanomethanide have been determined by X-ray crystallography. Together with (Z)-{4-[1-cyano-3-(diethylamino)-2-propenylium]phenyl}dicyanomethanide [(III): Cole, J. C., Howard, J. A. K., Cross, G. H. & Szablewski, M. (1995). Acta Cryst. C51, 715–718], these compounds exhibit varying degrees of quinoidal characteristics and yet have predominantly zwitterionic ground state character. The presence of high dipole moments in the crystal can thus be inferred and are substantiated by measurements of high solution-state dipole moments. Negative solvatochromism (hypsochromism on increasing solvent polarity) confirms the zwitterionic character in at least two of the series.

Crystal structures of amino substituted dicyanoquinodimethanes with potential nonlinear optical applications

The crystal structures of chiral aminomethylpyrrolidine-substituted 7,7,8,8-tetracyanoquinodimethane (TCNQ) and dipyrrolidine-substituted 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4) have been determined by single crystal X-ray crystallography. The TCNQ derivative crystallised in a noncentrosymmetric space group (P212121) and showed powder second harmonic generation (SHG) comparable to that of urea. An interesting case of H-bonding observed in its crystal structure is discussed. The TCNQF4 derivative is the first in the class of amino-substituted tetrafluoro-TCNQ derivatives. It has been observed to crystallise in a centrosymmetric space group (P21/c). Semi-empirical quantum chemical calculations using crystal structure geometries indicate moderate and negative (30–50×10-30 esu) hyperpolarisabilities. Crystal structural features and computational results are compared with known analogous systems.

Highly polar chromophores in poled polymer films characterised by second harmonic generation

Abstract Evidence for exceptionally high dipole moments in a series of nonlinear optical chromophores doped into poly(methyl methacrylate) films has been found. The strength of the poling field/dipole interaction is measured directly from incidence angle dependence of the second harmonic (SH) intensity generated from corona poled films. Values of the dipolar orientation parameter, , greater than 0.5 can be achieved for externally applied fields of only MV m −1 . Quantum chemical calculations of the evolution of the dipole moments of the compounds versus local reaction field support the conclusion that extremely high dipole moments characterise these molecules.