No abstract

The dynamics of 4-n-pentyl-4′-cyanobiphenyl (5CB) nematic liquid crystalline thin films have been studied in real time using step-scan Fourier transform infrared spectroscopy (FTIR). In these studies, the liquid crystal was confined in a nanocavity defined and bounded by an interdigitated gold electrode array. The gold microstructures were microfabricated on a zinc selenide (IR-transparent) window. The 5CB interactions with the ZnSe substrate result in surface-induced ordering of the ultrathin layers (on the order of 40 nm). As the films increase in thickness, the nanoscale organization induced by the surface layer becomes a less significant contributor to the overall bulk structure of the sample. Time-resolved FTIR studies have enabled the measurement of rate constants for the orientation and relaxation of the thin films under an applied electric field as a direct function of confinement dimensions. Cell thicknesses ranging from 40 to 300 nm were studied. The measured rate behaviors demonstrate the strong effects of the interactions occurring between the surfaces of the ZnSe crystals and the 5CB on the dynamics of the liquid crystalline assembly. Time-resolved studies reveal kinetically inhomogeneous line shapes for thicker films while ultrathin films maintain kinetically homogeneous peaks, suggesting the development of liquid crystalline domains or other inhomogeneities over this length scale in the transition from the surface layer to bulk.

An interdigitated array electrode (IDA) is fabricated on an IR-transparent substrate for use in electrooptical measurements using Fourier transform infrared (FT-IR) spectroscopy. The fabrication of the IDA and its unique sampling geometry for transmission spectroscopy is detailed. The gold IDA was patterned on the ZnSe substrate using a photolithographically defined liftoff process. An IR flow cell was modified to enable the substrate containing the IDA to be used for electrooptical measurements in transmission. The utility of the electrooptical cell is demonstrated by application to two model systems. In the first, changes in the conductivity and spectral features of a receptor membrane (Nafion) upon dehydration are measured. In the second, the measurement of the electric-field-induced orientation of a liquid-crystalline film of 4-n-pentyl-4'-cyanobiphenyl was conducted in a new experimental geometry, with IR transmission normal to the direction of the applied electric field. Difference spectroscopy facilitated the observation of the change in orientation of the liquid crystal with applied potential. These systems demonstrate the general utility of the fabrication scheme described here. It also illustrates its facile adaptation to infrared difference spectroscopy as a means of studying complex phenomena in electrochemical and electrooptical systems.

Abstract. Electrogenerated chemiluminescence (ECL)with tris(4,7-diphenyl-1,10-phenanthrolinedisulfonic acid)ruthenium(II) (RuBPS) in solution and immobilized on an electrode surface is investigated. Flow injection analysis with a thin layer electrochemical cell modified for ECL detection is used to determine the analytical utility of solution phase RuBPS and RuBPS immobilized in a cationic polypyrrole derivative. The solution phase reaction of RuBPS with oxalate is investigated with regard to the dependence of ECL emission on RuBPS concentration, carrier stream flow rate, and pH. In the parameter range studied, ECL intensity is not linear with the concentration of RuBPS in the sample. A maximum ECL intensity is observed with a RuBPS concentration of approximately 250 gM. Slower linear velocities give greater ECL intensities which is the opposite of what is observed for Ru(bpy)~ + and oxalate. Greater ECL intensity is observed at lower pHs for oxalate and at higher pHs for proline. RuBPS ECL with oxalate yields a working curve with a linear range from 0.1-100 btM oxalate. Solution phase ECL is observed for RuBPS and other amines such as NADH, proline, tripropylamine, and antibiotics including streptomycin and gentamicin. RuBPS is also immobilized by electrochemical polymerization of 1-methyl-3-(pyrrol-1-ylmethyl)pyridinium chloride (MPP) in the presence of RuBPS. This polymer-modified electrode yields ECL for oxalate and for amines.