Real-time determination of molecular orientation in Langmuir–Blodgett monolayers on integrated-optical structures

Abstract

Waveguide linear-dichroism measurements were used in determining the molecular orientation of N-(sulfopropyl)-4-(p-dioctylaminostyryl) pyridinium in Langmuir-Blodgett monolayers upon 150-microm-thick glass (Ti:Zn) substrates in real time. Acquisition of complete spatial-decay curves in t - 1 s with P(laser) = 100 microW was made possible by combining propagation in integrated-optical structures with direct imaging of monolayer or glass fluorescence onto the active area of a charge-coupled device camera. Multichannel detection permitted measurement of optical waveguide loss coefficients and ultimately of molecular orientations in approximately 10(-3) of the amount of time required by previous methodology. The effects of sample preparation and materials processing on the accuracy and precision of dichroic ratios are discussed. Observed dichroic ratios for a hemicyanine dye revealed no measurable change in the orientation of the electronic transition-dipole moment when Langmuir-Blodgett films were deposited at different surface pressures. These results are in good agreement with the results from singly resonant second-harmonic generation experiments.

Dipole Orientation Distributions in Langmuir−Blodgett Films by Planar Waveguide Linear Dichroism and Fluorescence Anisotropy

“…Optical techniques have become the predominant strategy for studying uniaxial (azimuthally symmetric) assemblies. Molecular orientation in organic films has been assessed by measuring absorbance linear dichroism (LD) in the infrared and UV−vis spectral regions, polarized luminescence, polarized Raman scattering, and second harmonic generation. − Each of these methods has its advantages and limitations, but sensitivity to a particular sample is usually the primary consideration in choice of method. The major difficulty is that by definition the sample comprises a small amount of material and is located at the interface between two immiscible phases.…”

“…This produces an evanescent path length that is orders of magnitude greater than that of a millimeter thick internal reflection element, with a concomitant sensitivity enhancement. − Using the planar IOW−ATR geometry to measure the linear dichroic ratio along with established interfacial optical theory, the mean tilt angle of the absorption dipoles in an IOW-supported molecular assembly can be determined relative to a laboratory-defined coordinate system. This approach has been used to study molecular orientation in organic monolayers and hydrated protein films. ,,, …”

Fabrication and Characterization of Uranium Oxide Doped Sol−Gel Planar Waveguides for Attenuated Total Reflectance Spectrometry

“…A planar integrated optical waveguide (IOW) is a substrate-supported, dielectric film, typically less than 1 μm thick, in which light propagates via total internal reflection in one or a few discrete modes . Relative to a conventional (i.e., much thicker) waveguide, an IOW supports a very high density of reflection sites per unit propagation length (using a ray optics approximation2a), which makes these devices particularly useful for evanescent spectral analysis of thin films − and optically based chemical sensing. − In many of these applications, measurements are performed by quantitating the attenuation of light propagating in the guided mode due to absorption of the evanescent field by a thin adlayer of chromophores at the IOW−superstrate interface. ,,− , …”

“…[9][10][11][12] In many of these applications, measurements are performed by quantitating the attenuation of light propagating in the guided mode due to absorption of the evanescent field by a thin adlayer of chromophores at the IOW-superstrate interface. 2,4,[6][7][8]12 One method used to measure mode attenuation in an IOW is to outcouple the light using a prism or diffraction grating and measure the intensity using a photodetec-tor. 2 Limitations of this method result from the fact that IOW attenuation measurements are usually conducted in a single-beam geometry.…”

Total Internal Reflection Fluorescence and Electrocapillary Investigations of Adsorption at a H2O−Dichloroethane Electrochemical Interface. 1. Low-Frequency Behavior

“…Di- N -butylaminonaphthylethenylpyridiniumpropylsulfonate ( I )belongs to a class of voltage-sensitive membrane probes, which have been used extensively in cell biology. − Our interests involve its use as a reporter of the interfacial processes occurring when planar membranes are subjected to external fields, such as those found in electroporation events. − Planar membranes offer both simplifications and complications relative to membranes in spherical geometries. On one hand, the planar membrane offers the opportunity to control the interaction of electromagnetic radiation with the fluorescent probe, e.g., by using total internal reflection to control the magnitudes of the interfacial field components. − On the other hand, planar membranes are metastable relative to deformations when placed in a perturbing environment, e.g., an external field. In this paper, we report the use of total internal reflection fluorescence (TIRF) and electrocapillary measurements to monitor the potential-dependent adsorption of I at bare and DLPC-modified H 2 O−DCE interfaces.…”