Understanding the optical spectroscopy of amphiphilic molecular rectifiers: a density functional approach.

We present results of first principles density functional theory calculations of the electronic and atomic structural properties of model Z-type Langmuir-Blodgett (LB) layers comprising amphiphilic quinolinium tricyanoquinodimethanide (Q3CNQ) chromophores. We find that the chromophore electronic ground state is not as clearly "zwitterionic" as required by models to explain electrical rectification purportedly seen in such systems. The computed visible region transitions are not what have been assumed to be the intervalence charge transfer bands seen in the visible region of molecules in Z-type LB films. Our own LB deposition and spectroscopic studies suggest that almost all visible region features previously seen may be ascribed to aggregates. The calculated lowest energy electronic excitation between HOMO and LUMO levels, which is located in the near infrared region, has a transition moment aligned approximately 9° off the molecular long axis, and has a normalized oscillator strength of 1 order of magnitude higher than those of the visible region transitions. This most dominant feature has been neglected from discussions of Langmuir-Blodgett layer rectification but our own deposition studies show no sign of this feature, indicating that the structure of the modeled system differs from that of typical experimental structures. The model indicates that such idealized LB layer structures cannot confidently be invoked to explain their experimental optical or electrical properties.

Surface anchoring structure of a liquid crystal monolayer studied via dual polarization interferometry.

The self-organization of liquid crystal molecules of 4- n -pentyl- 4' -cyanobiphenyl (5CB) forming an oriented monolayer by condensation from the vapor phase onto a silicon oxynitride surface has been observed using the evanescent wave dual slab waveguide dual polarization mode interferometry (DPI) technique. Two distinct stages to the layer formation are observed: After the formation of a layer of molecules lying prone on the surface, further condensation begins to densify the layer and produces a gradual mutual alignment of the molecules until the fully condensed, fully aligned monolayer is reached. At this limit the full coverage 5CB monolayer on this surface and at a temperature of 25 degrees C , is found to be anchored with an average molecular axis polar angle of 56+/-1 degrees and with a measured thickness of 16.6+/-0.5A . These results are in reasonable agreement with the molecular dimensions provided by molecular models. The apparent precision and accuracy of these results resolves some wide disparity between earlier studies of such systems. Previous difficulties in determining optogeometrical properties of such ultrathin birefringent films using ellipsometry or in the need for complex modeling of the film layer structure using x-ray reflectivity are overcome in this instance. We provide a technique for analyzing the dual polarization data from DPI such that the bulk refractive index values, when known, can be used to determine the orientation and thickness of a layer that is on the nanometer or subnanometer scale.