Determination of Molecular Anisotropy in Thin Films of Discotic Assemblies Using Attenuated Total Reflectance UV−Visible Spectroscopy

Abstract

We report here an investigation of absorbance anisotropy in highly ordered, single bilayer (ca. 5.6 nm) Langmuir-Blodgett (LB) thin films of discotic liquid-crystalline phthalocyanines, using a recently introduced broad-band attenuated total reflectance (ATR) spectroscopic technique, capable of measuring dichroism in such films in the UV--visible optical region down to absorbances of ca. 0.003 absorbance units. On the basis of the ATR measurements of LB-deposited films, a thorough treatment was established to determine the ensemble average of the Cartesian components and the associated optical anisotropy of transition dipoles in the molecular film. In an effort to recover order parameters of molecular orientation, those results were interpreted with a circular dipole model, which is the expected model for the isolated molecule based on symmetry properties. We measured a strong dipole component normal to the film plane that cannot be explained in terms of a truly circular model, indicating that the molecular transition dipoles were perturbed upon aggregation. The utility of the experimental approach was further demonstrated by (a) investigating the effect of substrate modifiers (methyl- and phenyl-terminated silanes) on the ordering within the phthalocyanine film and (b) the effect of water immersion and re-annealing of the thin film on molecular ordering and optical anisotropy.

Polarization-resolved spectroscopy imaging of grain boundaries and optical excitations in crystalline organic thin films

“…In solution or amorphous films, there is no preferential orientation of the molecular plane; therefore, no LD is observed in such cases. It is expected, however, that in a single crystal the long-range molecular order preserves the selection rules, hence the LD can be observed on a macroscopic scale 21 .…”

“…The LD values are positive across the scanned area with a 25% difference between adjacent stripes. Since the LD is proportional to cos (2 ϕ ) 21 , where ϕ is the angle that defines the stacking axis orientation with respect to the polarizer axis ( Fig. 2 ), the LD contrast observed in sample A corresponds to a relative angle Δ ϕ ∼5 0 between stacking axes from adjacent stripes.…”

Liquid-crystalline phthalocyanine-based nanostructures

“…The potentials uses of such materials in electronics attract attention to the complexes; therefore, the num ber of works devoted to the properties of mesomorphic sandwich complexes with various substituent groups is increasing [73,121].…”

Applications of the Planar Fiber Optic Chip

“…Such fiber optic sensor architectures employ signal transduction through a fragile cylindrical probing interface, which can be problematic for several applications where a robust platform or planar deposition technologies are required. Clearly, a supported planar interface would be advantageous for using standard planar deposition technologies such as Langmuir-Blodgett (LB)-deposited thin-films (Doherty et al, 2005;Flora et al, 2005) and planar supported lipid bilayers. (McBee et al, 2006) In addition, due to its more robust supported platform, a planar design would be amenable for integration into microfluidic systems and sensor arrays.…”