Visualization of the Molecular Shape and the Orientational Distribution in the Langmuir−Blodgett Film of a Bent-Shaped Achiral Molecule As Studied by Surface Second-Harmonic Generation

Abstract

We have studied the anisotropic molecular orientational distributions of two-dimensional bent-shaped molecular monolayers by optical surface second-harmonic generation (SHG). The molecular monolayer of 1,3-phenylene bis[4-(((4-dodecylphenyl)imino)methyl)benzoate] (12-PIMB) was prepared by the Langmuir−Blodgett (LB) technique. Oblique incident surface SHG measurements from the monolayer showed that the bent-shaped molecules preferably aligned toward the dipping direction. The analysis of the experimental results provided the nonlinear-optical tensor components of the 12-PIMB LB monolayer. Using these components and a simple superposition model, the molecular orientational distribution function was determined. We successfully showed that the distribution map provides a view of the molecular shape.

Incorporation in Langmuir and Langmuir–Blodgett films of symmetric fluorine substituted bent-core liquid crystals: Morphological and optical properties

“…Thus, according to the MakerFringes method, angle dependent SHG-measurements were performed for mono-and multilayer LB-structures implementing both significant P-In/P-Out and S-In/P-Out laser beam polarization geometries as explained before. The estimated NLO-coefficients are in good agreement with those reported in the literature for bent-shaped compounds composed of several benzene rings [5,18,[34][35][36][37][38][39][40].…”

“…In general, the experimental data may deviate from this approach for excessive multilayer depositions, where the system may drastically turn into a mechanically unstable configuration. Up to date, only a few studies of monolayer bent-core based LB-systems have been reported in the literature [5,6,8,9,12]. Indeed, the improvement of multilayer LB-systems with the fascinating bent-core "banana" compounds seems to be a challenging task of current interest, since a collapse of an excessive number of deposited layers seems to occur for this kind of compounds, as is shown in this work.…”

“…6a), the corresponding tensorial components of the second-order susceptibility were approximately evaluated using basic angle dependent SHG-measurements. As mentioned, we have considered here for a straightforward analysis, the simplified axial C 2V symmetry approximation accepted for this kind of compounds under ferroelectric polar arrangements (including LB-monolayers and prepared EO-film cells [5,34]), taking into account the one-dimensional monomeric unit approximation and the nearly rodlike shape of these molecules [5,12]. In the special case of bent-core compounds, the dipole moment is perpendicular to the long molecular axis, as shown in Fig.…”

“…Specifically, these properties are due to the presence of the Fluorine atom and the carboxyl-and oxy-groups on the molecular wings producing a net polarization along the symmetry axis of the molecular bent direction (like an arrow in a bow). Recent studies have proven that such systems lead to the observation of important optical nonlinearities since the nonvanishing components along the conjugation length of the two benzylideneaniline wings produce large molecular hyperpolarizabilities [5,17,18].…”

“…Some research groups have demonstrated that it is possible to produce stable monolayer arrangements of two-dimensional bananashaped mesogens and they have studied the dielectric, ferroelectric and antiferroelectric properties, the anisotropy as well as the orientational distribution of the corresponding molecular structures by surface second-harmonic generation (SHG), where the monolayer is transferred onto a glass substrate using the LB-technique [5][6][7][8][9][10][11][12]. Despite these efforts, there still exists a huge need of experimental data in order to better understand the alignment properties of these kinds of compounds in LB-systems; this is a difficult task due to the complexity of these mesogens and the number of imaginable configurations they can achieve in such structures.…”