POLAR MICRO STRUCTURES OF THE B₂- AND B₄-PHASES OF BENT-SHAPED LC-MOLECULES RESOLVED BY NONLINEAR OPTICAL MICROSCOPY

Abstract

A nonlinear optical (NLO) microscope has been developed based on recording the second harmonic generation (SHG) signals of the B 2 -and B 4 -phases of bent shaped liquid crystalline (LC) materials. We observed that the microstructures of the polar order can be accurately characterized by 2D scanned images implementing this device. Specifically, we have investigated two bent-core compounds, which have either B 4 or B 2 phases at room temperature and exhibit SHG activity. The NLO microscope was calibrated according to the Maker-fringes method allowing the in situ evaluation of the relative SHG efficiency. The developed NLO microscopy device may serve as an important tool to characterize liquid crystalline features at a microscopic scale for both fundamental and applied research.

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].…”

“…Indeed, the performed P-In/P-Out and S-In/P-Out polarizing angle dependent SHG-measurements allowed us to roughly estimate the average polar orientation and main NLO-tensor coefficients of the system. This was possible since the SHG-technique is a very susceptible tool for studying the polar arrangement of molecular layers, and the bent-core compounds exhibit, due to an inherent 2D-geometrical structure and 1D-highly polar configuration, a single media which allows molecular ordering inspections by indirectly monitoring their respective polar axis, as has been previously proven by scanning NLO-SHG-microscopy [18,41]. So far, only bent-core based monolayer LB-films have been investigated according to the SHGtechnique, in this work we have proven that it is possible to develop multilayer Z-type LB-films with predictable quadratic NLO-response for a moderate number of deposited layers, where the film structure exhibits a homogeneous and adequate architecture.…”

“…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].…”

Optical Properties of Fluorinated Bent-Core Liquid Crystals Confined in Anodic Alumina Nanotubes

“…The AFE arrangement can be easily switched by suitable external electric fields, to a ferroelectric (FE) state. On a molecular level, the structure of bent-shaped compounds show a non-centrosymmetric distribution of the conjugated p-electrons-system resulting in a permanent dipole moment along the bent direction, which is a key point for NLO-applications [38][39][40][41][42][43].…”

“…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 bend direction. 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 [46][47][48].…”