Preparation, structural and NLO-optical characterization of LB-molecular films from asymmetric Bent-Core liquid crystals

Abstract

Molecular mono- and multilayered films of a polar asymmetric bent-core (``banana-shaped'') liquid crystalline (LC) compound with hydrocarbon end-chains were prepared by the Langmuir-Blodgett (LB) technique. Langmuir films were characterized by surface pressure isotherms and Brewster angle microscopy (BAM). Likewise, LB-films deposited onto glass substrates were characterized by UV-VIS spectroscopy, the optical second harmonic generation (SHG) technique and atomic force microscopy (AFM). Results show that the asymmetric structure of bent-core liquid crystals may promotes an unstable multi-layered ($n > 10$ LB-layers) LB-architecture which leads to a rapid collapse of Z-type arrangements, giving rise to a drastic decrease of the nonlinear optical (NLO) properties and film quality. Indeed, measurements evidence a tolerable and uniform molecular coverage on the glass substrates with anisotropic orientational distribution for a moderate number of layers only ($n \leq 10$ LB-layers); where, according to NLO-experimental data, the net molecular polarization is aligned outward the substrate layer. This observation leads us to implement a simplified model based on the monomeric rod-like approximation, in order to estimate significant NLO-tensorial components and an effective molecular hyperpolarizability $\beta_{\textrm{eff}}$ -coefficient along the polar axes of the 2D-polar LC-compound within the mechanical stable LB-monolayer arrangements.