JultikaUniversity of Oulu repository

Abstract

Azobenzene is a broadly studied molecule that forms liquid crystal phases under certain circumstances. It has two distinctive geometries, a flat trans and a bent cis isomer. Through a reversible photoisomerisation process, the molecule can be forced to transform from one isomer to another. Because only the trans isomer forms a nematic liquid crystal phase at room temperature, this process additionally triggers a phase transition. Unique qualities of azobenzene and its derivatives are a key to multiple practical applications, such as using polymeric azobenzene films as image storage.

The primary goal of the study was to identify liquid crystal (LC) phases of trans- and cis-azobenzene by building phase diagrams from the simulation results. A bulk of 2000 azobenzene molecules was studied with Monte-Carlo simulations at constant pressure and temperature. The molecules were modelled with a coarse-grained two-site Gay-Berne (GB) potential, which was created by fitting the model into quantum-chemical azobenzene dimer interaction energies. Separate cis and trans parametrisations were acquired; this made mixed cis-trans simulations arduous to implement, so the carried out simulations only contained either isomer.

The phase structure of the two isomers was presented along with thermodynamic quantities and structural parameters. As a result, a crystal-nematic-isotropic phase structure was identified in the trans isomer with well-defined transition temperatures, and a more vague crystal-isotropic structure in the cis isomer. There are two reasons for the lack of interesting LC phases in the cis simulations. Firstly, the molecule itself is relatively short and has a sharper bending angle than many LC-forming bent-core molecules. Secondly, the least squares fit resulted in an almost-spherical parametrisation, a form that is very unlikely to form LC phases. Trans simulation results were anticipated to some extent, based on previous one-site GB studies with a similar parametrisation. A significant divergence here is the absence of the smectic phase, though, indicated by a comparatively small translational order parameter value. In previous related studies, smectic-A and smectic-B phases have been identified by using a similar single-site GB parametrisation and even by using a rodlike two-site GB model.