REPOSITORY > RESULTS

Abstract

Deuteron nuclear magnetic resonance was used to study the phase separation behavior of photoisomerizable
liquid crystal diheptylazobenzene (7AB). Under UV illumination, the elongated
trans molecules are converted to boomerang shaped cis form, creating a binary trans-7AB
and cis-7AB mixture. It was demonstrated that the concentration of both components can be
controlled dynamically using UV-illumination stimulated trans-to-cis photoisomerization and
thermally-induced cis-to-trans backisomerization. The so far elusive temperature-concentration
phase diagram of such mixture has been determined by comparative analysis of the behavior in
bulk, thin planar cell, and Anopore confining geometries.
Detailed information on mixing-demixing and isotropic-nematic phase segregation mechanisms
was obtained by using a newly constructed deuterium NMR probehead which made it
possible to perform in-situ UV illumination. In the nematic phase, the spectra of selectively
deuterium-labeled photoisomerized 7AB consisted of two doublets, attributed to orientationally
ordered trans and cis molecules, respectively. The temperature profiles of the order parameters
of both components were determined from the T-dependent frequency splittings of the doublets,
taking into account the average quadrupolar frequency nQ = 53 kHz, estimated from the powder
pattern spectrum of crystallized 7AB. In the isotropic phase, NMR spectrum of bulk material
exhibited a narrow central line.
In order to avoid the problems related to illumination-created spatial inhomogeneity of bulk
samples, 7AB was confined to cylindrical pores of Anopore membranes. The flat geometry of
membranes allowed for easy implementation of uniform illumination, resulting in a homogeneous
concentration profile throughout the sample. In confined samples, a weak splitting of the isotropic
central line was observed reflecting the surface-induced order. Similar to the nematic phase, the
splitting could be controlled by changing either the temperature or the concentration.
The experimental temperature-concentration phase diagram was constructed by performing
the measurements in the backisomerization regime, where sample heating effects due to nonresonant
absorption of light are absent. Since the timescale of the backisomerization process was
several hours, deuteron NMR experiments could be performed in a quasi-equilibrium regime.
The concentration of cis molecules was shown to decay exponentially with time, whereas
the characteristic decay times were found to be thermally activated with activation energy
Ea = 0:68 eV. In the phase diagram, boundary lines separating the nematic and isotropic phases
from the region of coexistence of both phases are clearly resolvable.
The measurements were modeled phenomenologically with an extended Landau-de Gennes
theory. By taking into account the concentration-dependence of the free energy terms, as well as
allowing for spatially-variable order parameter, we were able to reproduce the experimentally
observed behavior of both nematic and surface-induced orientational order, including the phase
boundary lines. The theory predicts straight isonematic lines in the temperature-concentration
phase diagram, a behavior also confirmed experimentally. Photoisomerized confined 7AB thus
behaves thermodynamically as a conventional binary nematic. Its main advantage, however, is
the dynamically variable concentration by means of UV illumination. This provides for an easy
way of accessing an arbitrary point in the temperature-concentration phase diagram.