Emission ellipsometry as a tool for luminescent liquid crystal phase transition identification

In the present work, we have studied the liquid crystalline properties of a luminescent molecule, which presents smectic mesophases, using the emission ellipsometry technique. A methodology with photoselection of the excitation light to improve the experiment is presented. An empirical model is proposed to explain the experimental results. The Stokes parameters of the emitted light allowed the correlation of changes in the polarization degree, ellipticity angle, and rotation angle with phase transitions.

Figure 1

(a) Variation of $P$ with phase transitions of a LC material between Cr and $I$. (b) Sketch of the crystalline domains’ behavior during phase transition processes.

Figure 2

(a) $d\chi /dT$ variation during the phase transitions Cr-$\mathrm{Sm}C-\mathrm{Sm}A-I$. (b) Rotation of the molecular planes inside the smectic-$A$ and -$C$ phases.

Figure 3

(a) Curve of the $d\psi /dT$ during the phase transitions Cr-$\mathrm{Sm}C-\mathrm{Sm}A-I$. (b) Molecular order during the phase transitions.

Figure 4

Molecular structure of the LC.

Figure 5

LC normalized absorption spectrum at room temperature and intensity of the emitted light $S_0$ as a function of the wavelength for temperatures between 30 °C and 225 °C. The sample was excited at 457 nm.

Figure 6

(a) Normalized Stokes parameters $S_i/S_0$; $i=1,2$, and 3; (b) $P$ parameter and first derivative $dP/dT$; (c) $\chi$ parameter and first derivative $d\chi /dT$; and (d) $\psi$ parameter and first derivative $d\psi /dT$ as a function of the temperature. The temperature varied at all ranges of the mesophases.