Freezing out all-optical poling dynamics of azophenylcarbazole molecules in polycarbonate

Abstract

In this work we have extended the application of a theoretical model describing the processes of all-optical poling of isomerizable molecules while taking into consideration the thermoisomerization related findings presented in the literature. A model describing all-optical poling transients using a three relaxation rate approach was contrasted with the experimental results of azophenylcarbazole doped polycarbonate measured in a wide range of temperatures from 150 to 300 K, thus covering the β transition for the host. By means of a long timescale and low temperature, we were able to better resolve the thermoisomerization and orientational diffusion processes. The cis → trans thermoisomerization relaxation rates k₁ and k₂ were obtained in the range 10⁻⁵ to 10⁻² s⁻¹ and the relaxation rate k₃ of the orientational diffusion of the trans isomer in the range 10⁻⁶ to 10⁻⁴ s⁻¹. The rates exhibited diverse temperature dependent behaviors: the two lowest (k₂,k₃) manifested Arrhenius type dependencies (E_k2 = 157 meV), whereas, the highest (k₁) showed a temperature dependence that is non-Arrhenius, or undistinguished for our experimental conditions. The latter was interpreted using the geometrical “adjustment” model. By investigating chromophore–polymer systems at temperatures far below T_g, we were able to uncover the situation when the cis → trans transition is switched-off and the orientational randomization is suppressed. Thus, we could consider the chromophores as “frozen”.