Kinetics of photoinduced ordering in azo-dye films: two-state and diffusion models

TL;DR

This paper models the kinetics of photoinduced molecular ordering in azo-dye films using two phenomenological approaches, revealing the transient biaxial anisotropy and uniaxial nature of initial and steady states.

Contribution

It introduces and compares a two-state model and a 2D diffusion model for understanding photoinduced ordering in azo-dye layers.

Findings

Transient structures are biaxially anisotropic.

Photosteady and initial states are uniaxial.

Models successfully interpret experimental data.

Abstract

We study the kinetics of photoinduced ordering in the azo-dye SD1 photoaligning layers and present the results of modeling performed using two different phenomenological approaches. A phenomenological two state model is deduced from the master equation for an ensemble of two-level molecular systems. Using an alternative approach, we formulate the two-dimensional (2D) diffusion model as the free energy Fokker-Planck equation simplified for the limiting regime of purely in-plane reorientation. The models are employed to interpret the irradiation time dependence of the absorption order parameters extracted from the available experimental data by using the exact solution to the light transmission problem for a biaxially anisotropic absorbing layer. The transient photoinduced structures are found to be biaxially anisotropic whereas the photosteady and the initial states are uniaxial.