Kinetic Monte Carlo Simulations for Birefringence Relaxation of Photo-Switchable Molecules on a Surface

TL;DR

This study uses kinetic Monte Carlo simulations to explore how photo-switchable molecules on a surface relax their birefringence, revealing that dense, low-temperature conditions lead to glass-like slow relaxation due to dynamic heterogeneity.

Contribution

It introduces a 2D molecular model with trans and cis isomers as needles and demonstrates the microscopic origin of glass-like relaxation in photo-switchable monolayers.

Findings

Power-law relaxation linked to dynamic heterogeneity at high density and low temperature.

Cis isomers inhibit heterogeneity, leading to exponential relaxation.

Simulation results align with experimental observations of birefringence decay.

Abstract

Recent experiments have demonstrated that in a dense monolayer of photo-switchable dye Methyl-Red molecules the relaxation of an initial birefringence follows a power-law decay, typical for glass-like dynamics. The slow relaxation can efficiently be controlled and accelerated by illuminating the monolayer with circularly polarized light, which induces trans-cis isomerization cycles. To elucidate the microscopic mechanism, we develop a two-dimensional molecular model in which the trans and cis isomers are represented by straight and bent needles, respectively. As in the experimental system, the needles are allowed to rotate and to form overlaps but they cannot translate. The out-of-equilibrium rotational dynamics of the needles is generated using kinetic Monte Carlo simulations. We demonstrate that, in a regime of high density and low temperature, the power-law relaxation can be traced to the formation of spatio-temporal correlations in the rotational dy- namics, i.e., dynamic heterogeneity. We also show that the nearly isotropic cis isomers can prevent dynamic heterogeneity from forming in the monolayer and that the relaxation then becomes exponential.