Recent experiments have demonstrated that in a dense monolayer ofphoto-switchable dye Methyl-Red molecules the relaxation of an initialbirefringence follows a power-law decay, typical for glass-like dynamics. Theslow relaxation can efficiently be controlled and accelerated by illuminatingthe monolayer with circularly polarized light, which induces trans-cisisomerization cycles. To elucidate the microscopic mechanism, we develop atwo-dimensional molecular model in which the trans and cis isomers arerepresented by straight and bent needles, respectively. As in the experimentalsystem, the needles are allowed to rotate and to form overlaps but they cannottranslate. The out-of-equilibrium rotational dynamics of the needles isgenerated using kinetic Monte Carlo simulations. We demonstrate that, in aregime of high density and low temperature, the power-law relaxation can betraced to the formation of spatio-temporal correlations in the rotational dy-namics, i.e., dynamic heterogeneity. We also show that the nearly isotropic cisisomers can prevent dynamic heterogeneity from forming in the monolayer andthat the relaxation then becomes exponential.
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