  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.
