  Using an isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport
model, we have studied the connection between the symmetry energy and features
of the dynamical dipole mode in fusion reactions with charge-asymmetric
entrance channel. The yield and angular distribution of the prompt photon
emission are extracted by a bremsstrahlung approach. The experimental data of
$^{36}$Ar+$^{96}$Zr at 16 MeV/nucleon and $^{32}$S + $^{100}$Mo at 9.3
MeV/nucleon are compared with IBUU model calculations, and the soft symmetry
energy is found to describe the data reasonably well.
