  In order to study isovector dipole response of neutron-rich nuclei, we have
applied a time-dependent method of antisymmetrized molecular dynamics. The
dipole resonances in Be, B and C isotopes have been investigated. In $^{10}$Be,
$^{15}$B, $^{16}$C, collective modes of the vibration between a core and
valence neutrons cause soft resonances at the excitation energy $E_x=10-15$ MeV
below the giant dipole resonance(GDR). In $^{16}$C, we found that a remarkable
peak at $E_x=14$ MeV corresponds to coherent motion of four valence neutrons
against a $^{12}$C core, while the GDR arises from the core vibration in the
$E_x >20$ MeV region. In $^{17}$B and $^{18}$C, the dipole strengths in the low
energy region decline compared with those in $^{15}$B and $^{16}$C. We also
discuss the energy weighted sum rule for the $E1$ transitions.
