  Charge dynamics in a dimer Mott insulating system, where a non-polar
dimer-Mott (DM) phase and a polar charge-ordered (CO) phase compete with each
other, are studied. In particular, collective charge excitations are analyzed
in the three different models where the internal-degree of freedom in a dimer
is taken into account. Collective charge excitation exists both in the
non-polar DM phase and the polar CO phase, and softens in the phase boundary.
This mode is observable by the optical conductivity spectra where the light
polarization is parallel to the electric polarization in the polar CO phase.
Connections between the present theory and the recent experimental results in
kappa-(BEDT-TTF)2Cu2(CN)3 are discussed.
