We show that colloidal nanoplatelets can self-assemble to form 1D superlattice. When self-assembled, an additional emission line appears in the photoluminescence spectrum at low temperatures. This emission line is a collective effect, not observed when the nanoplatelets are not self-assembled. It is attributed to the longitudinal optical (LO) phonon replica of the band-edge exciton and its presence in self-assembled nanoplatelets only is explained using a model based on the efficient photons re-absorption between neighboring nanoplatelets. The presence of phonon replica at low temperature in ensemble measurements suggests the possibility to design a laser, based on self-assembled nanoplatelets.