To understand the excitation migration process between the phenylene (Ph) moieties in the framework of periodic mesoporous organosilicas (PMOs), the exciton–exciton annihilation behavior between the Ph moieties is studied. The dependence of the excitation light intensity on the fluorescence decay curve of the Ph exciton was successfully analyzed with a two-dimensional exciton diffusion and collision model. The quenching distance and diffusion length of the Ph exciton are estimated to be 0.4–0.8 nm and at least 4 nm, respectively. It is suggested that the Ph exciton is delocalized between up to three Ph moieties and moved between approximately nine Ph moieties in the pore wall of Ph–PMO.