The comparative photodegradation studies of a free photosensitizer Radachlorine and Radachlorine being a part of the triple selenium-containing nanosystem (PS-2) based on the amphiphilic molecular brush (AMB) with a cellulose backbone and side chains of polymethacrylic acid (bound Radachlorine) have been performed using UV/visible spectroscopy, atomic force microscopy, and luminescence. The effect of the light exposure time on the optical density and luminescence intensity of Radachlorine in both states was established. The AMBs were found to hinder the association of selenium nanoparticles in the solution forming discrete spherical nanostructures before irradiation. But the nanostructures acquired an ellipsoidal shape after 60 min exposure; in addition, there was a tendency towards their association. On the basis of the UV/visible spectroscopy data for the PS-2 obtained before and after irradiation, the values of the band gap energy and diameter of the selenium nanoparticles were calculated.