AbstractSummary: New monomeric benzo[de]isoquinoline‐1,3‐dione blue emitting fluorophores, containing hindered amine stabilizer fragment have been synthesized under phase transfer catalysis conditions and their ability for “one‐step” optical bleaching and stabilization of poly(methyl methacrylate) has been demonstrated. During the co‐polymerization, no changes in the chromophoric system of the fluorophore occurred. The chemical bonding of the synthesized monomers in the polymer chain was confirmed spectrophotometrically. The absorption and some basic photophysical characteristics of the low and high molecular weight fluorescent brighteners have been determined. Photostability of the new monomeric fluorophores and their influence on the photostability of the blue emitting side‐group co‐polymers obtained have been established. A significant stabilizing effect against photodegradation was determined. magnified image