Six new fluorescent derivatives of 1,8-naphthalimide were synthesized. Three were dyes, and three were fluorescent whitening agents (FWAs) containing a tetramethylpiperidine (TMP) stabilizer fragment. The FWAs were obtained under phase-transfer catalysis conditions. Five of the compounds were copolymerized with methyl methacrylate, so copolymers with an intense color and/or fluorescence stable against solvents were obtained. The chemical bonding of the synthesized monomers in the polymers was confirmed spectrophotometrically. The participation of the monomer compounds did not significantly affect the process of copolymerization or the molecular masses of the obtained copolymers. The quantity of chemically bonded naphthalimide monomer in the copolymers was determined to be over 60%. The spectral properties of the compounds and their photostability in solution and in the copolymers were studied. The influence of the compounds on the photostability of the copolymers was determined. The compounds, especially those containing a stabilizer (TMP) fragment in their molecules, showed a positive stabilizing effect on the photodegradation of poly(methyl methacrylate). Polyamide fabrics with 2-allyl-6-hydrazino-benzo [de]isoquinoline-1,3-dione, 2-allyl-6-(2-aminoethylamino)-benzo [de]isoquinoline-1,3-dione, andisoquinoline-6-yl) acetohydrazide were dyed, and materials with an intense yellow color and fluorescence were obtained. Cotton fabrics were whitened with 2-(2,2,6,6-tetramethyl-piperidin-4-yl)-6-methoxy-benzo[de]isoquinoline-1,3-dione, 2-(2,2,6,6-tetramethyl-piperidin-4-yl)-6-allyloxybenzo [de]isoquinoline-1, 3-dione, and 2-[2-(2,2,6,6-tetramethyl-piperidin-4-yl)-1,3-dioxo-2,3-dihidro-1H benzo [de]isoquinoline-6-oxy]ethyl-2-methacrylate, and materials with bright whiteness and intense bluish fluorescence were obtained.