Abstract: To create a durable bioactive coating we investigated the effectiveness of the photochemical surface modification of polyethylene terephthalate (PET) by bifunctional dyes of phenothiazine series and selected the optimal conditions of the process. For that purpose a series of the bifunctional photosensitizers containing aryl azide group forming covalent bonds with the polymer matrix under UV and generating singlet oxygen under visible light exposure was synthesized. We have synthesized a series of amides of toluidine blue O (TBO) with 2– , 3– and 4– positions of benzoic acid substituted by the azide groups. Surface modification was carried out by irradiation of solutions of the synthesized bifunctional dyes when the samples of PET were submerged into acetonitrile by the low pressure mercury-quartz lamp. Based on the comparison of the UV spectra of PET films before and after the photochemical modification we found that the most effective modifier was TBO–4– azidobenzoic acid with concentration of 3 × 10^-3M, but the least effective modifier was TBO–2–azidobenzoic acid. Obviously it is caused by the sterically hindered insertion of a nitrene formed in the course of the azidobenzoic acid photolysis into the C=O bond of the polymer matrix.