Abstract: The present invention generally relates to the formation, chemistry and application of biologically active compositions. More particularly, the present invention relates to certain dyes, specifically porphyrin and chlorin derivatives, in combination with inventive polymers, i.e. light-cleavable polymers, that can be used as photosensitizer compositions for a wide range of light irradiation treatments such as photodynamic therapy of cancer, infections and other diseases. The dye derivatives may either be adsorbed on, or incorporated in, or attached to specific polymers, which as well form part of the invention.