Abstract: Biologically active compounds are provided that can be used as photosensitizers for diagnostic and therapeutic applications, particularly for PDT of cancer, infections and other hyperproliferative diseases, fluorescence diagnosis and PDT treatment of a non-tumorous indication such as arthritis, inflammatory diseases, viral or bacterial infections, dermatological, ophthalmological or urological disorders as well as providing methods to obtain them in pharmaceutical quality. One embodiment consists of a method to synthesize a porphyrin with a defined arrangement of meso-substituents and then converting this porphyrin system to a chlorin system by dihydroxylation or reduction, and if more than one isomer is formed separate them by chromatography either on normal or reversed phase silica. In another embodiment the substituents on the porphyrin are selected to direct the reduction or dihydroxylation to the chlorin so that a certain isomer is selectively formed.