Photobleaching of chlorins in homogeneous and heterogeneous media

Comparative spectroscopic studies and quantum chemical calculations involving sensitizers chlorin (C), meso-tetraphenyl chlorin (MTPC), octaethyl chlorin (OEC), pheophorbide a (PPa) and chlorin e6 (Ce6) have been performed. Upon irradiation, all except chlorin bleached detectably in a homogeneous system (DMF). The order of photobleaching efficiencies was Ce6>PPa>OEC>MTPC>C. Combining these results with data from quantum calculations suggest that the efficiency of photobleaching is determined by the substituent groups in the photosensitizer. In a heterogeneous system (DPPC liposomes), however, only Ce6 photobleached significantly. This might be caused by the change of polarity of the environment around sensitizer molecules. In addition, the effects of additives that enhance or decrease photobleaching rates have also been assessed. The results suggest that the mechanisms of sensitizer photodegradation are complex and media-dependent.