Effects of intramolecular excitation energy transfer and electron transfer on the photosensitizer activities of phosphorus(V) porphyrin heterotrimer and pentamer

Porphyrin heterotrimer and pentamer constructed with phosphorus(V) porphyrins (P(V)porphyrin) and free-base porphyrins were synthesized to evaluate their photosensitizer activities. Since the excitation energy of free-base porphyrins is smaller than that of P(V)porphyrins, intramolecular energy transfer from the P(V)porphyrin unit to the free-base porphyrin unit was observed by time-resolved fluorescence spectra measurement. The redox potential of the P(V)porphyrin unit was higher than that of the free-base porphyrin; the intramolecular electron transfer occurred after the intramolecular energy transfer. The quantum yields of singlet oxygen production by these multimers were significantly smaller than those of reference monomer compounds for the P(V)porphyrin and free-base porphyrin units. These multimers bound to the surface of human serum albumin; however, protein photodamage by these multimers was barely observed. In conclusion, the combination with a free-base porphyrin unit greatly suppresses the photosensitizer activity of the P(V)porphyrin unit.