Photosensitization of singlet oxygen via two-photon-excited fluorescence resonance energy transfer in a water-soluble dendrimer

A novel approach for the sensitization of singlet oxygen has been developed which utilizes indirect excitation of the photosensitizer by two-photon-excited fluorescence resonance energy transfer (FRET) from separate chromophores assembled into a dendrimer. This approach effectively enhances the two-photon excitation efficiency of a known photosensitizer, without the sort of chromophore modifications that could lead to loss of photosensitization and other desirable photophysical properties. Photosensitization of singlet oxygen via excitation wavelengths transmissive to human body tissue (750-1000 nm) could alleviate the depth limitations of photodynamic therapy. The dendritic photosensitizer was prepared by grafting two-photon-absorbing chromophores and water-solubilizing moieties to a known multivalent porphyrin photosensitizer. Efficient FRET (> 99% quenching of donor emission) between the peripheral donor two-photon-absorbing chromophores and the central acceptor photosensitizer at the core of the dendrimer was demonstrated under two-photon excitation conditions in an aqueous medium. Photosensitized production of singlet oxygen was monitored through chemical trapping and oxygen luminescence. Both methods independently demonstrated enhanced two-photon-induced singlet oxygen generation upon incorporation of two-photon-absorbing chromophores capable of efficient FRET to the photosensitizer.