A new approach to two-photon excited photodynamic therapy has been developed. A dendritic array of eight donor chromophores capable of two-photon absorption (TPA) was covalently attached to a central porphyrin acceptor. Steady-state fluorescence measurements demonstrated that the donor chromophores transfer excited-state energy to the porphyrin with 97% efficiency. Two-photon excitation of the donor chromophores at 780 nm resulted in a dramatic increase in porphyrin fluorescence relative to a porphyrin model compound. Enhanced singlet oxygen luminescence was observed from oxygen-saturated solutions of the target compound under two-photon excitation conditions.

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.

A conformationally flexible, generation-2,3 poly(aryl ether) dendrimer favors quantitative cascade fluorescence resonance energy transfer without the appearance of undesired chromophore self-quenching interactions such as excimer formation.

Utilization of two-photon (TP) excited fluorescence resonance energy transfer (FRET) within a light-harvesting dendrimer has proven to be a reliable method for the enhancement of the effective TP absorption efficiency of many FRET acceptor molecules. This light-harvesting approach has enabled TP photosensitization of singlet oxygen from a porphyrin in both aqueous and organic media using wavelengths more transmissive to human body tissue (750−1000 nm). This ability to utilize near-infrared irradiation to induce photochemical reactions is especially attractive for applications including in vivo photochemistry, oxygen sensing, and photodynamic cancer therapy. In efforts to further the applicability of this concept, we synthesized an array of novel photosensitizers by metalation of the porphyrin core with aluminum, silver, and zinc. Time-resolved fluorescence, transient absorption measurements, and TP excitation experiments demonstrated efficient TP excited FRET to produce the porphyrin excited state triplet which subsequently generates singlet oxygen by the aluminum and zinc metalated species. Singlet oxygen photosensitization efficiency was found to be most efficient using aluminum followed by zinc and least efficient using silver. In fact, silver metalated photosensitizers were found to be nonfluorescent and incapable of generating a measurable amount of singlet oxygen. With the proper choice of inserted metals, it was possible to tune the efficiency of TP induced singlet oxygen production.

The synthesis and photophysical characterization of a generation 1 dendrimer containing four coumarin 2 laser dyes at the periphery and a perylenebis(dicarboximide) derivative at the core are described. It was found that 99% of the UV light absorbed by the peripheral coumarin 2 donor chromophores is transferred through fluorescence resonance energy transfer (FRET) to the core acceptor. Emission from the core via FRET is observed in the near-infrared with a 6.2-fold amplification relative to direct excitation.

A novel fluorescence resonance energy transfer (FRET) system containing a two-photon absorbing dye and a nile red chromophore has been synthesized. Upon two-photon excitation by laser at 815 nm this molecule displays efficient energy transfer from the two-photon absorbing dye to the nile red moiety, with an 8-fold increase in emission compared to the model compound. Similarly, single-photon excitation of the two-photon absorbing moiety at 405 nm results in >99% energy-transfer efficiency, along with a 3.4-fold increase in nile red emission compared to direct excitation of the nile red chromophore at 540 nm. This system provides an effective way to use IR radiation to excite molecules that, by themselves, have little or no two-photon absorption.

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Polymers containing coumarin and Ru(BpyMe2)3 2+ chromophores were synthesized using a grafting approach and also a copolymerization approach. It was found that the solubility of polymers made from Ru-containing monomers was higher than that observed for comparable polymers obtained by grafting of Ru complexes onto bpy-containing prepolymers. The resulting bichromophoric macromolecules exhibited enhanced absorption and luminescence properties compared to the single Ru(BpyMe2)3 2+ complexes due to a very efficient (>95%) energy transfer between the coumarin donor dyes and the ruthenium chromophores. In addition, a novel system in which two different donor dyes are present together with the Ru complex was synthesized and also found to deliver efficient energy transfer to the ruthenium complexes.