Light-Harvesting Chromophores with Metalated Porphyrin Cores for Tuned Photosensitization of Singlet Oxygen via Two-Photon Excited FRET

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.