Efficient Singlet Oxygen Photogeneration by Zinc Porphyrin Dimers upon One- and Two-Photon Excitation.

The development of photodynamic therapy (PDT) at depth requires photosensitizers which have both sufficient quantum yield for singlet oxygen generation and strong two-photon absorption. Here, we show that this can be achieved by conjugated linkage of zinc porphyrins to make dimers. We determined the quantum yield of generation of 1O2 , ϕΔ, by measuring emission at 1270 nm using a near-infrared streak camera and found it to increase from 15% for a single porphyrin unit to 27-47% for the dimers with a conjugated linker. Then, we recorded the spectra of two-photon absorption cross section, σ2, by a focus-tunable Z-scan method, which allows for nondestructive investigation of light-sensitive materials. We observed a strong enhancement of the two-photon absorption coefficient in the dimers, especially those with an alkyne linker. These results lead to an excellent figure of merit for two-photon production of singlet oxygen (expressed by the product σ2 × ϕΔ) in the porphyrin dimers, of around 3700 GM, which is very promising for applications involving treatment of deep tumors by PDT.