Unexpected Disparity in Photoinduced Reactions of C60 and C70 in Water with the Generation of O2•- or 1O2

Well-defined fullerene-PEG conjugates, C-60-PEG (1) and two C-70-PEG (2 and 3 with the addition sites on ab-[6,6] and cc-[6,6]-junctions), were prepared from their corresponding Prato monoadduct precursors. The resulting highly water-soluble fullerene-PEG conjugates 1-3 were evaluated for their DNA-cleaving activities and reactive oxygen species (ROS) generation under visible light irradiation. Unexpectedly, photoinduced cleavage of DNA by C-60-PEG 1 was much higher than that by C-70-PEG 2 and 3 with higher absorption intensity, especially in the presence of an electron donor (NADH). The preference of photoinduced ROS generation from fullerene-PEG conjugates 1-3 via the type II (energy transfer) or the type I (electron transfer) photoreaction was found to be dependent on the fullerene core (between C-60 and C-70) and functionalization pattern of C-70 (between 2 and 3). This was clearly supported by the electron transfer rate obtained from cyclic voltammetry data and computationally estimated relative rate of each step of the type II and the type I reactions, with the finding that type II energy transfer reactions occurred in the inverted Marcus regime while type I electron transfer reactions proceeded in the normal Marcus regime. This finding on the disparity in the pathways of photoinduced reactions (type I versus type II) provides insights into the behavior of photosensitizers in water and the design of photodynamic therapy drugs.

Automated summary

The study found that photoinduced cleavage of DNA by C-60-PEG was higher than that by C-70-PEG under visible light irradiation, especially in the presence of an electron donor. The preference of photoinduced ROS generation from fullerene-PEG conjugates was dependent on the fullerene core and functionalization pattern of C-70, with different pathways (type I versus type II) determined by electron transfer rates and computationally estimated reaction rates.