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

Abstract

Well-defined fullerene-PEG conjugates, C60-PEG (1) and two C70-PEG (2 and 3 with the addition sites on ab-[6,6] and cc-[6,6]-junctions), were prepared from their corresponding Prato mono-adduct 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 C60-PEG 1 was much higher than C70-PEG 2-3 with higher absorption intensity, especially in the presence of electron donor (NADH) in line with the observed ROS generated via the type I electron transfer pathway. The preference of photoinduced ROS generation from fullerene-PEG conjugates between the type II (energy transfer) and the type I (electron transfer) was found to be difference dependent on the fullerene core (between C60 and C70 )cores and functionalization patterns of C70 (between 2 and 3). This was clearly supported by the electron transfer rate obtained from cyclic voltammetry data and computationally calculated relative rate of each step of the type II and the type I reac-tions, 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 new finding on disparity in the pathways of photoinduced reaction (type I versus type II) provide a new insight in the behaviors of photosensitizers in water and in the design of photodynamic therapy drugs