Light-Harvesting Antenna and Proton-Activated Photodynamic Effect of a Novel BODIPY-Fullerene C60 Dyad as Potential Antimicrobial Agent

A covalently linked BODIPY-fullerene C-60 dyad (BDP-C-60) was synthesized as a two-segment structure, which consists of a visible light-harvesting antenna attached to an energy or electron acceptor moiety. This structure was designed to improve the photodynamic action of fullerene C-60 to inactivate bacteria. The absorption spectrum of BDP-C-60 was found to be a superposition of the spectra of its constitutional moieties, whereas the fluorescence emission of the BODIPY unit was strongly quenched by the fullerene C-60. Spectroscopic, calculations, and redox studies indicate a competence between photoinduced energy and electron transfer. Protonating the dimethylaminophenyl substituent through addition of an acidic medium led to a substantial increase in the fluorescence emission, triplet excited state formation, and singlet molecular oxygen production. At physiological pH, photosensitized inactivation of Staphylococcus aureus mediated by 1M BDP-C-60 exhibited a 4.5 log decrease of cell survival (>99.997%) after 15min irradiation. A similar result was obtained with Escherichia coli using 30min irradiation. Moreover, proton-activated photodynamic action of BDP-C-60 turned this dyad into a highly effective photosensitizer to eradicate E. coli. Therefore, BDP-C-60 is an interesting photosensitizing structure in which the light-harvesting antenna effect of the BODIPY unit combined with the protonation of dimethylaminophenyl group can be used to improve the photoinactivation of bacteria.