Efficient Photoinduced Charge Separation in a BODIPY-C-60 Dyad

A donor acceptor dyad composed of a BF2-chelated dipyrromethene (BODIPY) and a C-60 fullerene has been newly synthesized and characterized. The two moieties are linked by direct addition of an azido substituted BODIPY on the C-60, producing an imino fullerene BODIPY adduct. The photoinduced charge transfer process in this system was studied by ultrafast transient absorption spectroscopy. Electron transfer toward the fullerene was found to occur selectively exciting both the BODIPY chromophore at 475 nm and the C-60 unit at 266 nm on a time scale of a few picoseconds, but the dynamics of charge separation was different in the two cases. Eletrochemical studies provided information on the redox potentials of the involved species and spectroelectrochemical measurements allowed to unambiguously assign the absorption band of the oxidized BODIPY moiety, which helped in the interpretation of the transient absorption spectra. The experimental studies were complemented by a theoretical analysis based on DFT computations of the excited state energies of the two components and their electronic couplings, which allowed identification of the charge transfer mechanism and rationalization of the different kinetic behavior observed by changing the excitation conditions.