Efficient Photoinduced Charge Separation in a BODIPY–C60 Dyad

A donor-acceptor dyad composed by a BF2-chelated dipyrromethene (BODIPY) and a C60 fullerene has been newly synthetized and characterized. The two moieties are linked by direct addition of an azido substituted BODIPY on the C60, producing an immino-fullerene-BODIPY adduct. The photoinduced charge-transfer process in this system was studied by ultrafast transient absorption spectroscopy. Electron transfer towards the fullerene was found to occur both selectively exciting the BODIPY chromophore at 475 nm and the C60 unit at 266 nm on a few picosecond timescale, 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 oxidised 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 to identify the charge transfer mechanism and rationalize the different kinetic behaviour observed by changing the excitation conditions.