Vectorial charge separation and selective triplet-state formation during charge recombination in a pyrrolyl-bridged BODIPY–fullerene dyad

A donor–acceptor dyad composed of BF2-chelated dipyrromethene (BDP or BODIPY) and fullerene connected with a pyrrole ring spacer, 1, has been newly synthesized and characterized. Because of α-carbon substitution and extended conjugation offered by the pyrrole ring, the optical absorbance and emission spectra of BDP macrocycle were found to be red-shifted significantly. Electrochemical studies provided information on the redox potentials while computational studies performed at the B3LYP/6-31G* level yielded an optimized geometry of the dyad that was close to that reported earlier for a BDP-C60 dyad covalently connected through the central boron atom, 2. The HOMO of the dyad was found to be on the BDP macrocycle, extended over the pyrrole bridging group, a property that is expected to facilitate electronic communication between the BDP and fullerene entities. The established energy level diagram using spectral, redox and optimized structural results predicted possibility of photoinduced electron transfer in both benzonitrile and toluene, representing polar and nonpolar solvents. However, such energy diagram suggested different routes for the charge recombination processes, that is, direct relaxation of the radical ion-pair in polar solvent while populating the triplet level of the sensitizer (3BDP* or 3C60*) in nonpolar solvent. Proof for charge separation and solvent dependent charge recombination processes were established from studies involving femto- and nano-second pump–probe spectroscopy. The measured rate of charge separation, kCS, for 1 was higher in both solvents compared to the earlier reported values for 2 due to electronically well-communicating pyrrole spacer. The charge recombination in toluene populated 3BDP* as an intermediate step while in benzonitrile it yielded directly ground state of the dyad. The present findings reveal the significance of pyrrole spacer between the donor and acceptor to facilitate charge separation and solvent polarity dependent charge recombination processes