Ultrafast Photoinduced Charge Separation Leading to High-Energy Radical Ion-Pairs in Directly Linked Corrole-C-60 and Triphenylamine-Corrole-C-60 Donor-Acceptor Conjugates

摘要

Closely positioned donor-acceptor pairs facilitate electron-and energy-transfer events, relevant to light energy conversion. Here, a triad system TPACor-C-60, possessing a free-base corrole as central unit that linked the energy donor triphenylamine (TPA) at the meso position and an electron acceptor fullerene (C-60) at the beta-pyrrole position was newly synthesized, as were the component dyads TPACor and Cor-C-60. Spectroscopic, electrochemical, and DFT studies confirmed the molecular integrity and existence of a moderate level of intramolecular interactions between the components. Steady-state fluorescence studies showed efficient energy transfer from (1)TPA* to the corrole and subsequent electron transfer from (1)corrole* to fullerene. Further studies involving femtosecond and nanosecond laser flash photolysis confirmed electron transfer to be the quenching mechanism of corrole emission, in which the electron-transfer products, the corrole radical cation (Cor(center dot+) in Cor-C-60 and TPA-Cor(center dot+) in TPACor-C-60) and fullerene radical anion (C-60(center dot-)), could be spectrally characterized. Owing to the close proximity of the donor and acceptor entities in the dyad and triad, the rate of charge separation, k(CS), was found to be about 10(11) s(-1), suggesting the occurrence of an ultrafast charge-separation process. Interestingly, although an order of magnitude slower than k(CS), the rate of charge recombination, k(CR), was also found to be rapid (k(CR) approximate to 10(10) s(-1)), and both processes followed the solvent polarity trend DMF > benzonitrile > THF > toluene. The charge-separated species relaxed directly to the ground state in polar solvents while in toluene, formation of (3)corrole* was observed, thus implying that the energy of the charge-separated state in a nonpolar solvent is higher than the energy of (3)corrole* being about 1.52 eV. That is, ultrafast formation of a high-energy charge-separated state in toluene has been achieved in these closely spaced corrole-fullerene donor-acceptor conjugates.