Photoinduced electron transfer in fullerene triads bearing pyrene and fluorene

Photochemical properties of pyrene and fluorene appended fulleropyrrolidine triads (AH1-C60-AH2; AH1 = pyrene and fluorene; AH2 = naphthalene and phenyl) are reported. Electrochemical studies using cyclic voltammetry technique and DFT calculations at B3LYP/3-21G(∗) method revealed that the charge-separated states in pyrene and fluorene appended triads are pyrene+-C60--AH2 and fluorene+-C60--AH2, respectively; however, no such charge-separated states could be established for naphthalene and phenyl appended triads. As demonstrated from the time resolved fluorescence, upon excitation of AH moiety in nonpolar solvents, energy transfer predominantly occurred from the singlet excited fluorophore to the C60 moiety, whereas in polar DMF charge-separation also contributed to the fluorescence quenching. Additionally, charge separation also occurred from the singlet excited C60 to the pyrene or fluorene entities of the triads in DMF. The rates and quantum yields of charge separation obtained by time-resolved emission studies were around 109 s−1 and 0.9-0.6 for pyrene-C60-AH2 and fluorene-C60-AH2 triads. Nanosecond transient absorption spectral studies performed by using 355 nm laser light on the triads, exhibited transient bands corresponding to the C60- and pyrene+ or fluorene+, thus establishing the occurrence of electron transfer in these triads in DMF. The rates of charge recombination obtained by monitoring the decay of the C60- were found to be around 106 s−1 in DMF which resulted in the lifetimes of the radical ion pairs up to 1000 ns indicating charge stabilization in pyrene-C60-AH2 and fluorene-C60-AH2 triads. The formations of long-lived charge-separated states, pyrene+-C60--AH2 and fluorene+-C60--AH2 in DMF, were rationalized by evaluating the Marcus parameters from the temperature dependence of the charge-recombination rate constants.