Structure-dependent photoinduced electron transfer in fullerodendrimers with light-harvesting oligophenylenevinylene terminals

Oligophenylenevinylene (OPV) -terminated phenylenevinylene dendrons G1-G4 with one, two, four, and eight "side-arms", respectively, were prepared and attached to C-60 by a 1,3-dipolar cycloaddition of azomethine ylides generated in situ from dendritic aldehydes and N-methylglycine. The relative electronic absorption of the OPV moiety increases progressively along the fullerodendrimer family C(60)G1-C(60)G4, reaching a 99:1 ratio for C(60)G4 (antenna effect). UV/Vis and near-IR luminescence and transient absorption spectroscopy was used to elucidate photoinduced energy and electron transfer in C(60)G1-C(60)G4 as a function of OPV moiety size and solvent polarity (toluene, dichloromethane, benzonitrile), taking into account the fact that the free-energy change for electron transfer is the same along the series owing to the invariability of the donor-acceptor couple. Regardless of solvent, all the fullerodendrimers exhibit ultrafast OPV --> C-60 singlet energy transfer. In CH2Cl2, the OPV-->C-60 electron transfer from the lowest fullerene singlet level (C-1(60)*) is slightly exergonic (Delta G(CS) approximate to 0.07 eV), but is observed, to an increasing extent, only in the largest systems C(60)G2-C(60)G4 with lower activation barriers for electron transfer. This effect has been related to a decrease of the reorganization energy upon enlargement of the molecular architecture. Structural factors are also at the origin of an unprecedented OPV --> C-60 electron transfer observed for C(60)G3 and C(60)G4 in apolar toluene, whereas in benzonitrile, electron transfer occurs in all cases. Monitoring of the lowest fullerene triplet state by sensitized singlet oxygen luminescence and transient absorption spectroscopy shows that this level is populated through intersystem crossing and is not involved in photoinduced electron transfer.