Photophysical properties of the ReI and RuII complexes of a new C60-substituted bipyridine ligand

The rhenium(I) and ruthenium(II) complexes of a fullerene-substituted bipyridine ligand have been prepared. Electrochemical studies indicate that some ground state electronic interaction between the fullerene subunit and the metal-complexed moiety are present in the Re(I) but not the Ru(II) complex. The photophysical properties have been investigated by steady-state and time-resolved UV/Vis-NIR luminescence spectroscopy and nanosecond laser flash photolysis in CH2Cl2 solution, and compared to those of the corresponding model compounds. Excitation of the methanofullerene moiety in the dyads does not lead to excited state intercomponent interactions. Instead, excitation of the metal-complexed unit shows that the lowest triplet metal-to-ligand-charge-transfer excited state ((3)MLCT) centered on the Re(I)- or Ru(II)-type unit is quenched with a rate constant of about 2.5 x 10(8) s(-1). The quenching is attributed to an electron-transfer (ElT) process leading to the reduction of the carbon sphere, as determined by luminescence spectroscopy for the Ru(II) dyad. Experimental detection of electron transfer in the Re(I) dyad is prevented due to the unfavorable absorption of the metal-complexed moiety relative to the fullerene unit. However, it can be postulated on the basis of energetic/kinetic arguments and by comparison with the Ru(II)-type array. The primary ElT process is followed by charge-recombination to give the lowest-lying fullerene triplet excited state (3C60) with quantitative yield, as determined by sensitized singlet oxygen luminescence experiments. Direct (3)MLCT-->3C60 triplet-triplet energy-transfer (EnT) does not successfully compete with ElT since it is highly exoergonic and located in the Marcus inverted region. The quantum yield of singlet oxygen sensitization (Phi(delta)) of the Re(I)-based dyad is found to be lower (0.80) than for the corresponding Ru(II) derivative (1.0). This is likely to be the consequence of different conformational structures for the two dyads, rather than a different yield of 3C60 formation.