Study of charge transfer complexes of [70]fullerene with phenol and substituted phenols

To improve the understanding of the charge transfer (CT) interaction of [70]fullerene with electron donors, interaction of [70]fullerene with a series of phenols, e.g., phenol, resorcinol and p-quinol were studied in 1,4-dioxan medium using absorption spectroscopy. An absorption band due to CT transition was observed in the visible region. The experimental CT transition energies (h nuCT) are well correlated (through Mulliken's equation) with the vertical ionisation potentials (I(D)v) of the series of phenols studied. From an analysis of this correlation degrees of charge transfer for the [70]fullerene-phenol complexes were estimated. The degrees of charge transfer in the ground state of the complexes have been found to be very low (<2%). The h nuCT values change systematically as the number and position of the -OH groups change on the aromatic ring of the phenol moiety. From the trends in the h nuCT values, the Hückel parameters (h(O) and k(C-O)) for the -OH group were obtained in a straightforward way and the values so obtained, viz., 1.91 and 1.0, respectively, are close to the ones (1.8 and 0.8) recommended by Streitwieser on the basis of other evidence. Oscillator strengths, transition dipole strengths and resonance energies of the [70]fullerene-phenol complexes were determined. Formation constants of the CT complexes were determined at four different temperatures from which enthalpies and entropies of formation of the complexes were estimated.