SOLVENT EFFECTS ON THE ELECTRONIC-SPECTRUM OF C-60

W-vis absorption spectra of C-60 have been measured in 15 different organic solvents. There are substantial shifts in lambda(max) values between gas phase spectra, n-hexane, aromatic solvents, and CS2. The solvatochromic shifts of two of the bands due to the symmetry-allowed transitions, the HOMO-LUMO Ao transition (1(1)T(1u)-1(1)A(g)) around 405 nm and the C transition (3(1)T(1u)-1(1)A(g)) around 330 nm, have been analyzed statistically in terms of a number of solvent parameters. These include the index of refraction, n, polarizability parameters, (n(2) - 1)/(2n(2) + 1) and (n(2) - 1)/(n(2) + 2), polarity parameter, (epsilon(2) - 1)/(2 epsilon(2) + 1), dielectric parameter, (epsilon - 1)/(epsilon + 2), molecular volume, V, Hildebrand solubility parameter, delta(H), pi* dipolarity/polarizability parameter and its polarizability correction term, delta(pi)*, and solubility. The general theory that the energy shift should be mainly dependent on the polarizability of the solvent is not obeyed; Although polarizability is a major contributor, parameters related to the polarity of the solvent, such as the polarity parameter, dielectric parameter, and pi*, are also statistically significant in determining the energy shift. By modifying the classical theory to take account of changes in quadrupole moments during the electron transition, good agreement between theoretical and observed solvatochromism has been established. This indicates that C-60 has a marked change in electron distribution upon excitation into the LUMO (and states of like symmetry) and suggests the formation of an axial quadrupole in the excited electronic state. This excited state is preferentially stabilized by polar solvents and those with a tendency to interact through pi-stacking.