SOLVENT EFFECTS ON THE ELECTRONIC-SPECTRUM OF C-60

W-vis absorption spectra of C-60 have been measured in 15 different or ganic solvents. There are substantial shifts in lambda(max) values bet ween gas phase spectra, n-hexane, aromatic solvents, and CS2. The solv atochromic shifts of two of the bands due to the symmetry-allowed tran sitions, the HOMO-LUMO Ao transition (1(1)T(1u)-1(1)A(g)) around 405 n m and the C transition (3(1)T(1u)-1(1)A(g)) around 330 nm, have been a nalyzed statistically in terms of a number of solvent parameters. Thes e include the index of refraction, n, polarizability parameters, (n(2) - 1)/(2n(2) + 1) and (n(2) - 1)/(n(2) + 2), polarity parameter, (epsi lon(2) - 1)/(2 epsilon(2) + 1), dielectric parameter, (epsilon - 1)/(e psilon + 2), molecular volume, V, Hildebrand solubility parameter, del ta(H), pi dipolarity/polarizability parameter and its polarizability correction term, delta(pi), and solubility. The general theory that t he energy shift should be mainly dependent on the polarizability of th e solvent is not obeyed; Although polarizability is a major contributo r, parameters related to the polarity of the solvent, such as the pola rity parameter, dielectric parameter, and pi, are also statistically significant in determining the energy shift. By modifying the classica l theory to take account of changes in quadrupole moments during the e lectron transition, good agreement between theoretical and observed so lvatochromism has been established. This indicates that C-60 has a mar ked change in electron distribution upon excitation into the LUMO (and states of like symmetry) and suggests the formation of an axial quadr upole in the excited electronic state. This excited state is preferent ially stabilized by polar solvents and those with a tendency to intera ct through pi-stacking.