We have investigated third-order nonlinear optical properties of charge-tra
nsfer (CT) complexes of C-60 with various aromatic amines with different io
nization potentials. The third-order nonlinear optical susceptibility \chi(
(3))\ measured by degenerate four-wave mixing shows a resonant enhancement
at the absorption band due to the CT transition. We have found that the sec
ond hyperpolarizability gamma of CT complexes increases with an increase of
the oscillator strength, which depends on the ionization potential. The hi
ghest hyperpolarizability among CT complexes studied here is 5.2 x 10(-28)
esu for C-60/aniline CT complex.