The resonant nonlinear optical (NLO) properties and excited state dynamics of pristine, chemically and photochemically modified C60 are studied at 590.5 nm by time-resolved degenerate four-wave mixing and nonlinear transmission. The chemically modified fullerenes consist of (a) composite films of C60 with meso-tetraphenyl porphine (TPP) prepared by vacuum vapor deposition, and (b) spin-coated films of solutions of C60 mixed with the conjugated polymer, poly-[2-methoxy,5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV). The photochemically modified fullerene, poly-C60, is a distribution of oligomers of single C60 molecules proposed to be linked together through four-membered rings. Photopolymerization of C60 leads to an enhancement of its physical properties without substantial alteration of its NLO properties. C60 and TPP form a ground state charge transfer complex with an enhanced NLO response. C60 and MEH-PPV form a charge transfer complex in the excited state which gives rise to more than an order of magnitude increase in the NLO response and the figure of merit. The dynamics of the NLO response for C60, poly-C60, C60:TPP, and C60:MEH-PPV films show strong fluence dependence. Bimolecular decay via exciton-exciton annihilation dominates the temporal response at high-fluence.