Sterically hindered fullerenyl chromophore dyad and triads, C 60 (>DPAF-C 9 ) x (x ) 1 and 2, respectively), in an acceptor-donor (A-D) molecular linkage of C 60 -(keto-fluorene) x were synthesized and fully characterized. Attachment of two 3,5,5-trimethylhexyl groups on C 9 of the fluorene ring moiety greatly improves their solubility and makes direct intermolecular aromatic stacking contacts more difficult. They are the first series of fullerene derivatives showing high three-photon absorptivity (3PA). Accordingly, C 60 (>DPAF-C 9 ) 2 exhibits 2PA and 3PA cross sections in the values of 0.824 × 10 -48 cm 4 s (or 82.4 GM) and 6.30 × 10 -25 cm 6 /GW 2 , respectively, in femtosecond region among the highest ones reported for many diphenylaminofluorene-derived AFX chromophores. Utilization of a keto linker located immediately between C 60 cage and fluorene chromophore moieties facilitates molecular polarization of the DPAF ring toward the C 60 cage. That may serve as the fundamental cause for correlation of enhanced A-D electron interactions to, ultimately, observed multiphoton absorption cross sections. By using nanosecond laser flash photolysis results taken at 355 nm as the reference, transient absorption data obtained from femtosecond pump-probe experiments at 800 nm unambiguously verified the occurrence of two-photon excitation processes of C 60 (>DPAF-C 9 ) in air-saturated benzene and subsequent efficient energy transfer from the two-photon pumped DPAF-C 9 moiety to the C 60 cage moiety.

A class of acceptor-keto-donor structures as hindered 9,9-di(3,5,5-trimethylhexyl)-2diphenylaminofluoreno-methano [60]fullerene C 60 (>DPAF-C 9 ) and the related bisadducts C 60 (>DPAF-C 9 ) 2 and C 60 (>DPAF-C 2 ) 2 were synthesized. They are derivatives of multiphoton absorptive C 60 (>DPAF-C 2 ) showing enhanced cross-sections of simultaneous two-photon absorption under laser excitation at 800 nm in nanosecond region. Molecular synthesis of these C 60 -DPAF conjugates involved the covalent attachment of a diphenylaminofluorene moiety to methano[60]fullerene via a keto linkage for increasing molecular acceptor-donor polarization of the chromophore in conjunction with the fullerene cage. Preparation of 7-(1,2-dihydro-1,2methanofullerene[60]-61-carbonyl)-9,9-dialkyl-2-diphenylaminofluorene C 60 (>DPAF-C n ) involved cyclopropanation of C 60 with a key synthon 7-a-bromoacetyl-9,9-dialkyl-2diphenylaminofluorene. Synthesis of this synthon was achieved by a three-steps procedure starting from 2-bromofluorene via dialkylation at C 9 of the fluorene ring, attachment of a diphenylamino group at C 2 of dialkylfluorene, and Friedel-Craft acylation of the a-bromoacetyl group at C 7 of diphenylaminofluorene. All C 60 -DPAF derivatives were fully characterized with the chemical structures confirmed by various spectroscopic analyses and validated by the single-crystal structural analysis data of C 60 (>DPAF-C 2 ). Strong solvent-sensitive fluorescence quenching phenomena of C 60 (>DPAF-C 2 ), C 60 (>DPAF-C 9 ), and C 60 (>DPAF-C 9 ) 2 were noticed, showing no fluorescence band above 700 nm in more polar solvents, such as DMF, PhCN, and THF, while in less polar solvents (toluene, CHCl 3 , and CS 2 ) a fullerenyl fluorescence band at 700-710 nm was observed. It was attributed to the occurrence of electron transfer via the singlet excited state of the fullerene moiety 1 C 60 *(>DPAF-C n ) in the former group of the solvents. On the contrary, energy transfer processes from DPAF-C n moiety to the fullerene cage are favored in the latter group of the solvents.

Synthesis of emerald green fullerenes (EF) C60[C(CH3)(CO2Et)2]6 and C60[C(CH3)(CO2-t-Bu)2]6 was performed by using hexaanionic C60 intermediate (C60-6) as a reagent in one-pot reaction for attaching six alkyl ester addends on one C60 cage. These EF compounds exhibit intense optical absorption over 600-940 nm, the longest optical absorption of the C60 cage among many [60]fullerene derivatives synthesized.