Synthesis and characterization of photoresponsive diphenylaminofluorenechromophore adducts of [60]fullerene

Abstract

A class of acceptor–keto–donor structures as hindered 9,9-di(3,5,5-trimethylhexyl)-2-diphenylaminofluoreno-methano[60]fullerene C₆₀(>DPAF-C₉) and the related bisadducts C₆₀(>DPAF-C₉)₂ and C₆₀(>DPAF-C₂)₂ were synthesized. They are derivatives of multiphoton absorptive C₆₀(>DPAF-C₂) showing enhanced cross-sections of simultaneous two-photon absorption under laser excitation at 800 nm in nanosecond region. Molecular synthesis of these C₆₀–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-1,2-methanofullerene[60]-61-carbonyl)-9,9-dialkyl-2-diphenylaminofluorene C₆₀(>DPAF-C_n) involved cyclopropanation of C₆₀ with a key synthon 7-α-bromoacetyl-9,9-dialkyl-2-diphenylaminofluorene. Synthesis of this synthon was achieved by a three-steps procedure starting from 2-bromofluorene via dialkylation at C₉ of the fluorene ring, attachment of a diphenylamino group at C₂ of dialkylfluorene, and Friedel–Craft acylation of the α-bromoacetyl group at C₇ of diphenylaminofluorene. All C₆₀–DPAF derivatives were fully characterized with the chemical structures confirmed by various spectroscopic analyses and validated by the single-crystal structural analysis data of C₆₀(>DPAF-C₂). Strong solvent-sensitive fluorescence quenching phenomena of C₆₀(>DPAF-C₂), C₆₀(>DPAF-C₉), and C₆₀(>DPAF-C₉)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, CHCl₃, and CS₂) 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 ¹C₆₀*(>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.