Synthesis and characterization of photoresponsive diphenylaminofluorene chromophore 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-Cn) 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-Cn) in the former group of the solvents. On the contrary, energy transfer processes from DPAF-Cn moiety to the fullerene cage are favored in the latter group of the solvents.