Various amphiphilic fullerene derivatives were prepared by functionalization of [5,6]fullerene-C60-Ih (C60) with malonate or bis-malonate derivatives obtained by esterification of the malonic acid mono-esters 5–7. Cyclopropafullerene 10 was obtained by protection of the carboxylic acid function of 6 as a tert-butyl ester, followed by Bingel addition to C60 and a deprotection step (Scheme 2). The preparation of 10 was also attempted directly from the malonic acid mono-ester 6 under Bingel conditions. Surprisingly, the corresponding 3′-iodo-3′H-cyclopropa[1,9][5,6]fullerene-C60-Ih-3′-carboxylate 11 was formed instead of 10 (Scheme 3). The general character of this new reaction was confirmed by the preparation of 15 and 16 from the malonic acid mono-esters 13 and 14, respectively (Scheme 4). All the other amphiphilic fullerene derivatives were prepared by taking advantage of the versatile regioselective reaction developed by Diederich and co-workers which led to macrocyclic bis-adducts of C60 by a cyclization reaction at the C-sphere with bis-malonate derivatives in a double Bingel cyclopropanation. The bis-adducts 37–39 with a carboxylic acid polar head group and four pendant long alkyl chains of different length were prepared from diol 22 and acids 5–7, respectively (Scheme 9). In addition, the amphiphilic fullerene derivatives 45, 46, 49, 54, and 55 bearing different polar head groups and compound 19 with no polar head group were synthesized (Schemes 11–13, 15, and 5, resp.). The ability of all these compounds to form Langmuir monolayers at the air-water interface was investigated in a systematic study. The films at the water surface were characterized by their surface pressure vs. molecular area isotherms, compression and expansion cycles, and Brewster-angle microscopy. The spreading behavior of compound 10 was not good, the two long alkyl chains in 10 being insufficient to prevent aggregation resulting from the strong fullerene-fullerene interactions. While no films could be obtained from compound 19 with no polar head group, all the corresponding amphiphilic fullerene bis-adducts showed good spreading characteristics and reversible behavior upon successive compression/expansion cycles. The encapsulation of the fullerene in a cyclic addend surrounded by four long alkyl chains is, therefore, an efficient strategy to prevent the irreversible aggregation resulting from strong fullerene-fullerene interactions usually observed for amphiphilic C60 derivatives at the air-water interface. The balance of hydrophobicity to hydrophilicity was modulated by changing the length of the surrounding alkyl chains or the nature of the polar head group. The best results in terms of film formation and stability were obtained with the compounds having the largest polar head group, i.e. 45 and 46, and dodecyl chains. Finally, the Langmuir films obtained from the amphiphilic fullerene bis-adducts were transferred onto solid substrates, yielding high-quality Langmuir-Blodgett films.
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Jul 27, 2009
Georgios Rotas + 1
Open Access