Peptide anchored Langmuir–Blodgett films of a fullerene amphiphile

A new amphiphilic derivative of fullerene C60 bearing an oligoglycyl tail (C60CHCOgly2OEt, 2) formed stable Langmuir floating films at the air–water interface. This occurred when the molecular assembly was stabilized by anchoring the amphiphilic C60ʹs to the aqueous subphase, via hydrogen bonding interactions between a dipeptide (Gly–L–Leu) dissolved in the water subphase, and the oligoglycyl chain. The compression (π−A) isotherm of the Langmuir floating film constructed in such a way showed no hysteresis, was steep, and evidenced that the monolayer collapsed at a surface pressure π⩾65 mN m−1, thus confirming that the film was tightly packed, extremely stable, and rigid. A limiting area per molecule of 89.1 Å2 was extrapolated, in agreement with the calculated cross-section area of the C60 fullerene. On the contrary, when the dipeptide was absent and pure water was used as the subphase, the π−A isotherm yielded a limiting area <55 Å2 which indicated the formation of multiple layers; moreover it showed significant hysteresis, the film was fragile, and it collapsed at π≈50 mN m−1. Once anchored by the dipeptide, the floating monolayer of 2 could be transferred onto hydrophobic quartz, glass and silicon substrates, by successive vertical dipping cycles, each cycle made up of two down-strokes and two up-strokes, to yield the Langmuir–Blodgett film. Up to 200 down- and up-strokes could be repeated reproducibly, a noteworthy result for non-covalently assembled LB films of fullerenes. The transfer ratio was 1.0, except for the second down-stroke of each cycle that gave a transfer ratio of zero, making the sequence of successful transfers: D, U, U, (cleaning and spreading), D, U, U, (cleaning and spreading), and so on (D=down-stroke, U=up-stroke). The total number of deposited layers was therefore 150. X-ray diffraction spectra were registered and exhibited a peak, which was fitted by a Montecarlo method of simulation to obtain the distribution of the repeat unit responsible for scattering; such distribution, with thickness between 20 and 60 Å, was consistent with the size of the amphiphile and the transfer sequence. The UV–Vis spectra of the LB film exhibited the characteristic C60 bands, and the absorption peaks in the 200–400 nm range were proportional to the number of layers, indicating that the deposition was reproducible and that the molecular environment of C60 in each layer remained constant.