Protonation-induced rearrangements in Langmuir films and redox properties of Langmuir–Blodgett films of 2-(n-alkyl)fulleropyrrolidines

The effect of protonation of the pyrrolidine ring nitrogen of 2-(n-alkyl)fulleropyrrolidines, C60pyr–Cm (m=4, 6, 8, 10 and 12), on the properties of the Langmuir and Langmuir–Blodgett (LB) films was investigated. The isotherms of both surface pressure (π) and surface potential change (ΔV) versus area per molecule (A) for the Langmuir films of C60pyr–Cm were determined simultaneously. It was found that the longer the alkyl chain of the fulleropyrrolidine the larger is the film compressibility, κ, i.e., κ(C60pyr–C4)=(2.1±0.4)×10−2 m mN−1, κ(C60pyr–C8)=(3.5±0.4)×10−2 m mN−1 and κ(C60pyr–C12)=(4.1±0.5)×10−2 m mN−1, as expected for the liquid surface films. The values of surface area at zero surface pressure (A1) differ in the range 0.6 nm2 molecule−1 (for m=4–8) to 1.4 nm2 molecule−1 (for m=10–12), indicating that all 2-(n-alkyl)fulleropyrrolidines form multilayer or aggregated films on neutral water subphase. However, acidification of the water subphase increases the A1 values for all investigated fulleropyrrolidines up to ca. 1.9 nm2 molecule−1, i.e., the value corresponding to maximum area occupied by a fulleropyrrolidine at horizontal orientation in a monolayer film. Apparently, 2-(n-alkyl)fulleropyrrolidinium cations formed at low pH are markedly de-aggregated in the films and their orientation is changed due to protonation of the pyrrolidine nitrogen. Similarly, however, less pronounced effects are observed if ionic strength of the subphase solution, I, is increased in the range 0≤I≤1.0 mol dm−3 NaCl. The Langmuir films formed on a water subphase were the most stable with respect to the LB transfer onto 5 MHz Au–quartz crystal vibrators. Simultaneous cyclic voltammetry and piezoelectric microgravimetry at an electrochemical quartz crystal microbalance of these films showed at least two electroreductions where the fulleropyrrolidine mono anions were stable with respect to dissolution.