Solid-state organization of semifluorinated alkanes probed by 19F MAS NMR spectroscopy

Bulk-phase self-assembly of a series of semifluorinated alkanes (SFAs) with hydrocarbon chains of varying length has been investigated by 19F NMR spectroscopy. At room temperature, a single 19F resonance for the terminal sCF3 group was observed at -81.7 ppm for perfluorododecylhexane (F12H6), whereas a sCF3 resonance was seen at -82.5 ppm for perfluorododecyldodecane (F12H12) and perfluorododecyleicosane (F12H20). This difference in chemical shift position is ascribed to the different molecular packing geometries, i.e., a monolayer lamellar structure for F12H6 vs a bilayer lamellar organization for F12H12 and F12H20. Moreover, in F12H12, a solid-solid phase transition from bilayer to monolayer lamellae can be followed by 19F NMR spectroscopy. 1H/19F-->13C CPMAS experiments indicated that the phase transition is accompanied by disordering of hydrocarbon chains, but does not involve a significant conformational change in the fluorocarbon chains. Yet, a change in the 19F T1 relaxation times was found to occur at the phase transition temperature, suggesting a change in the packing environments of the fluorocarbon chains. Two-dimensional exchange NMR experiments yielded cross-peaks between terminal sCF3 and inner sCF2CH2s moieties for the high-temperature monolayer phase, providing clear evidence for the spatial proximity between these groups. On the basis of these findings, we propose a model for the phase transition involving bilayer lamellae and monolayer lamellae with hydrocarbon and fluorocarbon interdigitation.