Solid-State Organization of Semifluorinated Alkanes Probed by 19F MAS NMR Spectroscopy
Young Joo Lee, Christopher G. Clark, Jr., Robert Graf, Manfred Wagner, Klaus Müllen and Hans Wolfgang Spiess*
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
J. Phys. Chem. B, 2009, 113 (5), pp 1360–1366
Abstract: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 CF3 group was observed at −81.7 ppm for perfluorododecylhexane (F12H6), whereas a CF3 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 CF3 and inner CF2CH2 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.