Solid-State Organization of Semifluorinated Alkanes Probed by 19F

Jan 13, 2009 - Bulk-phase self-assembly of a series of semifluorinated alkanes (SFAs) with hydrocarbon chains of varying length has been investigated ...
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J. Phys. Chem. B 2009, 113, 1360–1366

Solid-State Organization of Semifluorinated Alkanes Probed by 19F MAS NMR Spectroscopy Young Joo Lee, Christopher G. Clark, Jr., Robert Graf, Manfred Wagner, Klaus Mu¨llen, and Hans Wolfgang Spiess* Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ReceiVed: September 22, 2008; ReVised Manuscript ReceiVed: December 16, 2008

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 19 F NMR spectroscopy. 1H/19F f 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. 1. Introduction Semifluorinate alkanes (SFAs) with chemical structure F(CF2)m(CH2)nH are considered as diblock molecules that are made up of two immiscible parts, fluorocarbon and hydrocarbon segments. Because of the incompatibility between the two moieties, SFAs often exhibit remarkable phenomena such as solid-solid phase transitions and supramolecular assemblies in both the solid and liquid states.1-3 In particular, F(CF2)12(CH2)nH with hydrocarbon chains of varying length, denoted as F12Hn, have been extensively studied, displaying various morphologies and solid-solid phase transitions as a function of the length of the hydrocarbon chain.1,2,4 These molecules can be categorized into three groups. (1) F12Hn species with short hydrocarbon chains (2 e n e 6) yield a single reflection in small-angle X-ray scattering (SAXS). Below the transition temperature, the d spacing is slightly smaller than the length of the molecule, whereas above the phase transition temperature, the d spacing becomes equal to the length of the fully extended molecule. Thus, a monolayer lamellar structure that is tilted with respect to the surface normal below transition temperature and perpendicular above the transition temperature has been proposed. (2) For hydrocarbon and fluorocarbon chains of similar lengths (8 e n e 14), the scattering profile shows a complicated pattern with a d spacing larger than the length of molecule below the transition temperature and a single reflection with a d value equal to the molecular length above the transition temperature. There has been some controversy regarding the packing structure of the low-temperature phase. Russell et al. suggested that tilted bilayer lamellae are formed at low temperature and that the translation of the molecules along their axes gives rise to an * Corresponding author. E-mail: [email protected].

antiparallel monolayer packing at high temperature.1,2 Lang et al. proposed double-layered lamellae with interdigitating hydrocarbon chains for the low-temperature phase.5 (3) For F12Hn with long hydrocarbon chains (14 e n e 20), a single reflection peak is observed without a significant change as a function of temperature, and a bilayer lamellar structure is suggested. Two different models involving cylindrical and ribbon-type network models have been proposed for the microstructures of these molecules.6,7 Much more complex structures can be generated using SFA-substituted side chains in block copolymers.8 Examinations of the bulk-phase assembly of SFAs have conventionally been carried out by small- and wide-angle X-ray scattering and Raman spectroscopy. However, the molecular organization of SFAs is far from being completely solved, and further independent experimental studies are necessary. Solidstate NMR spectroscopy enables structural determination taking advantage of the sensitivity of NMR spectroscopy to the local environment without requiring long-range order. In particular, 19 F NMR spectroscopy has several advantages, including a good resolution due to widely dispersed chemical shifts and a high sensitivity resulting from a high magnetogyric ratio as well as a high natural abundance. Moreover, 19F NMR chemical shift values are sensitive to the local environment, a fact that has been widely exploited in studies of micelles and surfactants in various solvents.9-11 Detailed 19F NMR studies on the local dynamics12 of fluorinated polymers in the bulk, as well as on their degradation under UV irradiation13 and electron beam treatment, have been performed.14 However, the effects of the local environment on the chemical shielding of 19F NMR in the solid state of SFAs are rarely investigated. Hence, the aim of this investigation was to determine the feasibility of using 19 F MAS NMR spectroscopy to elucidate the morphology of

10.1021/jp808406e CCC: $40.75  2009 American Chemical Society Published on Web 01/13/2009

Solid-State Organization of Semifluorinated Alkanes

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TABLE 1: Thermal Characteristics and Molecular Packing of Semifluorinated Alkanes (F12Hn) SFA

Tm Ttr (°C)a (°C)b

F12H6 44 F12H12 80 F12H20 nod

79 89 100

structure monolayer lamellae: tilted f perpendicularc bilayer lamellae f monolayer lamellae bilayer lamellae

a Solid-solid phase transition temperature. b Melting temperature. Structural change can be accomplished by translation of the molecules along the molecular axis. This results in the structural change from monolayer lamellae with a tilt angle to lamellae perpendicular to the surface normal. d No transition observed. c

semifluorinated compounds and to explore structural and dynamic properties accounting for the molecular packing assembly. In this context, three SFAs representative of the three packing arrangement mentioned above, perfluorododecylhexane (F12H6), perfluorododecyldodecane (F12H12), and perfluorododecyleicosane (F12H20) (see Table 1) were investigated using 19 F and 19F/1H f 13C CPMAS NMR spectroscopy. This approach offered an opportunity to correlate the respective temperature-dependent packing motifs based on X-ray scattering results with 19F chemical shifts. It was found that NMR experiments not only resolved the ambiguities of the proposed structures, but also facilitated the exploration of the dynamic processes that are inherently responsible for the molecular packing rearrangements at elevated temperature. As a result, a clear mechanistic picture of the temperature-induced changes in the assembly behavior is presented. 2. Experimental Section 2.1. Materials. Perfluorododecylhexane (F12H6), perfluorododecyldodecane (F12H12), and perfluorododecyleicosane (F12H20) were synthesized by addition of perfluoroalkyl iodide to a terminal olefin following the methods in ref 1. 2.2. Solid-State NMR Spectroscopy. 19F nuclei suffer from a large chemical shielding anisotropy and strong homo- and heteronuclear dipolar couplings, which result in a severe line broadening, especially for solid-state NMR spectroscopy. However, recent advances in fast magic angle spinning (MAS) and multipulse sequences can effectively reduce line broadening, allowing a high-resolution solid-state NMR spectrum to be acquired.12,15-18 19F MAS NMR experiments were performed at 470.54 MHz on a Bruker DSX500 spectrometer, equipped with a 2.5-mm fast MAS probe. All 19F MAS NMR spectra were acquired at spinning frequencies of 30 kHz and with a 90° pulse length of 2.5 µs. 19F two-dimensional (2D) exchange spectra were acquired with various mixing times using either a NOESY- (nuclear Overhauser enhancement spectroscopy-) type pulse sequence or the radio-frequency-driven dipolar recoupling (RFDR) scheme. 1H f 13C and 19F f 13C cross-polarization (CP) MAS NMR experiments were performed at an operating frequency of 125.77 MHz on a Bruker DSX500 spectrometer. Two-pulse phase modulation (TPPM) was used for 1H decoupling, and rotor-synchronized 180° pulses with xy-16 phase cycling were used for 19F decoupling.16 Two-dimensional 19 F-13C heteronuclear correlation (HETCOR) spectra were acquired using the recoupled polarization transfer-heteronuclear single quantum correlation (REPT-HSQC) sequence19 combined with rotor-synchronized 180° pulses with xy-16 phase cycling for 19F decoupling during acquisition. The recoupling time was one rotor period (30 kHz) with equal excitation and reconversion periods. T1 relaxation times were measured using

Figure 1. (a) 19F MAS NMR spectra of various SFAs acquired at 55 °C for F12H20 and F12H12 and 40 °C for F12H6 with a spinning frequency of 30 kHz in each case. (b) Schematic representation of molecular packing structure of monolayer lamellae and bilayer lamellae and their local sCF3 environments. Fluorocarbon and hydrocarbon chains are denoted in yellow and green, respectively. The local environment of the terminal sCF3 group is marked with a red circle.

the inversion-recovery method at various temperatures. The sample temperature was calibrated using Pb(NO3)2 taking into account frictional heat caused by the high spinning frequency.20 The real (calibrated) temperatures are given in the figures and throughout the text. 3. Results and Discussion 3.1. 19F MAS NMR Spectra of Various SFAs. To investigate the effects of the packing motif on the 19F NMR chemical shifts, 19F MAS NMR spectra of various SFAs were recorded (Figure 1a). 19F MAS NMR spectra of F12H6, F12H12, and F12H20 exhibit large upfield shifts from reference CFCl3 at 0 ppm. The wide chemical shift range of 19F NMR spectroscopy yields distinctive resonances for different chemical groups. The resonance at the highest frequency is assigned to the terminal sCF3 group (approximately -82 ppm,) and the dominant resonance in the lower-frequency region is attributed to the sCF2s groups in the backbone (-122 ppm). The resonances on the higher-frequency and lower-frequency sides of the main sCF2s backbone signal are assigned to the sCF2s unit adjacent to the sCH2s units (-115 ppm, sCF2CH2s) and

1362 J. Phys. Chem. B, Vol. 113, No. 5, 2009 the sCF2s unit adjacent to the terminal sCF3 group (-127.0 ppm, sCF2CF3), respectively. Most intriguing is that the terminal sCF3 group and sCF2CF3 group exhibit differences in chemical shift position for different SFAs. These groups give rise to resonances at higher frequencies (by 0.7 ppm) for F12H6 than for F12H12 and F12H20. In contrast, no noticeable variation in the chemical shift position depending on the hydrocarbon chain length was observed for the CF2 backbone. The sensitivity of the 19F chemical shift of terminal sCF3 groups to the local environments has been observed in various systems, including micelles and aggregates of surfactants in solution and self-assembled monolayers.9,10,17 For example, a 19 F NMR signal was observed at -84 ppm for sCF3 groups in fluorocarbon aggregates, whereas a resonance at -82 ppm was observed for the sCF3 groups when the polymers were unaggregated and dissolved in water. Given the trends of 19F chemical shifts in solution, we assume that the 19F chemical shifts of solid materials must be affected by the local environments, which depend on the crystal structure, morphology, packing geometry, self-assembly, and so on. Terminal sCF3 groups in bilayer lamellae are surrounded by the fluorine atoms of neighboring fluorocarbon chains, whereas terminal sCF3 in monolayer lamellae reside in a local chemical environment containing both hydrocarbon and fluorocarbon moieties, as shown in Figure 1b. Thus, the magnetic shielding of fluorine nuclei is increased for bilayer lamellae, resulting in a decrease in chemical shift. However, fluorine atoms in inner fluorocarbon chains are not influenced by the different morphologies, rendering their chemical shifts insensitive to the change in the packing scheme. Our assumption is supported by observations from solution NMR spectra in various solvent mixtures as shown in the Supporting Information. The 19F NMR signal of a terminal sCF3 group shifts to lower frequency when the mixture contains a higher concentration of fluorocarbon. This is consistent with the trend observed for various SFAs. The 19F NMR spectra shown in Figure 1a were acquired below the transition temperatures of the SFAs. Under these experimental conditions, F12H6 exhibits a monolayer lamellar structure, whereas F12H12 and F12H20 form bilayer lamellae. Terminal sCF3 groups give rise to a signal at lower frequency for bilayers (-82.5 ppm for F12H12 and F12H20) than for a monolayer lamellar structure (-81.8 ppm for F12H6). Again, a 19F NMR resonance of the sCF2s unit adjacent to the terminal sCF3 group (sCF2CF3) is observed at lower frequency for bilayer lamellae (-127.0 ppm for F12H12 and F12H20) than for the monolayer structure in F12H6 (-126.5 ppm). It is noteworthy that additional minor resonances are also observed at -81.7 and -126.4 ppm for F12H12. This is due to the presence of mixed phases in F12H12. As discussed in the Introduction, for F12H12, various phases with similar energies exist. Thus, small amounts of the monolayer structure can be trapped during the cooling procedure and cocrystallize with the bilayer phase, which leads to the minor resonances. Overall, our experimental results show that 19F NMR spectroscopy is a useful probe for elucidating the packing assembly of various fluorinated compounds. 3.2. Variable-Temperature 19F MAS NMR Spectra of Various SFAs. 19F NMR spectra were acquired at various temperatures in order to follow the structural changes that occur as a result of the solid-solid phase transition. Considering F12H20 initially, no significant change in the spectral pattern was observed in the entire temperature range, as shown in Figure 2. The behavior of F12H6 was similar: no noticeable change was observed by NMR spectroscopy except for motional narrowing at high temperatures. This can be explained by the

Lee et al.

Figure 2. Variable-temperature 19F MAS NMR spectra of F12H20 acquired with a spinning frequency of 30 kHz. The temperature was calibrated taking into account the frictional heat induced by fast spinning.

local environments based on monolayer lamellae and bilayer lamellae. No solid-solid phase transition was found for F12H20 in thermal analysis and XRD measurements.1,2 A single 19F resonance over the whole temperature range is in agreement with single-phase behavior of F12H20. In the case of F12H6, several packing geometries exist, including both parallel and antiparallel packing, which can produce the scattering profile of monolayer lamellae (Figure 3).2 Among the possible geometries, a parallel packing geometry was suggested as the energetically most favorable structure according to semiemprical energy calculations. However, all of these structures provide similar local environments surrounding the terminal sCF3 groups, suggesting that the structural changes related to parallel vs antiparallel packing and tilted vs perpendicular orientation cannot be clearly distinguished by NMR spectroscopy. Therefore, no change in the NMR peak position is expected as a function of temperature. The experimental results for F12H6 are consistent with this assumption. In contrast, significant changes in the NMR spectra as a function of temperature that can be assigned to structural changes were observed for F12H12 (Figure 4). As the temperature increased, additional resonances of sCF3 and sCF2CF3 groups started to appear at -81.6 and -126.2 ppm and gradually increased in intensity at the expense of the original sCF3 (-82.4 ppm) and sCF2CF3 (-127.0 ppm) resonances. By 90 °C, which is above the transition temperature, the original resonances due to sCF3 and sCF2CF3 disappeared, leaving only the additional resonances at -81.3 and -125.7 ppm. However, the signals from the inner sCF2s backbone remained unchanged. The high-frequency shift of terminal sCF3 and sCF2CF3 resonances indicates that the phase transition must lead to a packing assembly in which terminal fluorines reside in local environments containing both hydro- and fluorocarbon chains. Russell et al. proposed a monolayer lamellar structure for the hightemperature phase of F12H12, with an antiparallel alignment of hydrocarbon and fluorocarbon chains (Figure 5c).2 However, this model appears unlikely considering the energy penalty paid for the mixing of incompatible hydrogenated and fluorinated chains. Taking into account the d spacing corresponding to the

Solid-State Organization of Semifluorinated Alkanes

Figure 3. Schematic model of the molecular packing of F12H6 taken from ref 2. The distance that will be observed in SAX is denoted on the right side of each diagram.

Figure 4. Variable-temperature 19F MAS NMR spectra of F12H12 acquired with a spinning frequency of 30 kHz.

length of the molecule, we propose a packing model for F12H12 at high temperature in which hydrocarbon and fluorocarbon chains are interdigitated, forming a monolayer lamellar-like structure as shown in Figure 5d. Russel et al. indicated higher energy for this model according to semiempirical energy calculations in which the chain interactions between adjacent layers was not considered. However, including adjacent layers and allowing interdigitation of hydrocarbon chains between the layers must be sufficient to fill the empty space between the

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Figure 5. Schematic model of the molecular packing of F12H12 taken from ref 2. The distance that will be observed in SAX is denoted on the right side of each diagram.

hydrocarbon chains and to lower the energy. The local environment of terminal sCF3 in this model is similar to the monolayer lamellar structure observed in F12H6, suggesting that our model would give rise to a 19F NMR signal at the same chemical shift position as F12H6. This is consistent with the 19F NMR spectra above the phase transition temperature. Experiments supporting this model are discussed in the following sections. 3.3. 19F f 13C and 1H f 13C CPMAS NMR Spectra of F12H12. 13C CPMAS NMR spectra utilizing polarization transfer from either fluorine or protons were acquired to elucidate the molecular processes that are responsible for the phase transition of F12H12. Depending on the spins for polarization and decoupling, fluorinated carbon segments and hydrogenated carbon segments can be investigated separately, which yields valuable information about the molecular packing and dynamics. First, 1H f 13C CPMAS NMR spectra were acquired below and above the transition temperature. As shown in Figure 6a, all resonances from methylene carbons shifted to lower frequency above the solid-solid phase transition temperature, indicating the presence of a significant amount of gauche defects. In particular, the dominant signal of the inner methylene carbons (sCH2s) observed at 32.6 ppm at low temperature shifted to 30.0 ppm above the transition temperature. This suggests that not only the hydrocarbon chain ends disorder, but also the inner parts of the hydrocarbon chains are conformationally disordered above the phase transition, which is in agreement with previous reports.21 Second, 19F f 13C CPMAS NMR spectra were obtained at various temperatures as shown in Figure 6b, and several resonances resulting from fluorocarbon chains were observed. The assignment was done based on a two-dimensional 19F-13C HETCOR experiment (Figure 7). In contrast to the hydrocarbon chains, no significant change was observed in 19F f 13C CPMAS NMR spectra of

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Lee et al.

Figure 8. Plots of the 19F T1 relaxation times of each chemical moiety as a function of inverse temperature for (a) F12H12 and (b) F12H20.

Figure 6. (a) 1H f 13C and (b) 19F f 13C CPMAS NMR spectra of F12H12 below and above the phase transition temperature. The spectra were acquired with rotor-synchronized π-pulse decoupling.

Figure 7. 19F-13C HETCOR NMR spectrum of F12H12 acquired at 55 °C, using the REPT-HSQC pulse sequence and the rotorsynchronized π-pulse decoupling scheme.

F12H12 at various temperatures (Figure 6b). This suggests that the phase transition observed for F12H12 cannot be ascribed to a conformational change of the fluorocarbon segments. Indeed, as the internal barrier to rotation about the CsC bond of CF2 groups is relatively high because of the steric hindrance of the large fluorine atoms, the conformational disordering of fluorocarbon chains is unlikely to occur at the phase transition. It is noteworthy that the signal assigned to CF2 adjacent to the hydrocarbon chains (sCH2CF2s, 120 ppm) is broad at low temperature and becomes narrow at high temperature. This can be explained by self-decoupling of protons resulting from the increased mobility of the hydrocarbon chains. Because a homonuclear decoupling of protons cannot be applied together with fluorine decoupling under our experimental setup, the carbon spins close to the hydrocarbon moiety must be affected by dipolar coupling between protons and carbons, which will

broaden the signal. However, at elevated temperatures, the hydrocarbon chain becomes disordered and mobile, leading to an averaging of the dipolar couplings to protons. As shown in Figure 7, the two-dimensional 19F-13C HETCOR spectrum clearly displays cross-peaks between fluorines and carbons that allow the carbons and fluorines to be assigned to different CFn groups. Our results show that two-dimensional 19 F-13C correlation spectra can be readily obtained using simple π-pulse decoupling,16 providing a valuable tool for the structural determination of fluorinated compounds. The applicability of the more advanced REPT-HSQC recoupling scheme has not been investigated for spin pairs between fluorine and carbon spins, although this method has been widely used for spin systems between protons and carbons. 3.4. 19F Spin-Lattice Relaxation Time as a Function of Temperature. Relaxation times provide information about the molecular dynamics on various time scales depending on the nature of the relaxation phenomena.22,23 For example, spin-lattice relaxation times (T1) are sensitive to the motions on the 10-7-10-9 s time scale and can be examined by varying the resonance frequency or measuring T1 at different temperatures. Typically, T1 relaxation times decrease as mobility increases in the slow motional limit, whereas T1 values increase as mobility increases under extreme narrowing conditions. The minimum value for T1 occurs when the mobility becomes comparable to the Larmor frequency.22 For hydrocarbons, a single T1 value for protons is often observed for different chemical moieties along the chain because of effective 1H spin diffusion. Spin diffusion is a transport process of spin polarization that is most effective in spin systems with high natural abundance and strong dipolar couplings. However, the contribution of spin diffusion is greatly reduced under fast MAS conditions and for large differences in resonance frequency, which is the case for 19F. To investigate the change in chain dynamics associated with the solid-solid phase transition, 19F spin-lattice relaxation times (T1) were measured at different temperatures (Figure 8). The temperature dependence of T1 relaxation times indicates that the terminal sCF3 groups are in the fast motional regime for both F12H12 and F12H20, which is probably due to threefold rotation of the CF3 groups. However, an intriguing

Solid-State Organization of Semifluorinated Alkanes difference was observed between the two materials. In the case of F12H20, T1 relaxation times of both sCF2s and sCF2CH2s groups increased with increasing temperature, whereas F12H12 exhibited a T1 maximum of sCF2s and sCF2CH2s resonances in the vicinity of the phase transition temperature. As the temperature was raised further above the phase transition, the T1 values gradually decreased and eventually became identical to those of the sCF3 group. Although the limited data set precludes quantitative analysis, this intriguing trend in T1 relaxation times of F12H12 allows us to address the following questions: First, what kind of molecular motion predominantly contributes to the relaxation behavior observed in the SFA systems? Second, can the relaxation behavior be used to support a structural picture associated with the phase transition? These issues are addressed in the remainder of this section. Relaxation is induced by fluctuations in the magnetic field at the resonance frequency by the molecular motion. Several mechanisms influence the relaxation behavior.23 Dipolar interactions including both intermolecular and intramolecular couplings, which depend on the distance between nuclei, can be modulated by molecular motion. This leads to fluctuations in the local magnetic field and influences the relaxation time. When the shielding at the nucleus is not highly symmetric, molecular motion can also modulate the local magnetic field surrounding the nuclei. This shielding anisotropy causes relaxation, and the relaxation time resulting from this effect depends on the static magnetic field. The motions in the condensed phase that might be responsible for the variation in relaxation time include conformational disordering, rotation about the chain axis, longitudinal displacements, and wobbling and wagging of the chain. Conformational fluctuations of the fluorocarbon chain can be ruled out based on the 19F f 13C CPMAS results. Another possible motion is rotation about the chain axis. For perfluoroalkanes, a solid-solid phase transition to a plastic-crystalline phase has been observed, which is associated with the onset of cooperative rotation of the chains.24 High internal molecular mobility related to a helical structure was reported for poly(tetrafluoroethylene) even at room temperature.25 In separate studies on poly(tetrafluoroethylene), a variation in the T1 relaxation times (approximately 1 s) as a function of temperature was observed and was ascribed to the reorientation of CF2 groups along their chain axis.26,27 It is not unlikely that the same types of motions occur in SFAs. Thus, we assume that rotation of the chains, probably coupled with wobbling and wagging motions of the chains, is the primary motional mechanism that governs the relaxation behavior of SFAs. We now address the second question stated above. The presence of a T1 maximum and the decrease in the relaxation times with increasing temperature seem to be counterintuitive. At first sight, this observation suggests that F12H12 undergoes a transition in molecular mobility from fast to slow motional regimes at elevated temperatures. Such a conclusion, however, is at variance with the continuous motional line narrowing observed for sCF2CH2s resonances above the phase transition temperature, which indicates a gradual increase in molecular mobility with temperature. Therefore, it is more likely that the decrease in the relaxation times is due to structural changes in the surrounding of fluorocarbon segments at the solid-solid phase transition. Given the fact that the relaxation times of sCF3 and sCH2CF2s converge when the transition is complete, we propose that spin diffusion between these groups occurs more efficiently in the high-temperature monolayer phase. This could shed light on the structural picture associated with the phase

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Figure 9. 19F-19F 2D exchange spectrum acquired at 85 °C with a 1-s mixing time.

transition. To obtain a structural picture, we considered the structural models under debate. Monolayer lamellar structures with hydrocarbon and fluorocarbon interdigitation exhibit a spatial arrangement in which sCF3 and sCF2CH2s groups reside close to each other. The lateral distance between the molecular chains has been determined as ∼5.5 Å,2 which leads to a F-F internuclear distance between CF3 and CF2CH2 of less than 5 Å. Because spin diffusion is based on the dipolar coupling of nuclear spins, the rate of spin diffusion is proportional to the inverse sixth power of internuclear distances. Thus, spatial proximity between two fluorine moieties in a monolayer lamellar structure with hydrocarbon and fluorocarbon interdigitation can indeed lead to efficient spin diffusion and correspondingly equivalent T1 relaxation times. In contrast, the distance between these fluorine moieties is large in other packing structures, so that spin diffusion cannot occur efficiently. Thus, the anomalous behavior of T1 relaxation times suggests that a monolayer lamellar structure with hydrocarbon and fluorocarbon interdigitation is a more plausible model for the high-temperature phase. 3.5. 19F-19F Two-Dimensional Correlation Experiments. Various morphologies associated with the phase transition are identified in 19F variable-temperature NMR experiments. However, information about relative 19F-19F proximity cannot be acquired by 19F 1D NMR experiments. To investigate both the spatial proximity between two phases and the connectivity between different chemical moieties, 19F-19F 2D exchange experiments were performed for F12H12 at a temperature where both phases, monolayer and bilayer lamellae, are present. Several interesting features can be extracted from the 2D exchange spectrum shown in Figure 9. First, cross-peaks for terminal sCF3 groups are observed between monolayer and bilayer phases, indicating that a magnetization transfer between the two phases occurs on the time scale of a few seconds. Two main mechanisms contribute to magnetization transfer in 2D exchange experiments: chemical exchange and cross-relaxation spin diffusion.28 The behaviors associated with chemical exchange such as gradual line broadening and coalescence of the signals23 were not observed in variable-temperature experiments. Therefore, chemical exchange can be ruled out, and the presence of cross-peaks can be ascribed to spin diffusion, suggesting spatial proximity between the two phases. This demonstrates that the phase transition occurs randomly throughout the system and a homogeneous mixture of the two phases exists over a broad temperature range. Second, intense cross-peaks were observed between sCH2CF2s and terminal sCF3 moieties only in the monolayer phase. This indicates that sCH2CF2s and terminal

1366 J. Phys. Chem. B, Vol. 113, No. 5, 2009 sCF3 moieties are close to each other in the monolayer phase, whereas the distance between these moieties is significantly larger in the bilayer phase. This observation confirms the monolayer structure with fluorocarbon and hydrocarbon interdigitation as proposed above. In contrast to 1H, the large chemical shielding anisotropy of 19 F can interfere with the dipolar coupling between 19F nuclei. Thus, a more detailed investigation of 19F-19F exchange experiments was carried out and is discussed in the Supporting Information.

Lee et al. 0207086) and from the Max Planck Society. The authors thank Sunna Scholz for synthetic support. Supporting Information Available: 19F solution-state NMR spectra of various mixtures of hydrocarbons and fluorocarbons (Figure S1), discussion of 19F-19F 2D correlation experiment with RFDR, and 19F-19F 2D exchange spectra acquired at 85 °C with a 5-ms mixing time using a NOESY-type three-pulse sequence (Figure S2) and RFDR recoupling (Figure S3). This material is available free of charge via the Internet at http:// pubs.acs.org.

4. Conclusions SFAs adopt different packing structures depending on the length of their hydrocarbon chains. Perfluorododecylhexane (F12H6) shows a monolayer lamellar structure, whereas perfluorododecyleicosane (F12H20) forms bilayer lamellae. Perfluorododecyldodecane (F12H12) undergoes a phase transition from bilayer lamellae to the monolayer lamellar structure. These packing motifs provide different local environments that can be exploited to establish the correlation between 19F chemical shifts and packing assembly. 19F chemical shifts of terminal sCF3 groups have been confirmed to be extremely sensitive to the packing geometry and to provide a fingerprint differentiating between monolayer and bilayer lamellae. In the case of F12H12, two discrete resonances for sCF3 are observed as a function of temperature, which can be attributed to a solid-solid phase transition from bilayer to monolayer lamellar structure. 1H/19F f 13C NMR experiments indicate that the phase transition is accompanied by disordering and melting of the hydrocarbon chains, but does not involve a significant structural change in the fluorocarbon segments. Under fast magic angle spinning conditions, large differences in the 19F chemical shifts allow for the determination of the 19F T1 relaxation times of the individual moieties, which provide further insight into both packing structure and molecular dynamics. The T1 relaxation times of the sCF2s groups of F12H12 decrease at the phase transition temperature, indicating a change in packing. Thus, we propose a structural picture of F12H12 exhibiting a bilayer lamellar structure in the low-temperature phase and monolayer lamellae with fluorocarbon and hydrocarbon interdigitation in the high-temperature phase. This monolayer scheme is confirmed by advanced 2D 19F-19F exchange experiments. These results demonstrate that 19F NMR spectroscopy is a powerful tool for elucidating the chemical structures of fluorinated compounds and investigating the molecular packing and dynamic process responsible for phase transitions. Acknowledgment. Financial support from the Deutsche Forschungsgemeinschaft (DFG, SFB625 in Mainz) is greatly acknowledged. C.G.C. is grateful for financial support from a U.S. National Science Foundataion MPS Distinguished International Research Fellowship (MPS-DRF) (Award DMR-

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