Unusual Dynamic Behavior in the Isotropic Phase of Banana

The dynamic behavior of the two liquid crystals was studied in their isotropic phases and in the nematic phase of ClPbis11BB by means of 2H NMR line w...
1 downloads 0 Views 147KB Size
Download PDF
J. Phys. Chem. B 2005, 109, 769-774

769

Unusual Dynamic Behavior in the Isotropic Phase of Banana Mesogens Detected by 2H NMR Line Width and T2 Measurements Valentina Domenici,*,† Marco Geppi,*,† Carlo Alberto Veracini,† Robert Blinc,‡ Andrija Lebar,‡ and Bosˇtjan Zalar‡ Dipartimento di Chimica e Chimica Industriale, UniVersita` di Pisa, Via Risorgimento 35, 56126 Pisa, Italy, and Department of Solid State Physics, Jozˇef Stefan Institute, JamoVa 39, 1000 Ljubljana, SloVenia ReceiVed: August 18, 2004; In Final Form: October 12, 2004

In this work the first experimental observation of a peculiar behavior in the isotropic phase of liquid crystals by means of 2H NMR is reported. In particular, two five-ring banana-shaped mesogens, the 1,3-phenylenebis{4,4′-(11-undecenyloxy)benzoyloxy}benzoate (Pbis11BB) and its 4-chloro homologue (ClPbis11BB), selectively deuterium labeled on their central rings, are the subject of our investigation. The dynamic behavior of the two liquid crystals was studied in their isotropic phases and in the nematic phase of ClPbis11BB by means of 2H NMR line width and spin-spin relaxation time (T2) analysis. The results obtained reveal that the unusual line broadening observed in the 2H NMR spectra in the isotropic phase, even far above the isotropic phase-mesophase transition, has a homogeneous nature, thus indicating the presence of reorientational motions much slower than in conventional isotropic liquid-crystalline phases.

Introduction Banana molecules, taking their name from their bent molecular shape, 1 represent one of the most surprising results in the study of soft matter in the past 10 years, as the fluid biaxial smectic phases, formed from these peculiar compounds, exhibit spontaneous polarization in the layer plane,2-4 even though without asymmetric carbons. Since their discovery, several classes of banana molecules have been synthesized and a rich variety of mesophases have been characterized,5,6 showing interesting chemical and physical properties, such as ferroelectricity-antiferroelectricity, piezo- and pyroelectricity, and secondorder nonlinear optical activity.7 The enormous scientific interest in these materials, apart from their huge potential in high-tech applications, is also focused on the molecular structural properties as well as on the origin of the chirality of these systems.8 Among the molecular properties, the value of the bending angle “R” (which is the angle between the two lateral wings of the bent molecules) seems to be of great relevance in determining the kind of mesophase formed. Characteristic R values for banana compounds, typically indirectly obtained from 13C NMR,9-11 range from 120° to 130°,5 even though quite different values can be found for compounds with one or more substitutions on the central ring. For example, two chlorine atoms in positions 4 and 6 on the central ring give a rodlike rather than a banana shape.9 Other evidence comes from the X-ray analysis of several compounds with the same central aromatic core, 4,6dichloro-substituted, and different lateral chains: the bending angle of these compounds ranges from 155.5° to 165.6° if all external aromatic rings are considered for such evaluation.12 Moreover, in this case, as well as for monochloro-substituted banana compounds,13,14 the presence of one or more chlorine * To whom correspondence should be addressed. (V.D.) Phone: +390502219289. Fax: +39-0502219260. E-mail: [email protected]. (M.G.) Phone: +39-0502219289. Fax: +39-0502219260. E-mail: [email protected]. † Universita ` di Pisa. ‡ Joz ˇ ef Stefan Institute.

atoms seems to be strictly related to the appearance of a nematic phase, rather than the typical Bi mesophases. In most of the cases examined, the nematic phase is the only phase formed, with a few exceptions;15 on the other hand, computer simulations,16 showing that above a critical bending angle the bent molecules can form a nematic phase, also confirm this behavior. Both experimental and theoretical studies therefore reveal a clear relationship among the chloro substitution in banana molecules, the large bending angle, and the occurrence of a nematic phase. A substantial contribution in understanding these relationships could come from NMR, which is a powerful technique to study molecular properties. In the field of banana LC systems, NMR spectroscopy has been used to obtain structural information, as the previously mentioned bending angle, and molecular and local order parameters,9,10 essentially by using 13C NMR techniques, while only a few papers concerned 2H NMR applications.14,17 The latter, putting aside its disadvantage of requiring deuteriumlabeled samples, can provide valuable information on orientational ordering and dynamics, which can be extracted and elaborated both for a molecule as a whole and for individual molecular fragments. One of the most peculiar advantages of studying the deuterium nucleus (I ) 1) lies in the possibility to gain insight into different contributions to molecular dynamics by measuring both the spin-lattice and the spin-spin relaxation times. From the low frequency-resolved T2, measurable by the CPMG sequence,18 to the quadrupolar and Zeeman T1 measurements, accessible by means of the Wimperis sequence,19 a wide range of frequencies can be covered, yielding detailed information on either slow motions (i.e., order director fluctuation) or fast reorientational motions (i.e., molecular and internal). Line shape and line width analysis can be used as an alternative approach to reveal dynamic processes with characteristic frequencies of the order of magnitude of the quadrupolar splitting. The analysis of the spin-spin relaxation times, determined via solid echo (or quadrupolar echo) pulse techniques,20,21 can further con-

10.1021/jp046278l CCC: $30.25 © 2005 American Chemical Society Published on Web 12/18/2004

770 J. Phys. Chem. B, Vol. 109, No. 2, 2005 CHART 1: Molecular Structure of the Two Labeled Banana-Shaped Samples

tribute to the quantification of the dynamic behavior of the system under study. In this work, we have investigated two banana-shaped liquid crystals, 1,3-phenylenebis{4,4′-(11-undecenyloxy)benzoyloxy}benzoate (Pbis11BB) and its 4-chloro homologue (ClPbis11BB), showing a B2 phase and a nematic phase, respectively. Both were deuterated at the central aromatic ring. However, the study of the isotropic phase of these compounds, rather than of their mesophases, is the central topic of this work, since the unusually broad 2H NMR lines observed are revealed to be particularly interesting. To identify the origin of this broadening and to determine the time scale of the molecular reorientational dynamics, we decided to employ deuterium NMR line width and quadrupolar echo T2 relaxation time measurements in a large range of temperatures above the isotropic phase-mesophase transition. Only recently, several groups have initiated investigations of the isotropic phase formed by banana molecules,22,23 but to our knowledge no papers have yet been published. We believe that the results discussed in the present paper are significant for the characterization of the dynamic behavior in isotropic phases formed by bent-core molecules. Moreover, a hypothesis of molecular packing in both the isotropic and nematic phases can be drawn. Experimental Section Samples. Two mesogens, 4-chloro-1,3-phenylenebis{4,4′-(11undecenyloxy)benzoyloxy}benzoate (ClPbis11BB-d3) and 1,3phenylenebis{4,4′-(11-undecenyloxy)benzoyloxy}benzoate (Pbis11BB-d4), selectively deuterated on the central ring (see Chart 1), have been investigated by means of 2H NMR. The synthesis and characterization of the nonlabeled and labeled compounds are reported in refs 24 and 25, respectively. The phase transition temperatures of the deuterated samples ClPbis11BB-d3 and Pbis11BB-d4, determined by means of differential scanning calorimetry, are, respectively

Iso, 355.2 K (SmC, 317.0 K), N, 310.4 K, Cr Iso, 366.5 K, B2, 342.9 K, Cr 2H

NMR. The 2H NMR experiments were carried out on a 9.40 T Varian InfinityPlus400 spectrometer (Pisa), working at 61 MHz for deuterium, and on a home-built spectrometer (Ljubljana), working at 58 MHz for deuterium, for the samples ClPbis11BB-d3 and Pbis11BB-d4, respectively. 2H NMR spectra were also recorded on a Varian VXR-300 spectrometer (Pisa) with a Larmor frequency of 46 MHz for the sample ClPbis11BBd3. In all cases the samples were macroscopically aligned within the magnet by slow cooling from the isotropic phase, the temperature was controlled within 0.2 K, and 20 min was allowed for thermal equilibration of the sample. The 2H spectra of ClPbis11BB-d3 were recorded on the Varian InfinityPlus400,

Domenici et al. either on cooling or on heating, every 2 K from 331 to 363 K and every 5 K from 363 to 413 K. Spectra were acquired by a single pulse sequence, either with or without 1H continuouswave decoupling, with a 90° pulse of 2.8 µs and 4000 scans. T2 measurements were performed by the quadrupolar echo (QE) sequence 20 (90x-τ-90y-τ) with the exorcycle26 phase scheme, and with 1H continuous-wave decoupling. The variable time τ ranged from 20 µs to 6 ms (25 and 18 values were used at temperatures lower and higher than 373 K, respectively). The number of scans was 500 and 8000 in the isotropic and nematic phases, respectively. 2H NMR spectra were recorded on the same sample on the Varian VXR-300 spectrometer every 2 K from 367 to 349 K, by using a single pulse sequence with a 90° pulse of 16 µs and 10000 scans. In the case of Pbis11BB-d4 the 2H spectra were recorded on the home-built spectrometer from 405 to 355 K, using the QE pulse sequence without 1H decoupling either to acquire 2H spectra in the whole temperature range or to measure spinspin relaxation times in the isotropic phase. The 90° pulse was 8.0 µs, the number of scans was 2096, and 25 values of τ ranging from 20 µs to 5 ms were used. For each temperature and for both samples, the experimental integrals of the signal as a function of τ were well reproduced by the equation27,28

I(τ) ) I(0) exp[-2τ/T2]

(1)

which was used to determine the T2 values. Due to instrumental limitation the T2 measurements could not have been performed in the B2 phase. Results and Discussion Before discussing in detail the behavior of 2H NMR spectra, one must point out the peculiar behavior of the two banana samples when introduced into the magnet. The procedure followed in all cases consists of heating the sample outside the magnet until the isotropic phase is reached and then slowly cooling the sample inside the magnet. In the case of the mesogen Pbis11BB-d4, we could not observe the expected quadrupolar splitting on passing from the isotropic phase to the B2 phase, even for extremely slow cooling. This indicates that the local directors were not aligned with the external magnetic field. The lack of alignment was also observed in ClPbis11BB-d3 in the magnetic field of 7.05 T; in the 9 T field, on the other hand, we were able to record the 2H NMR spectra shown in Figure 1, which demonstrate the onset of macroscopic orientational order. The explanation for this behavior is that banana mesophases are quite viscous5 as compared to other commonly studied liquid-crystalline mesophases. Consequently, it is quite difficult to orient the molecules in a bulk banana LC phase by an external magnetic field. Moreover, the B2 phase is not only very viscous, but also characterized by a very high degree of local order, resulting in a very different symmetry with respect to the isotropic phase: this is another factor that prevents the alignment of the molecules on entering the mesophase from the isotropic phase. In Figure 1 two sets of variable-temperature 2H NMR spectra, recorded at 61 MHz on the sample ClPbis11BB-d3, are reported. The transition temperature from the isotropic phase to the nematic phase is between 355 and 353 K. It must be noticed that only one quadrupolar doublet is present in the spectra recorded in the nematic phase instead of the two expected on the basis of molecular geometry, because the two quadrupolar splittings contributing to the 2H spectrum differ only in their

Unusual Dynamic Behavior in Banana Mesogens

J. Phys. Chem. B, Vol. 109, No. 2, 2005 771

Figure 1. 2H NMR spectra of the ClPbis11BB-d3 mesogen: (left) spectra recorded every 5 K in the isotropic phase from 413 K (bottom) to 363 K (top), (right) spectra recorded from 363 K (bottom) to 331 K (top) every 2 K, from the isotropic phase to the crystal phase.

Figure 2. 2H NMR spectra of the Pbis11BB-d4 mesogen recorded on cooling the sample in the isotropic phase from 405 to 359 K.

signs, but not in their absolute values. However, a detailed analysis of the spectra recorded in the nematic phase in terms of orientational order and molecular structure is reported in ref 14, while in this paper we focus on the line width analysis of the 2H NMR signals, particularly in the isotropic phase. It can be observed that the isotropic peak is unusually broad even far above the transition temperature: the line width of the isotropic signal slowly increases from 400 Hz to about 10 kHz on cooling from 413 to 355 K, and this behavior is reproducible on heating. Significant contributions to the line width ascribable to pretransitional effects can be ruled out on the basis of both the very large temperature range across which the line broadening persists and the similarity between the cooling and heating behaviors. In the nematic phase the line width, calculated as the average line width of the two peaks of the quadrupolar doublet, is practically constant over the nematic range, and amounts to about 7.5 kHz. 2H NMR spectra for the sample Pbis11BB-d were recorded 4 at 58 MHz (see Figure 2). The uncommonly large line

Figure 3. 2H NMR line width (Hz) vs temperature (K) obtained on cooling the sample ClPbis11BB-d3. Full and empty circles refer to single-pulse (SP) spectra at 46 and 61 MHz, respectively. Empty tilted squares and squares represent the line widths obtained at 61 MHz by using 1H decoupling SP and QE sequences, respectively. Empty triangles indicate the homogeneous line widths calculated from the experimentally determined T2.

broadening is observed as well for the isotropic peak in a wide temperature range above the I-B2 phase transition. As mentioned above, no quadrupolar splitting is observed in the B2 phase, but a further broadening of the isotropic-like spectral peak takes place on lowering the temperature below 359 K (isotropic transition) until the signal becomes so broad that it cannot be resolved any longer from the baseline noise. For the ClPbis11BB-d3 sample, the line width at half-height (∆νh/2exp), calculated from the 2H NMR spectra recorded at 61 and 46 MHz by using various pulse sequences (quadrupolar echo20 and single pulse, with or without 1H decoupling), is reported in Figure 3. Clearly, the experimental values depend neither on the pulse sequence nor on the magnetic field strength or on the 1H decoupling, since the differences among the various sets of measurements are, at each temperature, within the experimental error. A more detailed description of the origin of the isotropic line width broadening can be given by measuring the spin-spin

772 J. Phys. Chem. B, Vol. 109, No. 2, 2005

Domenici et al.

Figure 4. 2H NMR line width (Hz) vs temperature (K) obtained on cooling the sample Pbis11BB-d4. Full and empty circles refer to the line widths obtained at 58 MHz by using the QE sequences and the homogeneous line widths calculated from the measured T2, respectively.

Figure 5. Measured T2 relaxation times (s) vs 1000/T (K-1) on a logarithmic scale for the ClPbis11BB-d3 sample in the isotropic phase. The solid line represents the best fit to eqs 2a,b and 3.

relaxation time T2 using the QE pulse sequence. T2 is determined by the fast reorientational motions with inverse correlation time τc-1 much higher than the quadrupolar frequency constant νq ) (3/2)(e2qQ/h) ) 250 kHz of the deuterium nucleus in the C-2H bond:29

1 π2 ) νq2[3J(0) + 5J(ωL) + 2J(2ωL)] T2 30

(2a)

where ωL is the Larmor frequency and the spectral densities

J(ω) ) 2τc/(1 + ω2τc2)

(2b)

Here, we have assumed that the C-2H bond reorientation dynamics can be described by a single correlation time τc. Due to a rather small (η ) 0.04) biaxiality of the electric field gradient tensor at the sites of aromatic deuterons, 30 we have disregarded the factor 1 + η2/3 on the right-hand side of eq 2a. T2 is related to the homogeneous part of the 2H NMR line width via ∆νhomo ) (πT2)-1. Figure 3 shows the experimental ∆νhomo(T) dependence, calculated from the T2(T) experimental points for ClPbis11BB-d3. It is evident that ∆νhomo coincides with the total line width ∆νh/2exp both in the isotropic phase and in the nematic phase. Thus, in the chlorinated sample, the line broadening is fully ascribable to the homogeneous contribution. Consequently, the C-2H bond reorientation dynamics, equivalently the deuterated ring reorientation dynamics, must take place on a time scale shorter than the deuteron NMR time scale νq-1 ) 4 × 10-6 s. In the case of Pbis11BB-d4, T2 relaxation times could be measured solely in the isotropic phase, since the very short relaxation times in the B2 phase prevented us from extending the measurements to temperatures below 370 K. The comparison exp between the experimentally determined total line width ∆νh/2 and the homogeneous line width ∆νhomo is shown in Figure 4. exp ∆νhomo is very similar to ∆νh/2 , as in the chlorinated sample. The slight disagreement of about 200 Hz between the two quantities, observed at all temperatures, can be attributed to inhomogeneities of the magnetic field across the relatively large sample.

Figure 6. Measured T2 relaxation times (s) vs 1000/T (K-1) on a logarithmic scale for the Pbis11BB-d4 sample. The solid line represents the best fit to eqs 2a,b and 3.

Recognizing the homogeneous nature of the uncharacteristically large line broadening in the isotropic phase, let us now, with the help of eqs 2a,b, extract the information on the time scale of the molecular reorientation dynamics, τc. In the isotropic phase it is plausible to assume that the reorientation process is thermally activated, i.e.

τc(T) ) τ∞ exp[Ea/kBT]

(3)

Here τ∞ is the preexponential factor and Ea the activation energy of the C-2H bond reorientation dynamics. Spin-spin relaxation times T2, plotted on a logarithmic scale vs 1000/T, are shown in Figures 5 and 6 for ClPbis11BB-d3 and Pbis11BB-d4, respectively, together with the theoretical fits to eqs (2a,b and 3). The best-fit parameters are τ∞ ) 7 × 10-18 s and Ea ) 71

Unusual Dynamic Behavior in Banana Mesogens

Figure 7. Schematic representation of the molecular packing in the isotropic (left) and nematic (right) phases formed by banana mesogens. Circles indicate entanglements which restrict the rotation of molecules around their long axes.

kJ/mol and τ∞ ) 1.5 × 10-16 s and Ea ) 60 kJ/mol, respectively for ClPbis11BB-d3 and Pbis11BB-d4. The preexponential factors are relatively low and differ by more than 1 order of magnitude. This may signify that the two molecules experience different intermolecular couplings and entanglements and may represent a precursor of the different symmetries of the order in the mesophases. The activation energies are 2-3 times higher than those found in the isotropic phases formed by conventional rodlike liquid-crystal molecules such as pentylcyanobiphenyl (5CB) and octylcyanobiphenyl (8CB)31,32 and are similar to the activation energies of conformational internal motions or molecular rotational diffusion (spinning and tumbling motion) for rodlike liquid crystals33,34 in ordered phases, such as nematic and SmA phases. However, the correlation times τc, which amount at T ) Tc + 10 K (Tc is the transition temperature from the isotropic phase into the mesophase) to ∼1 × 10-7 and ∼4 × 10-8 s, respectively, for ClPbis11BB-d3 and Pbis11BB-d4, are at least 2 orders of magnitude longer than in the calamitic liquid crystals. For instance, τc ) 1.4 × 10-10 s at T ) Tc + 10 K in 5CB. 31 This experimental observation is in agreement with recent studies performed on similar banana mesogens in their isotropic phase by means of either light scattering23 or dielectric spectroscopy.35 It must be noticed that long-time exponential orientational relaxation behavior was also observed in conventional nematics by optical spectroscopy methods36 and explained in terms of the pretransitional nematic fluctuations, using Landau-de Gennes theory. However, in our case the uncommonly slow dynamics persists in the isotropic phase of the investigated banana samples even at temperatures high above Tc. Moreover, in the present case the 2H NMR line width in the isotropic phase is much larger than that usually observed in rodlike mesogens experiencing Landau-de Gennes pretransitional effects, and similar between the two banana molecules, although they give rise to very different mesophases (nematic and B2). These considerations allow us to reliably rule out the possibility that this unusual behavior can be exclusively associated with Landau-de Gennes pretransitional effects, specifically predicted for isotropic-nematic transitions.32 A possible explanation of this behavior is the occurrence of local molecular entanglements (see Figure 7), which, due to the bent shape of banana molecules, might hinder fast reorientations about long molecular axes. Conclusions The behavior of two banana mesogens in their isotropic phase, detected by 2H NMR, turns out to be atypical when compared

J. Phys. Chem. B, Vol. 109, No. 2, 2005 773 to that of conventional isotropic liquid-crystalline phases. A substantial line broadening, observed at temperatures much higher than the isotropic phase-mesophase transition temperature (in a range of 60 and 40 K for ClPbis11BB-d3 and Pbis11BB-d4, respectively), and the rapid transverse magnetization decay are both suggestive of a dynamics characteristic of partially oriented samples. In particular, the isotropic 2H NMR line width is of homogeneous nature. It reflects a dynamic character of the observed line broadening and rules out the pretransitional quasi-static orientational ordering as a possible line broadening mechanism. The motions that contribute to the spin-spin relaxation time T2 in the isotropic phase, i.e., internal and overall molecular reorientations, were revealed to be at least 2 orders of magnitude slower than in the isotropic phases of conventional liquid crystals, with unusually high (about 70 kJ/mol) activation energies. Moreover, the preliminary observation that the two investigated species of banana molecules exhibit 2H NMR spectra with sharp isotropic peaks, when dissolved in different solvents, including liquid-crystalline ones, confirms that the slow reorientational dynamics is inherent to ClPbis11BB and Pbis11BB in the isotropic phase and implies peculiar molecular interactions among banana molecules, possibly their entanglement. Acknowledgment. This work was financially supported by PRIN 2003 (Italian MIUR). We thank Katalin Fodor-Csorba for furnishing the deuterated samples, Antal Jakli for interesting discussions at the ILCC conference in Ljubljana (2004), and Marija Vilfan for stimulating conversation concerning relaxation mechanisms in the isotropic phase. References and Notes (1) Niori, T.; Sekine, F.; Watanabe, J.; Furukawa, T.; Takezoe, H. J. Mater. Chem. 1996, 6, 1231. (2) Prost, J.; Barois, P. J. Chem. Phys. 1983, 80, 65. (3) Petschek, R. G.; Wiefling, K. M. Phys. ReV. Lett. 1987, 59, 343. (4) Tournilhac, F.; Blinov, L. M.; Simon, J.; Yablonski, S. V. Nature 1992, 359, 621. (5) Pelzl, G.; Diele, S.; Weissflog, W. AdV. Mater. 1999, 11, 707. (6) Shen, D.; Pegenau, A.; Diele, S.; Wirth, I.; Tschierske, C. J. Am. Chem. Soc. 2000, 122, 1593. (7) Kentischer, F.; MacDonald, R.; Warnick, P.; Heppke, G. Liq. Cryst. 1998, 25, 341. (8) Heppke, G.; Moro, D. Science 1998, 279, 1872. (9) Weissflog, W.; Lischka, Ch.; Diele, S.; Pelzl, G.; Wirth, I.; Grande, S.; Kresse, H.; Schmalfuss, H.; Hartung, H.; Stettler, A. Mol. Cryst. Liq. Cryst. 1998, 333, 203. (10) Diele, S.; Grande, S.; Kruth, H.; Lischka, Ch.; Pelzl, G.; Weissflog, W.; Wirth, I. Ferroelectrics 1998, 212, 169. (11) Pelzl, G.; Grande, S.; Jakli, A.; Lischka, Ch.; Kresse, H.; Schmalfuss, H.; Wirth, I.; Weissflog, W. Liq. Cryst. 1999, 26, 401. (12) Hartung, H.; Settler, A.; Weissflog, W. J. Mol. Struct. 2000, 526, 31. (13) Pelzl, G.; Eremin, A.; Diele, S.; Kresse, H.; Weissflog, W. J. Mater. Chem. 2002, 12, 2591. (14) Dong, R. Y.; Fodor-Csorba, K.; Xu, J.; Domenici, V.; Prampolini, G.; Veracini, C. A. J. Phys. Chem. B 2004, 108, 7694. (15) Niori, T.; Yamamoto, J.; Yokoyama, H. Mol. Cryst. Liq. Cryst. 2004, 409, 475. (16) Memmer, R. Liq. Cryst. 2002, 29, 483. (17) Madsen, L. A.; Digemans, D. J.; Nakata, M.; Samulski, E. T. Phys. ReV. Lett. 2004, 92, 145505. (18) Luz, Z.; Meiboom, S. J. Chem. Phys. 1963, 39, 366. (19) Wimperis, S. J. Magn. Reson. 1990, 86, 46. (20) Powles, J. G.; Strange, J. H. Proc. Phys. Soc. 1963, 82, 6. (21) Luz, Z.; Meiboom, S. J. Chem. Phys. 1963, 39, 366. (22) Domenici, V.; Fodor-Csorba, K.; Veracini, C. A.; Blinc, R.; Lebar, A.; Zalar, B. European Conference of Liquid Crystals, Jaca, Spain, 2003; Poster P12. (23) Stojadinovi, S.; Liao, G.; Sawade, H.; Fodor-Csorba, K.; Weissflog, W.; Peltz, G.; Jakli, A.; Sprunt, S. International Liquid Crystals Conference, Ljubljana, Slovenia, 2004; Poster STRP106.

774 J. Phys. Chem. B, Vol. 109, No. 2, 2005 (24) Fodor-Csorba, K.; Vajda, A.; Galli, G.; Jakli, A.; Demus, D.; Holly, S.; Gacs-Baitz, E. Macromol. Chem. Phys. 2002, 203, 556. (25) Fodor-Csorba, K.; Holly, S.; Gacs-Baitz, E.; Domenici, V.; Veracini, C. A. To be submitted for publication. (26) Bodenhausen, G.; Freeman, R.; Turner, D. L. J. Magn. Reson. 1977, 27, 511. (27) Vold, R. R.; Vold, R. L.; Szeverenyi, N. M. J. Chem. Phys. 1981, 85, 1934. (28) Vold, R. R.; Vold, R. L. J. Magn. Reson. 1981, 42, 173. (29) Abragam, A. Principles of Nuclear Magnetism; Oxford University Press: New York, 1961. (30) Dong, R. Y. Nuclear Magnetic Resonance of Liquid Crystals; Springer-Verlag: New York, 1997.

Domenici et al. (31) Vilfan, M.; Apih, T.; Gregorovic´, A.; Zalar, B.; Lahajnar, G.; Zˇ umer, S.; Hinze, G.; Bo¨hmer, R.; Althoff, G. Magn. Reson. Imaging 2001, 19, 433. (32) Rizi, V.; Ghosh, S. K. NuoVo Cimento D 1998, 15, 661. (33) Catalano, D.; Chiezzi, L.; Domenici, V.; Geppi, M.; Veracini, C. A. J. Phys. Chem. B 2003, 107, 10104. (34) Calucci, L.; Catalano, D.; Fodor-Csorba, K.; Forte, C.; Veracini, C. A. Mol. Cryst. Liq. Cryst. 1999, 331, 1869. (35) Salfetnikova, J.; Zhuchnova, T.; Dantlgraber, G.; Tschierske, C.; Kresse, H. Liq. Cryst. 2002, 29, 155. (36) Gottke, S. D.; Cang, H.; Bagchi, B.; Fayer, M. D. J. Chem. Phys. 2002, 116, 6339.