CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201301036

Conformational Properties and Orientational Order of a de Vries Liquid Crystal Investigated through NMR Spectroscopy Valentina Domenici,*[a] Moreno Lelli,[b] Mario Cifelli,[a] Vera Hamplova,[c] Alessandro Marchetti,[b] and Carlo Alberto Veracini[a] Solid-state and liquid-state NMR spectroscopic techniques are used to describe at molecular level the behaviour of a de Vries liquid crystal (namely the mesogen 9HL) at the SmA–SmC* transition, which is characterized by the absence of the layer shrinkage, typical of non-de Vries smectogens. Previous 2 H NMR studies on the same smectogen, performed at a different magnetic field (from 4.70 to 18.80 T), provided evidence of the occurrence of a tilt of one of the three phenyl rings, constituting the aromatic core of 9HL, at the SmA–SmC* phase tran-

sition. In this work, the study is extended to the whole rigid aromatic core of the 9HL. In particular, the variable temperature behavior of the mesogen studied by 1D 13C NMR cross-polarization (CP) and 2D 1H–13C PDLF (proton-encoded 13C-detected, local field) NMR experiments made possible the characterization of the conformational and orientational properties in the two smectic phases. These results are compared with various proposed models invoked to describe the SmA–SmC* transition in de Vries smectogens at a molecular level.

1. Introduction The so called de Vries liquid crystals are a category of mesogens discovered by the crystallographer Adriaan de Vries several decades ago.[1–3] These smectic liquid crystals exhibit high potentialities for new applications in the field of electro-optic devices, such as ferroelectric (FLC) and antiferroelectric (AFLC) displays.[4] The peculiarity of de Vries liquid crystals is the substantial constant layer spacing within the smectic SmA and SmC* phases (no layer shrinkage, as shown in Scheme 1). This prevent the so-called chevron defects (zig-zag imperfections) at the SmA–SmC* transition that is the main limit in quality and performance of the current electro-optic devices.[4] For this reason, the investigation of the molecular conformation in de Vries liquid crystals as well as the perturbation in the molecular organization under the influence of external fields are at the centre of interest of both industry and academy involving many research teams.[4–17]

[a] Dr. V. Domenici, Dr. M. Cifelli, Prof. C. A. Veracini Dipartimento di Chimica e Chimica Industriale (DCCI) Universit di Pisa Via Risorgimento 35, 56126 Pisa (Italy) Fax: (+ 39) 050-2219260 E-mail: [email protected] [b] Dr. M. Lelli, Dr. A. Marchetti Universit de Lyon 1, CNRS/ENS Lyon Centre de RMN  Trs Hauts Champs 5 rue de la Doua, 69100 Villeurbanne (France) [c] Dr. V. Hamplova Liquid Crystal Group, Department of Dielectrics Institute of Physics, Academy of Sciences of the Czech Republic Na Slovance 2, 182 21 Prague (Czech Republic) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201301036.

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Scheme 1. Sketch of the shrinkage effect occurring at the transition between the SmA and the SmC* phase in normal ferroelectric liquid crystals. ^ and ^z refer to the local mesophase director and to the normal The vectors n to the smectic layers, respectively. dA and dC indicate the average layer thickness in the SmA and in the SmC*, respectively.

In addition to a modest layer shrinkage (less than 5 %)[4] at the SmA–SmC* transition, the de Vries SmA phases have a very large electro-clinic effect,[18] which is related to the presence of a significant tilt of constituent molecules. In fact, it has been shown that the application of external electric fields determines an increase of the induced tilt angle, or “optical tilt”, whose temperature dependence is well described by the Landau mean field theory.[19] Moreover, de Vries SmA phases are characterized by a large soft-mode absorption,[20, 21] as detected by dielectric measurements, and a high birefringence.[20, 22] Different models have been proposed to explain the properties of these mesogens: de Vries proposed the “random diffuse cone model”[3] supposing that the molecules are tilted with respect to the layer normal already in the SmA phase. However, the azimuthal angle is randomly distributed with no correlation in the smectic layer (see Scheme 2 a) and the SmA–SmC* ChemPhysChem 2014, 15, 1485 – 1495

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Scheme 2. Four models for the molecular organization in the SmA phase of de Vries smectogens: a) random diffuse cone model, b) conformational change model, c) interdigitation model, d) cluster diffuse cone model.

phase transition is a disorder–order transition in the molecular azimuthal angle. Later on, other possible explanations were proposed, supported by other different experimental techniques, such as calorimetry, birefringence, small angle X-ray scattering (SAXS) and electro-optic methods.[4–18] According to a second model, proposed by Diele and referred to as the “conformational change model”,[23] molecules experience a different average conformation in the two smectic phases and the absence of a layer shrinkage can be explained in terms of different orientations of both rigid core and lateral chains passing from the SmA to the SmC* phases (see Scheme 2 b). On the other hand, the occurrence of a conformational change at the SmA–SmC* transition is supported by theoretical and experimental works in “standard” ferroelectric LCs, too.[24–26] Depending on the geometry and chemical structure of the systems, conformational changes are compatible with no shrinkage at SmA–SmC* transition. A third model was proposed, independently, by several teams: Saunders et al.,[27] Gorkunov et al.[28] and Osipov et al.[29] Within this model, referred to as the “interdigitated model”, the low-shrinkage (or no-shrinkage) layers transition (i.e. NLS) arises when in the SmA phase the orientational order is low and strongly temperature dependent, so that the increase in orientational order observed at the SmA–SmC* transition is compensated by the tilt effect on layers spacing. Within this third model (see Scheme 2 c) an exceptionally low first-order orientational parameter (and possibly a high second-order one) should be a common feature of SmA materials undergoing typical de Vries transitions. This model seems particularly suited to describe nano-segregated systems,[16, 17, 30] such as fluorinate or organosiloxane derivatives, which have small nematic and high smectic orientational order. For the same materials, a different behaviour of molecules at the interface between consecutive smectic layers in the two phases has also been invoked. Within this model, in the SmA phase, molecules are not tilted, but partially interdigitated,[31] thus justifying a smectic layer spacing smaller than the effective molecular length. On the contrary, in the SmC* phase, molecules are tilted, but interdigitation does not occur anymore, resulting in a layer spacing similar to that in the SmA phase. Recently, a fourth model was proposed,[5, 15] based on 2 H NMR investigations at different magnetic field strength of  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org a de Vries compound selectively 2H-labeled in the aromatic core. These studies[5, 15] demonstrated the presence of an average tilt of the deuterated moiety in the SmA phase, which increases within the SmC* phase by decreasing the temperature. The evidence of a strong magnetic effect[5, 15] on the orientational order of the deuterated aromatic fragment within the SmA phase lead us to formulate a variation of the “diffuse cone model” by substituting the random distribution in azimuthal angle with a locally ordered or cluster distribution in azimuthal directions (see Scheme 2 d). This NMR study[5] showed also that the smA phase of the investigated de Vries smectogen has a quite high orientational order, typical of regular smectic phases; this result was also confirmed by X-ray and Raman techniques on the same smectogen.[32] In the present work, we show that a combination of different NMR techniques[33–40] can help in understanding basic molecular and conformational properties within the whole de Vries SmA mesophase and at the SmA–SmC* transition, thus adding fundamental information to discriminate among the theoretical models reported in the literature. In particular, we focus our attention to the representative de Vries smectogen 9HL, a (S)-hexyl-lactate derivative, which consists of a rigid core of three benzoate moieties and a lactate unit, and two linear alkyl chains of 6 and 9 carbon atoms respectively (see Figure 1).[41, 42] A 13C NMR study of the 9HL in its mesophases is

Figure 1. Optimized geometry of the smectogen 9HL-d2.

reported here, starting from the assignment of the carbons through a combination of 2D experiments (1H-13C HETCOR,[43] 1 H–13C PDLF (proton-detected local field),[44] and 13C13 C INADEQUATE[45] (incredible natural abundance double quantum transfer experiment) to the determination of the isotropic and anisotropic components of the 13C chemical shift. The determination of 13C chemical shift tensor principal elements dii and their relative orientation is done by combining data from the literature and line-shape analysis of the 2D PASS (phase-adjusted spinning sidebands) experiment[46] performed in the solid state of 9HL. In the last part of the paper, the values of the 13C chemical shift anisotropy (CSA) for the various carbon sites, as well as the values of the 1H–13C dipolar couplings obtained from the PDLF experiments[47] are analyzed in the whole mesophase range to get consistent and independent information about the local and molecular main orientational order parameters and the tilt of the aromatic fragments in the two smectic phases. These results will be finally discussed in terms of molecular modelling of the de Vries smectic phases.

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The assignment of the observed 13C chemical shift (hdi) in the SmA phase (Figure 3) was performed by comparing data reported in the literature[48–50, 52] for similar compounds, and it was validated by the analysis of 2D 1H–13C HETCOR[53] experiments acquired at different mixing time (reported in the Supporting Information). This assignment has been used for the analysis of the 2D PDLF experiments, and it was confirmed also from the analysis of 2D PASS experiments carried out in the solid phase, as described in the following paragraphs. A typical 13 C NMR spectrum of 9HL in the SmA phase (at T = 391 K) and the relative assignment of the Figure 2. Structure of 9HL-d2 molecule and assignment of the carbon sites (top); 13C NMR spectrum of 9HL in sodifferent carbon sites is reported lution (bottom). in Figure 3. The 13C proton-decoupled spectra recorded in the aligned 2. Results and Discussion liquid crystalline phases, as shown in Figure 4, present resolved peaks whose shifts depend on the CSA tensor (magnitude of 2.1. 13C NMR Spectra of 9HL in the Isotropic and in the the tensor principal components d33 ; d22 ; d11 and the relative Liquid Crystalline Phases orientation of the principal axes 1, 2 and 3) and on the degree The assignment of the 13C NMR chemical shifts of 9HL and 9HL-d2 was done in a CDCl3 solution (see Figure 2) with the help of 1D 13C, and 2D 1H-13C HMBC and 1H-13C HSQC exTable 1. Assignment of the 13C sites (three aromatic rings) and relative isotropic chemical shift diso, as determined in solution, by INADEQUATE experiperiments (reported in the Supporting Information). Data [48–50] ments in the bulk and by DFT calculations in vacuum. The principal values of available in the literature for similar compounds and the chemical shift tensor, dii, as determined by 2D PASS experiments for the DFT calculations in vacuum of the molecule 9HL were used carbons sites which were analyzed are also reported. to support the assignment of all 13C sites. The solution 13 13 diso d33 C diso diso d22 d11 C NMR spectrum of 9HL in CDCl3 acquired at room tem(PASS, (PASS, (PASS, site (solution, (INADEQUATE, (DFT 13 perature and the relative C site assignment is reported in CDCl3, T = 411 K) in T=295 K) T=295 K) T=295 K) iso Figure 2. The values of isotropic chemical shift, d , for the T = 298 K) vacuo) three-ring aromatic core are also reported in Table 1. 10 164.02 164.47 167.35 250.36 131.64 114.68 This solution assignment has been used as a guide for 11 114.40 115.30 111.17 – – – the assignment of the 9HL in the bulk. Moreover, 2D 13C– 12 132.50 132.69 137.65 233.31 156.44 8.74 13 13 121.05 122.11 122.97 – – – C INADEQUATE spectra were acquired in the bulk isotropic 14 132.50 132.69 134.97 233.31 156.44 8.74 phase of 9HL at T = 411 K, and the spin systems were char15 114.40 115.30 119.73 – – – acterized thanks to 2Q resonance pathways (see the Sup16 164.55 164.08 164.36 251.17 142.61 98.13 porting Information), whose values are also reported in 17 155.05 155.74 160.54 256.92 141.00 70.44 18 122.00 122.06 123.92 – – – Table 1, for comparison. Notably, the assignments obtained 19 131.80 131.53 136.34 229.91 154.52 7.70 independently in CDCl3 solution and in the bulk through 2D 20 127.45 128.16 127.51 – – – INADEQUATE are in good agreement, while they show 21 131.80 131.53 134.33 229.91 154.52 7.70 some small deviations with respect to the DFT values also 22 122.00 122.06 121.97 – – – 23 165.32 165.08 164.36 258.44 134.59 111.09 reported in Table 1. 13 24 155.90 156.42 159.84 256,92 141,00 70,44 C cross-polarization (CP) NMR spectra of the 9HL, in the 25 122.40 122.33 124.42 – – – bulk aligned mesophases, were also recorded as a function 26 132.05 131.95 134.59 229.91 154.52 7.70 of temperature by using a CP sequence with 1H decoupling 27 126.60 127.11 128.23 – – – 28 132.05 131.95 135.50 229.91 154.52 7.70 (SPINAL-64)[51] during the acquisition, in order to determine 29 122.40 122.33 121.91 – – – the trend of order parameters of all carbon sites as a func30 164.05 163.61 168.15 255.95 120.93 112.57 tion of the temperature.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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www.chemphyschem.org axial symmetry and the measured simply written as Equation (1):[52, 54] hdi  dISO

13

C chemical shifts can be

  2 1 ¼ P2 ðcosðbÞÞðd33  d22 Þ þ ðd22  d11 Þ SZZ þ 3 2 1 þ ðd11  d22 cos2 ðbÞ  d33 sin2 ðbÞÞD 3

Figure 3. Assignment of the carbon sites of 9HL on the static 13C CP NMR spectrum in the SmA phase.

Figure 4. Variable-temperature stacked 13C CP NMR static spectra of 9HL sample acquired on a 5 mm static probe at 500 MHz (temperatures decrease from top to bottom).

of order of the aligned phase (namely the orientational order parameters).[52, 54] In principle, the molecular orientational order parameters can be exploited from the analysis of the 13C chemical shift of the static aligned liquid crystalline phase once the CSA tensor of each investigated 13C spin is known. In the case of the aromatic core of the 9HL, the carboxyphenyl moieties can be considered as rigid fragments with uni 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ð1Þ

with SZZ and D ¼ ðSYY  SXX Þ the principal order parameters of the molecular fragment, hdi the 13C chemical shifts measured in the aligned liquid crystalline phase and dISO the isotropic chemical shift. The principal component of the CSA principal axis frame (PAS) is conventionally chosen as d33  d22  d11 . The angle b spans between the direction of d33 of the carbon PAS and the para axis of the phenyl ring (Z in the reference ring frame). The principal components of the CSA tensor can be experimentally determined by solid-state NMR of the 9HL in the microcrystalline powder phase. The CSA principal components characterize the powder pattern lineshapes of each carbon in the crystalline static powder spectra. As well, at moderate magic-angle spinning (MAS) frequencies, the CSA principal components modulate the pattern of the sidebands intensities of solid powder samples. The analysis of the intensities of the sideband patterns makes possible to determine the principal components of the CSA tensor,[55] but in fact the analysis of the sideband intensity in 1D low-MAS spectra is strongly hampered by the severe overlap among the sidebands pattern of different 13C spins. A successful approach is to use 2D NMR techniques in order to separate the isotropic and anisotropic contributions in different dimensions, with a significant increase in resolution. This is for example the approach developed in 2D PASS[56] or the 2D SUPER (separation of undistorted powderpatterns by effortless recoupling)[57] experiments. In particular, the 2D PASS experiment, operating under slow magical angle spinning MAS rate, does not average out the powder chemical shift anisotropy, but spread it out in spinning sidebands (SSBs) that move along the second dimension as a function of their frequency order, allowing the reconstruction of the powder pattern of the single carbon signal.[56] A 2D PASS NMR spectrum acquired in the not-oriented microcrystalline phase of 9HL (T = 295 K, MAS frequency 2.0 kHz) is shown in Figure 5 A. In principle, all single CSA powder patterns could be isolated from the spectrum. In practice, some overlapping can still be present due to lack of resolution and the small differences in the isotropic chemical shifts of the 13C nuclei of the 9HL, as can be seen from the zeroth-order spectrum shown in Figure 5 A. Nevertheless, several resonances can be resolved, enabling the reconstruction of the corresponding SSB powder pattern correctly. With this in mind, the analysis to determine the CSA principal components was limited to these carbon sites. The assignment of the solid-state isotropic resonances (SSB zeroth-order spectrum) was done on the bases of the strict correspondence with the spectrum acquired in the bulk isotropic phase and reported in Figure 2. Once the intensities of each SSB peak were measured, the principal components of the CSA tensor can be determined by fitting the exChemPhysChem 2014, 15, 1485 – 1495

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www.chemphyschem.org dANISO ¼ dISO  d33 h¼

ð2Þ

ðd22  d11 Þ dANISO

where dISO ¼ ðd33 þ d22 þ d11 Þ=3 is the isotropic chemical shift. In the following the convention d33  d22  d11 is applied. Once the best values of dANISO and h are obtained from the fitting optimization, the three principal components d33 ; d22 ; d11 can be determined from Equation (2) and the isotropic chemical shift dISO . The fitted values of d33 ; d22 ; d11 for the nine carbon sites analyzed (those having sufficient resolution) are reported in Table 1.

Figure 5. A) 2D PASS NMR spectrum of the 9HL in the solid state with different sidebands orders. Experimental (circles) and fitted (solid curves) sidebands profiles for the carbon site 24 (B) and carbon site 30 (C).

perimental pattern with the pattern simulated by the software SPIN EVOLUTION[58] under the same conditions of MAS frequency. Figure 5 B and 5 C show the excellent agreement among the calculated and the experimental values. The SSB powder pattern is fully determined once the dANISO and h are known [Eq. (2)]:[52, 55]

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We focused the attention on the aromatic rings composing the more rigid part of the 9HL. For each aromatic ring we were able to determine the CSA principal components for three carbon sites. The orientation of the CSA principal axes can be determined on the basis of the symmetry of the aromatic ring and by comparison with the values reported in literature.[52, 54] The orientational order parameters for the three phenyl rings can be thus determined on the bases of Equations (1) and (2). From the orientation of the CSA principal axes, and assuming a regular hexagonal geometry for the phenyl ring, the angle b in Equation (1) can be assigned to b = 08 and b = 608 for quaternary and ternary carbons, respectively.[52, 54] For the carbonyl atoms, the angle b was optimized to a values around 308, which is in good agreement with previously published results.[47–50, 52, 54] At this point, from the 13C chemical shifts measured at different temperatures in the aligned SmA phase it is possible to fit the orientational order parameters, SZZ and D, for each aromatic ring on the bases of Equations (1) and (2). In this fitting, it is assumed that the CSA tensor remains constant along the temperature interval, and that the 13C atoms in each carbonyl phenyl ring have the same order parameters.[47–50, 52, 54] Figure 6 reports the fitted order parameters vs temperature in the SmA phase, for the three aromatic moieties. The values of Szz for the three rings are close each other, within experimental error. The minimum and maximum values of Szz are ~ 0.66 (at higher temperature) and ~ 0.83 (at lower temperature), respectively. These values are typical of rod-like smectogens in the SmA phase.[36–38] With respect to the trend of Szz determined by previous 2H NMR studies for 9HL-d2,[5, 15] selectively deuterated on the first aromatic ring close to the achiral chain, the values reported in Figure 6 a are slightly higher at lower temperatures, showing a steeper increase by decreasing the temperature. However, the values of the order parameter Szz, obtained by CSA analysis, are affected by an error of 6–8 %, as shown in Figure 6 a by the error bars, which is higher than the that associated to the 2H NMR data analysis. From the CSA analysis, the biaxiality (D) of each aromatic ring was determined, as reported in Figure 6 b. These values are small (0.05  0.02) and they are of the order of magnitude of the biaxiality found in rod-like smectogens in their SmA phases.[37, 38] This is an indication of a fast reorientation of the aromatic rings around the long molecular axis (typical values of the spinning diffusion constant, D k , are 109–1011 s1).[34, 37, 38] ChemPhysChem 2014, 15, 1485 – 1495

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www.chemphyschem.org where i and j represent the two nuclei (namely 1H and 13C), rij is the distance between the two nuclei, Sij is the order parameter referred to the direction of rij, and K’ij is the dipolar constant [Eq. (4)]: K 0 ij ¼ gH gC h=ð8 p2 Þ

ð4Þ

with gH and gC the gyromagnetic ratios of the protons and carbons respectively. In the case of the PDLF experiments, the observed dipolar resonance splitting Dij is given by Equation (5): jDij j ¼ 2 kDij

ð5Þ

where k is the scaling factor typical of 1H DUMBO decoupling used in the indirect dimension (here k = 0.47, as explained in the following). In this work, we focus on the analysis of the three aromatic rings of the 9HL. In Figure 7 a scheme of several dipolar couplings between aromatic carbons and close protons in

Figure 6. Values of Szz (a) and biaxiality Dbiax (b) of the three rings of 9HL (ring close to the achiral chain: grey circles; central ring: empty circles; ring close to the lactate unit: black circles) as determined from the analysis of 13C chemical shift anisotropies (see the text).

In the case of 9HL, the measured 1H and 2H longitudinal and transverse relaxation times (not reported here) confirm that the dynamic properties of its aromatic core are similar to common smectogens, and this is also in agreement with Raman[32] and Dielectric Spectroscopy results.[41]

Figure 7. Example of 1H–13C dipolar couplings for the first aromatic ring of the 9HL (with carbon labels 10, 11, 12, 13, 14 and 15) and the relevant geometrical features (r is the proton-carbon distance, and q is the relative angle).

a phenyl ring is shown. In particular, it is possible to see a short-range dipolar coupling, between the carbon C11 and the proton H11, and two long-range dipolar couplings, between carbon C10 and proton H11, and between carbon C11 and proton H12. If we neglect the biaxiality of the ring, the observed dipolar couplings can be related to the main orientational order, Szz, referred to the para axis of the ring (see Figure 7), given by Equation (6): jDij j ¼ 2 kK ij Szz P2 ðcosqÞ

ð6Þ

2.2. 1H–13C PDLF NMR Experiments 2D 1H–13C PDLF[44] experiments were performed on the 9HL sample in the SmA and SmC* phases, as described in the Experimental Section. This experiment has the aim to obtain the residual dipolar couplings between each 13C site of the 9HL molecule and the protons in close proximity.[52, 59] In our analysis we have neglected[47] the effect of the scalar coupling, Jij, focusing on the direct dipolar coupling, Dij, defined according to Equation (3): Dij ¼ Sij K 0 ij =ðr ij Þ3  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ð3Þ

with Kij = K’ij/(rij)3, P2(cosq) is a second-order Legendre polynome and q the angle between rij and the para axis, z. For 9HL, the scaling factor k of Equation (6) has been determined by comparing the experimental splitting Dij with the value of the order parameter, Szz, independently determined by 2H NMR at the same magnetic field (1H Larmor frequency of 500 MHz). 9HL-d2 is the same compound, selectively deuterated in positions C11 and C15.[5, 15] In particular, the value of Szz at T = 343 K (in the SmA phase) is 0.765, and, if we assume a perfect hexagonal geometry (with rC11-H11 = 1.09  and qH11-C11-C12 = 1208),[52] ChemPhysChem 2014, 15, 1485 – 1495

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CHEMPHYSCHEM ARTICLES KC11-H11 is equal to 22 686 Hz. By considering the observed scalar dipolar coupling j DC11-H11 j = 2050 Hz at the same temperature, the scaling factor k results equal to 0.47, in agreement with the literature.[60] In the case of the aromatic carbon C11, long-range dipolar couplings can also be observed. For instance, an observed j Dij j in the range of about 1000–1100 Hz corresponds to the coupling between C11 and H12 (with rC11[52] which is H12 = 2.17 , qH12-C11-C12 = 268 and KC11-H12 = 2955 Hz) consistent with the same scaling factor k. However, due to the crowded number of splitting peaks overlapping in the centre of dipolar dimension of the PDLF (smaller splittings, around the 0 axis of the indirect spectral dimension), we do not analyse the long-range couplings, but only the stronger direct dipolar couplings, assuming a constant scaling factor k = 0.47. An example of complete 2D PDLF spectrum in the SmA phase of 9HL is reported in Figure 8, where the horizontal scale corresponds to the carbon chemical shift (in ppm) and

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Figure 9. 1H-13C NMR slices corresponding to the 13C carbon site “31” in the lactate unit of 9HL at different temperatures: from the SmA (bottom) to the SmC* (top).

Figure 10. Values of Szz of the three rings of 9HL (ring close to the achiral chain: grey circles; central ring: empty circles; ring close to the lactate unit: black circles) as determined from the analysis of PDLF scaled dipolar couplings. Solid and dashed curves are guidelines only. Figure 8. Example of 2D PDLF NMR spectrum of 9HL recorded at T = 391 K.

the vertical one shows the scaled 13C–1H residual dipolar splittings, j Dij j . In Figure 9, the F1 trace of the carbon site “31” (in the lactate unit of 9HL) at different values of the temperature from the SmA to the SmC* phase is reported. For the purpose of the present study, we focus on the rigid aromatic region. In particular, the order parameters, Szz of the three rings, referring to their para axes, were determined from the measured direct dipolar couplings of the three rings j DCiHi j by Equation (6). The trends of the orientational order, Szz, in the SmA and SmC* phases of 9HL, of the three rings, are reported in Figure 10. As shown in Figure 10, in the SmA phase the values of the orientational order Szz range from a minimum of 0.66 to a maximum of 0.80, which is typical of SmA phases formed by rodlike mesogens and it is in quite good agreement with the values previously determined by 2H NMR spectroscopy of 9HL 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

d2 as well as with those reported in Figure 6, determined independently from the CSA analysis. It should be noted, however, that the PDLF analysis has a higher precision than the CSA one (the error here is lower than 2 %, as seen from the error bars in Figure 10). As a consequence, the PDLF study enables us to see significant differences among the three rings within the investigated temperature range. In particular, in the SmA phase, the most aligned ring is the central one, while the two lateral rings have approximately the same orientational order parameter. When the SmA–SmC* phase transition occurs, the values of Szz for all rings decrease, showing typical behavior of ferroelectric SmC* phases, which are not unwound by the magnetic field.[39, 40] The apparent decreasing of Szz as increasing the temperature is due to the increasing tilt angle of the fragment zaxis with respect to the helical axis of the SmC* phases, which is parallel to the external magnetic field.[39, 40] Interestingly, the Szz of the lateral ring closest to the chiral chain (lactate unit) presents a sensitively decreasing in the SmC* phase with reChemPhysChem 2014, 15, 1485 – 1495

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spect to the other two rings. This fact can be interpreted with the occurrence of a higher tilt of the para axis of this ring with respect to the other two rings which are almost collinear (see Figure 11). By comparing the values of Szz of the three rings in * Þ at T = 320 K) with the extrapolated the SmC* phase (i.e. SðSmC zz

Figure 11. Sketch of the two different averaged conformations in the SmA and SmC* phases, taking into account the obtained values of the order parameters of the three rings of the smectogen 9HL.

ones from the SmA phases, SðexSmAÞ , (see also curves in zz Figure 10) a tilt # of about 158 for the two almost collinear rings and a tilt # of about 208 for the ring close to the lactate unit can be evaluated, according to Equation (7): ffi1 0sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðexSmAÞ Szz 2 ðSmC Þ þ 1 * Szz B C # ¼ arccos@ A 3

ð7Þ

2.3. The SmA–SmC* Transition of 9HL in Terms of the Present Models In the previous two sections, the temperature-dependent 13C chemical shift hdi and 1H–13C PDLF scalar dipolar couplings of the aromatic core of the de Vries smectogen 9HL have been analyzed in order to obtain the local order parameters, Szz and Dbiax, of the three aromatic rings. The two sets of experimental data, 13C chemical shift and 1H–13C PDLF scalar dipolar couplings, were analyzed independently. Previous 2H NMR studies[5, 15] of the 9HL-d2, selectively deuterated on the first aromatic ring close to the achiral chain, showed the presence of a small tilt (up to 108) already in the SmA phase, which is rather unusual in normal SmA phases. The existence of a tilt in the SmA phase was shown by 2H NMR investigations performed at various magnetic fields, ranging from 4.70 to 18.8 T, indicating a parallelism between the well-known electroclinic effect and an analogous magnetoclinic effect. Based on this 2 H NMR evidence,[5] we proposed a model, the “cluster diffuse cone model”, with the purpose of explaining this large magnetoclinic effect, which cannot be understood at a single-molecule level. This experimental finding, however, concerned a par 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ticular fragment of the smectogen, namely one of the three rings of the aromatic core, so that it was not possible to generalize the result to the whole molecule. This aspect motivated us to extend the study at least to the rigid core, which is well represented by the three-ring core. A first aspect concerns the determination of the values of the local order parameter, Szz, for the three aromatic rings, which is representative of the average orientation of the para axis of the three rings. The analysis of 13C chemical shift as a function of temperature in the SmA phase, reported in Section 2.1, provides a general indication that the three rings are oriented along the same direction, taking into account the error bars, as shown in Figure 6 a. The analysis of the direct 1H– 13 C dipolar couplings for the three rings, as obtained by the 2D PDLF spectra, and reported in Section 2.2, gives a more precise result, indicating that the central aromatic ring is less tilted with respect to the other two external rings (see Figure 10). This average orientation of the three rings forming the rigid core of 9HL is schematized in Figure 11 (on the left). The most important result is that these solid-state NMR techniques (2H NMR, 13C NMR and 2D PDLF 1H-13C NMR) show, independently, that in the SmA phase, there is a rather high orientational order parameter, Szz, up to 0.78  0.83 close to the SmA– SmC* phase transition. Moreover, the orientational order parameter in the SmA phase is approximately the same for the three rings, which can be considered aligned on average along a preferred direction (see zaro in Figure 11, on the left). This high value of the orientational order is not consistent with those found, for instance, in nanosegregated de Vries systems,[16, 17, 30] for which the “interdigitated model” is invoked to explain the de Vries behaviour. In the case of the 9HL smectogen, in order to discriminate among the existing models, we need to take also in account the results obtained by NMR spectroscopy in the SmC* phase. In previous 2H NMR investigations of the 9HL,[5, 15] the experimental data in the SmC* phase showed the occurrence of a large tilt of the deuterated aromatic ring, up to a value of 288 at low temperatures, with respect to the SmC* helix. This large tilt, which is typical of ferroelectric liquid crystals, was not expected in de Vries SmC* phases, as X-ray diffraction shows a small layer shrinkage at the SmA–SmC* phase transition. The absence of a layer shrinkage and the occurrence of the large tilt in the SmC* phase of the deuterated ring of 9HL were not easy to be interpreted by recurring the sole “cluster diffusion cone model”. In the present work, however, the analysis of the direct 1H– 13 C dipolar couplings for the three rings of 9HL, as obtained by the 2D PDLF spectra in the SmC* phase, gives a possible explanation. In fact, as shown in Figure 10, the order parameter of the three rings is far from being similar each other. In particular, one of the three rings, the one close to the chiral lateral chain, has much larger tilt, while the other two rings are almost collinear. This is represented in Figure 11 (on the right). The large tilt of the ring close to the lactate unit clearly indicates a change in the averaged conformation of the 9HL at the SmA–SmC* transition.

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CHEMPHYSCHEM ARTICLES The occurrence of a conformational change is also supported by the trends of the measured 13C chemical shift hdi of the alpha methylenoxy carbons (31) and (34) in the chiral lateral chain (region of 40  75 ppm) at the SmA–SmC* transition, as reported in Figure 4. In particular, hdi of carbon (34) is about 71–72 ppm in the whole SmA phase and it decreases until 64 ppm in the SmC* phase; on the contrary, the observed chemical shift hdi of carbon (31), closer to the aromatic region, ranges from 52 ppm to 39 ppm in the SmA phase and it increases up to 46 ppm in the SmC* phase. This change in temperature-dependence of hdi, opposite for the two carbon sites is an indication of a different configuration of the lactate lateral chain in the SmA and SmC* phases, similarly to what has been previously observed in two smectogens having a lactate chain as the 9HL.[25, 26, 38] In Figure 11, the two different conformations are schematized, in a way which could be consistent with the experimental finding of no layer shrinkage at the SmA–SmC* phase transition, since in the SmA phase, the lateral chain is tilted with respect to the layer normal and in the SmC* phase it is almost along that direction. This hypothesis should be further investigated, for instance by DFT calculations, but at the moment, the present analysis of the experimental NMR data in the aromatic core, suggests that both the “conformational change model”, first proposed by Diele,[21 ]and our “cluster diffusion cone model”[5] are well-suited to the case of 9HL and they can indeed coexist in the explanation of all the experimental results obtained so far by different techniques.[5, 15, 32]

3. Conclusions In this work we have extended the NMR study of a well-known de Vries smectogen, namely 9HL, to the whole rigid core, constituted of three rings and the carbonyl groups bonded to them. Several experimental solid-state 13C NMR techniques were applied in order to have a consistent and solid analysis in terms of local orientational order parameters, able to describe the average orientation of the three rings constituting the rigid core of 9HL as a function of temperature from the SmA to the SmC* phases. In particular, the global fitting of the temperature-dependent 13C chemical shift hdi in the SmA phase confirmed that the orientational order parameters, SZZ and D, are typical of rod-like smectogens and in particular, the values of order parameters for the three rings are similar, ranging between ~ 0.66 to ~ 0.83 in the SmA phase. The independent analysis of the scaled direct 1H–13C dipolar couplings, j DCi-Hi j , for the three aromatic rings in the SmA and in the SmC* phases, allowed us to obtain important information about the relative orientation of these three rings each other. In particular, in the SmC* phase, the first aromatic ring closer to the chiral lateral chain is much more tilted of the other two, which are almost collinear, thus indicating the occurrence of a conformational change at the SmA–SmC* phase transition. These experimental findings are at the basis of our hypothesis of average conformations in the two smectic phases, which is  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org consistent with the experimental observation of no layer shrinkage at the mesophase transition. Concerning the known models invoked to explain the behaviour of the de Vries SmA–SmC* phase transition, the present results based on 13C NMR and 1H–13C NMR techniques, combined with those previously obtained by 2H NMR spectroscopy on the same smectogen 9HL, show that the two models, “cluster diffusion cone” and “conformational change” ones, well adapt to the case of 9HL. The “cluster diffusion cone” model, in fact, is the only one which can explain the magnetoclinic effect, namely the increase of tilt of the aromatic fragments in the SmA phases under the effect of external magnetic fields, while the “conformational change” model, which was clearly confirmed by the present NMR study, is able to conciliate the occurrence of a large tilt of the rigid core in the SmC* phase with the evidence of no layer shrinkage at the SmA– SmC* phase transition.

Experimental Section Samples Samples (9HL and 9HL-d2) were synthesized and characterized according to the literature procedure previously reported in ref. [41].

NMR Experimental The 13C NMR experiments on the 9HL sample were performed on a Bruker-Biospin NMR spectrometer Advance III operating at 11.7 T (500 MHz of Proton Larmor frequency). The isotropic 13C NMR spectrum of the sample was acquired in CDCl3 at room temperature with a 13C 908 hard pulse of 13 ms and acquisition time under proton decoupling of 1.4 s, with a number of scans of 64 and a repetition delay of 10 s. The 13C NMR spectrum in 9HL bulk isotropic phase was acquired at T = 411 K with a 13C 908 hard pulse length of 8.25 ms and acquisition time of 34 ms, under proton decoupling, 1024 scans and a repetition delay of 2 s. At the same temperature the 2D-INADEQUATE spectrum was also acquired, with 128 scans and 32 768  512 real points (F2  F1) with acquisition times of 412 ms and 256 ms for the direct and indirect dimensions. A repetition time of 2 s was chosen and direct dimension acquisition was carried out under proton decoupling. All chemical shift assignments discussed in the text are referred to the standard 13C resonance of the tetramethylsilane (TMS). The 1D 13C CP NMR spectra were acquired in the smectic phases of 9HL in a solid-state MAS 4 mm Bruker probe. The sample was packed into a 4 mm zirconia rotor and spectra were acquired on oriented sample under static conditions in the temperature range 398–340 K in steps of 3 K. A prescan delay of 450 s was used to stabilize the temperature before any temperature change. The 908 pulses of 1H and 13C were set to 5 and 10 ms, respectively, the contact time for the cross polarization was optimized to 2.5 ms. The acquisition time was set to 34 ms, the spectra window to 50 kHz and 320 scan with a repetition time of 5 s were performed for those experiments. A 70 kHz spinaL-64 proton decoupling was applied during the acquisition time. The 2D 1H–13C PDLF NMR experiments were performed on the same sample in the temperature range of 391–331 K in steps of 6 K and with a temperature stabilization of 450 s. The 908 pulses of 1 H and 13C were set to 5 and 11.3 ms, respectively; 1.5 ms contact time was used for cross polarization in the liquid crystalline phase for these experiments. Spectra were acquired with 8 scans and ChemPhysChem 2014, 15, 1485 – 1495

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CHEMPHYSCHEM ARTICLES 1024  256 real points (F2  F1) with acquisition times of 34 ms and 32 ms for the direct and indirect dimensions. A repetition time of 3 s was chosen to ensure full relaxation between each scan. The 2D CP PASS experiment was performed in the solid phase of 9HL with the solid-state probe. The sample was inserted in a 4 mm rotor and spun at 2.0 kHz to create a spinning side band pattern in the spectrum second dimension. The 908 pulses of 1H and 13C were set to 2.5 and 5 ms, respectively, leading to a 10 ms 1808 13C pulse, while the delays between the 13C 1808 pulses have been set according to reference.[56] The contact time for the cross polarization was optimized to 1.6 ms in the solid phase. The spectrum was acquired with 1215 scans and 1024  16 real points (F2  F1) with acquisition times of 34 ms for the direct dimension F2. A repetition time of 3 s was chosen to ensure full relaxation between each scan. A 70 kHz spinaL-64 proton decoupling was applied during the acquisition time.

DFT Optimization The molecular structure of 9HL (see Figure 1) was built up by GaussView 4.1, and all calculations were done with the Gaussian 03 computational package. The geometry of 9HL was optimized in vacuum with DFT methods by exploiting the B3LYP/6-311G(d) combination of hybrid functional and basis set. The 13C nuclear shielding tensors were calculated at the DFT level of theory using GIAO approach.

Data Analysis NMR data were processed through TopSpin 3.2, 2D PASS NMR spectra were simulated and fitted by Spinevolution software (version 3.4.2) and the analysis of most of the NMR data in terms of orientational order parameters was performed by using homemade programs written in Mathematica 5.0 software (copyright 1988–2003, Wolfram Research).

Acknowledgements We acknowledge the financial support “Egide” (programme Galile) from the Universit Italo-Francese (Programma Galileo 2009/ 2010 and 2012/2013). This work was partially founded by the COST D35 WG13-05 project (between the Italian and Czech groups). We would like to thank Prof. Lyndon Emsley for helpful discussions and Prof. James Emsley for showing the right setup of the static probe. Keywords: conformations · liquid crystals · orientational order · phase transitions · solid-state NMR spectroscopy [1] A. de Vries, Mol. Cryst. Liq. Cryst. Lett. Sect. 1977, 41, 27. [2] A. de Vries, J. Chem. Phys. 1979, 71, 25; A. de Vries, A. Ekachai, N. Spielberg, Mol. Cryst. Liq. Cryst. Lett. Sect. 1979, 49, 143. [3] A. de Vries, Mol. Cryst. Liq. Cryst. Lett. Sect. 1970, 11, 361. [4] J. P. F. Lagerwall, F. Giesselmann, ChemPhysChem 2006, 7, 20. [5] A. Marchetti, V. Domenici, V. Novotna, M. Lelli, M. Cifelli, A. Lesage, C. A. Veracini, ChemPhysChem 2010, 11, 1641. [6] Q. X. Song, A. Bogner, F. Giesselmann, R. P. Lemieux, Chem. Commun. 2013, 49, 8202. [7] Z. V. Kost-Smith, P. D. Beale, N. A. Clark, M. A. Glaser, Phys. Rev. E 2013, 87, 050502. [8] M. Osipov, G. Pajak, Phys. Rev. E 2012, 85, 021701. [9] Y. P. Panarin, V. Panov, O. Kalinovskaya, J. K. Vij, J. Mater. Chem. 1999, 9, 2967.

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Received: November 11, 2013 Published online on January 31, 2014

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