13C NMR study of molecular ordering in a discotic columnar mesophase

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13C NMR study of molecular ordering in a discotic columnar mesophase

V. Rutar, R. Blinc, M. Vilfan, A. Zann, J.C. Dubois

To cite this version:

V. Rutar, R. Blinc, M. Vilfan, A. Zann, J.C. Dubois. 13C NMR study of molecular or- dering in a discotic columnar mesophase. Journal de Physique, 1982, 43 (5), pp.761-765.

�10.1051/jphys:01982004305076100�. �jpa-00209448�

13C NMR study of molecular ordering ^{in a} discotic columnar mesophase

V. Rutar, ^R. Blinc, ^{M. Vilfan}

J. Stefan Institute, ^E. Kardelj University ^of Ljubljana, Ljubljana, Yugoslavia

A. Zann and J. C. Dubois

Laboratoire Central de Recherches Thomson-CSF, ⁹¹⁴⁰¹ Orsay, ^France (Reçu le 5 octobre 1981, accepté ^le 13 janvier 1982)

Résumé.

La dépendance ^en température de l’orientation moléculaire des noyaux aromatiques ^et des chaînes

aliphatiques ^{dans la} phase ^cristal liquide colonnaire de l’hexapentyloxy-triphénylène ^{a été} déterminée en utilisant la spectroscopie de résonance magnétique nucléaire de haute résolution du 13C. On a trouvé que les chaînes ^ont

une configuration déployée (diablo). Les noyaux ^et les chaînes sont coplanaires ^et orientés de telle façon que le

champ magnétique ^{extérieur est dans le} plan ^de la molécule. Les noyaux aromatiques ^sont presque complètement

orientés (Sar

0,85 ± 0,10) ^et la mobilité des segments ^{de chaîne} augmente sensiblement quand la distance au

noyau aromatique augmente.

Abstract.

The temperature dependence of the molecular ordering of the aromatic rings ^{and the} aliphatic ^chains

in the columnar liquid crystalline phase ^of hexapentoxy-triphenylene has been determined using high ^resolution proton ^enhanced 13C NMR spectroscopy. The chains have been found to be preferentially in the extended (diablo) configuration. ^The rings and chains are coplanar ^and ordered in such a way that the external magnetic ^{field lies}

in the molecular plane. ^{The aromatic} rings ^are nearly completely ^ordered (Sar

^{0.85 ±} 0.10) whereas the mobi-

lity of the chain segments significantly ^{increases with} increasing ^distance from the aromatic core.

Physics ^Abstracts

61.30G - 64.70E

76.60C

1. Introduction.

Discotic liquid crystalline phases [1-3] ^are usually ^formed by molecules which contain a flat rigid ^aromatic part ^to which several

alkyl ^{chains are bonded in a} symmetric ^{way. In the} majority of the discotic mesophases the disc-like molecules are stacked one on the top of the other in columns. The positions of the molecules within

a column can be either ordered or not, and there is

no correlation between the positions of molecules in different columns.

Whereas the molecular ordering ⁱⁿ liquid crystal-

line phases ^formed by « rod-like » molecules is relative-

ly well understood _very little is known about the details of the molecular ordering in the discotic

mesophases. Only recently the first deuterium NMR

study ^{of a discotic} mesophase ^{has been} published [4].

In this contribution we report ^a study of the mole- cular ordering ^{in the discotic columnar} phase ^of hexapentoxy-triphenylene using high resolution 13C NMR spectroscopy ^{which has been} applied ^{for the}

first time to « rod-like » liquid crystals by ^{Pines and} Chang [5].

2. Experimental.

^The sample used in this study

was synthesized in the Thomson-CSF laboratory ^as

described earlier [2]. ^The transition from the solid into the columnar phase ^takes place ^at T. ^{= 69°C and}

from the columnar into the isotropic phase ^at T c ^{= 119 OC. It has been} observed that the transition temperature Tc is somewhat lower in the present sample ^{what can be} attributed to impurities [6]. X-ray

diffraction studies of hexapentoxy-triphenylene ^have

shown [3] that there exists within a column a one-

dimensional positional ordering of the aromatic parts of the molecules and that the system belongs ^{to the} Dh. class where the columns form a regular hexagonal

arrangement.

The high resolution 13C NMR spectra ^{were obtained}

on a superconducting magnet (Bo ^{= 6.3 T)} using ^a

Bruker SXP 4-100/270 spectrometer system. ^{In the} isotropic phase ^13C free induction decays ^were directly accumulated and Fourier transformed. In the discotic mesophase the molecular order induces

significant 1 H_13C dipolar coupling enabling ^{one to}

use the proton ^enhanced 13C cross-polarization

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01982004305076100

762

Fig. l. - High resolution 13C NMR spectra ^of hexapen- toxy-triphenylene ^{in the} isotropic ^and liquid crystalline phases. ^{In the columnar} mesophase single ^{contact cross-} polarizations ^{with a} mixing ^{time 7 ms were} used. All che- mical shifts are reported ^{downfield from the external TMS.}

technique [7]. Well resolved 13C signals ^{have been}

obtained with the help ^{of this} technique ^{in the discotic} mesophase (Fig. 1). All chemical shifts were measured downfield from the isotropic position ^{of the} methyl

group and later converted ^to the common TMS scale

by adding 14 ppm. Thus ^we adopted ^the convention,

that chemical shifts decrease with increasing shielding.

The measurements were done on cooling ^the sample,

and decrease of temperature ^never exceeded 0.3 OC in

a minute. Some spectra ^were checked also on heating

from the solid into the columnar phase ^{and no diffe-}

rence was observed within the spectrometer resolution.

3. Results.

Both in the isotropic ^{as well as in the}

columnar mesophase three aromatic and four ali-

phatic (one of them with double intensity) 13C ^reso-

nance lines have been observed. The number of 13C lines thus equals (Fig. 2) the number of chemically non-equivalent ^carbons.

Our measurements clearly ^{show that on} going ^into

the mesophase the aromatic 13C lines are shifted

downfield, while the aliphatic ^ones slightly, ^but distinctly ^move upfield (Fig. 2). ^{Since the most shielded}

axis of ring ^carbons nearly coincides with the normal to the molecular plane, ^we definitely conclude that the

Fig. ^2.

Temperature dependence ^{of the ’ 3C chemical}

shifts in hexapentoxy-triphenylene. ^The assignment ^{of the} 13C resonances is as well shown. Note that the difference between the line positions in the columnar and isotropic phase ^is proportional ^{to the order} parameter.

« discs » are preferentially aligned ^{in such a} way that the direction of the external magnetic field lies within the aromatic plane (i.e. Bo ^is perpendicular ^{to the}

normal of the ^« disc »).

The temperature dependence ^{of the 13C line} posi-

tions shows an abrupt change ^{at the} isotropic-colum-

nar transition temperature T c ^and

contrary ^{to the} nematic phase ^{in MBBA} [5]

nearly ^no temperature dependence ^{in the} mesophase.

4. Discussion.

The existence of narrow 13C NMR lines in the columnar mesophase requires the presence of molecular motions and conformational changes,

which are fast on the NMR time scale as defined by

the anisotropy ^{of the 13C} chemical shift tensors

(- ¹⁰ kHz). These motions are as well responsible ^for

the fact that the number of nonequivalent carbon sites is the same in both phases.

In order to explain ^the spectra ^{we shall use a model} of the columnar mesophase, which includes the

following ^motions (Fig. 3) :

(i) rotation of whole molecules around the normal

to the disc plane, ^{which we} call the aromatic rotation

axis,

Fig. ^3.

A schematic representation ^{of molecular motions}

used to explain ^3C chemical shifts in the columnar meso-

phase. ^The whole molecule rotates around the normal to the

disc, ^which preferentially points perpendicular ^{to the external} magnetic ^field. Aliphatic ^chains additionally ^{rotate and} fluctuate, ^{therefore the} aliphatic ^director

^{which moves} together ^{with the central} part of the molecule

is intro- duced.

(ii) fluctuations of this axis around the preferred

aromatic director,

(iii) aliphatic ^chains additionally ^{« rotate » around}

their long ^axes (aliphatic ^{« rotation »,} which is in fact

a simple description for fast chain isomerization via diffusion of« kink » defects etc.), ^which

(iv) ^fluctuate around the preferred extended confi-

guration.

The motions are supposed ^{to be} independent, ^there-

fore all of them ^« _{average »} the chemical shift tensors

separately. ^The convention that subscripts ^ar (and al) correspond ^{to aromatic} (and aliphatic) motion will be used to make clear, ^which type ^of averaging ^{will be} taken into account in the following step-by-step ^calcu-

lations.

The fast ^« aromatic » rotation leads to an axially symmetric ^{aromatic tensor. The} components ^{of the}

tensor in the frame, ^where the smallest principal ^{axis is} parallel ^to the aromatic rotation axis, ^become

i I 3 ^{and ao i ( Q 1 + (J 2)’ since the most shield-}

ed axis of aromatic 13C tensors nearly coincides with

ring normal, ^and Q1 > a2 1> (J 3 denote the principal

values in the rigid lattice molecular frame.

The aromatic rotation axis fluctuates making ^an angle 9(t) ^{with the} preferential ^direction.

The anisotropy ^AJ >c,

_{Ql - Q II} ^{of the aro-} matic 13C tensors in the columnar mesophase ^{is there-}

fore effectively ^{reduced to :}

Here Sar ^{is the «} aromatic >> order parameter ^defined

as Sar = 2 cos’,9(t) - ’ >, ^{Since in a magnetic}

resonance experiment ^{we measure the} component ^of the 13C chemical shift tensor along the direction of the external magnetic field, ^we find the measured value _Urn for aromatic carbons in the mesophase ^as

ranged ^to

The order parameter of the aromatic rings ^{is thus} directly proportional ^{to the} difference between the chemical shifts obtained in the columnar and iso-

tropic phase. The values of aO - aO ^{were calculated}

from the 13C chemical shift tensors in substituted benzenes [8, 9], since relevant data for hexapentoxy- triphenylene ^{are not} available. For carbon positions (6)

and (7) measurements on a model substance p-dietho- xybenzene [8] ^{gave us} Ql - 6 j ⁼ 126 ppm and 145 ppm respectively, while chemical shift tensors for durene (1,2,4,5-tetramethylbenzene) [9] ^{were used}

to obtain a° - QII ⁼ 174 ppm for carbon position (8).

All consequent results for the aromatic order para- meter lie within Sar ^{= 0.85 ± 0.10, where uncer-} tainty ^{is due} mostly ^{to the} inaccuracy of the data ^on

« static >> tensors [8-10]. ^{It should be} noted, ^{that the} temperature dependence of the aromatic order _para- meter was determined with a better relative accuracy

(Fig. 4) ^{for a} representative ring ^carbon (position 6).

The value of Sar ^increases abruptly ^on cooling ^{from the} isotropic into the columnar phase but within - 5 OC the value Sar ⁼ 0.82 is reached and it remains nearly

constant in the whole mesophase.

The aromatic order parameter, obtained in our 13C study, ^agrees within the experimental ^{error with the}

value S ⁼ 0.90-0.95 reported ^{in a recent 2D NMR} experiment ^{on a} selectively deuterated analogue hexahexyloxy-triphenylene [4].

The aliphatic ^{chains are more} mobile than the central disc, ^since they experience ^not only ^{the aro-}

matic rotation and fluctuations, ^{but also conforma-}

tional changes

^{described as} aliphatic ^{rotation -}

and fluctuations around the preferred chain direction.

Since the most shielded component ^{of the} methylene

Fig. ^4.

Temperature dependence of the order parameter S. For the ring ^carbon (6) the aromatic order parameter Sar

is shown, ^{while the} plotted aliphatic ^order parameter Sal

characterizes relative fluctuations of the aliphatic 0--CHZ-

group (position 5) around the preferred ^extended configura-

tion. See ^{the text} for details and remarks on accuracy.

764

chemical shift tensors coincides with the long ^{axis of}

the chain [11 ], aliphatic ^rotation (Fig. 3) ^{leads to an}

axially 1- ⁼ 2 (arl ^{JL symmetric +} a2). The ^tensor axis of rotation ^{with ao ii} independently ^{- (J 3 and}

fluctuates around the preferred ^extended configuration,

so that an instant value of the component ar(t) along

the aliphatic ^director equals

where 0(t) ^{measures the} angle between the preferred

and instant orientation of the chain segment. ^After usual rearranging ^{we obtain}

Since fluctuations are fast, only ^the averaged ^value

Q )al ^{will be used in the next} calculation. We define

Sal = ! ^cos’ 9(t) - i B ^{as an} aliphatic order para- meter which will characterize chain fluctuations around the aliphatic ^director. Equation (5) ^thus

becomes

Similarly ^{we obtain}

As the aliphatic ^{director rotates} together ^{with the}

whole molecule around the aromatic axis (Fig. 3) ^the components parallel ( all >0,.,,) ^and perpendicular (( ^{(J 1.} >0,.,,) ^{to the ring} normal become

Here again ^{we must take into account} fluctuations around the aromatic director as in equations (1) ^and (2).

The measured aliphatic 13C chemical shift _Qm in the columnar mesophase ^is expressed by

After inserting equations (6) ^and (7) ^into (8) ^and (9)

we finally ^obtain

Equation (10) ^{- based on our} simple ^{model of}

molecular motion in the columnar phase (Fig. 3 ) - clearly shows that the anisotropy ^{of the} aliphatic

chemical shift tensors is effectively ^reduced by aliphatic (Sal) ^{and aromatic} (S.,) fluctuations as well as by ^the

rotation of the whole molecule. Small, ^but distinctly upfield ^{shifts of the} methylene resonances, which are

observed ^on cooling through Tc

(Fig. 2), perfectly

agree with the prediction ^of equation (10).

The aliphatic ^order parameter Sa, ^{is obtained from} equation (10) ^as

Taking 03C31, Q2 and _Q3 from references [8,11], ^{we find} Sal ~ 0.3 for the bulk methylene groups (positions ³

and 4 in figure 2), S., - 0. 1 ^for the terminal methylene

group (position 2) ^and Sal ^{= 0 for the} methyl group.

The _accuracy of the calculation is limited by spectral

resolution (- ^0.5 ppm) ^{as well as} by ^the systematic

error in the value for Sar ^{due to} inaccuracy ^{of the}

« rigid lattices aromatic 13C tensors. Since the ali-

phatic ^order parameter Sal ^is proportional ^{to the diffe-}

rence between the measured chemical shifts in the

isotropic ^{and columnar} phase (11), ^{we were able to} study ^the temperature dependence ^{with a} significantly

better relative accuracy. It ^was found that Sal ^{for the} O-CH2- group (position 5) increases from Sal - ^0.7

close to Tc up ^to Sal - ^{0.95 near} the transition into the solid phase (Fig. 4), ^if Q° - Q II ^{= 51 ppm is}

calculated from the ^« static >> tensor of p-diethoxy-

benzene [8].

The above data confirm that :

(i) ^the aliphatic ^order parameter ^{for the} methylene segments depends ^{on the} position in the chain and it decreases from the central disc towards the end of the

tail,

(ii) ^on cooling through the columnar mesophase ^a

decrease of chain mobility is observed at the

0--CH2- ^group.

It should be noted that the extended (diablo) ^confi- guration ^{of chains was used in our} calculation, ^since

the observed 13C chemical shift spectra definitely

exclude the axial (octopus) configuration. ^Downfield shifts, characteristic for chains perpendicular ^{to the} magnetic field, ^were namely ^not observed. Our results agree with the fact [3] that the average spacing ^between

the molecules in a column is only ^3.6 A, whereas the

spacing ^between the columns equals ^{18.94 A which is} only slightly less than the diameter of the fully ^extended

molecule.

5. Conclusions.

The obtained results show that :

(i) The aromatic discs and the aliphatic ^tails prefe- rentially order in such ^a _way that they ^are coplanar ⁱⁿ

the columnar mesophase ^and that the external magne- tic field lies in the molecular planes.

(ii) ^The aliphatic ^{chains were found to be} preferen- tially in the extended (diablo) configuration.

(iii) ^{The aromatic} rings ^are nearly completely

ordered whereas the mobility of the chain segments significantly increases with increasing ^{distance from}

the aromatic ^core.

(iv) ^{In addition to} «nematic» fluctuations the molecules ^were found to rotate in the columnar phase

around the normals to the aromatic rings.

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