A deuterium nmr study of solute molecules dissolved in the discotic mesophase of p-n-hexahexyloxytriphenylene

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A deuterium nmr study of solute molecules dissolved in the discotic mesophase of p-n-hexahexyloxytriphenylene

D. Goldfarb, Z. Luz, H. Zimmermann

To cite this version:

D. Goldfarb, Z. Luz, H. Zimmermann. A deuterium nmr study of solute molecules dissolved in the discotic mesophase of p-n-hexahexyloxytriphenylene. Journal de Physique, 1982, 43 (2), pp.421-430.

�10.1051/jphys:01982004302042100�. �jpa-00209410�

421

A deuterium NMR study ^of solute molecules dissolved in the discotic mesophase ^of p-n-hexahexyloxytriphenylene

D. Goldfarb, ^{Z. Luz}

The Weizmann Institute of Science, ⁷⁶¹⁰⁰ Rehovot, ^Israel

and H. Zimmermann

Max-Planck-Institüt für Medizinische Forschung, ^D-6900 Heidelberg, Germany

(Rep le 19 août 1981, révisé le 26 octobre, accepte le 28 octobre 1981)

Résumé. 2014 On présente ^les spectres ^{RMN du deutérium de douze} composés deutérés, principalement aromatiques,

dissous dans la mésophase discotique ^de p-n-hexahexyloxytriphénylène normal. Les splittings quadrupolaires ^des

deutériums du soluté sont en général beaucoup plus petits que ^ceux observés dans des cristaux liquides ^thermo- tropiques ^{et en même} temps ^{sont très fortement} dépendants ^{de la} température, changeant ^{souvent de} signe lorsque l’on fait varier la température ^{dans la} région ^{de la} mésophase. ^{Les résultats sont} interprétés par ^un modèle dans

lequel les ^{molécules du soluté} peuvent occuper deux sites ayant ^des paramètres ^{d’ordre de} signes opposés. ^On

propose d’identifier les deux sites (I) ^{à des} molécules intercalées dans la structure en colonnes, ^et (II) ^à des molécules situées dans la région aliphatique des chaînes mésogènes latérales. Les résultats sont en accord avec un modèle dans lequel ^un équilibre dynamique rapide (à l’échelle de temps RMN) ^{s’établit entre les deux} sites, lequel ^est déplacé ^{du site I au} site II par ^un accroissement de température. Ces résultats indiquent que les paramètres magné- tiques ^sont différents pour chaque ^site.

Abstract

Deuterium NMR spectra ^of twelve, mostly aromatic, deuterated compounds ^{dissolved in the discotic} mesophase ^{of normal} p-n-hexahexyloxytriphenylene ^are reported. ^The solute deuterium quadrupole splittings

are in general ^{much smaller than} normally ^{observed in} thermotropic liquid crystals ^{and at the same time} they

exhibit a conspicuous ^dramatic temperature dependence, ^often changing sign ^{as the} temperature ^{is varied within} the mesophase region. The results are interpreted ^{in terms of a model} in which the solute molecules can occupy two sites having ^order parameters ^of opposite signs. ^{The two sites are} tentatively identified with (I) ^molecules

intercalated within the columnar structures, ^and (II) molecules within the aliphatic region of the mesogen side chains. The results are consistent with a model in which there is fast dynamic equilibrium (on ^{the NMR} timescale)

between the two sites, ^which shifts from site I to site II with increasing temperature. The results also indicate that the magnetic parameters ^{in the two sites are different.}

J. Physique ⁴³ (1982) ^{421-430 FTVRIER} 1982, ¹

Classification

Physics ^Abstracts

61.30 - 76.70 ^K

61.30

76.70 K

1. Introduction.

In a previous publication [1] ^we

have reported deuterium NMR measurements on the discotic mesophase ^of p-n-hexahexyloxytriphenylene (THE6). ^This compound belongs ^{to a} novel class of

liquid crystals composed ^of disc-like, rather than rod- like molecules, which exhibit new types ^{of mesomor-} phic phases [2, 3]. Several different types ^{of discotic} mesophases ^{have been} identified, including ^nematic

and various columnar smectic mesophases [4-9].

The mesophase ^{of THE6} belongs ^{to the} Dho ^class [ 10,1 ]

whose structure is characterized by columns of

regularly ^stacked molecules, arranged ^{in an} hexago-

nal _array (Fig. 1). The deuterium NMR measurements on this mesophase ^indicate [1] ^{that the} orientational

order of the rigid (aromatic) ^{core of} the molecule is

relatively high (0.90-0.95) ^and only weakly tempera-

ture dependent, while the side chains seem to be quite

disordered. These properties ^{suggest that the disco-}

tics could serve as hosts for probe molecules with

a variety of orientational ordering, depending ^{on the}

nature of the guest compounds. ^We would, e.g.,

expect that aromatic probes ^will prefer ^to intercalate between the host molecules within the columns and thus achieve a high degree ^of orientation, ^{while ali-} phatic compounds ^would prefer ^to dissolve within the side chain region ^{with a} considerably ^smaller

orientational order.

The purpose of the present ^{work is to} investigate

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01982004302042100

these expectations experimentally ^with particular emphasis ^{on aromatic} probes. ^{As in our} previous

Fig. ^1.

^{A schematic} diagram ^{of the columnar} mesophase describing ^the stacking of molecules into columns, ^and

their hexagonal arrangement.

study ^{we use deuterium NMR} of deuterated probe compounds. Preliminary proton ^{NMR measure-}

ment with normal probe ^molecules gave unresolved and unintelligible spectra. ^On the other hand, ^the

deuterium spectra ^are usually well resolved and can be

readily interpreted ^{in terms} of the molecular ordering.

The results were quite surprising in that the orienta- tional order parameters of all molecules studied, including large fused-ring aromatics, ^were very low

compared ^to corresponding values in usual thermo-

tropic liquid crystals, and at the ^same time they ^were

also strongly temperature dependent, ^often changing signs within the mesophase temperature range. We

interprete these results in terms of a model in which the solute molecules diffuse rapidly ^{between various}

sites of different orientational order. Such models were

employed previously ^for liquid crystalline ^solutions [11, 12] ^{but it} appears that in the present ^{case of the} Dho ^discotic mesophase the effect is particularly pronounced.

2. Basic equations ^and definitions.

The deute- rium NMR spectrum ⁱⁿ liquid crystalline phases ^is

dominated by ^the quadrupole interaction. The _spec- trum of a single deuteron consists of a doublet with

an overall splitting given by [13] :

This equation applies ^{to a uniaxial} mesophase ^as e.g.

the discotic phase ^under consideration, ^{and a mole-}

cule with D2 ^or C2, symmetry. ^The term e2 qqlh ^is

the deuterium principal component ^{of the} quadru- pole interaction tensor assumed to be along ^{the C-D}

bond direction ^{is the} asymmetry parameter,

11 ⁼ ( V n~ - V,)/V,4, ^{and Saa, Sbb and S~~ are the ele-}

ments of the ordering matrix of the probe ^molecules.

The axes q, ~, ~, ^{are the} principal direction of the

quadrupole ^tensor (where ~ ^{is taken to coincide}

with the C-D bond direction) ^{and a,} b, ^{c are the mole-}

cular fixed coordinates such that c coincides with the molecular C2 (or higher C) ^{axis, and ac, bc are sym-} metry planes. Finally, ^a, #5 y are Euler angles ^which

transform the 11(Ç coordinate system ^{to that of} abc,

and 00 is ^the angle between the magnetic field direc- tion and the director. From ^our previous ^studies [1] J

on the neat mesogen and on a benzene solution we

know that under the experimental conditions used in the present work i.e. in which the samples ^{are allow-}

ed to cool slowly inside the magnetic field from the

isotropic liquid (see ^the Experimental Section) ^the mesophase ^{consists of many} domains whose direc- tors lie in a plane perpendicular ^to the external field,

and are randomly distributed in this plane. ^Thus

for all domains 00 in equation (1) ^is n/2 ^{and the term}

’(3 ^cos2 00 - 1) ^{becomes -} 2.

Most of the solutes ^on which we report ^{below are} aromatic compounds. ^{For the final} presentation ^of , the ^{results on these} compounds ^we have chosen a new set of labelling, ^x, y, ^z such that ^z is the normal to the molecular plane ^{and x} is the direction of the shor- test dimension within the plane. ^{We shall be} mainly

concerned with the S~~ element of the ordering ^matrix.

Thus S~~ > 0 means that the molecular plane ^{of the}

solute prefers ^to align parallel ^{to the} mesogen mole- cules while Szz ⁰ corresponds ^to the situation in which the solute molecules prefer ^the perpendicular

orientation. For molecules having ^a symmetry ^axis Cn with n > ^{3 we} take the direction of this axis paral-

lel to z so that only the element S~~ ^is required ^to

describe the molecular orientation.

For the calculation of the ordering matrix elements

we used the quadrupole parameters given in table I.

These parameters ^were determined in various solids

and liquid crystalline solvents and they depend sligh-

tly ^{on the} system used for the measurements. However

the variations in these parameters ^are quite ^small

423

Table I.

The list of deuterated solutes and data

concerning the THE6 solutions which were studied in the present ^work.

(*) Numbers in brackets correspond ^to the references for the quadrupole interaction constant.

and did not significantly affect the resulting ^{values of}

the S~~’s. ^In practice ^{the term} in q ^{made a} very little contribution to the final results and therefore for q gg ^{0.05 it was} generally neglected. ^{For the case} of pyridine (vide infra) for which more extensive

experimental ^{values were} obtained, ^all q-values ^were

included in the computation ^{of the} S;;’s.

3. Experimental.

^The synthesis of p-n-hexahexy- loxytriphenylene (THE6) is described in references [4]

and [1]. ^{In the neat form its} phase transition tempera-

tures are 68 OC and 99 ~C for solid to mesophase,

and mesophase ^to isotropic respectively. ^Solutions

of probe ^{molecules in THE6 were} prepared by adding weighted ^amounts of solutes into known quantities

of _mesogen. The concentrations used ranged ^between

2 and 7 wt. %, and resulted in lowering ^{of the} clearing temperature by ^several degrees. ^Some of the deuterat- ed compounds (benzene, naphthalene, triphenylene)

were obtained commercially while others were lefto-

vers from previous studies in the Heidelberg ^labora- tory. The list of compounds studied, the solute con-

centrations and the corresponding clearing tempera-

tures are summarized ⁱⁿ table I, ^{in which the} quadru- pole interaction constants used for the calculation and the corresponding references are also indicated.

NMR MEASUREMENTS. - The NMR ^measure- ments were performed ^{on a} Bruker WH-270 spectro-

meter operating ^at 41.45 MHz for deuterium, ^and using ^the pulsed Fourier transform mode. The tem-

perature ^was controlled with a BST 100/700 ^unit

and its absolute value ^was calibrated using ^{a Fluke}

2190A digital thermometer. For convenience of

presentation the results in each case are given ^rela-

tive to the corresponding clearing temperature, Tc.

Five millimeter sample ^tubes containing approxi- mately 0.2 g of solvent plus solute which were sealed

under reduced _pressure were used. Before the actual NMR measurement the samples ^were heated several times to above the clearing point and shaken vigo- rously ^{in order to achieve a} homogeneous ^solution.

They ^{were then} placed ^{into the} preheated ^NMR probe

head and the temperature slowly ^{lowered to the desired}

value. Depending ^{on the} probe concentration, ^bet-

ween 100 and 2000 FID signals ^were accumulated.

4. Results and discussion.

In this section we

present the results of the deuterium quadrupole splittings ^{of the various} probe ^molecules investigat-

ed. We first consider the more symmetric probe molecules, ^i.e. benzene, triphenylene, acetonitrile and

cyclohexane for which a single ordering ^matrix

element is sufficient for the analysis ^{of the} quadru- pole splitting. ^These examples ^will bring ^{out the}

main points characterizing ^{the behaviour of solute}

molecules in the mesophase. ^We then describe results for several other probe molecules with lower _symme- try (D2 ^and C2,) ^{for which two} elements of the order-

ing ^{matrix are} required ^{for the} analysis.

4.1 BENZENE-d6 : THE TWO SITE MODEL.

Exam-

ples of deuterium NMR spectra ^{of a 5.0 wt.} % ^solu-

tion of deuterated benzene (consisting ^{of a} 2 : 3 ratio

C6D6 : C6H6) in normal THE6 at different tempera-

tures are shown in figure ^{2. The same} general ^beha-

viour of the splitting ^{was observed in two} other solu- tions that we have studied viz. containing ^{1.6 and}

6.6 wt. % benzene-d6. ^The magnitude of the observed

splitting, vQ ¹⁹ in the latter solution is plotted ^{in the}

upper part ^of figure ^{3a versus the} temperature ^diffe-

rence, T~ - T, ^{from the} clearing point. Since ben-

Fig. 2. - 20 NMR spectra ^{of a 5.0} w~ % ^{solution of}

deuterated benzene in THE6 at different temperatures.

The deuterated benzene consists of a 2 : 3 ratio of

C6D6 : C6H6.

Fig. ^3.

^The experimental quadrupolar splitting vQ I

and the derived order parameter Szz ^for (a) ^{6.6 wt.} % ^ben- zene-d6 ^and (b) ^{3.1 wt} % triphenylene-d12 in THE6 respec-

tively. ^{The two sets of data in the lower} parts ^{of the} figure correspond ^{in each case to the two} possible signs ^of _vQ.

For triphenylene Szz ^was calculated from the deuteron data of position ^2.

zene has a C6 ^{axis of} symmetry ^the expression ^for vQ

(taking fl ⁼ n/2 ⁱⁿ equation (1)) ^{reduces to}

so that the order parameter, Szz, ^is linearly ^{related to} _vQ.

The behaviour of vQ for the benzene probe ^{is consi-} derably different from that of the discotic bulk solvent,

and is quite unexpected : while for the aromatic deuterons of the neat mesogen, large ^and essentially temperature independent quadrupole splittings ^have

been observed [1], ^the magnitude of the benzene

splitting ^is very small ^even compared ^{to that found}

for benzene in normal thermotropic liquid crystals,

and at the same time its temperature dependence ^is quite ^dramatic

ⁱⁿ particular ^{note the} vanishing ^of vQ

at a particular temperature ^{within the} mesophase region. ^This temperature ^{does not} correspond ^to any

mesomorphic ^or other discontinuous change ^{in the}

neat mesogen. Also ^we have checked the deuterium

splittings ^{of a} perdeuterated mesogen solvent contain-

ing ^{5 wt} % benzene and found that they were essen-

tially ^{the same as in the neat mesogen and} only very

slightly ^affected by the presence of the solute. Since within the mesophase region ^we expect ^the quadru- pole splitting ^{to vary} continuously, ^the vanishing of vQ for the benzene solute must correspond ^{to a} point

at which its order parameter changes sign.

Since the sign _{of vQ is} ^not known it is not possible

to tell in which direction it crosses the zero line, ^i.e.

whether vQ ^{increases or} decreases with increasing

temperature. ^{The two} possibilities ^are plotted ^{in the}

lower part ^of figure ^{3a. In} principle ^{one could deter-}

mine the sign ^of vQ ^{if the} spectrum would exhibit features due to other interactions for which the abso- lute signs ^{are known,} e.g. the indirect coupling ^bet-

ween the deuterons. However the resolution of the deuterium spectrum ^{is much too} low for these inter- actions to show _up. We have tried to record the 1 H NMR spectrum of normal benzene dissolved in THE6 with the hope that it will provide ^the sign ^{of the} dipo-

lar interaction, ^{but were unable to obtain} high ^reso-

lution proton spectra of this solute.

To explain the results of the benzene solutions we

propose ^a model in which the solute molecules occupy several sites with different ordering matrix elements

undergoing ^fast (on ^{the NMR} timescale) dynamic equilibrium. ^In principle ^{there could be} many ^« sites »

representing ^different structures and different solute- solvent complexes. However in order to explain ^the

main features of figure ^{3a it is} sufficient to consider

just ^{two sites} having S~~ ^{values of} opposite signs, ^and

a population ratio that is temperature dependent ^The

average order parameter ^{is then} given by :

where the superscript ^I and II refer to the two sites,

and we assumed that e2 qQ/h ^{is the same in both of}

them. For the discotic columnar mesophase ^{this is a}

very plausible ^{model on chemical} grounds : ^{one site} (site I) ^would correspond ^to benzene molecules inter- calated within the discotic columns, ^with S’,, > 0,

since the benzene molecules will most likely ^{tend to}

lie parallel ^to the aromatic planes of the mesogen molecules. The second, ^less favourable, ^site (site II)

is identified with the ^« aliphatic ^» region ^{between the}

columns which is the space occupied by ^{the side}

chains of the mesogen molecules. To be consistent with the experimental ^{results we must assume that} S’i ^is negative. This conclusion is ^more difficult to

rationalize and it indicates that in the aliphatic region

the benzene molecules prefer ^an orientation in which their plane ^is perpendicular ^to that of the triphenylene rings.

For a more quantitative analysis ^{of the results we}

express the population ^ratio P II~P ^{I of the two sites} by ^{an Arrhenius} equation ^{of the} type

where OH is a positive enthalpy difference (H" - H’)

between the two sites, ^{and K a} weighting factor related to the entropy difference between them. To simplify

the analysis ^{we assume that the} temperature depen-

dence of S~~ ^comes mainly ^from changes ^{in P’ and PIT}

while S:z ^and Siz ^are temperature independent. ^The

latter parameters reflect the ordering in the various

sites of the _mesogen molecules, ^{which as} indicated

425

above was found to be only very weakly temperature dependent. With this assumption ^{the ratio} Szz/Siz

can be expressed ^{in terms of a} temperature To ^at

which vQ vanishes [11] :

substituting ⁱⁿ equation (3) ^and using equation (4)

and the relation P ^I + P" ⁼ 1 gives

where the second (approximate) equality applies

since we consider the region ^{where T -} To. ^{Since we}

assumed OH and S:z ^{to be} positive equation (6) predicts ^that vQ ^would change ^from positive ^to negative ^{as T} changes ^{from below to above} To.

Thus we adopt ^the plot ^indicated by ^{a full line in} figure 3a rather than the one with a dashed line.

We were unable to obtain a meaningful quantita-

tive fit of the results of figure ^{3a to} equation (6) using

all three parameters, S’z, ^{AH and AS} ( = R ^In x) ^as

free variables. Fixing S§z in the range 0.05 ^to 0.8 _gave best fit values for AH and AS which varied in the

narrow ranges 4.1 ^to 2.4 kcaL/mol-1 ^{and 11.0 to}

13.0 e.u. respectively. ^For Slz lower than 0.02 unrea-

sonably high ^{values of DH and AS were} obtained,

and the fit did not converge. The values obtained for AH and AS in the converging region ^seem quite

reasonable : the result for AH is typical for solute- solvent interaction and in our model it reflects the

specific interaction of the benzene with the aromatic

core of the mesogen molecules in site I, while the

high value of AS reflects the higher degree ^{of disorder}

in site II. The actual magnitude ^{of the} parameters should however not be taken too seriously ^{due to}

the simplified ^model used and the _many assumptions

made.

4. Z TRIPHENYLENE-d12 : ^SITE DEPENDENT MAGNETIC PARAMETERS.

Examples of deuterium spectra ^of

a 3.1 wt. % ^solution triphenylene in THE6 and a plot

of the corresponding splittings ^versus temperature

are shown in figures ⁴ and 3b. The two peaks ^{in the} spectrum correspond ^to positions ^{1 and 2 as indi-}

cated in the figures. ^It may be ^seen that the centres of these doublets do not coincide. This asymmetry is due to the chemical shift difference between the two positions, ^{and on} the basis of previous ^NMR

measurements [16] ⁱⁿ isotropic liquids ^the high ^field

doublet was identified with position 2. We have taken the data of this nucleus for the calculation of

Fig. ^4.

^{As in} figure ^{2 for a 3.1 wt} % solution of tri-

phenylene-d12 in THE6. The numbering ^{of the} peaks correspond ^to the deuteron positions ^{as indicated in the}

structural formula in figure ^3.

S~~ in the lower part ^of figure ^{3b. The} general ^beha-

viour of vQ ^and thus of S~~ ^is very similar to that of benzene although the overall changes ^{in the} splittings

are much larger. ^{A similar} analysis ^{as done above}

for the benzene (Eq. (6)) using the deuteron data of

position ² gave converging ^{results of 2.1-}

3.7 kcal./mol- 1 ^{for AH} and 6.3-8.4 e.u. for AS in the range 0.2 S:z 0.8. For lower values of Siz ^no

convergence ^was obtained. Although the results for the thermodynamic parameters ^seem reasonable, ^the resulting values for Sii ^{are about - 0.6, which is}

just outside the allowed region for motional constants.

We have so far disregarded ^the difference in the

quadrupolar, splitting between the two positions ¹

and 2. On first sight ^it might appear that this is due

to a small difference in the factor eZ qQ/h ^{in the two}

deuteron positions. However it is clear that this

cannot be enough. Firstly ^{because the} difference in

splitting ^is quite large compared ^to the overall split- tings : ^when triphenylene ^{is dissolved in normal} thermotropic liquid crystals its deuterium NMR also exhibits a small difference in splittings ^for positions ¹

and 2, ^but this difference amounts at most to a few tenth of a percent ^{However most} significant ^{is the}

fact that the quadrupole splittings ^{of the two deute-}

rons do not vanish simultaneously, while from equa- tion (3) ^the temperature ^{at which} vQ ^{= 0} should be

independent ^{of the} quadrupole parameter ^{and should}

coincide for both deuterons.

To account for this anomalous effect we slightly modify ^{our two} site model and allow the quadrupole

interaction parameter A ^{= - 3 e2} qQ (3 cos’ # - 1)/8 ^h (Eq. (2)), for each of the deuterons to be different in the two sites, ^{so that} equation (3) ^{now reads}

where i refers to the deuteron position ^{in the} triphe- nylene. ^It is clear that if the change ^{in A on} going

from site I to II is different for the two deuterons,

i.e. A lI/A i ^5~ Ai’/A2 ^{then the} vanishing ^of v’ ^{will not}

occur at the same temperature ^{as that for} vQ.

The number of parameters ^{in the} equations ^for vQ is now much too large ^to attempt ^{a best fit} analysis

of the experimental results. An order of magnitude

estimate indicates that a difference of ^a few percent in the change ^{of A on} going from site I to II can

explain ^{the observed} results. Whether this change ^is

due to redistribution of the electronic charge ^{in the} probe ^{molecules or} whether it is due to changes ⁱⁿ

the molecular geometry ^on going ^{from one site to}

the other cannot be decided from the experimental

results. It should be noted that similar effects i.e. site

dependent geometry changes of solutes in liquid crystalline ^{solvents were noticed} previously ^{in the}

NMR spectra of several molecules [11, 12].

4.3 ALIPHATIC PROBES: I CYCLOHEXANE- d 12, ^ACETO- NITRILE- d3 ^AND HEXYLALCOHOL-1 d2 . - The deute- rium NMR of each of these compounds in the THE6

mesophase solution consists of a single doublet. The temperature dependence of vQ ~ ^I derived from the spectra ^is plotted ⁱⁿ figure 5. For acetonitrile and

hexylalcohol the results are similar to those obtained in normal thermotropic liquid crystals, ^{i.e. a} gradual

Fig. ^5.

^{The deuterium} quadrupole interaction constants

I vQ ^{I for} cyclohexane-d, 2, acetonitrile-d3 ^and hexylalco-

hol-I d2 ^{and the} corresponding Szz I ^{for the first two} compounds. ^{Note the} change in scale for the cyclohexane

data.

decrease in vQ ^with increasing temperature, although

the splittings ^are relatively ^{small. It seems that these} probes ^dissolve predominantly ^{in the} aliphatic region (site II) ^{where the} magnitude ^{of the} ordering ^{is low.}

The order parameter for acetonitrile which has a C3

axis of symmetry ^was calculated from the equation

where we have taken ~3 ^{to be} equal ^to the tetrahedral

angle [17]. Since the sign ^of vQ ^{is not} known and there is no obvious _way to guess it on chemical

grounds ^{we have used} equation (8) ^to calculate the

magnitude ^of Szz ^{as shown on the} right ordinate in

figure ^{5. A} similar calculation for hexylalcohol ^{is not} possible because of its low symmetry ^and high ^flexi- bility.

The quadrupole splitting ^{of the} cyclohexane probe

is ^even smaller than for the other examples ^{in this}

section and its temperature dependence ^{is abnormal,}

i.e. the splitting increases with increasing tempera-

ture. As for the cases of benzene and triphenylene ^we interprete this behaviour in terms of the two site model in which the increase in I vQ ^{I is due to the} equilibrium shift from site I to site II, ^{which have} opposite sign ^for Szz. ^{Here too it} is difficult to guess the preferred orientations in the two sites and accord-

ingly ^we plot ^the magnitude ^of S~~ ^{for this} compound.

Since only ^a single ^{doublet is observed for the} cyclo-

hexane deuterons due to fast ring interconversion we use their _average quadrupole ^constant for the cal- culation of SZZ [ 18] :

where in the last equality ^we approximated ~3 by ^0.

(In reality ^{it is 2.60} [18].)

4.4 FUSED-RINGS AROMATIC COMPOUNDS WITH D2

SYMMETRY : I NAPHTHALENE-d8, ANTHRACENE-d10 ^AND PYRENE-djo.

^{In the} remaining part ^{of this} paper

we shall discuss probe molecules with symmetry C2~

or D2 ^{and thus two} ordering matrix elements are

required ^{to describe their} ordering ^{in the} mesophase.

To derive these constants from equation (1) ^{we need} quadrupole data from at least two inequivalent

deuterons. Since the signs ^{of the} vQ’s ^{are not known}

there are four possible ^sets of solutions for the S;;’s, corresponding ^{to the} following ^four possible ^combi-

nations for the two vQ’s : (a) + + ; (b) - - ; (c) ^{+ - ;}

427

and (d) - ^+. The results of the S;;’s ^{for cases} (a)

and (b) ^are identical in magnitude ^but opposite ⁱⁿ sign, ^and similarly ^{for cases} (c) ^and (d). ^{We shall}

therefore present only ^{two sets} of results for each

compound - ^{the other two} possibilities ^{are obtained} by inverting ^the signs ^of S~~ ^{and of} S.,.,-Sy. ^{The sets}

of results that we shall prefer ^to present will be those that seem more likely ^on the basis of the model des- cribed in the beginning ^{of this} section, ^{i.e. we shall} prefer ^{the solutions} for which Szz ^becomes alge- braically ^smaller (more negative) ^with increasing temperature. ^{This is the} expected change ⁱⁿ Szz ^{if we}

assume that _upon heating, ^{more of} the intercalated molecules shift to ’the aliphatic ^chain region.

Examples ^of spectra ^for perdeuterated naphtha- lene, anthracene and pyrene ^are shown in figure ^6.

Note that for the latter two compounds only ^two

doublets are observed due to the effective equiva-

lence of positions ¹ and 9 in anthracene and posi-

tions 2 and 3 in pyrene. This equivalence ^{also serves}

to identify ^{the NMR} signals. ^In naphthalene ^the peak assignment ^{was made on the basis of the small}

chemical shift [19] ^{between the two doublets as well}

as by comparison ^{with the} spectrum ^of naphthalene-

1 di. ^The temperature dependence ^{of the various} I vQ ~’s ^{of these} probes ^are shown in the upper dia- grams of figure ^{7 and the} corresponding ^calculated S~~’s (for ^{two sets of} signs ^{as indicated in the} figure

and the magnetic parameters [20, 15] indicated in table I) ^are plotted underneath each case.

4.5 AROMATIC PROBES WITH C2, SYMMETRY : PYRI-

DINE- d s ^AND NITROBENZENE- d:5. ^{- These two com-}

Fig. ^6.

^{2D NMR} spectra of solutions of naphthalene-d, (64 DC), anthracene-d 10 (72 °C~ ^and pyrene-d 10 (62 °C’~ ⁱⁿ THE6, ^where the numbers in brackets correspond ^{to the} temperatures ^at which the spectra ^{were recorded. The} compositions ^{of the solutions are} given ^{in table I and the} labelling ^of the deuterium positions ^{are indicated in the}

structural formulae in figure ^7.

pounds ^have C2, ^{symmetry and three} inequivalent deuterons, ^{due to} the deviation of the ring ^structure

from perfect hexagon. ^{Indeed as} may be seen in

figure ^{8 three} distinct doublets appear in the deute-

Fig. ^7.

^{The deuterium} quadrupolar splittings I v. ^{for the same solutions as in figure 6, and two out of the four possible}

sets of the ordering matrix elements for each compound. ^{The other two sets are obtained} by ^{inverting the} signs ^{of all} S;;.

Fig. ^8.

^{As in} figure ^{6 for} pyridine-ds (67 ~C~, ^nitro- benzene-ds (52 OC), o-xylene-dio (74 OC) ^and p-xylene-d,, (61 °Q. The deuteron labelling ^{are as in} figures ^{9 and 10.}

rium spectra ^{of these} compounds. ^The corresponding plots of I vQ ^{I versus} temperature ^are depicted ⁱⁿ figure ^{9. The} identification of the peaks ^{was based}

on their relative intensities and chemical shifts. Since there are now three sets of data one might hope ^that

certain choices of relative signs ^for vQ ^can be elimi- nated For nitrobenzene this is however not the case

since as may be ^seen from the plots of I vQ 1, positions

1 and 2 are nearly equivalent The situation is thus similar to that for the group of probes naphthalene, anthracene, pyrene. The results for SzZ plotted ⁱⁿ figure ^{9 were} calculated using geometrical para- meters and e2 qQ/h ^values given ^{in the literature} [21, 22], ^and assuming that the C-D bonds lies along

the direction of the corresponding CCC bisectors.

For pyridine ^on the other hand we could perform

a more complete analysis : firstly because both deu- terium quadrupole [23] ^and geometrical [24] ^{data for}

this compound ^are available, ^and secondly ^{because as}

may be seen in figure ^{9 the} vQ’s ^{for the various deute-}

rons in the mesophase ^{solution seem to be} changing independently. ^Indeed from the four possible pairs

of sign combinations only ^one pair ^{was consistent}

with all experimental data and the solution for

‘which Szz ^decreases algebraically is shown in the bottom part ^{of the} figure.

Fig. ^9.

^{The deuterium} quadrupole splitting I vQ ^{I for}

solutions of pyridine-ds ^and nitrobenzene-ds ⁱⁿ THE6, and several possible ^{sets of} Si’s ^{for each} compound ^The composition of the solution is given in table I.

4.6 PARA- AND ORTHO-XYLENE-d10. - The situa- tion here is similar to that as for the molecules dis- cussed in subsections 4.4 and 4.5 except ^{that the} CD3 groups ^are considered as a pseudo-aromatic

deuteron with an effective quadrupole interaction constant

Fig. ^10.

^{The deuterium} quadrupole splitting I vQ ^{I for}

solutions of o-xylene-dio ^and p-xylene-d,, ⁱⁿ THE6,

and possible ^{sets of} Sii’s ^{for each} compound. The compo-

sition of the solutions is givenrin ^{table I.}

429

where a is the angle ^{between the} methyl ^{C-D bond}

and the local symmetry ^{axis of} this group. Examples

of spectra, experimental ^values for I vQ ^{I and the}

derived possible values for the elements of the order-

ing ^{matrix are} given ⁱⁿ figures ^{8 and 10.}

5. Summary and conclusions.

The orientational order of solute molecules dissolved in the discotic

mesophase ^{of THE6 as measured} by ^{2D NMR has}

several unexpected features : the magnitude ^of

the solute order parameters ^is considerably ^smaller

than in normal thermotropic liquid crystals despite

the fact that the ordering ^{of the} triphenylene moiety

of ^the mesophase ^molecules is very high. ^Moreover

while the order parameter ^{of the neat} mesophase ^is essentially temperature independent that of the solute molecules changes dramatically ^with temperature and often changes sign within the mesophase region.

These effects can be qualitatively explained ^{in terms}

of ^a model involving ^{two solvation sites with} oppo- site signs of motional constants, ^and assuming ^fast

diffusion of the probe molecules between the sites. It is natural to associate (although impossible ^to prove)

these two sites with : (1) probe molecules intercalated within the stacked columns of the mesogen, and (II)

molecules dissolved in the space between the columns which is occupied by ^the aliphatic ^{chains of the}

mesogen. Planar aromatic molecules will tend ^to

align parallel ^to the mesogen molecules in site I while in site II they ^{will be} highly disordered and

apparently prefer ^a perpendicular orientation. The strong temperature dependence ^{of the} probe’s ^orien-

tational order is then attributed to a temperature driven shift of equilibrium between the two sites.

Specifically ^{we assume that at low} temperatures ^{site I} (intercalation) ^is preferred ^{while at} high tempera-

tures entropy effects drive the molecules to site II

(i.e. the side chain region). ^{It is} important ^to emphasize

that the two-site model represents ^the simplest ^one

that can be made to fit the experimental observation.

In reality ^there may be a large number of discrete

or perhaps ^a continuous _{range of} sites. It is not

possible ^to identify ^or characterize such sites from the NMR data, ^{but the} picture ^{of two sites corres-} ponding ^to intra- and inter-columnar regions ^seems

attractive. Also the semi-quantitative ^{estimate for}

the enthalpy ^and entropy difference between the two

sites is consistent with the above interpretation.

Perhaps ^some support for the two-site model can

be obtained by performing ^similar experiments ⁱⁿ polymorphic ^{discotics which} exibit both a columnar

phase ^{and a nematic} phase ^{in which there is no} (or

much less) ^molecular stacking. ^{We would then} expect that in the columnar phase ^the probe ^mole-

cules behave as in the THE6 mesophase ^{while in the}

nematic phase ^the characteristic features of two sites will not appear ^or will be much less pronounced

At the moment we do not possess materials suitable for such experiments.

The analysis of the results indicates a further

important fact, namely ^{that the} probe ^molecules might ^{have different} magnetic parameters ^{in the two} sites. In fact for the particular probe ^of perdeuterated triphenylene the results ^can only ^{be understood}

under the above assumption. ^Whether the difference in the magnetic parameter ^{in the two} sites reflect redistribution of electronic charges ^{due to different}

solvent solute interaction ^or perhaps geometrical changes ^on going ^{from one site to the other cannot}

be inferred from the experimental results. Geome- trical changes in molecules on changing ^{sites has}

been invoked previously [11, 12] ^to explain ^anoma-

lous dipolar splittings ⁱⁿ liquid crystals, ^{but it seems}

that the effect on the quadrupole splitting ^{is consi-} derably ^more significant

Acknowledgments.

This research was supported by the National Research Council of Israel and by

the U.S.-Israel Binational Science Foundation.

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