Thermotropi Biaxial Nematis:[1℄
Highly Desirable Materials, Still Elusive ?
Klaus Praefke
TehnisheUniversitatBerlin,
Jebensstrae 1,J437,10623 Berlin,Germany
Reeivedon12Deember,2001
Onthe basis and with the inlusion of results desribed in a previous paper [2℄, submitted for
publiationalmosttwoyearsago, weadd someommentsonveryreent developmentsregarding
boththedesignandsynthesisofthermotropinematogensoflow-moleularweight,supposedbiaxial
inharater, aswellas puzzles, stillunsolved,withmethods of theirunambiguousidentiation.
Ingeneral,this updatemay demonstratetheomplexityand problemati nature inherent inthis
topialsubjetofresearhindiatedbyastruggleoveraveryrelevant,butverydiÆultmatter.
I Introdution
Theexisteneofabiaxialnemati(N
b
)phasehasbeen
asubjetofgrowinginterestsine 1980[3℄, aboutten
yearsafteritspredition[4℄. However,whereasthe
mi-ellar typeofbiaxialnematiphase[3℄hasintheourse
of timebeenquite well establishedexperimentally,
in-luding reentstudies onhanges ofthe miellarform
inapolydispersesystemofsolution[5-7℄,theourene
andobservationofrelatedthermotropi N
b
phasesofa
ertainnumberofsingle,low-masshemialompounds
are still open to question for physisists and syntheti
hemists[8-18℄.
This assessment of the situation and the
ommit-mentto new tasksenouraged severalresearh groups
to an inreasing degree, mainly in Europe, also to
fo-us their eorts on the design, synthesis, and
identi-ation - the latter measure: a non-trivial task - of
low-massmoleulessupposedtoexhibitbiaxialnemati
behaviour. Hitherto,arealbreakthroughonthat
fasi-nating,butverydiÆulttopiofresearh,possiblyalso
signiant for tehnologial appliations [19℄ of suh
materials, has notyet been ahieved. Oneis tempted
toomparethisstrugglewiththesearhforaneedlein
ahaystak. Further ommentson thisdeient
situa-tionareknowninagreaternumber[20℄. Nevertheless,
an indisputableexampleof athermotropi biaxial
ne-matiphaseissurelyexpetedtobefoundinthenearer
II Reetions on the design of
thermotropi biaxial nematis
In ontrast to alyotropi statein whih an aggregate
ofmoleulesistheprinipalfeature, thestrutural
ele-mentofthermotropimesogensisamoleule,asin
stan-dard moleular systems. This fundamental dierene
with all itsonsequenes for the onstrutionof these
twomesomorphistatesdoes,surelyinthelatterase,
ausemajorsynthetidiÆulties. Indeedthey,atually,
still seem to be invinible, e.g. regarding pure
ther-motropi systemswith biaxialnematiproperties
on-sistentlyaepted by thesientiommunity.
With-out doubt, it appears neessary that their moleular
shapeshould deviate from thelassialalamiti type
possessingapproximateylindrialsymmetry.
At rst glane, it seemsto be lear that theissue
of phase biaxiality is well based on moleular
biaxial-ity but not neessarily on shape biaxiality only [15℄,
andyet mostworkhasbeendonedesigningsuh
ther-momesogens aording to geometrial priniples, i.e.,
making them somewhat shorter and broader [13 (the
overview)℄. For instane, it wassuggested quiteearly
andmorermly[21℄thatN
b
behaviourmaymostlikely
bedevelopedbyompounds ontainingstrutural
fea-tures both of a rod- and a dis-like moiety, as
de-pited by skethes of a few models some years later
[22℄. Themoleular biaxialityneedsto allowfor quite
pronounedlateralorganisation, andashape riterion
[13, 23℄ of 5:3:1 wassuggested within the onstraints
that themoleules oughtnottobeexposed toomuh.
Moleular biaxiality an be promoted by
or at least leadingtoaertain degreeof orderin that
diretion[15℄. Inthemeantime,asanbelearnedfrom
theliterature[13℄,variousorganimoleuleshavebeen
prepared and investigated by several researh groups
hopingthesematerials,designedinthemannerreferred
to above, would exhibit the N
b
phase as a result of
hindrane in rotation abouttheir longaxis. As many
of suh ativities have shown, however, it turned out
thatsevereproblemshavearisenfromthefatthatalso
broadermoleulesofpurematerials,nevertheless,strive
toredue theeetoftheirboard-likemoleularshape
to aminimum. Forinstane, veryfastonformational
hangesareonereasonthataloalN
b
situation,ifatall
formed onlyinverynarrowtemperatureranges,isnot
lastingoverawideareaduringasuÆientlylongtime.
Thefatthatthealamitimoleulesinausualnemati
phaserotatearoundtheirshortandlongaxesonatime
sale of millionsperseond orevenabout onemillion
timesfasteraroundthelatteronedemonstratesthe
in-herentproblems faedherewiththermotropi
nemato-gens, attributed to show biaxial behaviour, regarding
theirdesignandrealizationbygeometrialriteria[21,
22℄ andhemialsynthesis,respetively.
In an extensive overview [13℄ it was tried to
sum-marize ativities (known so far and doumented
un-til the end of 1997) on the design of low-molar-mass
thermomesogensinthesearhforbiaxialnematiliquid
rystalsdisussing ahugenumberofsuhpure
hemi-al ompounds verydierent in their basimoleular
strutures and broader as usually known for lassial
alamitis. But sofar, among all of these many
sub-stanessynthesized andtested throughonosopyand
birefringene measurements, it seems not one is fully
aepted yet asa biaxialnemati, f. the overviewin
[13℄.
Furthermore, another feasibility of the reationor
promotionofthebiaxialharaterofthenematiphase,
fousedonhere,isoneivableandmaylieintheanks
ofthemoleulesduetointermoleularlyside-to-side
in-teratingforesofattrationandrepulsion. This
situa-tion results, desirably, in a ertain redution of their
mobility,i.e. again,espeiallythehindraneofrotation
abouttheirlongaxis,assumedthehiefrequirementfor
theourreneofbiaxiality,see above. Inthis
onne-tionitisimportanttolearnfromomputersimulation
studies [24℄ that thermotropibiaxial nematis indeed
anexist,agreatstimulantforfurtherworkonthe
de-sign and preparation of promising new substanes as
well as on the development and expert use of
indis-putableanalytialmethodsinthisimportantsetionof
synthetiorganiandphysialhemistry.
However,suhlateraleets resultingin
intermole-ularattrationsshouldberatherbalanedinstrength,
sineotherwise-inthesespeiases-undesired
phy-sial (e.g. smeti [25℄ or even rystalline) properties
are to be feared. From a syntheti viewpoint, the
For instane, situations of moleular fae-to-fae
in-terations [25℄ or strong hydrogen bonding, the
lat-ter always straight (!) in geometry, would give rise
to other, a more rigid kind of supramoleular
stru-tures, onsequently, dierent in behaviour ompared
totheir thermomesomorphipreursors. On theother
hand, sine the introdution [26℄ of the rst laterally
long-(exible-)hain (illustratedby: )substituted
(mainly)standard nematis (illustrated by: ) in the
early eightiesfollowed by a greaternumber of various
analoguesuptoourdays[27-29℄,itisknownthat this
kindof hemialalteration of their respetive
double-intoatriple-ended( ! )struture[30℄usually
keepsthemesogenifeatureoftheinitialdouble-ended
one. Anyway, biaxial nemati phenomena have never
beenobservedinonnetionwiththisoranyothertype
of laterally monografted nematogens [26-29,31℄. The
same is true [31℄ with nematogens having two
long-hainsubstituentsmostlyinoppositelateralarenyl
po-sitions[28,32-35℄. Surprisingly,thefatofpreservation
of the type of mesophase has, to our knowledge, not
reeived general attention in view of using the
triple-ended [30℄ nemati mesogens ( ) for further
syn-theti purposes direted toward biaxial nematis. Of
ourse, this ould only work with a suitable hemial
funtion xpresentatthe tip(s)of theimplanted
long-hain spaer(s) as depited in the following example:
x : The single instane, up to now, of suh a
synthetiapproahhasbeendesribedalreadyover
f-teen years ago[36℄ and refers to aC=C-double bond
intheterminalpositionx. Severalexamplesofstarting
mesogens, eah monotropial nemati, yielded the
re-spetivehigh-moleular-massprodutsafter
polymeri-sation, eah now exhibiting an enantiotropi nemati
phase. Theassignment[36℄ ofthesemesophases as
bi-axialnemati,amongthemtherstexampleofahiral
nematione,appearsalsonottohavebeenuniversially
aeptedas suh [12℄; forinstane, to ourknowledge,
noneof these interestingasesof anemati phasehas
notyet beenexamined with 2
H-NMR.Although these
partiular liquid rystalline produts are (side-hain)
polymers,anyhow,thisbuildingprinipleoflateral
at-tahment deservesfurthersynthetiwork,seebelow.
Beside thesepolymers[36,37℄in whih xforms the
polymeri hain onstituting the moleular bakbone,
afewyearslater[27℄,aseond(idential)alamiti
ne-matiunit( )hasbeenputoninstead,furnishingnow
twin moleules [27a℄ or ligated twins [27b℄ whih, in
mostases,arealsothermomesomorphi,mainly
sme-ti[38-42℄,but not(biaxial)nemati [31℄ inharater.
TheirformalH or\atamaran"-likemoleular shape
maybeillustratedbythefollowingsketh: .
Otherstrutural variationsin thispartiulargroup
of twin materials began in Germany [43-45℄and
Eng-land [46℄ in the early nineties, after the rst
matogeni properties are learly determined by the
dominating, alkyl substituted, at \superdis"
seg-ment pentakis[(4-alkylphenyl)ethynyl℄phenyl depited
herein Fig. 1.
Figure1. Thebasimoleularstrutureof\superdis"-rod
ethers,RorR',e.g.,pentylorlongalkyl,respetively[43℄.
Basedonpreursorsofthisbigdisotishape,
dis-dis[43℄andalsosomedis-rod[45,46℄twinompounds,
the latter provided with dierent groups alamiti in
struture, assuh thermomesomorphi (smeti), and
ovalently,alwaysterminally, linkedto the\open"end
of R' have beenprepared and studied arefully.
Un-fortunately,thedis-roddimerspreparedsofarturned
outtobeeithernon-liquidrystalline[45℄orareatbest
monotropi nemati-(disoti) [45,46℄. On the other
hand,suh dis-disdimers,atually,exhibit the
ther-motropi nemati-(disoti)phase enantiotropiallyas
expeted, but its biaxiality stated bythe authors [43℄
isdisputedsinethen[e.g.: 9,12,15,18,24℄.
Asresearhdevelopedotherdis-roddimershaving
therod-likemoietyforthersttimelaterally attahed
to the(long) spaerat that \superdis" segment,
rep-resented in Fig. 1 and used [44℄ here too, ould now
indeedbesynthesizedandalsopuried[48℄. Thisvery
visousmaterialpossessesamonotropinemati phase
of asubstantialmoleular biaxialityashas been
har-aterized by 2
H-NMR spetrosopy. Their results
in-diatethattheside-onwayoflinkingtherod-likepart
withthealkoxy-\superdis",seeFig.1,introduedhere
suessfully forthe designof asingle moleularentity
ouldindeedbeapromisingapproah[44℄towards
low-molar-massthermotropibiaxialnematis [48℄.
Moreover,these Britishresearhersfoundstilla
se-ondnewapproahto therealisationofthebiaxial
ne-mati phase [49℄. This ase is based on spei
side-uorinatedPalladium(II)omplexes,seeFig. 2,dueto
thefatthatlikepartsofamoleuleprefertoassoiate
leadingheretomiro-segregationatthemoleularlevel;
2
H-NMRandx-raystudiesprovetheirndings[49℄.
Figure 2. The moleular strutures of the two isomeri
Pd(II) omplexes, R=R'=hexyloxy or R=hexyloxy and
R'=hexyl,botharemonotropiallynemati[49℄.
In the fae of instrutive omments published
re-ently [18℄ on bent-ore or banana-shaped moleules
and the exhibition of the biaxial nemati phase a
third ativityin this eld, theauthor hasjust learned
about, an be mentioned here [50℄. Subjet are
po-lar liquid rystal materials derived from
2,5-bis(4-hydroxyphenyl)-1,3,4-oxadiazolbyesteriationwith
4-substituted benzoi aid arrying either a heptyl or a
dodeyloxy group, having a 134 bend angle of a
rigid, boomerang-typeore [f. 22℄ with adipole
mo-mentof4D.ReportedareresultsofX-raydiration
andeletro-optialmeasurementsontheirmesophases:
a nemati and twosmeti ones. It is onluded that
the three systems the authors investigated showa
bi-axialnemati phasebelowtheisotropiliquid[50℄.
Probably another kind of lateral eet (see
fur-ther above) or interation with intra- and
inter-moleular onsequenesmust beinvolvedin thephase
behaviour of very interesting novel, oligomeri, at
least 75-membered gigantoheteroyles [51℄ based on
eah three units of
1-(4'-hydroxybiphenyl-4-yl)-2-(4-hydroxyphenyl)butaneand exible spaers. This type
of yli multimeri liquid rystals has no hain ends
and yet exhibits eah an enantiotropi nemati phase
on heating [51,52℄. This mesophase formation is
ex-plained as a onsequene of moleular alignment
fol-lowinga\self-parallelisation"of the\ultralarge"rings
[52℄. Obviously,thereasonforthisintramoleular
pro-essisduetoeletronitrans-annular (here,naturally,
alsointermoleular)eetsofattrationandrepulsion
- in other words: to through-spae-interations. This
yieldsasomewhathigherrigidity ofsuhrings,butstill
remainingexibleenough,ombinedwith their
widen-ingduetotheparallelarrangement,i.e. intramoleular
doublingofthiknessversussimplelinearmoleules. It
irum-longaxisofthese and, imaginably, similargiantyli
aswellasheteroylimaterials(f. above)toan
opti-maldegreeallowingbiaxialityoftheirnematiphaseto
our strongenough andbeoming detetable by
var-ious methods. However,to ourknowledge, no
exami-nationwith 2
H-NMRhasyet beenarried out.
Hope-fully,this stimulatingtypeofnemati materialwill
in-duemoresynthetiworkonthisimportantsubjetfor
whih,rstofall,themethodsofpreparation,isolation,
and puriation of suh and other \ultralarge" yli
produts must furnish substantially higher hemial
yieldslying,atpresent[51,52℄,belowveperentonly.
Apart from their mesogeni relevane suh
gigantoy-lesmayalsobeofinterestintheeldofnanohemistry.
In summary, it seems apparent that the design of
nematogens with suÆientbiaxialityto exhibit anN
b
phaseremainsanespeiallyhallengingtask. Syntheti
hemists have still muh to do here; this topi is
er-tainlynotbyanymeansexhausted. Therefore,onean
hope that thermotropi N
b
phases will soon beome
indisputably real so that physiists and other people,
eventually, an start studying suh material in detail
andlooking fortehnologialappliations.
III Remarks on the
identi-ation of thermotropi N
b
phases
The highdegree ofdisappointment, but nevertheless
-sinereently[48-50℄ { alsowithsomeinreasinghope
of suess regarding attempts to prepare examplesof
this important kind ofnematis stressed in the
previ-oushapteris,fromtheviewpointofasynthetiorgani
hemist,stillburdenedbytheunertainty[12,13,15,18℄
stemming from most of the methods employed so far
in the identiation of the mesophase and
interpreta-tionoftheresultsobserved. Severalimportantonerns
outlined in detail reently [12,13,15,18℄must be taken
into aountwhen athermotropi N
b
phaseis subjet
to investigation.
Theadvantageofthe 2
H-NMRspetrosopyis[e.g.,
8,11,18℄that bulk samples,eitherdeuteriatedassuh
-but, unfortunately, veryseldom the ase { ordoped
(less sensitive) with suitable deuteriatedsolvents, an
bestudiedsothat surfaeeets arenegligiblein
on-trast to optialmethods, suh asonosopystudiesof
nemati materials, where surfae or (ell) wall eets
[53℄ may,misleadingly,indue aweakphasebiaxiality.
Leadingasesinwhihthistehniquehasbeenusedare
suhinvestigationselegantlyarriedoutbyG.R.
Luk-hurst[8,11,18℄ orthedetermination [54℄of the
magni-IV Conlusion and Outlook
This short insight into the still prevailingsituation of
averydeliatematter of liquidrystalresearh shows
some of the problems whih must be brought under
ontrolinordertorealizetruethermotropiN
b
materi-als. Theirsyntheseswere,are,andwillalwaysbetriky
balaningats.
However,onthebasisofso-alledlateraleets
be-tween the moleules in a nemati phase disussed in
hapter 2 and their irumspet utilization, an
opti-mistivieworevenapromisingperspetiveseemstobe
possiblefortherealizationof,hopefully,unquestionable
thermotropiN
b
phases. Presumably,twin,bis-twin,or
gigantoylimoleulesouldbethebestandidatesfor
asuessfulN
b
-hunting.
In addition, mixtures between their homologues
wouldalsobeofinterest;thesemayevenbemoreuseful
thanpurematerialssinetheompoundsthenan
sta-bilizeeahotherbetter,similartothestateoflyotropi
systems.
Aknowledgement
The invitation to the 5th Ibero-Amerian
Work-shop on Complex Fluids and Their Appliation, the
IBERO2001meeting,inMaringa,Parana,Brazil,byits
Organizing Committee and Ibero-Amerian Board as
wellastheexellentandourteoushospitalityby
Pro-fessorDr. L.R.Evangelista,Universidade Estadualde
Maringa,UEM,andhisgroupofjudiiousandfriendly
olleagues, o-workers, andassistants is aknowledged
withordialgratitude.
In partiular, the author is very muh indepted
to his numerous former o-workers as well as to his
perfet tehniian and ommitted expert in omputer
appliation and waste management, Mr. A.
Ek-ert, for their engaged and very suessful
ollabora-tion in our various branhes of researh work. With
great pleasure he also thanks his olleagues, the
Pro-fessors S. Chandrasekhar, Centre for Liquid
Crys-tal Researh, Bangalore, India, D.W. Brue,
Uni-versity of Exeter, U.K., G.R. Lukhurst,
Univer-sity of Southampton, U.K., Satyendra Kumar, Kent
StateUniversity,Ohio, U.S.A., C.Tshierske,
Martin-Luther-Universitat, Halle-Wittenberg, Germany, Mrs.
Lia Q. do Amaral and A.M. Figueiredo Neto, both
Universityof S~ao Paulo, Brazil,and L.R.Evangelista,
seeabove,Brazil, forfruitful sientidisussions and
preprintsofpapersonthehottopiofthermotropi
bi-axial nematis by D.W. Brue, G.R. Lukhurst (both
U.K.) and Satyendra Kumar (U.S.A.) during the last
monthsof2001.
Furthermore,variouskindsofsupportforourwork
by the Tehnishe Universitat Berlin and, espeially,
for nanial support by the Deutshe F
Aus-nervaFoundation,Heidelberg,Germany,inonnetion
with TheWeizmann Institute of Siene, Rehovot,
Is-rael,theompanyE.Merk,Darmstadt,Germany,
No-vartisInternationalAG.,Basel,Switzerland,theNATO
andEuropeanUnion,bothBrussels,Belgium,overthe
yearsaregratefully reognized.
Referenes
[1℄ Part 134 of liquid-rystalline ompounds, based on
an invited leture, here revised, entitled \Dealing
With Thermotropi N
b
Mesogens, A Struggle Under
BigDiÆulties",at the5thIbero-AmerianWorkshop
on Complex Fluids and Their Appliations, Maringa,
Parana, Brazil, September9-12, 2001; part 133: A.
Nesrullajev,B.BilginEran,D.Singer,N.Kazani,and
K.Praefke, MaterialsResearhBulletin,inthepress
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ritial omments about this lyotropi N
b
system
have been derived from a study of a phase diagram:
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J.Phys.Chem.,B 102,7132(1998).
[4℄ M.J. Freiser, Phys. Rev.Lett., 24, 1041 (1970).- See
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Tit-telbah,J.Chem.Phys.,107,9252(1997).
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[14℄ S.Chandrasekhar,Geetha,G.Nair,D.S.ShankarRao,
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[15℄ See a report by 23 partiipants about The Oxford
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261-293 (1998), and before that also subjetof a private
ommuniationbyG.R.Lukhurst,November7,1997,
totheauthorofthispaper.
[16℄ K.PraefkeinPhysialProperties of Liquid Crystals:
Nematis, EMIS Datareviews Series No. 25, hapter
1.2, p. 17, eds.: D.A.Dunmur,A. Fukuda, and G.R.
Lukhurst, published by INSPEC, The Institution of
EletrialEngineers,London,U.K.,2001.
[17℄ G.R.Lukhurst,seereferene[16℄,hapter2.1,p.315.
[18℄ G.R.Lukhurst,ThinSolidFilms,393,40(2001).
[19℄ R. Shashidhar (ed.), SPIE. Proeedings Series,
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(1996).
[20℄ Othersayingsalsostressingthisunpleasant,unfullled
stateofworknotseldomheardfromtheinitiatedareas
follows: \deliate synthetiundertakings";\a projet
withthedangertoget onesngersburned"`;\awork
on the edge of a preipie"`; \a desire for expeting
theunexpeted";\aquikwaytoablamage"`;\awalk
onatightrope: extremelydangerous"; \awalkovera
mineeld"; \amatterstilltoomuhshroudedin
mys-tery".
[21℄ S. Chandrasekhar, Mol. Cryst. Liq. Cryst., 124, 1
(1985).
[22℄ K.Praefke,B.Kohne,B.Gundogan,D.Singer,D.
De-mus,S.Diele, G.Pelzl,andU.Bakowsky,Mol.Cryst.
Liq.Cryst.,198,393(1991);shematipresentationof
somebiaxialshapemodelsintwo-dimensional
proje-tions,partlyshowninthispaper:
Unfortunately, the boomerang- or banana-like model
(right-handside)forbent moleules,sinethenineties
agreatstimulusinliquidrystalresearh,hasnotbeen
dared by us to be inluded in this artoon already
nearly one deade earlier simply due to the
regret-table irumstane that the rst is-tetraoxadealin
derivatives (bent, \roof-like" in struture, and
not exhibitanythermomesophase.Their lineartrans
-tetraoxadealin analogues with pairs of equal
sub-stituentsfrom methoxyto hexyloxy, however, do
dis-play nemati, smeti A or both of these phases on
heating,seeB.Kohne,K.Praefke,R.ShabanaOmar,
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