joaquın barbera et al- self-organization of nanostructured functional dendrimers

8/3/2019 Joaqun Barbera et al- Self-organization of nanostructured functional dendrimers

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Self-organization of nanostructured functional dendrimers {

Joaqu n Barbera,a

Bertrand Donnio,b

Lionel Gehringer,b

Daniel Guillon, *b Mercedes Marcos, a Ana Omenat a

and Jose Luis Serrano *a

Received 17th February 2005, Accepted 23rd May 2005First published as an Advance Article on the web 16th June 2005DOI: 10.1039/b502464a

In this article, we review some of our recent work on the development and study of the properties of self-organizing supermolecular liquid crystalline dendritic materials. Prior to this purpose, the stateof the art in the field of molecular design, structure and properties of liquid-crystalline dendrimers(LCDs) will be briefly reviewed, and illustrated by a selection of pioneering examples. The first typeof LC dendrimer takes into account the location of their functional elements (mesogens) in theperiphery of the macromolecules and its topology of attachment, and the nature of the mesogen. Wewill show how functionality can be in-built into such materials so that self-organising functionalsystems can be created. Side-chain liquid-crystalline dendrimers exhibiting lamellar, columnar andnematic phases are shown. The mechanism of mesophase formation (mesogen interaction versusmicrophase separation) is also discussed in relation to the molecular structure (the nature of the

mesogen and of the dendritic matrix, dendritic generation number). The second type of LCdendrimer introduced presents a different topology in that the mesogens are now inserted within thedendritic matrix and in the periphery. Owing to their particular constitution, it will be shown thatthe mesophases of the lamellar and columnar types possess unusual morphologies.

I. Introduction

Two different approaches are usually considered in the elabora-tion of nanomaterials. 1 The first one is the top-down appro-ach which has been developed these last decades and whichconsists of pushing the limit of the lithography techniques

down to the nanometer scale. However, there are intrinsiclimitations to going beyond 100 nm with such techniques. Thesecond, more recent one is the bottom-up approach whichconsists of building up nanostructures and assembling themfrom individual atoms, molecules or macromolecules. 2 Withthis in mind, dendrimers have been considered, during the lastdecade, as promising materials for the fabrication andassembly of nanostructures. More specifically, dendrimersfunctionalised with mesogenic groups are of great interest forthe generation of self-organized nanostructures a la carte.

Since their discovery in the late 1970s, 3 and the adjustmentsof perfectly controlled iterative synthetic processes, thechemistry of dendrimers has led to the most impressivedevelopments and rapidly expanding areas of current

a Qu mica Organica. Instituto de Ciencia de Materiales de Aragon.Universidad de Zaragoza C.S.I.C., 50009 Zaragoza, Spain.E-mail: [email protected] Institut de Physique et Chimie des Materiaux de Strasbourg. Groupedes Materiaux Organiques. 23, rue du Loess, F-67037 Strasbourg,France. E-mail: [email protected]{ This work was funded as part of the EU Research Training NetworkLCDD, Supermolecular Liquid Crystal Dendrimers.

Dr Joaqu n Barbera received hisPhD in 1986 under the super-vision of Profs. E. Melendez and J. L. Serrano. After working with Dr A. M. Levelut in thePhysics Laboratory of Solids(CNRS-University Paris Sud),he obtained a permanent position in the University of Zaragoza, where he teachesOrganic Chemistry. His scientific activities have been carried out in the Liquid Crystal Group,and his current research ismainly devoted to the structural studies of liquid crystals.

Dr Bertrand Donnio was born in 1967 (Bretagne) and graduated in chemistry from the University of Rennes in 1991.

He obtained his PhD fromSheffield University (UK) in1996 under the supervision of Prof. D. W. Bruce. After two postdoctoral fellowships (inNeuchatel, Switzerland, Prof.R . D e sc h en a ux , a n d i nFreiburg, Germany, Prof. H.Finkelmann), he obtained a permanent position as Charge de Recherche (CNRS) at theIPCMS-GMO. He is interested in the elaboration of liquid crystalline materials (designand synthesis of dendrimers,elastomers, metallomesogens,

conjugated organic mesogens), and in the study of the supra-molecular organisation within LC mesophases (small angle X-raydiffraction, dilatometry).

Joaqun Barbera Bertrand Donnio

FEATURE ARTICLE www.rsc.org/materials | Journal of Materials Chemistry

This journal is The Royal Society of Chemistry 2005 J. Mater. Chem. , 2005, 15 , 40934105 | 4093


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into multicomponent nanosize objects in which each con-stituent element could be tailored independently.

The most important class of liquid-crystalline dendrimers(LCDs) is represented by the side-chain liquid crystallinedendrimers as schematically represented in Fig. 1. The overallstructure of such side-group LCDs (this terminology is usedby analogy to side-chain liquid crystal polymers) consists of a flexible branched network emanating from a single multi-valent initiator core and mesogenic or pro-mesogenic unitsattached at the termini of the branches. The control of themolecular conformation in such side-chain LC dendrimerswill be considered thoroughly in section II of this paper. Thisfield has now well expanded and one can distinguish severaladditional sub-classes of LCDs including main-chain systems(see section III), shape-persistent LCDs, 17 supramoleculardendromesogens, 18 fullerene-containing liquid crystals, 19

metallo-mesogenic-dendrimers 20 and the so-called meso-morphic polypedes. 21

To avoid confusion in the following, let us note that thedendritic systems so far mentioned are distinct from hyper-branched polymers, often referred to as dendrimers, amisleading term, since they are characterized by randomlybranched structures with a high degree of branching (withoutany cross-linking) and broad molecular weight distributions, a

consequence of their statistical mode of construction. 22 Twoapproaches have also been successfully applied to generatemesomorphism in these systems. One is the random poly-merization of mesogenic monomers containing at least twodifferent functional groups A and B (AB m -type monomerswith m 2 and A ? B) to yield main-chain systems, 23,24

whereas the other one consists of the grafting of mesogenic

groups at the periphery of pre-existing hyperbranchedpolymers, i.e. side-chain systems. 25

II. Control of molecular conformation in side-chainLC dendrimers

The term side-chain liquid crystal dendrimers is assigned, inanalogy with the polymer terminology, to those materialswhich are built by the functionalization of the periphery of a pre-formed dendrimer (poly(amidoamine) PAMAM, poly-(propyleneimine) PPI, carbosilane, poly(siloxane), carbosila-zane, etc. ) with units that promote the formation of supramolecular organisations giving rise to liquid crystal

mesophases15,2628

(Fig. 1). For instance, the introduction of rod-like or disk-like units at the periphery of the originaldendrimer leads to dendritic architectures that display liquidcrystalline properties. These LC dendrimers combine in onesingle macromolecule two main components with oppositetendencies: on one hand, the central isotropic dendriticarchitecture, and on the other hand, the structural anisotropic(pro)mesogenic units. The branches of the dendritic core tendto be isotropically distributed in space because of entropicforces, resulting in a pseudo-spherical morphology (star-burstshape in dendrimer terminology). The (pro)mesogenic units atthe periphery, in contrast, are subjected to strong anisotropicinteractions. In general, enthalpy gains over entropy in thesesystems, with the result of a mesophase formation. In addition,the incompatible chemical natures of the dendrimer core andthe (pro)mesogenic units forces the microsegregation of thesetwo constituent parts at the molecular level, which also favoursthe formation of mesophases. In summary, the mesomorphicproperties of these LC dendrimers (phase type, transitiontemperatures and thermodynamic stability) are determined by

Fig. 1 Schematic 2D representation of an end-group dendrimer of the second generation with a 4-fold core connectivity ( N C 5 4) and aternary branch multiplicity ( N B 5 3). The mesogen can be attachedterminally or laterally to yield (a) end-on and (b) side-on LCDs.

Dr Ana Omenat obta ined her PhD degree f rom theUniversity of Zaragoza, and a ft er work i ng wi th P rof .Ghedini in the University of C al ab ri a a nd i n P hi l ip sResearch Laborator ies inEindhoven, she joined theLiquid Crystal Group at theUniversity of Zaragoza. Herresearch activities have alwaysbeen related to liquid crystals:low and high molecular weightorganic and metallorganicmaterials.

Prof. Jose Luis Serrano received his PhD in Chemistry in1980 from the University of Zaragoza (Spain). In 1996,

he obtained the position of Full Professor of OrganicChemistry at the Universityof Zaragoza. Since 1985, hehas belonged to the Instituteo f M at e ri a ls S c ie n ce o f Aragon (ICMA), and nowhe is Vice-Director of theInstitute of Nanoscience of Aragon (INA). During thelast few years, Prof. Serranosresearch has been devoted to the use of liquid crystalsas a tool in supramolecularchemistry, dendrimers and polymers in order to obtain

functional materials with relevant optical and electrooptical properties.

Ana Omenat Jose Luis Serrano



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the enthalpy/entropy balance, the degree of chemical incom-patibility of the constituent parts, their size and the structureof the (pro)mesogenic unit itself.

Pioneering reports

Among this category of LC dendrimers, Meijer et al. 27 havedescribed two series of poly(propyleneimine) dendrimers

functionalised with pentyloxy and decyloxy cyanobiphenylmesogens at the periphery (Fig. 2). X-Ray diffractionexperiments have shown that all the mesophases observedare smectic A in nature. One striking feature is the almostconstant value of the layer spacing whatever the generationnumber for a given spacer. The fact that the layer spacing doesnot vary as a function of the number of peripheral mesogenicunits implies that the dendritic core should be in a pronounceddistorted conformation, which seems to indicate an extensionwith increasing generation number occurring only in twodimensions, i.e. in a plane parallel to the smectic layers.Lattermann et al. 28 have also considered poly(propyleneimine)dendrimers of different generations but substituted with 3,4-bis(decyloxy)benzoyl groups (Fig. 2). Firstly, these compoundsexhibit also mesomorphic behaviour with a melting tempera-ture decreasing with increasing molar mass. Secondly, theobserved mesophases have a columnar nature.

Carbosilane LC dendrimers

Shibaev et al. ,15 and to a lesser extent Frey et al. ,29 havereported the thermotropic behaviour of carbosilane LC

dendrimers. They showed that for the lower generations thereis no significant influence of the dendritic architecture.Structural studies on two series of dendrimers, containingeither terminal cyanobiphenyl and methoxyphenyl benzoategroups, show the existence of disordered smectic phases(smectic A and C) up to the fourth generation despite theglobular shape of the dendritic core, 30 with layers of mesogenic

groups alternating with layers formed by the carbosilanedendritic cores and aliphatic spacers. It is interesting to note,here also, that the layer spacing is slightly dependent ongeneration number but for the high generations only, in therange between 40 and 50 A . The fifth generation of thecarbosilane LC dendrimers, containing 128 terminal cyanogroups, forms not only a lamellar phase but also a supra-molecular columnar nanostructure. 31 A smectic A phase existsat low temperature, for which the molecular organisation is thesame as for the previous generations with a layer spacing of 53 A despite the presence of the large number of peripheralmesogenic groups. At higher temperature, this lamellarmesophase transforms into a rectangular columnar phase,

and finally into a hexagonal columnar phase. The explanationof this behaviour given by the authors is based on a change of the molecular shape. With the increase of the temperature, thecarbosilane LC dendrimers become less elongated and theirshape more circular, favouring the formation of ellipsoidalcolumns. Each column would consist of the stacking of ellipsoidal molecules, elongated in a plane perpendicular to thecolumnar axis. Further increase of the temperature leads to amore symmetrical shape of the LC dendrimer becomingcircular and inducing the formation of a hexagonal phase.The columns would result from the stacking of the circularmolecules flattened in a direction parallel to the columnar axis.In both rectangular and hexagonal phases, the surface of the

columns is covered by the mesogenic groups while their innerpart consists of soft dendritic cores. A similar trend has beenobserved with carbosilane dendrimers terminated with meso-genic groups based on anisic acid derivatives. 32

More recently, Shibaev and coworkers have also consideredphotosensitive LC dendrimers. The interest in such com-pounds is due to the new opportunities provided by suchsystems in the production of various optical devices and thepreparation of materials suitable for optical data storage.Moreover, the development of photoactive dendrimers capableof forming liquid-crystalline phases is of particular interest,because a low viscosity is expected to induce a quick responseof the dendrimers to the action of an external field such as, for

example, light irradiation. Thus, fast response photosensitivematerials can be generated. The first example concerns a liquidcrystalline carbosilane dendrimer of the first generation withazobenzene terminal groups, exhibiting a smectic A mesophasesuch as described above for other terminal mesogenicgroups. 33 In this case, the azobenzene terminal moiety servesthe dual function, i.e. on one hand, its rigid anisometric shapeensures the development of a mesomorphic state, and, on theother hand, the presence of azo-chromophores ensures that theLC dendrimer is sensitive to light irradiation. It was shownthat the E Z photopolymerization of the azobenzene groupsproceeds both in solution and in film under UV irradiation.This process is reversible photochemically and thermally.

Fig. 2 Poly(propyleneimine) dendrimers with two types of terminalmesogenic groups.

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PAMAM and PPI LC dendrimers

In the last years, a systematic study of the properties of side-chain LC dendrimers has been undertaken on poly(amido-amine) (PAMAM), poly(propyleneimine) (PPI) dendimers. Inall cases, it was found that the enthalpic gain of the mesogenicunits arranged as in a classical liquid crystalline mesophasedominates over the entropic tendency of the dendrimer core to

adopt a globular isotropic conformation. The flexibility of thedendritic core (PAMAM, PPI) allows the macromolecule toadopt a microphase separated molecular conformation whichgives rise to various types of liquid crystalline supramolecularorganisations. The chemical structure of the (pro)mesogenicunits (shape, number of terminal alkoxy chains) determinesthe type of mesophase formed (nematic, lamellar, columnar).The structural study of the mesophases by X-ray diffractiontechniques has led to the proposal of idealised models for boththe dendrimer molecules and the supramolecular arrangementsin the different mesophases.

LC dendrimers obtained by functionalization of commercialamino-terminated PAMAM and PPI with mesogenic units

with one terminal alkoxy chain, namely 4-alkoxybenzoyl-oxysalicylaldehyde, display nematic and smectic mesophases. 34

In this case, the molecular model proposed consists of acylinder in which the dendrimer core occupies the central slaband the mesogenic units are arranged parallel to each other,extending up and down from the molecule centre (Fig. 3a).This model explains the mesogenic behaviour of thesePAMAM and PPI derivatives since the dendrimeric super-molecules can be considered as large rods that would beordered parallel to each other promoting the supramolecularorder typical of smectic mesophases (Fig. 3b).

This model also shows that the dendritic core deformsstrongly with increasing generation number, since the layer

spacing stays around 45 nm whatever the molecular weight(Fig. 4). This deformation takes place in two dimensions in aplane parallel to the smectic layers. Systematic calculationsperformed from X-ray diffraction and volume data indicatethat the diameter of the dendrimer cylinder increases from1.3 nm up to 6 nm when going from the lowest generation upto the fourth one containing 64 peripheral mesogenic units.

Dendrimers incorporating mesogenic units with twoterminal alkoxy chains (3,4-dialkoxybenzoyloxysalicylalde-hyde) exhibit a hexagonal columnar mesophase. 35 Thesedendrimers cannot be arranged in a molecular cylindricalmodel like that proposed for the smectic phases, since thecross-sectional area of the terminal chains is larger thanthe area occupied by the mesogenic units. In this case, thedendrimers tend to adopt a different and more stableconformation, so that the mesogenic units can be accommo-dated optimally. It consists of a disk-like radial arrangementthat allows the filling of the space in all the three distinct

Fig. 3 (a) Schematic representation of the molecular model for dendrimers with one-terminal-chain mesogenic units. (b) Model for the smectic Asupramolecular organisation.

Fig. 4 Variation of the size of the elementary dendrimer cylinder as afunction of generation number (G n). d is the layer spacing, i.e. theheight of the cylinder.



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regions: central core, rigid part of the mesogenic units andterminal chains. Therefore, the dendritic molecules fill up thickdisks or disk equivalents: for the low generations, two or threemolecules are needed to form a complete disk, and from thethird generation onwards, just one molecule constitutes a disk,whose thickness depends on the generation number (Fig. 5a).The arrangement of these disks within supramolecularorganisations gives rise to cylindrical columns and thereforeto the hexagonal columnar mesophase (Fig. 5b).

This proposed model was further justified by MD simula-tion which reproduces the paving of the hexagonal lattice of the dendrimers in a flattened wedge conformation. The resultof the calculation evidenced a good filling of the availablevolume. An enhancement of the micro segregation over theentire simulation experiment time was also observed, con-tributing to the stabilization of the structure. Furthermore, thecompensation of the molecular areas at the dendritic/mesogensinterfaces implies the tilt of the peripheral rigid segments withrespect to the radial directions.

It is interesting to note that the diameter of the columns doesnot vary significantly with the generation number. Indeed itvaries irregularly between 5.3 nm and 6.7 nm, despite the fact

that the molecular weight of one single dendrimer varies from2660 up to 48500 Daltons when going from the lowestgeneration up to the fourth generation. In other words, thisclearly indicates that the dendritic core deforms strongly in onemain direction corresponding to that of the columnar axis,whereas the mesogenic units are arranged radially (Fig. 5a) toensure efficient lateral interactions within and betweencolumnar slices. Similarly to the case of lamellar phasesdescribed above, systematic calculations performed fromX-ray diffraction and volume data indicate the highestgeneration dendrimer (containing 64 peripheral mesogenicgroups) can fill the equivalent of a disk 1.9 nm thick, whereasthe third generation dendrimer containing 16 peripheral

mesogenic groups fills a disk 0.5 nm thick. It has been foundthat, whatever the generation, there is on average 16 mesogenicunits over 0.5 nm along the columnar axis.

To conclude this section, let us emphasize that both lamellarand columnar mesophases can be obtained with dendrimers,even with those of high molecular weight, the stability of thecorresponding phases being ensured by lateral interactionsbetween the (pro)mesogenic units and by a significantdeformation of the conformation of the dendritic core. The

difference in the symmetry of the mesophase is only related toa small difference in the molecular design of the peripheralmesogenic groups. Those with only one terminal end-chainproduce lamellar mesophases, whereas those with two terminalend-chains lead to columnar mesophases. Let us point out alsothat the dimensions (a few nanometers) of the elementarydendrimer cylinder can be tuned according to the generationnumber, but the fundamental dimensions of the mesophasestructure (layer spacing for the lamellar phases and inter-columnar distance for the columnar phases) do not depend onthe size of the dendrimer itself.

In the field of liquid crystals, it is well known that many of the materials displaying a columnar mesophase are constituted

by disk-like (discotic) molecules. To the best of our knowledge,very little attention has been paid to LC dendrimers containingdiscotic mesogenic units, namely based on hexasubstitutedderivatives of triphenylene. These compounds are also of special interest because of their photoconductive properties.A series of PPI-based dendrimers that incorporate discotictriphenylene mesogenic units at their periphery has beenprepared and their properties investigated. 36 All the dendri-mers except that of the first generation show a hexagonalcolumnar mesophase in a wide temperature range. The XRDcharacterisation of this mesophase reveals that its parametersare practically independent of the dendritic generationconsidered and indicate the existence of a hexagonal sublattice

Fig. 5 (a) Schematic representation of the molecular model for dendrimers with two-terminal-chain mesogenic units. (b) Model for the hexagonalcolumnar supramolecular organisation.



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in the hexagonal columnar mesophase. This hexagonalsublattice would be generated by the segregated columnscontaining either the stacked triphenylenes or the dendrimer(Fig. 6). As shown in Fig. 6, each dendrimer column issurrounded by six columns of triphenylene moieties. In thismodel, the flexible dendritic part adopts a cylindrical arrange-ment with the four generations having a similar diameter buttheir height increasing. Once more, the interactions betweenthe mesogenic units determine the supramolecular structureand the dendritic central core adopts the elongated conforma-tion necessary to allow this molecular arrangement.

Side-chain LC dendrimers exhibiting the nematic meso-phase, which is the most disordered liquid crystalline phase,can also be achieved by the appropriate choice of the meso-genic unit(s) attached to the dendrimer central core. Forexample, side-chain PPI-derived dendrimers containing meso-genic units with short terminal alkoxy chains (ethoxy, butoxyand pentoxy) or mesogenic units attached laterally, have beenprepared in a similar approach to that employed in side-chainliquid crystal polymers. 37 In such cases, the side-by-sidemolecular arrangement typical of lamellar phases is disfa-voured, thus promoting a nematic order (Fig. 7) and indeedside-on dendrimers, and some end-on dendrimers bearingmesogens with short aliphatic chains, display a nematicmesophase. This result indicates that these nanophase-struc-tured dendrimers lead to macromolecules with LC propertiessimilar to those of comparable low mass compounds.

Tuning the mesophase with codendrimers

As described above, the presence of one or two terminalalkoxy chains in the chemical architecture of the mesogenicunits induces a drastic modification in the thermotropicbehaviour of the dendrimer (from lamellar to columnarstructures). The introduction of both types of mesogenic units

(containing one and two terminal chains) within the samedendritic structure, i.e. the preparation of random codendri-mers, should open a larger possibility of tuning the symmetryof the mesophase exhibited by the material. A series of codendrimers was synthesised, in which both types of mesogenic units were introduced in various proportions 38

(Fig. 8). The study of the phase diagram obtained for thesecodendrimers reveals that, for intermediate compositions, twoother mesophases appear between those (smectic A andhexagonal columnar, respectively) of the homodendrimers.Codendrimers with a small content of the two-terminal-chaincomonomer exhibit a smectic C phase below the smectic Aphase. The appearance of the tilted smectic mesophase is aconsequence of the increase in the total number of terminalchains. The tilt of the molecule affords a larger area of theideal cylinder base, which allows the accommodation of allthe terminal chains (Fig. 8a). Larger contents of the two-terminal-chain comonomer in the codendrimers favour theoccurrence of a rectangular columnar mesophase. As thenumber of terminal chains increases, their accommodationwithin a cylindrical structure becomes unlikely. This elemen-tary dendritic cylinder is deformed into some kind of parallelepiped structure, which in turn promotes the existenceof a rectangular columnar mesophase (Fig. 8b). Thus,depending upon the respective concentration of the twotypes of monomers, orthogonal and/or tilted lamellar meso-phases, or else rectangular and/or hexagonal mesophases canbe obtained.

Tuning the dimensions of the nano-dendritic object with bulkyperipheral groups

In order to further investigate the influence of the structure of the mesogenic units on the mesomorphic behaviour of thedendrimers, bulky units have been introduced at the periphery

Fig. 6 Schematic representation of the model for the hexagonal columnar mesophase in LC dendrimers with discotic mesogenic units.



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of the fourth and fifth generations of amino-terminatedPAMAM and PPI (Fig. 9), with the aim of achievingcubic mesophases, since the increased volume around thedendritic core could force the dendrimer to adopt a globularconformation. 39

However, all the dendrimers prepared show only ahexagonal columnar mesophase, even those bearing the largestmesogenic unit with nine terminal chains. In contrast to thecase of the bulky dendrons reported by Percec et al. which canself-organize into micellar cubic mesophases, 40 the coating of the present PAMAM and PPI dendrimers is not sufficient to

lead to a globular shape of the dendrimer able to generate suchcubic mesophases.

A model of organization of these dendritic supermolecules,deduced from X-ray diffraction results and theoretical calcula-tions made therefrom, has been proposed. This molecularmodel implies a cylinder, whose inner part is occupied by thedendritic core and the bulky mesogenic units spreading aroundit, as shown in Fig. 10.

The dendritic core adopts an extended conformation whichis possible due to the great flexibility (conformational freedom)of the PAMAM and PPI skeletons. In this way, the molecular

Fig. 8 (a) Schematic representation of the molecular model for codendrimers with one- and two-terminal-chain mesogenic units (SmC phase).(b) Molecular model for codendrimers with one- and two-terminal-chain mesogenic units (rectangular columnar phase).

Fig. 7 General structure of nematic dendrimers with end-on and side-on mesogenic units.



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at every level of the dendritic hierarchy, and the dendrimers areforced to adopt constrained and regular structures. As such, theanisometricbranches do not radiate isotropicallyas in side-chaindendrimers, but, in contrast, favour preferentially an anisotropicorder by a gain in the enthalpy of the system in order to producethe most stable structure.

These main-chain dendrimers may represent an interestingalternative for the development of original molecular materialshaving a new architecture (Fig. 12b). As far as we are aware,the most closely related compounds which have been reportedare the so-called willow-like dendrimers synthesized by Percecet al. some years ago. 24,41 Shibaev et al. also foresaw theelaboration of such compounds, but have not yet reported ontheir synthesis and physical properties. 42

Octopus LC dendrimers

As already mentioned, conventional end-group dendrimers,where the core matrix is either PAMAM, PPI or carbosilane,

are usually built up using a divergent approachthat isconstructed outwards from a central core, and the mesogenicmoieties are attached to the periphery at the final stage of thesynthesis. For the presently described dendrimers, thissynthetic approach appeared unsatisfactory as a number of additional steps were preliminarily required to synthesize themesogenic moiety at each generation, before moving up to the

next generation, with the consequent diminishing overallreaction yields hampered by reactivity and purificationproblems. Therefore, in order to improve the reactivity of the connecting sites and facilitate the purification of thedendritic materials, a modular synthesis was elaborated for thepreparation of the branches. Each constituent part wasprepared separately and later assembled selectively together.The anisotropic units selected were a tolane-based or astilbene-based moiety, due to both their thermal stability andchemical versatility. Such a poor mesogenic segment, despiteits anisotropy and rigidity, was also chosen in order to testwhether mesomorphism could be induced solely by thedendrimerization process. The dendritic branches, function-

alized by a focal acid group, were then coupled to a small tetra-podand core unit, bearing four amino groups, to yield the finaldendrimer (Fig. 13). 43 The monodispersity and the analyticalcharacterization of the compounds were achieved by SEC,MALDI-TOF and various spectroscopic techniques.

Homolithic (X 5 Y) and heterolithic (X ? Y) dendrimerswere synthesized and all of them were liquid crystals. Meso-morphism was thus induced by the precise assembling of thesenon-mesogenic units within the dendritic frame. Moreover, themesophase stability was found to greatly depend on thelocalization of the various units within the heterolithic systems.

The dendrimers bearing only one aliphatic end-chain at theextremity of the outer tolane unit exhibit smectic behaviour.

Fig. 12 Schematic 2D representation of side-chain (a) and main-chain (b) liquid crystalline dendrimers.

Fig. 13 The different octopus dendrimers.



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The X-ray diffraction patterns are characterized by thepresence of 34 orders of reflection corresponding to a welldefined lamellar stacking. On the basis of these X-ray patterns,the high temperature phase has been assigned as a disorderedsmectic A phase, whereas the other, low-temperature, phase isdue to an extra in-layer order, such as a hexatic smectic Bphase, the layer spacing being rather large, in the range

1012 nm. This confirms the prolate conformation of thedendrimers in both smectic phases with the peripheralanisotropic units being almost perpendicular to the layernormal direction. In this case, due to geometric constraints, themesogenic groups of the first row of the arborescence, locatedin the smectic sub-layer, ought to be tilted to match themolecular area of roughly two mesogenic groups, in order tooccupy a larger apparent surface area (Fig. 14). Of course, therigid units contained in these aromatic slabs are tilted withrespect to the layer normal, but the tilt is not necessarilycorrelated, i.e. short-range tilt order. In other words, therelative disordered distribution of the rigid parts yields anapparent zero tilt angle, and then a uniaxial smectic sub-layer.

This model is consistent with the molecular conformationdeduced from MD calculations, and also explains thedifference between the periodicity and the molecular length.The SmB-to-SmA transformation corresponds to the loss of the hexatic order consequent on the lateral disorganization of the anisotropic units of the outer slabs. 44

Therefore, the morphology of the smectic phases generatedby such multiblock molecules is quite unique in that itpossesses a two-level molecular organization, each beingdependent on the other one. It consists of an internal sub-layer made of tilted rigid segments with no correlation of thetilt, flanked by outer slabs inside which the mesogenic groupsare arranged perpendicular to the layer (Fig. 14). Molecular

modeling supports this view of strongly segregated multilayerstructures, with interfaces between the various molecular parts.Obviously, these interfaces are not so well defined due tothermal fluctuations. Nevertheless, let us point out that due to

this peculiar structural feature, such layered mesophasescannot exactly be described as purely SmA or SmB phases.

As for the other set of dendrimers bearing two or threealiphatic chains at the extremity of the outer tolane or stilbenepart, they all exhibit a columnar mesophase with a hexagonalsymmetry. The formation of columnar mesophases in non-discotic systems, and particularly with polycatenar meso-

gens,45

is a consequence of the mismatch between the surfaceareas of the aromatic cores and the cross-section of thealiphatic chains, resulting in the curvature of all the interfaces,as has been discussed with side-chain LCDs bearing poly-catenar end-groups. In the present case, in order to compen-sate the discrepancy between the cross-sections of both theanisometric segments and the chains, one can also imagine theformer to be tilted and distributed in a splay fashion, withrespect to the columnar axis, also resulting in the curvature of the interfaces. 46 Indeed, the parameters of the hexagonallattices obtained experimentally, a 5 910 nm, correspondfairly well to the diameter of the dendrimers in flattenedconformation, ranging between 10 and 11 nm as estimated by

MD simulation. It is therefore highly probable that theoctopus preferably adopts an oblate shape within the columnsthat is a flattened or a wedge-like conformation with theanisotropic blocks lying more or less in the 2D hexagonallattice plane, rather than a prolate conformation (cylindrical)as in the smectic systems.

The formation of the mesophase results from the self-assembling process of octopus molecules necessarily adoptingpre-defined shapes as for the dendromesogens described byPercec et al. ,18,47 where columnar structures are generatedfrom the self-assembling of the most stable molecularconformations having either a wedge-like or half-disc shape.The overall molecular conformations of the dendrimers in the

mesophase are driven by the steric congestion of the terminalaliphatic chains and depend on the segregation between thedifferent constituent blocks. In the present case, one or twomolecular conformations likely predominate to satisfy thegeometrical requirements. The formation of supramoleculardisks or columns results from the molecular association of these two types of dendritic conformations, as shown inFig. 15. One possibility consists of assembling together two orthree dendrimers with solely the wedge-like conformation andanother one of associating three octopuses with both wedge-like and flattened conformations in a 2 : 1 mixture. Clearly,one has to bear in mind that these various arrangements mustco-exist in the columns since it is not possible to favour one

over the other. Then, the resultant columns further self-organize into a 2D hexagonal lattice. Considering the diblock,alternated chemical nature of these octopus dendrimers, anonion morphology for the columns is most probable. 44

In these dendrimers, the morphology of the mesophase isalso determined by the number of alkyl chains grafted on theperipheral mesogenic group. Indeed, the change in the numberof terminal chains per end group (one or two) modifies therelationships between the hard parts and the soft parts, andconsequently the molecules will adopt either a parallel (prolate) or a flat (oblate) conformation. The formation of the smectic lamellar phases is the result of the paralleldisposition of the mesogenic groups on both sides of the focal

Fig. 14 Model for the octopus conformation and organization withinthe smectic phases.



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tetravalent core, the dendrimer adopting the shape of a giant

elongated multipode, and then organizing into layers. Incontrast, the grafting of additional terminal chains at theperiphery prevents such a parallel disposition of the pro-mesogenic groups, which are forced to be radially arrangedaround the central moiety: the dendrimer can adopt theshape of a flat-tapered object which will self-arrangeinto supramolecular columns. Although the expected meso-morphism cross-over from lamellar to columnar structures wasobserved with increasing the number of terminal chains, 33,34

the two-phase behavior or the induction of intermediate meso-phases (such as bicontinuous cubic or rectangular columnarphase 37 for instance) is nevertheless absent within a singledendrimer.

IV. Conclusion

In conclusion, it is shown that the induction of mesomorphicproperties 14,16 can be extended to various types of dendrimersof high molecular weight, even with non-mesomorphicbuilding blocks (dendrimerisation promotes LC phases). Thisis achieved through a precise chemical design, combined withthe role of different types of intermolecular interactions. Theinteractions between the polarizable promesogenic unitscombined with the nanophase segregation contribute to thestabilization of the aromatic sublayers (in the smectic phases)and of the columnar cores (in the columnar phases). In the case

of side-chain LC dendrimers, the presence of one chain permesogenic unit favours their parallel arrangement; hence thelamellar morphology is promoted. The increased aliphaticchain density due to the introduction of two chains permesogenic unit, imposes a curved interface with the promeso-genic units arranged radially, hence inducing the columnarmesomorphism. The same is true in the case of main-chainLC dendrimers. The high density of aliphatic chains likelyimposes also curved interfaces at all hierarchical levels of the dendrimers, i.e. between the promesogenic units (internaland peripheral), the aliphatic spacers and the terminalchains respectively, forcing the molecules to adopt a wedge-like conformation, thus promoting their self-assembling

towards a columnar organization with an onion internalmorphology. As for the smectic-like phases, the dendrimersadopt a parallel conformation, and due to the particularnature of the molecules, the segments of the first generation,which are located in the smectic sub-layer, are tilted leadingto a two-level molecular organization. In addition, and veryimportantly, the individual dendritic nano-objects forming

the elementary bricks of the high-level self-organization, canbe tuned in shape and size through suitable molecular design,for example by using mesogenic groups of different typesand by building codendrimers or by using bulky promesogenicgroups.

Thus, the structural concept of these LC dendrimersproves to be innovative and versatile as it offers many newopportunities in the design of a wide range of multicomponentsystems with specific properties for potential novel applica-tions. Indeed, it appears that large flexibility and freedomare allowed in the choice of the elementary anisotropic bricksof these LCDs, without the suppression of the mesomorphicproperties. Additionally, such bricks can be independently

interchanged, and the stability of the mesophases accordinglymodulated, proving the good sensitivity of the dendriticscaffold with the nature and the mutual arrangement of themesogenic segments (spatial location), and the intimaterelationships with the mesomorphic properties. The highsensitivity of such dendrimers with the surrounding environ-ment (properties versus molecular structure) could be, inprinciple, beneficial to access certain kinds of molecularsensors, i.e. to use such supermolecules as tools to test howproperties in general may be altered or modulated upondelicate external stimulation. Systems with photoactivemoieties can also be envisaged in view of optoelectronicapplications.

Acknowledgements

This work has been supported by the CICYT of Spain and theFEDER funds (EU) under the projects MAT2002-04118-C02-01 and MAT2003-07806-C01, by the European Union underthe RTN Project Supermolecular Liquid Crystal Dendrimers(LCDD) (HPRN-CT2000-00016), and by the DiputacionGeneral de Aragon. B.D. and D.G. would like to thankC. Bourgogne and B. Heinrich for technical help and usefuldiscussions.

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