IntroductionFullerodendrimers, 1 which combine the outstanding electrochemical 2 and photophysical 3 properties of C 60 with the unique structural features of dendrimers, 4 generated fascinating studies in supramolecular chemistry and materials science. 5 Dendrimers play two major roles depending upon their architecture and functionalities: they increase the solubility of C 60 in organic solvents 6 or in water 7 (solubilizing effect), and they isolate C 60 from the external environment such as oxygen and solvent molecules (protection effect). 8 Both effects can be adjusted to specific experimental conditions by synthetic chemistry at the dendrimer level. Dendrimers prevent also the formation of aggregates resulting from strong interactions between C 60 units; highly ordered Langmuir and Langmuir-Blodgett films were so obtained. 9 With the view to construct supramolecular fullerene materials, whose properties could be of interest in nanotechnology (e.g., molecular switches, solar cells), we became interested in fullerene-containing thermotropic liquid crystals. We developed two concepts to design liquid-crystalline fullerenes: 10 in the first concept, C 60 was functionalized with liquid-crystalline malonates by applying the Bingel reaction 11 (leading to mesomorphic methanofullerenes 12 ), and in the second one, C 60 was functionalized with liquid-crystalline aldehydes and N-methylglycine or an amino acid derivative by applying the 1,3-dipolar addition reaction 13 (leading to mesomorphic fulleropyrrolidines 14 ). A great variety of liquid crystals was obtained, such as fullerene-ferrocene dyads (smectic A phases), 12a,b,d,14b fullerene-OPV conjugates (smectic A phases), 14a fullerene-TTF dyads (smectic A and B phases), 12g a hexa-adduct of C 60 (smectic A phase), 12c a chiral C 60 derivative (cholesteric phase), 12f and dendritic liquid-crystalline methanofullerenes (smectic A phases; an additional short-range nematic phase was observed for the second-generation dendrimer). 12e A bis(methano)fullerene was also reported (mesophase not identified). 15 Two other approaches were described: in the first one, C 60 was complexed by mesomorphic cyclotriveratrylene (CTV) derivatives (nematic and cubic phases), 16 and in the second one, five aromatic groups were attached around one pentagon of C 60 , yielding conical molecules (columnar phases). 17 Whereas methanofullerenes undergo retro-Bingel reaction upon chemical 18 and electrochemical 19 reduction, fulleropyrrolidines lead to stable reduced species. 13 Dendritic liquid-crystalline fulleropyrrolidines would represent an interesting family of electroactive macromolecules, as they would combine the electrochemical behavior 2 of C 60 with the rich mesomorphism of dendrimers. 20 We describe, herein, the synthesis, characterization, liquid-crystalline properties, and supramolecular organization of the dendritic liquid-crystalline fulleropyrrolidines 1-7 (Charts 1 and 2). By means of cyclic voltammetry experiments carried out on the secondgeneration dendrimer 2, it is de...
Janus-type liquid-crystalline fullerodendrimers were synthesized via the 1,3-dipolar cycloaddtition of two mesomorphic dendrons and C60. By assembling poly(aryl ester) dendrons functionalized with cyanobiphenyl groups, displaying lamellar mesomorphism, with poly(benzyl ether) dendrons carrying alkyl chains, which display columnar mesomorphism, we could tailor by design the liquid-crystalline properties of the title compounds as a function of each dendron size. The liquid-crystalline properties were examined by polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction. Depending on the dendrimer generations, smectic (SmC and/or SmA phases) or columnar (Colr-c2mm or Colr-p2gg phases) mesomorphism was obtained. The supramolecular organization is governed by (1) the adequacy of the cross-sectional area of the dendrons, (2) the microsegregation of the dendrimer, (3) the deformation of the dendritic core, and (4) the dipolar interactions between the cyanobiphenyl groups. Comparison of the mesomorphic properties of two fullerodendrimers with those of model compounds (fullerene-free analogues) indicated that the C60 unit does not influence the type of mesophase that is formed. Molecular properties determined in solution (permanent dipole moment, specific dielectric polarization, molar Kerr constant) confirm that microsegregation persists in solution and strengthen the models proposed for the structure of the mesophases.
A second-generation cyanobiphenyl-based dendrimer was used as a liquid-crystalline promoter to synthesize mesomorphic bisadducts of [60]fullerene. Liquid-crystalline trans-2, trans-3, and equatorial bisadducts were obtained by condensation of the liquid-crystalline promoter, which carries a carboxylic acid function, with the corresponding bisaminofullerene derivatives. A monoadduct of fullerene was also prepared for comparative purposes. All the compounds gave rise to smectic A phases. An additional mesophase, which could not be identified, was observed for the trans-2 derivative. The supramolecular organization of the monoadduct derivative is governed by steric constraints. Indeed, for efficient space filling, adequacy between the cross-sectional areas of fullerene (∼100 Å 2 ) and of the mesogenic groups (∼22-25 Å 2 per mesogenic group) is required. As a consequence, the monoadduct forms a bilayered smectic A phase. The supramolecular organization of the bisadducts is essentially governed by the nature and structure of the mesogenic groups and dendritic core. Therefore, the bisadducts form monolayered smectic A phases. The title compounds are promising supramolecular materials as they combine the self-organizing behavior of liquid crystals with the properties of fullerene.
The title compounds were synthesized by applying the 1,3-dipolar cycloaddition reaction of aldehyde-based poly(benzyl ether) dendrimers and sarcosine (N-methylglycine) to [60]fullerene (C 60 ). The dendritic building blocks used to functionalize C 60 displayed cubic and hexagonal columnar phases. The fullerene derivatives showed rectangular columnar phases of c2mm symmetry.The design of supramolecular [60]fullerene (C 60 ) assemblies, in which the [60]fullerene-containing molecular units are organized in a specific and controllable manner, is an elegant and appealing way to exploit the physical properties of C 60 in materials science (e.g., photoactive dyads and triads, 1 photovoltaic devices 2 ). Grafting dendrimers onto C 60 (fullerodendrimers) allowed this goal to be reached, as demonstrated by the formation of ordered and stable Langmuir and Langmuir-Blodgett films 3 and micelles.
Liquid-crystalline methanofullerodendrimers were synthesized via the Bingel addition reaction of mesomorphic malonate derivatives and C 60 . Second-and third-generation poly(benzyl ether) dendrons were selected as liquid-crystalline promoters to induce columnar mesomorphism. Based on a convergent and modular synthetic methodology, symmetrical (two identical dendrons) and non-symmetrical (two different dendrons) dendrimers were prepared, as well as hemidendrimers (only one dendron). The liquid-crystalline properties of the malonates and fullerodendrimers were investigated by polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction. All the malonates give rise to hexagonal columnar phases of p6mm symmetry. As for the fullerodendrimers, the second-generation hemidendrimer shows a rectangular columnar phase of c2mm symmetry, while the other materials give rise to hexagonal columnar phases of p6mm symmetry.
a Functionalization of [60]fullerene with liquid-crystalline dendrimers and a dibutylaniline-based phenylenevinylene moiety leads to supramolecular materials, the fluorescence of which responds to acid-base stimuli.The design of organic materials which can be used for reversible optical data storage and for the construction of photochemical switches requires the synthesis and assembly of components, the physical properties of which can be modulated by light.1 Photonic processes display properties superior to those of electron processes 2 for the following main reasons: (1) in the wavelength domain, multiple processing is achievable, (2) they present a high signal to noise ratio, and, more importantly, (3) since energy and electron transfer processes can occur on a subpicosecond timescale, it is possible to produce devices that respond with equal rapidity.On the other hand, liquid crystals (LCs) are soft materials with great potential for sophisticated applications in advanced technologies.3 LCs exhibit unique properties such as self-organizing behavior within a precise temperature range or a change in refractive index by changing their alignment. Furthermore, LCs are of interest as supramolecular platforms in, e.g., solar cell technology (e.g. fullerene associated with oligophenylenevinylene derivatives) 4 and for the development of photoactive switches (e.g., fullerene associated with ferrocene). 5We report, herein, on the synthesis, liquid-crystalline properties, electrochemical and photophysical behavior of compounds 1a and 1b (Scheme 1) which contain three subunits, i.e.(1) a donor unit formed by two dibutylanilines located at the periphery of a phenylenevinylene-based dendron, (2) [60]fullerene (C 60 ) as an electron acceptor unit, and (3) a secondor third-generation poly(arylester) dendron carrying four or eight cyanobiphenyl mesogenic units, respectively, as a liquid-crystalline promoter. As we will see below, it is possible to control the fluorescence of both the donor and acceptor units by protonation.The synthesis of 1a and 1b (Scheme 1) required the reaction of second- (4a) 6 or third-generation (4b) 7 dendron carrying a carboxylic acid function ( Fig. 1) with thionyl chloride in CH 2 Cl 2 to prepare the corresponding acid chlorides followed by their in situ condensation with fulleropyrrolidine 2 in CH 2 Cl 2 .Fulleropyrrolidine 2 was prepared in 28% yield by [3+2] dipolar cycloaddition of C 60 with the azomethine ylide 8 generated in situ from aldehyde 3 and glycine in chlorobenzene. Aldehyde 3 was prepared in 91% yield using the Scheme 1 Reagents and conditions: (i) t BuOK, THF, r.t., 2 h, then 1 M HCl, 91%; (ii) C 60 , glycine, chlorobenzene, reflux, 6 h, 28%; (iii) 4a or 4b, thionyl chloride, CH 2 Cl 2 , reflux, 7 h, then 2, CH 2 Cl 2 , pyridine, r.t., 30 min, quantitative yields.
Liquid-Crystalline Janus-Type Fullerodendrimers Displaying Tunable Smectic-Columnar Mesomorphism. -(LENOBLE, J.; CAMPIDELLI, S.; MARINGA, N.; DONNIO, B.; GUILLON, D.; YEVLAMPIEVA, N.; DESCHENAUX*, R.; J. Am. Chem. Soc. 129 (2007) 32, 9941-9952; Inst. Chim., Univ. Neuchatel, CH-2000 Neuchatel, Switz.; Eng.) -Bartels 48-089
A second-generation cyanobiphenyl-based dendrimer was used as a liquid-crystalline promoter to synthesize mesomorphic bisadducts of [60]fullerene. Liquid-crystalline trans-2, trans-3, and equatorial bisadducts were obtained by condensation of the liquid-crystalline promoter, which carries a carboxylic acid function, with the corresponding bisaminofullerene derivatives. A monoadduct of fullerene was also prepared for comparative purposes. All the compounds gave rise to smectic A phases. An additional mesophase, which could not be identified, was observed for the trans-2 derivative. The supramolecular organization of the monoadduct derivative is governed by steric constraints. Indeed, for efficient space filling, adequacy between the cross-sectional areas of fullerene (∼100 Å 2 ) and of the mesogenic groups (∼22-25 Å 2 per mesogenic group) is required. As a consequence, the monoadduct forms a bilayered smectic A phase. The supramolecular organization of the bisadducts is essentially governed by the nature and structure of the mesogenic groups and dendritic core. Therefore, the bisadducts form monolayered smectic A phases. The title compounds are promising supramolecular materials as they combine the self-organizing behavior of liquid crystals with the properties of fullerene.
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