Supramolecular Fullerene Materials: Dendritic Liquid-Crystalline Fulleropyrrolidines Ste ´ phane Campidelli, † Julie Lenoble, † Joaquı ´n Barbera ´ ,* ,‡ Francesco Paolucci,* ,§ Massimo Marcaccio, § Demis Paolucci, § and Robert Deschenaux* ,† Institut de Chimie, Universite ´ de Neucha ˆ tel, Avenue de Bellevaux 51, Case Postale 2, 2007 Neucha ˆ tel, Switzerland, Quı ´mica Orga ´ nica, Facultad de Ciencias-Instituto de Ciencia de Materiales de Arago ´ n, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain, and Dipartimento di Chimica “G. Ciamician”, Universita ` di Bologna, via Selmi 2, 40126 Bologna, Italy Received June 25, 2005; Revised Manuscript Received July 20, 2005 ABSTRACT: [60]Fullerene-containing liquid-crystalline dendrimers were synthesized from the first to the fourth generation by applying the 1,3-dipolar cycloaddition reaction from a mesomorphic dendritic- type aldehyde derivative, sarcosine (N-methylglycine) or glycine and C 60. The cyanobiphenyl unit was used as a liquid-crystalline promoter. With the exception of the first-generation fullerene dendrimer, which was found to be nonmesomorphic, all fullerene-based dendrimers gave rise to a smectic A phase. The liquid-crystalline fullerenes led to two different supramolecular organizations within the smectic layers: for the second-generation dendrimers, the molecules are oriented in a head-to-tail fashion within the layers; for each molecule the cyanobiphenyl units point in the same direction. For the dendrimers of third and fourth generations, the dendritic core extends laterally, parallel to the layer planes; the mesogenic units are oriented above and below the dendritic core. For the aldehyde precursors, only one organization inside the layers was obtained, similar to the one observed for the third and fourth fullerene- based dendrimers. Cyclic voltametry investigations displayed several one-electron and multielectron reduction processes; no significant interaction in the ground state between the fullerene and the dendrimer was noticed. The title compounds showed the typical electrochemical stability of fulleropyrrolidines. Introduction Fullerodendrimers, 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 chem- istry and materials science. 5 Dendrimers play two major roles depending upon their architecture and function- alities: 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 condi- tions 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 meso- morphic fulleropyrrolidines 14 ). A great variety of liquid crystals was obtained, such as fullerene-ferrocene dyads (smectic A phases), 12a,b,d,14b fullerene-OPV conju- gates (smectic A phases), 14a fullerene-TTF dyads (smec- tic 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 (smec- tic 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 re- action 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 macro- molecules, as they would combine the electrochemical behavior 2 of C 60 with the rich mesomorphism of den- drimers. 20 We describe, herein, the synthesis, characterization, liquid-crystalline properties, and supramolecular or- ganization of the dendritic liquid-crystalline fullero- pyrrolidines 1-7 (Charts 1 and 2). By means of cyclic voltammetry experiments carried out on the second- generation dendrimer 2, it is demonstrated that such materials are electrochemically stable. The cyanobi- phenyl derivatives, acting as mesomorphic groups, are located at the periphery of the dendritic core. The dendrimers were prepared by applying a convergent synthetic methodology. 21 The mesomorphic properties of 2 were already reported. 10b * Corresponding author. E-mail: robert.deschenaux@unine.ch. † Universite ´ de Neucha ˆ tel. ‡ Universidad de Zaragoza-CSIC. § Universita ` di Bologna. 7915 Macromolecules 2005, 38, 7915-7925 10.1021/ma051359g CCC: $30.25 © 2005 American Chemical Society Published on Web 08/20/2005