A Highly-Ordered 3D Covalent Fullerene Framework**

A highly-ordered 3D covalent fullerene framework is presented with a structure based on octahedrally functionalized fullerene building blocks in which every fullerene is separated from the next by six functional groups and whose mesoporosity is controlled by cooperative self-assembly with a liquid-crystalline block copolymer. The new fullerene-framework material was obtained in the form of supported films by spin coating the synthesis solution directly on glass or silicon substrates, followed by a heat treatment. The fullerene building blocks coassemble with a liquid-crystalline block copolymer to produce a highly ordered covalent fullerene framework with orthorhombic Fmmm symmetry, accessible 7.5 nm pores, and high surface area, as revealed by gas adsorption, NMR spectroscopy, small-angle X-ray scattering (SAXS), and TEM. We also note that the 3D covalent fullerene framework exhibits a dielectric constant significantly lower than that of the nonporous precursor material.

Abstract: Ah ighly-ordered 3D covalent fullerene framework is presented with as tructure based on octahedrally functionalized fullerene building blocks in whiche very fullerene is separated from the next by six functional groups and whose mesoporosity is controlled by cooperative self-assembly with aliquid-crystalline blockcopolymer.The new fullerene-framework material was obtained in the form of supported films by spin coating the synthesis solution directly on glass or silicon substrates,followed by aheat treatment. The fullerene building blocks coassemble with aliquid-crystalline blockcopolymer to produce ah ighly ordered covalent fullerene framework with orthorhombic Fmmm symmetry,a ccessible 7.5 nm pores,a nd high surface area, as revealed by gas adsorption, NMR spectroscopy, small-angle X-rays cattering (SAXS), and TEM. We also note that the 3D covalent fullerene framework exhibits adielectric constant significantly lower than that of the nonporous precursor material.
Anentirelynewbranchofchemistrywasdevelopedafterthe discovery of fullerenes in 1985 and after access to C 60 on ap reparative scale was gained in 1990. [1] Thes tructural uniqueness of the C 60 molecule sparked the interest of materials scientists to use it as ab uilding block for novel materials with intriguing properties. [2] Versatile two-dimensional and three-dimensional exohedral modification options (i.e.amodification outside the spherical molecule) stem from the multifunctionality of the fullerenes,t hus making them attractive precursors for macromolecular and supramolecular chemistry. [2][3] Fullerenep olymers are being developed to integrate the intriguing properties of C 60 molecules with the good processability and excellent mechanical stability of polymers. [4] Them ost straight-forward approach to obtain ap olymer using solely C 60 molecules is photopolymerization. The resulting fullerene polymer is nonsoluble,s table,a nd highly crosslinked, but it is disordered and no control over the resulting structure is possible. [5] More sophisticated strategies polymerize the C 60 or functionalized fullerene derivatives with the addition of auxiliary monomers to incorporate the fullerene core into ap olymer chain. [6] By using this method, however, only af ew weight percent of fullerene functionalities can be introduced into the polymer chain. [7] In adifferent approach, as tar block polymer with aC 60 core was designed to self-assemble into different thin-film structures ranging from lamellar to gyroidal, but without any intermolecular connection. [8] Other researchers have crosslinked as urfactant-like fullerene derivative that resulted in a2 D structure. [9] An alternative approach with metal-coordinated connections in between the fullerene derivatives produced 2D-layered structures with very small pores. [10] Despite the synthetic efforts in the field of fullerene polymers and other fullerene-based materials,t oo ur knowledge no examples exist of covalently crosslinked fullerene materials with three-dimensional order and stable high porosity.T he introduction of ordered porosity is expected to create additional desirable features,such as high surface area and molecular discrimination, that would be beneficial for catalytic applications or electronic interactions of the fullerene pore-wall material with molecular guests. [11] Herein we demonstrate the first example of as table covalent fullerene framework exhibiting ahighly-periodic 3D pore system with around 7.5 nm pore diameter.T his high porosity was achieved by developing an evaporation-induced self-assembly (EISA) strategy of af ullerene precursor templated by al iquid-crystalline block copolymer inducing high periodicity and porosity.I nt his context, fullerene molecules can be modified at many points on their surface, potentially resulting in am ultitude of adducts with different symmetries and av arying number of functionalities.T his would lead to ac omplicated coassembly behavior between the precursor and template,and likely produce limited order in the final product.
We surmised that ahexafunctionalized C 60 derivative with T h symmetry would be beneficial for the construction of aw ell-defined highly-ordered three-dimensional porous framework. Ther esulting covalent fullerene framework could show extraordinary thermal stability and interesting electrical properties.
Theb uilding block for the 3D covalent fullerene framework-a molecular hexafunctionalized fullerene-was synthesized by applying the template-directed activation method developed by Hirsch et al. (Scheme 1). [12] We successfully synthesized and purified the resulting hexa-adduct with a T h -symmetric octahedral addition pattern. The 13 CNMR spectrum of the product demonstrates this high symmetry and the purity of the precursor (see the Supporting Information, Figure S1).
Thin films of the fullerene framework were produced in an EISA process by spin coating an ethanol solution of the hexafunctionalized fullerene and ab lock copolymer as template onto different substrates,s uch as glass or transparent conductive oxides.T he resulting films proved to have ahighly ordered mesostructure as demonstrated by means of small-angle X-ray scattering (SAXS), as shown in Figure 1.
Forthe as-synthesized film, anarrow reflection with afull width at half maximum (FWHM) value of 0.058 8 was detected at 0.858 8,indicating ahighly-ordered structure with a d spacing of 10.4 nm along the film normal. After ah eat treatment at 100 8 8Ca nd template extraction with ethanol, the d spacing decreased to 7.8 nm. Likewise,a fter as econd thermal treatment at 300 8 8Cf or 1h under an itrogen atmosphere,t he d value decreased further to 6.9 nm. This is in good agreement with the typically reported uniaxial shrinkage of mesoporous thin films along the substrate normal. [13] Figure 2shows TEM micrographs of fullerene-framework films after thermal treatment at 300 8 8Cr ecorded in cross section and plan view,that is,perpendicular to and along the substrate normal, respectively.T he cross section (Figure 2a  . .

Angewandte
Communications 7578 www.angewandte.org shrinkage after heat treatment, the structure becomes orthorhombic,w ith space group Fmmm and lattice basis vectors a orh = a cub , b orh = (b cub + c cub )S,and c orh = c cub -b cub ,where S is the shrinkage factor.I nt he orthorhombic setting,t he indexing of the observed lattice planes changes as follows:(01 1) cub becomes (002) orh and (011) cub becomes (020) orh .T he relationship of the initial cubic structure and the orthorhombic structure is depicted in Figure S4. Films with the same space groups and orientations with respect to the substrate exist for carbon and metal oxides. [14] Thes ymmetry change of cubic films with Im3 m symmetry in [011] orientation along the film normal to orthorhombic Fmmm in [010] orientation is discussed in detail by Falcaro et al. [13a] Thel ocal chemical structure of the fullerene framework was examined by 13 Ca nd 29 Si solid-state NMR spectroscopy (Figure S3 A). The 13 Cc ross polarization magic-angle spinning (CP-MAS) NMR spectrum of the film removed from the substrate corresponds very well with that of the fullerene precursor,w hich confirms the integrity of the molecular structure of the precursor in the fullerene framework (see the Supporting Information, Section 4). Thec lear absence of Q units [Q n = Si(OSi) n (OH) 4-n ]ataround d = À100 ppm in the solid-state 29 Si MAS NMR spectra (Figures S3 B, C) shows that there is negligible hydrolytic SiÀCb ond cleavage and that the siloxane-bridged organic linkers are retained intact in the fullerene framework under the synthetic conditions employed.
Thermogravimetric measurements give additional evidence that the fullerene framework, and especially the molecular structure,a re stable in nitrogen up to 300 8 8C ( Figure S5).
Thea ccessible porous structure of the fullerene framework after template removal by solvent extraction was examined by nitrogen sorption of the film material. As shown in Figure 3, the isotherm shows at ypical type-IV shape,c ommonly found with mesoporous materials.T he hysteresis indicates astructure with large,cage-like pores that is typical for cubic mesostructures. [14b] Thee stimated pore-size distribution shows as harp maximum at 7.5 nm (Figure 3, inset). Them aterial shows aB ET (Brunauer-Emmett-Teller) specific surface area of 494 m 2 g À1 and at otal pore volume of 0.34 cm 3 g À1 .W en ote that the internal voids of the fullerene moieties are not detected by the nitrogen adsorption measurements.M oreover,a tt he highpressure end of the isotherm we observe ac omplete lack of textural porosity,which confirms the high crystallinity of this material. Additionally,a tl ow pressure no discontinuity between adsorption and desorption branches is visible, implying quite ar igid open framework with essentially no swelling and shrinking.
Theelectronic properties of the fullerene-framework film were subsequently investigated. In contrast to the unfunctionalized fullerene,t he hexafunctionalized fullerene shows nearly no light absorption above l = 300 nm ( Figure S6). [15] This can be explained by the attenuation of the conjugated

Angewandte
Chemie fullerene p-electron chromophore by virtue of transforming six double bonds into cyclopropane moieties. [16] TheU V/Vis absorption spectrum of the fullerene-framework film resembles that of the precursor solution, with maxima at l = 238 nm and 275 nm ( Figure S6). Thea bsence of any shift for the porous structure indicates that the single fullerene precursor molecules are very well dispersed in the framework without electronic coupling and aggregation, similar to the situation in solution. [2] Thea bsence of aggregation can be explained by the molecular structure of the precursor with T h symmetry, where the six malonate molecules are added to the fullerene cage in an octahedral pattern. This allows the fullerene molecules in the framework to be fully and omnidirectionally separated and thus electronic coupling can be avoided.
To probe the influence of the C 60 side groups on electronic mobility,f ield-effect mobility measurements with bottomgate and top-contact device configurations were performed. Thetransconductance characteristics of the monoadduct, the hexa-adduct without silane groups,a nd the final fullerene framework after template extraction are shown in Figure 4a. During sweeps of the gate voltage at aconstant source-drain voltage (V SD = 20 V), the two hexa-adduct films do not show any significant current, whereas the monoadduct device shows the characteristic signature of an n-type semiconductor. An electron mobility of the order of 10 À4 cm 2 V À1 s À1 was measured for the monoadduct. Thec onductance curves (Figure 4a,i nset) for two constant values of the gate voltage (V G ), one close to the threshold voltage (V Th )and the second value 20 Va bove,s how n-type characteristics including saturation of the monoadduct transistor,w hereas no current was detected for both hexa-adduct films.
In conclusion, the measurements show that the modification of C 60 with six side chains causes the electron mobility to decrease drastically.Compared to the electron mobility of C 60 which is of the order of 1-2.5 cm 2 V À1 s À1 , [17] the value for the monoadduct is already decreased by four orders of magnitude.T his observation is consistent with previous studies of mobilities in fullerene derivatives,w hich show that the mobility decreases with increasing distance between the fullerenes. [18] Thee lectronic properties of am aterial are not only defined by its electron mobility,w hich characterizes the velocity of electrons inside the material in response to an electric field, but also by the dielectric constant k,w hich relates to the polarizability of the material. Impedance measurements made with sandwich-type devices containing dense films of the hydrolyzed precursor give k values of around 4 ( Figure 4b). In contrast to this,t heoretical studies predict that with an etwork of fullerene cores connected by various linkers ultra-low k values,e ven below 2, can be achieved. [19] Thestructure of the linking side chains we used is more complex than the ones used for the calculations,which were shorter and sometimes only alkyl chains.Weassume that the higher content of C À Oa nd Si À Ob onds present in our precursor yields ahigher polarizability and lower porosity and therefore ah igher dielectric constant. [20] Fore xample,t he structure includes twelve siloxane groups per fullerene core, which is two times more than assumed in the study by Hermann et al. [19a] We observe that the k value for the porous compared to the dense fullerene-framework film is reduced to around 3 ( Figure 4b).
We have demonstrated the synthesis of an ew hexafunctionalized C 60 adduct with octahedral symmetry,w hich was employed as ab uilding block to create the first example of ahighly-ordered three-dimensional covalent fullerene framework. Thin films of this material were synthesized by block copolymer template-directed, evaporation-induced selfassembly,r esulting in ap eriodic orthorhombic structure with Fmmm symmetry that, after template removal, revealed apore size of around 7.5 nm. As aresult of the functionalization of the fullerene,t he resulting material has greatly differing electronic properties compared to pristine C 60 .T he dielectric constant of the porous fullerene framework was . . decreased relative to its dense analogue showing values of around 3and 4, respectively,over awide frequency range.Itis envisioned that the method of template-directed self-assembly of as urface-functionalized C 60 to make ah ighly-ordered 3D covalent fullerene framework could in principle be extended to other organic molecules.F or example,t he method could be employed to prepare periodic porous conjugated polymer and covalent organic framework [21] analogues with potential applications in areas such as catalysis,gas storage,a nd drug release.

Experimental Section
Thed etailed synthetic procedures for the modified fullerene precursors as well as characterization can be found in the Supporting Information.
Fullerene-framework films were synthesized by adding HCl (6 mL of a0.2 m solution) to asolution of PluronicF127 (12 mg,0.95 mmol) in ethanol (200 mL). This surfactant solution together with the precursor C 60 R 6 (3;18.9 mg,5.0 mmol) in ethanol (200 mL) was stirred at room temperature for 3h.The aged solution was used to spin coat glass and indium tin oxide (ITO)s ubstrates at various speeds from 500 to 1000 rpm. Thedried films were heat treated for 18 hat100 8 8C. Thes urfactant template was extracted into ethanol under reflux for 10 hinfour cyclesw ith fresh solvent for each cycle.