Single-molecule-magnet carbon-nanotube hybrids.

Carbon nanotubes (CNTs) hold great promise for sensing and nanoelectronics, as core components of chemical and biological ultra-sensitive probes and of field-effect transistors (FETs). CNT–SQUID devices in particular could constitute magnetic detectors with single-molecule sensitivity, thus offering a viable route to the long-sought readout of magnetic information stored in individual single-molecule magnets (SMMs). SMMs are metal-ion clusters with a large easy-axis magnetic anisotropy, exhibiting a magnetic hysteresis loop at low temperature and suggested as components for quantum computing and molecular spintronics. To date, the chemistry needed to bridge the domains of CNTs and SMMs has remained unexplored. CNT hybrids with gold or magnetic nanoparticles, proteins, enzymes, or luminescent molecules are currently under intense investigation. 8] The resulting materials usually entail a large number of nanoparticles or molecules per CNT, whereas CNT–SMM detectors and spintronic devices require the sequential addition of a small but very controlled number of nanomagnets. Grafting through covalent bonds might introduce electron scattering centers that may limit the performance of CNT devices. By contrast, noncovalent p-stacking interactions with pristine CNTs should largely preserve the CNT conductance, while guaranteeing SMM–CNT interaction. Herein we report the assembly of CNT–SMM hybrids using a tailor-made tetrairon(III) SMM, [Fe4(L)2(dpm)6] (1; Hdpm= dipivaloylmethane), designed to graft onto the walls of CNTs. The ligand L (H3L= 2-hydroxymethyl-2-(4(pyren-1-yl)butoxy)methylpropane-1,3-diol), features an alkyl chain with a terminal pyrenyl group and was synthesized as in Figure 1a. Reduction of 4-pyren-1-yl-butyric acid gives 4-(1-pyrenil)butanol, which is then coupled with 4-bromomethyl-1-methyl-2,6,7-trioxa-bicyclo[2.2.2]octane. A twosteps deprotection of the trimethylol function affords H3L, which is finally treated with the preformed complex [Fe4(OMe)6(dpm)6] (2) to give 1 in excellent yield (95%). The molecular structure of 1 (Figure 1b,c), determined by single-crystal X-ray diffraction, shows a tetrairon(III) propeller-like core with idealized D3 symmetry held together by two triply deprotonated H3L ligands lying at opposite sides of the molecular plane (see Supporting Information). The molecular size of 1 is 1.6–2.3 nm (av.: 1.9 nm). Low-temperature high-frequency (HF)-EPR spectra at 190 and 230 GHz (Figure 2a) and variable-temperature magnetic-susceptibility measurements show the presence of an S= 5 high-spin ground state with an easy-axis magnetic anisotropy (D= 0.409 cm ; Supporting Information). Indeed, single-crystal magnetic measurements reveal a hysteresis loop below 1 K with characteristic quantum-tunneling steps (Figure 2b), confirming the SMM behavior. CNT–FETs were obtained by electron-beam lithography on degenerately n-doped silicon wafers covered with a 300 nm thick SiO2 layer. Single CNTs were located by atomic force microscopy (AFM) and connected by palladium leads separated by 300 nm gaps. The hybrids were then produced by immersion of the CNT–FETs in a 3.1 10 m solution of 1 in 1,2-dichloroethane (DCE) for 30 min, followed by extensive washing with pure DCE. H NMR, ESI-MS, and fluorescence techniques demonstrate that the complex is completely stable in solution in the conditions used for the deposition (Supporting Information). The grafting was reiterated to follow the progressive addition of SMMs. After each treatment a few SMMs were found to stick onto the CNT (Figure 3a), while some others were also located on the surrounding surface. The isostructural complex containing H3L’= 2-hydroxymethyl-2-phenylpropane-1,3-diol did not graft onto CNTs in the same experimental conditions. This result is a strong indication that 1 has been grafted as a result of the pyrenyl functionalities. [*] Dr. L. Bogani, Dr. N. Bendiab, Dr. W. Wernsdorfer Institut N el, CNRS 25 Av. des Martyrs, 38042 Grenoble, Cedex 9 (France) Fax: (+33)4-7688-1191 E-mail: lbogani@hotmail.com