High-Resolution Characterization of Liquid-Crystalline [60]Fullerenes Using Solid-State Nuclear Magnetic Resonance Spectroscopy Sergey V. Dvinskikh, †,‡ Kazutoshi Yamamoto, † David Scanu, § Robert Deschenaux, § and Ayyalusamy Ramamoorthy* ,† Biophysics and Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109-1055, and Institut de Chimie, UniVersite ´ de Neucha ˆtel, AVenue de BelleVaux 51, Case postale 158, 2009 Neucha ˆtel, Switzerland ReceiVed: April 15, 2008; ReVised Manuscript ReceiVed: July 5, 2008 Liquid-crystalline materials containing fullerenes are valuable in the development of supramolecular switches and in solar cell technology. In this study, we characterize the liquid-crystalline and dynamic properties of fullerene-containing thermotropic compounds using solid-state natural abundance 13 C NMR experiments under stationary and magic angle spinning sample conditions. Chemical shifts spectra were measured in isotropic, liquid-crystalline nematic and smectic A and crystalline phases using one-dimensional 13 C experiments, while two-dimensional separated local-field experiments were used to measure the 1 H- 13 C dipolar couplings in mesophases. Chemical shift and dipolar coupling parameters were used to characterize the structure and dynamics of the liquid-crystalline dyads. NMR data of fullerene-containing thermotropic liquid crystals are compared to that of basic mesogenic unit and mesomorphic promoter compounds. Our NMR results suggest that the fullerene-ferrocene dyads form highly dynamic liquid-crystalline phases in which molecules rotate fast around the symmetry axis on the characteristic NMR time scale of ∼10 -4 s. 1. Introduction The self-organization in two-dimensional (2D) and three- dimensional (3D) space offered by the liquid crystals is an ideal vehicle to explore and control the organization of matter on the nanometer scale. 1 Design of functional supramolecular mesomorphic materials, which combine the organization be- havior of liquid crystals with the properties of the fullerenes, is expected to have a strong impact on the future development of materials science. 1 Because of the unique photophysical and electrochemical properties of C 60 , 2 the latter was combined with ferrocene, 3 tetrathiafulvalene (TTF), 4 or oligophenylenevinylene (OPV) 5 and a mesomorphic promoter to develop liquid- crystalline donor-acceptor dyads for which photoinduced electron and/or energy transfer could be achieved. Such dyads are appealing for the design of supramolecular switches 2 and in solar cell technology. 6 The above liquid crystals gave rise to smectic B and/or smectic A phases in agreement with the nature and structure of the liquid-crystalline addends grafted onto C 60 . On the other hand, no studies on the high-resolution structural and dynamical characterization of these molecules have been reported so far. Such investigations would better our under- standing of the general properties of these materials. In this study, we use high-resolution solid-state NMR (nuclear magnetic resonance) techniques to characterize the liquid-crystalline and dynamic properties of fullerene-containing thermotropic liquid crystals (compounds 3 and 4), liquid-crystalline promoter (compound 2), and mesogenic unit (compound 1) (Figure 1). The peralkylated ferrocene unit was oxidized to see a possible influence of the charge on the NMR properties. NMR spectroscopy is a powerful experimental tool to investigate the structure and dynamics of crystalline, noncrystal- line, and nonsoluble materials at atomistic-level resolution. 7,8 Particularly, it is impressive that the applications of solid-state NMR spectroscopy are not limited by the molecular size, solubility, or phase of system under study. Therefore, high- resolution information on molecular structure and mobility can be obtained from isotropic, solid, and liquid-crystalline phases of a variety of materials. 7,9,10 Carbon-13 NMR spectroscopy has several advantages for studying mesophases. Isotropic “solution- like” carbon-13 chemical shift spectra with excellent signal-to- noise ratios can be obtained even from nonisotropic samples and without the need for an isotopic enrichment in the sample. Because 13 C chemical shift resonances from chemically non- equivalent sites are typically well-resolved, it is easy to identify the chemical groups of the molecule under study. In addition, simple one-dimensional (1D) 13 C isotropic chemical shift measurements can reveal the crystalline and amorphous proper- ties of the sample. Anisotropic spin interactions, such as chemical shifts or dipolar couplings, observed in 1D or 2D spectra provide information on molecular ordering, structure, and phase transitions. 7,9-15 The experiments were carried out on both stationary and magic angle spinning (MAS) experi- mental conditions. 2. Experimental Section 2.1. Materials. The compounds under investigation were prepared by adapting previously described procedures; 3 the syntheses will be reported in a subsequent paper. The phase transition temperatures are reported in Figure 2. 2.2. NMR Experiments. NMR experiments were performed at a magnetic field of 9.4 T on a Chemagnetics/Varian Infinity 400 MHz spectrometer at the resonance frequencies of 400 and 100 MHz for protons ( 1 H) and carbons ( 13 C), respectively. A 5 mm double-resonance MAS probe was used. The measurements were performed on samples either at stationary or MAS * To whom correspondence should be addressed. Tel: 734-647-6572. E-mail: ramamoor@umich.edu. † University of Michigan. ‡ Present address: Division of Physical Chemistry and Industrial NMR Centre, Department of Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden. § Universite ´ de Neucha ˆtel. J. Phys. Chem. B 2008, 112, 12347–12353 12347 10.1021/jp803265z CCC: $40.75  2008 American Chemical Society Published on Web 09/09/2008