| Abstract
| - We report the highly convergent synthesis and solid-state characterization of six crystalline“molecular compasses” consisting of a central phenylene rotor with polar substituents, or compass needle,and two trityl groups axially connected by acetylene linkages to the 1,4-positions. Compounds with fluoro-,cyano-, nitro-, amino-, diamino-, and nitroamino substituents are expected to emulate the parent compoundwhich was shown to form crystals where the central phenylene can rotate about its 1,4-axis with rateconstants in the 103 −106 s-1 dynamic ranges near ambient temperature, depending on crystal morphology.With data from single-crystal X-ray diffraction analysis, solid-state CPMAS 13C NMR, differential scanningcalorimetry (DSC), and thermogravimetric analysis (TGA), it is shown that a relatively small structuralperturbation by a single polar group (F, CN, NO2, NH2) results in isomorphous structures with analogousproperties. In analogy to the parent compound, crystals grown from benzene formed clathrate structuresin the space group P1̄ with one molecular compass and two benzene molecules per unit cell. Solvent-freecrystals with the same space group obtained by a first-order phase transition between 60 and 130 °C wereshown to be spectroscopically identical to those obtained by slow solvent evaporation from a mixture ofCH2Cl2 and hexanes. A qualitative analysis of the positionally disordered phenylene groups in terms of theexpected solid-state rotational dynamics suggests a nonsymmetric, 2-fold rotational potential, or a processinvolving full 360° turns.
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