An Investigation of π−π Packing in a Columnar Hexabenzocoronene by Fast Magic-Angle Spinning and Double-Quantum 1H Solid-State NMR Spectroscopy

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Abstract

1H NMR methods employing very fast magic-angle spinning (MAS) are applied to the investigation of both the structure and dynamics of an alkyl-substituted hexa-peri-hexabenzocoronene (HBC), which is known to form a columnar mesophase with a very high one-dimensional charge carrier mobility. For the crystalline phase, three distinct aromatic resonances are identified in the single-quantum MAS spectrum. The observation of these distinct aromatic resonances is explained in terms of the differing degrees to which the aromatic protons experience the ring current of adjacent layers. Using the rotor-synchronized double-quantum MAS method, definite proton−proton proximities are identified, which are shown to be in agreement with the known crystal structure of unsubstituted HBC. In the liquid-crystalline phase, axial motion of the HBC disks leads to the averaging of the three sites into a single resonance. Furthermore, the reduction of the dipolar coupling caused by this motion is quantitatively investigated by an analysis of double-quantum MAS spinning sideband patterns. For all spectra, the resolution is significantly improved for a compound where the α-carbon positions have been deuterated; the synthesis of this deuterated compound, in particular the final cyclodehydrogenation step, is described.

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