A 1H double-quantum magic-angle spinning solid-state NMR investigation of packing and dynamics in triphenylene and hexabenzocoronene derivatives

Solid-state H-1 NMR methods employing very-fast magic-angle spinning (MAS) are applied to hexa-alkyl-substituted hexa-peri-hexabenzocoronenes (HBC) as well as an alkoxy-triphenylene derivative. Firstly for the HBC derivatives, the effect of changing the alkyl substituent from i-propyl to I-butyl on the structure adopted in the solid phase is investigated. From single-quantum MAS spectra, differences are apparent between the two cases, most notably in the aliphatic region, where for the t-butyl derivative, two peaks are resolved. The different structures adopted become much mon apparent in rotor-synchronised two-dimensional double-quantum (DQ) MAS spectra, where definite proton-proton proximities can be identified, taking advantage of the significant resolution improvements afforded by the extension to a second frequency dimension. In both cases, different aromatic H-1 resonances are resolved. This can be explained in terms of the differing degrees to which the protons experience the ring current due to the aromatic pi-electrons of a nearby aromatic core. For the i-propyl derivative, the observed pattern of DQ peaks is the same as that observed in a previous study of an analogous hexa-n-dodecyl substituted HBC molecule, and thus the aromatic cores can be assumed to also pack as in unsubstituted HBC. For the t-butyl derivative, the steric bulk of the substituents leads to a different packing arrangement as identified in a separate X-ray single crystal investigation-the complicated arrangement of aromatic and aliphatic DQ peaks is shown to be in excellent agreement with this structure. For the triphenylene-based molecule, which exists in a liquid-crystalline phase at room temperature, the dynamics is investigated by an analysis of DQ MAS spinning-sideband patterns (recorded at a H-1 Larmor frequency of 700.1 MHz), with, in particular, differential mobility along the alkyl chains being identified. (C) 2000 Elsevier Science B.V. All rights reserved.