Jian He - Conformational Analysis of o-Phenylenes: Helical Oligomers with Frayed Ends

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      Publication Details (including relevant citation   information):

      J. Org. Chem., 75 (24), 8627–8636(2010)

    DOI: 10.1021/jo1021025

    Publication Date (Web): November 17, 2010


      The o-phenylenes represent a fundamental class of   conjugated polymers that, unlike the isomeric   p-phenylenes, should exhibit rich conformational   behavior. Recently, we reported the synthesis and   characterization of a series of o-phenylene oligomers   featuring unusual electronic properties, including surprisingly   long-range delocalization as measured by UV−vis spectroscopy and   hypsochromic shifts in fluorescence maxima with increasing   length. To rationalize these properties, we hypothesized that the   oligomers predominantly assume a stacked helical conformation in   solution. This assertion, however, was supported by only indirect   evidence. Here we present a thorough investigation of the   conformational behavior of this series of o-phenylenes   by dynamic NMR spectroscopy and computational chemistry. EXSY   experiments, in combination with other two-dimensional NMR   techniques, provided full 1H chemical shift   assignments for at least the two most prevalent conformers for   each member of the series (hexamer to dodecamer). GIAO density   functional theory calculations were then used to relate the NMR   data to specific molecular geometries. We have found that the   o-phenylenes do indeed assume stacked helical   conformations with disorder occurring at the ends. Thus, the   o-phenylene motif appears to have great potential as a   means to organize arenes into predictable three-dimensional   arrangements. Our results also illustrate the power of   1H NMR GIAO predictions in the solution-phase   conformational analysis of oligomers, particularly those with a   high density of aromatic subunits.

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