Enantiomerically Pure\nHelical Bilayer Nanographenes:\nA Straightforward Chemical Approach

The semiconductor properties of nanosized graphene fragments,\nknown\nas molecular nanographenes, position them as exceptional candidates\nfor next-generation optoelectronics. In addition to their remarkable\noptical and electronic features, chiral nanographenes exhibit high\ndissymmetry factors in circular dichroism and circularly polarized\nluminescence measurements. However, the synthesis of enantiomerically\npure nanographenes remains a significant challenge. Typically, these\nmaterials are synthesized in their racemic form, followed by separation\nof the enantiomers using high-performance liquid chromatography (HPLC).\nWhile effective, this method often requires expensive instrumentation,\nextensive optimization of separation conditions, and typically yields\nanalytical quantities of the desired samples. An alternative approach\nis the enantioselective synthesis of chiral molecular nanographenes;\nhowever, to date, only two examples have been documented in the literature.\nIn this work, we present a straightforward chemical method for the\nchiral resolution of helical bilayer nanographenes. This approach\nenables the effective and scalable preparation of enantiomerically\npure nanographenes while avoiding the need for HPLC. The incorporation\nof a BINOL core into the polyarene precursor facilitates the separation\nof diastereomers through esterification with enantiomerically pure\ncamphorsulfonyl chloride. Following the separation of the diastereomers\nby standard chromatographic column, the hydrolysis of the camphorsulfonyl\ngroup yields enantiomerically pure nanographene precursors. The subsequent\ngraphitization, achieved through the Scholl reaction, occurs in an\nenantiospecific manner and with the concomitant formation of a furan\nring and a heterohelicene moiety. The absolute configurations of the\nfinal enantiomers, <i>P</i>-<b>oxa[9]HBNG</b> and <i>M</i>-<b>oxa[9]HBNG</b>, have been determined using X-ray\ndiffraction. Additionally, electrochemical, photophysical, and chiroptical\nproperties have been thoroughly evaluated.