Syntheses and properties of graphyne fragments: trigonally expanded dehydrobenzo[12]annulenes

We present herein the synthesis and properties of the largest hitherto unknown graphyne fragment, namely trigonally expanded tetrakis(dehydrobenzo[12]annulene)s (tetrakis-DBAs). Intramolecular three-fold alkyne metathesis reactions of hexakis(arylethynyl)DBAs 9 a and 9 b using Fürstner's Mo catalyst furnished tetrakis-DBAs 8 a and 8 b substituted with tert-butyl or branched alkyl ester groups in moderate and fair yields, respectively, demonstrating that the metathesis reaction of this protocol is a powerful tool for the construction of graphyne fragment backbones. For comparison, hexakis(arylethynyl)DBAs 9 c-g have also been prepared. The one-photon absorption spectrum of tetrakis-DBA 8 a bearing tert-butyl groups revealed a remarkable bathochromic shift of the absorption cut-off (λcutoff ) compared with those of previously reported graphyne fragments due to extended π-conjugation. Moreover, in the two-photon absorption spectrum, 8 a showed a large cross-section for a pure hydrocarbon because of the planar para-phenylene-ethynylene conjugation pathways. Hexakis(arylethynyl)-DBAs 9 c-e and 9 g and tetrakis-DBA 8 b bearing electron-withdrawing groups aggregated in chloroform solutions. Comparison between the free energies of 9 e and 8 b bearing the same substituents revealed the more favorable association of the latter due to stronger π-π interactions between the extended π-cores. Polarized optical microscopy observations, DSC, and XRD measurements showed that 8 b and 9 e with branched alkyl ester groups displayed columnar rectangular mesophases. By the time-resolved microwave conductivity method, the columnar rectangular phase of 8 b was shown to exhibit a moderate charge-carrier mobility of 0.12 cm(2)  V(-1)  s(-1) . These results indicate that large graphyne fragments can serve as good organic semiconductors.