2020 年 78 巻 2 号 p. 140-147
Antiaromaticity of bond-alternated planar cyclooctatetraene (COT) was predicted to be substantially high based on nucleus-independent chemical shift (NICS), while its destabilization effect was shown to be negligibly small in contrast to the expected character for antiaromatic compounds. The planar structure of typical COT ring is the transition state in the ring inversion process and hence hardly accessible by spectroscopic means. Thus, several molecules with planarized COT cores have been designed and synthesized to investigate the antiaromaticity. By comparing their NICS values, planar COT cores constrained with less aromatic and less strained frameworks were shown to have higher antiaromaticity. To further experimentally compare the antiaromaticity of COT with higher 4n π-systems, a method to evaluate relative hardness of 4n π-systems (n≥2) as a new experimental measure of antiaromaticity was developed. Based on the relative hardness, the high antiaromaticity of planar COT core constrained with one dithieno［3,4-b：3’,4’-d］thiophene unit was confirmed. Furthermore, the retention of high antiaromaticity of planar COT constrained with two dithieno［3,4-b：3’,4’-d］thiophene units after peracid oxidation was also experimentally proved, which is in sharp contrast with the general notion that antiaromatic compounds are unstable. The antiaromatic planar COT core caused narrowing of HOMO-LUMO gap and this feature was successfully applied to ambipolar semiconductor for field-effect transistor.