Tuning Antiaromaticity Through Meso‐Substituent Orientation in Core‐Modified Isophlorins

The antiaromaticity of dithiadioxaisophlorins is controlled by the steric bulkiness of meso-substituents through their tilt angle relative to the macrocycle. Bulkier substituents maintain larger angles, preserving stronger antiaromaticity, while smaller groups facilitate π-conjugation with the macrocycle, diminishing antiaromaticity. This structure–property relationship enables rational design of functional antiaromatic systems with precisely tunable electronic characteristics.

Abstract

Antiaromaticity is an essential principle in organic chemistry that significantly influences the properties of cyclic π-conjugated systems. This study systematically investigated the effects of meso-substituents on the antiaromaticity of dithiadioxaisophlorins (S₂O₂Iphs), which are stable isophlorin derivatives with strong antiaromatic characteristics. We synthesized two new S₂O₂Iphs derivatives with 3,5-bis(trifluoromethyl)phenyl and 5-cyanothien-2-yl substituents along with a previously reported pentafluorophenyl-substituted analog. Notably, a derivative with sterically demanding 2,6-dichlorophenyl substituents could not be isolated despite being detected by mass spectrometry, highlighting the delicate balance between electronic stabilization, and steric effects. Through comprehensive analysis using ¹H NMR spectroscopy, UV/vis absorption spectroscopy, and multiple computational approaches (NICS, GIMIC, HOMA/HOMAc, and AV1245/AV_min), we demonstrate that the steric bulkiness of meso-substituents predominantly determines their tilt angle relative to the isophlorin macrocycle, critically influencing antiaromaticity. Bulkier substituents maintain larger tilt angles, preserving stronger antiaromaticity, while less bulky groups allow greater π-conjugation with the macrocycle, thereby reducing antiaromaticity. These findings provide valuable guidance for the design of functional antiaromatic materials with tunable electronic properties.

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