Remarkable Isomeric Effects on Near‐Infrared Absorption and Aromaticity of Core‐Modified Phthalocyanines Incorporating Two Phenolic Groups

ABSTRACT

Two regioisomers of redox‐switchable near‐infrared (NIR) absorbing phthalocyanine analogues, dihydroxydicarbahemiporphyrazines, have been synthesized and their structure‐property relationships have been investigated. Both isomers were readily oxidized by oxidants, and the resulting two‐electron oxidized forms exhibited remarkable isomeric effects. The two‐electron oxidized forms of the anti‐isomers exhibited distinct NIR absorption over 900 nm in chloroform, and the maximum wavelength change in the absorption spectra upon redox process was over 550 nm. In contrast, weak and broad bands were observed in the 500–1000 nm region for the corresponding syn‐isomers. The ¹ H NMR spectra indicated the presence of diatropic ring current effect in the two‐electron oxidized form of the anti‐isomer with arylthio substituents. No such ring current effect was observed for the syn‐isomer. Density functional theory (DFT) calculations suggested that a nonaromatic asymmetric tautomer with a smaller static electric dipole moment plays a pivotal role in the optical properties and aromaticity in the syn‐isomers.