Intermediate diradical character and thermal cis–trans isomerization of near-infrared absorbing thionated squaraine dyes

Abstract

We synthesized squaraine dyes incorporating 4-membered thionated oxocarbon and chalcogenopyrylium moieties, which displayed intermediate diradical character as confirmed by bond-length analysis and temperature-dependent behaviors observed in ¹H NMR and ESR spectroscopy. The ¹H NMR signals of the cyclobutenedithione dyes disappeared at lower temperatures compared to the cyclobutenedione analogues, indicating a lower thermal transition to the excited triplet state and thus greater diradical character. Moreover, we identified cis–trans isomerization in the thionated dyes. Notably, an increase in diradical character lowered the activation energy of this isomerization, facilitating the structural transition between cis and trans isomers. These dyes exhibit strong near-infrared absorption, with slight temperature-dependent shifts likely arising from the closely matched electronic transition energies of the isomers.