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.

中文翻译：

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近红外吸收硫化方丁染料的中间双自由基特性和热顺反异构化


我们合成了掺入 4 元硫代氧碳和硫属吡啶部分的方方碱染料，其显示出中间的双自由基特性，这在 ¹H NMR 和 ESR 光谱中观察到的键长分析和温度依赖性行为得到了证实。与环丁烯二酮类似物相比，环丁烯二酮染料的 ¹H NMR 信号在较低温度下消失，表明向激发的三重态的热转变较低，因此具有更强的双自由基特性。此外，我们在硫代染料中鉴定了顺反异构化。值得注意的是，二自由基特性的增加降低了这种异构化的活化能，促进了顺式和反式异构体之间的结构转变。这些染料表现出很强的近红外吸收，异构体的紧密匹配的电子跃迁能可能引起了轻微的温度依赖性变化。