Selective Hg²⁺ Sensing Behaviors of Rhodamine Derivatives with Extended Conjugation Based on Two Successive Ring-Opening Processes

A novel class of rhodamine derivatives with two spirolactam moieties have been synthesized, and their two stereoisomers of cis- and trans-forms have been successfully separated and isolated, as well as structurally characterized by X-ray crystallography. Attributed to the successive ring-openings of two spirolactam moieties, different solution color, electronic absorption and emission responses were exhibited upon addition of various concentrations of mercury­(II) ion. Arising from two successive ring-opening processes in the presence of various concentration of Hg­(II) ion, two reporting states with different spectroscopic properties were suggested, that is, the first state showing pink color (absorption maximum at 496 nm) but no emission, while the second state giving purple color (absorption maximum at 593 nm) and red emission (emission maximum at 620 nm). The mechanism of such different spectroscopic responses was also proposed and has also been supported by computational studies. An extension of the present work to the study of the corresponding chemodosimeters from the compounds with two spirolactam groups has been made, in which a stoichiometric and irreversible Hg­(II)-promoted reaction of thiosemicarbazides was utilized to form 1,3,4-oxadiazoles.本研究合成了一类含两个螺内酰胺（spirolactam）结构单元的新型罗丹明（rhodamine）衍生物，并成功分离得到其顺式、反式两种立体异构体，同时通过X射线晶体衍射法完成了结构表征。得益于两个螺内酰胺结构单元的连续开环反应，该类衍生物在加入不同浓度的汞(II)离子(mercury(II) ion)后，可呈现出差异化的溶液颜色、电子吸收光谱及荧光发射响应。在不同浓度汞(II)离子存在时，经由两次连续开环过程，该类衍生物可形成两种具备不同光谱特性的响应状态：第一状态呈现粉色（吸收峰波长为496 nm）且无荧光发射，第二状态则呈现紫色（吸收峰波长为593 nm）并伴随红色荧光发射（发射峰波长为620 nm）。针对此类差异化光谱响应的作用机制，本研究提出了相关假说，并通过计算化学研究予以验证支持。本研究还将该工作拓展至基于含双螺内酰胺结构单元的化合物开发对应的化学传感探针（chemodosimeter）的研究领域，利用化学计量比且不可逆的汞(II)离子介导的氨基硫脲（thiosemicarbazides）反应，成功合成了1,3,4-噁二唑（1,3,4-oxadiazoles）类产物。