Synthesis, Photophysical Properties, and Practical Hg(II)-Sensing Applications of a Rigidified Phenolo-Merocyanine Dye.

Among the strategies available to improve photophysical properties of organic fluorophores intended for imaging and sensing, structural modifications allowing for scaffold rigidification are particularly effective for enhancing yellow emission and photostability. In order to develop novel high-performance phenolic fluorophores, we have explored the synthesis of a conformationally restrained analog of flexible Brooker's merocyanine. We had the idea of applying a rigidification approach previously devised for cyanine dyes. Thus, the implementation of an acid-catalyzed domino double cyclization process allowed the synthesis of fluorescent phenol KR306. A comprehensive photophysical study has highlighted its outstanding optical properties in neutral aq. buffered conditions (a brightness of 71,500 M-1 cm-1 at pH 7.5 for a yellow emission centered at ca. 560 nm). The hydroxyl group of KR306 can be modified to regulate its fluorescence properties. This feature has been demonstrated through the preparation of chromo-fluorogenic thiophosphinate and thiophosphonate esters suitable for mercuric cations Hg(II) sensing in neat aq. buffers. The validation of these chemodosimeters has highlighted superior sensing performances of thiophosphinate derivative 5. Furthermore, its ability to be activated by microorganisms in kefir grains doped with Hg(II) ions, was confirmed by simply observing a color change by the naked eye.