Advanced pH Indicators Consisting of BODIPY and Coumarin Fluorophores with a Guanidinyl Structure Capable of Methanol Recognition

Abstract

Guanidine is strongly basic and has a high molecular recognition ability. We previously developed a guanidine-bearing benzoquinoline for the fluorescence detection of MeOH by exploiting the strong basicity of guanidine. The benzoquinoline fluorophore quantitatively detected MeOH in EtOH with moderate sensitivity and a limit of detection (LOD) of 16.5 %. In this study, we developed two guanidinyl boron dipyrromethene (BODIPY) and two guanidinyl coumarin fluorophores. These fluorophores have higher molar extinction coefficients (ϵ) than those of benzoquinolines, which suggests that they could have improved detection sensitivity. The BODIPY fluorophores exhibited acid-responsive turn-on fluorescence for MeOH, similar to the benzoquinoline fluorophore. Their high ϵ values led to an enhanced MeOH detection sensitivity of 1.3 % in EtOH. The coumarin derivatives exhibited acid-responsive turn-off fluorescence. In addition, the absorbance spectral shift of the protonated and neutral forms of coumarin enabled the ratiometric detection of MeOH (LOD=0.85 % and 0.57 %, respectively). This study demonstrates the utility of guanidine-based fluorophores in molecular recognition and promotes the development of similar fluorophores in analytical chemistry.