Synthesis and Application of Ratiometric and "Turn-On" Fluorescent pH Sensors: An Advanced Organic Undergraduate Laboratory

An upper-division organic chemistry laboratory experiment exploring fluorescent sensing over two laboratory periods and part of a third is described. Two functionally distinct pH-responsive sensors are prepared through a dehydrative three-component coupling reaction. During the abbreviated (<1 h) first laboratory period, students set up reactions to prepare the two compounds. The second period involves purification using salt metathesis and characterization using standard techniques. The third period is spent exploring the fluorescent characteristics of the two compounds through visual (eye) and fluorometer analysis of solutions at varying pH. The two sensors show either a "turn-on" (photoinduced electron transfer, PET) or "ratiometric" (intramolecular charge transfer, ICT) response to protonation with these distinctions deriving from sterically induced differences in conformation. This experiment is appropriate for demonstrating steric effects on reactivity and macroscopic properties, multicomponent coupling reactions, carbonyl reactivity, aromatization, purification of ionic organic molecules, absorbance and emission of light by organic molecules, frontier molecular orbital theory, and fluorescence sensing.