Furo[3,2-c]coumarin-derived Fe³⁺ Selective Fluorescence Sensor: Synthesis, Fluorescence Study and Application to...

Furocoumarin (furo[3,2-c]coumarin) derivatives have been synthesized from single step, high yielding (82-92%) chemistry involving a 4-hydroxycoumarin 4 + 1 cycloaddition reaction. They are characterized by FTIR, ¹H-NMR, and, for the first time, a comprehensive UV-Vis and fluorescence spectroscopy study has been carried out to determine if these compounds can serve as useful sensors. Based on the fluorescence data, the most promising furocoumarin derivative (2-(cyclohexylamino)-3-phenyl-4H-furo[3,2-c]chromen-4-one, FH), exhibits strong fluorescence (ФF = 0.48) with long fluorescence lifetime (5.6 ns) and large Stokes' shift, suggesting FH could be used as a novel fluorescent chemosensor. FH exhibits a highly selective, sensitive and instant turn-off fluorescence response to Fe³⁺ over other metal ions which was attributed to a charge transfer mechanism. Selectivity was demonstrated against 13 other competing metal ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Mn²⁺, Fe²⁺, Al³⁺, Ni²⁺, Cu²⁺, Zn²⁺, Co²⁺, Pb²⁺ and Ru³⁺) and aqueous compatibility was demonstrated in 10% MeOH-H₂O solution. The FH sensor coordinates Fe³⁺ in a 1:2 stoichiometry with a binding constant, K_a = 5.25 × 10³ M^-1. This novel sensor has a limit of detection of 1.93 µM, below that of the US environmental protection agency guidelines (5.37 µM), with a linear dynamic range of ~28 (~2-30 µM) and an R² value of 0.9975. As an exemplar application we demonstrate the potential of this sensor for the rapid measurement of Fe³⁺ in mineral and tap water samples demonstrating the real-world application of FH as a "turn off" fluorescence sensor.