Pyrazoline-functionalized polyurethanes as selective fluorescent sensors for Fe3+ ions

Pyrazolines are a growing class of five membered nitrogen containing organic compounds valued for their luminescence, easy synthesis, stability, and strong metal binding ability, which makes them widely used as fluorescent sensors for detecting metal ions. In this paper, a pyrazoline derivative, namely 4-(1,3-diphenyl-4,5-dihydro-1 H -pyrazol-5-yl)phenyl (2-bromoethyl)carbamate, was attached to a polyurethane chain via a quaternization reaction, a strategy deliberately chosen to lead to a material that would concurrently exhibit the useful properties of the pyrazoline core and of the polyurethane elastomer. The pyrazoline units were incorporated in varying concentrations (1, 3 and 5 mol%), and the resulting materials were analysed by structural characterization techniques (FTIR, 1 H NMR, 13 C NMR) and optical methods. The pyrazoline monomer shows UV-Vis absorption in the near-ultraviolet region (356-364 nm) and solvent-dependent blue fluorescence (445-467 nm). The pyrazoline-polyurethane samples dissolved in THF display absorption maxima near 360 nm and fluorescence emission maxima at 446 nm, with fluorescence intensities increasing proportionally with the pyrazoline content. The chemosensing behaviour of all pyrazoline-based polyurethane solutions was evaluated in the presence of a broad range of metal ions, the most significant quenching effect being observed in the presence of Fe 3+ ions, the stoichiometry between the pyrazoline-polyurethane and this metal ion being 1:2, as determined by the Job's plot analysis. The samples have high association constants (K a ranging from 4.66 × 10 3 to 8.37 × 10 3 M −1 ) and low detection limit values (1.6-3.16 μM). The selectivity evaluation shows the strong quenching effect induced by Fe 3+ ions even in the presence of competing metal ions.