Given our aim of using pyrazolyl-phosphine oxides 3 and 5 in fluorescence applications, we investigated their photophysical properties in solution. Both 3 and 5 display absorption maxima (λabs) in the UV region (3 = 274-287 nm, 5 = 279-298 nm) with molar absorption coefficients of 4000-13000 M -1 .cm -1 depending on the solvent (Table ). Both 3 and 5 exhibit fluorescence, with very large Stokes shifts (139-189 nm) across a range of solvents. The fluorescence intensities and quantum yields of 3 and 5 differ significantly, however, with 3 displaying greater fluorescence intensities and higher quantum yields (Table ). Choice of solvent also has a significant effect on intensity and quantum yield, with more electron-donating solvents yielding lower fluorescence intensities and quantum yields. This is presumably due to such donating solvents disrupting the stabilising intramolecular interactions between the adjacent P=O and O-H units on the pyrazole unit. We next evaluated the fluorescence behaviour of pyrazolyl-phosphine oxides 3 and 5 in the presence of a series of metal ions. We examined the interactions of 3 and 5 with a variety of alkali and alkaline earth metals (Li + , Na + , K + , Mg 2+ , Ca 2+ ), transition metals (Cr 3+ , Mn 2+ , Fe 2+ , Fe 3+ , Cu 2+ , Zn 2+ ), a main-group metal (Al 3+ ), and a lanthanide metal (La 3+ ). Due to the poor solubility of 3 and 5 in water, and the poor solubility of many metal salts in organic solvents, a 1:1 MeCN/H2O mixture was used as the solvent for all metal-sensing experiments, as this was found to fully dissolve both the metal salts, and 3 and 5. To obtain an overview of the interactions of 3 and 5 with the metal ions, 5 equivalents of the corresponding metal salt were mixed with 1 equivalent of pyrazolylphosphine oxide 5. For the majority of metals (Li + , Na + , K + , Mg 2+ , Ca 2+ , Mn 2+ , Zn 2+ ), the fluorescence intensity remained largely unchanged (F/F0 = 0.93-1.00, Figure ). However, in the presence of Fe 3+ , the fluorescence intensity becomes almost completely quenched (F/F0 = 0.07). Less pronounced quenching was also observed in the presence of Fe 2+ (F/F0 = 0.78) and Cu 2+ (F/F0 = 0.72). Notably, two of Compound Solvent λabs (nm)