Searching for other systems that may serve as phosphorus-based metal sensors, we explored the possibility of a phosphorus-functionalised pyrazole unit. Although free pyrazole is not fluorescent, appropriately-substituted pyrazoles often display fluorescence, with high quantum yields and large Stokes shifts; these advantages make them particularly useful in metal sensing applications. Phosphoryl-substituted pyrazoles and pyrazolines have been widely studied in medicinal applications and in the agrochemical industry as herbicides and insecticides. A 4-phosphoryl pyrazolone, bearing a phosphine oxide unit, was reported in 1996 but no further reactivity was explored. Tris(pyrazolyl)phosphine oxides featuring P-N bonds have also been studied as scorpionate ligands for metal centres. Very recently, Song, Wang and coworkers developed a synthesis of axially chiral pyrazol-3-ylphosphine oxides bearing atropisomeric arylpyrazole moieties. Furthermore, Weigand and co-workers have recently reported a range of 4phosphoryl pyrazolones. These molecules exist in equilibrium between the keto (pyrazolone) and enol (hydroxypyrazole) tautomers; the majority crystallised as the latter and feature an -OH unit adjacent to the phosphonate group (Figure ). This arrangement creates a binding pocket that has been demonstrated to coordinate metals including lanthanides, actinides, alkali metals and alkaline earth metals. However, no luminescence was reported for any of these molecules.