Abstract: The present invention relates to novel compounds which are capable as acting as fluorescent sensors or which are precursors for these and for the use of these for the assay of biological processes such as posttranslational modifications of biological molecules such as phosphorylation, de-phosphorylation, proteolytic cleavage, phosphodiesterase mediated hydrolysis of cyclic nucleotides, methylation, acetylation of proteins peptides, DNA, lipids and the detection of biomolecule interactions (e.g., protein-protein interactions). A small molecule sensor is described which can associate to phosphorylated biological targets via metal ion—phosphate association. The association event can be monitored as fluorescence quench, sensitized emission, fluorescence polarization or a combination thereof.