Both complexes were further studied regarding their ESIPT-ON state upon titration. PolyP8 sensing of weakly emissive 6 was investigated by using fluorescence spectroscopy. Upon addition of polyP8 sodium salt into a solution of 6 (2.5 μM in 10 mM HEPES buffer at pH = 7.4), the fluorescence emission intensity (λex= 254 nm) centred at λem=434 nm was progressively enhanced within 2 minutes of polyP8 addition. An eightfold enhancement in the fluorescence intensity was observed when 10 equiv. of polyP8 were added (Fig. ). We attribute these significant changes in the spectral properties to the removal of Cu 2+ from the ligand's coordination sphere by polyP8, releasing the ESIPT fluorophore characterized by its large Stokes shift. During excitation at 254 nm, we observed a polyP8 concentration-dependent fluorescence increase at 434 nm (Fig. ). The titration resulted in a sigmoidal curve, by using nonlinear regression analysis of the polyP8 induced emission data. Notably, the fluorescence intensity did not decrease with time (Fig. ), demonstrating stability of the released fluorophore. To determine the complexation stoichiometry between 6 and polyP8, Job's plot analysis was conducted on the basis of the emission intensity at 434 nm (Fig. ). The result suggested that a 1:2 stoichiometry was found between 6 and polyP8, while [6] + [polyP8] = 5 µM was maintained throughout the experiment. Probe 7 (5 µM concentration) behaved in a similar way and showed ESIPT-ON under titration with polyP8 (excitation at 250 nm, emission at 430 nm; see Fig. ). Selectivity studies of the probes towards polyP over other phosphates: Given the excellent ESIPT-ON properties of the sensors in response to polyP8, we were now interested in profiling the selectivity of 6 and 7 with respect to other condensed phosphates. Both probes 6 and 7 were incubated with a wide range of biologically relevant (condensed) phosphates, like monophosphate (Pi), pyrophosphate (PPi), several abundant nucleotides such as ATP, AMP, ADP, GTP, GMP and GDP under the same conditions of 10 mM HEPES at pH = 7.4 with 2 minutes of incubation time.