Five structurally related europium(III) complexes of heptadentate macrocyclic ligands bearing azaxanthone or azathiaxanthone chromophores have been evaluated comparatively as responsive probes of the intracellular environment. Protein binding (HSA) and oxy-anion binding constants have been measured by titrimetric analysis, examining ratiometric emission changes for three examples, and systems exhibiting selectivity for citrate or hydrogencarbonate identified. The cytotoxicity of each ligand and complex has been assessed and correlated with the observed chemical stability profile in competitive aqueous media. The europium complex of L² is non-toxic, exhibits a large change in its emission spectral profile to variation in HCO₃ ⁻ (or citrate) concentration allowing ratiometric analysis, and localises in cellular mitochondria. Such features augur well for its future application as a responsive probe in microscopy to monitor local changes in pHCO₃.

Observation of two-photon excitation (760 nm) and emission of two responsive water soluble europium complexes is reported with cross-sections of up to 2 GM. Two-photon excitation spectra have also been measured, acquisition being achieved by the use of a cavity-dumped mode locked Ti-sapphire laser. Time-gated detection is used to differentiate the ligand fluorescence and metal centred emission in these europium complexes.