Successive additions of Fe 3+ (0 to 8 equivs., over 23 steps) to a solution of 5 yielded a continuous decrease in fluorescence intensity, with the emission wavelength remaining the same. (Figure ). When plotted for a fixed emission wavelength, a linear relationship between emission intensity and concentration of Fe 3+ (see ESI) was observed. Stern-Volmer analysis revealed a linear behaviour at lower Fe 3+ concentrations ([Fe 3+ ] ≤ 100 μM, KSV = 3502 ± 333 M -1 ; see ESI), with an upward deviation at higher concentrations. Furthermore, no change in the absorption wavelength was observed via UV/Vis spectroscopy upon increasing concentration of Fe 3+ (see ESI). Similar behaviour via both fluorescence and UV/Vis spectroscopy was observed upon titration of Fe 3+ into a solution of 3 (see ESI). Given the increased fluorescence intensity observed upon the interaction of 3 with Al 3+ , we also titrated Al 3+ (0 to 8 equivs., over 23 steps) into a solution of 3. This behaved differently; a non-linear increase in fluorescence intensity was observed, with a clear blue-shift in emission wavelength at higher concentrations of Al 3+ (411 nm vs 461 nm; Figure ). When plotted for a fixed emission wavelength, a sigmoidal curve shape was observed, in contrast to the linear relationship seen with Fe 3+ (see ESI). This was also accompanied by a clear red-shift in the main absorbance band in the UV/Vis spectrum for 3 upon increasing concentrations of Al 3+ , from 274 to 296 nm ESI). These results are consistent with a binding mechanism between 3 and Al 3+ ions, in which the corresponding 3/Al 3+ complex forms at higher Al 3+ concentrations and displays increased fluorescence intensity compared to 3, yielding a 'turn-on' response.