To investigate this binding further, we analysed the stoichiometry of a possible 3/Al 3+ complex using Job's method of continuous variation. This displayed a clear inflection point (see ESI), indicative of complex formation. While we have been unable to obtain additional structural information by X-ray crystallography, we assume that Al 3+ ions bind in between the two adjacent oxygen sites on 3, similarly to the structures reported by Weigand. Given the use of donor solvents, these may also be supported by donating MeCN or H2O ligands. By contrast, a Job plot for and FeCl3 showed no inflection point (see ESI). These data are consistent with a dynamic quenching mechanism for Fe 3+ , rather than binding of the metal ion. This alternative mechanism yields a 'turn-off' response for Fe 3+ , rather than the 'turn-on' response observed for Al 3+ . Indeed, the majority of previously reported fluorescent sensors for Al 3+ rely on a 'turn-on' response upon binding of Al 3+ between hard donor sites, while a 'turn-off' response to Fe 3+ is more common due to the paramagnetic nature of Fe 3+ . However, dual-responsive systems, in which different metal ions can be sensed by the same system via different mechanisms to produce different responses, are much scarcer. Such systems tend to be bases on photo-responsive stimuli, or chemical reactions induced by the metal ion. With 5 only showing a pronounced change in fluorescence in the presence of Fe 3+ , we investigated whether 5 was able to selectively detect Fe 3+ in a solution of competing metal ions. A mixture of 12 different metal ions (Li + , Na + , K + , Mg 2+ , Ca 2+ , Cr 3+ , Mn 2+ , Fe 2+ , Cu 2+ , Zn 2+ , La 3+ , Al 3+ , each 1 equiv.) was thus added to 5, followed by titration of this solution with Fe 3+ (Figure ). Addition of the competing ions to 5 yielded a minor drop in fluorescence intensity (F/F0 = 0.85). Stepwise addition of Fe 3+ subsequently yielded significant decreases in intensity at higher concentrations of Fe 3+ (F/F0 at 1 equiv. of Fe 3+ = 0.71; F/F0 at 5 equiv. of Fe 3+ = 0.27). As a control, the same experiment was then repeated, but with the concentrations of the other 12 metal ions increased alongside the concentration of Fe 3+ (Figure ). This yielded comparable decreases in intensity when compared to when the concentration of the competing ions is held at 1 equivalent (Figure ; F/F0 at 1 equiv. of [M n+ ] = 0.56; F/F0 at 5 equiv. of [M n+ ] = 0.12). This highlights that quenching by Fe 3+ is the dominant factor in the fluorescence quenching of 5, and demonstrates the selectivity of fluorescence quenching of 5 with Fe 3+ , even in the presence of competing ions.