An isoquinoline-based ligand, N,N,N',N'-tetrakis(1-isoquinolylmethyl)-2-hydroxy-1,3-propanediamine (1-isoHTQHPN), exhibits a fluorescence increase at 475 nm upon addition of 1 equiv. of Zn(2+) (IZn/I0 = 12, ϕZn = 0.023). This fluorescence enhancement turns and then decreases sharply after the addition of more than 1 equiv. of Zn(2+) reaching a constant minimum intensity with more than 2 equiv. of Zn(2+). In contrast, the fluorescence intensity at 353 nm continues to increase until the signal saturates at ca. 5 equiv. of Zn(2+). This observation can be explained by the formation of a fluorescent mononuclear complex ([Zn(1-isoHTQHPN)](2+)) followed by a non-fluorescent dinuclear complex ([Zn2(1-isoTQHPN)](3+)) at 475 nm during the titration of 1-isoHTQHPN with Zn(2+). Both the mono- and dinuclear complexes were characterized by UV-vis, fluorescence, (1)H NMR, ESI-MS, X-ray crystallography and TDDFT calculations. The fluorescence enhancement at 475 nm is Zn(2+)-specific; Cd(2+) induces a much smaller emission increase (ICd/I0 = 3.7, ICd/IZn = 31%). The Zn(2+)/Cd(2+) selectivity of the fluorescent response correlates with the difference in excimer-forming ability derived from the Cd-Nisoquinoline and Zn-Nisoquinoline bond distances.