OFF-ON-OFF fluorescent response of N,N,N',N'-tetrakis(1-isoquinolylmethyl)-2-hydroxy-1 ,3 -propanediamine (1-isoHTQHPN) toward Zn super(2+)

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 super(2+) (I sub(Zn)/I sub(0) = 12, phi sub(Zn) = 0.023). This fluorescence enhancement turns and then decreases sharply after the addition of more than 1 equiv. of Zn super(2+) reaching a constant minimum intensity with more than 2 equiv. of Zn super(2+). In contrast, the fluorescence intensity at 353 nm continues to increase until the signal saturates at ca. 5 equiv. of Zn super(2+). This observation can be explained by the formation of a fluorescent mononuclear complex ([Zn(1-isoHTQHPN)] super(2+)) followed by a non-fluorescent dinuclear complex ([Zn sub(2)(1-isoTQHPN)] super(3+)) at 475 nm during the titration of 1-isoHTQHPN with Zn super(2+). Both the mono- and dinuclear complexes were characterized by UV-vis, fluorescence, super(1)H NMR, ESI-MS, X-ray crystallography and TDDFT calculations. The fluorescence enhancement at 475 nm is Zn super(2+)-specific; Cd super(2+) induces a much smaller emission increase (I sub(Cd)/I sub(0) = 3.7, I sub(Cd)/I sub(Zn) = 31%). The Zn super(2+)/Cd super(2+) selectivity of the fluorescent response correlates with the difference in excimer-forming ability derived from the Cd-N sub(isoquinoline) and Zn-N sub(isoquinoline) bond distances.