OFF-ON-OFF fluorescent response of ,,′,′-tetrakis(1-isoquinolylmethyl)-2-hydroxy-1,3-propanediamine (1-isoHTQHPN) toward Zn

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+ ( I Zn / I 0 = 12, Zn = 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 ([Zn 2 (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 ( I Cd / I 0 = 3.7, I Cd / I Zn = 31%). The Zn 2+ /Cd 2+ selectivity of the fluorescent response correlates with the difference in excimer-forming ability derived from the Cd-N isoquinoline and Zn-N isoquinoline bond distances. 1-isoHTQHPN exhibits OFF-ON-OFF fluorescent response toward increasing concentration of Zn 2+ ions due to the specific intramolecular excimer formation in a mononuclear complex.