OFF–ON–OFF fluorescent response of N,N,N′,N′-tetrakis(1-isoquinolylmethyl)-2-hydroxy-1,3-propanediamine (1-isoHTQHPN) toward Zn2+

Abstract

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 Zn²⁺ (I_Zn/I₀ = 12, ϕ_Zn = 0.023). This fluorescence enhancement turns and then decreases sharply after the addition of more than 1 equiv. of Zn²⁺ reaching a constant minimum intensity with more than 2 equiv. of Zn²⁺. In contrast, the fluorescence intensity at 353 nm continues to increase until the signal saturates at ca. 5 equiv. of Zn²⁺. 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 Zn²⁺. Both the mono- and dinuclear complexes were characterized by UV-vis, fluorescence, ¹H NMR, ESI-MS, X-ray crystallography and TDDFT calculations. The fluorescence enhancement at 475 nm is Zn²⁺-specific; Cd²⁺ induces a much smaller emission increase (I_Cd/I₀ = 3.7, I_Cd/I_Zn = 31%). The Zn²⁺/Cd²⁺ 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.