Zinc-Specific Fluorescent Response of Tris(isoquinolylmethyl)amines (isoTQAs)

Isoquinoline-based tetradentate ligands with C3-symmetry, tris­(1- or 3-isoquinolylmethyl)­amine (1- or 3-isoTQA), have been prepared and their zinc-induced fluorescence enhancement was investigated. Upon excitation at 324 nm, 1-isoTQA shows very weak fluorescence (ϕ = ∼0.003) in DMF/H2O (1/1) solution. In the presence of zinc ion, 1-isoTQA exhibits fluorescence increase (ϕ = 0.041) at 359 and 470 nm. This fluorescence enhancement at 470 nm is specific for zinc. However, 3-isoTQA exhibited a smaller fluorescence enhancement upon zinc complexation (ϕ = 0.017, λem = 360 and 464 nm) compared with 1-isoTQA. Crystal structures of zinc complexes of isoTQAs demonstrate the diminished steric crowding and shorter Zn–Naromatic distances compared with isoTQENs (N,N,N′,N′-tetrakis­(isoquinolylmethyl)­ethylenediamines) leads to a higher fluorescent response toward zinc relative to cadmium.