Tris(8-methoxy-2-quinolylmethyl)amine (8-MeOTQA) as a highly fluorescent Zn2+ probe prepared by convenient C3-symmetric tripodal amine synthesis

Abstract

A convenient synthesis of C₃-symmetric tribenzylamine (TBA) derivatives has been investigated. The reaction of benzyl chlorides with acetaldehyde ammonia trimer (1) in the presence of base afforded tribenzylamines in high yields. This efficient method allows the diverse synthesis of TPA (tris(2-pyridylmethyl)amine) and TQA (tris(2-quinolylmethyl)amine) derivatives. Among the TQA compounds prepared, tris(8-methoxy-2-quinolylmethyl)amine (8-MeOTQA, 4) exhibited superior properties as a fluorescent zinc probe with high quantum yield (ϕ_Zn = 0.51) and high sensitivity (limit of detection (LOD) = 3.4 nM). The X-ray crystallographic analysis of [Zn(8-MeOTQA)]²⁺ revealed that the steric and electronic effect of 8-methoxy substituents kicks out the solvent and counterion molecules from the metal coordination sphere, resulting in short Zn–N_quinoline coordination distances (2.04–2.07 Å). The pseudo hexacoordinate complex of 6-methoxy derivative, [Zn(6-MeOTQA)(DMF)(ClO₄)]⁺, exhibited longer Zn–N_quinoline distances (2.07–2.19 Å) and much smaller fluorescence intensity (ϕ_Zn = 0.027). The replacement of one of the three 8-methoxyquinolines with pyridine also afforded much less fluorescent zinc complex (ϕ_Zn = 0.095) due to the solvent coordination (Zn–N_quinoline = 2.05–2.18 Å for [Zn(8-MeOBQPA)(CH₃OH)]²⁺).