A quinoline-based heptadentate ligand, N,N,N',N'-tetrakis(2-quinolylmethyl)-3-oxa-1,5-pentanediamine (TQOPEN), exhibits a fluorescence increase (I/I = 25, ϕ = 0.017) at 428 nm upon addition of 1 equiv of Cd. In contrast, 1 equiv of Zn induces a negligible fluorescence change due to weak interaction (I/I = 2.5, I/I = 10%). In comparison with TQOPEN, the thia and aza derivatives TQSPEN and TQNPEN exhibit improved Cd/Zn selectivity and higher Cd-binding affinity, respectively. The solid-state structures of mononuclear Cd and hydroxide-bridged dinuclear Zn complexes of TQOPEN were elucidated by X-ray crystallography. Although the crystal structure of the TQOPEN-Cd complex exhibits a six-coordinate metal center, in which one quinoline weakly interacts with the Cd center (Cd···N = 3.303(3) Å), a H NMR study at 233 K suggests that all quinolines interact with the Cd center to form a symmetrical seven-coordinate structure in solution. Theoretical calculations (TDDFT) support the flexible coordination environment around the Cd center, leading to intramolecular excimer formation with two quinoline moieties in the excited state. The importance of a heptadentate structure was further demonstrated by the lack of Cd specificity with hexadentate ligands TriQOPEN (N,N,N'-tris(2-quinolylmethyl)-3-oxa-1,5-pentanediamine) and TQCPEN (N,N,N',N'-tetrakis(2-quinolylmethyl)-1,5-pentanediamine).

TACN (1,4,7-triazacyclononane) derivatives with three 6-methoxy-2-quinolylmethyl or 1-isoquinolylmethyl moieties were examined as fluorescent zinc sensors. Upon the addition of zinc, 6-MeOTQTACN (5) exhibited a 9-fold fluorescence increase at 420 nm (λex = 341 nm, ϕZn = 0.070). Fluorescence enhancement is specific for zinc and cadmium, although cadmium induces smaller increases (ICd/I0 = 3.6 and ICd/IZn = 40%). The isoquinoline analog 1-isoTQTACN (6) exhibits minimal fluorescence enhancement upon zinc binding. TPEN (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylene-diamine) does not extract zinc from the 6-MeOTQTACN-Zn complex (5-Zn). The quantum yield, metal ion selectivity and metal binding affinity differences between TACN and ethylenediamine (EN) skeletons in quinoline-based ligands are discussed based on the X-ray crystallographic analysis of zinc and cadmium complexes, demonstrating the superiority of quinoline-TACN conjugates.

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TQPHEN (N,N,N',N'-tetrakis(2-quinolylmethyl)-1,2-phenylenediamine) and its methoxy-substituted derivatives, 6-MeOTQPHEN (N,N,N',N'-tetrakis(6-methoxy-2-quinolylmethyl)-1,2-phenylenediamine) and TriMeOTQPHEN (N,N,N',N'-tetrakis(5,6,7-trimethoxy-2-quinolylmethyl)-1,2-phenylenediamine), were examined as fluorescent Cd(2+) sensors. Although the TQPHEN exhibits a negligible fluorescence response toward Zn(2+) due to weak binding affinity in DMF-H2O (1:1), a 6-fold fluorescence enhancement at 392 nm was observed in the presence of 1 equiv. of Cd(2+). Comprehensive X-ray crystallographic analyses of TQPHEN-Zn(2+) and TQPHEN-Cd(2+) complexes reveal that significant distortion in the Zn(2+) complex plays a key role in the Cd(2+) specificity of TQPHEN. The TriMeOTQPHEN exhibits excellent sensitivity and selectivity for Cd(2+) detection (ICd/I0 = 44, ICd/IZn = 20 and LODCd = ∼10 nM (LOD = limit of detection)). On the other hand, the trans-1,2-cyclohexanediamine derivative TQDACH (N,N,N',N'-tetrakis(2-quinolylmethyl)-trans-1,2-cyclohexanediamine) exhibits high Zn(2+) specificity in the fluorescence response and extremely high Zn(2+) binding affinity (Dalton Trans., 2013, 42, 9688). Subtle differences in the PHEN and DACH backbone significantly alter the stability with a specific metal and the fluorescence response of tetrakisquinoline-based fluorescent probes.

A convenient synthesis of C3-symmetric tribenzylamine (TBA) derivatives has been investigated. The reaction of benzyl chlorides with acetaldehyde ammonia trimer () 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, ) exhibited superior properties as a fluorescent zinc probe with high quantum yield (ϕZn = 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-Nquinoline coordination distances (2.04-2.07 Å). The pseudo hexacoordinate complex of 6-methoxy derivative, [Zn(6-MeOTQA)(DMF)(ClO4)](+), exhibited longer Zn-Nquinoline distances (2.07-2.19 Å) and much smaller fluorescence intensity (ϕZn = 0.027). The replacement of one of the three 8-methoxyquinolines with pyridine also afforded much less fluorescent zinc complex (ϕZn = 0.095) due to the solvent coordination (Zn-Nquinoline = 2.05-2.18 Å for [Zn(8-MeOBQPA)(CH3OH)](2+)).