Zinc-specific intramolecular excimer formation in TQEN derivatives: fluorescence and zinc binding properties of TPEN-based hexadentate ligands

Abstract

Zn(2+)-induced fluorescence enhancement of the TPEN (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine)-based ligand, N,N-bis(1-isoquinolylmethyl)-N',N'-bis(pyridylmethyl)ethylenediamine (N,N-1-isoBQBPEN, 1b), has been investigated. Upon Zn(2+) binding, 1b shows a fluorescence increase (ϕZn = 0.028) at 353 and 475 nm. The fluorescence enhancement at longer wavelengths is due to intramolecular excimer formation of two isoquinolines and is specific for Zn(2+); Cd(2+) induces very small fluorescence at 475 nm (ICd/IZn = 10%). The excimer formation of the [Zn(1b)](2+) complex in the excited state is supported by the time-dependent DFT calculation. Neither long-wavelength fluorescence nor excimer formation is observed in the Zn(2+) complex of N,N'-1-isoBQBPEN (2b). The quinoline analog N,N-BQBPEN (1a) exhibits similar but significantly smaller excimer formation. Thermodynamic and kinetic comparisons of Zn(2+) binding properties of ethylenediamine-based hexadentate ligands with pyridines and (iso)quinolines are comprehensively discussed.

A Tetrakisquinoline Analog of Calcium Indicator Quin2 for Fluorescence Detection of Cd2+

“…On the other hand, Cd 2+ ‐bound TQquin2 exhibits significant components with elongated lifetimes [ τ = 3.3 ns (67 %) and 7.5 ns (33 %)]. The obtained long lifetime of the Cd 2+ complex with unsubstituted quinoline chromophore suggests the involvement of intramolecular excimer component as mentioned previously , , …”

“…The mechanism of fluorescence enhancement of TQquin2 is due to the combination of inhibition of PeT (photo‐induced electron transfer) and CHEF (chelation enhanced fluorescence) of central and side‐arm quinolines . In addition, we consider that the intramolecular excimer formation with unsubstituted quinoline rings may be involved , , . The BAPTQ exhibits lower energy emission at 413 nm in methanol in the presence of Cd 2+ , which suggests the intramolecular excimer formation arising from the unsubstituted tetrakisquinoline moiety for this ligand.…”

“…[31] In addition, we consider that the intramolecular excimer formation with unsubstituted quinoline rings may be involved. [21,32,33] The BAPTQ exhibits lower energy emission at 413 nm in methanol in the presence of Cd 2+ , which suggests the intramolecular excimer formation arising from the unsubstituted tetrakisquinoline moiety for this ligand. The TriMeOBAPTQ-Cd 2+ complex showed lower-energy shift of the emission wavelength, indicating the similar fluorescence mechanism with unsubstituted BAPTQ.…”

Carbohydrate‐Appended TQNPEN [N,N,N′,N′‐Tetrakis(2‐quinolylmethyl)‐3‐aza‐1,5‐pentanediamine] Derivatives for Fluorescence Detection of Intracellular Cd2+

“…As discussed previously, the Cd 2+ ‐induced fluorescence enhancement of TQNPEN derivatives is attributed to combination of chelation enhanced fluorescence (CHEF) and inhibition of photoinduced electron transfer (PeT) process, as well as involvement of an intramolecular excimer emission contribution from adjacent quinoline rings . The emission maxima and fluorescence lifetimes of sugar‐appended TQNPEN‐Cd 2+ complexes were in good agreement with those of parent TQNPEN‐Cd 2+ complex (Table and Figure S5–8), indicating the attached carbohydrate induces very small perturbation to the fluorescence properties of the Cd 2+ complex.…”

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

“…The Cd 2+ titration of 5,6,7-TriMeOTQA is shown in Fig. 46,47 Thus, the Cd 2+specific fluorescence spectral change of 6 is indicative for the excimer formation with seven or higher coordinate structure for the metal-bound species. † As mentioned above, introduction of more than two methoxy groups induces long-wavelength shift of fluorescence maximum (λ em ) upon Zn 2+ /Cd 2+ binding.…”