Effect of methoxy substituents on fluorescent Zn²⁺/Cd²⁺ selectivity of bisquinoline derivatives with a N,N′-dimethylalkanediamine skeleton

Abstract

BQDMEN (N,N’-bis(2-quinolylmethyl)-N,N’-dimethylethylenediamine) and its 6-methoxyquinoline derivative (6-MeOBQDMEN) are fluorescent Zn2+ sensors with minor response to Cd2+ (IZn/ICd = 3.9 for BQDMEN and IZn/ICd = 2.2 for 6-MeOBQDMEN in the presence...

Cd2+-Specific Fluorescence Response of Methoxy-Substituted N,N-Bis(2-quinolylmethyl)-2-methoxyaniline Derivatives

“…The Cd 2+ complex with 7 crystallized as a bis(μ-chloro)dinuclear complex (Figure a). A similar dicadmium complex with tetradentate ligand BQDMEN ( N , N ′-bis(2-quinolylmethyl)- N , N ′-dimethylethylenediamine) has been reported . Although this ethylenediamine complex exhibited apparent interligand stacking interaction between quinoline moieties in the bis(μ-chloro)dicadmium complex, no such interquinoline interaction was detected in the present complex, probably due to the steric constraints in the N , N -bis(2-quinolylmethyl)amine structure.…”

“…The methoxy substitution on the quinoline ring increases the metal binding affinity by the electron-donating effect and often affords fluorescent Cd 2+ selectivity. 27,34,35,43 So, we prepared 6-MeOBPDTQ (2) and TriMeOBPDTQ (3) and examined their fluorescence responses with metal ions. Changes in UV−vis spectra, including those for BPDTQ (1), are shown in Figure S6.…”

“…A similar dicadmium complex with tetradentate ligand BQDMEN (N,N′-bis(2-quinolylmethyl)-N,N′-dimethylethylenediamine) has been reported. 43 Although this ethylenediamine complex exhibited apparent interligand stacking interaction between quinoline moieties in the bis(μ-chloro)dicadmium complex, no such interquinoline interaction was detected in the present complex, probably due to the steric constraints in the N,N-bis(2-quinolylmethyl)amine structure. The distorted Cd 2 Cl 2 diamond core structure of the 7-Cd 2+ complex found in the difference in two Cd−Cl distances (Cd−Cl = 2.45 and 2.86 Å) and deviation from the orthogonality in Cl−Cd−Cl and Cd−Cl−Cd angles (83 and 97°, respectively) reflects the weak interaction of monomeric [Cd(BQMOA)Cl] + units in the dinuclear complex due to the lack of the interquinoline stacking interaction.…”

“…Such a distortion was not seen in the [Cd 2 (μ-Cl) 2 (BQDMEN) 2 ] 2+ complexes (Cd−Cl = 2.56−2.59 Å; Cl−Cd−Cl = 88−91°; and Cd−Cl−Cd = 88−91°). 43 The ESI-MS measurement of BQMOA derivatives with 3 equiv of Cd 2+ 43 The above experimental results conclude that the Cd 2+ complex with BQMOA derivatives could exist in a possible equilibrium between monomer [Cd(L)Cl] + and dimer ([Cd(L)Cl] + ) 2 , i.e., [Cd 2 (μ-Cl) 2 (L) 2 ] 2+ , in solution. However, the significant fluorescence enhancement in the 9-Cd 2+ system by addition of KCl (Figure 8b) strongly supports the extensive formation of bis(μchloro)dicadmium complex in the fluorescence measurement solution.…”

“…This is quite different from TriMeOBQD-MEN derivatives in our previous study, in which significantly long fluorescence lifetimes (τ = ∼30 ns) for Cd 2+ complexes, but no such long lifetimes for Zn 2+ complexes, were observed. 43 This is in good agreement with the above crystal structure of the bis(μ-chloro)dinuclear Cd 2+ complex with 7 (Figure 9a), in which no interligand quinoline−quinoline stacking interaction, namely, possibility of excimer emission, was observed. It should be noted that the compositions of the fluorescence lifetime for Cd 2+ complexes with 6 and 9 are quite similar, indicating a similar fluorescence mechanism for these complexes.…”