A new versatile solvatochromic amino-macrocycle. From metal ions to cell sensing in solution and in the solid state

Abstract

A new fluorescent NBD-polyaza-macrocycle sensor (L) was synthesized. The coordination of Cu(ii) and Zn(ii) in acetonitrile switches on the fluorescence with different emission wavelengths. Cu(ii) complexes showed solid-state fluorescence. Both L and Cu-complex interact with human cell line (U937) highlighting the cell membrane by fluorescence microscopy.

Multi‐Use NBD‐Based Tetra‐amino Macrocycle: Fluorescent Probe for Metals and Anions and Live Cell Marker

“…Solution studies : The absorption spectra of free L in acetonitrile show the two typical bands of 4,7‐disubstituted benzofurazan moieties at 345 nm ( ε =8000 cm −1 mol −1 dm 3 ) associated with π–π* transition and at 486 nm ( ε =19 000 cm −1 mol −1 dm 3 ) attributed to intramolecular charge transfer transition (CT)24 (Figure 3). In this solvent, L is very weakly fluorescent ( λ em =543 nm, Φ em <0.01) and the low fluorescence quantum yield can be explained in terms of photoinduced electron transfer (PET) from the lone pair of the three non‐conjugated nitrogen atoms of the macrocycle to the CT excited state of NBD 16…”

“…At the present, there are few known examples of chemosensors able to sense chloride by the enhancement of fluorescence; for example, two interesting OFF–ON fluorescent sensors for Cl − based on rotaxane displacement14 and on chemosensing ensemble15 approach have been reported in the last year. Recently, we reported a tetra‐aza‐macrocycle conjugated with the high performing NBD (4‐amino‐7‐nitrobenzo[1,2,5]oxadiazole) fluorophore, L 16 (4‐(7‐nitrobenzo[1,2,5]oxadiazole‐4‐yl)‐1,7‐dimethyl‐1,4,7,10‐tetra‐azacyclododecane) that is capable of acting as an OFF–ON sensor for Cu II and Zn II in acetonitrile solution and can interact with live cells to switch ON fluorescence emission. In the present work, we extend the study of the behavior of this ligand to Cd II and we consider the use of the related complexes [Zn L ] 2+ , [Cd L ] 2+ and [Cu L ] 2+ as metallo‐receptors for halide anions.…”

“…NMR spectroscopy studies : The 1 H NMR spectrum of free L shows five aliphatic and two aromatic resonances (Figure 4): a singlet at δ =2.30 ppm integrating six protons (6 H) attributed to the resonance of H5 (H5, 6 H), four triplets at δ =2.50 ppm (H6, 4 H), δ =2.67 ppm (H7, 4 H), δ =2.76 ppm (H4, 4 H) and δ =4.17 ppm (H3, 4 H), a doublet at δ =6.38 ppm (H2, 1 H), and a doublet at δ =8.37 ppm (H1, 1 H); this is in agreement with the 13 C spectrum, which exhibits a total of eleven signals (six for the aromatic and five for the aliphatic resonances),16 and indicates a C s symmetry of L mediated on the NMR time scale. The 1 H NMR spectra of [Zn L ] 2+ and [Cd L ] 2+ species, obtained by adding 1 equiv M II as perchlorate salt as well as by using the preformed complexes, exhibit the downfield shift of all resonances, with the exception of H3, which shifts upfield, with respect to free L (Figure 4).…”

“…In fact, given that the Zn(1)⋅⋅⋅N(4) distance is longer than the sum of the covalent (N) and ionic (Zn) radii of nitrogen and zinc atoms,18 but shorter than the sum of their van der Waals radii (2.94 Å)19 this Zn(1)⋅⋅⋅N(4) contact can be considered nothing more than a secondary intramolecular interaction (Table 1 and Figure 1) so that in compound 1 the coordination of the zinc cation can be described as “4+1” 20. It is to be noted that in the related X‐ray structures of the copper complexes [Cu L Cl](ClO 4 ) and [Cu L Cl](PF 6 )⋅(H 2 O)16 as well, the Cu⋅⋅⋅N(4) distance is significantly longer (0.1 Å) with respect to the others. However, the hybridization of N(4) is undoubtedly sp 3 and, as a consequence, all the nitrogen atoms of the macrocyclic base coordinate the copper cation.…”

“…The strong downfield shift of H2 together with the slow exchange between the [Zn L Cl] + and [Zn L ] 2+ species is in agreement and could be explained by the formation of a H bond involving H2 and Cl − coordinated to Zn II . In other words, a 3D arrangement similar to that observed in the crystal structure of the [Cu L Cl] + cation16 could be also hypothesized for the [Zn L Cl] + species in solution; this further H bond stabilization can be also the cause of the lower species exchanging, on the NMR time scale, with respect to the Cd II system. This hypothesis is supported by the results of an energy minimization of the complex cations [Zn L in Cl] + and [Zn L out Cl] + each one featuring the starting geometry of the experimental compounds [Zn L Cl] + and [Cu L Cl] + , respectively ( L in and L out denote the different arrangement of L about the metal cations, Figure 2).…”