Switching on the Fluorescence Emission of Polypyridine Ligands by Simultaneous Zinc(II) Binding and Protonation

Abstract

The synthesis and characterization of the two new open-chain ligands 1,15-bis-[6-(2,2'-bipyridyl)]-2,5,8,11,14-pentaaza-octadecane (L1) and 1,15-bis-[2-(1,10-phenanthroline)-9-methyl]-2,5,8,11,14-pentaazaoctadecane (L2), both featuring a tetraethylenpentaamine chain linking via methylene bridges the 6 and 2 positions of two identical 2,2'-bipyridyl (bpy) and 9-methyl-1,10phenanthroline (9-methyl-phen) moieties respectively, are reported. Their protonation and binding ability for Cu 2 + , Zn 2 + , Cd 2 + and Pb 2 + have been studied by coupling potentiometric titrations with UV-vis absorption and fluorescence emission measurements in water. L1 and L2 afford stable mono-and dinuclear complexes, in which the metal ion is bound by a single bpy or 9-methyl-phen unit and the amine groups on the aliphatic chain. However, L1 displays a greater binding ability for Cu 2 + and Zn 2 + with respect to L2, the stability constants of the [ML1] 2 + complexes being 21.8 (Cu 2 + ) and 19.4 (Zn 2 + ) log units vs 20.34 and 16.8 log. units for the corresponding L2 species. Among all the metal ions tested, only the Zn 2 + complex with L2 features an enhanced fluorescence emission at neutral pH, thanks to the simultaneous binding of one Zn 2 + ion and H + ion(s), that inhibits any possible photoinduced electron transfer (PET) process from the amine donors to the excited phen moiety. Binding of a second metal switches off the emission again.[a] Dr.by the structural characteristics of the ligand and its Zn 2 + complexes, including the number and disposition of amine donors, the overall rigidity of the molecular backbone, and, in the case of macrocyclic ligands, the dimension of the cavity. In these complexes, the heteroaromatic nitrogens are unequivocally coordinated to the metal cations. However, the stiffening of the macrocyclic structure imposed by the heteroaromatic moieties may not allow the simultaneous coordination to the metal cations of all the nitrogen donors of the binding unit. Interestingly enough, the 'benzylic' amine groups, close to the heteroaromatic unit, are not coordinated, or weakly coordinated, to the metal cation. Therefore, their lone pairs are able to quench the fluorophore excited state thanks to a photoinduced electron transfer (PET) process. As a consequence, normally fluorescent Zn 2 + complexes result to be scarcely emissive. [8a,b,22-24] In this paper, we extend this study to two open-chain ligands, L1 and L2 in Scheme 1, containing a pentaamine aliphatic chain linked at its extremities to two equal 2,2'bipyridyl (bpy) or 9-methyl-1,10-phenanthroline (9-methylphen) moieties, respectively. Our purpose is the analysis of the effects on both metal binding and sensing of the presence of two fluorogenic units linked by a flexible chain.Therefore, complex stability and fluorescence emission in the presence of metal cations will be compared to those of the previously reported ligands L3 [24] and L4 [8a] (Scheme 1), which contain the same aliphatic pentaamine chain linking the 6,6' and 2,9 positions of a single...

Protonation of cyclen-based chelating agents containing fluorescent moieties

“…The equipment and procedure used were as previously described. 73 The standard potential E° and the ionic product of water (pK w = 13.83(1) at 298.1 ± 0.1 K in 0.10 M NMe 4 Cl) were determined by Gran′s method. 74 At least three measurements (with about 100 data points for each) were performed for each system in the pH ranges 2−10.5.…”

A New Family of Macrocyclic Polyamino Biphenolic Ligands: Acid-Base Study, Zn(II) Coordination and Glyphosate/AMPA Binding

“…Indeed, besides their typically high water solubility, a fundamental pre-requisite for the application in real matrices, the high number of positively charged polyammonium groups formed in solution allows for the efficient interaction with anionic guests, stabilized via hydrogen bonding and charge-charge interactions [31,32]. Moreover, these frameworks can bind metal ions [33][34][35][36][37], affording promising metallo-receptors for anionic targets [38,39]. More in particular, a number of Zn(II) coordination compounds have been shown to possess promising features as receptors for anions of environmental and biological importance, mainly thanks to the presence of coordinatively unsaturated metal centers which can in turn act as suitable anchoring sites for the coordination of anionic guests [40][41][42].…”

Phenanthroline Derivative Fluorescent Probe for Rapid and Sensitive Detection of Silver(I)

“…3C that the fluorescence quenching efficiencies were relatively low for pH 3.0, because the protonation of MPP weakened its complexation with silver ions, resulting in lower fluorescence quenching efficiencies. 38 From pH 4.0 to 12.0, the quenching efficiencies of silver ions to the MPP were greater than 90%. This result indicates that MPP can be used to detect Ag + in a wide pH range, which is very useful for analyzing complex real samples.…”