Controlling the molecular aggregation. An amphiphilic Schiff-base zinc(II) complex as supramolecular fluorescent probe

Abstract

A designed amphiphilic Schiff-base bis(salicylaldiminato)zinc(II) complex, 1, forms defined aggregates in dilute solutions of non-coordinating solvents. The switching to the monomer can be driven by addition of a coordinating species and involves a dramatic enhancement of the fluorescence emission.

Supramolecular Aggregation of a New Substituted Bis(salicylaldiminato)zinc(II) Schiff-Base Complex Derived from trans-1,2-Diaminocyclohexane

“…Addition of a Lewis base to solutions of non-coordinating solvents of ZnL aggregates generally leads to disaggregation with the formation of monomeric adducts [6][7][8][9][10][11]. 1 H NMR studies of (R)-2 in mixtures of non-coordinating/coordinating (CDCl3/DMSO-d6) solvents further support the existence of various aggregate species in the former solvent ( Figure S3).…”

“…The 1 H NMR spectrum of (R)-2 in solution of the coordinating DMSO-d 6 solvent (1.0 × 10 −2 M) shows the presence of a single set of signals (Figure 1), independent from the concentration, indicating the existence of monomeric species in solution, as usually observed for Zn(II) Schiff-base complexes [6][7][8][9][10][11]. To further investigate about the structure of (R)-2 in solution, we performed diffusion-ordered NMR spectroscopy (DOSY) measurements, by using a known internal reference to estimate the molecular mass of species in solution [5][6][7][8][9][10].…”

“…For this family of complexes, we have established that the chemical shift of the imine hydrogens is diagnostic of their aggregation mode [5][6][7][8][9][10][11]. In particular, the switching from pentacoordinated monomeric adducts in coordinating solvents to dimeric aggregates in chloroform solutions has always accompanied by an up-field shift of these signals, because of the involved hydrogens lie under the shielding zone of the π electrons of a conjugated system [7][8][9][10][11].…”

“…In recent years we have been involved in the study of the aggregation properties of bis(salicylaldiminato)zinc(II) Schiff-base complexes, derivatives from substituted salicylaldehydes and 1,2-diamines [5][6][7][8][9][10][11]. Their aggregation properties are related to the Lewis acidic character of the metal center which, in turn, is strongly connected to the nature of the bridging diamine [12].…”

The Supramolecular Attitude of Metal–Salophen and Metal–Salen Complexes

“…Nevertheless, in the presence of strong coordinating species, aggregation is prevented. An observable sign of the ongoing disaggregation is the dramatic enhancement of fluorescence emission in Zn-salophen derivatives [29]. Clearly the aggregation properties of the complexes are related to the Lewis acidic character of the metal center which, in turn, is deeply connected to the nature of the bridging diamine.…”