Zinc(II) Schiff Base Complexes and their Aggregation/Deaggregation Properties: Versatile and Multifunctional Materials as Chemosensors and Building Blocks for New Supramolecular Architectures

Abstract

In this PhD thesis the synthesis, characterization, and study of the aggregation/deaggregation properties of a series of amphiphilic bis(salicylaldiminato)ZnII Schiff base complexes, involving different bridging diamino groups, and their applications as molecular probes, nonlinear optical (NLO) and vapochromic materials, as well as synthons for the formation of new fibrillar and branched supramolecular nanostructures, are reported. Through detailed 1H NMR, DOSY NMR and optical spectroscopic studies, it is found that these species always form aggregates in solution of non-coordinating solvents. The degree of aggregation is related to the nature of the bridging diamine. In coordinating solvents or in the presence of coordinating species, a complete deaggregation of all complexes occurs because of the axial coordination to the ZnII ion, accompanied by considerable changes of the 1H NMR and optical absorption and fluorescence spectra. Moreover, an easy switch-on of the NLO response upon addition of a Lewis base, such a pyridine, with formation of a 1:1 adduct, or 1,2-bis-(4-pyridyl)ethane, with formation of a 2:1 adduct, is observed. As the amount of the coordinating species for the complete deaggregation of the complexes is dependent of the Lewis character of the ZnII ion, an order of the Lewis acidic character can be established for the aggregate species in non-coordinating solvents. The effect of the alkyl chain length seems to play a minor role in the aggregation properties, since 1H NMR data, optical absorption and fluorescence spectra remain almost unaltered upon changing the chain lengths. The complexes having benzene ring bridge form fibrillar nanostructures whose width is influenced by the degree of interdigitation of side alkyl chains. The Lewis acid properties of the complexes having 2,3-diaminomaleonitrile bridge, with respect a series of primary, secondary and tertiary aliphatic amines, and several alkaloids, are also investigated. Through the analysis of fluorescence titrations, it is found that the binding interaction, the selectivity and the sensitivity of these complexes are strongly influenced by the steric characteristics of the nitrogen based donors and then by their Lewis basicity, leading to high selectivity, in the micromolar range, and sensitivity for pyridine-based, cinchona alkaloids, primary and alicyclic amines. A distinct selectivity is also observed along the series of secondary or tertiary amines, paralleling the increasing steric hindrance at the nitrogen atom. Moreover, these complexes have been also involved as reference Lewis acids, to build up a reliable Lewis basicity scale in dichloromethane for amines and various common solvents whose trend is influenced by the steric hindrance of both the Lewis bases and the reference Lewis acid. The these complexes have also investigated as vapochromic materials, able to change their colour upon exposure to volatile Lewis bases, allowing their application as chemosensors for volatile Lewis bases in the solid state. Finally, new branched nanostructures have been achieved for a ZnII Schiff base complex having an alkyl ammonium bromide in the alkyl side chains. Unlike to studies on the aforementioned amphiphilic complexes, in this case the control of the supramolecular architecture is governed by intermolecular Zn---Br interactions.