The synthesis, characterization, ¹H NMR, optical absorption and fluorescent properties of a series of amphiphilic Schiff-base bis(salicylaldiminato)zinc(II) complexes are reported. Detailed ¹H NMR, DOSY NMR, optical absorption and fluorescence spectroscopy studies indicate the existence of aggregate species in solutions of non-coordinating solvents. The degree of aggregation is related to the nature of the bridging diamine. Chloroform solutions of complexes where the bridging diamine contains a naphthalene or the pyridine nucleus are always characterized by the presence of defined dimer aggregates, whereas oligomeric aggregates are likely formed by complexes where the bridging diamine contains a benzene ring. In coordinating solvents or in the presence of coordinating species, a complete deaggregation of the complexes occurs, because of the axial coordination to the Zn^II ion, accompanied by considerable changes in the ¹H NMR and optical absorption spectra. The effect of the alkyl chains length seems to play a minor role in the aggregation properties, as noticed by ¹H NMR data, optical absorption and fluorescence spectra, which remain almost unaltered on changing the chain length.

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.

A luminescent bis(salicylaldiminato)zinc(II) Schiff-base complex, 1, is characterized by a concentration dependent second-order nonlinear optical response, related to the degree of aggregation of the complex in a dichloromethane solution. The formation of the monomeric adduct, by addition of a Lewis base, such as pyridine, to concentrated solutions of 1, leads to a switch-on of the quadratic hyperpolarizability. This represents an unprecedented mode of NLO switching in molecular materials.