Photophysical behaviour of 4-hexyloxysalicylaldimies and their copper(II) complexes

• The binding mode of alkoxysalicylaldimine ligands was deduced for the first time.
• Alkoxysalicylaldimine ligands bind to metal ions in an intermediate tautomeric state.
• In solution the free alkoxysalicylaldimine ligand exists in the enol-imine tautomer.
• Strong solvent dependency is seen for the free ligand but not for the complexes.

Salicylaldimines are popular ligands used in the synthesis of anisotropic complexes which can form liquid crystal phases (metallomesogens). They tautomerise via exchange of the phenol proton, giving rise to solvent-dependent photophysical behaviour. In this study the photophysical behaviour of a family of 4-alkoxysalicylaldimine ligands and their corresponding copper(II) complexes were examined using electronic absorption spectroscopy and density functional theory (DFT). The solvent-dependent photophysical behaviour exhibited by the ligands is due to changes in the position of the tautomeric equilibrium as the degree of hydrogen-bonding character of the solvent is modified. Furthermore, the ligands bind in an intermediate state in the copper(II) complex, with the possible existence of a third tautomer. The electronic absorption properties of these compounds are strongly affected by changes in the environment around the molecule.