Controlling the Phase Behaviour and Photophysical Properties of Metallomesogens Through Structural Modification
Authors:   , Greenbank William Albert 
Published: November 2021

<p>Metallomesogens (metal-containing liquid crystals) have been of interest to chemists since the early 1980s. Since this period, many of the studies published on metallomesogens have focussed on the synthesis of novel metallomesogens, and studies of their phase behaviour. As a result there is a substantial body of knowledge of their synthesis and phase behaviour, however many of these studies have overlooked the interesting physical properties that transition metals or lanthanides may impart to the mesophase (liquid crystal state). The studies that have been carried out suggest that the optical and photophysical properties resulting from their self assembly are very different to those observed in the crystalline or isotropic liquid phases, and are highly dependent on the structure of the mesophase. A series of salicylaldimine-based ligands and copper(II) complexes with a variety of structural modifications were synthesised and characterised. The structure, phase behaviour and phase relaxation kinetics of these compounds in the crystalline state were studied using differential scanning calorimetry (DSC), single crystal X-ray crystallography and variable temperature powder X-ray diffraction. The mesomorphism of the compounds was studied using small angle X-ray scattering (SAXS) and polarised optical microscopy (POM). The photophysical properties of the complexes and ligands were studied in the solution phase using ultraviolet-visible (UV-vis) spectroscopy. It was found that the smallest complexes (copper(II) N-alkyl,4-alkoxysalicylaldimine complexes) were not metallomesogens, but did exhibit multiple crystalline phases that formed as a result of changes in the conformation of the N-alkyl chains. The transition temperatures of these crystalline phase changes were strongly dependent on the length of the alkyl chains due to kinetic phenomena. The extension of the rigid core of the complex via synthesis of an N-(4-butylphenyl) derivative was successful in inducing mesomorphism in both the complex and the ligand. The ligand formed an enantiotropic nematic mesophase, while the complex formed a monotropic smectic A mesophase. The structural differences between the non-mesomorphic complexes, the mesomorphic ligand and the mesomorphic complex indicate that the determining factor in the formation of mesophases is the magnitude of lateral interactions between the molecules, which is governed by the size and shape of the rigid core. Further attempts at inducing mesomorphism by formation of bimetallic copper complexes were unsuccessful due to chemical instability. The photophysical properties of the compounds showed that the salicylaldimine ligands exist in solution in a tautomeric equilibrium, which can be influenced by the hydrogen-bonding character of the solvent. The ligands also show evidence of photochromism, while the complexes exhibit LMCT bands, both features which could affect and be affected by self assembly. It was also determined from their UV-vis spectra and DFT studies that the ligands bind to the metal centre in a manner which is intermediate to the two tautomeric forms, but close to the higher energy keto-amine tautomer. These results demonstrate that structural modification can be used to control both the phase behaviour and physical properties of salicylaldimine complexes. The compounds studied here also show potential to exhibit a variety of self assembly-dependent photophysical properties in the mesophase and would be good candidates for future research in this area.</p>