Liquid-crystalline oxovanadium(IV) complexes accessed from bidentate [N, O] donor salicylaldimine Schiff-base ligands

Bhattacharjee, Chira R.; Das, Gourab; Mondal, Paritosh

doi:10.1080/00958972.2011.619262

“…Energetically there is very little difference between the nematic and liquid phases. As such the heat of a nematicisotropic transition can be very low, 54 typically less than 1 kJ mol −1 and if the transition is broad it can be very difficult to observe.…”

Section: Structure and Phase Behaviour Of L3(46)mentioning

confidence: 99%

“…Peaks in this area and of this intensity in salicylaldimine compounds' spectra are generally assigned to a π-π* transition localised on the imine C=N chromophore. 54,74,75 This peak would therefore be expected to decline in intensity as the proportion of the enol-imine tautomer is reduced by the addition of a hydrogen-bonding solvent. This explanation is not consistent with the data presented in Figures 6.2.1 and 6.2.3.…”

Section: Spectral Features Of Ligandsmentioning

confidence: 99%

Controlling the Phase Behaviour and Photophysical Properties of Metallomesogens Through Structural Modification

Greenbank¹

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<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>

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“…This assignment is consistent with reported studies of other salicylaldimine-based compounds. 54,74,75 The behaviour of band B is, at first glance, harder to rationalise by this means. Peaks in this area and of this intensity in salicylaldimine compounds' spectra are generally assigned to a π-π* transition localised on the imine C=N chromophore.…”

Section: Spectral Features Of Ligandsmentioning

confidence: 99%

“…DSC is used extensively in liquid crystal research to find the temperatures and enthalpies of phase transitions with extreme precision. 4,20,26,54 There are, however, some issues in its application to liquid crystals. Unlike some other techniques, DSC is unable to confirm that a mesophase has formed; it can only show that multiple phase transitions have occurred.…”

Section: Future Workmentioning

confidence: 99%

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Controlling the Phase Behaviour and Photophysical Properties of Metallomesogens Through Structural Modification

Greenbank¹

0

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<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>

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

Greenbank

¹

,

McGrath

²

2014

Journal of Photochemistry and Photobiology A: Chemistry

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Liquid-crystalline oxovanadium(IV) complexes accessed from bidentate [N, O] donor salicylaldimine Schiff-base ligands

Cited by 10 publications

References 54 publications

Controlling the Phase Behaviour and Photophysical Properties of Metallomesogens Through Structural Modification

Controlling the Phase Behaviour and Photophysical Properties of Metallomesogens Through Structural Modification

Controlling the Phase Behaviour and Photophysical Properties of Metallomesogens Through Structural Modification

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

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