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Structural Correspondence of Solution, Liquid Crystal, and Crystalline Phases of the Chromonic Mesogen Sunset Yellow
dataset
posted on 2014-08-06, 00:00 authored by Wenchang Xiao, Chunhua Hu, Damien J. Carter, Shane Nichols, Michael
D. Ward, Paolo Raiteri, Andrew L. Rohl, Bart KahrThe azo dye, sunset yellow, is a
prototypical, chromonic liquid
crystal in which assembly in aqueous solution at high volume fraction
leads to lyotropic mesophases with a “package of properties
distinct in almost every aspect” (Lydon, J. Curr. Opin. Colloid Interface
Sci. 2004, 8, 480). In particular, the isotropic to nematic transition
in such phases, the consequence of stacking of dye molecules in chains,
is difficult to bring into correspondence with athermal theories for
rigid rods as well as modifications that consider chain interactions
with one another. Chromonic mesogens, small molecules that stack to
form lyotropic liquid crystals, prompt structural questions that have
yet to be answered; a full understanding of structure should inform
colligative properties. Herein, the single crystal structure of a
guanidinium salt of the sunset yellow dianion, a known chromonic mesogen,
is reported. The compound crystallizes as a dihydrate, tetrahydrofuran
solvate in the orthorhombic space group Pnna, with a = 6.8426(5) Å, b = 20.048(1) Å, c = 21.466(2) Å. The sunset yellow molecules, point
group approximately Cs, are disordered about a crystallographic diad axis. The structure
is informative because pairwise interactions in the disordered crystal
structure show a remarkable correspondence with the stereochemistry
of sunset yellow molecules in solution and in the liquid crystal phase.
The solution structure is here simulated by the combination of molecular
dynamics, metadynamics, and quantum chemical computations. The comparable
disorder in the fluid and solid states suggests the possibility that
stacked aggregates adhere to growing crystals intact. Computations
were used to evaluate proposals that stacking faults and branching
points lower the X-ray correlation lengths while preserving extended
structures. Evidence is found for stacking faults but not branches.
The solution stereochemistry and stereodynamics has implications for
the geometry of long rods, for which understanding is a prerequisite
for reckoning properties of vexing chromonic mesophases.
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Colloid Interface Scilyotropic mesophaseschain interactionschromonic mesophasesform lyotropiccrystal phaseCrystalline Phasesathermal theorieschromonic mesogendye moleculestetrahydrofuran solvateChromonic mesogenssolution structurecompound crystallizespairwise interactionsLiquid Crystalquantum chemical computationscrystal structure showpoint groupdiad axisguanidinium saltÅ.Chromonic Mesogen Sunset YellowThe azo dyevolume fractioncrystal structurereckoning propertiesspace group Pnnanematic transitionStructural Correspondencesolution stereochemistrycolligative propertiessunset