Structural Correspondence of Solution, Liquid Crystal, and Crystalline Phases of the Chromonic Mesogen Sunset Yellow

Abstract

The 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 Interfac. 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 C s , 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 evaluated proposals that stacking faults and branching pointslower 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.

Effect of Crowding Agent Polyethylene Glycol on Lyotropic Chromonic Liquid Crystal Phases of Disodium Cromoglycate

“…They are also evident in numerical simulations by Sidky and Whitmer [38]. Finally, NMR data and numerical simulations by Xiao et al [39] demonstrated that similar shift junctions can be observed in another LCLC Sunset Yellow; the authors concluded that the shift junctions lead to the underestimated length of the chromonic aggregates by X-ray scattering; Y-junctions were found to be of a transient nature, devolving in pairs of disconnected stacks.…”

Where Is the Most Hydrophobic Region? Benzopurpurine Self-Assembly at the Calcite–Water Interface

“…It is well known that organic dyes in aqueous solution tend to form stacks at high concentration, 42,57−60 and BPP has been reported to form aggregates as long as several micrometers. 42 As per our previous work on the Sunset Yellow dye, 61 we calculated the free energy to separate two BPP molecules in water using metadynamics simulations. 54 The computed binding free energy is −31.5 or −35.5 kJ/mol ( Figure S1), depending on the choice of collective variable, which is similar to the value for Sunset Yellow (−28 kJ/mol 61 ) The Journal of Physical Chemistry C Article and an experimental estimate (∼−26 kJ/mol 42 ).…”

Elasticity, viscosity, and orientational fluctuations of a lyotropic chromonic nematic liquid crystal disodium cromoglycate

“…60 Interestingly, recent structural studies of sunset yellow support the existence of stacking features in the form of lateral shis. 64 The stacking defects illustrated in Fig. 7 are polar.…”