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

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• 作者：Wenchang Xiao ; Chunhua Hu ; Damien J. Carter ; Shane Nichols ; Michael D. Ward ; Paolo Raiteri ; Andrew L. Rohl ; Bart Kahr
• 刊名：Crystal Growth & Design
• 出版年：2014
• 出版时间：August 6, 2014
• 年：2014
• 卷：14
• 期：8
• 页码：4166-4176
• 全文大小：581K
• ISSN：1528-7505

文摘

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 鈥減ackage of properties distinct in almost every aspect鈥?(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) 脜, b = 20.048(1) 脜, c = 21.466(2) 脜. 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 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.