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Assembly structure and free energy change of a chromonic liquid crystal formed by a perylene dye
摘要： The nematic (chromonic N) and isotropic phases formed by aqueous solutions of a perylene dye are investigated by x-ray diffraction and visible light absorption spectroscopy. X-ray diffraction shows that the molecules assemble into columns one molecule wide. The change in the visible absorption spectrum with concentration indicates that the molecules stack in a more or less isodesmic manner and that the free energy change associated with the addition or removal of a single molecule from a stack is 16–20 kBT. This range is appreciably higher than those of any previously-measured simple chromonic systems, i.e. those with single molecule-wide columns. It is suggested that the large free energy change promotes the formation of extremely long columns of molecules, and in turn leads to the low concentration at which mesophase formation takes place.
关键词： chromonic,isodesmic,scission energy,perylene dye,assembly structure,lyotropic chromonic,Liquid crystals
更新于2025-11-19 16:56:35
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Understanding the Role of Aggregation in the Broad Absorption Bands of Eumelanin
摘要： In this work, we investigate the relationship between the complex hierarchical assembly structure of eumelanin, its characteristic broad absorption band, and the highly unusual nonlinear dynamics revealed by pump-probe or transient absorption microscopy. Melanin-like nanoparticles (MelNPs), generated by spontaneous oxidation of dopamine, were created with uniform but adjustable size distributions, and kinetically controlled oxidation was probed with a wide range of characterization methods. This lets us explore the broad absorption bands of eumelanin models at different assembly levels, such as small subunit fractions (single monomeric and oligomeric units and small oligomer stacks), stacked oligomer fractions (protomolecules) and large-scale aggregates of protomolecules (parental particles). Both the absorption and pump-probe dynamics are very sensitive to these structural differences, or to the size of intact particles (a surprising result for an organic polymer). We show that the geometric packing order of protomolecules in long-range aggregation are key secondary interactions to extend the absorption band of eumelanin to the low energy spectrum, and produce drastic changes in the transient absorption spectrum.
关键词： broad absorption band,aggregation,geometric packing order,hierarchical assembly structure,Eumelanin
更新于2025-09-10 09:29:36