Chromonic self-assemblies in a series of dialkyl-thiacarbocyanine dyes and generalization of a facile route for the synthesis of fluorescent nanostructured silica fibers

In the search for new chromonic liquid crystals, we have explored the self-assembly behavior in water of a series of high-purity dialkylthiacarbocyanine dyes with different alkyl chain lengths and acetate as counterion. The dimer model was used to fit the UV–vis spectra and estimate the dissociation energy between molecules, which increases with the alkyl chain length suggesting that not only aromatic π–π interactions but also entropic/steric effects contribute to the aggregation process. NMR spectra suggest the existence of aromatic stacking interactions within the aggregates. At high concentrations, the dyes form nematic and hexagonal chromonic liquid crystals (CLC) in water within a limited range of alkyl chain lengths (ethyl to butyl); for longer lengths (pentyl) no liquid crystals were observed because of insufficient dye solubility. From small angle X-ray scattering patterns, it can be inferred that dye aggregates have cylindrical morphology with a multimolecular cross-section. The dye aggregates template the formation of silica nanofibers synthetized via sol–gel method in alkaline media. After removing excess dye, the silica/dye nanofibers showed high fluorescence emission with superb photochemical stability. The present report demonstrates a generalized route for the wet synthesis of nanostructured silica nanofibers with tunable optical properties.