Induced Circular and Linear Dichroism of Achiral Cationic Dye Molecules Adsorbed on TEMPO-Oxidized Cellulose Nanofibers in Aqueous Suspensions

Chiroptical properties of cellulose nanofibers (CNFs), especially optical chirality-inducing abilities, are crucial subjects to be explored for the development of CNF as a new class of optical materials. This study investigated experimentally the induced circular dichroism (CD) of the positively charged achiral dye molecules thioflavin T (ThT), toluidine blue O (TBO), and methylene blue (MB), adsorbed on 2,2,6,6-tetrametylpiperidine (TEMPO)-oxidized CNFs in aqueous suspensions. The achiral dyes exhibited CD spectra of the sigmoidal positive exciton-coupling shape, suggesting the effect of the right-handed helicity of the fiber. The observed CD intensities were proportional to the concentrations of the bound dye molecules. The TBO-CNF systems could be used for a reproducible titration method of the total amount of carboxylate ions on the surface of CNF, which gave a value of 2.0 ± 0.3 mmol/g. The Langmuir isotherm was applicable to the adsorption equilibrium between CNFs and the dye molecules, though a stacking effect was suggested in ThT systems, and a dimerization effect was found in MB systems. Furthermore, it was discovered that sweeping the dye-adsorbed 0.2% CNF suspension by a Cu wire in a 1 mm optical cell induced the positive and negative linear dichroism (LD) spectra of the dye molecules, depending on the vertical and horizontal sweeping directions, respectively. The hydrodynamic orientation of CNFs by the "wire-sweeping" was confirmed by the measurement of the linear birefringence (LB) of CNFs. A comparison between the angular dependences of the LD of the dye and the LB of the CNF in an identical sample suggested that the averaged angle between the electron transition moment of the adsorbed dye molecule and the long axis of the fiber was between 45° and 135°.