A Spectroscopic Investigation of Halogen Bonding for the Adsorption Process of Grafted Chitosan With Fluorescein Versus Rose Bengal Dye Systems

ABSTRACT Chitosan was modified by grafting of aniline via cross‐linking with urea and confirmed by spectroscopic (FT‐IR, 1 H‐NMR) and TGA results. Adsorption studies of chitosan and its grafted form were carried out in single‐component solutions of fluorescein (FL) and rose bengal (RB), along with their binary (FL+RB) mixtures. In single‐component dye solutions, grafted chitosan revealed enhanced uptake of FL and preferential uptake of RB. Dye adsorption selectivity employed a binary dye mixture (50:50 molar ratio), where nearly complete adsorption of RB, whereas FL remained in the aqueous phase. RB dye adsorption selectivity was accounted for by halogen bonding. The adsorption mechanism was investigated by 2D‐NOESY and fluorescence spectroscopy, which reveal the role of π–π interactions between the adsorbent and dyes. The role of halogen bonding between the amine, aniline rings, and carbonyl functional groups of the modified chitosan with RB was corroborated by XPS results. Halogen bonding was supported by 2D [ 1 H‐ 15 N] HMBC of glucosamine (as the chitosan monomer model) and Nʺ‐phenyl‐N‐β‐ d ‐glucopyranosyl urea (as the grafted chitosan monomer model), which was studied with and without RB, along with IR spectroscopy of RB supports the key role of halogen bonding in the adsorption mechanism.