Structure and hydrophilicity of azo‐dye‐derived rotaxane: density functional theory approach

We investigated the most probable molecular structure, energy, and corresponding properties, including the solvation energy and binding free energy in solution, for non‐encapsulated azo dyes using quantum mechanical density functional theory (DFT). The structures of non‐encapsulated azo dyes with six conformations were optimised to find the most stable conformation with the lowest energy, but the energy differences among the conformations were within ~2.2 kcal mol⁻¹. The LUMO–HOMO gaps were also similar, meaning that their interaction with light would be similar. The energies of alpha‐cyclodextrin (α‐CD) encircling the phenyl ring fragments were significantly higher than other conformations, because α‐CD tends preferably to encircle azo fragments to create a thermodynamically stable form. From solvation free energy calculations, the hydrophobic non‐encapsulated azo dye and hydrophilic α‐CD showed reasonable values for solvation free energy with respect to dimethyl sulphoxide (DMSO) and water: the former has more solvation in DMSO, while the latter has more solvation in water. Rotaxane formation is thermodynamically feasible in water (−3.06 kcal mol⁻¹) but not in DMSO (25.56 kcal mol⁻¹) according to calculation of binding free energy in solution state.