Complexes of dihexyl 2,6-naphthalenedicarboxylate with α- and β-cyclodextrins: Fluorescence and molecular modelling

Fluorescence techniques and molecular mechanics (MM) were used to study the inclusion complexes of dihexyl 2,6-naphthalenedicarboxylate (DHN) with α- and β-cyclodextrins (CDs). Stoichiometries, formation constants and the changes of enthalpy and entropy upon inclusion were also obtained by measuring the variation, with CD concentration and temperature, of the relative intensity of two peaks shown in all steady-state emission spectra. Results agree with the formation of 1:1 αCD:DHN and 2:1 βCD:DHN stoichiometry complexes. MM calculations in the presence of water were employed to study the formation of different stoichiometry complexes of DHN with both α- and βCDs. For the most stable structures of 1:1 complexes a large portion of DHN is exposed to the solvent making it possible for another CD to approach. Driving forces for 2:1 inclusion processes may be dominated by non-bonded van der Waals CD–DHN interactions. However, due to the different structures of the 2:1 complexes, an important electrostatic interaction appears between both βCDs in the βCD2:DHN complex. This interaction does not exist between αCDs in the αCD2:DHN complex. Most of this contribution is due to the intermolecular hydrogen bonding formation between secondary hydroxyl groups of both βCDs. The average of the lifetime measurements 〈τ〉 with the CD added also supports the formation of such stoichiometry complexes.