Solubilization of 1,4,5,8-naphthalenediimides and 1,8-naphthalimides through the formation of novel host-guest complexes with α-cyclodextrin

Solubility studies were performed in aqueous solutions of 1,8-naphthalimides and 1,4,5,8-naphthalenediimides (Scheme 1). A large solubility increase was found for N-butyl-1,8-naphthalimide (MBN) and N,N'-dibutyl-1,4,5,8-naphthalenediimide (DBN) in the presence of a-cyclodextrin (alpha-CD), indicating the formation of inclusion complexes. The presence of the N-butyl group is required for complex formation, leading to the conclusion that the butyl groups are the binding sites for alpha-CD. Accordingly, the analysis of solubility isotherms for the systems MBN/alpha-CD and DBN/alpha-CD showed the presence of 1:1 complexes for the former and of both 1:1 and 1:2 complexes for the latter. Association constants for the two systems were estimated, giving K = 470 M-1 for the MBN/alpha-CD complex and K-11 = 1316 M-1 and Kit = 329 M-1 for the stepwise association constants in the DBN/alpha-CD system. MBN undergoes hydrolysis in water, but the reaction is inhibited upon complexation with alpha-CD. The remarkable solubilization in water and stabilization toward hydrolysis makes these novel complexes of imides and diimides with alpha-CD potentially useful in the pharmaceutical applications known for these imides, as well as in the preparation of new materials, like polyimide-based polyrotaxanes.