Encapsulation and Stabilization of a Donor–Acceptor Stenhouse Adduct Isomer in Water Inside the Blue Box: A Combined Experimental and Theoretical Approach

We synthesized two types of donor–acceptor Stenhouse adducts (DASAs), a new type of photochromic molecules showing dual color in two different isomeric forms in solution phase, using Meldrum acid (DASA-Mel) and barbituric acid (DASA-Bar), along with a naphthalimide derivative to obtain interesting fluorescence properties. DASA-Mel was found to have fast photochromic conversion in comparison to DASA-Bar, evident from ultraviolet–visible (UV–vis) and fluorescence spectroscopic studies. The colored form of DASA-Mel was encapsulated inside the water-soluble Stoddart’s blue box and became soluble in water much faster than DASA-Bar. Interestingly, the competitive encapsulation experiment showed that DASA-Mel was selectively encapsulated inside the blue box in water whereas DASA-Bar was mostly separated out from the solution after centrifugation, and this phenomenon was confirmed by ¹H and DOSY NMR and mass spectroscopies. Moreover, we found through density functional theory (DFT) optimization that the open form of DASA-Mel was more stable during the encapsulation reaction in a water medium in comparison to DASA-Bar. The calculated binding energies of encapsulated DASA-Mel and DASA-Bar are −10.2 and −9.9 kcal/mol, respectively, clearly showing that the former is more stable by 0.3 kcal. Consequently, the organic macrocycle selectively separating one kind of DASA from a mixture by encapsulation in water is reported for the first time with experimental and theoretical support in the literature.