Synergistic Effect of Intramolecular Charge Transfer toward Supramolecular pKa Shift in Cucurbit[7]uril Encapsulated Coumarin Dyes

This article presents the process and mechanism of supramolecular pKa shift in two bichromophoric coumarin laser dyes, namely, coumarin 7 (C7), (ΔpKa = 4.6) and coumarin 30 (C30), (ΔpKa = 3.0), achieved by introducing a synthetic macrocyclic receptor, cucurbit[7]uril (CB7), in aqueous media. The intramolecular charge transfer, from the diethylamino coumarin moiety toward the benzimidazolyl moiety and its protonation, even at pH ∼8, is facilitated by the interaction of the cucurbituril host in a 2:1 (CB7/dye) stoichiometric ratio. The CB7 macrocycle interacts with C7/C30 dyes in a stepwise manner with binding constants of the order of K1 ≅105 M–1, K2 ≅104 M–1 for both C7 and C30 dyes. This study underlines a structure–property relationship to explain the host induced changes in the stereoelectronic distributions in the guest dyes supporting the supramolecular pKa shifts and is appropriately established by both experimental and theoretical considerations. On the other hand, the increased solubility (>250 times) and enhancement in fluorescence intensity (>13-fold) of the coumarin dyes in the presence of CB7 also find applications for developing aqueous dye laser systems where this supramolecular strategy will largely suppress the disadvantages of low solubility, aggregation, lower emission, or low stability of the dye in aqueous medium.