Modulation of Protolytic Equilibrium of Bichromophoric Coumarin‐30 Dye with Cucurbit[8]uril Encapsulation

Abstract

Host‐guest interaction of macrocyclic receptor, cucurbit[8]uril (CB8), with neutral bichromophoric coumarin 30 (C30) dye has been investigated using various spectroscopic tools. Absorption and fluorescence emission results along with 1H NMR data have demonstrated the existence of host‐guest complex of C30 dye with CB8 host in a 1:2 and 3:2 (CB8:C30) stoichiometries. Comparative studies with the photophysical aspects of the protonated C30 dye (at pH 3) confirmed that interaction of neutral C30 dye (at pH 7) with CB8 has led to the host assisted protonation of C30 in the CB8:C30 complex, registering an upward supramolecular pKa shift of about 3 units. The 3:2 stoichiometric complex exhibits strong excimer emission and is dictated by the structural features of C30. Due to this complexation not only we accomplish modulation in the photophysics of the dye, but also it enhances the aqueous solubility of C30, especially at pH 7. Taking advantage of CB8 assisted tunable pKa shift, currently we are pursuing the prospects of using photostable coumarin dyes for cellular imaging studies as well as their applicability as aqueous dye laser systems.

Cucurbit[n]uril Host‐Guest Complexes of Acids, Photoacids, and Super Photoacids

“…This work has been extended to the related coumarin 7 ( 35 ) and coumarin 30 ( 36 ) dyes with CB[7] and p K a shifts of 4.6 and 3.0, respectively, with 2 : 1 stoichiometries, are observed for the protonated dyes, as also seen with coumarin 6 ( 34 ) . With coumarin 30, CB[8] forms 1 : 2 and 3 : 2 (CB[8] : 36 ) stoichiometries, with an increase in the p K a value of 3.2 units in the presence of excess CB[8] …”