Influence of the Cucurbit[7]uril Structure on Photophysical Properties of the Encapsulated Styryl Dye

The interaction between the styryl dye 4-{(E)-2-[4-(dimethylamino)phenyl]vinyl}-1-methylpyridinium iodide (DASPI) and cucurbit[7]uril (CB[7]) in aqueous solution was studied by optical spectroscopy methods. Cucurbit[n]urils are cavitands composed of n glycoluril units linked by methylene bridges [1]. Due to their negatively charged portals, cucurbiturils can form complexes with cationic styryl dyes of suitable size. This complexation alters the dye photophysical properties, such as fluorescence in the case of 1:1 complexes (binding constant logK₁ = 5.5) [2].

It was previously found that the formation of 1:2 inclusion complexes leads to a red shift in the absorption band to around 330 nm and the appearance of an additional fluorescence band at 450 nm (binding constant logK₂ = 5.1) [2]. Such changes were previously attributed to protonation of the dye [3].

However, the direct measurements of pH show that dissolving 10⁻⁵ M CB[7] in water does not significantly change the acidity of the solution (pH = 5.67), so that the observed effects cannot be attributed to dye protonation. Indeed, it can be shown [2], that the molar fraction of the protonated dye ≈1 only when pK_a > pH.

The experimentally determined value of pK_a=3.23 for DASPI at 23°C [2], so that significant protonation of the dye occurs only when pH≲3.5. Therefore, at the working pH of 5.67, protonation contributes is neglected.

To explain the effect, we hypothesize that the effect arises from the influence of the electrostatic field generated by the negatively charged portals of cucurbit[7]uril on the conjugated π-electron system of the dye.