Fluorescent Switch Behavior of 1, 2-Bis(4-pyridyl) Ethylene Controlled by pH in Aqueous Solution

With the decrease of pH value from 8.45 to −1.0, the UV-Vis absorption and fluorescent spectra of 1,2-bis(4-pyridyl) ethylene(BPE) took on the same changing trend at four different successive pH stages: 8.45—7.20, 7.20—5.62, 5.62—2.60, and 2.60—1.0, namely, no change, decrease, increase, and decrease again. Among these, in a range of 7.20—5.62, the fluorescence wavelength blueshifted from 418 to 359 nm, but the UV-Vis absorption wavelength, in contrast, redshifted from 285 to 298 nm. The fluorescence intensity of BPE had a drop even to quench upon a decline in the pH value from 2.60 to −1.0 probably owing to its cation-π interaction to reduce the π electron cloud density of BPE. Two dissociation constants, pK_a1(4.30±0.01) and pK_a2(5.65±0.04), were obtained based on fluorescence data. The changes of fluorescence spectra indicate that BPE has “off-on-off” switch behavior. The fluorescent spectra of BPE were nearly independent on the presence of α- and β-cyclodextrins.