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 lour 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 295 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-pi interaction to reduce the T 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 alpha- and beta-cyclodextrins.