Excited-State Proton Transfer Reaction of Pyranine in Aqueous Sugar and Alcohol Solutions Investigated by Fluorescence Spectroscopy

摘要

The fluorescence emission behaviors of excited pyranine (PyOH*) and deprotonated pyranine (PyO-*) were studied to explore the effects of sugar or alcohol on the excited-state proton transfer (ESPT) reaction of pyranine in aqueous solutions. The fluorescence intensity ratio, Q between the two emission bands of pyranine varies significantly with the type and concentration of sugar or alcohol in aqueous solution. The results clearly show that a higher Q value (more PyOH* and less PyO-*) reflects a lower ESPT rate due to less free water molecules for solvation of PyO-* and H+. Higher Q values were observed in sugar solutions while lower values in alcohol solutions. Larger alcohols yielded higher Q values and the values for polyols (ethylene glycol and glycerin) were between those of methanol and ethanol, revealing that the hydroxyl groups in polyols form intramolecular hydrogen-bond. The location of the hydroxyl group was also found to affect the intensity ratio, Q. The conductivities of 0.01 M HCl in aqueous sugar and alcohol solutions are consistent with the Q values observed, and the inverse of Q show a quadratic relation with conductivity. The enthalpy and entropy changes for the deprotonation reaction of PyOH* in fructose and ethanol aqueous solutions were also determined, assuming that the deprotonation reaction was an equilibrated process. The higher H in fructose solution reveal that deprotonation of PyOH* in sugar solution is energetically less favored than in ethanol solution because solvation of fructose requires more water molecules.