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

Abstract

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 ΔH 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.