Excited State Proton Transfer in Indole-2-carboxylic Acid and Indole-5-carboxylic Acid

Abstract

Intramolecular excited state proton transfer in indole-2-carboxylic acid (I2C) and indole-5-carboxylic acid (I5C) was investigated in various solvents in acidic, basic, and neutral media by steady state and time-resolved fluorescence spectroscopy. Hidden dual fluorescence of I2C in polar and nonpolar solvents and distinct dual fluorescence of I5C in nonpolar solvents and broad structureless fluorescence band in polar solvent (which is composed of two fluorescence bands) are assigned to be arising out from Franck−Condon excited state and from proton transferred excited state, i.e., from zwitterionic form, respectively. The modulation of proton transfer equilibrium constant in β-CD cavity has also been investigated for both the molecules. For I5C it has been observed that in protic solvents intramolecular proton transfer is blocked by H-bond formation with solvent molecules surrounding it but when it enters into the β-CD cavity intramolecular proton transfer in the excited state could be observed again. The...