Excited state proton transfer in indole-2-carboxylic acid and indole-5-carboxylic acid

Intramolecular excited state proton transfer in indole-2-carboxylic acid (I2C) and indore-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 beta-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 beta-CD cavity intramolecular proton transfer in the excited state could be observed again. The excited state proton transfer equilibrium constant was calculated from a fluorescence band shape analysis, and its large solvent dependence arises primarily from local solute-solvent interaction. As the instrument time resolution was 500 ps, only monoexponential fluorescence decay could be observed for all excitation wavelengths and this showed that the proton transfer (rise time) was considerably faster than fluorescence decay. Structural change and large dipole moment in the excited state as revealed from quantum chemical calculations with AM1 Hamiltonian point to the tendency of proton transfer in first excited singlet state and also hindrance of that in the ground state.