FTIR and fluorescence studies on the ground and excited state hydrogen-bonding interactions between 1-methylindole and water in water-triethylamine mixtures

The ground and excited state π-hydrogen-bonding interactions between 1-methylindole, MI, and water have been investigated in water-triethylamine, water-TEA, mixtures. FTIR measurements performed on the OH stretching bands of the water-TEA clusters show that, upon MI addition, the typical bands of the water-TEA system at 3348 cm−1, 3440 cm−1, 3545 cm−1 and 3682 cm−1 diminish, whereas two new absorption bands at 3316 cm−1 and 3654 cm−1 grow up. These spectral changes have been rationalised assuming the formation of only one 1:1 water-MI complex, in which the dangling protons in the water-TEA clusters are hydrogen bonded to the π-cloud of the MI aromatic ring. Steady state and time resolved fluorescence measurements provide additional proofs on the ground state formation of a fluorescent OH ⋯ π hydrogen bonded complex. The relevance that the present and the previously reported results could have on the indole ring photophysics is discussed.