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Hydrogen‐bonding and Proton Transfer Interactions between Harmane and Trifluoroethanol in the Ground and Excited Singlet States

A study of the hydrogen‐bonding and proton transfer reactions of the ground and excited states of harmane (1‐methyl‐9H‐pyrido/3,4‐b/indole) and its N9‐methyl derivative with 2,2,2‐trifluoroethanol in cyclohexane is reported. Spectral measurements (UV–visible, Fourier trans‐form IR, steady‐state and time‐resolved fluorescence) show the formation of fluorescent ground‐state hydrogen‐bonded complexes. The results have been interpreted assuming a tautomeric equilibrium between a 1:1 hydrogen‐bonded complex and its 1:2 proton transfer tautomer (hydrogen‐bonding ion pair). Upon excitation to its singlet excited state, the proton transfer tautomer of harmane reacts with an additional 2,2,2‐trifluoroethanol molecule to give a zwitterionic exciplex, which fluoresces at longer wavelength.