Influence of hydrogen bonding in the ground and the excited states of the isomers of the β-carboline anhydrobase (N2-methyl-9H-pyrido[3,4-b]indole) in aprotic solvents

The ground and excited state hydrogen bonding interactions between N2-methyl-9H-pyrido[3,4-b]indole, BCA, and 1,1,1,3,3,3-hexafluoropropan-2-ol, HFIP, are comparatively studied in the aprotic solvents cyclohexane and toluene by absorption, steady state and time resolved fluorescence measurements. The different photophysical behaviours of the BCA-HFIP hydrogen bond complexes in these solvents definitively confirm the existence of two ground state BCA isomers. As previously proposed [A. Sánchez-Coronilla, C. Carmona, M.A. Muñoz, M. Balón, Chem. Phys. 327 (2006) 70.] we assume quinoid, Q, and zwitterionic, Z, structures for these isomers. Upon excitation, the hydrogen bond adducts of each isomer give dual fluorescence emitting from their locally excited states, LE, and from their intramolecular charge transfer states, ICT. In the hydrogen bond adducts of the Q form, the ICT process is favoured while it is disfavoured for the corresponding adducts of the Z form. The implication that these results could have on the current mechanistic interpretation of the excited state intramolecular proton transfer and phototautomerism of the betacarbolines is discussed.