Temperature effect on excited-state proton transfer reactions of β-carboline in different acetic-acid mixtures

The temperature dependence of the conjugated dual hydrogen bonding between β-carboline and guest molecules possessing a bifunctional hydrogen-bonding group is described in terms of their electrostatic interactions. In media with a high dielectric constant value, spectral analysis indicates the existence of neutral and cation species. When the solvent possesses a low dielectric constant, the system becomes much more complex, detecting up to five different species in the S1 state in dichloromethane–acetic acid mixtures. Surprisingly, in all mixtures a shift of the prototropic process toward the cation was observed upon cooling from 310 K up to 200 K.