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

The temperature dependence of the conjugated dual hydrogen bonding between beta-carboline and guest molecules possessing a bifunctional hydrogen-bondi ng group is described in terms of their electrostatic interactions. In medi a with a high dielectric constant value, spectral analysis indicates the ex istence of neutral and cation species. When the solvent possesses a low die lectric constant, the system becomes much more complex, detecting up to fiv e different species in the S-1 state in dichloromethane-acetic acid mixture s. Surprisingly, in all mixtures a shift of the prototropic process toward the cation was observed upon cooling from 310 K up to 200 K. (C) 1999 Elsev ier Science B.V. All rights reserved.