Kinetics and mechanism of ultrafast adiabatic intermolecular and intramolecular proton-transfer reactions of a protonated trimethylpyrichrominium ion in its fluorescent state

Abstract

The fluorescence behaviour of acidic solutions of a doubly charged protonated trimethylpyrichrominium ion (11-HA²⁺) has been studied as a function of time, acid strength and temperature. The compound exhibits triple fluorescence arising from emission by the ion (11-HA²⁺)* protonated on the 11-position, the ion (1-HA²⁺)* protonated on the 1-position and the deprotonated ion (A⁺)*. Both intermolecular and pseudo-intramolecular proton transfer from the electronically excited ion (11-HA²⁺)* are observed. The kinetics of these proton-transfer processes has been studied by following the evolution of the fluorescence with a streak camera. In the range 0 ⩽ pH ⩽ 4 only intermolecular proton transfer from (11-HA²⁺)* to proton acceptor sites (H₂O) in the solvent takes place, resulting in (A⁺)*. This leads to four-level laser action, owing to the creation of population inversion between A⁺ and (A⁺)*. At pH < 0 an equilibrium between (A⁺)* and (1-HA²⁺)* is established. What was previously thought to be intramolecular proton transfer from the 11 to the 1-position in the excited ion is in fact a two-step pseudo-intramolecular process involving deprotonation and reprotonation. The deprotonation step is temperature dependent and exhibits a temperature-independent kinetic isotope effect k_H/k_D= 1.7. It is shown that the temperature independence of k_H/k_D may be understood on the basis of thermally unassisted proton transfer in a well defined proton donor–acceptor complex.