Concerted or Stepwise Mechanism? CASPT2 and LC-TDDFT Study of the Excited-State Double Proton Transfer in the 7-Azaindole Dimer

Excited-state double proton transfer (ESDPT) in the 7-azaindole dimer is investigated using the complete active space second-order perturbation theory (CASPT2) method and the long-range corrected time-dependent density functional theory (LC-TDDFT) method. These methods are employed for geometry optimizations as well as single-point energy calculations of the excited-state potential energy profiles along the reaction paths. It is shown that three main reaction routes involving double proton transfer exist. In the first route, the ESDPT reaction takes place in the locally excited state through a single transition state following the concerted mechanism in which each proton-transfer process occurs simultaneously without forming any stable zwitterionic intermediate. The concerted ESDPT reaction is found to proceed asynchronously in C-s symmetry rather than synchronously in C-2h symmetry. In the second and third routes, on the other hand, the ESDPT reaction takes place following the stepwise mechanism in which each proton-transfer process occurs sequentially forming a neutral intermediate in the charge-transfer state. The calculated energy profiles of the three routes exhibit a lower barrier in the first route than in the other routes, suggesting that the ESDPT in the gas phase is likely to follow the asynchronous concerted mechanism at the lowest excitation energy.