Semiclassical dynamics simulations of proton transfer
Many molecules undergo excited-state intramolecular proton transfer (ESIPT) as a result of an electronic excitation. This is the case, for instance, of 10-hydroxybenzo[h]quinoline (HBQ) [35], 2-(2'-hydroxyphenyl)benzothiazole (HBT) [40], and [2,2 '-Bipyridyl]-3,3 '-diol (BP(OH)2) [44].
In these three cases the ESIPT dynamics is ultrafast and takes place in less than 100 fs.
We have performed excited-state dynamics simulations for the investigation of ESIPT processes. These simulations, carried out at TDDFT and CC2 levels, have helped to understand the structural deformations that trigger the proton transfer [35] (see movie below, left).
Especifically in the case of HBT in gas phase, the dynamics simulations also predicted the occurrence of cis-trans isomerization after the ESIPT [40] (see movie below, right), followed by internal conversion. This prediction has been confirmed by experiments also reported in Ref.[40].
|
|
|
|
The graph shows the time evolution of the NO distance during the ESIPT. |
Isomerization after ESIPT. The gaphs show the time evolution of the potential energy (left) and of the torsional angle (right). |
Dynamics simulations together with the experimental results from E. Riedle in the LMU (Munich), have allowed us to achieve a very complete view of the proton transfer cycle, including the regeneration of the initial molecule.
In the case of BP(OH)2 double proton transfer may occur. There is an ongoing discussion in the literature about whether the two transfers are simultaneous or sequential. The simulations clearly show that what has been experimentally determined as concerted transfer is in fact a combination of two sequential proton transfers separated by a small delay below the present experimental resolutions [44].
A short review on these topics can be found in Ref.[39].
