The origin of ultrafast proton transfer: Multidimensional wave packet motion vs. tunneling

We investigate the reaction kinetics of ultrafast excited state intramolecular proton transfer (ESIPT) and discuss the possible origins of the process: tunneling of the reactive proton, vibrationally enhanced tunneling, and multidimensional wave packet dynamics of the entire system. Comparison of the measured kinetics for the protonated and the deuterated form of 2-(2′-hydroxyphenyl)benzothiazole (HBT) to numerical simulations allows us to ascribe the characteristic 50 fs time found for the ESIPT solely to a ballistic wave packet motion along skeletal coordinates that mainly affect the donor acceptor distance. Tunneling is not found to be decisive.

Femtosecond studies on deuterated 2-(2′-hydroxyphenyl)benzothiazole exclude tunneling contributions in ultrafast excited state intramolecular proton transfer.73Research highlights► Excited state ultrafast proton transfer governed by multidimensional wave packet motion. ► Tunneling contribution insignificant for dynamics. ► Comparison of proton and deuteron transfer allows unambiguous analysis. ► Proton transfer modeling compared to 20 fs-pump-probe measurements.