Ultrafast excited-state proton transfer processes: Energy surfaces and on-the-fly dynamics simulations

The excited-state intramolecular proton transfer (ESIPT) is reviewed for several benchmark systems [o-hydroxybenzaldehyde (OHBA), salicylic acid and 2-(2′-hydroxyphenyl)-benzothiazole (HBT)] in order to verify the applicability of the time-dependent density functional theory (TDDFT) and the resolution-of-the-identity approximate second-order coupled cluster (RI-CC2) methods. It was found that these approaches are very well suited for the description of ESIPT processes. A comparative investigation of previous and new excited-state dynamics simulations is performed for HBT, 10-hydroxybenzo[h] quinoline (HBQ), and [2,2′-bipyridyl]-3,3′-diol (BP(OH) ₂). The time scale for the ESIPT process in these systems ranges in the time interval of 30-40 fs for HBT and HBQ and amounts to about 10 fs for the first proton transfer step in BP(OH)₂. The dynamics simulations also show that the proton transfer in HBT is strongly supported by skeletal modes and the proton plays a rather passive role, whereas in HBQ a semipassive mechanism is found due to its increased rigidity in comparison to HBT. The special role of the double proton transfer in BP(OH)₂ is discussed as well.