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.