Hybrid QM/QM Simulations of Excited-State Intramolecular Proton Transfer in the Molecular Crystal 7-(2-Pyridyl)-indole

A subtractive implementation of the QM/QM hybrid method for the description of photochemical reactions occurring in molecular crystals is presented and tested by applying it in a simulation study of the ultrafast intramolecular excited-state proton transfer reaction in the crystal form of 7-(2-pyridyl)-indole, an organic compound featuring an intramolecular hydrogen bond within a six-membered ring. By propagating molecular dynamics on the excited-state potential energy surface, a mean proton transfer time was calculated as 80 fs. The reaction mechanism is discussed in terms of three-dimensional reaction coordinate diagrams. Proton transfer was found to be barrierless and to be strongly coupled to vibrational modes of the photoexcited molecule that modulate the proton donor–acceptor distance. Some 300 fs after the initial photoexcitation, the excited state molecule reached an S1/S0 conical intersection through the mutual twist of the pyridyl and indolyl moieties.