Theoretical insights into ultrafast excited state proton transfer coupled with twisted intramolecular charge transfer mechanism in 7-(2ʹ-pyridyl)indole: Effect of hydrogen bonding

Abstract

The ultrafast proton transfer (PT) process in the excited state of 7-(2′-pyridyl)indole (abbreviated as 7PyIn) has been elucidated by on-the-fly dynamics simulations using the algebraic-diagrammatic construction scheme of second order (ADC(2)) method under the resolution of identity (RI) approximation. The tautomers of 7PyIn alone and associated with an acetonitrile molecule in the 7PyIn-CH3CN complex are most likely found after the excited state intramolecular proton transfer (ESIntraPT) within an ultrafast time scale between 40 and 60 fs. While a methanol molecule in the complex of 7PyIn-CH3OH could establish both ESIntraPT and excited-state intermolecular proton transfer (ESInterPT). ESInterPT of 7PyIn-CH3OH possibly occurs through a concerted process via the intermolecular hydrogen bonding bridge of the methanol molecule almost twice faster than that of ESIntraPT. This slower circumstance of the ESIntraPT process found in 7PyIn-CH3OH may be caused by the competition between formations of the intramolecular and intermolecular hydrogen bonds between 7PyIn and methanol. In addition, after the accomplishment of excited state proton transfer (ESPT), the initiation of conical intersection could be taking place by the structural twist of 7PyIn. The geometries, energies, proton probabilities, and provided mechanisms via the dynamics simulations of 7PyIn and its solvent clusters exhibiting ESIntraPT and ESInterPT are explored to provide the dynamic pictures of the ESPT processes at the molecular level.