Theoretical study of excited-state proton transfer of 2,7-diazaindole·(H2O)2 cluster via hydrogen bonding dynamics

Abstract

A new chromophore, 2,7-diazaindole (2,7-DAI), has been designed to surpass the limitation of 7-azaindole (7AI). It exhibits remarkable water catalyzed proton-transfer properties. Excited-state proton transfer (ESPT) has been investigated based on the time-dependent density functional theory method. The calculated vertical excitation energies in the S₀ and S₁ states agree well with the experimental values. Proton transfer couples with hydrogen-bonding dynamics between the 2,7-diazaindole and the surrounding water molecules. Hydrogen bond strengthening has been testified in the S₁ state based on a comparison of primary bond lengths and hydrogen bond energy that is involved in the intermolecular hydrogen bond between the S₀ and S₁ states. Frontier molecular further suggest that the electron density changes between the ground and excited states serve as basic driving forces for proton transfer. We determined the potential-energy curves of the S₀ and S₁ states to characterize the ESPT process. This work explains that the ESPT process for 2,7-DAI·(H₂O)₂ clusters at the molecular level, and highlights the importance of hydrogen bonding in ESPT.

Keywords: Excited state proton transfer; Hydrogen-bonding; Potential energy curves; Time-dependent density functional theory.