This paper identified a new excited-state proton transfer (ESPT) mechanism for 2,6-diazaindoles (2,6-DAI) in aqueous (HO) solution based on time-dependent density functional theory. The calculated results show that the excited-state three proton transfer reaction cannot occur because the 2,6-DAI with two water molecules do not form hydrogen bond wires; this finding was different from those reported in previous experiments (Chung et al. J. Am. Chem. Soc. 2017, 139, 6396-6402). 2,6-DAI with three water molecules form 2,6-DAI·(HO) clusters, whereas 2,6-DAI with four water molecules form 2,6-DAI·(HO) cluster. These clusters participate in the ESPT reaction. To determine the ESPT mechanism of 2,6-DAI·(HO) and 2,6-DAI·(HO) clusters, we constructed the potential energy curves of S and S states. The results confirmed the simultaneous presence of both 2,6-DAI·(HO) and 2,6-DAI·(HO) clusters and only one proton transfer pathway. By calculating the transition states of 2,6-DAI·(HO) and 2,6-DAI·(HO) clusters, we found that the ESPT reaction is a consistent mechanism. Our work investigated the number of water molecules involved in the ESPT and paved the way to further study the intermolecular hydrogen bonding interactions in the biological field.

Our theoretical results show that the fluorescence quenching of DFAN is caused by PET mechanism and the large Stokes shift fluorescence emission of DFAH is not due to the ESIPT mechanism. Due to the absence of PET process, enol structure emits strong fluorescence, which is inconsistent with the fluorescence mechanism in the experiment.

As the dielectric constant decreases, the ESIPT reaction occurs more easily and TICT is good at emitting double fluorescence.