Investigation of the intramolecular hydrogen bonding interactions and excited state proton transfer mechanism for both Br-BTN and CN-BTN systems

In the present work, two novel Br-BTN and CN-BTN compounds have been investigated theoretically. We in-depth explore the excited state hydrogen bonding interactions and the excited state intramolecular proton transfer (ESIPT) behaviors for the Br-BTN and CN-BTN system. We firstly verify the formation of hydrogen bond effects of O–H⋯N based on reduced density gradient (RDG) versus sign(λ(2))ρ. The simulated primary bond lengths and bond angles as well as infrared (IR) vibrational spectra reveal that the hydrogen bond O–H⋯N should be strengthened in the excited state. Combining the frontier molecular orbital (MO) investigations, we infer that the charge transfer phenomenon (from HOMO to LUMO) around hydrogen bonding moieties reveals the tendency of ESIPT reaction. Particularly, the increased electronic densities around proton acceptor atoms facilitate attracting a hydrogen proton, which plays a decisive role in opening the ESIPT reaction. Via constructing potential energy curves in both S(0) and S(1) states, the ultrafast ESIPT process can be verified which explains previous experimental characteristics. Furthermore, via searching the transition state (TS) structure and constructing the intrinsic reaction coordinate (IRC) reaction path, we check and confirm the ESIPT mechanism for both Br-BTN and CN-BTN systems. We sincerely hope that our theoretical work could guide novel applications based on Br-BTN and CN-BTN compounds in future.