Theoretical study on the mechanism for the excited-state double proton transfer process of an asymmetric Schiff base ligand

Excited-state double proton transfer (ESDPT) in the 1-[(2-hydroxy-3-methoxy-benzylidene)-hydrazonomethyl]-naphthalen-2-ol (HYDRAVH(2)) ligand was studied by the density functional theory and time-dependent density functional theory method. The analysis of frontier molecular orbitals, infrared spectra, and non-covalent interactions have cross-validated that the asymmetric structure has an influence on the proton transfer, which makes the proton transfer ability of the two hydrogen protons different. The potential energy surfaces in both S-0 and S-1 states were scanned with varying O-H bond lengths. The results of potential energy surface analysis adequately proved that the HYDRAVH(2) can undergo the ESDPT process in the S-1 state and the double proton transfer process is a stepwise proton transfer mechanism. Our work can pave the way towards the design and synthesis of new molecules.

Automated summary

In this study, the ESDPT process in the ligand was investigated using theoretical methods. It was found that the asymmetric structure influences proton transfer, resulting in different transfer abilities for the two hydrogen protons. The potential energy surface analysis revealed that HYDRAVH(2) can undergo ESDPT in the S-1 state through a stepwise proton transfer mechanism.