The order of multiple excited state proton transfer in ternary complex of norharmane and acetic acids

- The hydrogen bond enhancing behavior in first excited state (S1) is verified through the analysis of geometric configurations, infrared spectra, frontier molecular orbitals and potential energy curves.
- The corresponding hydrogen bond intensities are visually exhibited by the RDG isosurfaces in the S1 state, and we confirmed that the order of hydrogen bond interaction intensity is N1-H2···O3 < O4-H5···O6 < O7-H8···N9 in NHM-2A, so the proton transfer of O7-H8···N9 is easier to occur.
- We can clearly understand the mechanism of ESPT reaction, which is a gradual process completed through the process NHM-2A → NHM-2AS → NHM-2AD → NHM-2AT based on the derivatives' barrier values in S1 state.