Theoretical insights into excited-state intramolecular proton transfer in 1,8-dihydroxydibenzo[a,h]phenazine

1,8-Dihydroxydibenzoa,h]phenazine (DHBP) is a new synthetic compound possessing two intramolecular hydrogen bonds; however, it has been found to exhibit the excited-state intramolecular single proton transfer (ESSPT) behaviour, in recent experiment. To explain the phenomenon reasonably, two combined methods of CASSCF/CASPT2 and DFT/TD-DFT have been employed to investigate the structural and spectral properties of its three tautomers, corresponding to the non-proton-transferred (E), the single-proton-transferred (SK) and the double-proton-transferred (DK) forms. These studies suggest that the E form is the global minimum in the S₀ state, while the SK form is the most stable in the S₁ state, both of which are responsible for the experimental absorption peak at 2.54 eV and emission band at 1.64 eV, respectively. Because of the relatively high energy barrier, the DK form will play no important role in the fluorescence emission of DHBP. The present results lend a good support to the experimental finding of single proton transfer (SPT).