Theoretical studies on the mechanism of excited-state intramolecular proton transfer in 1,8-dihydroxydibenzo[a,c]phenazine

The photoinduced reaction mechanism of excited-state intramolecular proton transfer (ESIPT) of 1,8-dihydroxydibenzo[a,c]phenazine (denoted as DHBP), a bis-phenol displaying ESIPT property, has been explored by employing the CASSCF/CASPT2 and DFT/TD-DFT methods. The two lowest singlet electronic states of three tautomers, namely the enol form (E), a keto form with single proton transferred (SK) and a keto form with double proton transferred (DK), have been optimized. These results indicate that the E tautomer is the system with the lowest energy in the ground S-0 state, while the SK tautomer is the most stable structure in the singlet S-1 state. Moreover, the structural changes of E from S-0 to S-1 result in a strengthened intramolecular hydrogen bond, which contributes to facilitate the following ESIPT reaction. In addition, the absorption and emission spectra of DHBP have also been simulated using TD-DFT. The vertical excitation energy of the E form and vertical emission energy of the SK form are found at 2.86 and 1.74 eV, respectively, in good agreement with the available experimental values of 2.89 and 1.72 eV, which implies that the fluorescence detected by Piechowska et al. may only arise from the SK tautomer. To give a clearer picture of the proton transfer process, the constrained energy profiles (CEPs) for the three DHBP tautomers in the S-0 and S-1 states have been constructed. These calculations of CEPs show that the energy barriers during the PT processes from the E/DK to the SK forms in the S-1 state are much lower than those of the others considered here and support the hypothesis of an ESIPT mechanism of only a singlet proton transfer rather than double proton transfer.