Photophysical properties and excited state proton transfer in 1,8-Dihydroxydibenzo[a,h]phenazine: A theoretical study

Abstract

The photophysical properties as well as proton transfer process from normal form (Enol) to tautomer form (Keto) in 1,8-Dihydroxydibenzo[a,h]phenazine [DHBP] have been theoretically investigated by TDDFT method. The potential energy profiles of the reaction paths for concerted proton transfer and stepwise proton transfer via single-proton transferred intermediate (INT) are examined, and the results show that proton transfer occurs in locally excited state following the stepwise mechanism at the lowest excitation energy in the gas phase. The concerted reaction path is found to be unstable with respect to anti-symmetric mode statically. The steady state emission band detected in Piechowska et al. experiment was assigned to INT, while emissions from Enol and Keto are less likely to be observed. The low intensity of the fluorescence at INT can be explained by the inter-system crossing between the lowest singlet state S1 with the triplet states T1 and T2 instead of the internal conversion from S1 to S0 introduced by torsion at INT structure at current computational level. This study provides new insight into the quenching mechanism in π-extended rigid molecule DHBP.