Hydrogen atom and water complex determine the excited state dynamics of 8-azaguanine

Aza-substituted nucleobases are important nucleobase derivatives due to their potential application as clinic medicines. However, photosensitivity of aza-nucleobases has been reported and understanding their excited state dynamics becomes important. Among all the aza-base, 8-azaguanine (8-AG) shows most complex and unique emission behavior. In this work, excited state dynamics of 8-AG in different solutions are investigated by using time-resolved spectroscopy and the results are discussed with the help of TDDFT calculations. Our data indicates that there are two effective non-radiative decay pathways exist in 9H-8-AG while the major fluorescence emission of 8-AG is originated from 8H-8-AG water complex. Moreover, we show that hydrogen atom at C₈ position and hydrogen bonding network around 8-AG can greatly affect its excited state dynamics. These results not only clarify the previous debate of the unusual fluorescence emission of 8-AG in solution but also provide evidence for deep understanding its photophysical and photochemical properties.