Relaxation Mechanisms of 5-Azacytosine

摘要

The photophysics and deactivation pathways of the noncanonical 5-azacytosine nucleobase were studied using the CASPT2//CASSCF protocol. One of the most significant differences with respect to the parent molecule cytosine is the presence of a dark ~1(n_Nπ) excited state placed energetically below the bright excited state ~1(ππ) at the Pranck—Condon region. The main photoresponse of the system is a presumably efficient radiationless decay back to the original ground state, mediated by two accessible conical intersections involving a population transfer from the ~1(ππ) and the ~1(ππ) states to the ground state. Therefore, a minor contribution of the triplet states in the photophysics of the system is expected, despite the presence of a deactivation path leading to the lowest ~3(ππ) triplet state. The global scenario on the photophysics and photochemistry of the 5-azacytosine system gathered on theoretical grounds is consistent with the available experimental data, taking especially into account the low values of the singlet—triplet intersystem crossing and fluorescence quantum yields .observed.