title: Relaxation Mechanisms of 5-Azacytosine.
abstract: 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)(nNπ*) excited state placed energetically below the bright excited state (1)(ππ*) at the Franck-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)(nNπ*) 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.
authors.affiliation: Instituto de Ciencia Molecular, Universitat de València , P.O. Box 22085, ES-46071 Valencia, Spain.
authors.forename: Angelo
authors.initials: A
authors.lastname: Giussani
authors.affiliation: Instituto de Ciencia Molecular, Universitat de València , P.O. Box 22085, ES-46071 Valencia, Spain.
authors.forename: Manuela
authors.initials: M
authors.lastname: Merchán
authors.affiliation: Instituto de Química, Departamento de Química Fundamental, NAP-PhotoTech the USP Consortium for Photochemical Technology, Universidade de São Paulo , Avenida Professor Lineu Prestes 748, 05508-000 São Paulo, Brazil.
authors.forename: João Paulo
authors.initials: JP
authors.lastname: Gobbo
authors.affiliation: Instituto de Química, Departamento de Química Fundamental, NAP-PhotoTech the USP Consortium for Photochemical Technology, Universidade de São Paulo , Avenida Professor Lineu Prestes 748, 05508-000 São Paulo, Brazil.
authors.forename: Antonio Carlos
authors.initials: AC
authors.lastname: Borin