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On the Decay of the Triplet State of Thionucleobases


Singlet oxygen production upon photosensitization plays a critical role for drugs based on thionucleobases. While for immunosuppressants its yield must be near zero, for phototherapeutic drugs it should be near the unity. In this work, we apply high-level quantum chemical modelling to investigate the decay of the triplet state of thionucleobases, a main determinant of the singlet oxygen yield. Working on CASPT2 optimizations of two prototypical thiothymines (2-thiothymine and 6-aza-2-thiothymine), we showed that the T1 state is characterized by two ππ* minima and by the intersection of T1 to the singlet ground state. On the basis of this topography, we propose a two-step mechanistic model, which, depending on the energetic balance between the two minima, may have as determining step either a slow transition between minima or a faster intersystem crossing to S0. Chemical kinetics modelling, as well as simulation of transient absorption spectra, confirmed that the two-step model can explain the experimental results available for both molecules. Moreover, through additional investigations of 2-thiocytosine and 6-thioguanine, we show that such T1 topography is a common theme for nucleobases. We also discuss how the triplet-state topography may be used to control the singlet oxygen yield, aiming at different medical applications.

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Publication details

The article was received on 30 Mar 2017, accepted on 20 Apr 2017 and first published on 20 Apr 2017

Article type: Paper
DOI: 10.1039/C7CP02050C
Citation: Phys. Chem. Chem. Phys., 2017, Accepted Manuscript
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    On the Decay of the Triplet State of Thionucleobases

    S. Bai and M. Barbatti, Phys. Chem. Chem. Phys., 2017, Accepted Manuscript , DOI: 10.1039/C7CP02050C

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