Ultrafast Excited-State Decay Mechanisms of 6-Thioguanine Followed by Sub-20 fs UV Transient Absorption Spectroscopy
Danielle C. Teles-Ferreira,
Cristian Manzoni,
Lara Martínez-Fernández,
Giulio Cerullo,
Ana Maria de Paula,
Rocío Borrego-Varillas
Affiliations
Danielle C. Teles-Ferreira
Instituto Federal de Minas Gerais, Campus Ouro Preto, Ouro Preto 35400-000, MG, Brazil
Cristian Manzoni
Istituto di Fotonica e Nanotecnologie-Consiglio Nazionale delle Ricerche (IFN-CNR), Piazza Leonardo da Vinci 32, I-20133 Milano, Italy
Lara Martínez-Fernández
Departamento de Química, Facultad de Ciencias and Institute for Advanced Research in Chemistry (IADCHEM), Campus de Excelencia UAM-CSIC, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain
Giulio Cerullo
Istituto di Fotonica e Nanotecnologie-Consiglio Nazionale delle Ricerche (IFN-CNR), Piazza Leonardo da Vinci 32, I-20133 Milano, Italy
Ana Maria de Paula
Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte 31270-901, MG, Brazil
Rocío Borrego-Varillas
Istituto di Fotonica e Nanotecnologie-Consiglio Nazionale delle Ricerche (IFN-CNR), Piazza Leonardo da Vinci 32, I-20133 Milano, Italy
Understanding the primary steps following UV photoexcitation in sulphur-substituted DNA bases (thiobases) is fundamental for developing new phototherapeutic drugs. However, the investigation of the excited-state dynamics in sub-100 fs time scales has been elusive until now due to technical challenges. Here, we track the ultrafast decay mechanisms that lead to the electron trapping in the triplet manifold for 6-thioguanine in an aqueous solution, using broadband transient absorption spectroscopy with a sub-20 fs temporal resolution. We obtain experimental evidence of the fast internal conversion from the S2(ππ*) to the S1(nπ*) states, which takes place in about 80 fs and demonstrates that the S1(nπ*) state acts as a doorway to the triplet population in 522 fs. Our results are supported by MS-CASPT2 calculations, predicting a planar S2(ππ*) pseudo-minimum in agreement with the stimulated emission signal observed in the experiment.
