Double Thionated Pyrimidine Nucleobases: Molecular Tools with Tunable Photoproperties

Sulfur-substitutednucleobases are DNA and RNA base derivativesthat exhibit extremely efficient photoinduced intersystem crossing(ISC) dynamics into the lowest-energy triplet state. The long-livedand reactive triplet states of sulfur-substituted nucleobases arecrucial due to their wide range of potential applications in medicine,structural biology, and the development of organic light-emittingdiodes (OLEDs) and other emerging technologies. However, a comprehensiveunderstanding of non-negligible wavelength-dependent changes in theinternal conversion (IC) and ISC events is still lacking. Here, westudy the underlying mechanism using joint experimental gas-phasetime-resolved photoelectron spectroscopy (TRPES) and theoretical quantumchemistry methods. We combine 2,4-dithiouracil (2,4-DTU) TRPES experimentaldata with computational analysis of the different photodecay processes,which are induced by increasing excitation energies along the entirelinear absorption (LA) ultraviolet (UV) spectrum. Our results showhow the double-thionated uracil (U), i.e., 2,4-DTU, appears as a versatilephotoactivatable instrument. Multiple decay processes can be initiatedwith different ISC rates or triplet-state lifetimes that resemblethe distinctive behavior of the singly substituted 2- or 4-thiouracil(2-TU or 4-TU). We obtained a clear partition of the LA spectrum basedon the dominant photoinduced process. Our work clarifies the reasonsbehind the wavelength-dependent changes in the IC, ISC, and triplet-statelifetimes in doubly thionated U, becoming a biological system of utmostimportance for wavelength-controlled applications. These mechanisticdetails and photoproperties are transferable to closely related molecularsystems such as thionated thymines.