Sulfur-substituted
nucleobases are DNA and RNA base derivatives
that exhibit extremely efficient photoinduced intersystem crossing
(ISC) dynamics into the lowest-energy triplet state. The long-lived
and reactive triplet states of sulfur-substituted nucleobases are
crucial due to their wide range of potential applications in medicine,
structural biology, and the development of organic light-emitting
diodes (OLEDs) and other emerging technologies. However, a comprehensive
understanding of non-negligible wavelength-dependent changes in the
internal conversion (IC) and ISC events is still lacking. Here, we
study the underlying mechanism using joint experimental gas-phase
time-resolved photoelectron spectroscopy (TRPES) and theoretical quantum
chemistry methods. We combine 2,4-dithiouracil (2,4-DTU) TRPES experimental
data with computational analysis of the different photodecay processes,
which are induced by increasing excitation energies along the entire
linear absorption (LA) ultraviolet (UV) spectrum. Our results show
how the double-thionated uracil (U), i.e., 2,4-DTU, appears as a versatile
photoactivatable instrument. Multiple decay processes can be initiated
with different ISC rates or triplet-state lifetimes that resemble
the distinctive behavior of the singly substituted 2- or 4-thiouracil
(2-TU or 4-TU). We obtained a clear partition of the LA spectrum based
on the dominant photoinduced process. Our work clarifies the reasons
behind the wavelength-dependent changes in the IC, ISC, and triplet-state
lifetimes in doubly thionated U, becoming a biological system of utmost
importance for wavelength-controlled applications. These mechanistic
details and photoproperties are transferable to closely related molecular
systems such as thionated thymines.