Abstract:
Thymine, a DNA nucleobase, when photoexcited, relaxes to the ground state on an
ultrafast timescale. In contrast, when one or both oxygen atoms in thymine are replaced
with sulphur (yielding three different thiothymines), the molecule on photoexcitation
populates its triplet excited state with near unity yield. Thiothymines also exhibit a
redshift in their absorption spectrum compared to thymine, and in their triplet state,
are able to photosensitize ground state oxygen to singlet oxygen. These photoproperties
make thiothymines potential drugs to be used in photodynamic therapy. Three major
questions are addressed in this work, all of which focus on understanding how position of
substitution affect these photoproperties. Wavefunction-based multi-reference electronic
structure methods, specifically, complete active space self-consistent field (CASSCF) and
its extensions have been used.
First, we explain the absorption spectra, where, substitution at the 4th position (4-
thiothymine) exhibits a significantly higher redshift as compared to thionation at the
2nd position (2-thiothymine). We find that this unexpected trend can be attributed to
an interplay between two types of delocalization in thymine: one due to conjugation
with a double bond and the other owing to the size of the atom substituted. This work
demonstrates that studying substitutions in isolation can be misleading, and the intrinsic
features of the parent molecule need to be considered. Also, this study shows that the
well-established idea of delocalization is not just affected by conjugation with another
pi system, but is also influenced by the size of atoms.
Second, we look at how position of substitution affects triplet lifetimes since 4-
thiothymine was found to have a longer triplet lifetime than 2-thiothymine. This dif-
ference arises from the decay dynamics of 2-thiothymine being distinct from that of
4-thiothymine and 2,4-dithiothymine. The findings regarding absorption spectra and triplet lifetimes of thiothymines were extended to another molecule, xanthine, to evaluate their generality. This molecule highlights the generality of absorption properties
amongst thiobases, while suggesting prudence in making predictions about excited state
properties, especially when excited state structures vary significantly from ground state
structures.
Finally, we have investigated the mechanism of photosensitization, and calculated
the rates of photosensitization of oxygen by thiothymines. Recently, a method built
on certain classical approximations was proposed. In our work, we have employed a
time-dependent variant of Fermi’s Golden rule, where the treatment is purely quantum
mechanical. We believe this is the first instance where this rate expression has been used
for bimolecular nonradiative energy transfer.
