<p>Minor
structural modifications to the DNA and RNA nucleobases have a significant
effect on their excited state dynamics and electronic relaxation pathways.<b> </b>In this study, the excited state
dynamics of 7-deazaguanosine and guanosine 5’-monophosphate are investigated in
aqueous and in a mixture of methanol and water using femtosecond broadband
transient absorption spectroscopy following excitation at 267 nm. The transient
spectra are collected using photon densities that ensure no parasitic
multiphoton-induced signal from solvated electrons. The data can be fit satisfactorily
using a two- or three-component kinetic model. By analyzing the results from
steady-state, time-resolved, computational calculations, and the methanol-water
mixture, the following general relaxation mechanism is proposed for both
molecules, L<sub>b</sub> ®
L<sub>a</sub> ®
<sup>1</sup>ps*(ICT) ®
S<sub>0</sub>, where the <sup>1</sup>ps*(ICT) stands for an intramolecular charge transfer excited
singlet state with significant ps*
character. In general, longer lifetimes for internal conversion are obtained
for 7-deazaguanosine compared to guanosine 5’-monophosphate. Internal
conversion of the <sup>1</sup>ps*(ICT)
state to the ground state occurs on a similar time scale of a few picoseconds
in both molecules. Collectively, the results demonstrate that substitution of a
single nitrogen for a methine (C-H) group at position seven of the guanine
moiety stabilizes the <sup>1</sup>pp* L<sub>b</sub> and L<sub>a</sub> states and alter the
topology of their potential energy surfaces in such a way that the relaxation
dynamics in 7-deazaguanosine are slowed down compared to those in guanosine
5’-monophosphate but not for the internal conversion of <sup>1</sup>ps*(ICT) state to the ground state.</p>