Top Curr Chem (Z) (2018) 376:24
1 3
adenine–uracil dinucleoside (ApU), a system exhibiting different excited-state
relaxation mechanisms that would be intricately obscured in 1D-PP experiments.
Early experiments had shown how adenine and uracil separately would feature
ultrafast signals [110–112], as well as long-lived signals arising from partial population of their dark
1
nπ* states [113], but that an additional signal would arise in
dimeric species, probably due to inter-nucleobase interactions [87]. This signal
could not be clearly disentangled, but it was associated with the ability of the
dimeric nucleosides/tides to attain stacking conformations that would lead to the
formation of excimer/exciplex or charge-transfer events. In order to ascertain this
possibility, we have proposed the use of 2DUV spectroscopy for characterizing,
with high temporal and spectral resolution, the spectroscopic fingerprints of various excited-state minima along the complex photoinduced pathways.
An initial theoretical study was carried out for the solvated ApU dinucleoside [60], where each of the decay channels exploited by the monomeric units
were investigated by means of CASSCF excited-state optimizations and conical
intersection characterizations. Figure 16 shows the equilibrium geometries of the
localized excited states, named here as
1
L b for adenine and S(U) for uracil, as well
as delocalized CT states as originated by excited-state geometry optimizations
from the FC region, associated with a GS optimized geometry with vicinal interacting nucleobases (Fig. 16a). Transition states and conical intersections along
the excited-state decay channels were also characterized, providing an estimate
of the timescale of the relaxation times, as schematically reported in Fig. 16b.
To disentangle among these relevant stationary points involved in the ApU photophysics, broadband 2DES spectra were computed on top of their structures,
resulting in the simulated 2DES maps depicted in Fig. 16c. As can be seen, in the
FC region (at zero or ultrashort t 2 waiting time, sub-100 fs), the main signals are
expected to arise from the
1
L a and S(U) traces, with peaks from the spectroscopic
1
L a state of adenine rapidly depleted due to ultrafast deactivation and weak CT
signals that are present only for those conformations where such non-localized
states are bright. Due to the presence of a sizeable energy barrier for the deactivation of S(U) of uracil in ApU, in contrast to those recently computed for uridine
in water [114], it was predicted that most spectral contributions in the sub-10-ps
timescale would come from the intense S(U) trace. At longer waiting times, in
the 10–100-ps range, the 2D maps would feature signals on the
1
L b trace (at Ω 1
around 34,000–36,000 cm
−1
) and a particularly intense fingerprint of the CT minimum, whose trace (at Ω 1 around 40,000 cm
−1
) would be well separated from the
1
L b trace, allowing quantification of the population of ApU conformations with
accessible CT states. Thus, these simulations could help assign distinctive signals arising from different excited-state decay channels that might be recorded in
upcoming 2DUV experiments, providing a state-specific characterization of the
ApU excited-state dynamics. Here, with such an example, we demonstrated how
2DUV could be an extremely powerful tool for elucidating nucleic acid photophysics in model systems.
102
Reprinted from the journal
1 3
adenine–uracil dinucleoside (ApU), a system exhibiting different excited-state
relaxation mechanisms that would be intricately obscured in 1D-PP experiments.
Early experiments had shown how adenine and uracil separately would feature
ultrafast signals [110–112], as well as long-lived signals arising from partial population of their dark
1
nπ* states [113], but that an additional signal would arise in
dimeric species, probably due to inter-nucleobase interactions [87]. This signal
could not be clearly disentangled, but it was associated with the ability of the
dimeric nucleosides/tides to attain stacking conformations that would lead to the
formation of excimer/exciplex or charge-transfer events. In order to ascertain this
possibility, we have proposed the use of 2DUV spectroscopy for characterizing,
with high temporal and spectral resolution, the spectroscopic fingerprints of various excited-state minima along the complex photoinduced pathways.
An initial theoretical study was carried out for the solvated ApU dinucleoside [60], where each of the decay channels exploited by the monomeric units
were investigated by means of CASSCF excited-state optimizations and conical
intersection characterizations. Figure 16 shows the equilibrium geometries of the
localized excited states, named here as
1
L b for adenine and S(U) for uracil, as well
as delocalized CT states as originated by excited-state geometry optimizations
from the FC region, associated with a GS optimized geometry with vicinal interacting nucleobases (Fig. 16a). Transition states and conical intersections along
the excited-state decay channels were also characterized, providing an estimate
of the timescale of the relaxation times, as schematically reported in Fig. 16b.
To disentangle among these relevant stationary points involved in the ApU photophysics, broadband 2DES spectra were computed on top of their structures,
resulting in the simulated 2DES maps depicted in Fig. 16c. As can be seen, in the
FC region (at zero or ultrashort t 2 waiting time, sub-100 fs), the main signals are
expected to arise from the
1
L a and S(U) traces, with peaks from the spectroscopic
1
L a state of adenine rapidly depleted due to ultrafast deactivation and weak CT
signals that are present only for those conformations where such non-localized
states are bright. Due to the presence of a sizeable energy barrier for the deactivation of S(U) of uracil in ApU, in contrast to those recently computed for uridine
in water [114], it was predicted that most spectral contributions in the sub-10-ps
timescale would come from the intense S(U) trace. At longer waiting times, in
the 10–100-ps range, the 2D maps would feature signals on the
1
L b trace (at Ω 1
around 34,000–36,000 cm
−1
) and a particularly intense fingerprint of the CT minimum, whose trace (at Ω 1 around 40,000 cm
−1
) would be well separated from the
1
L b trace, allowing quantification of the population of ApU conformations with
accessible CT states. Thus, these simulations could help assign distinctive signals arising from different excited-state decay channels that might be recorded in
upcoming 2DUV experiments, providing a state-specific characterization of the
ApU excited-state dynamics. Here, with such an example, we demonstrated how
2DUV could be an extremely powerful tool for elucidating nucleic acid photophysics in model systems.
102
Reprinted from the journal