Top Curr Chem (Z) (2018) 376:24
1 3
C, D). Further, less intense peaks (A, B) appear at around 18,000 cm
−1
. To underscore that different higher-lying excited states are probed relative to those previously
reported for the 2DUV spectra at the FC, i.e. t 2 = 0 (see Fig. 14d, e), we use lettercode (A–D) to label the ESA peaks. As a consequence of the Markovian approximation, the three traces along Ω 1 adopt an identical peak structure with a progressively
decreasing intensity of the overtones. The physical interpretation behind this is that,
on a picosecond timescale, excess energy acquired by exciting the overtones would
be dissipated, and probing would become insensitive to the pump wavelength. Riedle and co-workers recently reported an experimental 2DUV spectrum of pyrene in
methanol at time delay t 2 = 1 ps, recorded in a collinear pump–probe set-up, utilizing
a narrowband pump–pulse pair, centered at around 32,000 cm
−1
, i.e. at the second
and third vibrational bands of the L a absorption, and a supercontinuum probe pulse
covering the UV–Vis spectral window (16,000–38,000 cm
−1
) [93]. This result represents one of the few experimental examples employing UV–Vis broad pulses for
2D spectroscopy. The spectrum is shown in Fig. 15b and compared to our simulations (note the inverted color code with respect to the usual color coding adopted in
other figures, in order to facilitate appropriate visual comparison with the experimental map). Remarkable agreement is achieved between theory and experiment
with regard to the GSB and the positions and relative intensities of the ESA signals.
Few differences could be found: (i) through the use of a narrowband pump–pulse
pair, the fundamental (0–0) transition was suppressed along Ω 1 in the experiment;
(ii) the ESA peaks were noticeably elongated along Ω 3 , which could be rationalized by the short lifetimes of the higher-excited states. The pyrene example, where
a simulated 2DUV spectrum adopting the excited-state equilibrium geometry was
successfully compared with the experimental spectrum recorded at a waiting time of
1 ps, demonstrates that even when equilibration has not been completely carried out
(time constant is 4 ps), the spectrum is dominated by the electronic structure of the
excited-state equilibrium geometry, and theoretical simulations can predict/interpret
2D spectroscopic fingerprints of excited-state minima. This outcome paves the way
for extending simulation of 2DUV spectra through our SOS//QM/MM to other biologically relevant systems, pursuing the idea that 2DUV fingerprints of excited-state
minima can be computed at a reasonable computational cost and could be directly
compared with experimental data recorded at appropriate t 2 delay times.
In this context, we have focused our efforts on 2DUV simulations for tracking the complex photophysics of nucleic acids, based on the extended benchmark studies illustrated in the previous sections. Recent efforts to obtain the first
experimental 2DUV spectra of DNA nucleobase monomers [21, 22] indicate the
concrete possibility of using this technique to understand the photophysics of
more realistic DNA/RNA models, e.g. multimeric species in short single or double strands. As mentioned in Sect. 3.2, dinucleoside monophosphates, i.e. dimers
of DNA/RNA nucleobases, represent excellent model systems to start exploring
nucleic acid photophysics from both a theoretical and experimental standpoint.
In fact, the presence of a neighboring base significantly affects the excited-state
dynamics of monomeric systems due to excimer/exciplex formation and a range
of charge/proton/hydrogen transfer/recombination events [104–107]. These
have been shown to be extremely difficult to separate and assess with standard
100
Reprinted from the journal
1 3
C, D). Further, less intense peaks (A, B) appear at around 18,000 cm
−1
. To underscore that different higher-lying excited states are probed relative to those previously
reported for the 2DUV spectra at the FC, i.e. t 2 = 0 (see Fig. 14d, e), we use lettercode (A–D) to label the ESA peaks. As a consequence of the Markovian approximation, the three traces along Ω 1 adopt an identical peak structure with a progressively
decreasing intensity of the overtones. The physical interpretation behind this is that,
on a picosecond timescale, excess energy acquired by exciting the overtones would
be dissipated, and probing would become insensitive to the pump wavelength. Riedle and co-workers recently reported an experimental 2DUV spectrum of pyrene in
methanol at time delay t 2 = 1 ps, recorded in a collinear pump–probe set-up, utilizing
a narrowband pump–pulse pair, centered at around 32,000 cm
−1
, i.e. at the second
and third vibrational bands of the L a absorption, and a supercontinuum probe pulse
covering the UV–Vis spectral window (16,000–38,000 cm
−1
) [93]. This result represents one of the few experimental examples employing UV–Vis broad pulses for
2D spectroscopy. The spectrum is shown in Fig. 15b and compared to our simulations (note the inverted color code with respect to the usual color coding adopted in
other figures, in order to facilitate appropriate visual comparison with the experimental map). Remarkable agreement is achieved between theory and experiment
with regard to the GSB and the positions and relative intensities of the ESA signals.
Few differences could be found: (i) through the use of a narrowband pump–pulse
pair, the fundamental (0–0) transition was suppressed along Ω 1 in the experiment;
(ii) the ESA peaks were noticeably elongated along Ω 3 , which could be rationalized by the short lifetimes of the higher-excited states. The pyrene example, where
a simulated 2DUV spectrum adopting the excited-state equilibrium geometry was
successfully compared with the experimental spectrum recorded at a waiting time of
1 ps, demonstrates that even when equilibration has not been completely carried out
(time constant is 4 ps), the spectrum is dominated by the electronic structure of the
excited-state equilibrium geometry, and theoretical simulations can predict/interpret
2D spectroscopic fingerprints of excited-state minima. This outcome paves the way
for extending simulation of 2DUV spectra through our SOS//QM/MM to other biologically relevant systems, pursuing the idea that 2DUV fingerprints of excited-state
minima can be computed at a reasonable computational cost and could be directly
compared with experimental data recorded at appropriate t 2 delay times.
In this context, we have focused our efforts on 2DUV simulations for tracking the complex photophysics of nucleic acids, based on the extended benchmark studies illustrated in the previous sections. Recent efforts to obtain the first
experimental 2DUV spectra of DNA nucleobase monomers [21, 22] indicate the
concrete possibility of using this technique to understand the photophysics of
more realistic DNA/RNA models, e.g. multimeric species in short single or double strands. As mentioned in Sect. 3.2, dinucleoside monophosphates, i.e. dimers
of DNA/RNA nucleobases, represent excellent model systems to start exploring
nucleic acid photophysics from both a theoretical and experimental standpoint.
In fact, the presence of a neighboring base significantly affects the excited-state
dynamics of monomeric systems due to excimer/exciplex formation and a range
of charge/proton/hydrogen transfer/recombination events [104–107]. These
have been shown to be extremely difficult to separate and assess with standard
100
Reprinted from the journal