Top Curr Chem (Z) (2018) 376:10
1 3
observed: i) three distinctive diagonal peaks are assigned to the photobleaching of
S 1 , S 2 , and S 3 excitons, ii) positive S 2 /S 1 (excitation/emission) and S 3 /S 1 cross-peaks
along the vertical at 1.8-eV emission energy are the signature of an ultrafast relaxation of S 3 and S 2 into S 1 in 30 fs, and iii) a positive signal at the S 1 /S 2 cross-peak due
to the shared electron level of the two excitons. The other 2DES maps allow tracking
the photoreaction dynamics. At T = 60 and 130 fs, the relaxation from S 3 and S 2 to
S 1 keeps happening (growth of the amplitude of the S 3 /S 1 and S 2 /S 1 cross-peaks).
The amplitude of the S 2 /S 2 peak decreases and the one of the S 3 /S 3 stays stable. The
dynamics of hot hole and electrons is faster than 500 fs, because the 2DES maps at
130 and 500 fs are similar.
By monitoring the decay dynamics of the diagonal and cross-peaks, a proposed
mechanism of the ultrafast hole and electron dynamics and their relaxation pathways for the first three excitons of CdTe NRs is given in Fig. 17b. A hot hole thermalization from 1Σ 3/2 to 1Σ 1/2 in the range of 30 fs, while higher energetic electrons
after S 3 pumping relax from Σ′ to Σ within 50 fs time constant. Those results were
unraveled only by 2DES and can design complex hybrid nanostructures that aim to
separately control the electron and hole dynamics.
The second example is a new approach described by Ruetzel et al. [7] that allows
tracking of the putative mechanisms involved in complexes photochemical reactions.
The main advantage of 2DES is to be able to probe the reactants axis (excitation
axis) at the same time that the products one (emission axis). The ring-open-6-nitroBIPS (Fig. 18a) in acetonitrile exists in a mixture of two stable merocyanine isomers,
which photoconvert through a photoisomerization process. The photoisomerization
was reported but the identification of the mechanism remained unknown because of
the spectral overlap of those isomers [83, 84]. Figure 18a shows the absorption spectrum of the different isomers of 6-nitro-BIPS: the dominant trans–trans–cis isomer
(TTC) and only 10% of merocyanine molecules exist in the trans–trans–trans isomer (TTT). The TTC isomer has an absorption peak at 557 nm, whereas the absorbance peak of TTT isomer is centered at 595 nm.
The photoisomerization was recorded for a long waiting time. In order to be able
to disentangle the mechanism, the 2DES data are Fourier-transformed along the
waiting time, T, to obtain a third-order 3D spectrum, represented on Fig. 18b. The
main contribution is observed in a plane at ν T = 0 cm
−1
. By contrast, the observed
vibrational motion leads to separated cross-peaks in the 3D spectrum at 176 cm
−1
,
which corresponds to a period of 190 fs centered at the crossing of TTC excitation
and TTT photobleaching emission. A second isolated cross-peak appears around
ν T = 363 cm
−1
(period of 90 fs) and is slightly shifted away from the crossing point
toward higher excitation frequencies, indicating that more excess energy is required
to initiate it. The location in the 3D spectrum reveals that oscillations of cross-peaks
connecting the two isomers give rise to these signatures. This analysis was possible
only by measuring 2DES data.
Figure 18c shows slices of the 3D spectrum in the excitation/emission plane
for the two main vibrational modes together with the phase. The main contributions are situated at TTC excitation and TTT emission wavenumbers. A vibrational
wavepacket motion in the excited state absorption band of TTT is enhanced in the
rectangle IV. Both modes are observable at the red edge of the stimulated emission
52
Reprinted from the journal
1 3
observed: i) three distinctive diagonal peaks are assigned to the photobleaching of
S 1 , S 2 , and S 3 excitons, ii) positive S 2 /S 1 (excitation/emission) and S 3 /S 1 cross-peaks
along the vertical at 1.8-eV emission energy are the signature of an ultrafast relaxation of S 3 and S 2 into S 1 in 30 fs, and iii) a positive signal at the S 1 /S 2 cross-peak due
to the shared electron level of the two excitons. The other 2DES maps allow tracking
the photoreaction dynamics. At T = 60 and 130 fs, the relaxation from S 3 and S 2 to
S 1 keeps happening (growth of the amplitude of the S 3 /S 1 and S 2 /S 1 cross-peaks).
The amplitude of the S 2 /S 2 peak decreases and the one of the S 3 /S 3 stays stable. The
dynamics of hot hole and electrons is faster than 500 fs, because the 2DES maps at
130 and 500 fs are similar.
By monitoring the decay dynamics of the diagonal and cross-peaks, a proposed
mechanism of the ultrafast hole and electron dynamics and their relaxation pathways for the first three excitons of CdTe NRs is given in Fig. 17b. A hot hole thermalization from 1Σ 3/2 to 1Σ 1/2 in the range of 30 fs, while higher energetic electrons
after S 3 pumping relax from Σ′ to Σ within 50 fs time constant. Those results were
unraveled only by 2DES and can design complex hybrid nanostructures that aim to
separately control the electron and hole dynamics.
The second example is a new approach described by Ruetzel et al. [7] that allows
tracking of the putative mechanisms involved in complexes photochemical reactions.
The main advantage of 2DES is to be able to probe the reactants axis (excitation
axis) at the same time that the products one (emission axis). The ring-open-6-nitroBIPS (Fig. 18a) in acetonitrile exists in a mixture of two stable merocyanine isomers,
which photoconvert through a photoisomerization process. The photoisomerization
was reported but the identification of the mechanism remained unknown because of
the spectral overlap of those isomers [83, 84]. Figure 18a shows the absorption spectrum of the different isomers of 6-nitro-BIPS: the dominant trans–trans–cis isomer
(TTC) and only 10% of merocyanine molecules exist in the trans–trans–trans isomer (TTT). The TTC isomer has an absorption peak at 557 nm, whereas the absorbance peak of TTT isomer is centered at 595 nm.
The photoisomerization was recorded for a long waiting time. In order to be able
to disentangle the mechanism, the 2DES data are Fourier-transformed along the
waiting time, T, to obtain a third-order 3D spectrum, represented on Fig. 18b. The
main contribution is observed in a plane at ν T = 0 cm
−1
. By contrast, the observed
vibrational motion leads to separated cross-peaks in the 3D spectrum at 176 cm
−1
,
which corresponds to a period of 190 fs centered at the crossing of TTC excitation
and TTT photobleaching emission. A second isolated cross-peak appears around
ν T = 363 cm
−1
(period of 90 fs) and is slightly shifted away from the crossing point
toward higher excitation frequencies, indicating that more excess energy is required
to initiate it. The location in the 3D spectrum reveals that oscillations of cross-peaks
connecting the two isomers give rise to these signatures. This analysis was possible
only by measuring 2DES data.
Figure 18c shows slices of the 3D spectrum in the excitation/emission plane
for the two main vibrational modes together with the phase. The main contributions are situated at TTC excitation and TTT emission wavenumbers. A vibrational
wavepacket motion in the excited state absorption band of TTT is enhanced in the
rectangle IV. Both modes are observable at the red edge of the stimulated emission
52
Reprinted from the journal