Top Curr Chem (Z) (2018) 376:10
1 3
devices, electronics, and others depend on the efficiency of the dynamics of each
states and how they interact to each other [58]. 2DES can bring some clues on these
phenomena.
The absorption spectrum of CdSe nanocrystals is broad with two low-energy
exciton bands. During those experiments, both bands were excited and probed
(Fig. 7b). The 2DES spectra show two diagonal and two anti-diagonal peaks corresponding to the two exciton bands and the two coupled exciton states, respectively.
In comparison to the simulation, those 2DES spectra are really blurry because of the
inhomogeneous broadening. This inhomogeneous broadening takes its origin in the
size distribution of the nanocrystals in solution.
Figure 7 shows that analyzing the 2DES spectra by their rephasing, non-rephasing and absorptive signals give some insights into the inhomogeneous broadening.
Indeed, photon-echo (rephasing signal) can improve the spectroscopic resolution by
removing inhomogeneous line broadening [17]. After this analysis, the two diagonal and anti-diagonal peaks appear clearly on the spectra. This allows to determine
that the binding energies of the ( �X 1 X 2 ⟩ ) and the ( �
� X 1 X 1 ⟩ ) biexcitons are similar. A
value of 25 meV for this energy reproduces the peaks very well in simulations. The
non-rephasing spectra do not contain photon-echo signal and therefore contain inhomogeneous contributions to both the diagonal and antidiagonal linewidths. We will
see in the section photoreactivity that the disentanglement of those state dynamics is
rendered possible by 2DES.
4 Excitation Energy Transfer (EET)
Excitation energy transfer (EET) in complex electronic systems has been successfully studied by 2DES. Among the advantages of 2DES over standard transient
absorption measurement is the capability of distinguishing between weakly or
strongly coupled systems (Fig. 8). In the case of a weakly coupled donor-acceptor
system (Fig. 8a), EET can be tracked in a 2DES experiment as the appearance of a
cross-peak at the donor (B)/ acceptor (A) frequencies in the 2DES map as a function of the waiting time T. In the strong coupling regime (Fig. 8b), the donor and
acceptor share a common ground state and the excitation becomes delocalized over
two excitonic states, α and β. Thus, at T = 0 the 2DES map reveals the existence
of separate cross-peaks at the αβ/βα positions, which evolve along T. In transient
absorption experiments, due to the lack of resolution over the excitation axis, these
cases will be indistinguishable. In particular, in photosynthetic complexes, the interchromophore electrostatic interaction gives rise to Coulombic couplings, which in
turn re-define energetically shifted delocalized and coupled states (namely exciton
[59]) that ultimately determine the photosynthetic optical properties. Accordingly,
the electronic absorption spectra are—in most cases—broad and congested, which is
beneficial for maximizing light absorption, but makes the analysis of these systems
extremely difficult.
2DES is the perfect tool to identify the chromophore couplings and track the
energy transfer pathways in protein-pigment complexes. In addition to spectrally
resolve excitation and emission frequencies with femtosecond resolution, this
38
Reprinted from the journal
1 3
devices, electronics, and others depend on the efficiency of the dynamics of each
states and how they interact to each other [58]. 2DES can bring some clues on these
phenomena.
The absorption spectrum of CdSe nanocrystals is broad with two low-energy
exciton bands. During those experiments, both bands were excited and probed
(Fig. 7b). The 2DES spectra show two diagonal and two anti-diagonal peaks corresponding to the two exciton bands and the two coupled exciton states, respectively.
In comparison to the simulation, those 2DES spectra are really blurry because of the
inhomogeneous broadening. This inhomogeneous broadening takes its origin in the
size distribution of the nanocrystals in solution.
Figure 7 shows that analyzing the 2DES spectra by their rephasing, non-rephasing and absorptive signals give some insights into the inhomogeneous broadening.
Indeed, photon-echo (rephasing signal) can improve the spectroscopic resolution by
removing inhomogeneous line broadening [17]. After this analysis, the two diagonal and anti-diagonal peaks appear clearly on the spectra. This allows to determine
that the binding energies of the ( �X 1 X 2 ⟩ ) and the ( �
� X 1 X 1 ⟩ ) biexcitons are similar. A
value of 25 meV for this energy reproduces the peaks very well in simulations. The
non-rephasing spectra do not contain photon-echo signal and therefore contain inhomogeneous contributions to both the diagonal and antidiagonal linewidths. We will
see in the section photoreactivity that the disentanglement of those state dynamics is
rendered possible by 2DES.
4 Excitation Energy Transfer (EET)
Excitation energy transfer (EET) in complex electronic systems has been successfully studied by 2DES. Among the advantages of 2DES over standard transient
absorption measurement is the capability of distinguishing between weakly or
strongly coupled systems (Fig. 8). In the case of a weakly coupled donor-acceptor
system (Fig. 8a), EET can be tracked in a 2DES experiment as the appearance of a
cross-peak at the donor (B)/ acceptor (A) frequencies in the 2DES map as a function of the waiting time T. In the strong coupling regime (Fig. 8b), the donor and
acceptor share a common ground state and the excitation becomes delocalized over
two excitonic states, α and β. Thus, at T = 0 the 2DES map reveals the existence
of separate cross-peaks at the αβ/βα positions, which evolve along T. In transient
absorption experiments, due to the lack of resolution over the excitation axis, these
cases will be indistinguishable. In particular, in photosynthetic complexes, the interchromophore electrostatic interaction gives rise to Coulombic couplings, which in
turn re-define energetically shifted delocalized and coupled states (namely exciton
[59]) that ultimately determine the photosynthetic optical properties. Accordingly,
the electronic absorption spectra are—in most cases—broad and congested, which is
beneficial for maximizing light absorption, but makes the analysis of these systems
extremely difficult.
2DES is the perfect tool to identify the chromophore couplings and track the
energy transfer pathways in protein-pigment complexes. In addition to spectrally
resolve excitation and emission frequencies with femtosecond resolution, this
38
Reprinted from the journal