1 3
Top Curr Chem (Z) (2018) 376:10
the chromophores, the ones below the diagonal, show a clear increase of the signal
intensity, associated with the downhill EET.
In Fig. 9d we show a series of 2DES maps recorded at different T times for the
whole photosynthetic cell [60]. The early map at T = 30 fs shows a series of diagonal
peaks attributed to the different transition in the steady-state absorption spectrum.
At later T times, one can clearly observe the formation and decay of cross-peaks
below the diagonal, revealing how energy flows within and between the individual
complexes in the apparatus. By examining the connectivity between different complexes and applying a global fitting method [60], it was possible to track the step-bystep energy flow through the entire unit (Fig. 9d) and observed for the first time that
the FMO complex serves as energy conduit between the chlorosome and the RC.
A second example of EET process resolved by 2DES is reported in Fig. 10 [63].
In this work, the isolated light-harvesting antenna complex (namely LH1) of a purple bacterium is studied with a two-color 2DES apparatus, where both transition
in the near-infrared and in the visible spectra region were covered with two different femtosecond broadband pulses. The absorption spectrum of the LH1 complex is
constituted of two chromophores (Fig. 10a), a carotenoid—namely, spirilloxanthin
(Spx)—whose 0–0 first optically allowed S 0 → S 2 transition peaks at 540 nm, and
a bacterio-chlorophyll (BChl) named B890 due to its Q y band peaking at 881 nm
(the higher energy Q x band of the B890 peaks at 585 nm). By exciting in the visible
range and detecting over both visible and near-IR ranges, it was possible to follow
all the photoinduced processes, namely, i) the Spx internal conversion, ii) the BChl
Q x → Q y internal conversion, and iii) the Spx → B890 EET process by tracking the
formation of several cross-peaks in the 2DES maps. In the degenerate 2DES experiment (Fig. 10b), the internal conversion for the bright S 2 state of the Spx to the dark
S 1 state is observed as the appearance of a negative cross-peak (color-coded in blue)
on within the first few hundreds of femtoseconds, assigned to the formation of an
excited state absorption from the S 1 state, at the 545/620 nm excitation/detection
cross-peak. Moreover, a second negative feature is assigned to the formation of a
parallel long-lived state, named S*, at the 545/570 nm excitation/detection crosspeak [64]. The degenerate 2DES map also shows that the diagonal peak of the Q x
of the B890 (585/585 nm excitation/detection) has become less pronounced after
T = 65 fs, indicating that population in the B890 moiety, has started the internal
conversion process to Q y , reducing the stimulated emission contribution to the positive signal on the diagonal.
The two-color 2DES maps (excitation axis in the visible range and detection in
the near-IR) in Fig. 10c show two positive cross-peaks at 875-nm detection wavelength that appear at the excitation wavelengths of 545 and 585 nm, corresponding to resonances of S 2 and Q x , respectively. The center wavelength of the positive
bands at 875 nm identifies these features as pure photobleaching contributions from
Q y , showing that at these early T times, the population is still entirely on Q x and
the optical probes which are specific for population in Q y . At later T times, the further evolution of the 2DES maps consists in the growth of the positive cross-peaks
at 875 nm and the appearance of two excited-state absorption peaks on the short
wavelength side, around 845 nm. Both the features at 875 and 845 nm are specific
optical probes for population on Q y , showing that both the Spx → B890 EET and
41
Reprinted from the journal
Top Curr Chem (Z) (2018) 376:10
the chromophores, the ones below the diagonal, show a clear increase of the signal
intensity, associated with the downhill EET.
In Fig. 9d we show a series of 2DES maps recorded at different T times for the
whole photosynthetic cell [60]. The early map at T = 30 fs shows a series of diagonal
peaks attributed to the different transition in the steady-state absorption spectrum.
At later T times, one can clearly observe the formation and decay of cross-peaks
below the diagonal, revealing how energy flows within and between the individual
complexes in the apparatus. By examining the connectivity between different complexes and applying a global fitting method [60], it was possible to track the step-bystep energy flow through the entire unit (Fig. 9d) and observed for the first time that
the FMO complex serves as energy conduit between the chlorosome and the RC.
A second example of EET process resolved by 2DES is reported in Fig. 10 [63].
In this work, the isolated light-harvesting antenna complex (namely LH1) of a purple bacterium is studied with a two-color 2DES apparatus, where both transition
in the near-infrared and in the visible spectra region were covered with two different femtosecond broadband pulses. The absorption spectrum of the LH1 complex is
constituted of two chromophores (Fig. 10a), a carotenoid—namely, spirilloxanthin
(Spx)—whose 0–0 first optically allowed S 0 → S 2 transition peaks at 540 nm, and
a bacterio-chlorophyll (BChl) named B890 due to its Q y band peaking at 881 nm
(the higher energy Q x band of the B890 peaks at 585 nm). By exciting in the visible
range and detecting over both visible and near-IR ranges, it was possible to follow
all the photoinduced processes, namely, i) the Spx internal conversion, ii) the BChl
Q x → Q y internal conversion, and iii) the Spx → B890 EET process by tracking the
formation of several cross-peaks in the 2DES maps. In the degenerate 2DES experiment (Fig. 10b), the internal conversion for the bright S 2 state of the Spx to the dark
S 1 state is observed as the appearance of a negative cross-peak (color-coded in blue)
on within the first few hundreds of femtoseconds, assigned to the formation of an
excited state absorption from the S 1 state, at the 545/620 nm excitation/detection
cross-peak. Moreover, a second negative feature is assigned to the formation of a
parallel long-lived state, named S*, at the 545/570 nm excitation/detection crosspeak [64]. The degenerate 2DES map also shows that the diagonal peak of the Q x
of the B890 (585/585 nm excitation/detection) has become less pronounced after
T = 65 fs, indicating that population in the B890 moiety, has started the internal
conversion process to Q y , reducing the stimulated emission contribution to the positive signal on the diagonal.
The two-color 2DES maps (excitation axis in the visible range and detection in
the near-IR) in Fig. 10c show two positive cross-peaks at 875-nm detection wavelength that appear at the excitation wavelengths of 545 and 585 nm, corresponding to resonances of S 2 and Q x , respectively. The center wavelength of the positive
bands at 875 nm identifies these features as pure photobleaching contributions from
Q y , showing that at these early T times, the population is still entirely on Q x and
the optical probes which are specific for population in Q y . At later T times, the further evolution of the 2DES maps consists in the growth of the positive cross-peaks
at 875 nm and the appearance of two excited-state absorption peaks on the short
wavelength side, around 845 nm. Both the features at 875 and 845 nm are specific
optical probes for population on Q y , showing that both the Spx → B890 EET and
41
Reprinted from the journal