Top Curr Chem (Z) (2018) 376:10
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the internal conversion from Q x to Q y are occurring on this time scale. The observation of energy transfers and internal conversion parallel pathways was only possible
thanks to the two-color 2DES experiment.
In the third example (Fig. 11), a pulse generated via continuum filamentation
spanning from 500 to 1300 nm is used to collect 2DES-WL (white light) spectra
to resolve energy transfer within a network of semiconducting carbon nanotubes
(CNTs) [36]. CNTs thin films are being explored for energy harvesting and optoelectronic devices because of their exceptional transport and optical properties. In
this study, the nanotubes in the film are in close contact, allowing the energy to flow
through the films. The film is composed primarily of four different diameter nanotubes, defined by their chiral indices: (7,5), (7,6), (8,6) and (8,7). Each tube exhibits
an S 1 transition in the near-infrared (corresponding to the optical bandgap) and an S 2
transition in the visible. The 2DES-WL map described in Fig. 11b, exhibits several
peaks, both on the diagonal as off diagonal. Each of the electronic transitions in the
absorption spectrum creates a pair of diagonal peaks in the 2DES-WL spectrum.
The negative peak on the diagonal corresponds to photobleaching (color-coded in
blue) of the direct bandgap transition, while the blue-shifted positive peak along the
detection axis is from an excited state absorption (color-coded in red). Moreover, the
cross-peaks appearance correlates the different tubes.
We discuss as an example, only a sub-quadrant of the 2DES-WL map. We choose
the S 1 /S 1 quadrant. Excitation of S 1 lies below the ionization threshold and so no
charges, but only excitons are generated. The cross-peaks appear on the lower half of
the diagonal, indicating that energy transfer occurs downhill from larger to smaller
bandgap tubes. The cross-peaks raise simultaneously, indicating that exciton transfer is equally probable between any downhill combination of nanotubes. Moreover,
the cross-peaks are round, whereas the diagonal peaks are elongated, which imply
uncorrelated energy transfer. This means that a photoexcited exciton created on a
random nanotube has equal probability of transferring to another nanotube anywhere within the acceptor’s distribution of energies. Thus, the inhomogeneity of the
bandgap transitions does not impact energy transfer. Thus, it is equally likely for
an S 1 exciton to relax to the next smallest bandgap nanotube as it is to relax directly
to the smallest bandgap nanotube.
Based on the similarity of the transfer rates and a lack of correlation, the transfer was found to be independent of the spectral overlap, ruling out Förster resonance energy transfer between bright states as the rate limiting mechanism. These
Fig. 10 a Arrangement of the LH1 complex and supramolecular architecture of the BChl B890 (purple)
and the Car Spx (orange) together with the absorption spectrum of LH1 complex extracted from Rsp.
rubrum (black line) and pulse spectra used in the 2DES experiments in the visible (NOPA1, green line)
and near-IR (NOPA2, red line) wavelength regions. b Degenerate 2DES maps of the LH1 complex
from Rsp. rubrum, for different waiting times T, following excitation by a sub-10-fs visible pulse, resonant with the S 0 → S 2 transition of the Car and the Qx transition of the BChl. c Two-color 2DES maps of
the LH1 complex for different waiting times T, with same excitation axis as in B but detection axis in the
near-IR. Adapted from [63], with the permission of AIP Publishing
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Reprinted from the journal
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the internal conversion from Q x to Q y are occurring on this time scale. The observation of energy transfers and internal conversion parallel pathways was only possible
thanks to the two-color 2DES experiment.
In the third example (Fig. 11), a pulse generated via continuum filamentation
spanning from 500 to 1300 nm is used to collect 2DES-WL (white light) spectra
to resolve energy transfer within a network of semiconducting carbon nanotubes
(CNTs) [36]. CNTs thin films are being explored for energy harvesting and optoelectronic devices because of their exceptional transport and optical properties. In
this study, the nanotubes in the film are in close contact, allowing the energy to flow
through the films. The film is composed primarily of four different diameter nanotubes, defined by their chiral indices: (7,5), (7,6), (8,6) and (8,7). Each tube exhibits
an S 1 transition in the near-infrared (corresponding to the optical bandgap) and an S 2
transition in the visible. The 2DES-WL map described in Fig. 11b, exhibits several
peaks, both on the diagonal as off diagonal. Each of the electronic transitions in the
absorption spectrum creates a pair of diagonal peaks in the 2DES-WL spectrum.
The negative peak on the diagonal corresponds to photobleaching (color-coded in
blue) of the direct bandgap transition, while the blue-shifted positive peak along the
detection axis is from an excited state absorption (color-coded in red). Moreover, the
cross-peaks appearance correlates the different tubes.
We discuss as an example, only a sub-quadrant of the 2DES-WL map. We choose
the S 1 /S 1 quadrant. Excitation of S 1 lies below the ionization threshold and so no
charges, but only excitons are generated. The cross-peaks appear on the lower half of
the diagonal, indicating that energy transfer occurs downhill from larger to smaller
bandgap tubes. The cross-peaks raise simultaneously, indicating that exciton transfer is equally probable between any downhill combination of nanotubes. Moreover,
the cross-peaks are round, whereas the diagonal peaks are elongated, which imply
uncorrelated energy transfer. This means that a photoexcited exciton created on a
random nanotube has equal probability of transferring to another nanotube anywhere within the acceptor’s distribution of energies. Thus, the inhomogeneity of the
bandgap transitions does not impact energy transfer. Thus, it is equally likely for
an S 1 exciton to relax to the next smallest bandgap nanotube as it is to relax directly
to the smallest bandgap nanotube.
Based on the similarity of the transfer rates and a lack of correlation, the transfer was found to be independent of the spectral overlap, ruling out Förster resonance energy transfer between bright states as the rate limiting mechanism. These
Fig. 10 a Arrangement of the LH1 complex and supramolecular architecture of the BChl B890 (purple)
and the Car Spx (orange) together with the absorption spectrum of LH1 complex extracted from Rsp.
rubrum (black line) and pulse spectra used in the 2DES experiments in the visible (NOPA1, green line)
and near-IR (NOPA2, red line) wavelength regions. b Degenerate 2DES maps of the LH1 complex
from Rsp. rubrum, for different waiting times T, following excitation by a sub-10-fs visible pulse, resonant with the S 0 → S 2 transition of the Car and the Qx transition of the BChl. c Two-color 2DES maps of
the LH1 complex for different waiting times T, with same excitation axis as in B but detection axis in the
near-IR. Adapted from [63], with the permission of AIP Publishing
▸
42
Reprinted from the journal