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Ultrafast photo-induced charge transfer unveiled by two-dimensional electronic spectroscopy
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Image of FIG. 1.
FIG. 1.

Linear absorption of [LuPc2]TBA+ in benzonitrile (blue line) and [LuPc2] in toluene (red line) shown together with the pulse spectrum used in the 2D experiments (grey). The inset shows the structure of [LuPc2] and the position of the TBA+ counter-ion.

Image of FIG. 2.
FIG. 2.

2D spectra of [LuPc2]TBA+ at different waiting times (t 2) indicated in the figure. The top row shows the experimental results, while the middle and bottom rows show the simulated total spectra and ground state bleach (GSB) plus stimulated emission (SE) contributions, respectively. All spectra have been normalized to the maximum of the total signal, and contour lines are drawn in 5% increments starting at 5%. Positive signal contributions (GSB + SE) are shown in red while negative (ESA) are shown in blue dash. The points used for the kinetics in Fig. 3 are shown for t 2 = 45 fs.

Image of FIG. 3.
FIG. 3.

Kinetics of the different points in the 2D spectra indicated in Fig. 2. (a) Diagonal peak kinetics for dp1 (black triangles), dp2 (red circles), and dp3 (blue squares, upper panel) together with the simulations (solid lines in the same color). The different curves have their own y-axis as indicated by the colors. (b) Cross-peak kinetics for cp1 (black triangles), cp2 (red circles), and cp3 (blue squares, upper panel) together with the simulated curves (solid lines).

Image of FIG. 4.
FIG. 4.

Pump-probe spectra of [LuPc2]TBA+ in benzonitrile with a focus on the NIR region for t 2 = 0.1, 0.6, 1.5, 3, and 10 ps (from red to blue). The inset shows the (normalized) kinetics at the frequencies indicated by the arrows in the main figure, i.e., at 10 870 cm−1 (red circles) and 12 500 cm−1 (blue circles). The solid lines are the corresponding fits. In pump-probe spectroscopy, the standard sign convention (i.e., ΔOD) is opposite to that in 2D, e.g., ESA gives rise to positive signal contributions.

Image of FIG. 5.
FIG. 5.

Pump-probe spectra of [LuPc2] in toluene for t 2 = 0.05, 0.2, 0.5, 1, 3, and 5 ps (from bottom to top). The spectra have been shifted for ease of viewing. Solid lines are fitted results based on a model discussed in the text. Negative signals correspond to SE/GSB in accordance with the convention in pump-probe.

Image of FIG. 6.
FIG. 6.

Diagrams illustrating selected singly excited states contributing to the model. The full set of states is obtained by permutations of A → B. Solid circles represent electrons, hollow circles represent holes. Black and red color refer to excitation of x- and y-orbital, respectively. The couplings between the states are indicated by J ex and J ct .

Image of FIG. 7.
FIG. 7.

Optimized ground state geometries at the BHLYP/SV(P) level of theory. (a) [LuPc2] and (b) [LuPc2]TBA+.

Image of FIG. 8.
FIG. 8.

(a) Linear absorption spectra of [LuPc2] (red squares) and [LuPc2]TBA+(blue circles). The solid curves show the simulations based on the model discussed in the text. For [LuPc2]TBA+, the stick-spectrum using a single realization of disorder is also shown. The four peaks correspond to the transitions to the states e 4 (14 273 cm−1), e 5 (14 778 cm−1), e 7 (16 153 cm−1), and e 8 (16 349 cm−1) in the exciton model. (b) Energy level structure in the exciton model for [LuPc2] (i) and [LuPc2]TBA+ (ii). Solid (dashed) lines correspond to states with large (small) transition dipole moment from the ground state. Solid (dashed) arrows indicate the major (minor) relaxation pathways discussed in the text.

Image of FIG. 9.
FIG. 9.

Dynamics of e 8 (black solid), e 7 (red dash), e 5 (blue dot), e 4 (magenta dashed-dotted), and e 1 (olive dashed-dotted-dotted) after initial population of e 8 (a) and e 7 (b).

Image of FIG. 10.
FIG. 10.

Kinetics of the simulated 2D spectra with (solid) and without (dashed) ESA contribution. (a) dp1 (black) and dp2 (red). (b) cp1 (black) and cp2 (red).

Image of FIG. 11.
FIG. 11.

Dominant orbitals involved in selected transitions for [LuPc2]TBA+ using TD-BHLYP/SV(P). The legend to the left refers to the number of the state in Table I, and the legend to the right to the features assigned in Fig. 2. The timescales are obtained from the analysis of the kinetics of the 2D spectra. The curved arrows indicate the different electron (ET) and hole-transfer (HT) processes taking place after excitation of the upper band in [LuPc2]TBA+.


Generic image for table
Table I.

Transition energies and oscillator strengths for the first 6 electronic states in [LuPc2]TBA+ at the BHLYP/SV(P) level of theory.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Ultrafast photo-induced charge transfer unveiled by two-dimensional electronic spectroscopy