Influence of excitonic coupling, static disorder, and coherent dynamics in action-2D electronic spectroscopy of a molecular dimer model

TL;DR

This work analyzes how excitonic coupling, static disorder, and coherent dynamics shape Action-2DES spectra in a minimal molecular-dimer model that includes second-excited states. Using a Lindblad framework and an explicit four-pulse A-2DES formalism with $K=\alpha J$, the authors dissect how cross peaks arise from incoherent mixing versus excitonic delocalization across weak-to-strong coupling. They demonstrate that intermediate coupling yields the strongest cross peaks with high-contrast waiting-time oscillations in A-2DES, while static disorder suppresses these features—especially those linked to incoherent mixing—and that cross peaks become more robust when coupling is strong. The results highlight A-2DES as a powerful tool for probing excitonic dynamics in small multi-chromophoric systems, with implications for interpreting spectra in real materials and for distinguishing coherent from incoherent contributions.

Abstract

We investigate the spectral features of Action-2D Electronic Spectroscopy (A-2DES) in a molecular dimer model across different regimes of excitonic coupling. By explicitly including a second-excited state for each chromophore, we simulate A-2DES spectra ranging from the non-interacting limit to the strong-coupling case, focusing on the significance of cross peaks. While for weak excitonic coupling, cross peaks can be understood as the incoherent mixing of linear signals of the two chromophores, these features reflect excitonic delocalization as the coupling increases. We highlight that A-2DES offers enhanced sensitivity to coherent excited-state dynamics, particularly in the intermediate-coupling regime, where it provides higher contrast compared to its coherent-detected counterpart. Finally, we show that static disorder reduces the relative amplitude of cross peaks compared to diagonal features in a way that depends on the excitonic coupling. Notably, the relative suppression of cross peaks decreases with the strength of the excitonic coupling, implying that spectral features related to incoherent mixing are less prominent in inhomogeneous samples. These findings support the potential of A-2DES for investigating excitonic dynamics in small multi-chromophoric systems.

Figures (6)

• Figure 1: Model of a molecular dimer, where each chromophore is described as a three-level electronic system. The states can be represented in the (a) localized site basis and (b) delocalized excitonic basis. The yellow boxes indicate the states of the one- and two-exciton manifolds interacting via excitonic coupling $J$ and $K$, respectively. The arrows represent the processes of Exciton Recombination (ER), Internal Conversion (IC), and Exciton-Exciton Annihilation (EEA).
• Figure 2: (a) Sequence of four laser pulses used in A-2DES experiments and emission of the incoherent signal. (b) Feynman diagrams for the considered system relative to Ground-State Bleaching (GSB), Stimulated Emission (SE), Excited-State Absorption I (ESAI), and Excited-State Absorption II (ESAII) pathways. (c) Position of the spectral features corresponding to the diagonal peaks (D1 and D2) and cross peaks (C1 and C2).
• Figure 3: (a-d) A-2DES spectra for different values of $2J/\Delta E$: (a) $0.0$, (b) $0.005$, (c) $0.1$, (d) $0.5$. (e) Comparison between the amplitudes of the spectral features as a function of $2J / \Delta E$ in A-2DES (solid lines) and C-2DES (dashed lines). The different regions correspond to: (white region) no cross peaks, (red region) incoherent mixing, (green region) ESA-free, (blue region) delocalization. (f-i) C-2DES spectra for different values of $2J/\Delta E$: (f) $0.0$, (g) $0.005$, (h) $0.1$, (i) $0.5$.
• Figure 4: Amplitudes of spectral features in A-2DES (solid lines) and C-2DES (dashed lines) as a function of the waiting time $t_{2}$ for (a) $2J/\Delta E = 0.1$, (b) $2J/\Delta E = 0.5$, and (c) $2J/\Delta E = 1.0$.
• Figure 5: A-2DES spectra for different values of excitonic coupling (columns) and static disorder (rows).