Direct observation of exciton–exciton interactions

Abstract

Natural light harvesting as well as optoelectronic and photovoltaic devices depend on efficient transport of energy following photoexcitation. Using common spectroscopic methods, however, it is challenging to discriminate one-exciton dynamics from multi-exciton interactions that arise when more than one excitation is present in the system. Here we introduce a coherent two-dimensional spectroscopic method that provides a signal only in case that the presence of one exciton influences the behavior of another one. Exemplarily, we monitor exciton diffusion by annihilation in a perylene bisimide-based J-aggregate. We determine quantitatively the exciton diffusion constant from exciton–exciton-interaction 2D spectra and reconstruct the annihilation-free dynamics for large pump powers. The latter enables for ultrafast spectroscopy at much higher intensities than conventionally possible and thus improves signal-to-noise ratios for multichromophore systems; the former recovers spatio–temporal dynamics for a broad range of phenomena in which exciton interactions are present.

From wavelike to sub-diffusive motion: exciton dynamics and interaction in squaraine copolymers of varying length

“…This technique facilitates a direct observation of exciton-exciton interactions. [48][49][50] In EEI2D, exciton-exciton annihilation is observable in the evolution of the signal amplitude as a function of population time. This technique can therefore be used not only to study EEI, but one can utilize the annihilation as a tool to probe single-exciton propagation dynamics even in the absence of spectral changes during transport.…”

Mapping of exciton–exciton annihilation in a molecular dimer via fifth-order femtosecond two-dimensional spectroscopy

“…52 The paths contributing to the signal are conventionally illustrated using double-sided Feynman diagrams. 64,66 In the present case, one finds seven diagrams 15,16 which are collected in Fig. 3.…”

“…Thus, in larger systems the phenomena related to the existence of exciton-exciton interactions will appear time delayed. 16 This delay is not present in the dimer case where laser excitation prepares excitons on the two monomers residing right next to each other. Investigations on larger aggregates are under way.…”

“…15 Recently, it was shown that it is possible to detect exciton-exciton interactions and to extract diffusion time constants from coherent two-dimensional (2D) spectra. 16 The EEA is connected to an appearance of spectral peaks during a pulse delay time (population time) and it becomes more effective if the laser fields interacting with a sample are of high intensity. This is due to the fact that, with increasing field strength, the number of prepared excitons becomes larger, and the probability that two excitons meet is enlarged.…”

“…Thus, it is difficult (if possible at all) to isolate a signal which reflects the two-exciton interactions. On the other hand, the appropriate fifthorder signal 16 arises if exciton-exciton interaction is present. Thus, we here address which features of the EEI 2D signal contain which type of information.…”