Local energetic disorder in molecular aggregates probed by the one-exciton to two-exciton transition

doi:10.1016/0009-2614(94)00379-3

Chemical Physics Letters

Volume 222, Issue 5, 27 May 1994, Pages 450-456

https://doi.org/10.1016/0009-2614(94)00379-3 Get rights and content

Abstract

We demonstrate a novel approach to probing the magnitude and degree of spatial correlation of local (molecular or atomic) energetic disorder in delocalized exciton systems. The approach is based on measuring the correlation between the ground state to one-exciton and the one-exciton to two-exciton transition frequencies using two-color pump—probe experiments. We apply this technique to aggregates of pseudo-isocyanine and find surprisingly strong intermolecular correlations in the disorder.

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The theoretical description and the properties of Frenkel excitons in noncovalently bonded molecular aggregates are reviewed from a multiexciton perspective of dissipative quantum dynamics. First, the photophysical and quantum chemical characterization of the monomeric dye building blocks is discussed, including the important aspect of electron-vibrational coupling within the Huang-Rhys model. Supplementing the model by the Coulombic interactions between monomers, the description of aggregates in terms of excitonic or vibrational-excitonic bands follows. Besides giving rise to complex absorption and emission line shapes, exciton-vibrational interaction is responsible for energy and phase relaxation, and thereby limits the size of coherent excitations in larger aggregates. Throughout, emphasis is put on the electronic three-level model as a minimum requirement to describe nonlinear spectroscopies including effects of two-exciton states such as excited state absorption and exciton–exciton annihilation. The experimentally observed characteristics of stationary absorption and fluorescence spectra of aggregates aswell as their temperature dependence are discussed. Examples for ultrafast spectroscopic experiments including pump-probe studies, photon echo and two-dimensional spectroscopy are presented and results on the size of coherence domains and on intraand interband relaxation are given. Finally, experimental signatures for exciton–exciton annihilation and their analysis with respect to the mobility of excitons are described.
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Citation Excerpt :
In the latter, two or more excitation quanta are shared by the molecules in one coherence domain. An experiment that has drawn particular attention during the past 15 years, is the pump-probe spectrum [4–18]. Typically, this spectrum reveals a dispersive line shape, characterized by a negative peak, due to bleaching and stimulated emission of the one-exciton states, and a blue-shifted positive (induced-absorption) peak, which is caused by the probe pulse inducing transitions from the one-exciton states created by the pump pulse to the two-exciton manifold.
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Local energetic disorder in molecular aggregates probed by the one-exciton to two-exciton transition

Abstract

Chem. Phys.

Chem. Phys.

Chem. Phys. Letters

Chem. Phys. Letters

Phys. Rev. Letters

Physics Today

Phys. Rev.

J. Chem. Phys.

Phys. Rev. Letters

J. Chem. Phys.