Quantum Dynamics and Spectroscopy of Excitons in Molecular Aggregates

O. Kuhn,S. Lochbrunner

Published 2011 in arXiv: Chemical Physics

ABSTRACT

Abstract 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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