PUB - Publikationen an der Universität Bielefeld

In this paper time-dependence of coherent energy transfer in a model oxacyanine (S9) Scheibe aggregate (or J-aggregate) in the presence of thiacyanine (S11) perturber chromophores is studied quantum mechanically. The mixed Scheibe aggregate is assumed to have a hexagonal two-dimensional (2D) brick layer structure. It is shown that the present model, based on the extended dipole-dipole resonance coupling energy among the monomer "bricks'', can account for the observed red-shift of the J-band (of the aggregate) with respect to the band of the monomers, for monomer of the same brick dimensions as estimated by Kuhn and co-workers, experimentally. Quantum propagation of the energy of an initial excitation in the aggregate is found to occur as a (probability) wave that can be strongly affected by the presence of the perturber chromophores. It is found that if a perturber is excited, a part of the excitation energy can be trapped at the site. More interestingly, it may also oscillate between two or more perturbers placed in the aggregate. This coherent oscillation of energy (that has been called a "sloshing'' effect) has been at first predicted for a quantum mechanical 1D molecular chain, is shown to occur also in the present 2D system of a mixed S9/S11-Scheibe aggregate. Under suitable conditions, the effect can be used to control the excitation transfer between the perturbing chromophores. For example, it is shown that more than 60% of the excitation energy can be transferred in this way within 1.5 ps between two perturber chromophores situated 80 Angstrom from each other. The density of states of the mixed aggregate and the excitation spectrum under various initial excitation conditions are investigated. It is suggested that the sloshing effect can be detected by resolving the associated strongly excited doublet line structure in the excitation spectrum of the perturbed aggregate. (C) 1999 American Institute of Physics. [S00219606(99)70507-6].