Picosecond spectroscopy of molecular aggregates

Date

1992-12

Journal Title

Journal ISSN

Volume Title

Publisher

Texas Tech University

Abstract

Cyanine dyes in aqueous solutions at higher concentrations are known to deviate from Beer's law. These spectral changes have been attributed to the formation of dimers and higher polymers, and may consist of a shift of the absorption peak or a splitting of the monomer band due to excitonic interactions. Dimerization of l,r-diethyl-2,2'-carbocyanine has been studied over a certain concentration range. The value of the dimerization equilibrium constant K has been determined. The dimer spectrum has been extracted from the composite spectra and analyzed in terms of a vibronic exciton coupling theory.

The study of exciton transport in simple molecular aggregates is crucial in understanding the dynamics of excitation energy transfer in light-harvesting antennae. The optical dynamics of J-aggregates of l,r-diethyl-2,2'-cyanine in sodium chloride solution, on colloidal silica and poly(vinylsulfonic acid, sodium salt) have been studied by using picosecond polarized pump-probe spectroscopy. The picosecond transient photobleaching signal of colloidal silica system decays biexponentially and appears to be independent of the laser intensity. The absorption anisotropy is constant and discussed in terms of the coherence properties of excitons. For the J-aggregates in the systems of sodium chloride and polyelectrolyte solutions, the absorption anisotropy decay and ground-state recovery kinetics are excitation-intensity dependent. The excitation-intensity dependence of the decay kinetics of J-aggregates in these two systems indicates that exciton annihilation is occurring. The results are discussed in terms of the dynamics of delocalized and localized excitons.

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