Resumen de Visible Absorption Spectroscopy and Structure of Cyanine Dimers in Aqueous Solution:: An Experiment for Physical Chemistry
Miin-Liang Horng, Edward L. Quitevis
The shape of the visible absorption spectra for cyanine dyes in aqueous solution is concentration dependent. This property is attributed to the formation of dimers and higher aggregates in solution. The shape changes are typically manifested by a splitting of the absorption bands or the appearance of new bands. These spectral features arise from exciton coupling between the molecules in an aggregate. In this experiment, an iterative procedure for obtaining the dimer spectrum from the composite spectra of concentrated dye solutions is used. Simple exciton theory is used to obtain information about the structure of the dimer from its spectrum. The iterative procedure and the application of simple exciton theory are demonstrated using the visible absorption spectra of 1,1'-diethyl-2,2'-cyanine in aqueous solution at different concentrations. In this experiment, concepts in quantum theory and spectroscopy are illustrated. These include energy delocalization, excitons, transition moments, and oscillator strengths. Expanding the scope of analysis to include dye aggregation and exciton theory enhances the pedagogical value of studying the visible absorption spectra of conjugated dyes in the physical chemistry curriculum.
