A cyclic dimer of the streptocyanine dye, 5-dimethylaminopenta-2,4-dienylidenedimethylammonium chloride, was prepared in a 29% purified yield by the nucleophilic displacement of the dimethylamine groups by 1,5-diazacyclooctane.  The pentamethine chromophores are held in close proximity in a face-to-face (H-aggregate) arrangement by the connecting propyl chains.  The dimer is similar in structure to a previously reported dimer 1 but with closer pi-stacking distance between the chromophores.  Exciton theory would predict that the closer pi-stacked dimer would have the larger blue shifted UV-Vis absorbance compared to the absorbance of the monomer, but the closer pi-stacking distance in this case leads to a smaller blue shifted absorbance.    The lower molar absorptivity as well as the larger Stokes shift in the emission spectra for the dimer suggest that the molecular orbital interactions between the chromophores are important  and that exciton theory alone cannot account for the optical properties of these dimers.  1) Okazaki et al, Bull. Chem. Soc. Jpn, 1997, 70, 2279

Optical properties of cyclic dimers of a pentamethine dye: Moving beyond exciton theory

240th National Meeting (2010)

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