Have you ever ruminated on the factors which affect the color of a given compound in solution? If so, let us help broaden your perspective as we examine the properties of pentamethine dyes. In order to better understand aggregate behavior and learn what types of interactions (exciton theory versus molecular orbital theory) contribute to observed UV-vis absorbance shifts at varying concentrations, research focused on synthesis of various dimers in order to evaluate the corresponding monomer chromophores under fixed geometries. Also, varying colors in different solvents (solvatochromism) and polymorphism were observed.
Research performed by Maggie Karr and Jai Pattur under Professor Charles Liberko included synthesis of (1) a previously known pentamethine (monomer) from the dihydrate sodium salt of 5-hydroxypenta-2,4-dienal with dimethylamine and dimethylamine tertrafluoroborate, or 2,4-dinitrophenyl pyridinium chloride and dimethylamine tertrafluoroborate, (2) a new compound, cyanine sandwich dimer (tied twice at each end), from 1,5-diazacyclooctane and pentamethine, or from 2,4-dinitrophenylpyridinium chloride.
Although not all projects met with success, we were able to synthesize sandwich dimer from pentamethine and 1,5-diazacyclooctane. UV-vis spectroscopy revealed that the monomer absorbed at 410 nm, whereas the dimer was blue shifted and absorbed at 400 nm. This is predicted for H-aggregates by exciton theory. Another group produced similar dimer from the same monomers and observed a much larger shift (410 nm to 380 nm). Since a slightly greater blue shift is predicted by exciton theory, the smaller blue shift observed here is believed to be the result of molecular orbital interactions.
Margaret Karr, ’03 Vinton, IA
Major: Biochemistry and Molecular Biology
Jai Pattur, ’04 Castle Rock, CO
Majors: Chemistry, Biochemistry and Molecular Biology, Philosophy
Sponsor: Charles Liberko