Self-assembled helices from 2,2 '-biimidazoles

2,2'-Biimidazoles were synthesized by palladium(0)-catalyzed coupling of 2- iodoimidazoles bearing an alkyl and an ester group at the 4- and 5-position s, respectively. The products were found to be fluorescent and moderately s oluble in organic solvents. Three biimidazoles were subjected to single cry stal X-ray diffraction analysis. In all three instances, adjacent molecules were found to be bound together in the solid state by pairs of N-H . . .N hydrogen bonds, forming twisted ribbon-like columns which resemble double h elices. The amount of helical twist observed between neighboring biimidazol e subunits in these helices varies with the identity of the alkyl and ester groups; in two cases it is approximately 60 degrees, whereas in the third it is about 90 degrees Mass spectra of six different biimidazoles display i ons with masses corresponding to dimers; this indicates that these compound s retain some affinity for each other in the gas phase. The three most solu ble biimidazoles also show mass spectrometric peaks ascribable to trimers a nd tetramers. The solution-phase aggregation tendencies of these latter thr ee compounds were studied by vapor pressure osmometry. In each case, the ap parent molecular weight in 1,2-dichloroethane solution is higher than would be expected for free monomers.