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-positions, respectively. The products were found to be fluorescent and moderately soluble in organic solvents. Three biimidazoles were subjected to single crystal 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 helices. The amount of helical twist observed between neighboring biimidazole 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 ions with masses corresponding to dimers; this indicates that these compounds retain some affinity for each other in the gas phase. The three most soluble biimidazoles also show mass spectrometric peaks ascribable to trimers and tetramers. The solution-phase aggregation tendencies of these latter three compounds were studied by vapor pressure osmometry. In each case, the apparent molecular weight in 1,2-dichloroethane solution is higher than would be expected for free monomers.