Final Report Summary - HELICALLENES (Helical Alleno-Acetylenic Oligomers with High Chiroptical Activity)
The principal goal of this project was to develop and study structurally-stable materials composed of all-carbon backbones, namely, alleno-acetylenes. These axially chiral building blocks can be obtained in their enantiopure form, and display outstanding chiro-optical properties. With the support of this grant we therefore perused the following projects, which resulted in new supramolecular materials, deeper understanding of their structure-property relations, and in the application of these materials as sensitive sensors for the detection of small molecules.
1. In a combined computational and experimental study of alleno-acetylenic macrocycles and oligomers with different linkers (acetylenes and diacetylenes), we have shown that strong Cotton effect is a hallmark of alleno-acetylenic molecules. In addition we have provided evidence that these molecules adopt a secondary helical structure, and that the handedness of these structures is dependent on the chirality of the constituting monomers. This work was recently published in Chemistry - A European Journal.
2. We have found that enantiopure alleno-acetylenic ligands assemble diastereoselectively upon the addition of zinc(II) salt to form triple-stranded helicates, which provide a sufficiently large helical cage (‘helicage’) for encapsulation of organic molecules. This is an unusual case for helicates, rarely containing large enough cavity for encapsulation of organic guests. The circular dichroism (CD) spectra of the helicates, which showed strong Cotton effects and exciton coupling, were found to be extremely sensitive to the nature of the guest molecules. Consequently, a series of non-chromophoric, achiral guests of different sizes, or regioisomers (1,3- and 1,4-dioxane) became distinguishable on the basis of their induced CD (ICD) spectra. Particularly high affinity towards 1,4-dioxane, a class 2B carcinogen routinely monitored in drinking water, allowed its selective detection at parts-per-million (ppm) levels in aqueous solutions. This example highlights the potential of such materials to function as detectors for various molecules. This work was recently published in Angewande Chemie.
3. Extended helical cages of alleno-acetylenes, bearing two separated cavities were synthesized and investigated. We have found that small guests bind independently to the two cavities. The binding of various guests was readily distinguishable by their ICD spectra, with stronger Cotton effect in comparison with the shorter helical cage. As far as we know, this is the first example of multiple-guests binding inside enantiopure helicates. A manuscript describing the above results is currently in preparation.
4. Homochiral strands of alleno-acetylenes selectively assemble upon the addition of silver(I) salt to yield double-stranded helicates. Upon increasing the solvent polarity, these helicates form interlocked structures (catenanes) with unprecedented morphology. The CD spectra show a significant increase in the Cotton effect for catenanes over the corresponding helicates, indicating stabilization of the secondary structure. Highly selective narcissistic self-sorting was demonstrated for a racemic mixture containing short and long catenanes, highlighting the potential of this method for the preparation of linear and homochiral [2n-2]pseudocatenanes from n nuclear helicates. A manuscript describing the above results is currently in preparation.
The scientific and socio-economic impact of this project stems from the potential application of these materials as sensitive and selective sensors for the detection of analytes in aqueous solutions, as demonstrated in project 2. In addition, the highly-selective assembly of the alleno-acetylenes demonstrated in projects 2-4, combined with their outstanding chiroptical properties, highlights their potential application of such materials in future opto-electronic devices.