Enantiopure, monodisperse alleno-acetylenic oligomers, and cyclooligomers

Enantiopure 1,3-diethynylallenes were used as monomers in the construction of monodisperse oligomers and cyclooligomers. The open-chain structures show a non-linear increase in the intensity of the chiroptical properties as the no. of monomeric units increases. This behavior is indicative of the formation of a secondary structure. Time dependent DFT (TDDFT) calcns. were used to simulate their UV/Vis and electronic CD spectra. By comparison between the simulated and exptl. spectra, a single-handed helical secondary structure was proposed for these chiral oligomers. Similarly, cyclooligomers were prepd. and characterized, in many cases by single crystal X-ray diffraction. A combination of spectroscopic techniques and TDDFT calcns. led to the formulation of simple guidelines for the design of chiral carbon-rich structures with strong chiroptical properties. These principles are related to the extinction coeff., conformational flexibility, and symmetry of the mol., and can be extended to understand the chiroptical properties of polymers.