DICOPPER(I) TREFOIL KNOTS AND RELATED UNKNOTTED MOLECULAR-SYSTEMS - INFLUENCE OF RING SIZE AND STRUCTURAL FACTORS ON THEIR SYNTHESIS AND ELECTROCHEMICAL AND EXCITED-STATE PROPERTIES

Five new dicopper(I) knots have been synthesized as well as their face-to-face isomers. The knots range from 80- to 90-membered rings, and their preparation yields depend crucially on structural parameters such as number of methylene fragments linking the two chelating units and length of the poly(ethyleneoxy) unit used in the cyclization reaction. The best yield was obtained for an 84-membered knotted ring with a -(CH2)6- connector: this relatively long fragment allows pronounced winding of the double-helix precursor and is thus favorable to the knotting reaction. The face-to-face complexes were in some instances the major products, being obtained in yields amounting to 24% in the case of the dicopper(I) bis(43-membered-ring) system. The electrochemical properties of the copper complexes also depend on their structure. The redox potential values of the Cu(II)/Cu(I) couple span over a wide range (approximately 0.5-0.75 V vs SCE), the most electrochemically stable copper(I) complex being the 84-membered knotted compound. In CH2Cl2 solution, both the Cu2 knots and their face-to-face isomers exhibit metal-to-ligand charge-transfer absorption bands in the visible region and emission bands in the red spectral region. The profile of the absorption spectra and the luminescence properties (lambda(max) quantum yield, lifetime, and rate of excited-state quenching by acetone) depend on the length of the connectors. In agreement with the electrochemical results, the -(CH2)6- linker has a pronounced shielding effect on the metal center as well as a special ability to impose geometrical constraint.