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 t heir 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 wind ing of the double-helix precursor and is thus favorable to the knottin g reaction. The face-to-face complexes were in some instances the majo r products, being obtained in yields amounting to 24% in the case of t he dicopper(I) bis(43-membered-ring) system. The electrochemical prope rties of the copper complexes also depend on their structure. The redo x 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 co pper(I) complex being the 84-membered knotted compound. In CH2Cl2 solu tion, both the Cu2 knots and their face-to-face isomers exhibit metal- to-ligand charge-transfer absorption bands in the visible region and e mission bands in the red spectral region. The profile of the absorptio n spectra and the luminescence properties (lambda(max) quantum yield, lifetime, and rate of excited-state quenching by acetone) depend on th e length of the connectors. In agreement with the electrochemical resu lts, the -(CH2)6- linker has a pronounced shielding effect on the meta l center as well as a special ability to impose geometrical constraint .