SYNTHESIS OF A VARIETY OF BICHROMOPHORIC BALL-AND-CHAIN SYSTEMS BASEDON BUCKMINSTERFULLERENE (C-60) FOR THE STUDY OF INTRAMOLECULAR ELECTRON AND ENERGY-TRANSFER PROCESSES

Diels-Alder reaction of C-60 with the 1,3-dienes 7e-h, 8a, 8b, and 8d- h affords the ''ball-and-chain'' systems 2e-h, 3a, 3b, and 3d-h bearin g two chromophores linked via a rigid, hybrid saturated polynorbornane -bicyclo[2.2.0]hexane (''norbornylogous'') hydrocarbon bridge. Analogo us reaction with the bis(diene) 9 affords the soluble dumbbell system 4 bearing two C-60, chromophores. The norbornylogous bridge is a stron g mediator of electron and energy transfer via a through-bond coupling mechanism. The norbornylogous donor-bridge-diene units 7d-h, 8a, 8b, and 8d-h were prepared in a straightforward manner from bicyclo[2.2.2] octane precursors by extending the bridges with linearly fused norborn ane-bicyclo[2.2.0]hexane moieties through execution of the tandem Mits udo-Smith series of reactions. The X-ray structure of the dimethoxyben zene-bridge-C-60 system 3a reveals favorable self-complementarity mani fested by the unusual packing structure of 3a in the crystal. Molecula r mechanics, semiempirical, and ab initio conformational analyses of c ompounds 2e, 3a, 3b, 3e, 3f, 3h, 68, and 70 (MM2, Sybyl, CVFF, AM1, HF /3-21G) were performed to quantify their ability to adopt two nondegen erate boat conformations, i.e,, extended and folded conformers, as wel l as their kinetic barrier of interconversion. A similar treatment of the C-60-bridge-C-60 system 4 revealed unusual preference for the fold ed-folded conformer (18.9 kcal/mol at CVFF level), which was not repro duced by the AM1 method (0.11 kcal/mol). The reduction potentials of t he systems 2e, 3a, and 3e were about 0.1-0.5 V more negative than C-60 , and the third reduction potential (E(3)) Of the 6-bond system 2e was 0.14 V more negative than the corresponding wave for the 10-bond syst em 3e. This shift was attributed to the closer proximity of the dimeth ylaniline donor group to the C-60 surface for 2e vs 3e.