Functionalised oligoenes with unusual topologies: Synthesis, electrochemistry and structural studies on redox-active [3]- and [4]-dendralenes

New [3]- and [4]-dendralenes bearing electron-donor 1,3-dithiole and ferroc ene substituents have been synthesised. Compounds 8, 15 and 17 have been ch aracterised by single-crystal X-ray diffraction. Two of the dithiole rings of 8 are conjugated (dihedral angle 9 degrees), while the third dithiole ri ng is almost orthogonal to this plane, and hence its pi-electron system is isolated. For the dendralene precursor molecule 15, the substituted cyclope ntadienyl ring, two C=C bonds and fused dithiole and dithiine rings compris e an extended pi-conjugated system. In molecule 17 the potential conjugatio n path C(6)C(3) C(4)C(5)-C5H5 is distorted by an 8 degrees twist around the C(3)-C(4) bond and a 7 degrees twist around the C(5)-C(21) bond, and the d elocalisation along the chain is insignificant. Solution electrochemical da ta demonstrate that the dendralenes are strong pi-electron donors, which gi ve rise to dication, radical trication or tetracation species. Spectroelect rochemical studies on compounds 7 and 10 suggest that the radical species a re situated within the linear 1,2-ethylenediylidene moieties and that a con formational change may occur at the dication redox stage. UV/Vis spectrosco pic data are consistent with poor cross-conjugation in these systems.