The objective of the research was to study the effects of alkylidene bridges on the conformations and the conformational behaviour of overcrowded homomerous bistricyclic aromatic ethenes (1). The isopropylidenebridged bistricyclic ethene 2 and 3 were synthesized by a reductive "dimerization" of 7, using TiCl 4-Zn-pyridine-THF. The methylene-bridged bistricyclic ethenes 4-6 were synthesized by LiAlH 4-AlCl 3-Et 2 O reductions of the corresponding bianthrones. The structures of 2-6 were established by 1 Hand 13 C-NMR spectroscopy and in the cases of 2 and 3, also by X-ray analysis. Compounds 2 and 3 adopted C i-anti-folded conformations with 53.0Њ and 28.8Њ folding dihedrals between pairs of benzene rings of tricyclic moieties. The central C 9 ᎐ ᎐ C 9Ј bond in 2 was essentially planar. A short C 9 ؒ ؒ ؒ C 10 distance of 2.81 Å in 2 indicated an intramolecular overcrowding effect in the highly folded bistricyclic ethene. Semiempirical PM3 and AM1 calculations of the anti-folded, syn-folded, twisted and orthogonally twisted conformations of 2 and 4 indicated that anti-folded 2 and 4 were the most stable conformations with folding dihedrals of 48.7Њ and 45.0Њ, respectively at AM1. A DNMR spectroscopic study of E, Z-isomerizations and conformational inversions gave ∆G c ‡ (E Z) = 99.6 kJ mol Ϫ1 (CDBr 3) and ∆G c # (inversion) = 97.9 kJ mol Ϫ1 (hexachlorobutadiene) in 5 and ∆G c ‡ (inversion) > 108 kJ mol Ϫ1 (benzophenone) in 3. These high energy barriers were interpreted in terms of less overcrowded fjord regions in the anti-folded ground-state conformations.

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The overcrowded bistricyclic aromatic enes (BAEs) [10-[10-(dicyanomethylene)-9(10H)-anthracenylidene]-9(10H)-anthracenylidene]propanedinitrile (7) and [10-[10-oxo-9(10H)-anthracenylidene]-9(10H)-anthracenylidene)]propanedinitrile (8) were synthesized by a condensation of bianthrone (2) with malononitrile in the presence of TiCl 4 and pyridine. The crystal and molecular structure of 7 were determined. It crystallizes in two polymorphic forms, belonging to the space groups P2 1 /c and P2 1 /n. DFT calculations of 7 and 8 show

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