1,3-Bicyclo ACHTUNGTRENNUNG[1.1.1]pentanediyl: The Shortest Rigid Linear Connector of Phenylated Photochromic Units and a 1,5-Dimethoxy-9,10- di(phenylethynyl)anthracene Fluorophore

BicycloACHTUNGTRENNUNG[1.1.1]pentane and its derivatives are not only of theoretical interest, but have attracted particular attention of organic and physical-organic chemists as peculiar fragments in oligomers, liquid crystalline compounds, supramolecular aggregates and monolayers, as well as compounds for drug design, etc. 1,3-Disubstituted derivatives—bicycloACHTUNGTRENNUNG[1.1.1]pentanes with blocked bridgehead positions—are rigid rod-like molecules which, in spite of their high strain energy, are remarkably thermally stable, resistant to oxygen and organometallic reagents. Bicyclo ACHTUNGTRENNUNG[1.1.1]pentane is transparent in the UV and visible light region, and because of its very short C–C distance of approximately 1.87 ), it can serve as a unique linker or bridge between two unsaturated molecular fragments, if their close proximity, yet without any significant p-conjugation, is desired. According to previous work from our own as well as from another group, 1,3diarylbicyclo ACHTUNGTRENNUNG[1.1.1]pentanes can be prepared. Thus, it ought to be possible to incorporate the bicycloACHTUNGTRENNUNG[1.1.1]pentane unit between any two aromatic systems, for example, between a fluorescent group and a photochromic part (Figure 1). MateAbstract: An excess of bis-1,3-(4iodophenyl)bicyclo ACHTUNGTRENNUNG[1.1.1]pentane, prepared in 63% yield by iodination of 1,3-diphenylbicycloACHTUNGTRENNUNG[1.1.1]pentane, was selectively mono-coupled with 9-ethynyl-1,5-dimethoxy-10-phenylethynylanthracene (26), and subsequently with the zinc derivatives of 1-(2-methyl/methoxy-4-methyl-5-phenylthiophen-3-yl)2-(2-methyl/methoxy-4-methylthiophen-3-yl)perfluorocyclopentenes (38H–41-H). Regioselective synthesis of the 2-unsubstituted thiophenes 38-H– 41-H required intermediate preparation of 2-trimethylsilyl-3,5-dimethyl-4bromothiophene (37) or 2-trimethylsilyl-5-methoxy-3-methyl-4-bromothiophene (40). Protection of the a-position of the thiophene ring with a 2-trimethylsilyl group blocks the rearrangement of the 4-lithio derivatives into the corresponding 2-lithiated thiophenes. With the bicycloACHTUNGTRENNUNG[1.1.1]pentane fragment linking the photochromic units 1– 3 and 1,5-dimethoxy-9,10-di(phenylethynyl)anthracene as a fluorescent part, quantitative resonance energy transfer between the excited state of the fluorophore (donor) and the closed form of the photochromic units 1–3 (acceptors) was observed. The closed forms of the methoxy-substituted photochromic units 2 and 3 are less resistant to UV light (313 nm) than the closed form of 1.