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The reversible Z-E photoswitching properties of the (Z) and (E) isomers of the severely constrained bridged azobenzene derivative 5,6-dihydrodibenzo[c,g][1,2]diazocine (1) were investigated quantitatively by UV/vis absorption spectroscopy in solution in n-hexane. In contrast to normal azobenzene (AB), 1 has well separated S(1)(n pi*) absorption bands, peaking at lambda(Z) = 404 nm and lambda(E) = 490 nm. Using light at lambda = 385 nm, it was found that 1Z can be switched to 1E with very high efficiency, Gamma = 92 +/- 3%. Conversely, 1E can be switched back to 1Z using light at lambda = 520 nm with approximately 100% yield. The measured quantum yields are Phi(Z-->E) = 72 +/- 4% and Phi(E-->Z) = 50 +/- 10%. The thermal lifetime of the (E) isomer is 4.5 +/- 0.1 h at 28.5 degrees C. The observed photochromic and photoswitching properties of 1 are much more favorable than those for normal AB, making our title compound a promising candidate for interesting applications as a molecular photoswitch especially at low temperatures. The severe constraints by the ethylenic bridge apparently do not hinder but favor the Z-E photoisomerization reactions.

Magnetic bistability, as manifested in the magnetization of ferromagnetic materials or spin crossover in transition metal complexes, has essentially been restricted to either bulk materials or to very low temperatures. We now present a molecular spin switch that is bistable at room temperature in homogeneous solution. Irradiation of a carefully designed nickel complex with blue-green light (500 nanometers) induces coordination of a tethered pyridine ligand and concomitant electronic rearrangement from a diamagnetic to a paramagnetic state in up to 75% of the ensemble. The process is fully reversible on irradiation with violet-blue light (435 nanometers). No fatigue or degradation is observed after several thousand cycles at room temperature under air. Preliminary data show promise for applications in magnetic resonance imaging.

The defining feature of aromatic hydrocarbon compounds is a cyclic molecular structure stabilized by the delocalization of pi electrons that, according to the Hückel rule, need to total 4n + 2 (n = 1,2, em leader ); cyclic compounds with 4n pi electrons are antiaromatic and unstable. But in 1964, Heilbronner predicted on purely theoretical grounds that cyclic molecules with the topology of a Möbius band--a ring constructed by joining the ends of a rectangular strip after having given one end half a twist--should be aromatic if they contain 4n, rather than 4n + 2, pi electrons. The prediction stimulated attempts to synthesize Möbius aromatic hydrocarbons, but twisted cyclic molecules are destabilized by large ring strains, with the twist also suppressing overlap of the p orbitals involved in electron delocalization and stabilization. In larger cyclic molecules, ring strain is less pronounced but the structures are very flexible and flip back to the less-strained Hückel topology. Although transition-state species, an unstable intermediate and a non-conjugated cyclic molecule, all with a Möbius topology, have been documented, a stable aromatic Möbius system has not yet been realized. Here we report that combining a 'normal' aromatic structure (with p orbitals orthogonal to the ring plane) and a 'belt-like' aromatic structure (with p orbitals within the ring plane) yields a Möbius compound stabilized by its extended pi system.

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