acetylene chermistry by F. Diederich, P. J. Stang, R. R. Tykwinski

By F. Diederich, P. J. Stang, R. R. Tykwinski

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5 Hybrid sp-sp3 Oligoacetylenic Molecules Several b-diynes (or “skipped” diynes [99]) X(CaCH)2 (X = CRl2, SiRl2, PRl, S,…) are known, but the elusiveness of the unsubstituted diethynyl ether (X = O) [100] motivated comparative ab initio theoretical comparisons with the analogues X = S [101] or X = Se [102]. 3 Cyclic Acetylenic Scaffolds ≠ R • R •• X X X • P R P P •• R R X = HO >> H2 N > O > S > NH > PH > CH R = C≡CH kT Reactivity of b-diynes: theoretically predicted Bergman-like cycloaromatization [103] and observed inversion of stereogenic propargylic phosphorus centers [105].

Even at low temperature, free ring twisting dominates in the gas phase, becoming almost periodic after a few picoseconds of equilibration, while random conformations are predicted in solution. This random twisting is expected to affect the fluorescence properties of these compounds significantly. Indeed, rigidity favors fluorescence, making intramolecular dissipation difficult. Molecular dynamics was also used to describe the arrangement of the chains of block copolymers containing one conjugated p-EP segment and one or two non-conjugated segments (e.

However, carbo-merization of the acceptor NO2 group remains essential. The properties of the NC4O2 group were thus investigated at higher levels of theory (DFT and CASSCF). Several nearly degenerate conformational minima were found, the sp carbon atoms exhibiting strong carbenic character. This suggests a high flexibility and instability of the NC4O2 group, thus discouraging attempts at experimental synthesis. Chromophores made up of ethynyl-carbo-phenylene units bridging pyridinium and phenolate end groups have also been envisioned [88], but no b0 exaltation was predicted.

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