HgCl2(Caf): Co‐crystallization of Mercuric Chloride and Caffeine

Colourless single crystals of the co‐crystallizate of mercuric chloride and caffeine, HgCl₂(Caf), were obtained from an ethanolic solution of mercuric chloride, HgCl₂, and caffeine (Caf) and recrystallized from hot water. The crystal structure (monoclinic, P2₁, Z = 2, a = 398.36(8), b = 1964.5(4), c = 809.6(2) pm, β = 99.24(3)°, Z = 2, R₁ = 0.0584 for 1430 F_o > 4σ(F_o)) contains helical chains (parallel to the 2₁ screw axis) of almost unaffected HgCl₂ molecules and caffeine molecules which are very weakly bound to one keto‐oxygen atom (O4) of one and N9 of a second caffeine molecule at distances of 282 and 281 pm, respectively. To the contrary, theoretical calculations show that the molecule HgCl₂(Caf)₂ is stable (in the gas phase at T = 0 K) with surprisingly strong bonding as indicated by the “tetrahedrization” of the molecule.     

Divalent ligand for intramolecular complex formation to streptavidin

Monovalent ligand and divalent ligand have been synthesized, and their thermodynamic parameters of complexation to avidin and streptavidin have been analyzed in terms of multivalent binding.     

Enantioselective synthesis of chiral porphyrin macrocyclic hosts and kinetic enantiorecognition of viologen guests

The construction of macromolecular hosts that are able to thread chiral guests in a stereoselective fashion is a big challenge. We herein describe the asymmetric synthesis of two enantiomeric C₂-symmetric porphyrin macrocyclic hosts that thread and bind different viologen guests. Time-resolved fluorescence studies show that these hosts display a factor 3 kinetic preference (ΔΔG^‡_on = 3 kJ mol⁻¹) for threading onto the different enantiomers of a viologen guest appended with bulky chiral 1-phenylethoxy termini. A smaller kinetic selectivity (ΔΔG^‡_on = 1 kJ mol⁻¹) is observed for viologens equipped with small chiral sec-butoxy termini. Kinetic selectivity is absent when the C₂-symmetric hosts are threaded onto chiral viologens appended with chiral tails in which the chiral moieties are located in the centers of the chains, rather than at the chain termini. The reason is that the termini of the latter guests, which engage in the initial stages of the threading process (entron effect), cannot discriminate because they are achiral, in contrast to the chiral termini of the former guests. Finally, our experiments show that the threading and de-threading rates are balanced in such a way that the observed binding constants are highly similar for all the investigated host–guest complexes, i.e. there is no thermodynamic selectivity.

Chiral guests display kinetic stereoselective threading through chiral porphyrin cages if their chirality is located at the chain ends and not in the centers, supporting the previously reported entron effect of threading.     

Boden

Ohne Zusammenfassung     

Direct time–space conversion of ultra-fast pulses based on interferometric time-of-flight cross correlation using a fast optical addressable spatial light modulator

To realize an ultra-fast spatio-temporal photonic interface system, we propose direct time–space conversion of ultra-fast pulses based on the interferometric time-of-flight cross correlation technique using a fast optical addressable spatial light modulator (OA-SLM). We show preliminary experiments using transverse-field photorefractive MQW devices as an OA-SLM compatible with ultra-fast pulses.     

Wasseruntersuchung

Ohne Zusammenfassung