Highly Sensitive, Easy-to-Use, One-Step Detection of Peroxide-, Nitrate- and Chlorate-Based Explosives with Electron-Rich Ni Porphyrins.

Homemade explosives, such as peroxides, nitrates, and chlorates, are increasingly abused by terrorists, criminals, and amateur chemists. The starting materials are easily accessible and instructions on how to make the explosives are described on the Internet. Safety considerations raise the need to detect these substances quickly and in low concentrations using simple methods. Conventional methods for the detection of these substances require sophisticated, electrically operated, analytical equipment. The simpler chemical detection methods are multistep and require several chemicals. We have developed a simple, one-step method that works similarly to a pH test strip in terms of handling. The analytical reaction is based on an acid-catalyzed oxidation of an electron-rich porphyrin to an unusually stable radical cation and dication. The detection limit for the peroxide-based explosive triacetone triperoxide (TATP), which is very frequently used by terrorists, is 40 ng and thus low enough to detect the substance without direct contact via the gas phase. It is sufficient to bring the stick close to the substance to observe a color change from red to green. Nitrates and chlorates, such as ammonium nitrate, urea nitrate, or potassium chlorate, are detected by direct contact with a sensitivity of 85-350 ng. A color change from red to dark brown is observed. The test thus detects all homemade explosives and distinguishes between the extremely impact-, shock-, and friction-sensitive peroxides and the less sensitive nitrates and chlorates by color change of a simple test strip.

Light-Controlled Destruction and Assembly: Switching between Two Differently Composed Cage-Type Complexes.

We report on two regioisomeric, diazocine ligands 1 and 2 that can both be photoswitched between the E- and Z-configurations with violet and green light. The self-assembly of the four species (1-Z, 1-E, 2-Z, 2-E) with CoII ions was investigated upon changing the coordination vectors as a function of the ligand configuration (E vs Z) and regioisomer (1 vs 2). With 1-Z, Co2 (1-Z)3 was self-assembled, while a mixture of ill-defined species (oligomers) was observed with 2-Z. Upon photoswitching with 385 nm to the E configurations, the opposite was observed with 1-E forming oligomers and 2-E forming Co2 (2-E)3 . Light-controlled dis/assembly was demonstrated in a ligand competition experiment with sub-stoichiometric amounts of CoII ions; alternating irradiation with violet and green light resulted in the reversible transformation between Co2 (1-Z)3 and Co2 (2-E)3 over multiple cycles without significant fatigue by photoswitching.

Photoswitchable Diazocine-Based Estrogen Receptor Agonists: Stabilization of the Active Form inside the Receptor.

Photopharmacology is an emerging approach in drug design and pharmacological therapy. Light is used to switch a pharmacophore between a biologically inactive and an active isomer with high spatiotemporal resolution at the site of illness, thus potentially avoiding side effects in neighboring healthy tissue. The most frequently used strategy to design a photoswitchable drug is to replace a suitable functional group in a known bioactive molecule with azobenzene. Our strategy is different in that the photoswitch moiety is closer to the drug's scaffold. Docking studies reveal a very high structural similarity of natural 17ß-estradiol and the E isomers of dihydroxy diazocines, but not their Z isomers, respectively. Seven dihydroxy diazocines were synthesized and subjected to a biological estrogen reporter gene assay. Four derivatives exhibit distinct estrogenic activity after irradiation with violet light, which can be shut off with green light. Most remarkably, the photogenerated, active E form of one of the active compounds isomerizes back to the inactive Z form with a half-life of merely several milliseconds in water, but nevertheless is active for more than 3 h in the presence of the estrogen receptor. The results suggest a significant local impact of the ligand-receptor complex toward back-isomerization. Thus, drugs that are active when bound but lose their activity immediately after leaving the receptor could be of great pharmacological value because they strongly increase target specificity. Moreover, the drugs are released into the environment in their inactive form. The latter argument is particularly important for drugs that act as endocrine disruptors.

Effect of an axial ligand on the self-assembly of molecular platforms.

Sub-monolayer amounts of trioxatriangulenium (TOTA) molecules functionalized with biphenyl on Ag(111) were investigated with scanning tunnelling microscopy. The molecule is comprised of a rod-shaped axial ligand and a triangular platform that tends to form hydrogen bonds in arrays. Two superstructures are observed, a hexagonal tiling and a phase of molecular double rows. While the former structure matches previous observations from other functionalized TOTA molecules the latter one was unexpected. Aided by density functional theory results, we analyse the observed intramolecular contrast and present a model of the new phase. We discuss possible interaction mechanisms underlying the molecular pattern.

Azimuthal Dipolar Rotor Arrays on Surfaces.

A set of dipolar molecular rotor compounds was designed, synthesized and adsorbed as self-assembled 2D arrays on Ag(111) surfaces. The title molecules are constructed from three building blocks: (a) 4,8,12-trioxatriangulene (TOTA) platforms that are known to physisorb on metal surfaces such as Au(111) and Ag(111), (b) phenyl groups attached to the central carbon atom that function as pivot joints to reduce the barrier to rotation, (c) pyridine and pyridazine units as small dipolar units on top. Theoretical calculations and scanning tunneling microscopy (STM) investigations hint at the fact that the dipoles of neighboring rotors interact through space through pairs of energetically favorable head-to-tail arrangements.

Triplet sensitization enables bidirectional isomerization of diazocine with 130 nm redshift in excitation wavelengths.

Diazocines are bridged azobenzenes with phenyl rings connected by a CH2-CH2 group. Despite this rather small structural difference, diazocine exhibits improved properties over azobenzene as a photoswitch and most importantly, its Z configuration is more stable than the E isomer. Herein, we reveal yet another unique feature of this emerging class of photoswitches. In striking contrast to azobenzenes and other photochromes, diazocine can be selectively switched in E → Z direction and most intriguingly from its thermodynamically stable Z to metastable E isomer upon successive excitation of two different triplet sensitizers present in solution at the same time. This approach leads to extraordinary large redshift of excitation wavelengths to perform isomerization i.e. from 400 nm blue to 530 nm green light (Z → E) and from 530 nm green to 740 nm far-red one (E → Z), which falls in the near-infrared window in biological tissue. Therefore, this work opens up of potential avenues for utilizing diazocines for example in photopharmacology, smart materials, light energy harvesting/storage devices, and out-of-equilibrium systems.

Highly Crowded Twisted Thienylene-Phenylene Structures: Evidence for Through-Space Orbital Coupling in a [4]Catenated Topology.

Sterically highly crowded and twisted thienylene-phenylenes are synthesized and structurally characterized. Single-crystal X-ray structure analyses and theoretical studies give evidence of through-space delocalization of π-electrons of peripheral (hetero)aromatic rings in toroidal and catenated topology.