Modification of Extended Open Frameworks with Fluorescent Tags for Sensing Explosives: Competition between Size Selectivity and Electron Deficiency

Three new electron‐rich metal–organic frameworks ( MOF‐1 – MOF‐3 ) have been synthesized by employing ligands bearing aromatic tags. The key role of the chosen aromatic tags is to enhance the π‐electron density of the luminescent MOFs. Single‐crystal X‐ray structures have revealed that these MOFs form three‐dimensional porous networks with the aromatic tags projecting inwardly into the pores. These highly luminescent electron‐rich MOFs have been successfully utilized for the detection of explosive nitroaromatic compounds (NACs) on the basis of fluorescence quenching. Although all of the prepared MOFs can serve as sensors for NACs, MOF‐1 and MOF‐2 exhibit superior sensitivity towards 4‐nitrotoluene (4‐NT) and 2,4‐dinitrotoluene (DNT) compared to 2,4,6‐trinitrotoluene (TNT) and 1,3,5‐trinitrobenzene (TNB). MOF‐3 , on the other hand, shows an order of sensitivity in accordance with the electron deficiencies of the substrates. To understand such anomalous behavior, we have thoroughly analyzed both the steady‐state and time‐resolved fluorescence quenching associated with these interactions. Determination of static Stern–Volmer constants (K_S) as well as collisional constants (K_C) has revealed that MOF‐1 and MOF‐2 have higher K_S values with 4‐NT than with TNT, whereas for MOF‐3 the reverse order is observed. This apparently anomalous phenomenon was well corroborated by theoretical calculations. Moreover, recyclability and sensitivity studies have revealed that these MOFs can be reused several times and that their sensitivities towards TNT solution are at the parts per billion (ppb) level.     

Sensing of Phosphates by Using Luminescent EuIII and TbIII Complexes: Application to the Microalgal Cell Chlorella vulgaris

Phenanthroline‐based chiral ligands L¹ and L² as well as the corresponding Eu^III and Tb^III complexes were synthesized and characterized. The coordination compounds show red and green emission, which was explored for the sensing of a series of anions such as F^?, Cl^?, Br^?, I^?, NO₃^?, NO₂^?, HPO₄^2?, HSO₄^?, CH₃COO^?, and HCO₃^?. Among the anions, HPO₄^2? exhibited a strong response in the emission property of both europium(III) and terbium(III) complexes. The complexes showed interactions with the nucleoside phosphates adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP). Owing to this recognition, these complexes have been applied as staining agents in the microalgal cell Chlorella vulgaris. The stained microalgal cells were monitored through fluorescence microscopy and scanning electron microscopy. Initially, the complexes bind to the outer cell wall and then enter the cell wall through holes in which they probably bind to phospholipids. This leads to a quenching of the luminescence properties.     

Conformational Control of Oligo(p‐phenyleneethynylene)s with Intrinsic Substituent Electronic Effects: Origin of the Twist in Pentiptycene‐Containing Systems

Dependence of the backbone planarity of oligo(p‐phenyleneethynylene)s (OPEs) on the intrinsic electronic character of substituents and on the nature of the solvent has been experimentally demonstrated with a series of center‐symmetrical five‐ring systems, pentiptycene‐pentiptycene‐arene‐pentiptycene‐pentiptycene, differing in the substituents on the central arene. In frozen 2‐methyltetrahydrofuran (MTHF), the adjacent pentiptycene units prefer to be in a mutually twisted orientation when the substituents are electron‐withdrawing (F and amido), resulting in a TPPT or TTTT conformation, whereas a planarized PPPP backbone is favored in the case of electron‐donating substituents (alkyl and alkoxy). The propensity to adopt the PPPP form is generally enhanced by replacing MTHF with either methylcyclohexane or mixed ethanol/methanol as solvent. These observations reveal that the twist between adjacent pentiptycene units in OPEs is a consequence of the electronic rather than steric effects of iptycenyl substituents. The electronic effect of iptycenyl substituents is manifested in decreased phenylene π polarizability as the net effect of both electron‐donating hyperconjugation and an electron‐withdrawing inductive effect. Variable‐temperature electronic absorption and emission spectroscopies are the critical tools for this work. Our findings provide important guidelines for conformational and electronic engineering of OPEs and for the design of novel iptycene‐based organic electronic materials.     

Can Low‐Valent Germanium Chemistry Be Practiced Under Ambient Conditions? A Tale of a Water‐Stable Yet Reactive Germylene Monochloride Complex

A germylene monochloride complex ((DPM)GeCl, 1 ) that is water stable was isolated for the first time. Interestingly, it reacts with cesium fluoride under ambient conditions (non‐inert atmosphere and water‐containing solvent) to afford water stable germylene monofluoride complex ((DPM)GeF, 2 ). Due to the usage of DPM (dipyrrinate) ligand, germylene monohalides 1 and 2 show fluorescence in the visible region at 555 and 538 nm, respectively. Compounds 1 and 2 are the first fluorescent germylene complexes and were characterized by multinuclear NMR spectroscopy. The structure of compound 1 was also proved by single crystal X‐ray diffraction studies.     

Enhanced Thermal Oxidation Stability of Reduced Graphene Oxide by Nitrogen Doping

Nitrogen‐doped reduced graphene oxide (N‐doped RGO) samples with a high level of doping, up to 13 wt. %, have been prepared by annealing graphene oxide under a flow of pure ammonia. The presence of nitrogen within the structure of RGO induces a remarkable increase in the thermal stability against oxidation by air. The thermal stability is closely related with the temperature of synthesis and the nitrogen content. The combustion reaction of nitrogen in different coordination environments (pyridinic, pyrrolic, and graphitic) is analyzed against a graphene fragment (undoped) from a thermodynamic point of view. In agreement with the experimental observations, the combustion of undoped graphene turns out to be more spontaneous than when nitrogen atoms are present.     

Reversible Re‐programing of Cell–Cell Interactions

The ability to engineer and re‐program the surfaces of cells would provide an enabling synthetic biological method for the design of cell‐ and tissue‐based therapies. A new cell surface‐engineering strategy is described that uses lipid‐chemically self‐assembled nanorings (lipid‐CSANs) that can be used for the stable and reversible modification of any cell surface with a molecular reporter or targeting ligand. In the presence of a non‐toxic FDA‐approved drug, the nanorings were quickly disassembled and the cell–cell interactions reversed. Similar to T‐cells genetically engineered to express chimeric antigen receptors (CARS), when activated peripheral blood mononuclear cells (PBMCs) were functionalized with the anti‐EpCAM‐lipid‐CSANs, they were shown to selectively kill antigen‐positive cancer cells. Taken together, these results demonstrate that lipid‐CSANs have the potential to be a rapid, stable, and general method for the reversible engineering of cell surfaces and cell–cell interactions.     

Brønsted Acid Catalyzed,Conjugate Addition of β‐Dicarbonyls to In Situ Generated ortho‐Quinone Methides—Enantioselective Synthesis of 4‐Aryl‐4H‐Chromenes

We describe herein a catalytic, enantioselective process for the synthesis of 4H‐chromenes which are important structural elements of many natural products and biologically active compounds. A sequence comprising a conjugate addition of β‐diketones to in situ generated ortho‐quinone methides followed by a cyclodehydration reaction furnished 4‐aryl‐4H‐chromenes in generally excellent yields and high optical purity. A BINOL‐based chiral phosphoric acid was employed as a Brønsted acid catalyst which converted ortho‐hydroxy benzhydryl alcohols into hydrogen‐bonded ortho‐quinone methides and effected the carbon–carbon bond‐forming event with high enantioselectivity.     

An Isolated Nitridyl Radical‐Bridged {Rh(N.)Rh} Complex

Photochemical activation of [(PNNH)Rh(N₃)] (PNNH=6‐di‐(tert‐butyl)phosphinomethyl‐2,2′‐bipyridine) complex 2 produced the paramagnetic (S=1/2), [(PNN)Rh?N^.‐Rh(PNN)] complex 3 (PNN^?=methylene‐deprotonated PNNH), which could be crystallographically characterized. Spectroscopic investigation of 3 indicates a predominant nitridyl radical (^.N^2?) character, which was confirmed computationally. Complex 3 reacts selectively with CO, producing two equivalents of [(PNN)Rh^I(CO)] complex 4 , presumably by nitridyl radical N,N‐coupling.