Construction of Multifunctional Covalent Organic Frameworks for Photocatalysis.

Covalent organic frameworks (COFs) have recently drawn intense attention due to their potential applications in photocatalysis. Herein, we report a multifunctional COF which consists of triphenylamine (TPA) and 2,2'-bipyridine (2, 2'-bipy) entities. The obtained TAPA-BPy-COF is a heterogeneous photocatalyst and can efficiently catalyze the oxidative coupling of thiols to disulfides. In addition, TAPA-BPy-COF can be further metalated by Pd(II) via 2,2'-bipy-metal coordination. The generated Pd@TAPA-BPy-COF can highly promote photocatalytic synthesis of 3-cyanopyridines via cascade addition/cyclization of arylboronic acids with γ-ketodinitriles in heterogeneous way. This work has demonstrated the way for the rational design and preparation of more efficient photoactive COFs for photocatalysis.

Reversible Photochromism of 4,4'-Disubstituted 2,2'-Bipyridine in the Presence of SO3.

We report an unusual photochromic behavior of 4,4'-disubstituted-2,2'-bipyridine. It was found that in the presence of a SO3 source and HCl, 2,2'-bipyridine-4,4'-dibutyl ester undergoes a color change from yellow to magenta in solution with maximum absorbance at 545 nm upon irradiation with 395 nm light. The photochromism is thermally reversible in solution. Different from the known bipyridine-based photoswitching pathways, the photo response does not involve any metal which form colored complexes or the formation of colored free radical cations like the photo-reduction of viologens. A combination of experimental and computational analysis was used to probe the mechanism. The results suggest the colored species to be a complex formed between N-oxide of the 2,2'-bipyridine-4,4'-dibutyl ester and SO2; the N-oxide and SO2 are formed from photoactivated oxidation of the bipyridine with SO3 serving as the oxygen source. This complex represents a new addition to the library of photoswitches that is easy to synthesize, reversible in solution, and of high fatigue resistance, making it a promising candidate for applications in photo-switchable materials and SO3 detection. We also demonstrated experimentally similar photochromic behaviors with 2,2'-bipyridine-containing polymers.

New Fluorescent Dye for the Detection of Zn2+ in Living Cells and Fixed Sections of the Rat Pancreas.

We report the synthesis and characterization of a new 4-methoxyphenyl-2,2'-bipyridine-based ligand, such as 12, bearing dipicolylaminomethyl core as a receptor unit, as a probe for the fluorescence "turn-on" detection of Zn2+. Thus, in the presence of Zn2+ the probe 12 exhibited a fluorescence enhancement with a Stokes shift of ~ 180 nm and photoluminescence quantum yields value of ~ 1.0. In addition, 12 exhibited higher binding constant for Zn2+ (~ 2 × 105 M-1) with the LOD reaching the nanomolar level (~ 0.1 × 10-9 M) compare to the previously reported probe 1. The stoichiometry and structure of the [Zn(12)]2+ and [Zn(1)]2+ complexes were supported by XRD analysis, DFT calculations and 1H NMR experiments. It was postulated that, as a result of binding of Zn2+, the sample exhibited a bright "on" state via the PET-ICT processes. Molecular docking studies and confocal fluorescence microscopy experiments demonstrated that the probe 12 could be used for the fluorescence detection of Zn2+ not only in artificially enriched with zinc salts live cells, but also in fixed tissues with cations are in a bound state. The high binding constant of compound 12 to Zn2+ cation allows it to be used for the accurate localization of pancreatic beta cells (islets of Langerhans).

Nanosecond heme-to-heme electron transfer rates in a multiheme cytochrome nanowire reported by a spectrally unique His/Met-ligated heme.

Proteins achieve efficient energy storage and conversion through electron transfer along a series of redox cofactors. Multiheme cytochromes are notable examples. These proteins transfer electrons over distance scales of several nanometers to >10 µm and in so doing they couple cellular metabolism with extracellular redox partners including electrodes. Here, we report pump-probe spectroscopy that provides a direct measure of the intrinsic rates of heme-heme electron transfer in this fascinating class of proteins. Our study took advantage of a spectrally unique His/Met-ligated heme introduced at a defined site within the decaheme extracellular MtrC protein of Shewanella oneidensis We observed rates of heme-to-heme electron transfer on the order of 109 s-1 (3.7 to 4.3 Å edge-to-edge distance), in good agreement with predictions based on density functional and molecular dynamics calculations. These rates are among the highest reported for ground-state electron transfer in biology. Yet, some fall 2 to 3 orders of magnitude below the Moser-Dutton ruler because electron transfer at these short distances is through space and therefore associated with a higher tunneling barrier than the through-protein tunneling scenario that is usual at longer distances. Moreover, we show that the His/Met-ligated heme creates an electron sink that stabilizes the charge separated state on the 100-µs time scale. This feature could be exploited in future designs of multiheme cytochromes as components of versatile photosynthetic biohybrid assemblies.

A Molecular Hybrid of the GFP Chromophore and 2,2'-Bipyridine: An Accessible Sensor for Zn2+ Detection with Fluorescence Microscopy.

The few commercially available chemosensors and published probes for in vitro Zn2+ detection in two-photon microscopy are compromised by their flawed spectroscopic properties, causing issues in selectivity or challenging multistep syntheses. Herein, we present the development of an effective small molecular GFP chromophore-based fluorescent chemosensor with a 2,2'-bipyridine chelator moiety (GFZnP BIPY) for Zn2+ detection that has straightforward synthesis and uncompromised properties. Detailed experimental characterizations of the free and the zinc-bound compounds within the physiologically relevant pH range are presented. Excellent photophysical characteristics are reported, including a 53-fold fluorescence enhancement with excitation and emission maxima at 422 nm and 492 nm, respectively. A high two-photon cross section of 3.0 GM at 840 nm as well as excellent metal ion selectivity are reported. In vitro experiments on HEK 293 cell culture were carried out using two-photon microscopy to demonstrate the applicability of the novel sensor for zinc bioimaging.

Dimethyl Derivatives of 2,2'-Bipyridine as Ligands in [W(CN)6(bpy)]2--Type Complexes.

The thermal decomposition of (XMebpyH)3(H3O)[W(CN)8]·3H2O (where XMebpy denotes 4Mebpy 4,4'-dimethyl-2,2'-bipyridine or 5Mebpy 5,5'-dimethyl-2,2'-bipyridine) in glycerol results in the coordination of XMebpy. Salts of the anion formula [W(CN)6(XMebpy)]2- were isolated for PPh4+ and/or AsPh4+ cations as well as for K+, Rb+, and Cs+ ones. X-ray single-crystal analyses for tetraphenyl-phosphonium and tetraphenyl-arsonium cations are described. IR, UV-Vis, and cyclic voltammetry data are presented. The results were compared with those for [W(CN)6(bpy)]2- (bpy = 2,2'-bipyridine) ion salts.

Engineering Single Cu Sites into Covalent Organic Framework for Selective Photocatalytic CO2 Reduction.

Photocatalytic CO2 conversion into value-added chemicals is a promising route but remains challenging due to poor product selectivity. Covalent organic frameworks (COFs) as an emerging class of porous materials are considered as promising candidates for photocatalysis. Incorporating metallic sites into COF is a successful strategy to realize high photocatalytic activities. Herein, 2,2'-bipyridine-based COF bearing non-noble single Cu sites is fabricated by chelating coordination of dipyridyl units for photocatalytic CO2 reduction. The coordinated single Cu sites not only significantly enhance light harvesting and accelerate electron-hole separation but also provide adsorption and activation sites for CO2 molecules. As a proof of concept, the Cu-Bpy-COF as a representative catalyst exhibits superior photocatalytic activity for reducing CO2 to CO and CH4 without photosensitizer, and impressively, the product selectivity of CO and CH4 can be readily modulated only by changing reaction media. Experimental and theoretical results reveal the crucial role of single Cu sites in promoting photoinduced charge separation and solvent effect in regulating product selectivity, which provides an important sight onto the design of COF photocatalysts for selective CO2 photoreduction.

Photochemically Mediated Toluene Oxidation through a Copper Complex.

A method is described to photochemically oxidize toluene selectively to benzaldehyde, an essential compound in the chemical industry. Copper(I) complexes with different ligands were applied in combination with [Ru(bipy)3 ](PF6 )2 and dioxygen as the oxidant. As a result, a "dioxygen adduct" copper complex, for example, a peroxido complex, is formed as the active species. The copper(II) complex obtained after oxidation can be photochemically reduced to the starting copper(I) species, and the process can be repeated continuously. The ligand tris(2-methylpyridyl)amine (tmpa) led to the highest conversion rates.

Source, Synthesis, and Biological Evaluation of Natural Occurring 2,2'-Bipyridines.

Molecules containing bipyridine scaffold are fascinating and versatile compounds in the field of natural product chemistry and drug discovery, and these molecules have possible therapeutic applications due to possession of potent biological activities such as antimicrobial, immunomodulatory, antitumor, and phytotoxic. Significant efforts have been devoted to isolating various 2,2' bipyridine compounds from natural sources, with antimicrobial, anti-cancer, and immunosuppressive properties. This review describes recent developments in isolation from different microbial origins, synthesis, and investigation of different kinds of biological activities of 2,2' bipyridines, with a particular emphasis on caerulomycins, collismycins, and related derivates thereof in detail.

Molecular imprinting electrochemiluminescence sensor based on nitrogen-doped carbon quantum dots /Ru(bpy)3@SiO2 for the determination of citrinin.

An electrochemiluminescence (ECL) sensor based on molecular imprinting polymer and SiO2 nanoparticles loaded Ru(bpy)3 and nitrogen-doped carbon quantum dots (NCQDs) is constructed for citrinin detection. The Ru(bpy)3 acts as ECL emitter, and the NCQDs cooperate with tri-n-propylamine (TPA) in solution as a coreactant to facilitate the luminescence. The citrinin imprinted poly(p-aminothiophenol) film is deposited on the surface of the luminophore by electrochemical method, which can immobilize the luminophore besides recognizing the target. The obtained ECL sensor exhibits high sensitivity, stability, and reproducibility. The change of ECL intensity and the logarithm of citrinin concentration display a good linear relationship in the range 1.0 to 100 pg mL-1, and the detection limit is 5 fg mL-1. When it is applied to the detection of citrinin contents in food sample (i.e., rice and millet) solutions, the RSD is less than 6.1%, and the recoveries for spiked standards range from 95.5 to 102.0%. Hence, this work provides a promising alternative for citrinin detection.

Insight into Unusual Supramolecular Self-Assemblies of Terthiophenes Directed by Weak Hydrogen Bonding.

A supramolecular self-assembly of semiconducting polymers and small molecules plays an important role in charge transportation, performance, and lifetime of an optoelectronic device. Tremendous efforts have been put into the strategies to self-organize these materials. In this regard, here, we present the self-organization of terthiophene and its methyl alcohol derivative with 4,4'-bipyridine (44BiPy). An unexpected 2D layered organization of 5,5″-dimethyl-2,2':5',2″-terthiophene (DM3T) and 44BiPy was obtained and analyzed. Single-crystal X-ray diffraction analysis revealed that DM3T and 44BiPy consist of stacked, almost independent, infinite 2D layers while held together by weak hydrogen bonds. In addition to this peculiar supramolecular arrangement of these compounds, the investigation of their photophysical properties showed strong fluorescence quenching of DM3T by 44BiPy in the solid state, suggesting an efficient charge transfer. On the other hand, the methyl alcohol derivative of terthiophene, DM3TMeOH, organized in a closed cyclic motif with 44BiPy via hydrogen bonds.

Fine-Tuning of the Optical and Electrochemical Properties of Ruthenium(II) Complexes with 2-Arylbenzimidazoles and 4,4'-Dimethoxycarbonyl-2,2'-bipyridine.

A series of cyclometalated complexes of ruthenium (II) with four different substituents in the aryl fragment of benzimidazole was synthesized in order to study the effect of substituent donation on the electronic structure of the substances. The resulting complexes were studied using X-ray diffraction, NMR spectroscopy, MALDI mass spectrometry, electron absorption spectroscopy, luminescence spectroscopy, and cyclic voltammetry as well as DFT/TDDFT was also used to interpret the results. All the complexes have intense absorption in the range of up to 700 nm, the triplet nature of the excited state was confirmed by measurement of luminescence decay. With an increase in substituent donation, a red shift of the absorption and emission bands occurs, and the lifetime of the excited state and the redox potential of the complex decrease. The combination of these properties shows that the complexes are excellent dyes and can be used as photosensitizers.

A Comprehensive Mechanistic Antibacterial and Antibiofilm Study of Potential Bioactive ((BpA)2bp)Cu/Zn Complexes via Bactericidal Mechanisms against Escherichia coli.

Bacterial resistance to antibiotics and host defense systems is primarily due to bacterial biofilm formation in antibiotic therapy. In the present study, two complexes, bis (biphenyl acetate) bipyridine Cu (II) (1) and bis (biphenyl acetate) bipyridine Zn (II) (2), were tested for their ability to prevent biofilm formation. The minimum inhibitory concentration and minimum bactericidal concentration of complexes 1 and 2 were 46.87 ± 1.822 and 93.75 ± 1.345 and 47.87 ± 1.345 and 94.85 ± 1.466 µg/mL, respectively. The significant activity of both complexes was attributed to the damage caused at the membrane level and was confirmed using an imaging technique. The biofilm inhibitory potential levels of complexes 1 and 2 were 95% and 71%, respectively, while the biofilm eradication potential levels were 95% and 35%, respectively, for both complexes. Both the complexes showed good interactions with the E. coli DNA. Thus, complexes 1 and 2 are good antibiofilm agents that exert their bactericidal actions possibly by disrupting the bacterial membrane and interacting with the bacterial DNA, which can act as a powerful agent to restrain the development of bacterial biofilm on therapeutic implants.

Noncovalent Modification of Cycloparaphenylene by Catenane Formation Using an Active Metal Template Strategy.

The active metal template (AMT) strategy is a powerful tool for the formation of mechanically interlocked molecules (MIMs) such as rotaxanes and catenanes, allowing the synthesis of a variety of MIMs, including π-conjugated and multicomponent macrocycles. Cycloparaphenylene (CPP) is an emerging molecule characterized by its cyclic π-conjugated structure and unique properties. Therefore, diverse modifications of CPPs are necessary for its wide application. However, most CPP modifications require early stage functionalization and the direct modification of CPPs is very limited. Herein, we report the synthesis of a catenane consisting of [9]CPP and a 2,2'-bipyridine macrocycle as a new CPP analogue that contains a reliable synthetic scaffold enabling diverse and concise post-modification. Following the AMT strategy, the [9]CPP-bipyridine catenane was successfully synthesized through Ni-mediated aryl-aryl coupling. Catalytic C-H borylation/cross-coupling and metal complexation of the bipyridine macrocycle moiety, an effective post-functionalization method, were also demonstrated with the [9]CPP-bipyridine catenane. Single-crystal X-ray structural analysis revealed that the [9]CPP-bipyridine catenane forms a tridentated complex with an Ag ion inside the CPP ring. This interaction significantly enhances the phosphorescence lifetime through improved intermolecular interactions.

Enhancement of Visible-Light-Driven Hydrogen Evolution Activity of 2D π-Conjugated Bipyridine-Based Covalent Organic Frameworks via Post-Protonation.

Photocatalytic hydrogen (H2 ) evolution represents a promising and sustainable technology. Covalent organic frameworks (COFs)-based photocatalysts have received growing attention. A 2D fully conjugated ethylene-linked COF (BTT-BPy-COF) was fabricated with a dedicated designed active site. The introduced bipyridine sites enable a facile post-protonation strategy to fine-tune the actives sites, which results in a largely improved charge-separation efficiency and increased hydrophilicity in the pore channels synergically. After modulating the degree of protonation, the optimal BTT-BPy-PCOF exhibits a remarkable H2 evolution rate of 15.8 mmol g-1 h-1 under visible light, which surpasses the biphenyl-based COF 6 times. By using different types of acids, the post-protonation is proved to be a potential universal strategy for promoting photocatalytic H2 evolution. This strategy would provide important guidance for the design of highly efficient organic semiconductor photocatalysts.

Iridium-Catalyzed ortho-Selective Borylation of Aromatic Amides Enabled by 5-Trifluoromethylated Bipyridine Ligands.

Iridium-catalyzed borylations of aromatic C-H bonds are highly attractive transformations because of the diversification possibilities offered by the resulting boronates. These transformations are best carried out using bidentate bipyridine or phenanthroline ligands, and tend to be governed by steric factors, therefore resulting in the competitive functionalization of meta and/or para positions. We have now discovered that a subtle change in the bipyridine ligand, namely, the introduction of a CF3 substituent at position 5, enables a complete change of regioselectivity in the borylation of aromatic amides, allowing the synthesis of a wide variety of ortho-borylated derivatives. Importantly, thorough computational studies suggest that the exquisite regio- and chemoselectivity stems from unusual outer-sphere interactions between the amide group of the substrate and the CF3 -substituted aryl ring of the bipyridine ligand.

Positive Cooperativity Induced by Interstrand Interactions in Silver(I) Complexes with α,α'-Diimine Ligands.

The allosteric positive cooperativity accompanying the formation of compact [CuI (α,α'-diimine)2 ]+ building blocks contributed to the historically efficient synthesis of metal-containing catenates and knotted assemblies. However, its limited magnitude can easily be overcome by the negative chelate cooperativity that controls the overall formation of related polymetallic multistranded helicates and grids. Despite the more abundant use of analogous dioxygen-resistant [AgI (α,α'-diimine)2 ]+ units in modern entangled metallo-supramolecular assemblies, a related thermodynamic justification was absent. Solid-state structural characterizations show the successive formation of [AgI (α,α'-diimine)(CH3 CN)][X] and [AgI (α,α'-diimine)2 ][X] upon the stepwise reactions of α,α'-diimine=2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen) derivatives with AgX (X=BF4 - , ClO4 - , PF6 - ). In room-temperature, 5-10 mM acetonitrile solutions, these cationic complexes exist as mixtures in fast exchange on the NMR timescale. Spectrophotometric titrations using the unsubstituted bpy and phen ligands point to the statistical (=non-cooperative) binding of two successive bidentate ligands around AgI , a mechanism probably driven by the formation of hydrophobic belts, that overcomes the unfavorable decrease in the positive charge borne by the metallic cation. Surprisingly, the addition of methyl groups adjacent to the nitrogen donors (6,6' positions in dmbpy; 2,9 positions in dmphen) induces positive cooperativity for the formation of [Ag(dmbpy)2 ]+ and [Ag(dmphen)2 ]+ , a trend assigned to additional stabilizing interligand interactions. Adding rigid and polarizable phenyl side arms in [Ag(Brdmbpy)2 ]+ further reinforces the positively cooperative process, while limiting the overall decrease in metal-ligand affinity.

Enhanced Photoelectrochemical Water Splitting of Black Silicon Photoanode with pH-Dependent Copper-Bipyridine Catalysts.

Since the water oxidation half-reaction requires the transfer of multi-electrons and the formation of O-O bond, it's crucial to investigate the catalytic behaviours of semiconductor photoanodes. In this work, a bio-inspired copper-bipyridine catalyst of Cu(dcbpy) is decorated on the nanoporous Si photoanode (black Si, b-Si). Under AM1.5G illumination, the b-Si/Cu(dcbpy) photoanode exhibits a high photocurrent density of 6.31 mA cm-2 at 1.5 VRHE at pH 11.0, which is dramatically improved from the b-Si photoanode (1.03 mA cm-2 ) and f-Si photoanode (0.0087 mA cm-2 ). Mechanism studies demonstrate that b-Si/Cu(dcbpy) has improved light-harvesting, interfacial charge-transfer, and surface area for water splitting. More interestingly, b-Si/Cu(dcbpy) exhibits a pH-dependent water oxidation behaviour with a minimum Tafel slope of 241 mV/dec and the lowest overpotential of 0.19 V at pH 11.0, which is due to the monomer/dimer equilibrium of copper catalyst. At pH ∼11, the formation of dimeric hydroxyl-complex could form O-O bond through a redox isomerization (RI) mechanism, which decreases the required potential for water oxidation. This in-depth understanding of pH-dependent water oxidation catalyst brings insights into the design of dimer water oxidation catalysts and efficient photoanodes for solar energy conversion.

Synthesis and Chemosensory Properties of New Cyanosubstituted 2,2'-Bipyridine Derivatives.

A series of novel 6-oxo-1,6-dihydro-[2,2'-bipyridine]-5-carbonitriles has been synthesized and characterized. Their photophysical properties in DMSO solution and aqueous medium as well as fluorescence response to the presence of metal ions have been investigated. The obtained 4-(4-methoxyphenyl)-6-oxo-1,6-dihydro-[2,2'-bipyridine]-5-carbonitrile has been shown as a selective fluorescent "turn-ON" probe for Cd2+ ions with LoD 0.359 µM, 1:1 metal-ligand ratio and binding constant of 5.2 × 104 M-1.

Synthesis, Optoelectronic and Photoluminescent Characterizations of Green Luminous Heteroleptic Ternary Terbium Complexes.

This article presents four ternary terbium complexes based on fluorinated 2-thenoyltrifluoroacetone (TTFA) and N donor bidentate neutral ligands. The prepared complexes were examined by elemental study, electrochemical analysis, spectroscopically and thermo-gravimetrically. Spectral analysis shows the bonding of Tb3+ ion with oxygen and nitrogen atom of diketone and neutral ligand respectively. Upon excitation in UV region, synthesized terbium complexes show luminescence in green region of electromagnetic spectrum. Photoluminescence emission spectra of complexes do not show any ligand based peak suggesting the effective transferal of energy from ligand to metal ion. Green emanation by terbium complexes is owing to intense peak ~547 nm (5D4 → 7F5). The outcome of emission data and CIE coordinates correlate with each other and affirms the utility of green luminous complexes as potential emissive material for optoelectronic gadgets applied in lighting system.