Alkaline Earth Metal Ion Induced Coil–Helix–Coil Transition of Lysine–Coumarin–Azacrown Hybrid Foldamers with OFF–OFF–ON Fluorescence Switching

A new metal‐ion‐responsive and fluorescent foldamer, OPLM₈ , composed of eight lysine–coumarin–azacrown units, has been designed and synthesized. The flexible OPLM₈ can be forced into a well‐defined helix structure only upon the addition of alkaline earth metal ions. The structural change is based on the crown ether moieties being positioned in the requisite arrangement along the peptide chain, that is, at i, i+4 spacing, such that the alkaline earth metal ions can mediate the formation of four sandwich complexes between them. Moreover, varying the chelator‐to‐metal‐ion ratio from 2:1 to 1:1 resulted in disassembly of the sandwich complexes leading to collapse of the helical structure to a random coil. These metal‐ion‐induced structural transitions could not only be monitored by the CD amplitude change but also easily probed by unique “OFF–OFF–ON” fluorescence intensity changes from 0.7‐fold to 14‐fold as the structure changed from the folded helix to a random coil. To further verify that the helix formation was indeed induced by metal‐ion complexation, two kinds of control octamers with only four metal‐ion chelators on the side chains were studied. One, which was capable of forming two sandwich complexes between the i and i+4 residues, displayed a negative Cotton couplet with the magnitude of its A value close to half that of OPLM₈ , and the second had four metal‐ion chelators positioned in the same turn, and hence was incapable of forming intramolecular metal complexes and showed different induced CD signals. Collectively, the photospectroscopic data and the results of the control studies suggest that alkaline earth metal ions can efficiently promote the flexible octamer OPLM₈ into a well‐organized helix by the formation of sandwich complexes between substituents at an i, i+4 spacing.     

Molecular Logic Gates and Switches Based on 1,3,4‐Oxadiazoles Triggered by Metal Ions

Organic molecular devices for information processing applications are highly useful building blocks for constructing molecular‐level machines. The development of “intelligent” molecules capable of performing logic operations would enable molecular‐level devices and machines to be created. We designed a series of 2,5‐diaryl‐1,3,4‐oxadiazoles bearing a 2‐(para‐substituted)phenyl and a 5‐(o‐pyridyl) group (substituent X=NMe₂, OEt, Me, H, and Cl; 1 a – e ) that form a bidentate chelating environment for metal ions. These compounds showed fluorescence response profiles varying in both emission intensity and wavelength toward the tested metal ions Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, and Pb²⁺ and the responses were dependent on the substituent X, with those of 1 d being the most substantial. The 1,3,4‐oxadiazole O or N atom and pyridine N atom were identified as metal‐chelating sites. The fluorescence responses of 1 d upon metal chelation were employed for developing truth tables for OR, NOR, INHIBIT, and EnNOR logic gates as well as “ON‐OFF‐ON” and “OFF‐ON‐OFF” fluorescent switches in a single 1,3,4‐oxadiazole molecular system.     

New 3‐(Heteroaryl)‐2‐iminocoumarin‐based Borate Complexes: Synthesis,Photophysical Properties,and Rational Functionalization for Biosensing/Biolabeling Applications

Members of a series of boron difluoride complexes with 3‐(heteroaryl)‐2‐iminocoumarin ligands bearing both a phenolic hydroxyl group (acting as a fluorogenic center) and an N‐aryl substituent (acting as a stabilizing moiety) have been synthesized in good yields by applying a straightforward two‐step method. These novel fluorogenic dyes belong to the family of “Boricos” (D. Frath et al., Chem. Commun.­ 2013 , 49, 4908–4910) and are the first examples of phenol‐based fluorophores of which the photophysical properties in the green‐yellow spectral range are dramatically improved by N,N‐chelation of a boron atom. Modulation of their fluorescence properties through reversible chemical modification of their phenol moieties has been demonstrated by the preparation of the corresponding 2,4‐dinitrophenyl (DNP) ethers, which led to a dramatic “OFF‐ON” fluorescence response upon reaction with thiols. Additionally, to expand the scope of these “7‐hydroxy‐Borico” derivatives, particularly in biolabeling, amine or carboxylic acid functionalities amenable to (bio)conjugation have been introduced within their scaffold. Their utility has been demonstrated in the preparation of fluorescent bovine serum albumin (BSA) conjugates and “Borico”‐DOTA‐like scaffolds in an effort to design novel monomolecular multimodal fluorescence‐ radioisotope imaging agents.     

Fluorescence Spectral Studies on the Coordination of Calix[4]‐arenes Bearing Boronic Acid Moieties with Monosaccharides

Coordination of ten calix[4]arenes bearing boronic acid moieties with five monosaccharides was studied by fluorescence spectrometry. The stability constants (K₂) of the complexes and Gibbs free energy change ( ‐ ΔG⁰) of the coordination reactions were calculated according to the modified Hilderbrand‐Benesi equation. The results obtained indicated that the coordination ability of D‐( ‐ )‐fructose with calix[4] arenes bearing boronic acid moieties was stronger than that of the other monosaccharides. And these calix[4]arene derivatives might be used for identification of L‐( ‐ )‐sorbose.     

Formation of Novel Dinuclear Lanthanide Luminescent Samarium(III), Europium(III), and Terbium(III) Triple‐Stranded Helicates from a C2‐Symmetrical Pyridine‐2,6‐dicarboxamide‐Based 1,3‐Xylenediyl‐Linked Ligand in MeCN

The synthesis of the C₂‐symmetrical ligand 1 consisting of two naphthalene units connected to two pyridine‐2,6‐dicarboxamide moieties linked by a xylene spacer and the formation of Ln^III‐based (Ln=Sm, Eu, Tb, and Lu) dimetallic helicates [Ln₂? 1 ₃] in MeCN by means of a metal‐directed synthesis is described. By analyzing the metal‐induced changes in the absorption and the fluorescence of 1 , the formation of the helicates, and the presence of a second species [Ln₂? 1 ₂] was confirmed by nonlinear‐regression analysis. While significant changes were observed in the photophysical properties of 1 , the most dramatic changes were observed in the metal‐centred lanthanide emissions, upon excitation of the naphthalene antennae. From the changes in the lanthanide emission, we were able to demonstrate that these helicates were formed in high yields (ca. 90% after the addition of 0.6 equiv. of Ln^III), with high binding constants, which matched well with that determined from the changes in the absorption spectra. The formation of the Lu^III helicate, [Lu₂? 1 ₃], was also investigated for comparison purposes, as we were unable to obtain accurate binding constants from the changes in the fluorescence emission upon formation of [Sm₂? 1 ₃], [Eu₂? 1 ₃], and [Tb₂? 1 ₃].     

Engineering Chemically Exfoliated Large‐Area Two‐Dimensional MoS2 Nanolayers with Porphyrins for Improved Light Harvesting

Molybdenum disulfide (MoS₂) is a promising candidate for electronic and optoelectronic applications. However, its application in light harvesting has been limited in part due to crystal defects, often related to small crystallite sizes, which diminish charge separation and transfer. Here we demonstrate a surface‐engineering strategy for 2D MoS₂ to improve its photoelectrochemical properties. Chemically exfoliated large‐area MoS₂ thin films were interfaced with eight molecules from three porphyrin families: zinc(II)‐, gallium(III)‐, iron(III)‐centered, and metal‐free protoporphyrin IX (ZnPP, GaPP, FePP, H₂PP); metal‐free and zinc(II) tetra‐(N‐methyl‐4‐pyridyl)porphyrin (H₂T4, ZnT4); and metal‐free and zinc(II) tetraphenylporphyrin (H₂TPP, ZnTPP). We found that the photocurrents from MoS₂ films under visible‐light illumination are strongly dependent on the interfacial molecules and that the photocurrent enhancement is closely correlated with the highest occupied molecular orbital (HOMO) levels of the porphyrins, which suppress the recombination of electron–hole pairs in the photoexcited MoS₂ films. A maximum tenfold increase was observed for MoS₂ functionalized with ZnPP compared with pristine MoS₂ films, whereas ZnT4‐functionalized MoS₂ demonstrated small increases in photocurrent. The application of bias voltage on MoS₂ films can further promote photocurrent enhancements and control current directions. Our results suggest a facile route to render 2D MoS₂ films useful for potential high‐performance light‐harvesting applications.     

Coordination Chemistry‐Assembled Multicomponent Systems Built from a Gable‐Like Bis‐Porphyrin: Synthesis and Photophysical Properties

Multiporphyrinic assemblies were quantitatively formed, in one step, from a gable‐like zinc(II) bis‐porphyrin ZnP₂ and free‐base porphyrins bearing pyridyl groups. The different fragments are held together by axial 4′‐N(pyridyl)–Zn interactions. Formation of a macrocycle ZnP₂?(4′‐cisDPyP) and a bis‐macrocycle (ZnP₂)₂?(TPyP) is discussed. The macrocycle and the bis‐macrocycle were crystallized and studied by X‐ray diffraction, which confirmed the excellent complementarity between the various components. Spectrophotometric and spectrofluorimetric titrations and studies reveal high association constants for both multiporphyrinic assemblies due to the almost perfect geometrical match between the interacting units. As expected, energy transfer from the zinc porphyrin component to the free‐base porphyrin quenches the fluorescence of the zinc porphyrin components in both compounds. But while in ZnP₂?(4′‐cis DPyP) sensitization of the emission of the free‐base porphyrin was observed, in (ZnP₂)₂?(TPyP) excitation of the peripheral Zn porphyrin units does not lead to quantitative sensitization of the luminescence of the free‐base porphyrin acceptor. An unusual HOMO–HOMO electron transfer reaction from ZnP₂ to the excited TPyP unit was detected and studied.     

Base‐Dependent Stereodivergent Intramolecular Aza‐Michael Reaction: Asymmetric Synthesis of 1,3‐Disubstituted Isoindolines

The nucleophilic addition (A_N) / intramolecular aza‐Michael reaction (IMAMR) process on Ellman’s tert‐butylsulfinyl imines, bearing a Michael acceptor in the ortho position, is studied. This reaction affords 1,3‐disubstituted isoindolines with a wide range of substituents in good yields and diastereoselectivities. Interestingly, careful choice of the base for the aza‐Michael step allows either the cis or the trans diastereoisomers to be exclusively obtained. This stereodivergent cyclization has enabled the synthesis of C₂‐symmetric bisacetate‐substituted isoindolines. In addition, bisacetate isoindolines bearing two well‐differentiated ester moieties are also noteworthy because they may allow for the orthogonal synthesis of β,β′‐dipeptides using a single nitrogen atom as a linchpin.     

Structure–property relationship study of the HPLC enantioselective retention of neuroprotective 7‐[(1‐alkylpiperidin‐3‐yl)methoxy]coumarin derivatives on an amylose‐based chiral stationary phase

The enantiomer separation of a number of racemic 7‐[(1‐alkylpiperidin‐3‐yl)methoxy]coumarin derivatives, some of which show outstanding in vitro multitarget neuroprotective activities, was successfully achieved on a polysaccharide‐based chiral stationary phase, bearing amylose tris(3,5‐dimethylphenylcarbamate) as a chiral selector, in normal polar mode (methanol and acetonitrile as the mobile phases). The majority of the screened selectands, especially those bearing 1‐(3‐X‐benzyl)piperidin‐3‐yl moieties, showed baseline enantiomer separations, and compound 8 (X = NO₂) was the best resolved (α = 2.01; R_S = 4.27). Linear free energy relationships, usefully complemented by molecular docking calculations, have the key role in enantioselective retention of aromatic interactions between π‐donor moieties in the chiral selector and π‐acceptor moieties in selectand, strengthened by hydrogen bond interaction between a hydrogen bond donor in the chiral selector and the hydrogen bond acceptor group(s) in the selectand. Statistically, reliable equations highlighted the importance of the substituent's size and substitution pattern (meta better than para) to affect the enantiorecognition of the title compounds. The chromatographic data support the scalability of the optimized experimental conditions for preparative purposes.     

Directing/Protecting‐Group‐Free Synthesis of Tetraaryl‐Substituted Pyrazoles through Four Direct Arylations on an Unsubstituted Pyrazole Scaffold

A directing/protecting‐group‐free synthesis of 1,3,4,5‐tetraaryl‐substituted pyrazoles was achieved through four transition metal‐catalyzed direct arylations. Various pyrazoles with four different aryl rings were obtained using readily available reagents from an unsubstituted pyrazole. Two aryl‐substituted pyrazoles showed intense violet fluorescence, high quantum yields (Φ_f=0.68, 0.64), and large Stokes shifts (19000, 15200 cm^?1).     

Fluorescence Studies of Pyrene‐Labeled Cholesterol‐Bearing Polymer Films

Copolymers of cholesteryl 6‐methacryloyloxy hexanoate and (1‐pyrenemethyl) 6‐methacryloyloxy hexanoate were prepared by free radical polymerization, with the amount of (1‐pyrenemethyl) 6‐methacryloyloxy hexanoate units (f_Py) in the polymers ranging from 0–100 mol‐%. The copolymers with f_Py ≤ 20.0 mol‐% show distinct liquid‐crystalline (LC) phase behavior. According to the fluorescence of pyrene in these copolymer LC films, each pyrene moiety is individually incorporated into the LC domain, and the interaction between the pyrene moieties is restricted.     

Synthesis,Structure and High Thermal Stability of a Novel Three‐dimensional Zinc(II) Complex with an Unsymmetrical Rigid‐flexible Ligand

A novel metal‐organic framework, [Zn(C₁₀H₈O₅)]_n ( 1 ) (C₁₀H₈O₅ = 2‐(4‐carboxylatophenoxy)propionate), was synthesized and characterized by elemental analysis, IR spectroscopy, X‐ray crystallography and thermogravimetric analysis. The crystal structure study reveals that each zinc atom is coordinated by four oxygen atoms from four different ligands to obtain a distorted tetrahedron. The rigid carboxyl group bridges two adjacent zinc atoms to form a dimer of eight‐membered rings, whereas the flexible carboxyl group bridges two adjacent dimers to form 1D chains along the a axis. Two adjacent 1D chains are interconnected by the ligands to produce 2D layers. These layers are further stabilized by intermolecular hydrogen bonds to construct a 3D framework showing high thermal stability (445 °C).     

Systematic Investigation of Molecular Arrangements and Solid‐State Fluorescence Properties on Salts of Anthracene‐2,6‐disulfonic Acid with Aliphatic Primary Amines

Organic salts of anthracene‐2,6‐disulfonic acid (ADS) with a wide variety of primary amines have been fabricated, and their arrangements of anthracene molecules and solid‐state fluorescence properties investigated. Single‐crystal X‐ray studies reveal that the salts show seven types of crystal forms and corresponding molecular arrangements of anthracene moieties depending on the amine, while anthracene shows only one form and arrangement in the solid state. Depending on the molecular arrangements, the ADS salts exhibit various solid‐state fluorescence properties: spectral shift (30 nm) and suppression and enhancement of the fluorescence intensity. Especially the ADS salt with n‐heptylamine (nHepA), which shows discrete anthracene moieties in the crystal, exhibits the highest quantum yield (Φ_F=46.1±0.2 %) in the series of ADS salts, which exceeds that of anthracene crystal (Φ_F=42.9±0.2 %). From these systematic investigations on the arrangements and the solid‐state properties, the following factors are essential for high fluorescence quantum yield in the solid state: prevention of contact between π planes of anthracene moieties and immobilization of anthracene rings. In addition, such organic salts have potential as a system for modulating the molecular arrangements of fluorophores and the concomitant solid‐state properties. Thus, systematic investigation of this system constructs a library of arrangements and properties, and the library leads to remarkable strategies for the development of organic solid materials.     

Hierarchic Supramolecular Interactions within Assemblies in Solution and in the Crystal of 2,3,6,7‐Tetrasubstituted 5,5′‐(Anthracene‐9,10‐diyl)bis[pyrimidin‐2‐amines]

The title compounds 6 and 7 were synthesized in good yield (Schemes 1 and 2), and their mode of assembly was studied both in solution, for the tetrakis(decyloxy) derivative 6 , and in the crystal, for the tetramethoxy analogue 7 . The pyrimidin‐2‐amine moieties of 6 and 7 can engage in three different supramolecular interactions: i) metal ligation via one of the pyrimidine N‐atoms, ii) cooperative double H‐bonding via the NH₂ group, and iii) π–π‐stacking interactions. In solution, coordination of the central Zn‐atom within the soluble porphyrinatozinc complex 19 leads to significant changes in the NMR and absorption spectra of 6 . In the absence of metal ligation, the next strongest interaction is H‐bonding which can operate in nonpolar or moderately polar solvents. In these cases, however, no stacking interaction or inclusion compounds could be put into evidence in the case of 6 by absorption, fluorescence, or NMR spectroscopies. The π‐stacking interactions were only observed in the crystal of 7 in conjunction with double H‐bonding. Slightly disordered DMSO molecules are also H‐bonded to the NH₂ groups of 7 , perturbing the expected packing. The present study illustrates some of the challenges inherent to directing hierarchical assembly processes in the solid state.     

Tautomeric Switching and Metal‐Cation Sensing of Ligand‐Equipped 4‐Hydroxy‐/4‐oxo‐1,4‐dihydroquinolines

Novel 4‐hydroxyquinoline (4HQ) based tautomeric switches are reported. 4HQs equipped with coordinative side arms (8‐arylimino and 3‐piperidin‐1‐ylmethyl groups) were synthesized to access O‐ or N‐selective chelation of Zn²⁺ and Cd²⁺ ions by 4HQ. In the case of the monodentate arylimino group, O chelation of metal ions induces concomitant switching of phenol tautomer to the keto form in nonpolar or aprotic media. This change is accompanied by selective and highly sensitive fluorometric sensing of Zn²⁺ ions. In the case of the bidentate 8‐(quinolin‐8‐ylimino)methyl side arm, NMR studies in CD₃OD indicated that both Cd²⁺ and Zn²⁺ ions afford N chelation for 4HQ, coexisting with tautomeric switching from quinolin‐4(1H)‐one to quinolin‐4‐olate. In corroboration, UV/Vis‐monitored metal‐ion titrations in toluene and methanol implied similar structural changes. Additionally, fluorescence measurements indicated that the metal‐triggered tautomeric switching is associated with compound signaling properties. The results are supported by DFT calculations at the B3LYP 6‐31G* level. Several X‐ray structures of metal‐free and metal‐chelating 4HQ are presented to support the solution studies.     

Approaches for synthesis of soluble poly(3,6‐dihydroxy‐2,7‐carbazole) with phenolic function

Poly(N‐alkyl‐3,6‐dihydroxy‐2,7‐carbazole) which should be soluble and have phenolic function was synthesized through different two routes. The former method was a direct synthesis by polymerizing a 2,7‐dibromo‐3,6‐dihydroxycarbazole monomer using Ni(cod)₂, which only gave a low molecular weight polymer. The latter method was an ether cleavage reaction of methoxy groups in a precursor polymer, poly(3,6‐dimethoxycarbazole), using BBr₃, which gave successfully the objective polymer that has a number average molecular weight of 4300 g/mol comparable to the precursor polymer. They showed large spectral changes in photoabsorption and fluorescence on addition of base. They also showed redox behavior similar to a hydroquinone/benzoquinone couple investigated by cyclic voltammetry. These new functions could be derived from the phenolic hydroxy group in the carbazole unit. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2039–2044     

Tetrakis(diethyl‐phosphonat)‐, Tetrakis(ethyl‐phenylphosphinat)‐ und Tetrakis(diphenylphosphin‐oxid)‐substituierte Phthalocyanine

Tetrakis(diethyl phosphonate), Tetrakis(ethyl phenylphosphinate)‐, and Tetrakis(diphenylphosphine oxide)‐Substituted Phthalocyanines The title compounds 7, 9 , and 11 are obtained by tetramerization of diethyl (3,4‐dicyanophenyl)phosphonate ( 5 ), ethyl (3,4‐dicyanophenyl)phenylphosphinate ( 8 ), and 4‐(diphenylphosphinyl)benzene‐1,2‐dicarbonitrile ( 10 ). The ³¹P‐NMR spectra of the phthalocyanines 7, 9 , and 11 and of their metal complexes present five to eight signals confirming the formation of four constitutional isomers with the expected C_4h, D_2h, C_2v, and C_s symmetry. In the FAB‐MS of the Zn, Cu, and Ni complexes of 7 and 9 , the peaks of dimeric phthalocyanines are observed. By gel‐permeation chromatography, the monomeric complex [Ni( 7 )] and a dimer [Ni( 7 )]₂ can be separated. These dimers differ from the known phthalocyanine dimers, i.e., possibly the P(O)(OEt)₂ and P(O)(Ph)(OEt) substituents in 7 and 9 are involved in complexation. The free phosphonic acid complex [Zn( 12 )] and [Cu( 12 )] are H₂O‐soluble. In the FAB‐MS of [Zn( 12 )], only the peaks of the dimer are present; the ESI‐MS confirms the existence of the dimer and the metal‐free dimer. In the UV/VIS spectrum of [Zn( 12 )], the hypsochromic shift characteristic for the known type of dimers from 660–700 nm to 620–640 nm is observed. As in the FAB‐MS of [Zn( 12 )], the free phosphinic acid complex [Zn( 13 )] shows only the monomer, an ESI‐MS cannot be obtained for solubility problems. The UV/VIS spectrum of [Zn( 13 )] demonstrates the existence of the monomer as well as of the dimer.     

Investigation of the Qx–Qy Equilibrium in a Metal‐Free Phthalocyanine

Phthalocyanines (Pcs) have attracted a lot of interest as small molecules for organic electronics. However, some excited‐state properties of metal‐free phthalocyanines, as for example, the dynamics of the transition between the nondegenerate Q_x and Q_y states in a metal‐free phthalocyanine, have not been fully established. This effect results in a blue‐shifted shoulder with low intensity in the Pc fluorescence spectrum. This shoulder was suggested to be related to emission from the more energetic Q_y state. By using ultrafast femtosecond transient absorption, we have found a clear equilibrium between the Q_x and Q_y state of metal‐free phthalocyanines in solution.     

Ethynylpyrene Linked Benzocrown Ethers as Fluorescent Sensors for Metal Ions

Substances containing ethynylpyrenes linked to either one or four benzocrown ethers were synthesized, and their absorption and fluorescence spectroscopic responses to metal ions were assessed. Addition of metal perchlorates to solutions of these substances promotes short wavelength shifts in their absorption and fluorescence maxima and increases in their fluorescence intensities. The magnitudes of the fluorescence intensity increases are dependent on the ring size and number of the crown ether and the nature of the metal cation. Association constants for complex formation were calculated using fluorescence intensity versus concentration data. Analysis using Job's plots showed that the substances containing one benzocrown ether moiety form 1:1 complexes with metal ions. Results of experiments employing repeated addition and removal of Mg(ClO₄)₂ demonstrate that the ON‐OFF fluorescence response can be repeated at least three times. Results of molecular orbital calculations show that complexation with metal ions lowers the energies of both the π and π* levels of the ethynylpyrene moiety and that in some cases the vacant orbital on the metal becomes the LUMO of the complex. An explanation of the spectroscopic changes promoted by metal ions is proposed in terms of electrostatic repulsion and structural regulation.     

A Metalloregulated Four‐State Nanoswitch Controls Two‐Step Sequential Catalysis in an Eleven‐Component System

The nanomechanical switch 1 with its three orthogonal binding motifs—the zinc(II) porphyrin, azaterpyridine, and shielded phenanthroline binding station—is quantitatively and reversibly toggled back and forth between four different switching states by means of addition and removal of appropriate metal‐ion inputs. Two of the four switching stages are able to initiate catalytic transformations (ON1, ON2), while the two others shut down any reaction (OFF1, OFF2). Thus, in a cyclic four‐state switching process the sequential transformation A + B + C → AB + C → ABC can be controlled, which proceeds stepwise along the switching states OFF1→ON1 (click reaction: A + B → AB )→OFF2→ON2 (Michael addition: AB + C → ABC )→OFF1. Two consecutive cycles of the sequential catalysis were realized without loss in activity in a reaction system with eleven different components.