Crowded Bis-(M-salphen) [M = Pt(II), Zn(II)] Coordination Architectures: Luminescent Properties and Ion-Selective Responses

Saccharide-Functionalized Poly(Zn-salphen)-alt-(m- and p-phenyleneethynylene)s as Dynamic Helical Metallopolymers

The study of metallopolymers with controllable helical sense remains in its infancy. We report arabinose-functionalized (Zn-salphen)-based conjugated polymers that display mirror-image circular dichroism spectra for L- and D-sugar sidechains respectively, signifying ordered (helical) coiling of the polymer backbone with opposite screw-sense preferences. The observation of different spectroscopic behavior and Cotton effects for a variety of solvents (in a reversible manner) and temperatures, ascribed to changes in the extent of intrachain (Zn⋅⋅⋅O(salphen) and π-stacking) interactions between Zn-salphen moieties, thus indicate the flexible, responsive and dynamic nature of the folded helical conformation in these systems. An application study signifying that activity can be governed by the structure and helical sense of the polymer is described.

A new 3D luminescent Ba-organic framework with high open metal sites: CO2 fixation, luminescence sensing, and dye sorption

A new synthesized 3D luminescent Ba-organic framework (1) may be used as a recyclable heterogeneous catalyst for fixation of CO₂ and has excellent response and sensitivity for pollutant ions. Moreover, 1 exhibits the particular selective sorption towards Congo red (CR) dye.

Homo- and Hetero-Oligonuclear Complexes of Platinum Group Metals (PGM) Coordinated by Imine Schiff Base Ligands

Chemistry of Schiff base (SB) ligands began in 1864 due to the discovery made by Hugo Schiff (Schiff, H., Justus Liebigs Ann. der Chemie 1864, 131 (1), 118-119). However, there is still a vivid interest in coordination compounds based on imine ligands. The aim of this paper is to review the most recent concepts on construction of homo- and hetero-oligonuclear Schiff base coordination compounds narrowed down to the less frequently considered complexes of platinum group metals (PGM). The combination of SB and PGM in oligonuclear entities has several advantages over mononuclear or polynuclear species. Such complexes usually exhibit better electroluminescent, magnetic and/or catalytic properties than mononuclear ones due to intermetallic interactions and frequently have better solubility than polymers. Various construction strategies of oligodentate imine ligands for coordination of PGM are surveyed including simple imine ligands, non-innocent 1,2-diimines, chelating imine systems with additional N/O/S atoms, classic N2O2-compartmental Schiff bases and their modifications resulting in acyclic fused ligands, macrocycles such as calixsalens, metallohelical structures, nano-sized molecular wheels and hybrid materials incorporating mesoionic species. Co-crystallization and formation of metallophilic interactions to extend the mononuclear entities up to oligonuclear coordination species are also discussed.

Dinuclear zinc(ii) salen-type Schiff-base complexes as molecular tweezers

In this contribution, the synthesis and the unusual aggregation/deaggregation properties in solution of two dinuclear ZnII Schiff-base complexes of tetradentate Schiff-base units, having non-conjugated spacers between each molecular unit, are reported in comparison to the mononuclear model complex. Through detailed 1H NMR, DOSY NMR, optical absorption, fluorescence emission, and multivariate analysis of optical absorption data, emerge some interesting findings. In solution of non-coordinating solvents, these Lewis acidic species are characterized as monomers stabilized by formation of intramolecular aggregates, having distinct spectroscopic properties in comparison to intermolecular aggregates derived from the mononuclear model analogue. Instead, in coordinating solvents they exhibit a typical behaviour, with formation of stable adducts showing a strong fluorescence. Deaggregation studies using pyridine as reference Lewis base allowed establishing a larger thermodynamic stability of these intramolecular aggregates, in comparison to intermolecular aggregates, even larger than that of aggregates of conjugated multinuclear complexes. The combined analysis of spectroscopic data upon deaggregation with ditopic Lewis bases unambiguously demonstrated the formation of stable 1 : 1 adducts, with higher binding constants in comparison to those related to monotopic species. Therefore, the present Lewis acidic, dinuclear complexes behave as molecular tweezers of ditopic guests having a strong Lewis basicity.

Crystal structures, red-shifted luminescence and iodide-anion recognition properties of four novel D-A type Zn(ii) complexes

Four D-A type Zn(ii) coordination complexes, [Zn(C29H29N3O2)·(CH3OH)]·(CH3OH) (1), Zn2(C74H90N6O4)·(CH3OH) (2), [Zn(C30H28N4O2)·(CH3OH)]·(CH3OH) (3) and [Zn(C38H44N4O2)·(C2H5OH)]·(C2H5OH) (4), were designed, synthesized, and studied. Their fluorescence properties in the solid state and in THF solution were comprehensively analysed based on their single-crystal structures. The results showed that the red-shift of fluorescence emission from complexes 1 to 4 was successfully achieved via the strategy of enhancing intramolecular charge transfer (ICT) effects by increasing the number of electron-pulling and pushing groups gradually. Meanwhile, because of the fluorescence recognition abilities of these four complexes towards iodide anions in THF, they could be regarded as potential fluorescent sensors for I- in this organic solution in the future.

Unexpected Structure of a Helical N4 -Schiff-Base Zn(II) Complex and Its Demetallation: Experimental and Theoretical Studies

A new Zn-N₄ -Schiff base L=((±)-trans-N,N'-Bis(2-aminobenzylidene)-1,2-diaminocyclohexane) complex was synthesized and fully characterized, showing an unexpected self-assembled double-stranded helicate structure. The X-ray crystal analysis of the Zn₂ L₂ complex ((C₄₀ H₄₄ N₈ Zn₂ ,CH₂ Cl₂ , a=14.2375(3) Å, b=16.7976(4) Å, c=16.1613(4) Å, monoclinic, P2₁ /n, Z=4) shows a centrosymmetrical structure in which zinc atoms are in distorted tetrahedral environments, revealing an M- (R, R) left-handed helicity in its asymmetric unit. However, it was observed that this dinuclear complex is thermodynamically unstable in the presence of small water amounts and undergoes demetallation into free N4-Schiff base ligand and ZnO nanoparticles. This hydrolysis process was thoroughly identified and monitored through detailed ¹ H NMR, DOSY NMR analysis. The reaction mechanism of this demetallation event was elucidated by using the DFT method, involving an activation energy smaller than 13 kcal/mol. Besides, a theoretical mechanism of the demetallation process is given for the first time.

Synthesis and Guest Recognition of Switchable Pt-Salphen Based Molecular Tweezers

Molecular tweezers are artificial receptors that have an open cavity generated by two recognition units pre-organized by a spacer. Switchable molecular tweezers, using a stimuli-responsive spacer, are particularly appealing as prototypes of the molecular machines that combine mechanical motion and allosteric recognition properties. In this present study, the synthesis of switchable molecular tweezers composed of a central terpyridine unit substituted in 4,4″ positions by two Pt(II)-salphen complexes is reported. The terpyridine ligand can be reversibly converted upon Zn(II) coordination from a free ‘U’-shaped closed form to a coordinated ‘W’ open form. This new substitution pattern enables a reverse control of the mechanical motion compared to the previously reported 6,6″ substituted terpyridine-based tweezers. Guest binding studies with aromatic guests showed an intercalation of coronene in the cavity created by the Pt-salphen moieties in the closed conformation. The formation of 1:1 host-guest complex was investigated by a combination of NMR studies and DFT calculations.

Study of the structure–bioactivity relationship of three new pyridine Schiff bases: synthesis, spectral characterization, DFT calculations and biological assays

Schiff bases exhibit a broad range of applications, including their use as catalysts, stabilizers, dyes, and intermediates in organic synthesis; and biological activities, such as antifungal properties.

Exploiting exciton coupling of ligand radical intervalence charge transfer transitions to tune NIR absorption

We detail the rational design of a series of bimetallic bis-ligand radical Ni salen complexes in which the relative orientation of the ligand radical chromophores provides a mechanism to tune the energy of intense intervalence charge transfer (IVCT) bands in the near infrared (NIR) region.

We detail the rational design of a series of bimetallic bis-ligand radical Ni salen complexes in which the relative orientation of the ligand radical chromophores provides a mechanism to tune the energy of intense intervalence charge transfer (IVCT) bands in the near infrared (NIR) region. Through a suite of experimental (electrochemistry, electron paramagnetic resonance spectroscopy, UV-vis-NIR spectroscopy) and theoretical (density functional theory) techniques, we demonstrate that bimetallic Ni salen complexes form bis-ligand radicals upon two-electron oxidation, whose NIR absorption energies depend on the geometry imposed in the bis-ligand radical complex. Relative to the oxidized monomer [1˙]⁺ (E = 4500 cm^–1, ε = 27 700 M^–1 cm^–1), oxidation of the cofacially constrained analogue 2 to [2˙˙]²⁺ results in a blue-shifted NIR band (E = 4830 cm^–1, ε = 42 900 M^–1 cm^–1), while oxidation of 5 to [5˙˙]²⁺, with parallel arrangement of chromophores, results in a red-shifted NIR band (E = 4150 cm^–1, ε = 46 600 M^–1 cm^–1); the NIR bands exhibit double the intensity in comparison to the monomer. Oxidation of the intermediate orientations results in band splitting for [3˙˙]²⁺ (E = 4890 and 4200 cm^–1; ε = 26 500 and 21 100 M^–1 cm^–1), and a red-shift for [4˙˙]²⁺ using ortho- and meta-phenylene linkers, respectively. This study demonstrates for the first time, the applicability of exciton coupling to ligand radical systems absorbing in the NIR region and shows that by simple geometry changes, it is possible to tune the energy of intense low energy absorption by nearly 400 nm.

Thermally Stable Zinc Disalphen Macrocycles Showing Solid-State and Aggregation-Induced Enhanced Emission

In order to investigate the solid-state light emission of zinc salphen macrocycle complexes, 7 dinuclear zinc salphen macrocycle complexes (1-7), with acetate or hexanoate coligands, are synthesized. The complexes are stable in air up to 300 °C, as shown via thermogravimetric analysis (TGA), and exhibit green to orange-red emission in solution (λ_em = 550-600 nm, PLQE ≤ 1%) and slightly enhanced yellow to orange-red emission in the solid state (λ_em = 570-625 nm, PLQE = 1-5%). Complexes 1, 2, 4, 5, and 7 also display aggregation-induced enhanced emission (AIEE) when hexane (a nonsolvent) is added to a chloroform solution of the complexes, with complex 4 displaying a 75-fold increase in peak emission intensity upon aggregation (in 0.25:0.75 chloroform:hexane mixture).

Red-shift in fluorescence emission of D–A type asymmetrical Zn(ii) complexes by extending the π–π stacking interaction

Two D–A type asymmetrical Zn(ii) coordination complexes, [ZnL¹(C₂H₅OH)] (1) and [ZnL²(DMF)]·DMF (2) were designed, synthesized, and studied. Through the strategy of expanding π–π stacking interaction, fluorescence emission is red-shifted remarkably.

Topologically diverse shape-persistent bis-(Zn-salphen) catalysts: efficient cyclic carbonate formation under mild conditions

By adopting a shape-persistent bimetallic design approach, high initial molecular turnover frequencies (up to 14,800 h(-1)) for coupling of CO2 with epoxides in conjunction with (n)Bu4NI, plus excellent yields under mild conditions (1 bar of CO2, 45 °C) have been achieved for catalysts containing cofacial Zn-salphen units.

Spectral and thermal characterization of salophen type Schiff base and its implementation as solid contact electrode for quantitative monitoring of copper(II) ion

Salophen templated Schiff base 2,3-bis(salicylaldimino)pyridine (H2IF) has been synthesized and fully characterized by a series of different spectroscopic methods and thermogravimetric analysis (TGA). It has been also further probed electrochemically and explored as a cation recognition ionophore in the form of a polymeric membrane as selective sensor for quantitative monitoring of Cu(2+). Dielectric properties of the membrane have been studied by electrochemical impedance spectroscopy (EIS). The potentiometric results have demonstrated that the sensor exhibits very good selectivity and sensitivity towards Cu(2+) over a wide variety of cations. The electrode has a linear response to Cu(2+) with a detection limit of 4.46×10(-8) and displays a Nernstian slope (29.14 mV/decade) between pH 3.0 and 6.0 with a fast response time less than 10s. The solid contact electrodes have been exploited over five mounts period with good reproducibility. The analytical availability of the proposed electrode has been evaluated by applying in the determination of Cu(2+) ions in water samples. The structural features and complexation of ionophore with Cu(2+) have been monitored by UV-Vis spectroscopy and spectral findings have been further supported by DFT and TD-DFT calculations.

Tunable Luminescence and Application in Dye-Sensitized Solar Cells of Zn(II)/Hg(II) Complexes: Methyl Substitution-Induced Supramolecular Structures Based on (E)-N-(6-Methoxypyridin-2-ylmethylene)arylamine Derivatives

Using Schiff-base ligands (E)-N-(6-methoxypyridin-2-yl)(CH═NAr) (where Ar = C6H5, L1; 2-MeC6H4, L2; 2,4,6-Me3C6H2, L3), six Zn(II)/Hg(II) complexes, namely, [ZnL1Cl2] (Zn1), [HgL1Cl2] (Hg1), [ZnL2Cl2] (Zn2), [HgL2Cl2] (Hg2), [ZnL3Cl2] (Zn3), and [HgL3Cl2] (Hg3) have been synthesized under solvothermal conditions. The structures of six complexes have been established by X-ray single-crystal analysis and further physically characterized by EA, FT-IR, (1)H NMR, and ESI-MS. The crystal structures of these complexes indicate that noncovalent interactions, such as hydrogen bonds, C-H···Cl, and π···π stacking, play essential roles in constructing the resulting supramolecular structures (1D for Hg3; 2D for Zn2, Hg2; 3D for Zn1, Hg1, and Zn3). Upon irradiation with UV light, the emission of complexes Zn1-Zn3 and Hg1-Hg3 could be finely tuned from green (480-540 nm) in the solid state to blue (402-425 nm) in acetonitrile solution. It showed that the ligand and metal cation can influence the structures and luminescence properties of complexes such as emission intensities and maximum wavelengths. Since these ligands and complexes could compensate for the absorption of N719 in the low-wavelength region of the visible spectrum and reduce charge recombination of the injected electron, the ligands L1-L3 and complexes Zn3/Hg3 were employed to prepare cosensitized dye-sensitized solar cells devices for investigating the influences of the electron-donating group and coordination on the DSSCs performance. Compared to DSSCs only being sensitized by N719, these prepared ligands and complexes chosen to cosensitize N719 in solar cell do enhanced its performance by 11-41%. In particular, a DSSC using L3 as cosensitizer displays better photovoltaic performance with a short circuit current density of 18.18 mA cm(-2), corresponding to a conversion efficiency of 7.25%. It is much higher than that for DSSCs only sensitized by N719 (5.14%).

Switchable platinum-based tweezers with Pt-Pt bonding and selective luminescence quenching

Molecular tweezers incorporating peripheral platinum salphen complexes and a central chelating terpyridine group have been synthesized. The terpyridine can be switched upon metal binding between a free 'W' shaped form and a coordinated 'U' form. The crystallographic structure of the zinc-closed molecular tweezers was obtained and presented a strong π-stacking between the Pt-salphen units associated with a Pt-Pt bond. The luminescence properties, notably in response to selected guest ions (Zn(2+), Pb(2+), Hg(2+)) and the resulting mechanical motion, have been investigated by UV-Vis and emission spectroscopy. While ion coordination to the terpy resulted in no significant changes in the luminescence, a selective intercalation of a second Hg(2+) associated with a large differential quenching was observed.

Structure and aggregation properties of a Schiff-base zinc(ii) complex derived from cis-1,2-diaminocyclohexane

The effect of the bridging diamine upon the aggregation properties of a Zn^II Schiff-base complex is reported. The X-ray crystal structure indicates the presence of an asymmetric dimer which is preserved even in solution.

Three new solvent-directed Cd(ii)-based MOFs with unique luminescent properties and highly selective sensors for Cu2+ cations and nitrobenzene

Three new solvent-directed MOFs were yielded in different solvent systems. The luminescent properties of the MOFs were studied via UV light at ambient temperature, especially for 3, which showed highly selective sensitivity for sensing Cu²⁺ ions and NB molecules.

Terpy(Pt-salphen)2 switchable luminescent molecular tweezers

The design and synthesis of switchable molecular tweezers based on a luminescent terpy(Pt-salphen)2 (1; terpy=terpyridine) complex is reported. Upon metal coordination, the tweezers can switch from an open "W"-shaped conformation to a closed "U"-shaped form that is adapted for selective recognition of cations. Closing of the tweezers by metal coordination (M=Zn(2+), Cu(2+), Pb(2+), Fe(2+), Hg(2+)) was monitored by (1)H NMR and/or UV/Vis titrations. During the titration, exclusive formation of the 1:1 complex [M(1)] was observed, without appearance of an intermediate 1:2 complex [M(1)2]. The crystallographic structure of the 1:1 complex was obtained with Pb(2+) and showed a distorted helical structure. Selective intercalation of Hg(2+) cations by the closed "U" form was observed. The tweezers were reopened by selective metal decoordination of the terpyridine ligand by using tris(2-aminoethyl)amine (tren) as a competitive ligand without modification of the Pt-salphen complex. Detailed photophysical studies were performed on the open and closed tweezers. Structured emission was observed in the open form from the Pt-salphen moieties, with a high quantum yield and a long lifetime. The emission is slightly modified upon closing with 1 equivalent of Zn(2+) or Hg(2+), whereas a dramatic quenching was obtained upon intercalation of additional Hg(2+).

Luminescent oligo(ethylene glycol)-functionalized cyclometalated platinum(II) complexes: cellular characterization and mitochondria-specific localization

A readily tunable series of non-planar oligo(ethylene glycol)-substituted phosphorescent Pt(II) complexes has been investigated as live cell imaging agents; suitable structural modifications can give good cellular uptake, traceable mitochondria-specific localization and potent cytotoxic characteristics towards HeLa cells.

Different Zn(II) cation coordination geometries in di-μ-acetato-bis{2-chloro-6-[(pyridine-2-ylmethylimino)methyl]phenol}dizinc(II) chloroform monosolvate

In the title compound, di-μ-acetato-κ(2)O:O;κ(2)O:O'-bis[(6-chloro-2-{(E)-[(pyridin-2-yl)methylimino]methyl}phenolato-κ(3)N,N',O)zinc(II)], [Zn2(C13H10ClN2O)2(C2H3O2)2]·CHCl3, the Zn(II) cation adopts a five-coordinate geometry and is coordinated by two N atoms and one O atom of a tridentate 6-chloro-2-{(E)-[(pyridin-2-yl)methylimino]methyl}phenolate ligand and two O atoms of two bridging acetate groups, but their coordination geometries differ. One Zn(II) cation adopts a distorted trigonal bipyramidal geometry and the other a square-pyramidal geometry. The two acetate ligands bridge two Zn(II) cations with mono- and bidentate coordination modes. The title compound exhibits a strong emission at 460 nm upon excitation at 325 nm with a quantum yield of 23.1%.