Pyridyl Units Research Articles

13C–1H dipolar couplings for probing rod-like hydrogen bonded mesogens

Hydrogen bonding is the most important non-covalent interaction utilised in building supramolecular assemblies and is preferred often as a means of construction of molecular, oligomeric as well as polymeric materials that show liquid crystalline properties. In this work, a pyridine based nematogenic acceptor has been synthesized and mixed with non-mesogenic 4-methoxy benzoic acid to get a hydrogen bonded mesogen. The existence of hydrogen bonding between the pyridyl unit and the carboxylic acid was established using FT-IR spectroscopy from the observation of characteristic stretching vibrations of unionized type at 2425 and 1927 cm−1. The mesogenic acceptor and the complex have been investigated using 13C NMR in solution, solid and liquid crystalline states. Together with the 2D separated local field NMR experiments, the studies confirm the molecular structure in the mesophase and yield the local orientational order parameters. It is observed that the insertion of 4-methoxy benzoic acid not only enhances the mesophase stability but also induces a smectic phase due to an increase in the core length of the hydrogen bonded mesogen.

Spin crossover in Co(ii) metallorods – replacing aliphatic tails by aromatic

Terpyridine ligands with ter- and quater-phenyl substituents at the 4' position provide bis(ligand)Co(II) complexes showing very different magnetic properties to those of their analogues with long-chain aliphatic substituents, with no evidence of "re-entrant" behavior involving multiple high- and low-spin species. Structure determinations of [Co(tptpy)2](BF4)2·CH3OH and [Co(qptpy)2](BPh4)2·3.5dmf·2H2O (dmf = N,N-dimethylformamide) show the metal ion centres in both to be relatively distant and that the "terpyridine embrace" observed to be partially retained in their analogues is completely lost. Consideration of available structural and magnetic data for these and other Co(II) complexes of functionalised terpyridines and terpyridine itself provides evidence that spin crossover behaviour may be regulated by face-to-face contacts of the pyridyl units of the head groups.

Multi-pyridine decorated Fe(ii) and Ru(ii) complexes by Pd(0)-catalysed cross couplings: new building blocks for metallosupramolecular assemblies

Eight metal complexes of the type [M(tpy)2](2+) (tpy = 2,2':6',2''-terpyridine) featuring four pendant pyridine rings are reported and characterised by NMR, MS, absorption spectroscopy and electrochemical methods. Palladium-mediated Suzuki and Sonogashira cross-coupling reactions were performed on both free 4'-(3,5-dibromophenyl)-tpy and its Ru(II) complex in good yields. The ready N-alkylation of the pendant pyridyl units has significant influence on the absorption and electrochemical reduction of the complexes, processes which are localised on the periphery and leaves the [Ru(tpy)2](2+) core essentially unaffected. The binding of metal ions by the free pyridines is also demonstrated as means of assembling larger ordered non-covalent structures.

Solution Structure of Ln(III) Complexes with Macrocyclic Ligands Through Theoretical Evaluation of 1H NMR Contact Shifts

Herein, we present a new approach that combines DFT calculations and the analysis of Tb(III)-induced (1)H NMR shifts to quantitatively and accurately account for the contact contribution to the paramagnetic shift in Ln(III) complexes. Geometry optimizations of different Gd(III) complexes with macrocyclic ligands were carried out using the hybrid meta-GGA TPSSh functional and a 46 + 4f(7) effective core potential (ECP) for Gd. The complexes investigated include [Ln(Me-DODPA)](+) (H(2)Me-DODPA = 6,6'-((4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(methylene))dipicolinic acid, [Ln(DOTA)(H(2)O)](-) (H(4)DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate), [Ln(DOTAM)(H(2)O)](3+) (DOTAM = 1,4,7,10- tetrakis[(carbamoyl)methyl]-1,4,7,10-tetraazacyclododecane), and related systems containing pyridyl units (Ln = Gd, Tb). Subsequent all-electron relativistic calculations based on the DKH2 approximation, or small-core ECP calculations, were used to compute the (1)H hyperfine coupling constants (HFCCs) at the ligand nuclei (A(iso) values). The calculated A(iso) values provided direct access to contact contributions to the (1)H NMR shifts of the corresponding Tb(III) complexes under the assumption that Gd and Tb complexes with a given ligand present similar HFCCs. These contact shifts were used to obtain the pseudocontact shifts, which encode structural information as they depend on the position of the nucleus with respect to the lanthanide ion. An excellent agreement was observed between the experimental and calculated pseudocontact shifts using the DFT-optimized geometries as structural models of the complexes in solution, which demonstrates that the computational approach used provides (i) good structural models for the complexes, (ii) accurate HFCCs at the ligand nuclei. The methodology presented in this work can be classified in the context of model-dependent methods, as it relies on the use of a specific molecular structure obtained from DFT calculations. Our results show that spin polarization effects dominate the (1)H A(iso) values. The X-ray crystal structures of [Ln(Me-DODPA)](PF(6))·2H(2)O (Ln = Eu or Lu) are also reported.

Formation of Oligomeric and Macrocyclic Ureas Based on 2,6-Diaminopyridine

The conversion of 1,3-bis-(6-amino-pyridin-2-yl)-urea (1) with N,N'-carbonyldiimidazole at high temperatures in DMSO yielded a mixture of defined cyclic trimers and tetramers. On the basis of model reactions, exchange reactions were evidenced, which convert the cyclic tetramer into a stable cyclic trimer. Linear even numbered oligomers were obtained in acetone under reflux where side reactions were suppressed. The pronounced tendency of cyclization is attributed to a preferred folded conformation of the urea bond between two pyridyl units.

Novel amphipathic photoluminescent copolymers containing fluorene, pyridine and thiophene moieties: Synthesis, characterization and self-assembly

In order to investigate the explicit electro-optical properties and self-assembly variations of the photoluminescent polymer with sterically hindered side chains, two novel polymers (P0 and P1) based on fluorene, pyridine and thiophene moieties were successfully synthesized through Suzuki coupling reaction. The molecular structures of the polymers were fully characterized by 1H NMR, 13C NMR, elemental analysis and gel-permeation chromatograph, respectively. The photophysical properties, energy band gap and self-assembly behaviors of polymer P0 and P1 were examined through UV–vis absorption, photoluminescent spectra, cyclic voltammetry, dynamic laser light scattering and transmission electron microscopy. The experimental results indicated that the polymers took on wide band gaps of 3.09 and 3.11 eV with blue–green emission in thin solid films. By combining supramolecular assembly nature of hydroxy, phenolic and pyridyl units with controlled micro-phase separation of well-defined amphipathic di-block copolymers, we have obtained stable vesicles from the fluorescent amphipathic polymers in aqueous solution. The size and morphology of the vesicles formed can be effectively tuned in a range of 1144 nm–291 nm. This work can serve as an excellent exploratory example for the fine self-assembly control principle in the amphipathic photoluminescent copolymer with sterically hindered side chains through the multiple weak interactions of the π-system.

Preparation of new supramolecular liquid-crystalline polyesters containing 3-chloro-4-(alkoxy)benzoic acids

Supramolecular liquid-crystalline polyester complexes based on intermolecular hydrogen bonds between the carboxylic group and the pyridyl moieties was prepared by using non-liquid-crystalline H-donors, [3-chloro-4-(butyloxy)benzoic acid (2a), 3-chloro-4-(octyloxy)benzoic acid (2b), 3-chloro-4-(dodecyloxy)benzoic acid (2c) and 3-chloro-4-(tetradecyloxy)benzoic acid (2d)] and H-acceptor-polyester containing pyridyl units. Intermolecular hydrogen bond formation was confirmed by Fourier transform infrared spectroscopy. The liquid-crystalline behavior of the complex formed was established by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The polyester complexes containing 2c and 2d donor components exhibit liquid crystalline mesophase and behave as side-chain liquid-crystalline polymers. Compared with unsubstituted parent acid, the presence of chloro group as a lateral substituent has a little negative effect on the induction of liquid crystallinity on the polyester complexes systems. The results show that the more stability of the obtained H-bonded complexes in comparison with analogues without 3-Cl substituents is due to the increased acidity of benzoic acid moiety.

Hydrogen-bonded liquid-crystalline complexes of polyester containing a pyridyl moiety with 4-(alkoxy)benzoic acid

Novel hydrogen-bonded polyester complexes from mesogenic 4-(butyloxy)benzoic acid (2a), 4-(octyloxy)benzoic acid (2b), 4-(dodecyloxy)benzoic acid (2c) and 4-(tetradecyloxy)benzoic acid (2d) as the hydrogen bond donors and a polyester (3) based on 2,6-pyridine dicarboxylic acid as the hydrogen bond acceptor were prepared by the melt method. The association by hydrogen bonding was confirmed by means of FTIR. The components were miscible up to 0.1 mole ratio of 2d and 2c versus the polyester repeat unit. The limiting mole ratio for 2b and 2a to 3 were 0.2 and 0.4, respectively. Phase separation in the supramolecular complexes occurred above these limiting values and a two phase system consisting of a polyester supramolecular complex and 4-(alkyloxy)benzoic acid was formed. This was due to weak hydrogen bonding between the low molecular acid (especially the acid with longer terminal alkoxy group) and pyridyl unit of polyester in comparison with thermodynamically more favorable dimerization of acid molecules. The liquid crystalline behavior of these supramolecular polymeric complexes was confirmed by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). It was found that 2a exhibits no mesomorphism when mixed with polyester. However, among the other acid derivatives, 2b exhibited stable mesogenic complexes.

A Metal-Directed Self-Assembled Electroactive Cage with Bis(pyrrolo)tetrathiafulvalene (BPTTF) Side Walls

A straightforward synthesis of a bis(pyrrolo)tetrathiafulvalene (BPTTF)-based tetratopic ligand bearing four pyridyl units is described. The first example of a TTF-based self-assembled cage has been produced from this redox-active ligand through metal-directed synthesis with a cis-coordinated square-planar Pt(II) complex. The resulting cage corresponds to a trigonal-prismatic structure, as shown by X-ray crystallography. A UV-vis titration indicated that the electron-rich cavity can be used to incorporate one molecule of tetrafluorotetracyano-p-quinodimethane (TCNQF(4)).

Dye Molecular Structure Device Open-Circuit Voltage Correlation in Ru(II) Sensitizers with Heteroleptic Tridentate Chelates for Dye-Sensitized Solar Cells

Dicarboxyterpyridine chelates with π-conjugated pendant groups attached at the 5- or 6-position of the terminal pyridyl unit were synthesized. Together with 2,6-bis(5-pyrazolyl)pyridine, these were used successfully to prepare a series of novel heteroleptic, bis-tridentate Ru(II) sensitizers, denoted as TF-11-14. These dyes show excellent performance in dye-sensitized solar cells (DSCs) under AM1.5G simulated sunlight at a light intensity of 100 mW cm(-2) in comparison with a reference device containing [Ru(Htctpy)(NCS)(3)][TBA](3) (N749), where H(3)tctpy and TBA are 4,4',4"-tricarboxy-2,2':6',2"-terpyridine and tetra-n-butylammonium cation, respectively. In particular, the sensitizer TF-12 gave a short-circuit photocurrent of 19.0 mA cm(-2), an open-circuit voltage (V(OC)) of 0.71 V, and a fill factor of 0.68, affording an overall conversion efficiency of 9.21%. The increased conjugation conferred to the TF dyes by the addition of the π-conjugated pendant groups increases both their light-harvesting and photovoltaic energy conversion capability in comparison with N749. Detailed recombination processes in these devices were probed by various spectroscopic and dynamics measurements, and a clear correlation between the device V(OC) and the cell electron lifetime was established. In agreement with several other recent studies, the results demonstrate that high efficiencies can also be achieved with Ru(II) sensitizers that do not contain thiocyanate ancillaries. This bis-tridentate, dual-carboxy anchor configuration thus serves as a prototype for future omnibearing design of highly efficient Ru(II) sensitizers suited for use in DSCs.

Poly[[diaqua-bis-[μ(4)-5-nitro-isophthalato-κ(4)O(1):O(1):O(3):O(3')]bis-[μ(3)-pyridine-4-carboxyl-ato-κ(3)O:O':N]tricobalt(II)] tetra-hydrate

The title compound, {[Co3(C6H4NO2)2(C8H3NO6)2(H2O)2]·4H2O}n, exhibits a two-dimensional layer-like structure in which the CoII ions exhibit two kinds of coordination geometries. One nearly octa­hedral CoII ion with crystallographic inversion symmetry is coordinated to six carboxyl­ate O atoms from four bridging 5-nitro­isophthalate (NIPH) ligands and two isonicotinate (IN) anions, while the other type of CoII ion binds with one N atom and one carboxyl­ate O atom from two IN anions, two carboxyl­ate O atoms from two different NIPH anions and one ligated water mol­ecule, displaying a distorted square-pyramidal coordination geometry. Three adjacent CoII ions are bridged by six carboxyl­ate groups from four NIPH ligands and two IN anions to form a linear trinuclear secondary building unit (SBU). Every trinuclear SBU is linked to its nearest neighbours in the ab plane, resulting in a two-dimensional layer-like structure perpendicular to the c axis. Along the a-axis direction neighbouring mol­ecules are connected through carboxyl­ate and pyridyl units of the IN anions, along the b axis through carboxyl­ate groups of the NIPH ligands. The H atoms of one free water mol­ecule are disordered in the crystal in a 1:1 ratio. Typical O—H⋯O hydrogen bonds are observed in the lattice, which include the following contacts: (a) between coordinated water mol­ecules and carboxyl­ate O atoms of the NIPH anions, (b) between lattice water mol­ecules and carboxyl­ate O atoms of the NIPH anions, and (c) between coordinated and lattice water mol­ecules. These inter­molecular hydrogen bonds connect the two-dimensional layers to form a three-dimensional supra­molecular structure.

Synthesis, solid-state structure and supramolecularity of [Cu(pyterpy)2](ClO4)2

The copper(II) polypyridyl complex [Cu(pyterpy)2](ClO4)2 (3) (pyterpy=4′-(4-pyridyl)-2,2′:6,2″-terpyridine) was prepared by the reaction of pyterpy (1) with stoichiometric amounts of [Cu(ClO4)2·6H2O] (2). The progress of the reaction was controlled by FT-IR and UV–vis spectroscopy. The title complex crystallized in the tetragonal space group I4(1)/a with unit cell dimensions of a=8.6277(1), b=8.6277(1), c=57.6398(10)Å, V=4290.55(12)Å3, and Z=4. The structure of 3 in the solid-state consists of discrete [Cu(pyterpy)2]+ ions with copper(II) in a distorted octahedral environment setup by two meridional coordinated tripodal 4′-(4-pyridyl)-2,2′:6,2″-terpyridine ligands of which the pyridyl unit stays free. Face-to-face π-interactions between terminal coordinated terpy C5N rings link adjacent [Cu(pyterpy)2]2+ units resulting in the formation of a 2D-polymer. The geometrical-to-geometrical centroid distance (d) is 3.568Å.

Ru(ii) complexes with N-heterocyclic carbene ligands or terpyridine analogues: synthesis, characterization, and electrochemical and proton-dependent spectrometric properties

Four ruthenium(II) complexes, BPT, BPN, BPPT, and BPPN, have been prepared and characterized by (1)H NMR, (13)C NMR, high resolution mass spectrometry (HRMS), elemental analysis, and X-ray crystallography. All the complexes incorporate a pyridyl unit on the terpyridine-type or N-heterocyclic carbene (NHC) ligand and the pyridyl unit can be protonated upon addition of 1.0 M HCl in diethyl ether. The proton-dependent absorption and luminescence spectrum were measured in CH(3)CN. In the case of BPT, the λ(max)(abs) was moved by 10 nm from 490 nm to 500 nm after the addition of 12 equiv. of HCl and the intensity of the emission spectrum increased. In contrast, in the case of BPN, the λ(max)(abs) was red-shifted by 43 nm from 424 nm to 467 nm and the emission was dramatically quenched upon the addition of the equiv. of HCl. However, there were no noticeable changes in the λ(max)(abs) values of BPPT and BPPN even after the addition of the HCl to a solution of those complexes. Moreover, BPN has a selective sensing property for a proton among many cations.

Molecular tectonics: zinc coordination networks based on centric and acentric porphyrins bearing pyridyl units

Two new ligands, one symmetric 1 and the other acentric 2, based on a porphyrin backbone bearing either two ethynylpyridyl or one pyridyl and one ethynylpyridyl coordinating groups connected to the porphyrin at two opposite meso positions have been designed and prepared. In the presence of a Zn(II) cation, they lead to the formation of neutral metallatectons 1-Zn and 2-Zn which self-assemble into coordination networks in the crystalline phase. Whereas the metallatecton 1-Zn leads exclusively to the formation of grid type 2D networks, 2-Zn generates two types of crystals with rod and rhombic morphologies. The rod type crystals are composed of a 1D zigzag type arrangement whereas crystals with rhombic morphology are composed of directional 2D grid type architecture. The packing of the latter leading to the formation of the crystal occurs in a centrosymmetric fashion causing thus the loss of directionality.

Two photoluminescent polymers based on fluorene and 2,4,6‐triphenyl pyridine: Synthesis and electroluminescence

AbstractTwo fluorene and triphenyl pyridine‐based linear and dendronized copolymers, P1 and P2, were synthesized and fully characterized by 1H‐NMR, 13C‐NMR, and matrix assistant laser desorption/ionization time‐of‐flight mass spectra, respectively. The absorption, photoluminescence (PL) behavior, and energy band gaps of P1 and P2 relative to those of polyfluorene end‐capped with benzene (P0) were examined through UV–vis, photoluminescent spectra, and cyclic voltammetry. The UV–vis absorption and PL emission behavior of P0 and P1 were hardly affected by molecular architecture, while those of P2 were strongly correlated with the dendronized molecular frameworks. Cyclic voltammetry studies indicated the lower highest occupied molecular orbital energy level and wider band gap of P2 thin solid film relative to those of P0 and P1. The new polymers were thermally stable up to 410°C. The better luminance and external quantum efficiencies of P1 relative to those of P0 in polymer light‐emitting diode (PLED) applications are due to improved electron injection, charge trapping and recombination at the pyridine sites. Through the experiments, it is found that the triphenyl pyridyl segments and excimers‐formation make pronounced contribution to long wavelength emission in P1‐based blue light‐emitting materials, and the analogous materials containing 2,4,6‐triphenyl pyridyl unit of P1 constitute highly attractive materials for white PLED applications. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Positively Charged Lanthanide Complexes with Cyclen-Based Ligands: Synthesis, Solid-State and Solution Structure, and Fluoride Interaction

The syntheses of a new cyclen-based ligand L(2) containing four N-[2-(2-hydroxyethoxy)ethyl]acetamide pendant arms and of its lanthanide(III) complexes [LnL(2)(H(2)O)]Cl(3) (Ln = La, Eu, Tb, Yb, or Lu) are reported, together with a comparison with some Ln(III) complexes of a previously reported analogue L(1) in which two opposite amide arms have been replaced by coordinating pyridyl units. The structure and dynamics of the La(III), Lu(III), and Yb(III) complexes in solution were studied by using multinuclear NMR investigations and density functional theory calculations. Luminescence lifetime measurements in H(2)O and D(2)O solutions of the [Ln(L(2))(H(2)O)](3+) complexes (Ln = Eu or Tb) were used to investigate the number of H(2)O molecules coordinated to the metal ion, pointing to the presence of an inner-sphere H(2)O molecule in a buffered aqueous solution. Fluoride binding to the latter complexes was investigated using a combination of absorption spectroscopy and steady-state and time-resolved luminescence spectroscopy, pointing to a surprisingly weak interaction in the case of L(2) (log K = 1.4 ± 0.1). In contrast to the results in solution, the X-ray crystal structure of the lanthanide complex showed the ninth coordination position occupied by a chloride anion. In the case of L(1), the X-ray structure of the [(EuL(1))(2)F] complex features a bridging fluoride donor with an uncommon linear Eu-F-Eu entity connecting two almost identical [Eu(L(1))](3+) units. Encapsulation of the F(-) anion within the two complexes is assisted by π-π stacking between the pyridyl rings of two complexes and C-H···F hydrogen-bonding interactions involving the anion and the pyridyl units.

Dynamics of Closure of Zinc Bis‐Porphyrin Molecular Tweezers with Copper(II) Ions and Electron Transfer

Zinc bis-porphyrin molecular tweezers composed of a N(4) spacer bound through pyridyl units to the meso position of porphyrins were synthesized, and the tweezers are closed by the coordination of a copper(II) ion inside the spacer ligand. The effect of the π-π interaction between the porphyrin rings in the closed conformation on the absorption spectra of multi-electron oxidized species and the reduction potentials were clarified by chemical and electrochemical oxidation of the closed form of the zinc bis-porphyrin molecular tweezers in comparison with the open form without copper(II) ion and the corresponding porphyrin monomer. The shifts in redox potentials and absorption spectrum of the porphyrin dication indicate a strong electronic interaction between the two oxidized porphyrins in the closed form, whereas there is little interaction between them in the neutral form. The dynamics of copper(II) ion coordination and subsequent electron transfer was examined by using a stopped-flow UV/Vis spectroscopic technique. It was confirmed that coordination of copper(II) occurs prior to electron-transfer oxidation of the closed form of the zinc bis-porphyrin molecular tweezers.

Size- and Shape-Selective Isostructural Microporous Metal–Organic Frameworks with Different Effective Aperture Sizes

Two isostructural metal-organic frameworks (MOFs) having micropores of the same "static aperture size" but different "effective aperture size" have been prepared using 5-(pyridin-3-ylethynyl)isophthalic acid as a ligand having two different types of functional units, an isophthalate (iph) unit and a pyridyl unit, simultaneously in a single ligand. The combination of iph unit and Cu (or Zn) ion led to two-dimensional layers of Kagomé (kgm) net topology, the layers being further pillared by the internal auxiliary pyridyl unit to form a three-dimensional microporous framework having two different types of cage-like pores, cage A and cage B, with different aperture sizes and shapes. (1) The MOFs can distinguish the adsorbates (N(2)/Ar) not based on the widely used kinetic diameters of the adsorbates but based on the minimum diameters of the adsorbates, which are more shape dependent. While cage A with a sufficiently large aperture size compared with the size of the adsorbates does not show any size selectivity, cage B with an approximate size match between the adsorbates and the pore apertures shows shape selectivity for the adsorbates. The smaller but spherically shaped Ar atom is not allowed into the pore with the oval-shaped aperture; however, the larger linear N(2) molecule is allowed into the pore with the oval-shaped aperture. (2) Even though the two isostructural MOFs have the same static aperture size of cage B, they show different size selectivity for the adsorbates based on the effective aperture size, which reflects the different extents of the framework flexibility.

A Catalyst for the Simultaneous Ring‐Opening Metathesis Polymerization/Vinyl Insertion Polymerization

Come together: Catalyst 1, which contains a diethylboryl-protected pyridyl unit, copolymerizes ethylene and cyclic olefins such as norborn-2-ene (NBE) and cis-cyclooctene (COE) to give multiblock copolymers. Both vinyl insertion polymerization and ring-opening metathesis polymerization are in effect within a single polymer chain, and the respective NBE and COE blocks can be obtained in different ratios.

Ru2(CO)4{OOC(CH2) n CH3}2L2 sawhorse-type complexes containing μ2-η2-carboxylato ligands derived from saturated fatty acids

The thermal reaction of Ru3(CO)12 with the saturated fatty acids (heptanoic, nonanoic, decanoic, tridecanoic, tetradecanoic, heptadecanoic, octadecanoic) in refluxing tetrahydrofuran, followed by addition of triphenylphosphine (PPh3) or pyridine (C5H5N), gives the dinuclear complexes Ru2(CO)4{OOC(CH2)nCH3}2L2 (1: n = 5, 2: n = 7, 3: n = 8, 4: n = 11, 5: n = 12, 6: n = 15, 7: n = 16; a: L = NC5H5, b: L = PPh3). The single crystal structure analysis of 1b, 2a, 3a, 4a and 5a reveals a dinuclear Ru2(CO)4 sawhorse structure, the diruthenium backbone being bridged by the carboxylato ligands, while the two L ligands occupy the axial positions at the ruthenium atoms. In 2a, π-π stacking interactions between adjacent pyridyl units of symmetry related molecules prevail, while in the longer alkyl chain derivatives 3a, 4a and 5a, additional van der Waals and electrostatic interactions between the alkyl chains take place as well in the packing arrangement of the molecules, thus giving rise to layers of parallel alkyl chains in the crystal.