Synapse-Mimicking Memristors Based on 3,6-Di(tpy)-9-Phenylcarbazole Unimer and Its Copolymer with Cobalt(II) Ions.

The title compound, unimer U (tpy stands for 2,2':6',2″-terpyridin-4'-yl end-group), by itself shows the memristor effect with a retention time of 18 h and persistence of 11 h. Its coordination copolymer with Co(II) ions, [CoU]n, exhibits multimodal resistance changes similar to the synaptic responses observed in biological systems. More than 320 cycles of potentiation and depression measured in continuous sequence occurred without observing a significant current change, confirming the operational stability and reproducibility of the device based on the [CoU]n polymer. The synaptic effect of a device with an indium tin oxide (ITO)/[CoU]n/top-electrode (TE) configuration is more pronounced for the device with TE = Au compared to devices with TE = Al or Ga. However, the latter TEs provide a cost-effective approach without any significant compromise in device plasticity. The detected changes in the synaptic weight, about 12% for pair-pulse facilitation and 80% for its depression, together with a millisecond trigger and reading pulses that decay exponentially on the time scale typical of neurosynapses, justify the device's ability to learn and memorize. These properties offer potential applications in neuromorphic computation and brain-inspired synaptic devices.

Pyrazinyl and pyridinyl bis-azomethines formation: an experimental and computational study.

Formation of bis-azomethines from hydrazine and heterocyclic aromatic carbaldehydes, namely pyridine-2-carbaldehyde and pyrazine-2-carbaldehyde, is studied using density functional theory. The theoretical investigation is correlated with experimental results obtained by means of NMR spectroscopy. The presence of bis-hemiaminal intermediates is evidenced by NMR spectra while surprisingly stable hemiaminal intermediate was isolated experimentally. Water, methanol and acetic acid were outlined to play a crucial role as active catalysts of elementary steps of the reaction mechanisms. The possible reaction sequences, i.e. addition-dehydration-addition-dehydration or addition-addition-dehydration-dehydration are investigated and discussed. Also, alternative mechanistic path via ionic mechanism was proposed for the formation of hemiaminals.

Hemiacetal-based dynamic systems: a new mechanistic insight.

The formation of hemiacetals from pyrazine trifluoromethylketone as a model receptor and four simple alcohols was studied by using quantum chemical calculations and NMR spectroscopy. Free energy profiles for four types of mechanistic pathways were calculated and discussed with respect to kinetic and thermodynamic measurements. We show that hemiacetal formation is facilitated by an assisted proton transfer process via a pseudo eight-membered transition state which brings the theory and experiment into close agreement. Also, a newly proposed mechanistic pathway for hemiacetal formation via a five-membered transition state leading to zwitterionic intermediates is discussed. Direct proton transfer in a pseudo four-membered transition state can be ruled out due to the high energy of transition states with respect to other mechanistic pathways. We also show that in the case of hemiacetals, water and alcohol molecules cannot account sufficiently for the H-transfer process via six-membered transition states.

Novel Metallo-Supramolecular Polymers with 1-Thioxophosphole Main-Chain Units and Remarkable Photoinduced Changes in Their Resonance Raman Spectra.

New low-bandgap unimers, with the central thiophene-(1-thioxophosphole)-thiophene (TPT) ring sequence and 2,2':6',2″-terpyridin-4'-yl (tpy) end groups connected to the central unit via conjugated linkers of different size, are prepared and assembled with Zn(II) and Fe(II) ions to metallo-supramolecular polymers (MSPs) that are studied regarding their properties. The most interesting feature of Zn-MSPs is the luminescence extended deep into the NIR region. Fe-MSPs not only show the metal-to-ligand charge transfer (MLCT) manifested by the MLCT band (an expected feature) but also an as-yet-undescribed remarkable phenomenon: specific damping of the bands of the TPT sequence in the resonance Raman spectra taken from solid Fe-MSPs using the excitation to the MLCT band (532 nm). The damping is highly reversible at the low laser power of 0.1 mW but gradually becomes irreversible as the power reaches ca. 5 mW. The revealed phenomenon is not shown by the same Fe-MSPs in solutions, nor by Fe-MSPs containing no phosphole units. A hypothesis is proposed that explains this phenomenon and its dependence on the irradiation intensity as a result of the interplay of three factors: (i) enhancement of the MLCT process by excitation radiation, (ii) the electron-acceptor character of the 1-thioxophosphole ring, and (iii) morphological changes of the lattice and their dependence on the population of new structures in the lattice.

Effects of the halogenido ligands on the Kumada-coupling catalytic activity of [Ni{ t BuN(PPh2)2-κ2P}X2], X = Cl, Br, I, complexes.

Novel nickel(ii) complexes bearing ( t butyl)bis(diphenylphosphanyl)amine and different halogenido ligands, [Ni(P,P)X2] = [Ni{ t BuN(PPh2)2-κ2P}X2], (X = Cl, Br, I) are prepared, characterized by IR and NMR spectroscopy, mass spectrometry and X-ray crystallography, and tested as catalysts in the Kumada cross-coupling reaction of model substituted iodobenzenes and p-tolylmagnesium bromide. The data obtained together with DFT calculations indicate that these new catalysts operate in the Ni(i)-Ni(iii) mode. The highest catalytic activity and selectivity are exhibited by [Ni(P,P)Cl2], which is most easily reduced by the used Grignard reagent to the Ni(i) state. This process is much more energy demanding in the case of the bromido and iodido complexes, causing the appearance of the induction period. [Ni(P,P)Cl2] is also very active in the cross-couplings of substrates with iodine atoms sterically shielded by ortho substituents. The data obtained are in good accordance with the described positive effect of the increased electron-releasing power of N-substituents R' on the overall catalytic performance of [Ni{R'N(PPh2)2-κ2P}X2] complexes.

Oleate-modified hyaluronan: Controlling the number and distribution of side chains by varying the reaction conditions.

In this work, low molecular weight hyaluronan was chemically modified by oleoyl moieties utilising mixed anhydrides methodology. The activation of oleic acid with benzoyl chloride in organic solvents miscible with water was followed by NMR spectroscopy. The product selectivity correlates with the solvent's Hildebrand solubility parameter. Furthermore, the effect of the solvent for the mixed anhydride formation was elucidated by density functional theory (DFT) and showed that the reactions are faster in acetonitrile or alcohols than in hexane. Furthermore, the solvent demonstrated to control the substituent distribution pattern along HA chain during esterification. An even distribution of substituents was observed in reactions performed in water mixed with ethers. The substituent distribution pattern clearly influenced the aggregation behaviour of amphiphilic HA, controlling the stability of the delivery system, while increasing the encapsulation capacity.

Iron (II) Metallo-Supramolecular Polymers Based on Thieno[3,2-b]thiophene for Electrochromic Applications.

Four new bis(tpy) unimers with different linkers between the thieno[3,2-b]thiophene-2,5-diyl central unit and terpyridine-4'-yl (tpy) end-groups: no linker (Tt), ethynediyl (TtE), 1,4-phenylene (TtPh) and 2,2'-bithophene-5,5'-diyl (TtB) are prepared, characterized, and assembled with Fe2+ ions to metallo-supramolecular polymers (Fe-MSPs). The Fe-MSP films prepared by spin-casting on Indium Tin Oxide (ITO) glass are characterized by atomic force microscope (AFM) microscopy, cyclic voltammetry, and UV/vis spectroscopy and studied for their electrochromism and effect of the unimer structure on their electrochromic performance. Of the studied MSPs, Fe-Tt shows the highest optical contrast as well as coloration efficiency (CE = 641 cm2 C-1) and the fastest optical response. This makes it an excellent candidate for possible use in electrochromic devices.

Kinetics of the Photoexcited States in Thin Films of Metallo-Supramolecular Polymers With Ditopic Thiophene-Bridged Terpyridine Ligands.

Managing the excited-state decay by a supramolecular structure is a crucial issue for organic photovoltaics. We show that in thin films of metallo-supramolecular polymers made of bis(terpyridine-4'-yl)terthiophenes and Z n 2 + coupling ions, the photoexcited states generated by ultrashort laser pulses at the wavelength of 440 nm decay by the bi-molecular annihilation predominantly controlled by the Förster transfer between singlet states. During this bi-molecular annihilation of singlet states, intermediate hot triplet pairs are formed, which subsequently dissociate into long-living diffusing triplet states. It explains a significant shortening of the triplet state rise time with increasing pump fluence. The diffusion coefficient of triplets showed power-law time dependence, with its exponent proportional to the pump fluence, decreasing thus the diffusivity of triplets.

Controlled Tuning of the Size of Ag-Hydrosol Nanoparticles by Nonstabilized THF and Detection of Peroxides in THF.

Surface plasmon extinction (SPE) spectra of plasmonic nanoparticles (NPs) are sensitive indicators of their composition, size, shape, interparticle interactions, and of the dielectric constant of their ambient. In this study, rapid changes in SPE spectra of Ag NPs suggesting variations in NP size and concentration were detected after addition of aged tetrahydrofuran (THF). Using time-dependent UV/vis spectroscopy combined with factor analysis, transmission electron microscopy imaging, selected-area electron diffraction, and energy-dispersive X-ray analysis, we observed that an over-limit amount of aged THF fully dissolved Ag NPs with no plasmon recovery. By contrast, an under-limit amount led to incomplete dissolution of Ag NPs and, after reaching the turnover point, to spontaneous recrystallization on residual Ag nuclei, as demonstrated by the SPE band intensity recovery to the original or even higher values. The newly formed Ag NPs were isometric, and their diameter was dependent on the added amount of THF. Furthermore, both Ag NP dissolution and recrystallization were caused by THF peroxides and their reduction products. Therefore, the dissolution of Ag NPs and the resulting hydrosol bleaching may be used as an indicator of the presence of peroxides in THF. Moreover, the reaction of aged THF with Ag NPs can be employed as a tool for tuning the size of Ag NPs in hydrosols.

Effect of the configuration of poly(lactic acid) and content of poly(oxyethylene) blocks to the structure and functional properties of poly(lactic acid)-block-poly(oxirane)-based nanofibrous electrospun polyester-ether-urethanes used as potential drug-delivery system.

Poly(lactic acid)-block-poly(oxirane)s (PLA-b-POE) of various compositions were prepared using a one-pot approach and then extended in a reaction with l-lysine diethyl ester diisocyanate, thereby forming polyester-ether-urethanes (PEU) with prolonged chains and units with increased degradability. The PEUs are processed by electrospinning to prepare degradable nanofibrous sheet materials with and without encapsulating the antibiotic Vancomycin (VAC). PLA block isomerism and POE blocks oligomeric content (1000 g/mol) affect the thermal properties, processability, nanofibrous sheet morphology, abiotic degradation, cytocompatibility, and encapsulated antibiotic release rate of prepared PEUs. Therefore, our findings provide an effective approach to tuning the functional properties of these advanced biocompatible materials. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 2378-2387, 2019.

Arrays of Ag and Au Nanoparticles with Terpyridine- and Thiophene-Based Ligands: Morphology and Optical Responses.

The assembly of Ag and Au nanoparticles (NPs) into nanoparticulate arrays mediated by terpyridine (tpy), 4'-(2-thienyl)terpyridine (T-tpy), and short α,ω-bis(tpy)oligothiophene ligands has been accomplished at the interface between the Ag or Au NP hydrosol and a solution of the molecular species in dichloromethane. The relationship between the morphology and the optical responses of the arrays has been investigated by advanced methods of TEM (transmission electron microscopy) image analysis and surface plasmon extinction (SPE) spectra. It has been established that the size of islands of closely spaced NPs rather than the average interparticle distance affects the extent of delocalization of the surface plasmon excitations and thus also the SPE spectra. Furthermore, the structure of surface-adsorbate complexes formed in these arrays has been investigated by SERS spectral measurements carried out as a function of the excitation wavelength. Photoinduced charge transfer (CT) transitions from the neutral Ags0 and Aus0 adsorption sites on metal NPs to antibonding orbitals of the adsorbates have been identified for Ag/tpy, Ag/T-tpy, Au/tpy, and Au/T-tpy nanoparticulate arrays. Although the surface-adsorbate complexes displaying a photoinduced CT are known for Ag NPs, the Aus0 surface complexes with this CT are newly reported. Bis(tpy)oligothiophenes were found to be attached to both Ag and Au NPs via the tpy group(s). The match between the interparticle distances within the NP islands and the lengths of the oligomers molecules indicates that the molecules act as interparticle linkers. In this case, unequivocal spectral marker band evidence of the Ags0 as well as Aus0 surface complex formation has not been obtained.

Morphology and Kinetics of Aggregation of Silver Nanoparticles Induced with Regioregular Cationic Polythiophene.

The aggregation kinetics of negatively charged borate-stabilized silver nanoparticles (NPs) induced by the cationic regioregular polythiophene polyelectrolyte poly{3-[6-(1-methylimidazolium-3-yl)hexyl]thiophene-2,5-diyl bromide} (PMHT-Br) and the morphology of formed aggregates have been investigated via ultraviolet-visible light (UV-vis) spectroscopy, transmission electron microscopy (TEM), zeta (ζ) potential measurements, dynamic light scattering (DLS), and time-resolved small-angle X-ray scattering (SAXS). Two or three populations of NPs are formed within milliseconds upon mixing the components, which differ in the mean size, extent of polymer coating, and time stability. These characteristics are primarily controlled by the PMHT-Br to Ag-NPs ratio. Population of single NPs of a mean size of ∼5 nm is present in every system and is mostly stable for a long time. At low ratios, the single NPs are most probably almost free of polymer chains and the second population includes slow, but in a limited extent, growing NPs in which single NPs might be interconnected by polymer chains. At the ratios corresponding to the charge balance in the system (ca. zero ζ-potential of NPs), the NPs aggregate, forming a second population that continuously grows in size, and finally undergo sedimentation. At the high ratios, three long-time stable populations of NPs are observed, having mean sizes of ca. 5, 13, and 35 nm; all NPs should be fully coated with PMHT-Br, giving them a positively charged stabilizing shell.

Alcohol-soluble bis(tpy)thiophenes: new building units for constitutional dynamic conjugated polyelectrolytes.

New building units (unimers) for metallo-supramolecular polymers 2,5-bis(2,2':6',2''-terpyridine-4'-yl)thiophene, M, and 5,5'-bis(2,2':6',2''-terpyridine-4'-yl)(2,2'-bithiophene), B, with ionic groups attached to thiophene rings are prepared by the modification of corresponding bromo-precursors and assembled with Zn(2+) and Fe(2+) ions into alcohol-soluble conjugated constitutional-dynamic polyelectrolytes (polyelectrolyte dynamers). Ionization of side groups only slightly affects the absorption spectra of unimers as well as dynamers but dramatically changes their solubility. Cyclic conformations of unimer molecules resulting from intramolecular interactions between tpy end-groups and cationic or polar (-CH2Br) side groups are proposed to explain the spectral conformity of the M- and B-type unimers and their dynamers and also inhibition of the ionization reaction with tpy end-groups. The absorption spectra and excitation profiles of Raman spectra show that mainly the red arm of the metal-to-ligand charge transfer band of Fe-dynamers is significantly contributed with transitions involving thiophene rings. The constitutional dynamics of Zn-dynamers is fast while that of Fe-dynamers is so slow that it allows effective separation of the dynamer to fractions in SEC columns. Electronic spectra and viscosity measurements proved that excess of Fe(2+) ions results in shortening of the dynamer chains and their end-capping by these ions.

Time-Resolved Transient Optical Absorption Study of Bis(terpyridyl)oligothiophenes and Their Metallo-Supramolecular Polymers with Zn(II) Ion Couplers.

α,ω-Bis(terpyridyl)oligothiophenes spontaneously assemble with Zn(II) ions giving conjugated constitutional dynamic polymers (dynamers) of the metallo-supramolecular class, which potentially might be utilized in optoelectronics. Their photophysical properties, which are of great importance in this field of application, are strongly influenced by the dynamic morphology. It was assessed in this study by using ultrafast pump-probe optical absorption spectroscopy. We identified and characterized relaxation processes running in photoexcited molecules of these oligomers and dynamers and show impacts of disturbed coplanarity of adjacent rings (twisting the thiophene-thiophene and thiophene-terpyridyl bonds by attached hexyl side groups) and Zn(II) ion couplers on these processes. Major effects are seen in the time constants of rotational relaxation, intersystem crossing, and de-excitation lifetimes. The photoexcited states formed on different repeating units within the same dynamer chain do not interact with each other even at very high excitation density. The method is presented that allows determining the equilibrium fraction of unbound oligothiophene species in a dynamer solution, from which otherwise hardly accessible values of the average degree of polymerization of constitutionally dynamic chains in solution can be estimated.

Polyacetylene-type networks prepared by coordination polymerization of diethynylarenes: new type of microporous organic polymers.

Microporous organic polymers (MOP) of a new type have been synthesised in high yields by a simple coordination polymerization of 1,3-diethynylbenzene, 1,4-diethynylbenzene and 4,4'-diethynylbiphenyl catalysed by [Rh(cod)acac] and [Rh(nbd)acac] complexes. The new MOPs are non-swellable polyacetylene-type conjugated networks consisting of ethynylaryl-substituted polyene main chains that are crosslinked by arylene linkers. Prepared MOP samples have a mole fraction of branching units (by (13)C CP/MAS NMR) from 0.30 to 0.47 and exhibit the BET (Brunaer-Emmett-Teller) surface up to 809 m(2) g(-1) and hydrogen uptake up to 0.69 wt% (77 K, H2 pressure 750 torr).

UV/Vis Study of the Alkali Salts of Poly(thiophen-3-ylacetic acid) in Water.

UV/Vis spectroscopic investigation of aqueous solutions of regio-irregular poly(thiophen-3-ylacetic acid) (PTAA) with and without methyl-ester groups in the presence of sodium and lithium ions as counterions is presented. The samples were carefully purified and characterized with respect to molar mass and the amount of -COOH groups present. We examined how the UV/Vis spectra of solution change with aging of PTAA solution, polymer concentration, addition of low molecular weight salt, temperature, and some other parameters. We show that results crucially depend on whether the spectra are taken for freshly prepared or mature solutions. We demonstrate the validity of the Beer-Lambert law for PTANa, PTAA/Na, and PTAA/Li (aged solution), and violation of this law for PTANa/Me (fresh) solutions in water; the latter system is instead found to exhibit an isosbetic point at 402 nm. We prove that UV/Vis spectra of polythiophene derivatives in water depend on the "age" (time after dissolution) of the solution. The inconsistencies among the spectroscopic data found in literature, as also the discrepancies with our own measurements on PTAA-based systems are discussed.

Colloidal systems of silver nanoparticles and high-regioregular cationic polythiophene with ionic-liquid-like pendant groups: Optical properties and SERS.

We report tuning of structure dependent optical properties of colloidal systems of borate-stabilized silver nanoparticles (Ag NPs) and polythiophene-based cationic polyelectrolyte with ionic-liquid like side groups: poly{3-[6-(1-methylimidazolium-3-yl)hexyl]thiophene-2,5-diyl bromide} (PMHT-Br) towards obtaining local electromagnetic field enhancement effects. Surface-enhanced Raman scattering (SERS) studies showed that the strong electromagnetic field enhancement is related to the formation of aggregates of Ag NPs achieved at the components ratio providing the charge balance between Ag NPs and cationic polythiophene, at which Ag NPs are nearly single-polymer-layer coated, their zeta potential is close to zero and they easily form aggregates in which the mean inter-particle distance enables the occurrence of desired plasmonic effects. Fluorescence quenching is efficient only in the systems with low concentrations of PMHT-Br, in which almost all polymer chains directly interact with the Ag NPs surface.

Morphology and optical responses of SERS active pi-conjugated poly(N-ethyl-2-ethynylpyridinium iodide)/Ag nanocomposite systems.

The influence of the poly(N-ethyl-2-ethynylpyridinium iodide) (PEEP-I) concentration on the morphology and optical properties of nanocomposite systems prepared by mixing the polymer solution with a hydrosol of ca. 9 nm Ag nanoparticles (NPs) was investigated by a combination of surface plasmon extinction (SPE) measurements, transmission electron microscope (TEM) imaging and surface-enhanced Raman spectroscopy (SERS). The PEEP-I concentration was found to have a strong impact on the assembly of Ag NPs and, consequently, on the optical responses of the composite systems. At low polymer concentrations in the composite (corresponding to ca. 50-1800 monomer units/NP), the formation of fractal aggregates was observed. In particular, the average fractal dimension D = 1.9 +/- 0.1 was determined for aggregates in the system with 5 x 10(-6) M polymer concentration. By contrast, in systems with polymer concentrations higher than about 1 x 10(-5) M, relatively small aggregates of Ag NPs with large interparticle distances were formed. The differences in the morphology of the composite systems with various polymer concentrations manifested themselves clearly in their SPE spectra. Furthermore, upon optical excitation with appropriate wavelengths (488.0 and 514.5 nm), the fractal aggregates acted as carriers of "hot spots", i.e. strong, localized, nanoscale optical fields, from which intense and well resolved SERS spectra of the polymer were obtained.

Characterization of substituted polyacetylene microstructure by pyrolysis gas chromatography.

A series of substituted acetylenes has been polymerized with WOC14/Ph4Sn metathesis catalyst and [Rh(cod)OMe]2 insertion catalyst, and the thermal degradation of the polyacetylenes prepared has been studied using pyrolysis capillary gas chromatography (Py-GC) with flame ionization and mass spectrometric detection to obtain information on the effect of the catalyst on the head-tail (H-T) isomerism of polyacetylenes (poly(phenylacetylene), poly[(4-methylphenyl)acetylene], poly(benzylacetylene), poly((2-fluorophenyl)acetylene], poly[(3-fluorophenyl)acetylene], and poly[(4-fluoro-phenyl)acetylenel). Cyclotrimers have been found to be the main pyrolysis products in all cases. Direct Py-MS connection was used to determine the temperature profiles of the released pyrolysis products. 1,3,5-Trisubstituted benzenes were found to be the predominant pyrolysis products of the polymers prepared with the insertion catalyst, which proves the presence of long head-to-tail sequences of monomeric units in these polyacetylenes. On the other hand, both 1,2,4- and 1,3,5-trisubstituted benzenes are present in significant amounts in the pyrolysis products of polymers prepared with the metathesis catalyst, which proves the presence of a significant content of the head-to-head (HH) and tail-to-tail (TT) linkages in these isomers of polyacetylenes. Contents of the regular (HT) and inverse (HH-TT) monomer linkages (RML and IML, respectively) in polymer chains were determined from the relative amounts of di-, tri-, and tetrasubstituted benzenes found in the Py-GC products.

Polybenzimidazole-supported [Rh(cod)Cl]2 complex: effective catalyst for the polymerization of substituted acetylenes.

The first heterogeneous catalyst which affords polymerization of substituted acetylenes into readily available high molecular weight polymers is reported. The catalyst (Rh/PBI) has been prepared by supporting di-mu-chloro-bis(eta4-cycloocta-1,5-diene)dirhodium(I), [Rh(cod)Cl]2, on commercial polybenzimidazole (PBI) porous beads by means of a simple quantitative adsorption from THF solution, and tested in polymerization of phenylacetylene, 4-fluorophenylacetylene, and 4-pentylphenylacetylene. The polymer molecules formed were found to be released from the Rh/PBI to surrounding solution during the polymerization performed in THF. Formation of high molecular weight ((M)w values up to 325,000) polymers in prevailing cis-transoid configuration has been observed with all monomers. In a comparison with free [Rh(cod)Cl]2 used as the homogeneous catalyst, the Rh/PBI can be used repeatedly, exhibits somewhat lower polymerization activity but almost no oligomerization activity, and provides polymers of higher molecular weight.