Tuning Reprocessing Temperature of Aliphatic Polyurethane Networks by Alkoxyamine Selection.

Recent studies have shown that the largest employed thermoset family, polyurethanes (PUs), has great potential to be reprocessed due to the dynamic behavior of carbamate linkage. However, it requires high temperatures, especially in the case of aliphatic PUs, which causes side reactions besides the desired exchange reaction. To facilitate the reprocessing of aliphatic PUs, in this work, we have explored the dynamic potential of alkoxyamine bonds in PU networks to facilitate the reprocessing under mild conditions considering their fast recombination ability. Taking advantage of the structural effect of the nitroxide and alkyl radicals on the dissociation energy, two different alkoxyamine-based diols have been designed and synthesized to generate PU networks. Our study shows that replacing 50 mol % of a nondynamic diol chain extender with these dynamic blocks boosts the relaxation times of the networks, enabling reprocessing at temperatures as low as 80 °C.

An integrated experimental and theoretical approach to probe Cr(vi) uptake using decorated halloysite nanotubes for efficient water treatment.

Halloysite nanotubes (HNTs) were surface functionalized using four distinct chemical moieties (amidoxime, hydrazone, ethylenediamine (EDA), and diethylenetriamine (DETA)), producing modified HNTs (H1-H4) capable of binding with Cr(vi) ions. Advanced techniques like FTIR, XRD, SEM, and EDX provided evidence of the successful functionalization of these HNTs. Notably, the functionalization occurred on the surface of HNTs, rather than within the interlayer or lumen. These decorated HNTs were effective in capturing Cr(vi) ions at optimized sorption parameters, with adsorption rates ranging between 58-94%, as confirmed by atomic absorption spectroscopy (AAS). The mechanism of adsorption was further scrutinized through the Freundlich and Langmuir isotherms. Langmuir isotherms revealed the nearest fit to the data suggesting the monolayer adsorption of Cr(vi) ions onto the nanotubes, indicating a favorable adsorption process. It was hypothesized that Cr(vi) ions are primarily attracted to the amine groups on the modified nanotubes. Quantum chemical calculations further revealed that HNTs functionalized with hydrazone structures (H2) demonstrated a higher affinity (interaction energy -26.33 kcal mol-1) for the Cr(vi) ions. This can be explained by the formation of stronger hydrogen bonds with the NH moieties of the hydrazone moiety, than those established by the OH of oxime (H1) and longer amine chains (H3 and H4), respectively. Overall, the findings suggest that these decorated HNTs could serve as an effective and cost-efficient solution for treating water pollution.

Bio-Based Polyhydroxyanthraquinones as High-Voltage Organic Electrode Materials for Batteries.

Organic materials have gained much attention as sustainable electrode materials for batteries. Especially bio-based organic electrode materials (OEMs) are very interesting due to their geographical independency and low environmental impact. However, bio-based OEMs for high-voltage batteries remain scarce. Therefore, in this work, a family of bio-based polyhydroxyanthraquinones (PHAQs)-namely 1,2,3,4,5,6,7,8-octahydroxyanthraquinone (OHAQ), 1,2,3,5,6,7-hexahydroxyanthraquinone (HHAQ), and 2,3,6,7-tetrahydroxyanthraquinone (THAQ)-and their redox polymers were synthesized. These PHAQs were synthesized from plant-based precursors and exhibit both a high-potential polyphenolic redox couple (3.5-4.0 V vs Li/Li+) and an anthraquinone redox moiety (2.2-2.8 V vs Li/Li+), while also showing initial charging capacities of up to 381 mAh g-1. To counteract the rapid fading caused by dissolution into the electrolyte, a facile polymerization method was established to synthesize PHAQ polymers. For this, the polymerization of HHAQ served as a model reaction where formaldehyde, glyoxal, and glutaraldehyde were tested as linkers. The resulting polymers were investigated as cathode materials in lithium metal batteries. PHAQ polymer composites synthesized using formaldehyde as linker and 10 wt % multiwalled carbon nanotubes (MWCNTs), namely poly(THAQ-formaldehyde)-10 wt % MWCNTs and poly(HHAQ-formaldehyde)-10 wt % MWCNTs, exhibited the best cycling performance in the lithium metal cells, displaying a high-voltage discharge starting at 4.0 V (vs Li/Li+) and retaining 81.6 and 77.3 mAh g-1, respectively, after 100 cycles.

Experimental and theoretical study of the effect of different functionalities of graphene oxide/polymer composites on selective CO2 capture.

There is a constant need for versatile technologies to reduce the continuously increasing concentration of CO2 in the atmosphere, able to provide effective solutions under different conditions (temperature, pressure) and composition of the flue gas. In this work, a combination of graphene oxide (GO) and functionalized waterborne polymer particles was investigated, as versatile and promising candidates for CO2 capture application, with the aim to develop an easily scalable, inexpensive, and environmentally friendly CO2 capture technology. There are huge possibilities of different functional monomers that can be selected to functionalize the polymer particles and to provide CO2-philicity to the composite nanostructures. Density functional theory (DFT) was employed to gain a deeper understanding of the interactions of these complex composite materials with CO2 and N2 molecules, and to build a basis for efficient screening for functional monomers. Estimation of the binding energy between CO2 and a set of GO/polymer composites, comprising copolymers of methyl methacrylate, n-butyl acrylate, and different functional monomers, shows that it depends strongly on the polymer functionalities. In some cases, there is a lack of cooperative effect of GO. It is explained by a remarkably strong GO-polymer binding, which induced less effective CO2-polymer interactions. When compared with experimental results, in the cases when the nanocomposite structures presented similar textural properties, the same trends for selective CO2 capture over N2 were attained. Besides novel functional materials for CO2 capture and a deeper understanding of the interactions between CO2 molecules with various materials, this study additionally demonstrates that DFT calculations can be a shorter route toward the efficient selection of the best functionalization of the composite materials for selective CO2 capture.

Mild Open-Shell Character of BODIPY and Its Impact on Singlet and Triplet Excitation Energies.

This study describes and rationalizes the electronic structure of BODIPY combining a large variety of quantum chemistry methods and computational tools. Examination of the obtained results using state-of-the-art electronic structure analyses provides a new and complete interpretation of the nature of low-lying electronic states in BODIPY and elucidates the limitations of excited-state methods in the computation of T1 and S1 energies, that is, systematic under- and overestimation of time-dependent density functional theory energies, respectively, and a large overestimation of the T1/S1 energy gap. Our analysis identifies the important role and physical origin of the mild open-shell character in the BODIPY ground state, that is, strong highest occupied and lowest unoccupied molecular orbital exchange interactions. The study provides guidelines for the accurate quantification of the T1/S1 gap, which is extremely relevant for the computational investigation of the photophysical properties of BODIPY and its derivatives. These conclusions should be taken into consideration in order to predict and interpret conspicuous photoactivated phenomena such as intersystem crossing, singlet fission, and triplet-triplet annihilation. Moreover, we believe that our study might provide new ideas and strategies for the analysis of other molecular chromophores.

Ionic Inter-Particle Complexation Effect on the Performance of Waterborne Coatings.

The performance of waterborne (meth)acrylic coatings is critically affected by the film formation process, in which the individual polymer particles must join to form a continuous film. Consequently, the waterborne polymers present lower performance than their solvent-borne counter-polymers. To decrease this effect, in this work, ionic complexation between oppositely charged polymer particles was introduced and its effect on the performance of waterborne polymer films was studied. The (meth)acrylic particles were charged by the addition of a small amount of ionic monomers, such as sodium styrene sulfonate and 2-(dimethylamino)ethyl methacrylate. Density functional theory calculations showed that the interaction between the selected main charges of the respective functional monomers (sulfonate-amine) is favored against the interactions with their counter ions (sulfonate-Na and amine-H). To induce ionic complexation, the oppositely charged latexes were blended, either based on the same number of charges or the same number of particles. The performance of the ionic complexed coatings was determined by means of tensile tests and water uptake measurements. The ionic complexed films were compared with reference films obtained at pH at which the cationic charges were in neutral form. The mechanical resistance was raised slightly by ionic bonding between particles, producing much more flexible films, whereas the water penetration within the polymeric films was considerably hindered. By exploring the process of polymer chains interdiffusion using Fluorescence Resonance Energy Transfer (FRET) analysis, it was found that the ionic complexation was established between the particles, which reduced significantly the interdiffusion process of polymer chains. The presented ionic complexes of sulfonate-amine functionalized particles open a promising approach for reinforcing waterborne coatings.

Theoretical Characterization of New Frustrated Lewis Pairs for Responsive Materials.

In recent years, responsive materials including dynamic bonds have been widely acclaimed due to their expectation to pilot advanced materials. Within these materials, synthetic polymers have shown to be good candidates. Recently, the so-called frustrated Lewis pairs (FLP) have been used to create responsive materials. Concretely, the activation of diethyl azodicarboxylate (DEAD) by a triphenylborane (TPB) and triphenylphosphine (TPP) based FLP has been recently exploited for the production of dynamic cross-links. In this work, we computationally explore the underlying dynamic chemistry in these materials, in order to understand the nature and reversibility of the interaction between the FLP and DEAD. With this goal in mind, we first characterize the acidity and basicity of several TPB and TPP derivatives using different substituents, such as electron-donating and electron-withdrawing groups. Our results show that strong electron-donating groups increase the acidity of TPB and decrease the basicity of TPP. However, the FLP-DEAD interaction is not mainly dominated by the influence of these substituents in the acidity or basicity of the TPB or TPP systems, but by attractive or repulsive forces between substituents such as hydrogen bonds or steric effects. Based on these results, a new material is proposed based on FLP-DEAD complexes.

Selective Chemical Upcycling of Mixed Plastics Guided by a Thermally Stable Organocatalyst.

Chemical recycling of plastic waste represents a greener alternative to landfill and incineration, and potentially offers a solution to the environmental consequences of increased plastic waste. Most plastics that are widely used today are designed for durability, hence currently available depolymerisation methods typically require harsh conditions and when applied to blended and mixed plastic feeds generate a mixture of products. Herein, we demonstrate that the energetic differences for the glycolysis of BPA-PC and PET in the presence of a protic ionic salt TBD:MSA catalyst enables the selective and sequential depolymerisation of these two commonly employed polymers. Employing the same procedure, functionalised cyclic carbonates can be obtained from both mixed plastic wastes and industrial polymer blend. This methodology demonstrates that the concept of catalytic depolymerisation offers great potential for selective polymer recycling and also presents plastic waste as a "greener" alternative feedstock for the synthesis of high added value molecules.

Synthesis of Functionalized Cyclic Carbonates through Commodity Polymer Upcycling.

Functionalized cyclic carbonates are attractive monomers for the synthesis of innovative polycarbonates or polyurethanes for various applications. Even though their synthesis has been intensively investigated, doing so in a sustainable and efficient manner remains a challenge. Herein, we propose an organocatalytic procedure based on the depolymerization of a commodity polymer, bisphenol A based polycarbonate (BPA-PC). Different carbonate-containing heterocycles are obtained in good to excellent yields employing BPA-PC as a sustainable and inexpensive source of carbonate, including functionalized six-membered cyclic carbonates.

Effect of Molecular Structure in the Chain Mobility of Dichalcogenide-Based Polymers with Self-Healing Capacity.

Recently, it has been shown that the reaction mechanism in self-healing diphenyl dichalcogenide-based polymers involves the formation of sulfenyl and selenyl radicals. These radicals are able to attack a neighbouring dichalcogenide bond via a three-membered transition state, leading to the interchange of chalcogen atoms. Hence, the chain mobility is crucial for the exchange reaction to take place. In this work, molecular dynamics simulations have been performed in a set of disulfide- and diselenide-based materials to analyze the effect of the molecular structure in the chain mobility. First of all, a validation of the computational protocol has been carried out, and different simulation parameters like initial guess, length of the molecular chains, size of the simulation box and simulation time, have been evaluated. This protocol has been used to study the chain mobility and also the self-healing capacity, which depends on the probability to generate radicals ( ρ ), the barrier of the exchange reaction ( Δ G ) and the mobility of the chains ( ω ). The first two parameters have been obtained in previous quantum chemical calculations on the systems under study in this work. After analyzing the self-healing capacity, it is concluded that aromatic diselenides (PD-SeSe) are the best candidates among those studied to show self-healing, due to lower reaction barriers and larger ω values.

Perovskite Solar Cells Based on Oligotriarylamine Hexaarylbenzene as Hole-Transporting Materials.

A cobalt-catalyzed cyclotrimerization of bis(aryl)alkyne is used as an innovative tool to obtain hole-transport materials (HTMs). The novel HTM containing six units of oligotriarylamine (HAB1), characterized by UV-vis, cyclic voltammetry, DFT, and thermogravimetric analysis, confirms its suitability as an efficient HTM in PSCs. A PCE of 17.5% was obtained in HAB1-containing PSCs, a performance comparable to that obtained with spiro-OMeTAD and with slightly better thermal stability.

Diselenide Bonds as an Alternative to Outperform the Efficiency of Disulfides in Self-Healing Materials.

Self-healing materials are a very promising kind of materials due to their capacity to repair themselves. Among others, dichalcogenide-based materials are widely studied due to their dynamic covalent bond nature. Recently, the reaction mechanism occurring in these materials was characterized both theoretically and experimentally. In this vein, a theoretical protocol was established in order to predict further improvements. Among these improvements, the use of diselenides instead of disulfides appears to be one of the paths to enhance these properties. Nevertheless, the physicochemical aspects of these improvements are not completely clear. In this work, the self-healing properties of several disulfides, diselenides, and mixed S-Se materials have been considered by means of computational simulations. Among all the tested species, diphenyl diselenide based materials appear to be the most promising ones due to the decrease on the reaction barriers, instead of weaker diselenide bonds, as thought up to now. Moreover, the radical formation needed in this process would also be enhanced by the fact that these species are able to absorb visible light. In this manner, at room conditions, selenyl radicals would be formed by both thermal dissociation and photodissociation. This fact, together with the lower energetic barriers needed for the diselenide exchange, makes diphenyl diselenides ideal for self-healing materials.

Improvement of the electrochemical and singlet fission properties of anthraquinones by modification of the diradical character.

In this work, theoretical methods of quantum chemistry are employed to estimate the effects that the structural modification of 1,5- and 9,10-anthraquinone molecules might produce in their electronic structure, in the pursuit of a common strategy to improve the electrochemical and singlet fission features of conjugated quinones. The proposed modifications are the following: (i) substitution of the carbonyl oxygen atom, (ii) insertion of heteroatoms in the carbon backbone, and (iii) introduction of electron-withdrawing and electron-donating substituents in different positions of the rings. This work shows how specific modifications of the electronic structure can be used to tune the electrochemistry and photophysics of the quinones, improving their suitability to be singlet fission sensitizers and cathode materials. As it was previously known, intermediate diradical characters favor the accomplishment of the singlet fission process. This can be achieved for 1,5-anthraquinone by introduction of B and Si atoms and electron-donating groups in low spin density positions of the carbon backbone or N atoms in high spin density positions. The introduction of heteroatoms or substituents in 9,10-anthraquinone hardly facilitates the singlet fission process. Regarding the electrochemistry, it is observed that the reduction potentials greatly depend on the nature of the modification rather than on the diradical character.

Probing the structures and bonding of auropolyynes, Au-(C≡C) n -Au- (n = 1-3), using high-resolution photoelectron imaging.

We report an investigation of a series of auropolyynes, Au-(C≡C) n -Au- (n = 1-3), using high-resolution photoelectron imaging and ab initio calculations. Vibrationally resolved photoelectron spectra are obtained, allowing the electron affinities of Au-(C≡C) n -Au to be accurately measured as 1.651(1), 1.715(1), and 1.873(1) eV for n = 1-3, respectively. Both the Au-C symmetric stretching and a bending vibrational frequency are observed for each neutral auropolyyne. Theoretical calculations find that the ground state of Au2C2 - has a linear acetylenic Au-C≡C-Au- structure, whereas the asymmetric Au-Au-C≡C- structure is a low-lying isomer. However, for Au2C4 - and Au2C6 -, our calculations show that the asymmetric Au-Au-(C≡C) n - isomers are the global minima and the Au-(C≡C) n -Au- symmetric structures become low-lying isomers. All the asymmetric Au-Au-(C≡C) n - isomers are found computationally to have much higher electron binding energies and are not accessible at the detachment photon energies used in the current study. For neutral Au2C2n , the Au-(C≡C) n -Au auropolyyne structures are found to be the global minima for n = 1-3. The electronic structures and bonding for Au-(C≡C) n -Au (n = 1-3) are compared with the corresponding Au-(C≡C) n and Au-(C≡C) n -H species.

Supramolecular-Enhanced Charge Transfer within Entangled Polyamide Chains as the Origin of the Universal Blue Fluorescence of Polymer Carbon Dots.

The emission of a bright blue fluorescence is a unique feature common to the vast variety of polymer carbon dots (CDs) prepared from carboxylic acid and amine precursors. However, the difficulty to assign a precise chemical structure to this class of CDs yet hampers the comprehension of their underlying luminescence principle. In this work, we show that highly blue fluorescent model types of CDs can be prepared from citric acid and ethylenediamine through low temperature synthesis routes. Facilitating controlled polycondensation processes, the CDs reveal sizes of 1-1.5 nm formed by a compact network of short polyamide chains of about 10 monomer units. Density functional theory calculations of these model CDs uncover the existence of a spatially separated highest occupied molecular orbital and a lowest unoccupied molecular orbital located at the amide and carboxylic groups, respectively. Photoinduced charge transfer between these groups thus constitutes the origin of the strong blue fluorescence emission. Hydrogen-bond-mediated supramolecular interactions between the polyamide chains enabling a rigid network structure further contribute to the enhancement of the radiative process. Moreover, the photoinduced charge transfer processes in the polyamide network structure easily explain the performance of CDs in applications as revealed in studies on metal ion sensing. These findings thus are of general importance to the further development of polymer CDs with tailored properties as well as for the design of technological applications.

Sulfenamides as Building Blocks for Efficient Disulfide-Based Self-Healing Materials. A Quantum Chemical Study.

The theoretical self-healing capacity of new sulfenamide-based disulfides is estimated by using theoretical methods of quantum chemistry. Starting from previously studied aromatic disulfides, the influence of inserting a NH group between the disulfide and the phenyl ring (forming the sulfenamide), as well as the role of the phenyl ring in the self-healing process is analyzed. Three parameters are used in the evaluation of the self-healing capacity: i) the probability to generate sulfenyl radicals, which is the first step of the process; ii) the effect of the hydrogen bonding, which affects the mobility of the chains; and iii) the height of the exchange reaction barrier. The insertion of the NH group notably decreases the bond dissociation energy and, therefore, increases the probability to produce sulfenyl radicals and helps the approach of these radicals to neighboring disulfides, favoring the self-healing process. The role of the phenyl rings is clearly observed in the reaction barriers, where the π-π stacking interactions notably stabilize the transition states, resulting in larger rate constants. Nevertheless, this stabilization is somewhat reduced in the aromatic sulfenamides, owing to a less effective π-π interaction. Therefore, the sulfenamide-based aromatic disulfides may be considered as promising candidates for the design of efficient self-healing materials.

Fullerene-Based Materials as Hole-Transporting/Electron-Blocking Layers: Applications in Perovskite Solar Cells.

Here we report for the first time an efficient fullerene-based compound, FU7, able to act as hole-transporting material (HTM) and electron blocking contact. It has been applied on perovskite solar cells (PSCs), obtaining 0.81 times the efficiency of PSCs with the standard HTM, spiro-OMeTAD, with the additional advantage that this performance is reached without any additive introduced in the HTM layer. Moreover, as a proof of concept, we have described for the first time efficient PSCs in which both selective contacts are fullerene derivatives, to obtain unprecedented "fullerene sandwich" PSCs.

Theoretical design of conjugated diradicaloids as singlet fission sensitizers: quinones and methylene derivatives.

The electronic structures of 206 carbonyls and methylene derivatives based on conjugated cyclic hydrocarbons are computationally studied in this work using theoretical methods of quantum chemistry. The singlet open-shell nature of the ground state and its influence on the low-lying excited states is analyzed for 90 carbonyl (quinone, Q), 90 methylene (quinone dimethide, QDM) and 26 carbonyl-methylene (quinone methide, QM) mixed derivatives in the pursuit of new promising candidates for singlet fission sensitizers. Non-negligible diradical character is observed for most of the studied molecules, which is mainly determined by the nature and the relative position of the substituting groups in the bare rings. In general, the methylene group enhances to a greater extent the diradical character and the following trend is observed: y0(QDM) > y0(QM) > y0(Q). This feature leads to a decrease in the energy of the S0 → S1 and, especially, the S0 → T1 transitions, facilitating the accomplishment of the singlet fission energetic conditions: 2T1-S1 ≤ 0 (C1) and 2T1-T2 < 0 (C2). For all the methylene derivatives, these transitions have π → π* character, while some carbonyl-containing molecules, in particular those with low diradical character, show transitions with n → π* character, due to the presence of the lone pairs of the oxygen atom. From the total set of 206 molecules analyzed, 10 molecules with intermediate diradical character may be considered as potential candidates to undergo singlet fission efficiently.

The role of non-covalent interactions in the self-healing mechanism of disulfide-based polymers.

In this work, a theoretical protocol based on classical molecular dynamics has been defined, in order to study weak non-covalent interactions in diphenyl disulfide based compounds. This protocol is then used to study the influence of hydrogen bonds and π-π stacking in four selected cases, namely, monosubstituted and amine ortho trisubstituted urea and urethane-based diphenyl disulfides. In all cases, it has been observed that hydrogen bonds are much more relevant than π-π stacking, which has little influence. In addition, hydrogen bonds are the responsible to maintain the polymeric chains close, so that the disulfides may reach the reacting region, even in urethane-based materials, where the lower amount of hydrogen bonds formed make the chains more flexible and mobile. Combining the results obtained by classical molecular dynamics with those obtained earlier by means of quantum mechanics, we conclude that there are two main factors that are relevant to the self-healing properties of disulfide-based materials: firstly, the capacity to generate sulfenyl radicals by breaking the disulfide S-S bond and, secondly, the ability of these radicals to attack neighboring disulfides. The former is dominated by the bond dissociation energy of the S-S bond, while the latter is strongly influenced by two other factors. On the one hand, the hydrogen bonding interactions established between chains, and on the other, the energy barriers for the attack of sulfur radicals to neighbor disulfides. We have defined three new parameters to estimate the influence of these features, with the aim of predicting the self-healing capacity of disulfides and related materials, which will help experimentalists in the development of improved materials.

π⋠⋠⋠H+ ⋠⋠⋠π Hydrogen Bonds and Their Lithium and Gold Analogues: MP2 and CASPT2 Calculations.

Molecular systems in which two simple π-electron species, acetylene and ethylene, are linked by a cation located between them are analyzed in this study. In particular, the C2 H2 ⋅⋅⋅M+ ⋅⋅⋅C2 H2 , C2 H4 ⋅⋅⋅M+ ⋅⋅⋅C2 H2 , and C2 H4 ⋅⋅⋅M+ ⋅⋅⋅C2 H4 complexes (M+ =H+ , Li+ , Au+ ) are calculated with the use of MP2 and CASPT2 methods. The Quantum Theory of Atoms in Molecules (QTAIM), energy decomposition analysis (EDA), and Natural Bond Orbital (NBO) approaches are applied to deepen the understanding of the nature of M+ ⋅⋅⋅π interactions in these complexes. It is found that the interactions in gold and proton complexes are characterized by at least partial covalency, whereas interactions in lithium complexes are rather electrostatic in nature.