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The slip and friction behavior of n-hexadecane, confined between organic friction modifier surfactant films adsorbed on hematite surfaces, has been studied using nonequilibrium molecular dynamics simulations. The influence of the surfactant type and coverage, as well as the applied shear rate and pressure, has been investigated. A measurable slip length is only observed for surfactant films with a high surface coverage, which provide smooth interfaces between well-defined surfactant and hexadecane layers. Slip commences above a critical shear rate, beyond which the slip length first increases with increasing shear rate and then asymptotes toward a constant value. The maximum slip length increases significantly with increasing pressure. Systems and conditions which show a larger slip length typically give a lower friction coefficient. Generally, the friction coefficient increases linearly with logarithmic shear rate; however, it shows a much stronger shear rate dependency at low pressure than at high pressure. Relating slip and friction, slip only occurs above a critical shear stress, after which the slip length first increases linearly with increasing shear stress and then asymptotes. This behavior is well-described using previously proposed slip models. This study provides a more detailed understanding of the slip of alkanes on surfactant monolayers. It also suggests that high coverage surfactant films can significantly reduce friction by promoting slip, even when the surfaces are well-separated by a lubricant. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations are reported of model overbased detergent nanoparticles on a model water surface which mimic their behavior on a Langmuir trough or large water droplet in engine oil. The simulations predict that the structure of the nanoparticle on a water surface is different to when it is immersed in a bulk hydrophobic solvent. The surfactant tails are partly directed out of the water, while the carbonate core maximizes its extent of contact with the water. Umbrella sampling calculations of the potential of mean force between two particles showed that they are associated with varying degrees with a maximum binding free energy of ca. 10 kBT for the salicylate stabilized particle, ca. 8 kBT for a sulfurized alkyl phenate stabilized particle, and ca. 5 kBT for a sulfonate stabilized particle. The differences in the strength of attraction depend on the proximity of nearest approach and the energy penalty associated with the disruption of the hydration shell of water molecules around the calcium carbonate core when the two particles approach. This is greatest for the sulfonate particle, which partially loses the surfactant ions to the solution, and least for the salicylate, which forms the weakest water "cage". The particles are separated by a water hydration layer, even at the point of closest approach. [ABSTRACT FROM AUTHOR]

Engine deposits can reduce performance and increase emissions, particularly for modern direct-injection fuel delivery systems. Surfactants known as deposit control additives (DCAs) adsorb and self-assemble on the surface of deposit precursors to keep them suspended in the fuel. Here, we show how molecular simulations can be used to virtually screen the ability of surfactants to bind to polyaromatic hydrocarbons, comprising a major class of carbonaceous deposits. We use molecular dynamics with the adaptive biasing force method to generate the potential of mean force as a function of the vertical distance between the surfactants and deposits in gasoline and diesel fuel surrogates. We find that a zwitterionic surfactant outperforms a conventional polyisobutylene succinimide for binding to these aromatic species. The amine groups in the succinimide headgroup only weakly adsorb on the polyaromatic deposit, while additional functional groups in the zwitterionic surfactant, particularly the quarternary ammonium ion, markedly enhance the binding strength. We decompose the adsorption free energies of the surfactants into their entropic and enthalpic components, to find that the latter dominates the attraction from these non-aqueous solvents. The adsorption free energy of both surfactants is slightly weaker from n -hexadecane (diesel) than iso-octane (gasoline), which is due to the larger steric barrier from stronger molecular layering of the former on the deposit. Density functional theory calculations of the adsorption of DCA fragments validate the force field used in the molecular dynamics simulations and provide further insights into the nature of the intermolecular interactions. The approach introduced here shows considerable promise for accelerating the discovery of novel DCAs to facilitate more advanced fuel formulations to reduce emissions.

Fluid exudation in cartilage under normal loading can be counteracted by a sliding-induced rehydration phenomenon, which has a hydrodynamic origin related to a wedge effect at the contact inlet. Similar to cartilage, hydrogels also exhibit tribological rehydration properties, and we mimic this phenomenon to restore hydration lubrication and overcome creeping. It occurs within a specific velocity range and is mainly dependent on the applied load and hydrogel network structures. Crucially, a certain velocity in the mixed lubrication regime can produce a hydrodynamic pressure peak at the wedge and drive the rehydration inflow to overcome the extrusion. At lower sliding velocities in the boundary lubrication regime, inflows are insufficient to counteract fluid exudation, whereas at higher velocities in the hydrodynamic lubrication regime, the inlet wedge effect would diminish. These results suggest that tribological rehydration offers a novel approach to enhancing load-bearing capacity and maintaining lubrication in the hydrogels.

The rational design of dielectric fluids for immersion cooling of batteries requires a molecular-level understanding of the heat flow across the battery casing/dielectric fluid interface. Here, we use nonequilibrium molecular dynamics (NEMD) simulations to quantify the interfacial thermal resistance (ITR) between hematite and poly-α-olefin (PAO), which are representative of the outer surface of the steel battery casing and a synthetic hydrocarbon dielectric fluid, respectively. After identifying the most suitable force fields to model the thermal properties of the individual components, we then compared different solid-liquid interaction potentials for the calculation of the ITR. These potentials resulted in a wide range of ITR values (4-21 K m 2 GW -1 ), with stronger solid-liquid interactions leading to lower ITR. The increase in ITR is correlated with an increase in density of the fluid layer closest to the surface. Since the ITR has not been experimentally measured for the hematite/PAO interface, we validate the solid-liquid interaction potential using the work of adhesion calculated using the dry-surface method. The work of adhesion calculations from the simulations were compared to those derived from experimental contact angle measurements for PAO on steel. We find that all of the solid-liquid potentials overestimate the experimental work of adhesion. The experiments and simulations can only be reconciled by further reducing the strength of the interfacial interactions. This suggests some screening of the solid-liquid interactions, which may be due to the presence of an interfacial water layer between PAO and steel in the contact angle experiments. Using the solid-liquid interaction potential that reproduces the experimental work of adhesion, we obtain a higher ITR (33 K m 2 GW -1 ), suggesting inefficient thermal transport. The results of this study demonstrate the potential for NEMD simulations to improve understanding of the nanoscale thermal transport across industrially important interfaces. This study represents an important step toward the rational design of more effective fluids for immersion cooling systems for electric vehicles and other applications where thermal management is of high importance.

Phosphate esters decompose on metal surfaces and form protective polyphosphate films. For many applications, such as in lubricants for electric vehicles and wind turbines, an understanding of the effect of electric fields on molecular decomposition is urgently required. Experimental investigations have yielded contradictory results, with some suggesting that electric fields improve tribological performance, while others have reported the opposite effect. Here, we use nonequilibrium molecular dynamics (NEMD) simulations to study the decomposition of tri- n -butyl phosphate (TNBP) molecules nanoconfined between ferrous surfaces (iron and iron oxide) under electrostatic fields. The reactive force field (ReaxFF) method is used to model the effects of chemical bonding and molecular dissociation. We show that the charge transfer with the polarization current equalization (QTPIE) method gives more realistic behavior compared to the standard charge equilibration (QEq) method under applied electrostatic fields. The rate of TNBP decomposition via carbon-oxygen bond dissociation is faster in the nanoconfined systems than that in the bulk due to the catalytic action of the surfaces. In all cases, the application of an electric field accelerates TNBP decomposition. When electric fields are applied to the confined systems, the phosphate anions are pulled toward the surface with high electric potential, while the alkyl cations are pulled to the surface with lower potential, leading to asymmetric film growth. Analysis of the temperature- and electric field strength-dependent dissociation rate constants using the Arrhenius equation suggests that, on reactive iron surfaces, the increased reactivity under an applied electric field is driven mostly by an increase in the pre-exponential factor, which is linked to the number of molecule-surface collisions. Conversely, the accelerated decomposition of TNBP on iron oxide surfaces can be attributed to a reduction in the activation energy with increasing electric field strength. Single-molecule nudged-elastic band (NEB) calculations also show a linear reduction in the energy barrier for carbon-oxygen bond breaking with electric field strength, due to stabilization of the charged transition state. The simulation results are consistent with experimental observations of enhanced and asymmetric tribofilm growth under electrostatic fields., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Implant-associated infections and delayed osseointegration are major challenges for the clinical success of titanium implants. To enhance antibacterial effects and promote early osseointegration, we developed a synergistic photothermal (PTT)/photodynamic (PDT) therapy strategy based on near-infrared (NIR) responsive biomimetic micro/nano titanate/TiO 2- X heterostructure coatings (KMNW and NaMNS) in situ constructed on the surface of titanium implants. Specifically, KMNW and NaMNS significantly enhanced photothermal conversion capabilities, achieving localized high temperatures of 48-51 °C and promoting substantial amounts of reactive oxygen species production under 808 nm irradiation. In vitro antibacterial experiments demonstrated that KMNW achieved the highest antibacterial rates against Staphylococcus aureus and Escherichia coli , at 98.78 and 98.33% respectively. Moreover, by mimicking the three-dimensional fibrous network of the extracellular matrix during bone healing, both KMNW and NaMNS markedly promoted the proliferation and osteogenic differentiation of osteoblasts. In vivo implantation studies further confirmed these findings, with KMNW and NaMNS exhibiting superior antibacterial performance under NIR irradiation─94.45% for KMNW and 92.66% for NaMNS. Moreover, KMNW and NaMNS also significantly promoted new bone formation and improved osseointegration in vivo. This study presents a promising PTT/PDT therapeutic strategy for dentistry and orthopedics by employing NIR-responsive biomimetic coatings to combat implant-associated infection and accelerate osseointegration.

Aqueous mixtures of oppositely charged polyelectrolytes and surfactants are useful in many industrial applications, such as shampoos and hair conditioners. In this work, we investigate the friction between biomimetic hair surfaces in the presence of adsorbed complexes formed from cationic polyelectrolytes and anionic surfactants in an aqueous solution. We apply nonequilibrium molecular dynamics (NEMD) simulations using the coarse-grained MARTINI model. We first developed new MARTINI parameters for cationic guar gum (CGG), a functionalized, plant-derived polysaccharide. The complexation of CGG and the anionic surfactant sodium dodecyl sulfate (SDS) on virgin and chemically damaged biomimetic hair surfaces was studied using a sequential adsorption approach. We then carried out squeeze-out and sliding NEMD simulations to assess the boundary lubrication performance of the CGG-SDS complex compressed between two hair surfaces. At low pressure, we observe a synergistic friction behavior for the CGG-SDS complex, which gives lower shear stress than either pure CGG or SDS. Here, friction is dominated by viscous dissipation in an interfacial layer comprising SDS and water. At higher pressures, which are probably beyond those usually experienced during hair manipulation, SDS and water are squeezed out, and friction increases due to interdigitation. The outcomes of this work are expected to be beneficial to fine-tune and screen sustainable hair care formulations to provide low friction and therefore a smooth feel and reduced entanglement.

Inflammatory bowel disease (IBD) is a serious public health issue because of its chronic and incurable nature. Common IBD drugs have limited efficacy and produce adverse effects, leading to an urgent need to develop new drugs and drug delivery systems. Curcumin (Cur) is a natural and nontoxic drug that is increasingly used in the treatment of IBD owing to its anti-inflammatory and antioxidant effects. Metal-polyphenol networks constructed from metal ions and polyphenols exhibit biological functionality while acting as an adhesive nanomaterial to encapsulate nano-Cur, thereby improving its solubility and drug release behavior. In this study, we prepared a Cur@Fe&TA nanodrug delivery system by constructing an Fe 3+ /tannic acid (TA) metal-polyphenol network with encapsulated Cur. The Cur@Fe&TA nanodrug exhibited good stability, drug release behavior, and biocompatibility. Based on the anti-inflammatory and antioxidant effects of Cur@Fe&TA, the gastrointestinal cytopathology in an IBD mouse model was effectively improved. The proposed Cur@Fe&TA nanomedicine delivery system has promising application and research value for the treatment of IBD by regulating levels of antioxidants and inflammatory cytokines., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Hybrid organic-inorganic perovskites (HOIPs) have attracted considerable attention in the past years as photoactive materials for low-cost, high-performance photovoltaics. Polaron formation through electron-phonon coupling has been recognized as the leading mechanism governing charge carrier transport and recombination in HOIPs. In this work, two types of MAPbBr 3 film samples deposited on different substrates (transparent insulating SrTiO 3 and a heterostructure mimicking a functioning photovoltaic cell) were photoexcited with above-bandgap radiation at 450 nm, and the effects of illumination on the sample were analyzed in the infrared region. The infrared absorbance detected at different powers of the photoexciting laser allowed us to obtain an estimate of the characteristic decay time of photoexcited polaron population of the order of 100-1000 ns. When focusing on the absorption features of the MA molecular cation in the region of the NH stretching modes, we observed the influence of hydrogen bonding and the effect of the polaron dynamics on the cation reorientation., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Dimethyl carbonimidodithioates, 2 derived from various primary aryl amines ( 1 ) by reacting with carbon disulfide and methyl iodide in dimethyl formamide in the presence of concentrated sodium hydroxide, are converted to the diaziridine derivatives, 3 by reacting with hydrazine in ethanol. The diaziridines, 3 on oxidation with lead tetraacetate in refluxing xylene, extrudes nitrogen, and intramolecular stabilization, particularly 1,2-carbon migration, takes place to give the product, 5 . The reaction may take place through the intermediates, diazirines, 4 , which have not been isolated. This work provides a new approach for the conversion of aryl amines having no α-methylene to aryl nitriles., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

The evolution of the liquid-vapor interface of a Lennard-Jones fluid is examined with molecular dynamics simulations using the intrinsic sampling method. Results suggest clear damping of the intrinsic profiles with increasing temperature. Investigating the surface stress distribution, we have identified a linear variation of the space-filling nature (fractal dimension) of the stress clusters at the intrinsic surface with increasing surface tension or, equivalently, with decreasing temperature. A percolation analysis of these stress networks indicates that the stress field is more disjointed at higher temperatures. This leads to more fragile (or poorly connected) interfaces which result in a reduction in surface tension.

Biomimetic liquid-repelling surfaces have been the subject of considerable scientific research and technological application. To design such surfaces, a flexibility-based oscillation strategy has been shown to resolve the problem of liquid-surface positioning encountered by the previous, rigidity-based asymmetry strategy; however, its usage is limited by weak mechanical robustness and confined repellency enhancement. Here, we design a flexible surface comprising mesoscale heads and microscale spring sets, in analogy to the mushroomlike geometry discovered on springtail cuticles, and then realize this through three-dimensional projection microstereolithography. Such a surface exhibits strong mechanical robustness against ubiquitous normal and shear compression and even endures tribological friction. Simultaneously, the surface elevates water repellency for impacting droplets by enhancing impalement resistance and reducing contact time, partially reaching an improvement of ∼80% via structural tilting movements. This is the first demonstration of flexible interfacial structures to robustly endure tribological friction as well as to promote water repellency, approaching real-world applications of water repelling. Also, a flexibility gradient is created on the surface to directionally manipulate droplets, paving the way for droplet transport.

Droplets impacting solid surfaces is ubiquitous in nature and of practical importance in numerous industrial applications. For liquid-repelling applications, rigidity-based asymmetric redistribution and flexibility-based structural oscillation strategies have been proven on artificial surfaces; however, these are limited by strict impacting positioning. Here, we show that the gap between these two strategies can be bridged by a flexibility-patterned design similar to a trampoline park. Such a flexibility-patterned design is realized by three-dimensional projection micro-stereolithography and is shown to enhance liquid repellency in terms of droplet impalement resistance and contact time reduction. This is the first demonstration of the synergistic effect obtained by a hybrid solution that exploits asymmetric redistribution and structural oscillation in liquid-repelling applications, paving the rigidity-flexibility cooperative way of wettability tuning. Also, the flexibility-patterned surface is applied to accelerate liquid evaporation.

Surfactant molecules, known as organic friction modifiers (OFMs), are routinely added to lubricants to reduce friction and wear between sliding surfaces. In macroscale experiments, friction generally decreases as the coverage of OFM molecules on the sliding surfaces increases; however, recent nanoscale experiments with sharp atomic force microscopy (AFM) tips have shown increasing friction. To elucidate the origin of these opposite trends, we use nonequilibrium molecular dynamics (NEMD) simulations and study kinetic friction between OFM monolayers and an indenting nanoscale asperity. For this purpose, we investigate various coverages of stearamide OFMs on iron oxide surfaces and silica AFM tips with different radii of curvature. We show that the differences between the friction-coverage relations from macroscale and nanoscale experiments are due to molecular plowing in the latter. For our small tip radii, the friction coefficient and indentation depth both have a nonmonotonic dependence on OFM surface coverage, with maxima occurring at intermediate coverage. We rationalize the nonmonotonic relations through a competition of two effects (confinement and packing density) that varying the surface coverage has on the effective stiffness of the OFM monolayers. We also show that kinetic friction is not very sensitive to the sliding velocity in the range studied, indicating that it originates from instabilities. Indeed, we find that friction predominately originates from plowing of the monolayers by the leading edge of the tip, where gauche defects are created, while thermal dissipation is mostly localized in molecules toward the trailing edge of the tip, where the chains return to a more extended conformation.

The origin of friction and wear in polycrystalline materials is intimately connected with their microstructural response to interfacial stresses. Although many mechanisms that govern microstructure evolution in sliding contacts are generally understood, it is still a challenge to ascertain which mechanisms matter under what conditions, which limits the development of tailor-made microstructures for reducing friction and wear. Here, we shed light on the circumstances that promote plastic deformation and surface damage by studying several face-centered cubic CuNi alloys subjected to sliding with molecular dynamics simulations featuring tens of millions of atoms. By analyzing the depth- and time-dependent evolution of the grain size, twinning, shear, and stresses in the aggregate, we derive a deformation mechanism map for CuNi alloys. We verify the predictions of this map against focused ion beam images of the near-surface regions of CuNi alloys that were experimentally subjected to similar loading conditions. Our results may serve as a tool for finding optimum material compositions within a specified operating range.

Artificial liquid-repellent surfaces have recently attracted vast scientific attention; however, achieving mechanical robustness remains a formidable challenge before industrialization can be realized. To this end, inspired by plateaus in geological landscapes, a self-compensating strategy is developed to pave the way for the synthesis of durable repellent surfaces. This self-compensating surface comprises tall hydrophobic structural elements, which can repel liquid droplets. When these elements are damaged, they expose shorter structural elements that also suspend the droplets and thus preserve interfacial repellency. An example of this plateau-inspired stratified surface was created by three-dimensional (3D) direct laser lithography micro-nano fabrication. Even after being subjected to serious frictional damage, it maintained static repellency to water with a contact angle above 147° and was simultaneously able to endure high pressures arising from droplet impacts. Extending the scope of nature-inspired functional surfaces from conventional biomimetics to geological landscapes, this work demonstrates that the plateau-inspired self-compensating strategy can provide an unprecedented level of robustness in terms of sustained liquid repellency.

Understanding the behavior of surfactant molecules on iron oxide surfaces is important for many industrial applications. Molecular dynamics (MD) simulations of such systems have been limited by the absence of a force field (FF), which accurately describes the molecule-surface interactions. In this study, interaction energies from density functional theory (DFT) + U calculations with a van der Waals functional are used to parameterize a classical FF for MD simulations of amide surfactants on iron oxide surfaces. The original FF, which was derived using mixing rules and surface Lennard-Jones (LJ) parameters developed for nonpolar molecules, was shown to significantly underestimate the adsorption energy and overestimate the equilibrium adsorption distance compared to DFT. Conversely, the optimized FF showed excellent agreement with the interaction energies obtained from DFT calculations for a wide range of surface coverages and molecular conformations near to and adsorbed on α-Fe 2 O 3 (0001). This was facilitated through the use of a Morse potential for strong chemisorption interactions, modified LJ parameters for weaker physisorption interactions, and adjusted partial charges for the electrostatic interactions. The original FF and optimized FF were compared in classical nonequilibrium molecular dynamics simulations of amide molecules confined between iron oxide surfaces. When the optimized FF was employed, the amide molecules were pulled closer to the surface and the orientation of the headgroups was more similar to that observed in the DFT calculations. The optimized FF proposed here facilitates classical MD simulations of anhydrous amide-iron oxide interfaces in which the interactions are representative of accurate DFT calculations.

Wettability of artificial surfaces is attracting increasing attention for its relevant technological applications. Functional performance is often achieved by mimicking the topographical structures found in natural flora and fauna; however, surface attributes inspired by geological landscapes have so far escaped attention. We reproduced a stratified morphology of plateaus with a bi-Gaussian height distribution using a three-dimensional direct laser lithography. The plateau-inspired artificial surface exhibits a hydrophobic behavior even if fabricated from a hydrophilic material, giving rise to a new wetting mechanism that divides the well-known macroscopic Wenzel and Cassie states into four substates. We have also successfully applied the plateau-inspired structure to droplet manipulation.

Wetting mechanisms on rough surfaces were understood from either a monolayer or a multiscale perspective. However, it has recently been shown that the bi-Gaussian stratified nature of real surfaces should be accounted for when modeling mechanisms of lubrication, sealing, contact, friction, acoustic emission, and manufacture. In this work, a model combining Wenzel and Cassie theories was put forward to predict the static contact angle of a droplet on a bi-Gaussian stratified surface. The model was initially applied to numerically simulated surfaces and subsequently demonstrated on hydrophilic steel and hydrophobic self-assembled monolayer specimens with preset bi-Gaussian stratified topographies. In the Wenzel state, both the upper and the lower surface components are fully wetted. In the Cassie state, the upper component is still completely wetted, while the lower component serves as gas traps and reservoirs. By this model, wetting evolution was assessed, and the existence of different wetting states and potential state transitions was predicted.

Mesoporous NiO films were deposited by means of a screen printing technique onto fluorine-doped tin oxide transparent electrodes and consequently sensitized with Erythrosin B (EryB) dye. The obtained colored NiO material was used as a working electrode in a three-electrode cell to study the evolution of the triple semiconductor/dye/electrolyte interface upon electrochemical polarization in dark conditions. The electrolyte was a solution of I 3 - /I - in acetonitrile, with the redox couple representing the typical redox shuttle of dye-sensitized solar cells (DSCs). The adopted electrochemical conditions were devised in order to simulate the actual electrical environment of the NiO/dye photocathode in a light-soaked DSC. The use of a benchmark sensitizer EryB and of the most widely used redox mediator I 3 - /I - is particularly meaningful for the study of the adsorption dynamics and the determination of possible degradative phenomena on the basis of the behavior of numerous analogue systems. Therefore, for the first time, the evolution of the NiO/EryB/I 3 - /I - multiple interface was investigated combining the electrochemical characterization with ex situ spectroscopic analysis by means of X-ray photoelectron spectroscopy. The resulting picture shows that EryB in the immobilized state promotes the redox processes based on the I 3 - /I - couple. Moreover, the EryB sensitizer inhibits the phenomena of recombination between the metal oxide semiconductor and the redox couple., Competing Interests: The authors declare no competing financial interest.

Classical lubrication theory is unable to explain a variety of phenomena and experimental observations involving soft viscoelastic materials, which are ubiquitous and increasingly used in e.g. engineering and biomedical applications. These include unexpected ruptures of the lubricating film and a friction-speed dependence, which cannot be elucidated by means of conventional models, based on time-independent stress-strain constitutive laws for the lubricated solids. A new modeling framework, corroborated through experimental measurements enabled via an interferometric technique, is proposed to address these issues: Solid/fluid interactions are captured thanks to a coupling strategy that makes it possible to study the effect that solid viscoelasticity has on fluid film lubrication. It is shown that a newly defined visco-elasto-hydrodynamic lubrication (VEHL) regime can be experienced depending on the degree of coupling between the fluid flow and the solid hysteretic response. Pressure distributions show a marked asymmetry with a peak at the flow inlet, and correspondingly, the film thickness reveals a pronounced shrinkage at the flow outlet; friction is heavily influenced by the viscoelastic hysteresis which is experienced in addition to the viscous losses. These features show significant differences with respect to the classical elasto-hydrodynamic lubrication (EHL) regime response that would be predicted when solid viscoelasticity is neglected. A simple yet powerful criterion to assess the importance of viscoelastic solid contributions to soft matter lubrication is finally proposed.

The control of luminous radiation has extremely important implications for modern and future technologies as well as in medicine. In this Review, we detail chemical structures and their relevant photophysical features for various groups of materials, including organic dyes such as metalloporphyrins and metallophthalocyanines (and derivatives), other common organic materials, mixed metal complexes and clusters, fullerenes, dendrimeric nanocomposites, polymeric materials (organic and/or inorganic), inorganic semiconductors, and other nanoscopic materials, utilized or potentially useful for the realization of devices able to filter in a smart way an external radiation. The concept of smart is referred to the characteristic of those materials that are capable to filter the radiation in a dynamic way without the need of an ancillary system for the activation of the required transmission change. In particular, this Review gives emphasis to the nonlinear optical properties of photoactive materials for the function of optical power limiting. All known mechanisms of optical limiting have been analyzed and discussed for the different types of materials.

The adsorption of I - and I 3 - anions, i.e., the two species constituting the most common redox couple of dye-sensitized solar cells (DSCs), onto the surface of screen-printed nanoporous NiO was studied by means of X-ray photoelectron spectroscopy (XPS). Nanoporous NiO films were deposited on transparent metallic fluorine-doped tin oxide (FTO) and polarized as working electrodes in a three-electrode cell with differently concentrated I - /I 3 - electrolytes to simulate the different conditions experienced by the NiO cathodes during the lifecycle of a p-type DSC (p-DSC) at those atomic sites not passivated by the dye. Bare NiO films were tested also as photocathodes of nonsensitized p-DSCs. The ex situ XPS analysis of I 4d ionization region of both reference and electrochemically treated NiO films showed that the presence of native and electrochemically generated Ni 3+ and Ni 4+ centers induces fast adsorption/desorption of I - ions and catalyzes their oxidation to I 3 - ions. The adsorption phenomena generated by I - and I 3 - species on nanoporous NiO electrodes can also induce an effect of electrochemical passivation toward a fraction of charged Ni sites. Such an effect would render these sites inactive for the further realization of those photoelectrochemical processes at the basis of the operation of a p-DSC.

For the successful development and application of lubricants, a full understanding of the nanoscale behavior of complex tribological systems is required, but this is difficult to obtain experimentally. In this study, we use nonequilibrium molecular dynamics (NEMD) simulations to examine the atomistic structure and friction properties of commercially relevant organic friction modifier (OFM) monolayers adsorbed on iron oxide surfaces and lubricated by a thin, separating layer of hexadecane. Specifically, acid, amide, and glyceride OFMs, with saturated and Z-unsaturated hydrocarbon tail groups, are simulated at various surface coverages and sliding velocities. At low and medium coverage, the OFMs form liquidlike and amorphous monolayers, respectively, which are significantly interdigitated with the hexadecane lubricant, resulting in relatively high friction coefficients. At high coverage, solidlike monolayers are formed for all of the OFMs, which, during sliding, results in slip planes between well-defined OFM and hexadecane layers, yielding a marked reduction in the friction coefficient. When present at equal surface coverage, OFMs with saturated and Z-unsaturated tail groups are found to yield similar structure and friction behavior. OFMs with glyceride head groups yield significantly lower friction coefficients than amide and particularly carboxylic acid head groups. For all of the OFMs and coverages simulated, the friction coefficient is found to increase linearly with the logarithm of sliding velocity; however, the gradient of this increase depends on the coverage. The structure and friction details obtained from these simulations agree well with experimental results and also shed light on the relative tribological performance of these OFMs through nanoscale structural variations. This has important implications in terms of the applicability of NEMD to aid the development of new formulations to control friction.

A computationally efficient classical molecular simulation technique is derived for ranking the pKa values of a set of chemically similar congeneric molecules in an implicit solvent model of water. This uses the deprotonation free energy of the titratable group in the gas and aqueous phases obtained by thermodynamic integration (TI). For a series of alcohols and acids a strong linear correlation is demonstrated between the experimental pKa and the deprotonation free energy difference in the gas and liquid phases. These calculations also show that classical TI is more efficient than slow-growth TI in calculating deprotonation free energies for the series of molecules considered herein.

Nanoporous NiO thin film electrodes were obtained via plasma-assisted microwave sintering and characterized by means of a combination of electrochemical techniques and X-ray photoelectron spectroscopy (XPS). The aim of this study is the elucidation of the nature of the surface changes introduced by the redox processes of this nanostructured material. NiO undergoes two distinct electrochemical processes of oxidation in aqueous electrolyte with the progress of NiO anodic polarization. These findings are consistent with the sequential formation of oxyhydroxide species at the surface, the chemical nature of which was assessed by XPS. Electronic relaxation effects in the Ni 2p spectra clearly indicated that the superficial oxyhydroxide species resulted to be β-NiOOH and γ-NiOOH. We also show for the first time spectral evidence of an electrochemically generated Ni(IV) species. This study has direct relevance for those applications in which NiO electrodes are utilized in aqueous electrolyte, namely catalytic water splitting or electrochromism, and may constitute a starting point for the comprehension of electronic phenomena at the NiO/organic electrolyte interface of cathodic dye-sensitized solar cells (p-DSCs).

Circular dichroism (CD) spectroelectrochemistry is used to determine the extent of singly occupied molecular orbital delocalization in mixed-valence multinuclear complexes, specifically the mixed-valence Ru(II)Ru(III) states of the four diastereoisomers of [(Ru(bpy)(2))(2)(bpt)](3+) [1; bpy = 2,2'-bipyridyl and bpt = 3,5-bis(pyrid-2'-yl)-1,2,4-triazolato anion]. The complex was found to be stable to thermal racemization in the three oxidation states, but photoracemization in the Ru(II)Ru(II) state was observed., (© 2011 American Chemical Society)

The synthesis of two asymmetric dinuclear complexes with the formula [M(bpy)(2)(bpt)Ru(tpy)Cl](2+), where M = Ru (1a), Os(2a); bpy = 2,2'-bipyridyl; Hbpt = 3,5-bis(pyridin-2-yl)1,2,4-triazole and tpy = 2,2',6',2''-terpyridine, is reported. The compounds obtained are characterized by mass spectrometry, (1)H NMR, UV/vis/NIR absorption, luminescence, and resonance Raman spectroscopy. Deuterium isotope labeling facilitates assignment of the (1)H NMR and resonance Raman spectra. The interaction between the two metal centers, mediated by the bridging 1,2,4-triazolato moiety in the mixed valent state, is assigned as type II based on the observation of metal to metal charge transfer absorption bands at 7090 and 5990 cm(-1) for 1a and 2a, respectively. The extent of localization of the emissive excited state was determined by transient resonance Raman and emission spectroscopy. Both 1a and 2a show phosphorescence at the same wavelengths; however, whereas for compound 1a the emission is based on the Ru(tpy)Cl- center, for 2a the emissive state is localized on the Os(bpy)(2)- unit. This indicates that also in the excited state there is efficient interaction between the two metal centers.

We report on the first observation of reverse saturable absorption by ferrocene (Fc) in toluene using nanosecond pulses at 532 nm. Pump and probe experiments in the visible spectral region show the existence of an excited triplet state with an intersystem crossing quantum yield S1 --> T1 of 0.085 and a molar extinction coefficient epsilon(Fc)(T) of 5650 L mol(-1) cm(-1) at 700 nm. The full understanding of the nonlinear optical behavior of Fc cannot be obtained, however, with a model that includes only the one-photon absorption from T1, but it is mandatory to consider also a simultaneous two-photon absorption from an excited singlet state of Fc (two-photon absorption cross section: 2.4 x 10(-41) cm4 s ph(-1) mol(-1)). The optical spectrum of the ground and triplet state of Fc are calculated within a TD-DFT approach considering several functionals (PBE, BLYP, LDA, OPBE) for the optimization of molecular geometry.

Tetrakis-2,3-[5,6-di-(2-pyridyl)pyrazino]porphyrazinatopalladium(II) [Py 8TPyzPzPd] ( 1) and the corresponding pentapalladated species [(PdCl 2) 4Py 8TPyzPzPd] ( 2), dissolved (c approximately 10 (-5)-10 (-6) M) in preacidified dimethylformamide ([HCl] approximately 10 (-4) M), behave as potent photosensitizing agents for the production of singlet oxygen, (1)O 2, with Phi Delta values of 0.89 +/- 0.04 and 0.78 +/- 0.05, respectively. The related octacation [(2-Mepy) 8TPyzPzPd] (8+) ( 3), examined under similar experimental conditions, exhibits lower Phi Delta values, that is, 0.29 +/- 0.02 (as an iodide salt) and 0.32 +/- 0.02 (as a chloride salt). In view of the very high values of Phi Delta, the photophysics of complexes 1 and 2 has been studied by means of pump and probe experiments using ns laser pulses at 532 nm as excitation source. Both complexes behave like reverse saturable absorbers at 440 nm because of triplet excited-state absorption. The lifetimes of the triplet excited states are 65 and 96 ns for the penta- and mononuclear species, respectively. Fluorescence quantum yields (Phi f) are approximately 0.1% for both 1 and 2. Such low Phi f values for the two complexes are consistent with the high efficiency of triplet excited-state formation and the measured high yields of (1)O 2. Time-dependent density-functional theory (TDDFT) calculations of the lowest singlet and triplet excited states of the mono- and pentapalladated species help to rationalize the photophysical behavior and the relevant activity of the complexes as photosensitizers for the (1)O 2 ( (1)Delta g) generation.

The photophysical properties of four axially substituted indium phthalocyanines, namely, 2,(3)-tetra- tert-butyl-phthalocyaninato indium chloride ( 1), 2,(3)-tetra-[(3,5-di- tert-butyl)-phenyloxy]-phthalocyaninato indium bromide ( 2), 2,(3)-tetra-[(3,5-di- tert-butyl)-phenyloxy]-phthalocyaninato indium iodide ( 3), and 2,3-octa-[(2-hexyl)-ethyloxy]-phthalocyaninato indium trifluoroacetate ( 4), have been investigated, and their optical limiting properties with nanosecond light pulses were evaluated. All complexes behave as reverse saturable absorbers in the range of 400-625 nm due to a triplet-triplet excited-state transition. Excited-state absorption cross sections and triplet state lifetimes are not significantly affected by the nature of the axial ligand. On the other hand, remarkable differences in the variation of nonlinear transmittance are observed for 1- 4 due to significantly different intersystem crossing rates. Heavier axial ligands in phthalocyanines 2 and 3 produce the largest variations of nonlinear transmission (heavy-atom effect). Complex 1 in polystyrene matrix shows reversible nonlinear absorption when incident fluence does not exceed 0.025 J cm (-2).

A series of five hemiporphyrazines (Hps) with different coordinating central atoms (H2, GeCl2, InCl, Pt, Pb), and the acyclic derivative 1,3-bis-(6'-amino-4'-butoxy-2'-pyridylimino)-1,3-dihydroisoindoline have been synthesized and their multiphoton absorption properties examined at the second harmonic frequency of the Nd:YAG laser in the nanosecond time regime. Metal-free and platinum Hps display saturation of optical transmittance within incident fluence values of 6 J cm(-2). Comparison with other similar molecular structures like phthalocyanines and related molecules shows that Hps are strong nonlinear absorbers. The experimental curves of nonlinear transmission at 532 nm have been fitted by means of a three-level model with the occurrence of simultaneous two-photon absorption from an excited state. In the sole case of the InCl complex we found that a five-level model is needed because of the participation of triplet states. Contrary to phthalocyanines, naphthalocyanines, and porphyrins, a heavy central atom does not improve the nonlinear absorption properties since a different excited states dynamic is involved. The large nonlinear absorption of Hps combined with the very small absorption in the visible spectral range makes these molecules a very interesting class of molecules for nonlinear optical applications.

The effect of the number of bromide substituents on the photophysical and nonlinear optical properties of the tetrabrominated naphthalocyanine Br4(tBu2PhO)4NcSi[OSi(Hex)3]2 (1) and the octabrominated naphthalocyanine Br8NcSi[OSi(Hex)3]2 (2) has been investigated through various spectroscopic techniques. Absorption and emission of 1 and 2 have been studied at room temperature and 77 K to determine the spectral properties of the ground and the excited states and the lifetimes and quantum yields of formation of the excited states. There is a moderate increase of the quantum yield of the triplet excited-state formation (PhiT = 0.10 vs 0.13) and a decrease of the triplet excited-state lifetime (tauT approximately 70 vs 50 mus) from 1 to 2. These can be attributed to the stronger heavy atom effect produced by the larger number of peripheral bromide substituents in 2 considering that an excited state with a triplet manifold is involved in the excitation dynamics of both complexes 1 and 2. The quantum yields of the singlet oxygen formation (PhiDelta) upon irradiation of 1 and 2 at 355 nm were also evaluated, and a value of PhiDelta(1) = PhiDelta(2) = 0.16 was obtained. In addition to that, octabrominated complex 2 displays a larger decrease of nonlinear optical transmission for nanosecond pulses at 532 nm with respect to the tetrabrominated complex 1. The nanosecond Z-scan experiments reveal that 1 and 2 exhibit both a reverse saturable absorption and a nonlinear refraction at 532 nm. However, both the sign and the magnitude of the nonlinear refraction change from 1 to 2. For picosecond Z-scan in the visible spectral region, these two complexes exhibit only reverse saturable absorption, and the excited-state absorption cross-section increases at longer wavelengths.

Three axially substituted complexes, 2,3-octa(3,5-di-tert-butylphenoxy)-2,3-naphthalocyaninato indium chloride (1a), 2,3-octa(3,5-di-tert-butylphenoxy)-2,3-naphthalocyaninato indium bromide (1b), and 2,3-octa(3,5-di-tert-butylphenoxy)-2,3-naphthalocyaninato indium iodide (1c) have been synthesized and their photophysical properties have been investigated. Optical power limiting of nanosecond (ns) and picosecond (ps) laser pulses at 532 nm using these complexes has been demonstrated. All complexes display strong Q(0,0) absorption and measurable emission in the near-infrared region and exhibit strong excited-state absorption in the range of 470-700 nm upon ns laser excitation. The different axial ligands show negligible effect on the linear absorption, emission, and transient difference absorption spectra. However, the excited-state lifetime, triplet excited-state quantum yield, and efficiency to generate singlet oxygen are affected significantly by the heavier axial ligand. Brominated and iodinated complexes 1b and 1c show higher triplet excited-state quantum yield, while chlorinated complex 1a has longer excited-state lifetime and is more efficient in generating singlet oxygen. The iodinated complex 1c displayed the best optical limiting due to the higher ratio of excited-state absorption cross section to ground state absorption cross section (sigma(eff)/sigma(0)).

The axially substituted complex chloro indium(III) 2-tetrabromo-3-tetra-(3,5-di-tert-butylphenyloxy)naphthalocyanine [Br4(tBu2PhO)4NcInCl (1); MW = 1996] has been synthesized for the first time, and its nonlinear transmission properties have been evaluated with the Z-scan technique in both open and closed aperture configurations at 532 nm for nanosecond pulsed radiation. The tetrabrominated complex 1 displayed a larger positive nonlinear absorption coefficient when compared to an analogous nonbrominated naphthalocyanine [(tBu2PhO)8NcInCl (2); MW = 2498]. The effect of the four Br atoms on the nonlinear optical behavior of 1 is evaluated, discussed, and compared with the nonlinear optical behavior of 2. It is shown that the bromination of the naphthalocyanine ring considerably improves the limiting properties of such a system when high-intensity radiations are produced by nanosecond laser pulses at 532 nm.

The multiphoton absorption properties of the axially substituted tetrapyrazinotetraazaporphyrinato complex Pyz(4)TAPInCl (1) are reported and interpreted. In particular, the nonlinear optical transmission of the complex and the excited states involved in the nonlinear absorption have been determined at the frequency of the second harmonic generation of a Nd:YAG laser in the nanosecond time regime. Pyz(4)TAPInCl has an excited-state absorption cross section larger than its ground state in the 460-540 nm spectral region, and it shows an optical limiting (OL) behavior at 532 nm, which derives from a sequential two-photon absorption with a larger absorption cross section of the excited triplet state with respect to the ground state. It results that the absorption cross section of 1 in the excited triplet state is 7.8 x 10(-18) cm(2) vs 0.9 x 10(-18) cm(2) of the ground state at the wavelength of OL analysis.

The synthesis, characterization, and various photophysical properties of axially substituted tetrapyrazinotetraazaporphyrinatotitanium(IV) oxide (Pyz(4)TAPTiO) (1), tetrapyrazinotetraazaporphyrinatovanadium(IV) oxide (Pyz(4)TAPVO) (2), tetrapyrazinotetraazaporphyrinatozirconium(IV) dihydroxide [Pyz(4)TAPZr(OH)(2)] (3) are reported. Nonlinear optical (NLO) properties of Pyz(4)TAPs 1-3 have been evaluated at 532 nm with nanosecond pulses for optical limiting (OL). It is found that the introduction of nitrogen atoms in the condensed rings of the tetrapyrrolic macrocycles together with the presence of axial substituents lead to an improvement of the excited-state absorption properties in comparison to phthalocyanines (Pcs). In the linear optical regime Pyz(4)TAPs 1-3 display a blue shift (about 50-60 nm) of the main UV-vis absorption bands with respect to Pcs and exhibit orange-red fluorescence, which can be observed with the eye in the case of 1. Frontier electronic orbitals of a Pyz(4)TAP could be depicted from available experimental data and the results of density functional theory (DFT) calculations. In the solid state, Pyz(4)TAPTiO (1) displays photoconducting properties.

A series of metal complexes of tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine, [Py(8)TPyzPzH(2)], having the general formula [Py(8)TPyzPzM].xH(2)O (M = Mg(II)(H(2)O), Mn(II), Co(II), Cu(II), Zn(II); x = 3-8) were synthesized by reaction of the free-base macrocycle with the appropriate metal acetate in pyridine or dimethyl sulfoxide under mild conditions. Clathrated water and retained pyridine molecules for the Mn(II) and Co(II) species are easily eliminated by heating under vacuum, the water molecules being recovered by exposure of the unsolvated macrocycles to air. Magnetic susceptibility measurements and EPR spectra of the materials in the solid state provide basic information on the spin state of the Cu(II), Co(II), and Mn(II) species. Colloidal solutions caused by molecular aggregation are formed in nondonor solvents (CH(2)Cl(2), CHCl(3)), a moderately basic solvent (pyridine), and an acidic solvent (CH(3)COOH), with the extent of aggregation depending on the specific solvent and the central metal ion. UV-vis spectral monitoring of the solutions after preparation indicates that disaggregation systematically occurs as a function of time leading ultimately to the formation of clear solutions containing the monomeric form of the porphyrazine. Cyclic voltammetry and thin-layer spectroelectrochemistry show that each compound with an electroinactive metal ion undergoes four reversible one-electron reductions, leading to formation of the negatively charged species [Py(8)TPyzPzM](n-) (n = 1 - 4). The stepwise uptake of four electrons is consistent with a ring-centered reduction, but in the case of the cobalt complex a metal-centered (Co(II) --> Co(I)) reduction occurs in the first process and only three additional reductions are observed. No oxidations are observed in pyridine or CH(2)Cl(2) containing 0.1 M tetrabuthylammonium perchlorate (TBAP). The nonlinear optical properties (NLO) of the species [Py(8)TPyzPzM] (M = 2H(I), Cu(II), Zn(II), Mg(II)(H(2)O)) have also been examined with nanosecond pulses at 532 nm in dimethyl sulfoxide solution. Reverse saturable absorption is shown by all of the [Py(8)TPyzPzM] species, which exhibit distinct behavior depending on the nature of M and extent of aggregation.

Following a previous report on the synthesis and physicochemical characterization of a novel class of porphyrazines carrying peripherally annulated seven-membered rings, i.e., tetrakis-2,3-(5,7-diphenyl-1,4-diazepino)porphyrazine [Ph(8)DzPzH(2)].4H(2)O and its metal derivatives [Ph(8)DzPzM].xH(2)O (x = 2-7, M = Mg(II)(H(2)O), Cu(II), and Zn(II)), a new more convenient procedure is reported here, allowing the preparation in high yields of the Li(I) and Na(I) derivatives of formulas [Ph(8)DzPzLi(2)].5H(2)O and [Ph(8)DzPzNa(2)].6H(2)O, which can be directly converted into other metal derivatives under mild conditions (room temperature) and in good yields. The series studied has been extended to include the Mn(II) and Co(II) complexes also reported here for the first time. Physicochemical characterization of the new "diazepinoporphyrazines" was based on fast atom bombardment (FAB) mass spectrometry and X-ray powder patterns, infrared (IR), electron paramagnetic resonance (EPR), and room-temperature magnetic susceptibility measurements. A detailed discussion of the UV-vis spectra emphasizes the role played by the external diazepine rings in electron delocalization through their tautomeric or protonated forms present in neutral, basic, and acidic media. The nonlinear optical effect of optical limiting for the different species [M = 2H, Mg(II)(H(2)O), Mn(II), Co(II), Cu(II), and Zn(II)] has also been measured. It has been observed that the extent of the optical limiting depends on the specific M center. The observed nonlinear optical features are analyzed and discussed in terms of the electronic and magnetic properties exhibited by some of the metal ions and taking into account the model of the excited-state absorption in which the nature of M determines the kinetics of formation of the highly absorbing state of the specific complex examined. As evidenced by the detailed electrochemical and spectroelectrochemical study carried out on this new class of macrocycles, one of the most important aspects is the facilitated electron delocalization for the oxidized and reduced species allowed by a 1H-6Htautomerism taking place on the peripheral diazepine rings.

The preparation of (octaaryltetraazaporphyrinato)indium(III) chlorides [aryl = phenyl (5a), p-tert-butylphenyl (6a), p-(trifluoromethyl)phenyl (7a), m-(trifluoromethyl)phenyl (8a)] and their reactions with aryl Grignard reagents XMgBr to give 5b-8b [X = p-(trifluoromethyl)phenyl] and 5c (R = 3,5-difluorophenyl) are described. The characterization of all compounds by UV-vis, FT-IR, and (1)H and (13)C NMR spectroscopy was performed. The hypsochromic shift of all bands in the absorption spectra of complexes 5a-8a is observed in the sequence 6a < 5a < 7a approximately 8a. This is associated with the increasing electron-withdrawing character of the aryl substituents in the periphery of the tetraazaporphyrin macrocycle. Compounds 8a,b are very good soluble in organic solvents with 8a exhibiting the higher photochemical stability among the various synthesized species. The optical limiting (OL) properties of the complexes have been studied and correlated with the structure of the (tetraazaporphyrinato)indium(III) complexes and the electronic nature of the different substituents. In particular, the OL effect at 532 nm increases on going from the series of compounds 5 to the series 8.