Hydrophilic Photocrosslinkers as a Universal Solution to Endow Water Affinity to a Polymer Photocatalyst for an Enhanced Hydrogen Evolution Rate.

A universal approach for enhancing water affinity in polymer photocatalysts by covalently attaching hydrophilic photocrosslinkers to polymer chains is presented. A series of bisdiazirine photocrosslinkers, each comprising bisdiazirine photophores linked by various aliphatic (CL-R) or ethylene glycol-based bridge chains (CL-TEG), is designed to prevent crosslinked polymer photocatalysts from degradation through a safe and efficient photocrosslinking reaction at a wavelength of 365 nm. When employing the hydrophilic CL-TEG as a photocrosslinker with polymer photocatalysts (F8BT), the hydrogen evolution reaction (HER) rate is considerably enhanced by 2.5-fold compared to that obtained using non-crosslinked F8BT photocatalysts, whereas CL-R-based photocatalysts yield HER rates comparable to those of non-crosslinked counterparts. Photophysical analyses including time-resolved photoluminescence and transient absorption measurements reveal that adding CL-TEG accelerates exciton separation, forming long-lived charge carriers. Additionally, the in-depth study using molecular dynamics simulations elucidates the dual role of CL-TEG: it enhances water penetration into the polymer matrix and stabilizes charge carriers after exciton generation against undesirable recombination. Therefore, the strategy highlights endowing a high-permittivity environment within polymer photocatalyst in a controlled manner is crucial for enhancing photocatalytic redox reactivity. Furthermore, this study shows that this hydrophilic crosslinker approach has a broad applicability in general polymer semiconductors and their nanoparticulate photocatalysts.

Predictive Molecular Models for Charged Materials Systems: From Energy Materials to Biomacromolecules.

Electrostatic interactions play a dominant role in charged materials systems. Understanding the complex correlation between macroscopic properties with microscopic structures is of critical importance to develop rational design strategies for advanced materials. But the complexity of this challenging task is augmented by interfaces present in the charged materials systems, such as electrode-electrolyte interfaces or biological membranes. Over the last decades, predictive molecular simulations that are founded in fundamental physics and optimized for charged interfacial systems have proven their value in providing molecular understanding of physicochemical properties and functional mechanisms for diverse materials. Novel design strategies utilizing predictive models have been suggested as promising route for the rational design of materials with tailored properties. Here, an overview of recent advances in the understanding of charged interfacial systems aided by predictive molecular simulations is presented. Focusing on three types of charged interfaces found in energy materials and biomacromolecules, how the molecular models characterize ion structure, charge transport, morphology relation to the environment, and the thermodynamics/kinetics of molecular binding at the interfaces is discussed. The critical analysis brings two prominent field of energy materials and biological science under common perspective, to stimulate crossover in both research field that have been largely separated.

Deep Eutectic Solvents: Molecular Simulations with a First-Principles Polarizable Force Field.

The unique properties of deep eutectic solvents make them useful in a variety of applications. In this work we develop a first-principles force field for reline, which is composed of choline chloride and urea in the molar ratio 1:2. We start with the symmetry adapted perturbation theory (SAPT) protocol and then make adjustments to better reproduce the structure and dynamics of the liquid when compared to first-principles molecular dynamics (FPMD) simulations. The resulting force field is in good agreement with experiments in addition to being consistent with the FPMD simulations. The simulations show that primitive molecular clusters are preferentially formed with choline-chloride ionic pairs bound with a hydrogen bond in the hydroxyl group and that urea molecules coordinate the chloride mainly via the trans-H chelating hydrogen bonds. Incorporating polarizability qualitatively influences the radial distributions and lifetimes of hydrogen bonds and affects long-range structural order and dynamics. The polarizable force field predicts a diffusion constant about an order of magnitude larger than the nonpolarizable force field and is therefore less computationally intensive. We hope this study paves the way for studying complex hydrogen-bonding liquids from a first-principles approach.

A Transferable Polarizable Force Field for Urea Crystals and Aqueous Solutions.

Urea is an important chemical with many biological and industrial applications. In this work, we develop a first-principles polarizable force field for urea crystals and aqueous solutions within the symmetry-adapted perturbation theory (SAPT) protocol with the SWM4-NDP model for water. We make three adjustments to the SAPT force field protocol: We augment the carbonyl oxygen atom of urea with additional interaction sites in order to address the "chelated" bent double hydrogen bonds in urea, we reduce the polarizability of urea by a factor of 0.70 to reproduce experimental in-crystal dipole moments, and we re-fit atomic pre-exponential parameters to correct the predicted liquid structure. We find that the resulting force field is in good agreement for the static and dynamic properties of aqueous urea solutions when compared to experiment or first-principles molecular dynamics simulations. The polarizable urea model accurately reproduces the crystal-solution phase diagram in the temperature range of 261 to 310 K; for which, it is superior to non-polarizable models. We expect that this force field will be useful in the modeling of complex biomolecular systems and enable studies of polarizability effects of solid-liquid phase behavior of complex fluids.

The Driving Force for the Association of Gemini Surfactants.

The self-assembly of surfactants into lyotropic liquid crystalline phases is interesting from a fundamental and practical perspective. The propensity for self-assembly is particularly interesting in Gemini surfactants which have a very low critical micelle concentration. In this work, we study the effect of headgroup identity on the driving force for the self-assembly of Gemini surfactants, using computer simulations of the potential of mean force (PMF). We find that surfactants with sulfonate headgroups have a greater tendency to assemble than those with carboxylate headgroups. The minimum in the PMF is about a factor of 2 deeper and occurs at shorter distances. Interestingly, the driving force is entropic with the carobxylate and energetic with the sulfonate headgroups. Analysis of different contributions suggests that these differences arise from surfactant headgroup electrostatics and size. The results provide an explanation for why the morphology diagram of the sulfonate surfactants is insensitive to temperature.

Low-symmetry sphere packings of simple surfactant micelles induced by ionic sphericity.

Supramolecular self-assembly enables access to designer soft materials that typically exhibit high-symmetry packing arrangements, which optimize the interactions between their mesoscopic constituents over multiple length scales. We report the discovery of an ionic small molecule surfactant that undergoes water-induced self-assembly into spherical micelles, which pack into a previously unknown, low-symmetry lyotropic liquid crystalline Frank-Kasper σ phase. Small-angle X-ray scattering studies reveal that this complex phase is characterized by a gigantic tetragonal unit cell, in which 30 sub-2-nm quasispherical micelles of five discrete sizes are arranged into a tetrahedral close packing, with exceptional translational order over length scales exceeding 100 nm. Varying the relative concentrations of water and surfactant in these lyotropic phases also triggers formation of the related Frank-Kasper A15 sphere packing as well as a common body-centered cubic structure. Molecular dynamics simulations reveal that the symmetry breaking that drives the formation of the σ and A15 phases arises from minimization of local deviations in surfactant headgroup and counterion solvation to maintain a nearly spherical counterion atmosphere around each micelle, while maximizing counterion-mediated electrostatic cohesion among the ensemble of charged particles.