A Bond-Based Machine Learning Model for Molecular Polarizabilities and A Priori Raman Spectra.

The use of machine learning (ML) algorithms in molecular simulations has become commonplace in recent years. There now exists, for instance, a multitude of ML force field algorithms that have enabled simulations approaching level accuracy at time scales and system sizes that significantly exceed what is otherwise possible with traditional methods. Far fewer algorithms exist for predicting rotationally equivariant, tensorial properties such as the electric polarizability.

A Molecular Expression for "Line Tension".

"Line tension", a concept that features in an additional term to the Young's equation, was introduced to describe the size dependence of contact angles of nanodroplets on surfaces. Although this concept describes the observations in a succinct, elegant manner, theorists have long had misgivings about the physical interpretation of the phenomenon. Papers have been published that attempt to nail down its value, which is reportedly very small (∼10 pN) and evidently even the sign has been uncertain.

Structural Disorder at the Edges of Rubrene Crystals Enhances Singlet Fission.

Materials that undergo singlet fission are of interest for their use in light-harvesting, photocatalysis, and quantum information science, but their ability to undergo fission can be sensitive to local variations in molecular packing. Herein we employ transient absorption microscopy, molecular dynamics simulations, and electronic structure calculations to interrogate how structures found at the edges of orthorhombic rubrene crystals impact singlet fission. Within a micrometer-scale spatial region at the edges of rubrene crystals, we find that the rate of singlet fission increases nearly 4-fold.

Accumulation and ordering of P3HT oligomers at the liquid-vapor interface with implications for thin-film morphology.

The morphology of semiconducting polymer thin films is known to have a profound effect on their opto-electronic properties. Although considerable efforts have been made to control and understand the processes which influence the structures of these systems, it remains largely unclear what physical factors determine the arrangement of polymer chains in spin-cast films. Here, we investigate the role that the liquid-vapor interfaces in chlorobenzene solutions of poly(3-hexylthiophene) [P3HT] play in the conformational geometries adopted by the polymers.

Imaging Heterogeneous 3D Dynamics of Individual Solutes in a Polyelectrolyte Brush.

Understanding molecular transport in polyelectrolyte brushes (PEBs) is crucial for applications such as separations, drug delivery, anti-fouling, and biosensors, where structural features of the polymer control intermolecular interactions. The complex structure and local heterogeneity of PEBs, while theoretically predicted, are not easily accessed with conventional experimental methods. In this work, we use 3D single-molecule tracking to understand transport behavior within a cationic poly(2-(,-dimethylamino)ethyl acrylate) (PDMAEA) brush using an anionic dye, Alexa Fluor 546, as the probe.

Mechanism for plasmon-generated solvated electrons.

Solvated electrons are powerful reducing agents capable of driving some of the most energetically expensive reduction reactions. Their generation under mild and sustainable conditions remains challenging though. Using near-ultraviolet irradiation under low-intensity one-photon conditions coupled with electrochemical and optical detection, we show that the yield of solvated electrons in water is increased more than 10 times for nanoparticle-decorated electrodes compared to smooth silver electrodes.

Aggregation of Charge Acceptors on Nanocrystal Surfaces Alters Rates of Photoinduced Electron Transfer.

Semiconductor nanocrystals (NCs) interfaced with molecular ligands that function as charge and energy acceptors are an emerging platform for the design of light-harvesting, photon-upconverting, and photocatalytic materials. However, NC systems explored for these applications often feature high concentrations of bound acceptor ligands, which can lead to ligand-ligand interactions that may alter each system's ability to undergo charge and energy transfer. Here, we demonstrate that aggregation of acceptor ligands impacts the rate of photoinduced NC-to-ligand charge transfer between lead(II) sulfide (PbS) NCs and perylenediimide (PDI) electron acceptors.

Single-Molecule Dynamics Reflect IgG Conformational Changes Associated with Ion-Exchange Chromatography.

Conformational changes of antibodies and other biologics can decrease the effectiveness of pharmaceutical separations. Hence, a detailed mechanistic picture of antibody-stationary phase interactions that occur during ion-exchange chromatography (IEX) can provide critical insights. This work examines antibody conformational changes and how they perturb antibody motion and affect ensemble elution profiles.

Predicting optical spectra for optoelectronic polymers using coarse-grained models and recurrent neural networks.

Coarse-grained modeling of conjugated polymers has become an increasingly popular route to investigate the physics of organic optoelectronic materials. While ultraviolet (UV)-vis spectroscopy remains one of the key experimental methods for the interrogation of these materials, a rigorous bridge between simulated coarse-grained structures and spectroscopy has not been established. Here, we address this challenge by developing a method that can predict spectra of conjugated polymers directly from coarse-grained representations while avoiding repetitive procedures such as back-mapping from coarse-grained to atomistic representations followed by spectral computation using quantum chemistry.

Isotope effects in aqueous solvation of simple halides.

We present a path-integral-molecular-dynamics study of the thermodynamic stabilities of DOH⋯ X and HOD⋯ X (X = F, Cl, Br, I) coordination in aqueous solutions at ambient conditions. In agreement with experimental evidence, our results for the F case reveal a clear stabilization of the latter motif, whereas, in the rest of the halogen series, the former articulation prevails. The DOH⋯ X preference becomes more marked the larger the size of the ionic solute.

Isotopic equilibria in aqueous clusters at low temperatures: Insights from the MB-pol many-body potential.

By combining path-integrals molecular dynamics simulations with the accurate MB-pol potential energy surface, we investigate the role of alternative potential models on isotopic fractionation ratios between H and D atoms at dangling positions in water clusters at low temperatures. Our results show clear stabilizations of the lighter isotope at dangling sites, characterized by free energy differences ΔG that become comparable to or larger than kT for temperatures below ∼75 K. The comparison between these results to those previously reported using the empirical q-TIP4P/F water model [P.

Direct observation of backbone planarization via side-chain alignment in single bulky-substituted polythiophenes.

The backbone conformation of conjugated polymers affects, to a large extent, their optical and electronic properties. The usually flexible substituents provide solubility and influence the packing behavior of conjugated polymers in films or in bad solvents. However, the role of the side chains in determining and potentially controlling the backbone conformation, and thus the optical and electronic properties on the single polymer level, is currently under debate.

Fluorescent Proteins Detect Host Structural Rearrangements via Electrostatic Mechanism.

The rational design of genetically encoded fluorescent biosensors, which can detect rearrangements of target proteins via interdomain allostery, is hindered by the absence of mechanistic understanding of the underlying photophysics. Here, we focus on the modulation of fluorescence by mechanical perturbation in a popular biological probe: enhanced Green Fluorescent Protein (eGFP). Using a combination of molecular dynamics (MD) simulations and quantum chemistry, and a set of physically motivated assumptions, we construct a map of fluorescence quantum yield as a function of a 2D electric field imposed by the protein environment on the fluorophore.

Electronic Excited State Lifetimes of Anionic Water Clusters: Dependence on Charge Solvation Motif.

An ongoing controversy about water cluster anions concerns the electron-binding motif, whether the charge center is localized at the surface or within the cluster interior. Here, mixed quantum-classical dynamics simulations have been carried out for a wide range of cluster sizes (n ≤ 1000) for (HO) and (DO), based on a nonequilibrium first-order rate constant approach. The computed data are in good general agreement with time-resolved photoelectron imaging results (n ≤ 200).

Impact of backbone fluorination on nanoscale morphology and excitonic coupling in polythiophenes.

Fluorination represents an important strategy in developing high-performance conjugated polymers for photovoltaic applications. Here, we use regioregular poly(3-ethylhexylthiophene) (P3EHT) and poly(3-ethylhexyl-4-fluorothiophene) (F-P3EHT) as simplified model materials, using single-molecule/aggregate spectroscopy and molecular dynamic simulations, to elucidate the impacts of backbone fluorination on morphology and excitonic coupling on the molecular scale. Despite its high regioregularity, regioregular P3EHT exhibits a rather broad distribution in polymer chain conformation due to the strong steric hindrance of bulky ethylhexyl side chains.

Relating Chromophoric and Structural Disorder in Conjugated Polymers.

The optoelectronic properties of amorphous conjugated polymers are sensitive to the details of the conformational disorder, and spectroscopy provides the means for structural characterization of the fragments of the chain that interact with light-"chromophores". A faithful interpretation of spectroscopic conformational signatures, however, presents a theoretical challenge. Here we investigate the relationship between the ground-state optical gaps, the properties of the excited states, and the structural features of chromophores of a single molecule poly(3-hexyl)-thiophene (P3HT) using quantum-classical atomistic simulations.

Communication: Isotopic effects on tunneling motions in the water trimer.

We present results of ring polymer molecular dynamics simulations that shed light on the effects of nuclear quantum fluctuations on tunneling motions in cyclic [H2O]3 and [D2O]3, at the representative temperature of T = 75 K. In particular, we focus attention on free energies associated with two key isomerization processes: The first one corresponds to flipping transitions of dangling OH bonds, between up and down positions with respect to the O-O-O plane of the cluster; the second involves the interchange between connecting and dangling hydrogen bond character of the H-atoms in a tagged water molecule. Zero point energy and tunneling effects lead to sensible reductions of the free energy barriers.

An insight into non-emissive excited states in conjugated polymers.

Conjugated polymers in the solid state usually exhibit low fluorescence quantum yields, which limit their applications in many areas such as light-emitting diodes. Despite considerable research efforts, the underlying mechanism still remains controversial and elusive. Here, the nature and properties of excited states in the archetypal polythiophene are investigated via aggregates suspended in solvents with different dielectric constants (ɛ).

Resonance Raman Spectroscopy of the T1 Triplet Excited State of Oligothiophenes.

The characterization of triplet excited states is essential for research on organic photovoltaics and singlet fission. We report resonance Raman spectra of two triplet oligothiophenes with n-alkyl substituents, a tetramer and hexamer. The spectra of the triplets are more complex than the ground state, and we find that density functional theory calculations are a useful starting point for characterizing the bands.

Surface Isotope Segregation as a Probe of Temperature in Water Nanoclusters.

Using ring polymer molecular dynamics simulations, we examine equilibrium and dynamical characteristics of solid-like, aqueous clusters that combine isotopic mixtures of HDO dilute in H2O, at temperatures intermediate between 50 and 175 K. In particular, we focus attention on the relative thermodynamic stabilities of the two isotopes at dangling hydrogen bond sites. The water octamer is analyzed as a reference system.

Isotopic Preferential Solvation of I(-) in Low-Temperature Water Nanoclusters.

We present results from ring polymer molecular dynamics experiments that provide microscopic insights into the characteristics of the isotopic stabilizations of H and D aqueous species in the first solvation shell of a halide I(-) ion in water nanoclusters at low temperatures. The analysis of the simplest I(-)·(HOD) dimer shows a clear propensity for the light isotope to lie at the non-hydrogen-bonded dangling position. Our results predict that, at T ∼ 50 K, I(-)·(DOH) isomers are three times more abundant than I(-)·(HOD) ones.

Application of a new ensemble conserving quantum dynamics simulation algorithm to liquid para-hydrogen and ortho-deuterium.

We apply the Feynman-Kleinert Quasi-Classical Wigner (FK-QCW) method developed in our previous work [Smith et al., J. Chem.