Photocurrent generation in organic photovoltaics (OPVs) relies on the dissociation of excitons into free electrons and holes at donor/acceptor heterointerfaces. The low dielectric constant of organic semiconductors leads to strong Coulomb interactions between electron-hole pairs that should in principle oppose the generation of free charges. The exact mechanism by which electrons and holes overcome this Coulomb trapping is still unsolved, but increasing evidence points to the critical role of hot charge-transfer (CT) excitons in assisting this process. Here we provide a real-time view of hot CT exciton formation and relaxation using femtosecond nonlinear optical spectroscopies and non-adiabatic mixed quantum mechanics/molecular mechanics simulations in the phthalocyanine-fullerene model OPV system. For initial excitation on phthalocyanine, hot CT excitons are formed in 10(-13) s, followed by relaxation to lower energies and shorter electron-hole distances on a 10(-12) s timescale. This hot CT exciton cooling process and collapse of charge separation sets the fundamental time limit for competitive charge separation channels that lead to efficient photocurrent generation.

We describe and illustrate a simple procedure for identifying a liquid interface from atomic coordinates. In particular, a coarse grained density field is constructed, and the interface is defined as a constant density surface for this coarse grained field. In applications to a molecular dynamics simulation of liquid water, it is shown that this procedure provides instructive and useful pictures of liquid-vapor interfaces and of liquid-protein interfaces. The InterfaceDefinitions of soft-matter interfaces at a molecular level can be ambiguous. 1,2 Due to molecular motions, interfacial configurations change with time, and the identity of molecules that lie at the interface also change with time. Generally useful procedures for identifying interfaces must accommodate these motions. Here, we present a simple and intuitive procedure for doing so. The procedure is based upon spatial coarse graining, it applies to reasonably arbitrary geometries, and it can be applied at any point in time so that it can be used to interpret time dependent phenomena and fluctuations. We find the procedure to be useful in a variety of contexts, a few of which are illustrated in this and the next section.The basic idea begins with the instantaneous density field at space-time point r,t, (1) where r i (t) is the position of the ith particle at time t, and the sum is over all such particles of interest. Rendering this field directly provides only vague impressions of interfaces. A more manageable field can be formed through coarse graining. Our choice of spatial coarse graining is a convolution with the normalized Gaussian functions (2) where r is the magnitude of r, ξ is the coarse graining length, and d stands for dimensionality. Applied to ρ(r,t) we have the coarse grained density fieldThe choice of ξ will depend upon the physical conditions under considerations. With ξ set, we define interfaces to be the (d -1)-dimensional manifold r = s for which (4) NIH Public Access where c is a constant. In other words, we define instantaneous interfaces to be points in space where the coarse grained density field has the value c. This coarse grained density changes with time as molecular configurations change with time, i.e., s = s(t) = s({r i (t)}).For a given molecular configuration, {r i (t)}, Eq. (4) can be solved quickly through interpolation on a spatial grid. Figure 1 illustrates what is found for one configuration of a slab of liquid water at conditions of water-vapor coexistence. Details of our simulations are described below in the Methods section. We have taken {r i (t)} to refer to the positions of all the oxygen atoms in the system, and because the bulk correlation length of liquid water is about one molecular diameter, we have used use ξ = 2.4Å; further, we have used c = 0.016Å −3 , which is approximately one-half the bulk density. The choice of coarse graining length, ξ, is just large enough so that the instantaneous density of bulk water contains few if any voids. See the Methods section for further discussion.The pictured ...

We have applied molecular dynamics and methods of importance sampling to study structure and dynamics of liquid water in contact with metal surfaces. The specific surfaces considered resemble the 100 and 111 faces of platinum. Several results emerge that should apply generally, not just to platinum. These results are generic consequences of water molecules binding strongly to surfaces that are incommensurate with favorable hydrogen-bonding patterns. We show that adlayers of water under these conditions have frustrated structures that interact unfavorably with adjacent liquid water. We elucidate dynamical processes of water in these cases that extend over a broad range of timescales, from less than picoseconds to more than nanoseconds. Associated spatial correlations extend over nanometers. We show that adlayer reorganization occurs intermittently, and each reorganization event correlates motions of several molecules. We show that soft liquid interfaces form adjacent to the adlayer, as is generally characteristic of liquid water adjacent to a hydrophobic surface. The infrequent adlayer reorganization produces a hydrophobic heterogeneity that we characterize by studying the degrees by which different regions of the adlayers attract small hydrophobic particles. Consequences for electrochemistry are discussed in the context of hydronium ions being attracted from the liquid to the metal-adlayer surface. E xtended metal interfaces play a fundamental role in aqueous electrochemistry, a field of principal importance in the advancement of renewable, clean energy sources (1, 2). In many processes that occur at metal interfaces, such as electrolysis, corrosion, and electrocatalysis, water is ubiquitous, often acting as both solvent and reactant (3). Although many studies exist that detail the behavior of water across small length scales and timescales (4-7) and at low temperatures (8-10), at present there is little understanding of the large length scale correlations and emergent behavior of water on metal surfaces, even though such effects are likely to influence function in important ways (11)(12)(13)(14). Here, we address this gap in knowledge with a theoretical model of the interactions between water and a metal surface. Specifically we illustrate how a metal surface can impose geometrical constraints within the adlayer of water, leading to a composite metal-water interface that is hydrophobic on large length scales. We further show how defects within the hydrogen-bonding patterns of the adlayer create transient regions of hydrophobic behavior that exist on small length scales and over long timescales. These results offer a microscopic explanation and generalization of previous experimental observations that have inferred hydrophobicity of a platinum surface at low temperatures (15-17).To study the aqueous metal interface we use a molecular model (6, 18) that neglects explicit electronic degrees of freedom beyond accounting for electronic polarization of the metal. Despite its relative simplicity, the model is in reaso...

Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications.

Polymer networks can have a range of desirable properties such as mechanical strength, wide compositional diversity between different materials, permanent porosity, convenient processability and broad solvent compatibility. Designing polymer networks from the bottom up with new structural motifs and chemical compositions can be used to impart dynamic features such as malleability or self-healing, or to allow the material to respond to environmental stimuli. However, many existing systems exhibit only one operational state that is defined by the material's composition and topology; or their responsiveness may be irreversible and limited to a single network property (such as stiffness). Here we use cooperative self-assembly as a design principle to prepare a material that can be switched between two topological states. By using networks of polymer-linked metal-organic cages in which the cages change shape and size on irradiation, we can reversibly switch the network topology with ultraviolet or green light. This photoswitching produces coherent changes in several network properties at once, including branch functionality, junction fluctuations, defect tolerance, shear modulus, stress-relaxation behaviour and self-healing. Topology-switching materials could prove useful in fields such as soft robotics and photo-actuators as well as providing model systems for fundamental polymer physics studies.

The results of molecular dynamics simulations of the properties of water in an aqueous ionic solution close to an interface with a model metallic electrode are described. In the simulations the electrode behaves as an ideally polarizable hydrophilic metal, supporting image charge interactions with charged species, and it is maintained at a constant electrical potential with respect to the solution so that the model is a textbook representation of an electrochemical interface through which no current is passing. We show how water is strongly attracted to and ordered at the electrode surface. This ordering is different to the structure that might be imagined from continuum models of electrode interfaces. Further, this ordering significantly affects the probability of ions reaching the surface. We describe the concomitant motion and configurations of the water and ions as functions of the electrode potential, and we analyze the length scales over which ionic atmospheres fluctuate. The statistics of these fluctuations depend upon surface structure and ionic strength.The fluctuations are large, sufficiently so that the mean ionic atmosphere is a poor descriptor of the aqueous environment near a metal surface. The importance of this finding for a description of electrochemical reactions is examined by calculating, directly from the simulation, Marcus free energy profiles for transfer of charge between the electrode and a redox species in the solution and comparing the results with the predictions of continuum theories. Significant departures from the electrochemical textbook descriptions of the phenomenon are found and their physical origins are characterized from the atomistic perspective of the simulations.

Gels formed via metal–ligand coordination typically have very low branch functionality, f, as they consist of ∼2–3 polymer chains linked to single metal ions that serve as junctions. Thus, these materials are very soft and unable to withstand network defects such as dangling ends and loops. We report here a new class of gels assembled from polymeric ligands and metal-organic cages (MOCs) as junctions. The resulting ‘polyMOC’ gels are precisely tunable and may feature increased branch functionality. We show two examples of such polyMOCs: a gel with a low f based on a M2L4 paddlewheel cluster junction and a compositionally isomeric one of higher f based on a M12L24 cage. The latter features large shear moduli, but also a very large number of elastically inactive loop defects that we subsequently exchanged for functional ligands, with no impact on the gel's shear modulus. Such a ligand substitution is not possible in gels of low f, including the M2L4-based polyMOC.

Charge-transfer (CT) states, bound combinations of an electron and a hole on separate molecules, play a crucial role in organic optoelectronic devices. We report direct nanoscale imaging of the transport of long-lived CT states in molecular organic donor-acceptor blends, which demonstrates that the bound electron-hole pairs that form the CT states move geminately over distances of 5-10 nm, driven by energetic disorder and diffusion to lower energy sites. Magnetic field dependence reveals a fluctuating exchange splitting, indicative of a variation in electron-hole spacing during diffusion. The results suggest that the electron-hole pair of the CT state undergoes a stretching transport mechanism analogous to an 'inchworm' motion, in contrast to conventional transport of Frenkel excitons. Given the short exciton lifetimes characteristic of bulk heterojunction organic solar cells, this work confirms the potential importance of CT state transport, suggesting that CT states are likely to diffuse farther than Frenkel excitons in many donor-acceptor blends.