Molecular dynamics simulations have been carried out for liquid water between flat hydrophobic surfaces. The surfaces produce density oscillations that extend at least 10 Å into the liquid, and significant molecular orientational preferences that extend at least 7 Å into the liquid. The liquid structure nearest the surface is characterized by ‘‘dangling’’ hydrogen bonds; i.e., a typical water molecule at the surface has one potentially hydrogen-bonding group oriented toward the hydrophobic surface. This surface arrangement represents a balance between the tendencies of the liquid to maximize the number of hydrogen bonds on the one hand, and to maximize the packing density of the molecules on the other. A detailed analysis shows that the structural properties of the liquid farther from the surface can be understood as effects imposed by this surface structure. These results show that the hydration structure of large hydrophobic surfaces can be very different from that of small hydrophobic molecules.

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.

The structure and dynamics of liquid water at the interface with three solid surfaces has been investigated via molecular dynamics simulation. The three surfaces include a flat hydrophobic surface, an atomically rough hydrophobic surface, and a contrasting, hydrophilic, fully hydroxylated silica surface. The results of analysis show that, as expected, the solvent near each of the two hydrophobic surfaces behaves essentially equivalently, with loss of hydrogen bonding at the interface. For the hydroxylated surface, surface–solvent hydrogen bonding is stronger than interactions in the bulk solvent, with the nearest solvent layer interacting specifically with up to three surface hydroxyl groups. Nevertheless, distinct structural perturbation of the solvent extends in every case no more than about 10 Å from the surface, and the perturbation is only strong in the immediate solvation layer. Furthermore, the corresponding dynamical perturbation of the solvent, as measured by the diffusion rates and reorientation times in comparison to the bulk, is always relatively small. For the hydrophilic case, it is largest, but even here it is less than a factor of 5 at the immediate interface and less than a factor of 2 in the second hydration layer. The residence time for solvent at the interface is found to be insensitive to the hydrophilicity of the surface. Calculated nuclear magnetic resonance (NMR) order parameters for the solvent are found to reflect solvent orientational ordering, but are shown not to be distinctive of the nature of that order.

The optical, electronic and mechanical properties of synthetic and biological materials consisting of polymer chains depend sensitively on the conformation adopted by these chains. The range of conformations available to such systems has accordingly been of intense fundamental as well as practical interest, and distinct conformational classes have been predicted, depending on the stiffness of the polymer chains and the strength of attractive interactions between segments within a chain. For example, flexible polymers should adopt highly disordered conformations resembling either a random coil or, in the presence of strong intrachain attractions, a so-called 'molten globule'. Stiff polymers with strong intrachain interactions, in contrast, are expected to collapse into conformations with long-range order, in the shape of toroids or rod-like structures. Here we use computer simulations to show that the anisotropy distribution obtained from polarization spectroscopy measurements on individual poly[2-methoxy-5-(2'-ethylhexyl)oxy-1,4-phenylenevinylene] polymer molecules is consistent with this prototypical stiff conjugated polymer adopting a highly ordered, collapsed conformation that cannot be correlated with ideal toroid or rod structures. We find that the presence of so-called 'tetrahedral chemical defects', where conjugated carbon-carbon links are replaced by tetrahedral links, divides the polymer chain into structurally identifiable quasi-straight segments that allow the molecule to adopt cylindrical conformations. Indeed, highly ordered, cylindrical conformations may be a critical factor in dictating the extraordinary photophysical properties of conjugated polymers, including highly efficient intramolecular energy transfer and significant local optical anisotropy in thin films.

We perform systematic molecular dynamics simulations of water confined between two nanoscale plates at T = 300K. We investigate the effect of pressure (-0.15 GPa< or = P< or =0.2GPa) and plate separation (0.4 nm < or =d < or =1.6 nm) on the phase behavior of water when the plates are either hydrophobic or hydrophilic. When water is confined between hydrophobic plates, capillary evaporation occurs between the plates at low enough P. The threshold value of d at which this transition occurs decreases with P (e.g., 1.6 nm at P approximately equal to -0.05 GPa, 0.5 nm at P approximately equal to 0.1 GPa), until, at high P, no capillary evaporation occurs. For d approximately equal to 0.6 nm and P > or =0.1 GPa, the system crystallizes into a bilayer ice. A P-d phase diagram showing the vapor, liquid, and bilayer ice phases is proposed. When water is confined by hydrophilic (hydroxylated silica) plates, it remains in the liquid phase at all P and d studied. Interestingly, we observe for this case that even at the P at which bulk water cavitates, the confined water remains in the liquid state. We also study systematically the state of hydration at different P for both kinds of plates. For the range of conditions studied here, we find that in the presence of hydrophobic plates the effect of P is to enhance water structure and to push water molecules toward the plates. The average orientation of water molecules next to the hydrophobic plates does not change upon pressurization. In contrast, in the presence of hydrophilic plates, water structure is insensitive to P. Hence, our results suggest that upon pressurization, hydrophobic plates behave as "soft" surfaces (in the sense of accommodating pressure-dependent changes in water structure) while hydrophilic walls behave as "hard" surfaces.