Free energy of metal-organic framework self-assembly.

Metal-organic frameworks (MOFs) represent an important class of materials. Careful selection of building blocks allows for tailoring of the properties of the resulting framework. The self-assembly process, however, is not understood, and without detailed knowledge of the underlying molecular mechanism, it is difficult to anticipate whether a particular design can be realized, or whether the material adopts a metastable, kinetically arrested state. We present a detailed examination of early-stage self-assembly pathways of the MOF-5. Enhanced sampling techniques are used to model a self-assembly in an explicit solvent (dimethylformamide, DMF). We identify several free energy barriers encountered during the assembly of the final MOF, which arise from structural rearrangements preceding MOF formation and from disrupted MOF-solvent interactions as formation proceeds. In all cases considered here, MOFs exhibit favorable entropic gains during the assembly. More generally, the strategy presented provides a step toward the experimental design characterizing the formation of ordered frameworks and possible sources of polymorphism.

Tenfold increase in the photostability of an azobenzene guest in vapor-deposited glass mixtures.

Improvements to the photostability of organic glasses for use in electronic applications have generally relied on the modification of the chemical structure. We show here that the photostability of a guest molecule can also be significantly improved-without chemical modification-by using physical vapor deposition to pack molecules more densely. Photoisomerization of the substituted azobenzene, 4,4'-diphenyl azobenzene, was studied in a vapor-deposited glass matrix of celecoxib. We directly measure photoisomerization of trans- to cis-states via Ultraviolet-visible (UV-Vis) spectroscopy and show that the rate of photoisomerization depends upon the substrate temperature used during co-deposition of the glass. Photostability correlates reasonably with the density of the glass, where the optimum glass is about tenfold more photostable than the liquid-cooled glass. Molecular simulations, which mimic photoisomerization, also demonstrate that photoreaction of a guest molecule can be suppressed in vapor-deposited glasses. From the simulations, we estimate that the region that is disrupted by a single photoisomerization event encompasses approximately 5 molecules.

Photostability Can Be Significantly Modulated by Molecular Packing in Glasses.

While previous work has demonstrated that molecular packing in organic crystals can strongly influence photochemical stability, efforts to tune photostability in amorphous materials have shown much smaller effects. Here we show that physical vapor deposition can substantially improve the photostability of organic glasses. Disperse Orange 37 (DO37), an azobenzene derivative, is studied as a model system. Photostability is assessed through changes in the density and molecular orientation of glassy thin films during light irradiation. By optimizing the substrate temperature used for deposition, we can increase photostability by a factor of 50 relative to the liquid-cooled glass. Photostability correlates with glass density, with density increases of up to 1.3%. Coarse-grained molecular simulations, which mimic glass preparation and the photoisomerization reaction, also indicate that glasses with higher density have substantially increased photostability. These results provide insights that may assist in the design of organic photovoltaics and light-emission devices with longer lifetimes.

Electronic structure at coarse-grained resolutions from supervised machine learning.

Computational studies aimed at understanding conformationally dependent electronic structure in soft materials require a combination of classical and quantum-mechanical simulations, for which the sampling of conformational space can be particularly demanding. Coarse-grained (CG) models provide a means of accessing relevant time scales, but CG configurations must be back-mapped into atomistic representations to perform quantum-chemical calculations, which is computationally intensive and inconsistent with the spatial resolution of the CG models. A machine learning approach, denoted as artificial neural network electronic coarse graining (ANN-ECG), is presented here in which the conformationally dependent electronic structure of a molecule is mapped directly to CG pseudo-atom configurations. By averaging over decimated degrees of freedom, ANN-ECG accelerates simulations by eliminating backmapping and repeated quantum-chemical calculations. The approach is accurate, consistent with the CG spatial resolution, and can be used to identify computationally optimal CG resolutions.

Influence of Vapor Deposition on Structural and Charge Transport Properties of Ethylbenzene Films.

Organic glass films formed by physical vapor deposition exhibit enhanced stability relative to those formed by conventional liquid cooling and aging techniques. Recently, experimental and computational evidence has emerged indicating that the average molecular orientation can be tuned by controlling the substrate temperature at which these "stable glasses" are grown. In this work, we present a comprehensive all-atom simulation study of ethylbenzene, a canonical stable-glass former, using a computational film formation procedure that closely mimics the vapor deposition process. Atomistic studies of experimentally formed vapor-deposited glasses have not been performed before, and this study therefore begins by verifying that the model and method utilized here reproduces key structural features observed experimentally. Having established agreement between several simulated and experimental macroscopic observables, simulations are used to examine the substrate temperature dependence of molecular orientation. The results indicate that ethylbenzene glasses are anisotropic, depending upon substrate temperature, and that this dependence can be understood from the orientation present at the surface of the equilibrium liquid. By treating ethylbenzene as a simple model for molecular semiconducting materials, a quantum-chemical analysis is then used to show that the vapor-deposited glasses exhibit decreased energetic disorder and increased magnitude of the mean-squared transfer integral relative to isotropic, liquid-cooled films, an effect that is attributed to the anisotropic ordering of the molecular film. These results suggest a novel structure-function simulation strategy capable of tuning the electronic properties of organic semiconducting glasses prior to experimental deposition, which could have considerable potential for organic electronic materials design.