The incorporation of metal-organic frameworks into advanced devices remains a desirable goal, but progress is hindered by difficulties in preparing large crystalline metal-organic framework films with suitable electronic performance. We demonstrate the direct growth of large-area, high quality, and phase pure single metal-organic framework crystals through chemical vapor deposition of a dimolybdenum paddlewheel precursor, Mo2(INA)4. These exceptionally uniform, high quality crystals cover areas up to 8600 µm2 and can be grown down to thicknesses of 30 nm. Moreover, scanning tunneling microscopy indicates that the Mo2(INA)4 clusters assemble into a two-dimensional, single-layer framework. Devices are readily fabricated from single vapor-phase grown crystals and exhibit reversible 8-fold changes in conductivity upon illumination at modest powers. Moreover, we identify vapor-induced single crystal transitions that are reversible and responsible for 30-fold changes in conductivity of the metal-organic framework as monitored by in situ device measurements. Gas-phase methods, including chemical vapor deposition, show broader promise for the preparation of high-quality molecular frameworks, and may enable their integration into devices, including detectors and actuators.

2D molecular crystal lattices form a compelling class of 2D materials with synthetically-tunable properties and the potential to spawn a new generation of devices for optoelectronics and energy sustainability.

Hydrogen bonding between 1,3-bis ureas is a commonly used motif in the assembly of supramolecular structures such as gels, capsules and crystals. The title compound, 1,3-bis­(m-cyanophenyl)­urea (mCyPU), has previously been shown to crystallize in both an anhydrous and monohydrate phase (α and H–I). An expanded search for polymorphs and cocrystals of mCyPU revealed a much greater diversity of solid forms including three additional polymorphs (β, δ, ε), a second hydrate (H–II) and two cocrystal phases with dimethyl sulfoxide and triphenylphosphine oxide. Analysis of the single crystal structures obtained in this study shows that the typical 1-dimensional H-bonding between 1,3-bis urea groups is disrupted by the presence of other H-bond acceptors including cyano, water, sulfoxide and phosphine oxide functionalities. Re-examination of α-mCyPU additionally showed both blade and plate-like morphologies could be obtained from different growth solvents, with crystals of the latter morphology exhibiting a grain boundary migration prior to melting.

Diarylureas frequently assemble into structures with one-dimensional H-bonded chain motifs. Herein, we examine the ability of triphenylphosphine oxide (TPPO) to disrupt the H-bonding motif in 14 different meta-substituted N,N′-diphenylureas (mXPU) and form cocrystals; 1:1 mXPU:TPPO cocrystals were obtained in 9 of 14 cases examined (64% success rate). Cocrystals adopt five different lattice types, all of which show unsymmetrical H-bonded [R2 1(6)] dimers between the urea hydrogens and the phosphine oxygen. Heterodimer (mXPU···TPPO) and homodimer (mXPU···mXPU) interaction energies, ΔE int, calculated using density functional theory at the B3LYP/6-31G­(d,p) level were used to rationalize the experimental results. A clear trend was observed in which cocrystals were experimentally realized only in cases in which the differences in heterodimer versus homodimer energy, ΔΔE int, were greater than ∼5.3–6 kcal/mol. Although calculated interaction energies are a simplified measure of the system thermodynamics, these results suggest that the relative ΔΔE int between heterodimers and homodimers is a good predictor of cocrystal formation in this system.

Diphenylurea (PU) crystallizes via slow evaporation from a range of solvents as phase pure α-PU or in a concomitant mixture with the less stable β-PU phase. Sublimation onto glass also yields α-PU in high phase purity. In contrast, sublimation onto siloxane-coated glass templates with various terminal groups (e.g., isocyanate, acetate, bromine) resulted in primarily β-PU. Scanning electron microscopy showed that the individual crystals on siloxanes exhibit a range of different morphologies (e.g., plates, needles, hollow tubes, spirals) which appear to be influenced by the terminal siloxane functionality. Under extended sublimation times (>5 h), mixtures of β and α become apparent on most siloxanes. Grazing incidence X-ray diffraction studies showed that the β:α ratio changes as a function of distance from the siloxane, with more α-PU appearing at higher angles. Diffraction line intensities also indicate strong preferred orientations for β-PU crystallites within the polycrystalline layer with the fastest growth direction (b-axis) parallel to the template, but more varied orientations for α-PU. This suggests α-PU may nucleate elsewhere in the sublimation apparatus rather than on the β-coated siloxane. This study demonstrates the utility of templatedirected sublimation methods as a way to engineer metastable phases but also illustrates some of the challenges associated with achieving high phase purity on templates with limited surface area.