Hydrogen-Bonded Fibrous Nanotubes Assembled from Trigonal Prismatic Building Blocks.

In reticular chemistry, molecular building blocks are designed to create crystalline open frameworks. A key principle of reticular chemistry is that the most symmetrical networks are the likely outcomes of reactions, particularly when highly symmetrical building blocks are involved. The strategy of synthesizing low-dimensional networks aims to reduce explicitly the symmetry of the molecular building blocks. Here we report the spontaneous formation of hydrogen-bonded fibrous structures from trigonal prismatic building blocks, which were designed to form three-dimensional crystalline networks on account of their highly symmetrical structures. Utilizing different microscopic and spectroscopic techniques, we identify the structures at the early stages of the assembly process in order to and understand the growth mechanism. The symmetrical molecular building blocks are incorporated preferentially in the longitudinal direction, giving rise to anisotropic hydrogen-bonded porous organic nanotubes. Entropy-driven anisotropic growth provides micrometer-scale unidirectional nanotubes with high porosity. By combining experimental evidence and theoretical modeling, we have obtained a deep understanding of the nucleation and growth processes. Our findings offer fundamental insight into the molecular design of tubular structures. The nanotubes evolve further in the transverse directions to provide extended higher-order fibrous structures [nano- and microfibers], ultimately leading to large-scale interconnected hydrogen-bonded fiber-like structures with twists and turns. Our work provides fundamental understanding and paves the way for innovative molecular designs in low-dimensional networks.

Field-driven cluster formation in two-dimensional colloidal binary mixtures.

We study size- and charge-asymmetric oppositely charged colloids driven by an external electric field. The large particles are connected by harmonic springs, forming a hexagonal-lattice network, while the small particles are free of bonds and exhibit fluidlike motion. We show that this model exhibits a cluster formation pattern when the external driving force exceeds a critical value. The clustering is accompanied with stable wave packets in vibrational motions of the large particles.

Polycrystalline Covalent Organic Framework Films Act as Adsorbents, Not Membranes.

Covalent organic framework (COF) membranes are of great promise for energy-efficient separations. Thick, polycrystalline COF films have been reported to separate dyes, salts, bacteria, and nanoparticles on the basis of size-selective transport through ordered pores. Here, we show that these materials function as adsorbents, not as size-sieving membranes. Binding isotherms of several dyes typical of the COF membrane literature to three COF powder samples illustrate that COFs are high-capacity adsorbents with affinities that span a range of 3 orders of magnitude, trends which map onto previously reported separation behavior. Computational results suggest that observed differences in adsorption can be correlated to variable entropic gains driving the adsorption process. Polycrystalline COF pellets show volume-dependent and flow-rate dependent "rejection" of dyes, consistent with an adsorption-based removal mechanism. Previous reports of thick, polycrystalline COF membranes used low flow rates and small dye volumes to probe rejection capabilities, where membrane and adsorbent behavior is not distinguishable. A mixed dye separation experiment in flow shows affinity-dependent performance. These results necessitate a careful reexamination of the COF membrane literature, as separations based on differential transport through 2D COF pores remain an important yet unrealized frontier.