Polymerization of monomers into periodic two-dimensional networks provides structurally precise, layered macromolecular sheets that exhibit desirable mechanical, optoelectronic, and molecular transport properties. Two-dimensional covalent organic frameworks (2D COFs) offer broad monomer scope but are generally isolated as powders comprising aggregated nanometer-scale crystallites. We found that 2D COF formation could be controlled using a two-step procedure in which monomers are added slowly to preformed nanoparticle seeds. The resulting 2D COFs are isolated as single-crystalline, micrometer-sized particles. Transient absorption spectroscopy of the dispersed COF nanoparticles revealed improvement in signal quality by two to three orders of magnitude relative to polycrystalline powder samples, and suggests exciton diffusion over longer length scales than those obtained through previous approaches. These findings should enable a broad exploration of synthetic 2D polymer structures and properties.

Significant interest exists in lead trihalides that present the perovskite structure owing to their demonstrated potential in photovoltaic, lasing, and display applications. These materials are also notable for their unusual phase behavior often displaying easily accessible phase transitions. In this work, time-resolved X-ray diffraction, performed on perovskite cesium lead bromide nanocrystals, maps the lattice response to controlled excitation fluence. These nanocrystals undergo a reversible, photoinduced orthorhombic-to-cubic phase transition which is discernible at fluences greater than 0.34 mJ cm−2 through the loss of orthorhombic features and shifting of high-symmetry peaks. This transition recovers on the timescale of 510 ± 100 ps. A reversible crystalline-to-amorphous transition, observable through loss of Bragg diffraction intensity, occurs at higher fluences (greater than 2.5 mJ cm−2). These results demonstrate that light-driven phase transitions occur in perovskite materials, which will impact optoelectronic applications and enable the manipulation of non-equilibrium phase characteristics of the broad perovskite material class.

Covalent organic frameworks (COFs) are highly modular porous crystalline polymers that are of interest for applications such as charge‐storage devices, nanofiltration membranes, and optoelectronic devices. COFs are typically synthesized as microcrystalline powders, which limits their performance in these applications, and their limited solubility precludes large‐scale processing into more useful morphologies and devices. We report a general, scalable method to exfoliate two‐dimensional imine‐linked COF powders by temporarily protonating their linkages. The resulting suspensions were cast into continuous crystalline COF films up to 10 cm in diameter, with thicknesses ranging from 50 nm to 20 μm depending on the suspension composition, concentration, and casting protocol. Furthermore, we demonstrate that the film fabrication process proceeds through a partial depolymerization/repolymerization mechanism, providing mechanically robust films that can be easily separated from their substrates.

Highly crystalline, monodisperse, imine-linked covalent organic framework nanoparticles were obtained under Sc(OTf)3-catalyzed conditions and enlarged by a slow monomer addition technique that prevents secondary nucleation.

Large singlet exciton diffusion lengths are a hallmark of high performance in organic-based devices such as photovoltaics, chemical sensors, and photodetectors. In this study, exciton dynamics of a two-dimensional covalent organic framework, 2D COF-5, is investigated using ultrafast spectroscopic techniques. After photoexcitation, the COF-5 exciton decays via three pathways: (1) excimer formation (4 ± 2 ps), (2) excimer relaxation (160 ± 40 ps), and (3) excimer decay (>3 ns). Excitation fluence-dependent transient absorption studies suggest that COF-5 has a relatively large diffusion coefficient (0.08 cm2/s). Furthermore, exciton–exciton annihilation processes are characterized as a function of COF-5 crystallite domain size in four different samples, which reveal domain-size-dependent exciton diffusion kinetics. These results reveal that exciton diffusion in COF-5 is constrained by its crystalline domain size. These insights indicate the outstanding promise of delocalized excitonic processes available in 2D COFs, which motivate their continued design and implementation into optoelectronic devices.

This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/smll.adma202002812.

One-dimensional nanostructures such as carbon nanotubes and actin filaments rely on strong and directional interactions to stabilize their high aspect ratio shapes. This requirement has precluded making isolated, long, thin organic nanotubes by stacking molecular macrocycles, as their noncovalent stacking interactions are generally too weak. Here we report high aspect ratio (>10), lyotropic nanotubes of stacked, macrocyclic, iminium salts, which are formed by protonation of the corresponding imine-linked macrocycles. Iminium ion formation establishes cohesive interactions that, in organic solvent (tetrahydrofuran), are two orders of magnitude stronger than the neutral macrocycles, as explained by physical arguments and demonstrated by molecular dynamics simulations. Nanotube formation stabilizes the iminium ions, which otherwise rapidly hydrolyze, and is reversed and restored upon addition of bases and acids. Acids generated by irradiating a photoacid generator or sonicating chlorinated solvents also induced nanotube assembly, allowing these nanostructures to be coupled to diverse stimuli, and, once assembled, they can be fixed permanently by cross-linking their pendant alkenes. As large macrocyclic chromonic liquid crystals, these iminium salts are easily accessible through a modular design and provide a means to rationally synthesize structures that mimic the morphology and rheology of carbon nanotubes and biological tubules.

The synthesis of periodic two-dimensional (2D) polymers and characterization of their optoelectronic behaviors are challenges at the forefront of polymer chemistry and materials science. Recently, we showed that layered 2D polymers known as 2D covalent organic frameworks (COFs) can be synthesized as single crystals by preparing COF particles as colloidal suspensions. Here we expand this approach from the condensation of boronic acids and catechols to the dehydrative trimerization of polyboronic acids. The resulting boroxine-linked colloids are the next class of 2D COFs to be obtained as single-crystalline particles, as demonstrated here for four 2D COFs and one 3D COF. Colloidal stabilization enables detailed structural analysis by synchrotron X-ray diffraction and high-resolution transmission electron microscopy. Solution fluorescence spectroscopy revealed that the COF crystallites are highly emissive compared to their respective monomer solutions. Excitation–emission matrix fluorescence spectroscopy indicated that the origin of this enhanced emission can be attributed to through-space communication of chromophores between COF sheets. These observations will motivate the development of colloidal COF systems as a platform to organize functional aromatic systems into precise and predictable assemblies with emergent properties.