Benjamin T. King

A two-dimensional polymer prepared by organic synthesis

Synthetic polymers are widely used materials, as attested by a production of more than 200 millions of tons per year, and are typically composed of linear repeat units. They may also be branched or irregularly crosslinked. Here, we introduce a two-dimensional polymer with internal periodicity composed of areal repeat units. This is an extension of Staudingers polymerization concept (to form macromolecules by covalently linking repeat units together), but in two dimensions. A well-known example of such a two-dimensional polymer is graphene, but its thermolytic synthesis precludes molecular design on demand. Here, we have rationally synthesized an ordered, non-equilibrium two-dimensional polymer far beyond molecular dimensions. The procedure includes the crystallization of a specifically designed photoreactive monomer into a layered structure, a photo-polymerization step within the crystal and a solvent-induced delamination step that isolates individual two-dimensional polymers as free-standing, monolayered molecular sheets.

A nanoporous two-dimensional polymer by single-crystal-to-single-crystal photopolymerization

In contrast to the wide number and variety of available synthetic routes to conventional linear polymers, the synthesis of two-dimensional polymers and unambiguous proof of their structure remains a challenge. Two-dimensional polymers-single-layered polymers that form a tiling network in exactly two dimensions-have potential for use in nanoporous membranes and other applications. Here, we report the preparation of a fluorinated hydrocarbon two-dimensional polymer that can be exfoliated into single sheets, and its characterization by high-resolution single-crystal X-ray diffraction analysis. The procedure involves three steps: preorganization in a lamellar crystal of a rigid monomer bearing three photoreactive arms, photopolymerization of the crystalline monomers by [4 + 4] cycloaddition, and isolation of individual two-dimensional polymer sheets. This polymer is a molecularly thin (~1 nm) material that combines precisely defined monodisperse pores of ~9 Å with a high pore density of 3.3 × 10(13) pores cm(-2). Atomic-resolution single-crystal X-ray structures of the monomer, an intermediate dimer and the final crystalline two-dimensional polymer were obtained and prove the single-crystal-to-single-crystal nature and molecular precision of the two-dimensional photopolymerization.

A Two-Dimensional Polymer from the Anthracene Dimer and Triptycene Motifs

A two-dimensional polymer (2DP) based on the dimerization of anthraceno groups arranged in a triptycene motif is reported. A photoinduced polymerization is performed in the crystalline state and gives a lamellar 2DP via a crystal-to-crystal (but not single-crystal to single-crystal) transformation. Solvent-induced exfoliation provides monolayer sheets of the 2DP. The 2DP is considered to be a tiling, a mathematical approach that facilitates structural elucidation.

The Largest Synthetic Structure with Molecular Precision: Towards a Molecular Object

Pushing the limits: A 200A - 10 Da structurally defined, linear macromolecule (PG5) has a molar mass, cross-section dimension, and cylindrical shape that are comparable to some naturally occurring objects, such as amyloid fibrils or certain plant viruses. The macromolecule is resistant against flattening out on a surface; the picture shows PG5 embracing the tobacco mosaic virus (TMV).

Synthesis of a covalent monolayer sheet by photochemical anthracene dimerization at the air/water interface and its mechanical characterization by AFM indentation

Covalent monolayer sheets in 2 hours: spreading of threefold anthracene-equipped shape-persistent and amphiphilic monomers at the air/water interface followed by a short photochemical treatment provides access to infinitely sized, strictly monolayered, covalent sheets with in-plane elastic modulus in the range of 19 N/m.

Large Area Synthesis of a Nanoporous Two-Dimensional Polymer at the Air/Water Interface

We present the synthesis of a two-dimensional polymer at the air/water interface and its nm-resolution imaging. Trigonal star, amphiphilic monomers bearing three anthraceno groups on a central triptycene core are confined at the air/water interface. Compression followed by photopolymerization on the interface provides the two-dimensional polymer. Analysis by scanning tunneling microscopy suggests that the polymer is periodic with ultrahigh pore density.

Quadrannulene: A Nonclassical Fullerene Fragment

The presence of nonhexagonal rings in an otherwise graphitic lattice induces curvature. Pentagons are common—twelve pentagons surrounded by hexagons make up C60. Larger rings are present in Stone–Wales defects, and the polyhedral formula of Euler mandates their existence in carbon nanotube Y-junctions. Except for a single, partially saturated example, four-membered rings in graphitic structures are, however, unknown. The smallest examples of these graphitic structures are the [n]circulenes, wherein a central n-sided polygon is surrounded by n-fused benzenoid rings. [7]Circulene, first prepared by Yamamoto, Nakazaki, and coworkers in 1983, is saddle shaped. [6]Circulene, or coronene, is the trivial, planar case, and it was first synthesized by Scholl andMeyer in 1932 but also occurs naturally. [5]Circulene, or corannulene, comprises 1/3 of the C60 skeleton and has been intensely studied, and it was first prepared by Lawton and Barth in 1971. Whereas a few pioneering attempts have been reported, [4]circulene has never been synthesized before. We report herein the preparation and characterization of a stable [4]circulene. By analogy with Lawton s naming of corannulene (Latin: cor, heart; annula, ring), we suggest the trivial name quadrannulene (Latin: quadra, square; annula, ring) for the [4]circulene parent. Hence, we name this derivative 1,8,9,16-tetrakis(trimethylsilyl)tetra-cata-tetrabenzoquadrannulene, abbreviated TMS4-TBQ. The IUPAC name and atom numbering are given in the Supporting Information. Our unoptimized five-step synthesis (Scheme1) provides TMS4-TBQ in very low yield. Hopf and co-workers recently summarized two synthetic strategies to the quadrannulene core: making the four-membered ring from [2,2]-paracyclo-

Two-dimensional polymers: concepts and perspectives

Creation of polymers comprised of repeat units that can create topologically planar macromolecules (rather than linear) has been the topic of several recent studies in the field of synthetic polymer chemistry. Such novel macromolecules, known as 2D polymers, are the result of advanced synthetic methodology which allows creation of monolayer sheets with a periodic internal structure and functional groups placed at predetermined sites under mild conditions. Given the promising potentials of 2D polymers, this feature paper aims at discussing the concept of these novel macromolecules from a topological viewpoint in Section 1. This is followed by spotlighting the expected behavior of 2D polymers in the context of polymer physics (entropy elasticity, strength, percolation, and persistence) and polymer chemistry (copolymers and growth kinetics) in Section 2. Section 3 delineates synthetic and analytical matters associated with 2D polymers followed by a brief final section highlighting the potential of these sheet-like macromolecules for application purposes. We hope this article will trigger the interest of chemists, physicists and engineers to help develop this encouraging new class of materials further such that societally relevant applications will be accessible in the market soon.

A Triphenylene-Based Triptycene with Large Free Volume Synthesized by Zirconium-Mediated Biphenylation

Biphenylation using (Li(THF)(4))(2) x Zr(biphe)(3) of hexabromotriptycenes bearing H (1-H) or Bu (1-Bu) at the bridgeheads gave triptycenes with triphenylene blades. The blades extend both perpendicular and parallel to the 3-fold axis and generate a large intramolecular free volume (IMFV) (1-H, AM1, 710 A(3); cf. triptycene, AM1 71 A(3)). Crystals of 1-H could not be obtained. Triptycene 1-Bu, in which the Bu groups fill the voids near the bridgehead, was crystalline. X-ray diffraction analysis revealed crystal packing with alternating, interlocked corrugated and distorted hexagonal layers.

t-Butyl Biphenylation of o-Dibromoarenes: A Route to Soluble Polycyclic Aromatic Hydrocarbons

Large, soluble polycyclic aromatic hydrocarbons (PAHs) have been synthesized using Zr-mediated and Stille-type biphenylation reactions. Both the Zr and Stille methodologies have been adopted to incorporate tert-butyl substituents, permitting the direct synthesis of alkylated PAHs that are much more soluble than their unsubstituted analogues. To demonstrate the utility of these methods and the importance of solubilizing functionality, several large PAHs were synthesized and crystallographically characterized. The scope of the Zr-mediated and Stille methodologies is shown to be complementary. The Stille methodology often gives higher yields but is ineffective for the introduction of strain and failed with some polybrominated arenes. In these difficult cases, the zirconium methodology is effective, albeit in low yields.