Linda S. Shimizu

Self-Assembled Phenylethynylene Bis-urea Macrocycles Facilitate the Selective Photodimerization of Coumarin

There is much interest in designing molecular sized containers that influence and facilitate chemical reactions within their nanocavities. On top of the advantages of improved yield and selectivity, the studies of reactions in confinement also give important clues that extend our basic understanding of chemical processes. We report here, the synthesis and self-assembly of an expanded bis-urea macrocycle to give crystals with columnar channels. Constructed from two C-shaped phenylethynylene units and two urea groups, the macrocycle affords a large pore with a diameter of ∼9 Å. Despite its increased size, the macrocycles assemble into columns with high fidelity to afford porous crystals. The porosity and accessibility of these channels have been demonstrated by gas adsorption studies and by the uptake of coumarin to afford solid inclusion complexes. Upon UV-irradiation, these inclusion complexes facilitate the conversion of coumarin to its anti-head-to-head (HH) photodimer with high selectivity. This is contrary to what is observed upon the solid-state irradiation of coumarin, which affords photodimers with low selectivity and conversion.

Thermal Reaction of a Columnar Assembled Diacetylene Macrocycle

Reported is a macrocyclic diacetylene that assembled into columns to afford porous crystals. Heating this assembly initiated a topochemical polymerization of the preorganized diacetylene units to give covalent conjugated polydiacetylenes. These stable conjugated materials maintained permanent porosity as evidenced by their type I gas adsorption isotherms with CO(2) (g). Such conjugated polymeric nanotubes could possess unusual properties for sensing and electronics.

Functional Materials from Self-Assembled Bis-urea Macrocycles

CONSPECTUS: This Account highlights the work from our laboratories on bis-urea macrocycles constructed from two C-shaped spacers and two urea groups. These simple molecular units assembled with high fidelity into columnar structures guided by the three-centered urea hydrogen bonding motif and aryl stacking interactions. Individual columns are aligned and closely packed together to afford functional and homogeneous microporous crystals. This approach allows for precise and rational control over the dimensions of the columnar structure simply by changing the small molecular unit. When the macrocyclic unit lacks a cavity, columnar assembly gives strong pillars. Strong pillars with external functional groups such as basic lone pairs can expand like clays to accept guests between the pillars. Macrocycles that contain sizable interior cavities assemble into porous molecular crystals with aligned, well-defined columnar pores that are accessible to gases and guests. Herein, we examine the optimal design of the macrocyclic unit that leads to columnar assembly in high fidelity and probe the feasibility of incorporating a second functional group within the macrocycles. The porous molecular crystals prepared through the self-assembly of bis-urea macrocycles display surface areas similar to zeolites but lower than MOFs. Their simple one-dimensional channels are well-suited for studying binding, investigating transport, diffusion and exchange, and monitoring the effects of encapsulation on reaction mechanism and product distribution. Guests that complement the size, shape, and polarity of the channels can be absorbed into these porous crystals with repeatable stoichiometry to form solid host-guest complexes. Heating or extraction with an organic solvent enables desorption or removal of the guest and subsequent recovery of the solid host. Further, these porous crystals can be used as containers for the selective [2 + 2] cycloadditions of small enones such as 2-cyclohexenone or 3-methyl-cyclopentenone, while larger hosts bind and facilitate the photodimerization of coumarin. When the host framework incorporates benzophenone, a triplet sensitizer, UV-irradiation in the presence of oxygen efficiently generates singlet oxygen. Complexes of this host were employed to influence the selectivity of photooxidations of 2-methyl-2-butene and cumene with singlet oxygen. Small systematic changes in the channel and bound reactants should enable systematic evaluation of the effects of channel dimensions, guest dimensions, and channel-guest interactions on the processes of absorption, diffusion, and reaction of guests within these nanochannels. Such studies could help in the development of new materials for separations, gas storage, and catalysis.

Examination of the structural features that favor the columnar self-assembly of bis-urea macrocycles.

Second-generation self-assembling bis-urea macrocycles were designed that form columnar structures in the solid state. The new macrocycles were constructed from more flexible building blocks yielding greater solubility and a more efficient synthesis. In addition, heteroatoms in the form of ether oxygens were incorporated in the walls of the macrocycles to provide additional recognition sites for guest encapsulation. We observed reduced fidelity of the stacking motif and in some cases the intermolecular urea-urea hydrogen bonds were disrupted by the formation of intramolecular hydrogen bonds. We also observed new offset assembly motifs that maintained the urea-urea interaction. These results suggest that the stacking of the arene units in the rigid first-generation systems was an important factor in guiding the formation of the columnar stacks.

Inclusion of electrochemically active guests by novel oxacalixarene hosts

We demonstrate for the first time the utility of oxacalixarenes as hosts and investigate the forces that influence the thermodynamics of binding.

Control of the Intramolecular [2+2] Photocycloaddition in a Bis-Stilbene Macrocycle

The intramolecular [2+2] photocyclization of a bis-stilbene macrocycle was studied. The reaction can be controlled by the insertion and removal of urea protecting groups. Upon UV-irradiation in both the solid state and DMSO solution, the free urea macrocycle undergoes a cis-trans photoisomerization that is followed by a [2+2] cycloaddition to afford a single product in high yield. The presence of the triazinanone urea protecting groups does not alter the cis-trans photoisomerization but greatly decreases the selectivity of the photocycloaddition step.

Assembled Columnar Structures from bis-urea Macrocycles

Presented is a review of our research on using macrocyclic bis-ureas as supramolecular building blocks. These macrocycles predictably self-assemble into columnar structures via strong urea–urea hydrogen bonds and pi–stacking interactions. We developed a facile synthetic route to these macrocyclic ureas, confirmed their self-assembly pattern, and are now assessing their potential as a supramolecular building block. A series of bis-urea macrocycles were synthesized and assembled to verify the fidelity of their self-assembly motifs. Ultimately, a large phenylether bis-urea macrocycle was synthesized that formed tubular assemblies containing a guest accessible channel. We have characterized the structure of the assembled nanotubes by NMR and X-ray crystallography and evaluated this new porous solid with respect to its binding properties and specificity. This porous self-assembled material is thermally and chemically stable and can reversibly bind and exchange a variety of guest molecules.

Crystalline Bis-urea Nanochannel Architectures Tailored for Single-File Diffusion Studies.

Urea is a versatile building block that can be modified to self-assemble into a multitude of structures. One-dimensional nanochannels with zigzag architecture and cross-sectional dimensions of only ∼3.7 Å × 4.8 Å are formed by the columnar assembly of phenyl ether bis-urea macrocycles. Nanochannels formed by phenylethynylene bis-urea macrocycles have a round cross-section with a diameter of ∼9.0 Å. This work compares the Xe atom packing and diffusion inside the crystalline channels of these two bis-ureas using hyperpolarized Xe-129 NMR. The elliptical channel structure of the phenyl ether bis-urea macrocycle produces a Xe-129 powder pattern line shape characteristic of an asymmetric chemical shift tensor with shifts extending to well over 300 ppm with respect to the bulk gas, reflecting extreme confinement of the Xe atom. The wider channels formed by phenylethynylene bis-urea, in contrast, present an isotropic dynamically average electronic environment. Completely different diffusion dynamics are revealed in the two bis-ureas using hyperpolarized spin-tracer exchange NMR. Thus, a simple replacement of phenyl ether with phenylethynylene as the rigid linker unit results in a transition from single-file to Fickian diffusion dynamics. Self-assembled bis-urea macrocycles are found to be highly suitable materials for fundamental molecular transport studies on micrometer length scales.

Applications of a Bis-Urea Phenylethynylene Self-Assembled Nanoreactor for [2 + 2] Photodimerizations

Confined environments can be used to alter the selectivity of a reaction by influencing the organization of the reactants, altering the mobility of trapped molecules, facilitating one reaction pathway or selectively stabilizing the products. This manuscript utilizes a series of potentially photoreactive guests to interrogate the utility of the one-dimensional nanochannels of a porous host to absorb and facilitate the reaction of encapsulated guests. The host is a columnar self-assembled phenylethynylene bis-urea macrocycle, which absorbs guests, including coumarin, 6-methyl coumarin, 7-methyl coumarin, 7-methoxy coumarin, acenaphthylene, cis-stilbene, trans-stilbene, and trans-β-methylstyrene to afford crystalline inclusion complexes. We examine the structure of the host:guest complexes using powder X-ray diffraction, which suggests that they are well-ordered highly crystalline materials. Investigations using solid-state cross-polarized magic angle spinning (13)C{(1)H}CP-MAS NMR spectroscopy indicate that the guests are mobile relative to the host. Upon UV-irradiation, we observed selective photodimerization reactions for coumarin, 6-methyl coumarin, 7-methyl coumarin, and acenaphthylene, while the other substrates were unreactive even under prolonged UV-irradiation. Grand Canonical Monte Carlo simulations suggest that the reactive guests were close paired and preorganized in configurations that facilitate the photodimerization with high selectivity while the unreactive guests did not exhibit similar close pairing. A greater understanding of the factors that control diffusion and reaction in confinement could lead to the development of better catalysts.

Self-assembly of a bis-urea macrocycle into a columnar nanotube

A bis-urea macrocycle 1 was synthesized and shown to form extended nanotubular columns by X-ray crystallography.