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

Abstract

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.

Modulating the reactivity of chromone and its derivatives through encapsulation in a self-assembled phenylethynylene bis-urea host

“…Previous work demonstrated that the $0.9 nm diameter nanochannels of the self-assembled phenylethynylene bis-urea host are accessible to gases including Xe and CO 2 [27,30]. The channels can also accommodate a range of organic guests including coumarins, acenaphthylene and stilbenes [21,27]. Confinement of guests within the nanochannels of host 1 facilitated the selective [2 + 2] photodimerization reactions of coumarin, 6-methyl coumarin, 7-methyl coumarin and acenaphthylene in good conversion.…”

“…In comparison, stilbenes and 7-methoxy coumarin were unreactive. We turned to GCMC simulations to probe the origin of these changes in reactivity and selectivity, which afforded good predictions for these simple aromatic guests [27]. Here, we test utility and scope of the previously employed GCMC simulation protocol to predict if chromone and its derivatives (1) will be absorbed by this host and (2) will be reactive inside the confined space of the host.…”

“…These crystal structure parameters were imported into the MAPS program and the coordinates for the nitrobenzene guests were removed. The chemical potentials (m) of the guests were calculated as previously described [27]. Next, 1 Â10 6 step Grand Canonical (mVT) Monte Carlo simulations were conducted for each guest using the Dreiding force field with predetermined guest chemical potentials (m).…”

“…All GCMC simulations were performed using the Monte Carlo for Complex Chemical Systems (MCCCS) Towhee plug-in built into Scienomics' Materials Processes and Simulations (MAPS) platform [23] as previously reported [27]. Each guest was built using the amorphous builder within MAPS and their chemical potentials were calculated on a systems containing 100 guest molecules via a 5 Â 10 4 step canonical MC simulation with MCCS Towhee and using the Widom insertion method and the Dreiding force field [28].…”

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

“…5À12 Phenylether bis-urea (1) and phenylethynylene bis-urea (2) self-assemble into similar columnar structures held together by hydrogen bonds with distances of 2.85À3.05 Å, which set a repeat distance of between 4.6 and 4.7 Å. 6,10 The cross-sectional size and shape of the columns differ as they are precisely determined by the molecular structure of the macrocycle building blocks. In 1, the cavity is seen to be roughly elliptical in shape with cross-sectional dimensions of ∼4.8 Å Â 3.7 Å, while in 2, the pore space cross-section is almost round with a diameter of ∼9 Å.…”