Putting Cocrystal Stoichiometry to Work: A Reactive Hydrogen-Bonded "Superassembly" Enables Nanoscale Enlargement of a Metal-Organic Rhomboid via a Solid-State Photocycloaddition.

Enlargement of a self-assembled metal-organic rhomboid is achieved via the organic solid state. The solid-state synthesis of an elongated organic ligand was achieved by a template directed [2 + 2] photodimerization in a cocrystal. Initial cocrystals obtained of resorcinol template and reactant alkene afforded a 1:2 cocrystal with the alkene in a stacked yet photostable geometry. Cocrystallization performed in the presence of excess template resulted in a 3:2 cocrystal composed of novel discrete 10-component hydrogen-bonded "superassemblies" wherein the alkenes undergo a head-to-head [2 + 2] photodimerization. Isolation and reaction of elongated photoproduct with Cu(II) ions afforded a metal-organic rhomboid of nanoscale dimensions that hosts small molecules in the solid state as guests.

Polyladderane Constructed from a Gemini Monomer through Photoreaction.

Polyladderane, the first polymer to contain the ladderane functional group, was synthesized from a gemini monomer through photoreaction in the solid state. The modular design of the gemini monomers used to create polyladderane allowed specific structural modification, resulting in the formation of two distinct polymer products. Monomers were synthesized by connecting two photoreactive units, either sorbic acids (monomer I) or 2-furanacrylic acids (monomer II), with a 1,4-butanediol linker. Single-crystal X-ray diffraction analysis of the monomers confirmed that they packed in the desired head-to-tail orientation and within a viable distance for photoreaction by electronically complementary interaction. Pre-organized gemini monomers were irradiated with UV light and monitored by FT-IR. Two polyladderanes with cis,anti,cis-[3]-ladderane as a characteristic functional group were constructed stereospecifically in 24-36 hours.

A biorenewable cyclobutane-containing building block synthesized from sorbic acid using photoenergy.

A novel cyclobutane-containing diacid building block, CBDA-3, was synthesized from sorbic acid using clean, efficient [2 + 2] photocycloaddition. This photoreaction can be performed using commercially available germicidal lamps, which represent a form of ECO-UV. SC-XRD showed that the cyclobutane ring in CBDA-3 has a unique semi-rigid character, unlike more rigid aromatic rings or more flexible types of aliphatic rings. C=C bonds present in the structure of CBDA-3 provide opportunities for derivatization which could be used to alter the characteristics of polymers made from this monomer. Additionally, TGA and DSC analysis showed CBDA-3 to have excellent thermal stability. These characteristics make CBDA-3 a promising building block with the potential to be used as a sustainable alternative to traditional petroleum-derived diacids. Finally, a facile and reliable Fischer esterification of CBDA-3 was performed to tune its melting point and solubility for different applications and to demonstrate the applicability of this building block in polymer synthesis.

A recyclable thermoset with built-in thermocleavable group developed from a cis-cyclobutane-1,2-dicarboxylic acid.

A novel class of recyclable thermoset has been developed from cis-3,4-diphenylcyclobutane-1,2-dicarboxylic acid (CBDA-4) due to its thermocleavability at high temperature. This key CBDA-4 building block was synthesized from ß-trans-cinnamic acid using a [2+2] photocycloaddition reaction. CBDA-4 was subsequently linked with glycerol via esterification to give a thermoset with Tg of 68 °C. The thermoset was heated to 300 °C to analyze its degradation. A key intermediate was successfully obtained after purification of the degraded polymer. NMR, FT-IR, HRMS, and single crystal X-ray diffraction confirmed the intermediate was glycerol cinnamate, which was the result of splitting cyclobutane in the polymer backbone at high temperature. Glycerol cinnamate was readily hydrolyzed reforming the starting materials glycerol and trans-cinnamic acid to complete the recycling loop.

Development of Red-Emissive Porphyrin Graphene Quantum Dots (PGQDs) for Biological Cell-Labeling Applications.

Red and near-infrared emission is a highly desirable feature for fluorescent nanoparticles in biological applications mainly due to longer wavelengths more easily being able to deeply penetrate tissues, organs, skin, and other organic components, while less autofluorescence interference would be produced. Additionally, graphene quantum dots (GQDs) that contain unique optical and electrical features have been targeted for their use in cell labeling applications as well as environmental analysis. Their most desirable features come in the form of low toxicity and biocompatibility; however, GQDs are frequently reported to have blue or green emission light and not the more advantageous red/NIR emission light. Furthermore, porphyrins are a subgroup of heterocyclic macrocycle organic compounds that are also naturally occurring pigments in nature that already contain the desired red-emission fluorescence. Therefore, porphyrins have been used previously to synthesize nanomaterials and for nanoparticle doping in order to incorporate the red/NIR emission light property into particles that otherwise do not contain the desired emission light. Meso-tetra(4-carboxyphenyl)porphine (TCPP) is one type of porphyrin with a large conjugated π-electron system and four carboxyl groups on its exterior benzene rings. These two key characteristics of TCPP make it ideal for incorporation into GQDs, as it would design and synthesize red-emissive material as well as give rise to excellent water solubility. In this work, TCPP is used in tangent with cis-cyclobutane-1,2-dicarboxylic acid (CBDA-2), a biomass derived organic molecule, to synthesize "green" porphyrin-based graphene quantum dots (PGQDs) with red-emission. The obtained PGQDs were characterized by various analytical methods. Utilizing TEM, HRTEM, and DLS the size distribution of the particles was determined to be 7.9 ± 4.1, well within the quantum dot range of 2-10 nm. FT-IR, XPS, and XRD depicted carbon, nitrogen, and oxygen as the main elemental components with carbon being in the form of graphene and the main porphyrin ring of TCPP remaining present in the final PGQDs product. Lastly, absorption and fluorescence spectroscopy determined the excitation wavelength at 420 nm and the emission at 650 nm which was successfully utilized in the imaging of HeLa cells using confocal microscopy.

Synthesis of Highly Near-Infrared Fluorescent Graphene Quantum Dots Using Biomass-Derived Materials for In Vitro Cell Imaging and Metal Ion Detection.

Graphene quantum dots (GQDs) are a subset of fluorescent nanomaterials that have gained recent interest due to their photoluminescence properties and low toxicity and biocompatibility features for bioanalysis and bioimaging. However, it is still a challenge to prepare highly near-infrared (NIR) fluorescent GQDs using a facile pathway. In this study, NIR GQDs were synthesized from the biomass-derived organic molecule cis-cyclobutane-1,2-dicarboxylic acid via one-step pyrolysis. The resulting GQDs were then characterized by various analytical methods such as UV-Vis absorption spectroscopy, fluorescence spectroscopy, dynamic light scattering, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Moreover, the photostability and stability over a wide pH range were also investigated, which indicated the excellent stability of the prepared GQDs. Most importantly, two peaks were found in the fluorescence emission spectra of the GQDs, one of which was located in the NIR region of about 860 nm. Finally, the GQDs were applied for cell imaging with human breast cancer cell line, MCF-7, and cytotoxicity analysis with mouse macrophage cell line, RAW 246.7. The results showed that the GQDs entered the cells through endocytosis on the fluorescence images and were not toxic to the cells up to a concentration of 200 µg/mL. Thus, the developed GQDs could be a potential effective fluorescent bioimaging agent. Finally, the GQDs depicted fluorescence quenching when treated with mercury metal ions, indicating that the GQDs could be used for mercury detection in biological samples as well.

Substitution reactions at tetracoordinate boron: synthesis of N-heterocyclic carbene boranes with boron-heteroatom bonds.

Boryl halide, carboxylate and sulfonate complexes of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (dipp-Imd-BH(2)X, X = halide or sulfonate) have been prepared from the parent borane dipp-Imd-BH(3) by (1) substitution reactions with R-X (X = halide or sulfonate), (2) reactions with electrophiles (like I(2) or NIS), or (3) acid/base reactions with HX (provided that HX has a pK(a) of about 2 or less). Dipp-Imd-BH(2)I is most conveniently prepared by reaction with diiodine while dipp-Imd-BH(2)OTf is best prepared by reaction with triflic acid. These and other less reactive complexes behave as electrophiles and can be substituted by a wide range of heteroatom nucleophiles including halides, thiolates and other sulfur-based nucleophiles, isocyanate, azide, nitrite, and cyanide. The resulting products are remarkably stable, and many have been characterized by X-ray crystallography. Several are members of very rare classes of functionalized boron compounds (boron azide, nitro compound, nitrous ester, etc.).

EPR studies of the generation, structure, and reactivity of n-heterocyclic carbene borane radicals.

N-Heterocyclic carbene boranes (NHC-boranes) are a new "clean" class of reagents suitable for reductive radical chain transformations. Their structures are well suited for their reactivity to be tuned by inclusion of different NHC ring units and by appropriate placement of diverse substituents. EPR spectra were obtained for the boron-centered radicals generated on removal of one of the BH(3) hydrogen atoms. This spectroscopic data, coupled with DFT computations, demonstrated that the NHC-BH(2)* radicals are planar pi-delocalized species. tert-Butoxyl radicals abstracted hydrogen atoms from NHC-boranes more than 3 orders of magnitude faster than did C-centered radicals, although the rate decreased markedly for sterically shielded NHC-BH(3) centers. Combinations of two NHC-boryl radicals afforded 1,2-bis-NHC-diboranes at rates which also depended strongly on steric shielding. The termination rate increased to the diffusion-controlled limit for sterically unhindered NHC-boryls. Bromine atoms were rapidly transferred to imidazole-based NHC-boryl radicals from alkyl, allyl, and benzyl bromides. Chlorine-atom abstraction was, however, much less efficient and only observed for sterically unhindered NHC-boryls reacting with allylic and benzylic chlorides. For an NHC-borane containing a bulky thexyl substituent at boron, the tertiary H atom of the thexyl group was selectively removed. The resulting beta-boron-containing alkyl radical rapidly underwent beta scission of the B-C bond with production of an NHC-boryl radical and an alkene.

Second-generation tags for fluorous chemistry exemplified with a new fluorous Mitsunobu reagent.

A new fluorous DEAD reagent bearing two perfluoro-tert-butyloxy groups with propylene spacers shows excellent promise for use in fluorous Mitsunobu reactions. Pure target products were obtained in good yields after removing fluorous byproducts by FSPE. The new reagent serves as a prototype for a greener second generation of fluorous reagents bearing tags that are not expected to degrade in the environment to compounds that are highly persistent or that bioaccumulate in higher organisms.

CBS reductions with a fluorous prolinol immobilized in a hydrofluoroether solvent.

A fluorous prolinol precatalyst bearing only 34 fluorine atoms has been immobilized in the hydrofluoroether solvent HFE-7500. The CBS reduction of acetophenone proceeded rapidly, in high yield and in high ee in the absence of any organic solvent. The organic product was stripped from the HFE-7500 phase with a polar solvent, and the HFE-7500 phase was reused "as is" with satisfactory results through eight runs. This process is an attractive prototype for the large-scale use, recovery, and reuse of fluorous organocatalysts.

Synthesis of deep-cavity fluorous calix[4]arenes as molecular recognition scaffolds.

Several lower-rim perfluoroalkylated (fluorous) calix[4]arenes have been synthesized by O-alkylation of the parent calix[4]arene. The compounds are formed in the cone conformation. They are soluble in several fluorous solvents and show promise for use in sensing, selective extractions and other applications.

Scalable preparation and property investigation of a cis-cyclobutane-1,2-dicarboxylic acid from ß-trans-cinnamic acid.

Scalable synthesis of ß-truxinic acid (CBDA-4) was accomplished by capturing and photodimerizing a metastable crystalline solid of trans-cinnamic acid. This synthetic approach builds a foundation for investigating the properties and applications of the useful diacid. The X-ray crystal structure of CBDA-4 was determined for the first time. The cyclobutane ring in CBDA-4 was cleaved upon heating, making it a promising building block for thermally recyclable/degradable materials.

New fluorous/organic biphasic systems achieved by solvent tuning.

Miscibility tests between sixty pairs of fluorous and organic solvents have been performed, and a number of biphasic systems based on hydrofluoroether solvents have been identified. Mutual solubilities of a series of fluorous and organic solvents have been measured to ascertain the compositions of the biphasic systems. A qualitative solvent tuning strategy based on solvent polarity and fluorophilicity/phobicity is introduced. Solvent tuning is then used to modulate the partition coefficients (P) of triarylphosphines with 0-3 fluorous tags. The results lay a foundation for future applications of these and related biphasic systems in catalysis and extraction.

Cyclobutane-1,3-Diacid (CBDA): A Semi-Rigid Building Block Prepared by [2+2] Photocyclization for Polymeric Materials.

A previously overlooked building block, cyclobutane-1,3-diacid (CBDA), is introduced to materials synthesis due to its great potentials. As an example of CBDA, α-truxillic acid or 2,4-diphenylcyclobutane-1,3-dicarboxylic acid, was readily synthesized from commercially available trans-cinnamic acid. This CBDA showed outstanding stability both in sunlight and upon heating. While its two carboxylic acid groups can be readily utilized in connecting with other molecules to form new materials, the cyclobutane ring was able to tolerate acid and base treatments showing good chemical stability. A series of cyclobutane-containing polymers (CBPs), namely poly-α-truxillates, were obtained by condensation between α-truxillic acid and diols including ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-petanediol, and 1,6-hexanediol. The structures of these poly-α-truxillates were analyzed by NMR, FT-IR, and HRMS. Powder X-ray diffraction results of the poly-α-truxillates indicated that they are semi-crystalline materials. Preliminary thermal, chemical, and photochemical tests showed that the poly-α-truxillates exhibited comparable stabilities to PET.

A recyclable fluorous organocatalyst for Diels-Alder reactions.

Chiral fluorous imidazolidinone catalyst 2 provides consistently high enantioselectivities in Diels-Alder reactions of dienes and alpha, beta-unsaturated aldehydes. The catalyst can be readily separated from the reaction products by fluorous solid-phase extraction, and recovered in excellent purity for direct reuse.

Synthesis of polymeric ladders by topochemical polymerization.

Two polymeric ladders were synthesized by topochemical polymerization. The critical assemblies with multiple reactive centers were characterized by single crystal X-ray diffraction. Approximately 64% and 70% of the mass of the two polymeric ladders can be derived from biomass, respectively.

A two-dimensional hydrogen bonded organic framework self-assembled from a three-fold symmetric carbamate.

A flexible organic sheet spontaneously assembled under mild conditions from a tri-carbamate. By introducing cyclohexyl side chains as 'pillars' between the sheets, a hydrogen bonded organic framework was formed which displayed a capacity for accommodating and releasing guest molecules. The guest size/shape selectivity of the lamellar framework was demonstrated by testing with various guests.

Shape-tunable hollow silica nanomaterials based on a soft-templating method and their application as a drug carrier.

A one-step soft-templating method for synthesizing shape-tunable hollow silica nanomaterials was developed in a reliable and highly reproducible way. For the first time, both nonspherical and spherical shapes with hollow interiors, including nanowire, nanospheres, and nanotadpole, were successfully obtained by simply changing the solvent. Poly(vinylpyrrolidone) (PVP)-water droplets were used as soft templates for the formation of hollow structures, while three different solvents, including 1-propanol, 1-pentanol, and ethanol, led to the designed shapes. It was found that the solvent, the formation of PVP-water droplets, the amount of ammonia, and the reaction time had great effects on the morphology of synthesized hollow nanomaterials. The effect of various factors on the morphology was systematically studied to propose a growth mechanism. The obtained hollow silica nanomaterials showed excellent reproducibility and great potential for a large-scale synthesis. Finally, the application of the developed hollow silica nanomaterials was demonstrated using the hollow spherical silica nanoparticles. Its drug-carrying ability was studied. The results could be extended for doping various target molecules into the hollow structures for a broad range of applications.