Organic solar cells typically employ only two organic semiconductors: a p-type Donor and a n-type Acceptor. Due to the intrinsic narrow absorption width of organic Donors, the binary solar cells exhibit a noticeably poor light-harvesting capability, which limits their highest efficiency achieved today to ~ 10 %. Ternary solar cells that mix two or more Donors of different absorption features, on the other hand, enjoy both an increased light absorption width, and an easy fabrication process associated with their simple structures. However, their fundamental working principles are still under investigation
In a Perspective, published in the Journal of Physical Chemistry Letters, investigators in the You Group offer their insights on the major governing mechanisms in these intriguing ternary solar cells. Through careful analyses of exemplary cases, they summarize the advantages and limitations of the three major mechanisms: charge transfer, energy transfer, and parallel-linkage. Further, they identify a few worthy future directions for these ternary solar cells. For example, incorporating singlet fission or upconversion materials into the energy transfer dominant ternary solar cells can theoretically breach the S-Q limit of single junction solar cells. This Perspective assures researchers working in this area that the feedback loop between fundamental understanding of mechanisms and materials selection will accelerate the efficiency improvement of these ternary solar cells.
Published in JACS, researchers in the Papanikolas and Waters groups, in collaboration with members of the Meyer group at Carolina Chemistry and the Papoian Group at the University of Maryland, describe how solid-phase peptide synthesis has been applied to the preparation of phosphonate-derivatized oligoproline assemblies containing two different RuII polypyridyl chromophores coupled via "click" chemistry.
In water or methanol the assembly adopts the polyproline II (PPII) helical structure, which brings the chromophores into close contact. Excitation of the assembly is followed by rapid, efficient intra-assembly energy transfer to the inner RuII. The oligoproline/click chemistry approach holds great promise for the preparation of interfacial assemblies for energy conversion based on a family of assemblies having controlled compositions and distances between key functional groups.
Work entirely designed, implemented, and interpreted by UNC undergraduates has been published in Biochemistry and is highlighted on the journal web page. Many viruses encode their genetic information in RNA molecules and these RNAs can have complex structures that are essential for efficient replication. The all-undergraduate team developed a model for the genome of the satellite tobacco mosaic virus, which is roughly the "hydrogen atom" of RNA viruses.
The UNC undergraduates discovered that the RNA genome has a complex higher-order structure with three domains, each of which corresponds to an essential viral function. This work is likely to broadly inform our understanding of the role of genome structure in the infectivity and pathogenesis of many RNA viruses, including those that infect humans.
The work was carried out as part of the UNC Undergraduate Transcriptome Project, an NSF-funded program developed in the Weeks Laboratory, designed to help undergraduates explore their potential for independent creativity, to fuel their passion for science, and to be a model for engaging undergraduates in a research university.
Tetrahydrofuran rings are common structural elements present in numerous biologically active naturally occurring molecules, including a number of lignans and polyether antibiotics. Perhaps owing to their prevalence in natural products, there have been a number of direct catalytic synthetic methods devised to construct this motif. Common strategies include carbonyl ylide dipolar cycloadditions, the Prins-Pinacol reaction, the Oshima–Utimoto reaction, and Lewis acid catalyzed [3+2] cycloadditions of donor–acceptor cyclopropanes and aldehydes.
Published in Angewandte Chemie, the Nicewicz Group now reports on the development of a new organocatalytic synthetic method for the construction of tetrahydrofurans employing simple and readily available allylic alcohols and alkenes. The reaction is catalyzed by a commercially available organic single electron photooxidant coupled with a redox-active hydrogen atom donor, and the method provides the direct synthesis of valuable tetrahydrofurans from common organic reagents. In combining successive polar and radical steps, polar-radical-crossover reactions have the potential to become a powerful strategy for future development of reactions relying on multiple bond-forming events.
Medicinal application of many complex natural products is precluded by the impracticality of their chemical synthesis. Pactamycin, the most structurally intricate aminocyclopentitol antibiotic, displays potent antiproliferative properties across multiple phylogenetic domains, but it is highly cytotoxic. A limited number of analogs produced by genetic engineering technologies show reduced cytotoxicity against mammalian cells, renewing promise for therapeutic applications.
For decades, an efficient synthesis of pactamycin amenable to analog derivatizations has eluded researchers. Published in Science, the Johnson Group now present a short asymmetric total synthesis of pactamycin. An enantioselective Mannich reaction and symmetry-breaking reduction sequence was designed to enable assembly of the entire carbon core skeleton in under five steps and control critical three-dimensional, stereochemical, functional group relationships. This modular route totals 15 steps and is immediately amenable for structural analog synthesis.
In a multi-institutional collaboration, members of the Sheiko Group report in JACS on the synthesis and characterization of a complex polymeric architecture based on a block copolymer with a cylindrical brush block and a single-chain polymeric nanoparticle block folded due to strong intramolecular hydrogen-bonds. The self-assembly of these constructs on mica surfaces was studied with atomic force microscopy, corroborating the distinct presence of block copolymer architectures.
The article received attention in C&EN, where Stephen Ritter describes how advances in polymer synthetic techniques now allow better control over the size and shape of polymers, allowing researchers to think more like architects, dreaming up exotic new polymer designs. One goal of the work from the Sheiko Group is to create macromolecules with functional properties for drug delivery, catalysis, chemical sensing, and other applications.
Most cellular RNA molecules function properly only when they fold into the correct three-dimensional shape. RNA folding is facilitated by helper molecules called chaperones. Chaperones cause some RNAs interact faster, induce other RNAs to change conformation, and work simultaneously across large distances. It was not clear how chaperones could have such wide-ranging molecular properties.
RNA structures contain base pairs, mostly involving guanosine-cytosine and adenosine-uridine pairs, that are stabilized by three versus two hydrogen bonds, respectively. Sometimes, due to the three bonds, the stronger guanosine-cytosine pairs get "stuck" when an RNA folds. Using chemical microscope technologies, invented in the Weeks laboratory, graduate student Jake Grohman discovered that RNA chaperones simply weaken stable three-bond pairs containing guanosine. In this way, RNA chaperones smooth out folding rough spots.
Because of its simplicity and potential universality, this mechanism has broad implications for understanding nucleic acid structure and RNA folding. The work is currently available online and will be published by the journal Science.
Researchers in the Papanikolas Group, in collaboration with colleagues in the Cahoon Group, both here at Carolina Chemistry, have developed a pump–probe microscope capable of exciting a single semiconductor nanostructure in one location and probing it in another with both high spatial and temporal resolution. Their findings are published in NanoLetters.
Experiments performed on Si nanowires enable a direct visualization of the charge cloud produced by photoexcitation at a localized spot as it spreads along the nanowire axis. The time-resolved images show clear evidence of rapid diffusional spreading and recombination of the free carriers, which is consistent with ambipolar diffusion and a surface recombination velocity of 104 cm/s. The free carrier dynamics are followed by trap carrier migration on slower time scales.
Researchers in the Meyer Group, in collaboration with colleagues from UNC's Department of Physics and Astronomy, RTI International, and Rutgers University, used orthorhombic Nb2O5 nanocrystalline films functionalized with [Ru(bpy)2(4,4′-(PO3H2)2bpy)]2+ as the photoanode in dye-sensitized photoelectrosynthesis cells (DSPEC) for hydrogen generation. As published in the journal Chemistry of Materials, they undertook a set of experiments to establish key properties—conduction band, trap state distribution, interfacial electron transfer dynamics, and DSPEC efficiency, to develop a general protocol for future semiconductor evaluation and for comparison with other wide-band-gap semiconductors.
The investigators found that, for a T-phase orthorhombic Nb2O5 nanocrystalline film, the conduction band potential is slightly positive (<0.1 eV), relative to that for anatase TiO2. Anatase TiO2 has a wide distribution of trap states including deep trap and band-tail trap states. Orthorhombic Nb2O5 is dominated by shallow band-tail trap states. Trap state distributions, conduction band energies, and interfacial barriers appear to contribute to a slower back electron transfer rate, lower injection yield on the nanosecond time scale, and a lower open-circuit voltage (Voc) for orthorhombic Nb2O5, compared to anatase TiO2. In an operating DSPEC, with the ethylenediaminetetraacetic tetra-anion (EDTA4–) added as a reductive scavenger, H2 quantum yield and photostability measurements show that Nb2O5 is comparable, but not superior, to TiO2.
Transport of single molecules in nanochannels or nanoslits might be used to identify them via their transit (flight) times. As the first of a series of articles resulting from a highly multi-disciplinary project, involving several institutions across the US and in Korea, sponsored by the NIH and the Korean Government and published in the Journal of Physical Chemistry B, members of the Soper Group present molecular dynamics simulations of transport of single deoxynucleotide 5'-monophoshates (dNMP) in aqueous solution under pressure-driven flow, to average velocities between 0.4 and 1.0 m/s, in 3 nm wide slits with hydrophobic walls. The simulation results show that, while moving along the slit, the mononucleotides are adsorbed and desorbed from the walls multiple times.
For the simulations, the estimated minimum slit length required for separation of the dNMP flight time distributions is about 5.9 μm, and the minimum analysis time per dNMP is about 10 μs. These are determined by the nature of the nucleotide–wall interactions, channel width, and by the flow characteristics. A simple analysis using realistic dNMP velocities shows that, in order to reduce the effects of diffusional broadening and keep the analysis time per dNMP reasonably small, the nucleotide velocity should be relatively high. Tailored surface chemistry could lead to further reduction of the analysis time toward its minimum value for a given driving force.
Researchers in the Waters Group, as described in an article published in JACS, utilized dynamic combinatorial chemistry to identify a novel small molecule receptor, A2D, for asymmetric dimethyl arginine, aRMe2, which is a post-translational modification, PTM, in proteins. It is known to play a role in a number of diseases, including spinal muscular atrophy, leukemia, lymphoma, and breast cancer.
The receptor exhibits 2.5–7.5-fold selectivity over the isomeric symmetric dimethyl arginine, depending on the surrounding sequence, with binding affinities in the low micromolar range. The affinity and selectivity of A2D for the different methylated states of Arg parallels that of proteins that bind to these PTMs. Characterization of the receptor–PTM complex indicates that cation−π interactions provide the main driving force for binding, loosely mimicking the binding mode found in the recognition of dimethyl arginine by native protein receptors.
Catalytic transformations of C1 feedstocks are a key foundation of the chemical industry. Formic acid is a C1 species that is especially difficult to convert to more valuable products. Formic acid is also readily produced from renewable resources such as CO2 or biomass. New transformations of formic acid are therefore needed to promote development of renewable C1 chemistry; conversion to methanol would represent a renewable route to a major commodity chemical and high energy density fuel. In 1911, Sabatier and Mailhe reported that some dimethoxymethane was produced upon thermolysis of formic acid over thorium oxide, thereby providing indirect evidence of methanol production. Given the great interest in the facile interconversion of various C1 chemicals, it is remarkable that one hundred years have passed without further reports on this matter.
A team of investigators, including the Miller Group, has set out to uncover new routes to methanol as part of the NSF Center for Enabling New Technologies Through Catalysis (CENTC). Published in Angewandte Chemie, that team now reports that a molecular iridium species catalyzes the disproportionation of formic acid to methanol, water, and CO2. This study represents the first well-defined example of such a reaction mode of formic acid. Methanol is produced under mild, aqueous conditions, without the use of any organic solvents or hydrogen gas.
Published in Chemical Science, and also highlighted in C&EN, the Lin Group prepared three metal–organic frameworks (MOFs) of the UiO-68 network topology and investigated for sorption of uranium from water and artificial seawater. The stable and porous phosphorylurea-derived MOFs were shown to be highly efficient in sorbing uranyl ions, with saturation sorption capacities as high as 217 mg U g−1 which is equivalent to binding one uranyl ion for every two sorbent groups.
Coordination modes between uranyl groups and simplified phosphorylurea motifs were investigated by DFT calculations, revealing a thermodynamically favorable monodentate binding of two phosphorylurea ligands to one uranyl ion. Convergent orientation of phosphorylurea groups at appropriate distances inside the MOF cavities is believed to facilitate their cooperative binding with uranyl ions. This work represents the first application of MOFs as novel sorbents to extract actinide elements from aqueous media.
Published in Macromolecules, Jason Rochette in the Ashby Group describes how the synthesis of a library of poly(ester urethane)s (PEUs) containing pendant photoresponsive moieties afforded through the incorporation of one of two novel bifunctional monomers resulted in degradable materials with a range of tunable thermal and mechanical properties.
Examination of these materials under physiological conditions displayed tunable degradation with rates faster than PCL-based materials, and initial biocompatibility studies exhibited negligible cytotoxicity for HeLa cells based on results of ATP assay. The ability to tune thermal properties also allowed specific polymer compositions to boast transition temperatures within a range of applicable temperature for thermal shape memory.
Photoinduced electrocyclic ring opening reactions in conjugated cycloalkenes are among the most elementary processes in organic chemistry. It is known that extremely fast sub-100fs internal conversion transitions precede ring opening in simple derivatives of cyclohexadiene; however, these dynamics had never been directly measured in solution because of insufficient time resolution. In an article published in New Journal of Physics, the Moran Group investigates α-terpinine using a variety of four-wave mixing spectroscopies with extraordinary 20fs time resolution in the deep UV spectral range. Overall, the study supports a picture in which photoexcitation initiates a wavepacket that undergoes several recurrences in C=C stretching coordinates before twisting of the ring sets the molecule free from the Franck-Condon region of the ππ* potential energy surface. It is thought that the momentum imparted within the first 100fs govern products yields by steering the system into particular relaxation channels.
The optical analogues of NMR carried out in this work utilize a specialized spectrometer operational in the deep UV spectra range. Extraordinary time resolution is achieved with techniques inspired by the optical physics community in which noble gases are used as media for laser pulse generation. In addition to elementary organic photochemistries, this unprecedented time resolution has been leveraged to study excited state deactivation processes in DNA, which are partly responsible for biological photoprotection. Technical aspects of the experiments and recent applications are discussed in an invited Perspective Article published in the Journal of Physical Chemistry Letters.
Materials capable of dynamically controlling surface chemistry and topography are highly desirable. Sarah Brosnan, Andrew Brown and Professor Valerie Ashby, as published in JACS, have designed a system that is uniquely able to remotely control the presented functionality and geometry at a given time by using a functionalizable shape memory material.
The group accomplished this by incorporating controlled amounts of an azide-containing monomer into a shape memory polymeric material. These materials are capable of physically changing surface geometry over a broad range of length scales from >1 mm to 100 nm. Using copper-assisted click chemistry, they can be functionalized with a variety of molecules to yield different surfaces. Combining these features gave materials that can change both the presented geometry and functionality at tunable transition temperatures.
Photoresists are light-sensitive resins used in a variety of technological applications. In most applications, however, photoresists are generally used as sacrificial layers or a structural layer that remains on the fabrication substrate. Researchers in the Allbritton Group, as described in the Journal of Micromechanics and Microengineering, have fabricated thin layers of patterned 1002F photoresist released to form a freestanding film. Films of thickness in the range of 4.5–250 µm were patterned with through-holes to a resolution of 5 µm and an aspect ratio of up to 6:1. Photoresist films could be reliably released from the substrate after a 12 h immersion in water. The Young's modulus of a 50 µm-thick film was 1.43 ± 0.20 GPa.
The investigators demonstrated the use of the films as stencils for patterning sputtered metal onto a surface, and these 1002F stencils were used multiple times without deterioration in feature quality. Furthermore, the films provided biocompatible, transparent surfaces of low autofluorescence on which cells could be grown. Culture of cells on a film with an isolated small pore enabled a single cell to be accessed through the underlying channel and loaded with exogenous molecules independently of nearby cells. Thus 1002F photoresist was patterned into thin, flexible, free-standing films that will have numerous applications in the biological and MEMS fields.