Supramolecular Self-Assembly of Engineered Polyproline Helices.

The ability to rationally design biomaterials to form desired supramolecular constructs presents an ever-growing research field, with many burgeoning works within recent years providing exciting results; however, there exists a broad expanse of promising avenues of research yet to be investigated. As such we have set out to make use of the polyproline helix as a rigid, tunable, and chiral ligand for the rational design and synthesis of supramolecular constructs. In this investigation, we show how an oligoproline tetramer can be specifically designed and functionalized, allowing predictable tuning of supramolecular interactions, to engineer the formation of supramolecular peptide frameworks with varying properties and, consequently, laying the groundwork for further studies utilizing the polyproline helix, with the ability to design desired supramolecular structures containing these peptide building blocks, having tunable structural features and functionalities.

A Reversibly Porous Supramolecular Peptide Framework.

The ability to use bio-inspired building blocks in the assembly of novel supramolecular frameworks is at the forefront of an exciting research field. Herein, we present the first polyproline helix to self-assemble into a reversibly porous, crystalline, supramolecular peptide framework (SPF). This framework is assembled from a short oligoproline, adopting the polyproline II conformation, driven by hydrogen-bonding and dispersion interactions. Thermal activation, guest-induced dynamic porosity and enantioselective guest inclusion have been demonstrated for this novel system. The principles of the self-assembly associated with this SPF will be used as a blueprint allowing for the further development of helical peptide linkers in the rational design of SPFs and metal-peptide frameworks.

Nanometer control in plasmonic systems through discrete layer-by-layer macrocycle-cation deposition.

In this work, we demonstrate that coordination interactions between Fe3+ and cucurbit[7]uril (CB[7]) can be utilised to build up defined nanoscale spacing layers in metallic nanosystems. We begin by characterising the layer-by-layer deposition of CB[7] and FeCl3·6H2O coordination layers through the use of a Quartz-Crystal Microbalance (QCM) and contact angle measurements. We then apply this layered structure to accurately control the spacing, and thus optical properties, of gold nanoparticles in a Nanoparticle-on-Mirror (NPoM) structure, which is demonstrated via normalising plasmon resonance spectroscopy.

Preferential binding of unsaturated hydrocarbons in aryl-bisimidazolium·cucurbit[8]uril complexes furbishes evidence for small-molecule π-π interactions.

Whilst cucurbit[n]urils (CBn) have been utilized in gas encapsulation, only the smaller CBn (n = 5 and 6) have utility given their small cavity size. In this work, we demonstrate that the large cavity of CB8 can be tailored for gaseous and volatile hydrocarbon encapsulation by restricting its internal cavity size with auxiliary aryl-bisimidazolium (Bis, aryl = phenyl, naphthyl, and biphenyl) guests. The binding constants for light hydrocarbons (C ≤ 4) are similar to those measured with CB6, while larger values are obtained with Bis·CB8 for larger guests. A clear propensity for higher affinities of alkenes relative to alkanes is observed, most pronounced with the largest delocalized naphthalene residue in the auxiliary Bis guest, which provides unique evidence for sizable small-molecule π-π interactions.

Plasmonic tunnel junctions for single-molecule redox chemistry.

Nanoparticles attached just above a flat metallic surface can trap optical fields in the nanoscale gap. This enables local spectroscopy of a few molecules within each coupled plasmonic hotspot, with near thousand-fold enhancement of the incident fields. As a result of non-radiative relaxation pathways, the plasmons in such sub-nanometre cavities generate hot charge carriers, which can catalyse chemical reactions or induce redox processes in molecules located within the plasmonic hotspots. Here, surface-enhanced Raman spectroscopy allows us to track these hot-electron-induced chemical reduction processes in a series of different aromatic molecules. We demonstrate that by increasing the tunnelling barrier height and the dephasing strength, a transition from coherent to hopping electron transport occurs, enabling observation of redox processes in real time at the single-molecule level.

Cucurbit[7]uril as a Supramolecular Artificial Enzyme for Diels-Alder Reactions.

The ability to mimic the activity of natural enzymes using supramolecular constructs (artificial enzymes) is a vibrant scientific research field. Herein, we demonstrate that cucurbit[7]uril (CB[7]) can catalyse Diels-Alder reactions for a number of substituted and unreactive N-allyl-2-furfurylamines under biomimetic conditions, without the need for protecting groups, yielding powerful synthons in previously unreported mild conditions. CB[7] rearranges the substrate in a highly reactive conformation and shields it from the aqueous environment, thereby mimicking the mode of action of a natural Diels-Alderase. These findings can be directly applied to the phenomenon of product inhibition observed in natural Diels-Alderase enzymes, and pave the way toward the development of novel, supramolecular-based green catalysts.

Aqueous interfacial gels assembled from small molecule supramolecular polymers.

The self-assembly of a stimuli-responsive aqueous supramolecular hyperbranched polymer from small molecules is reported. This system is composed of ditopic and tritopic guest-functionalised molecules that are able to form heteroternary supramolecular complexes with the macrocyclic host cucurbit[8]uril (CB[8]). We demonstrate that the supramolecular hyperbranched polymer formed is responsive to both photo- and chemical stimuli, exhibiting reversibility. Furthermore, this system is shown to assemble at liquid-liquid interfaces, which upon gelation, is observable on the micrometre scale. This self-healing supramolecular network can act as a soft matter barrier for aqueous microdroplets, inhibiting their coalescence.

Observing Single Molecules Complexing with Cucurbit[7]uril through Nanogap Surface-Enhanced Raman Spectroscopy.

In recent years, single-molecule sensitivity achievable by surface-enhanced Raman spectroscopy (SERS) has been widely reported. We use this to investigate supramolecular host-guest chemistry with the macrocyclic host cucurbit[7]uril, on a few-to-single-molecule level. A nanogap geometry, comprising individual gold nanoparticles on a planar gold surface spaced by a single layer of molecules, gives intense SERS signals. Plasmonic coupling between the particle and the surface leads to strongly enhanced optical fields in the gap between them, with single-molecule sensitivity established using a modification of the well-known bianalyte method. Changes in the Raman modes of the host molecule are observed when single guests included inside its cavity internally stretch it. Anisotropic intermolecular interactions with the guest are found which show additional distinct features in the Raman modes of the host molecule.

Mechanistic insight into the formation of tetraarylazadipyrromethenes.

The tetraarylazadipyrromethene chromophore class has gained increasing attention in the past decade for a diverse set of scientific interests and applications. The most direct synthetic route available for their generation is heating of 4-nitro-1,3-diarylbutan-1-ones with an ammonium source in an alcohol solvent. Despite the practical simplicity, the reaction pathway(s) for these conversions are lengthy and unclear. To gain insight into the steps involved, (15)N labeling experiments with MS and NMR analysis were utilized for conversion of 4-nitro-1,3-diphenylbutan-1-one 1 into tetraphenylazadipyrromethene 2 with (15)NH(4)OAc. To permit examination of later stages of the reaction sequence to 2, the (15)N-labeled potential intermediate 3,5-diphenyl-1H-pyrrol-2-amine 10 was synthesized. A study of the dimerization pathway utilizing (15)N-labeled 10 revealed an unprecedented nitrogen rearrangement in the final stages of the pathway involving a ring-opening/closing of a pyrrole ring. Utilizing (15)N labeling experiments we have shown that 2,4-diphenylpyrrole 8 can also react under the reaction conditions with 3,5-diphenyl-2H-pyrrol-2-imine 7 (from oxidation of 10) to produce 2. Overall in the conversion of 1 into 2, two related pathways are ongoing concurrently; the first involves a dimerization of 3,5-diphenyl-2H-pyrrol-2-imine 7, and the other a reaction of 7 with 2,4-diphenylpyrrole 8.

Cellular uptake mediated off/on responsive near-infrared fluorescent nanoparticles.

Fluorescence imaging, utilizing molecular fluorophores, often acts as a central tool for the investigation of fundamental biological processes and offers huge future potential for human imaging coupled to therapeutic procedures. An often encountered limitation with fluorescence imaging is the difficulty in discriminating nonspecific background fluorophore emission from a fluorophore localized at a specific region of interest. This limits imaging to individual time points at which background fluorescence has been minimized. It would be of significant advantage if the fluorescence output could be modulated from off to on in response to specific biological events as this would permit imaging of such events in real time without background interference. Here we report our approach to achieve this for the most fundamental of cellular processes, i.e. endocytosis. We describe a new near-infrared off to on fluorescence switchable nanoparticle construct that is capable of switching its fluorescence on following cellular uptake but remains switched off in extracellular environments. This permits continuous real-time imaging of the uptake process as extracellular particles are nonfluorescent. The principles behind the fluorescence off/on switch can be understood by encapsulation of particles in cellular organelles which effect a microenvironmental change establishing a fluorescence signal.

BF2-azadipyrromethenes: probing the excited-state dynamics of a NIR fluorophore and photodynamic therapy agent.

BF(2)-Azadipyrromethene dyes are a promising class of NIR emitter (nonhalogenated) and photosensitizer (halogenated). Spectroscopic studies on a benchmark example of each type, including absorption (one and two photon), time-resolved transient absorption (ps-ms) and fluorescence, are reported. Fast photodynamics reveal that intense nanosecond NIR fluorescence is quenched in a brominated analog, giving rise to a persistent (21 µs) transient absorption signature. Kinetics for these changes are determined and ascribed to the efficient population of a triplet state (72%), which can efficiently sensitize singlet oxygen formation (ca. 74%), directly observed by (1)Δ(g) luminescence. Photostability measurements reveal extremely high stability, notably for the nonhalogenated variant, which is at least 10(3)-times more stable (Φ(photodeg.) = < 10(-8)) than some representative BODIPY and fluorescein dyes.

New on-bead near-infrared fluorophores and fluorescent sensor constructs.

The facile synthesis and photophysical characterization of new on-bead fluorophores and fluorescent sensors are described. The unique covalent immobilization strategy results in highly fluorescent beads with sharp emission profiles between 650 and 800 nm. Illustrative examples include imaging in an aqueous cellular environment and adaptation to include off/on sensing functionality, proven by a prototypical detection of gaseous HCl.

Co(II), Ni(II), Cu(II) and Zn(II) complexes of tetraphenylazadipyrromethene.

The synthesis, crystallographic and spectroscopic properties of four divalent isomorphous metal complexes of tetraphenylazadipyrromethene are described.

B,O-chelated azadipyrromethenes as near-IR probes.

Complementary synthetic routes to a new class of near-IR fluorophores are described. These allow facile access (four synthetic steps) to the core fluorophore and substituted derivatives with emissions between 740 and 780 nm in good quantum yields.