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.

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.Surprisingly few fluorescent probes emit in the near-IR region with high quantum yields. 1, 2 Such compounds are valuable for intracellular imaging because autofluorescence in cells tends to obscure emissions at wavelengths below approximately 550 nm, but this factor becomes less of an issue at longer wavelengths. Probes that emit in the 750 -900 nm region are, therefore, relatively easy to visualize in vivo. 3 Cyanine dyes are currently the most widely used probes for this wavelength region, but their quantum yields and photostabilities are sub-optimal. 4 Thus there is need for new fluorescent probes that emit efficiently above 750 nm. Dyes based on the 5-6-5 fused BODIPY ring system A are popular in biotechnology because they tend to have relatively sharp fluorescence emission characteristics, and high quantum yields. 5 However, the emission wavelengths of most BODIPY dyes are between 530 and 630 nm, and this is sub-optimal for intracellular or tissue imaging. Previous work by Burgess et al ,6-8 and others, 9,10 successfully explored a modification strategy for shifting BODIPYfluorescence emissions to the red ( Figure 1). Adaptation of the core fluorophore scaffold by intramolecular B-O ring formation to produce the 6,6-5-6-5-6,6 ring system B gave a ca 65 nm bathochromatic shift (compare C and D), 11 but the fluorescence emission wavelengths were less than 700 nm. 12O'Shea and co-workers have demonstrated that BF 2 chelated tetraaryl-substituted azadipyrromethenes E, fluoresce with wavelength maxima between 670-720 nm with paraoriented electron donating groups being particularly effective in enhancing the red-shifted fluorescence, eg F (Figure 2). 13-19 This is close to the 750 -1,400 nm region at which light permeation through tissue is most effective. Subsequently, it was shown that derivatives with extended aromatic groups do emit in near-IR region. 20 The premise of this paper is that it would be advantagous to "hybridize" the structure B and E to shift their emissions beyond 750 nm. On the basis of the structures shown in Figure 1 it was therefore logical to introduce an intramolecular B,O-chelate on a tetraaryl aza-BODIPY scaffold eg G. The studies described here feature compound 1 of this type; it emerges that they can emit in this region.Scheme 1 shows one approach to the synthesis of the target class 1 from the precursors 2a-d which can be synthesized in four steps from commercially available materials. 21 A solid state X-ray structure determination of 2a illustrated that both 2-MeOC 6 H 4 rings lie out of the plane of the central chromophore with torsion angles of 42.7° and 61.4° (Supporting Information). Compounds 1 were obtained from the corresponding 2 by treatment with boron tribromide in dichloromethane. We anticipated that demethylation of the...

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.

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

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.

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.

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.

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.