Local Environments Created by the Ligand Coating of Nanoparticles and Their Implications for Sensing and Surface Reactions.

ConspectusThe ligand shells of colloidal nanoparticles (NPs) can serve different purposes. In general, they provide colloidal stability by introducing steric repulsion between NPs. In the context of biological applications, the ligand shell plays a critical role in targeting, enabling NPs to achieve specific biodistributions. However, there is also another important feature of the ligand shell of NPs, namely, the creation of a local environment differing from the bulk of the solvent in which the NPs are dispersed. It is known that charged ligand shells can attract or repel ions and change the effective charge of a NP through Debye-Hückel screening. Positively charged ions, such as H+ (or H3O+) are attracted to negatively charged surfaces, whereas negatively charged ions, such as Cl- are repelled. The distribution of the ions around charged NP surfaces is a radial function of distance from the center of the NP, which is governed by a balance of electrostatic forces and entropy of ions and ligands. As a result, the ion concentration at the NP surface is different from its bulk equilibrium concentration, i.e., the charged ligand shell around the NPs has formed a distinct local environment. This not only applies to charged ligand shells but also follows a more general principle of induced condensation and depletion. Polar/apolar ligand shells, for example, result in a locally increased concentration of polar/apolar molecules. Similar effects can be seen for biocatalysts like enzymes immobilized in nanoporous host structures, which provide a special environment due to their surface chemistry and geometrical nanoconfinement. The formation of a local environment close to the ligand shell of NPs has profound implications for NP sensing applications. As a result, analyte concentrations close to the ligand shell, which are the ones that are measured, may be very different from the analyte concentrations in bulk. Based on previous work describing this effect, it will be discussed herein how such local environments, created by the choice of used ligands, may allow for tailoring the NPs' sensing properties. In general, the ligand shell around NPs can be attractive/repulsive for molecules with distinct properties and thus forms an environment that can modulate the specific response. Such local environments can also be optimized to modulate chemical reactions close to the NP surface (for example, by size filtering within pores) or to attract specific low abundance proteins. The importance hereby is that this is based on interaction with low selectivity between the ligands and the target molecules.

Fluorotrifluoromethyl Group Installation: Tetrasubstituted Tertiary Stereocenters Containing C-F and C-CF3 Bonds via Copper-Mediated Allylic Substitution.

Tetrasubstituted tertiary centers containing a fluorotrifluoromethyl (FTF) motif were obtained from secondary 3-fluoro-3-trifluoromethyl allylic phosphates by a copper(I)-mediated allylic substitution reaction using Grignard reagents. The reaction features a broad substrate scope, high γ-regioselectivity, and excellent (E)-stereoselectivity. Two examples of chirality transfer from enantiopure secondary fluorine-substituted allylic phosphates to the corresponding scalemic products indicate the stereoselective potential of the method to install FTF stereocenters.

Chemical Doping by Fluorination and Its Impact on All Energy Levels of π-Conjugated Systems.

Halogenation of organic molecules causes chemical shifts of C1s core-level binding energies that are commonly used as fingerprints to identify chemical species. Here, we use synchrotron-based X-ray photoelectron spectroscopy and density functional theory calculations to unravel such chemical shifts by examining different partially fluorinated pentacene derivatives. Core-level shifts occur even for carbon atoms distant from the fluorination positions, yielding a continuous shift of about 1.8 eV with increasing degree of fluorination for pentacenes. Since also their LUMO energies shift markedly with the degree of fluorination of the acenes, core-level shifts result in a nearly constant excitation energy of the leading π* resonance as obtained in complementary recorded K-edge X-ray absorption spectra, hence demonstrating that local fluorination affects the entire π-system, including valence and core levels. Our results thus challenge the common picture of characteristic chemical core-level energies as fingerprint signatures of fluorinated π-conjugated molecules.

Shape control in 2D molecular nanosheets by tuning anisotropic intermolecular interactions and assembly kinetics.

Since molecular materials often decompose upon exposure to radiation, lithographic patterning techniques established for inorganic materials are usually not applicable for the fabrication of organic nanostructures. Instead, molecular self-organisation must be utilised to achieve bottom-up growth of desired structures. Here, we demonstrate control over the mesoscopic shape of 2D molecular nanosheets without affecting their nanoscopic molecular packing motif, using molecules that do not form lateral covalent bonds. We show that anisotropic attractive Coulomb forces between partially fluorinated pentacenes lead to the growth of distinctly elongated nanosheets and that the direction of elongation differs between nanosheets that were grown and ones that were fabricated by partial desorption of a complete molecular monolayer. Using kinetic Monte Carlo simulations, we show that lateral intermolecular interactions alone are sufficient to rationalise the different kinetics of structure formation during nanosheet growth and desorption, without inclusion of interactions between the molecules and the supporting MoS2 substrate. By comparison of the behaviour of differently fluorinated molecules, experimentally and computationally, we can identify properties of molecules with regard to interactions and molecular packing motifs that are required for an effective utilisation of the observed effect.

Vicinal ketoesters - key intermediates in the total synthesis of natural products.

This review summarizes examples for the application of vicinal ketoesters such as α-ketoesters, mesoxalic esters, and α,ß-diketoesters as key intermediates in the total synthesis of natural products utilizing their electrophilic keto group as reactive site. Suitable key reactions are, e.g., aldol additions, carbonyl ene reactions, Mannich reactions, and additions of organometallic reagents. The vicinal arrangement of carbonyl groups allows the stabilization of reactive conformations by chelation or dipole control.

Preussochromone Puzzle: Structural Revision of Preussochromones E and F by Total Synthesis.

A stereoselective synthesis of the proposed and actual structures of the natural products preussochromones E and F is reported. The key step is a ring-closing metathesis to close the five-membered ring and install the trans configuration of the annulated five-six ring system. The analysis of the 3J NMR couplings of the isolated natural product with the synthesized compound revealed its real structure with a cis annulation, which could also be synthesized using an intramolecular aldol reaction of a vic-tricarbonyl compound.

Regioselective Fluorination of Acenes: Tailoring of Molecular Electronic Levels and Solid-State Properties.

Optoelectronic properties of molecular solids are important for organic electronic devices and are largely determined by the adopted molecular packing motifs. In this study, we analyzed such structure-property relationships for the partially regioselective fluorinated tetracenes 1,2,12-trifluorotetracene, 1,2,10,12-tetrafluorotetracene and 1,2,9,10,11-pentafluorotetracene that were further compared with tetracene and perfluoro-tetracene. Quantum chemical DFT calculations in combination with optical absorption spectroscopy data show that the frontier orbital energies are lowered with the degree of fluorination, while their optical gap is barely affected. However, the crystal structure changes from a herringbone packing motif of tetracene towards a planar stacking motif of the fluorinated tetracene derivatives, which is accompanied by the formation of excimers and leads to strongly red-shifted photoluminescence with larger lifetimes.

Visible-Light-Induced Photoannulation of α-Naphthyl Cyclopropane Carboxylic Esters to Functionalized Dihydrophenalenes.

A general synthetic entry to functionalized dihydrophenalenes has been found using naphthyl-cyclopropane esters as starting materials. The desired annulation was possible with visible light, Ir(Fppy)3 as photocatalyst, BnNMe2 or DABCO as electron donor, HAT-catalyst, and proton source. A broad scope of substituted naphthyl and azanaphthyl derivatives provided the photoannulation products in high yield. Deuteration studies support a photoredox mechanism involving the photoreductive cyclopropane opening to an enolate radical followed by an aryl radical trapping.

Biphenylene network: A nonbenzenoid carbon allotrope.

The quest for planar sp2-hybridized carbon allotropes other than graphene, such as graphenylene and biphenylene networks, has stimulated substantial research efforts because of the materials' predicted mechanical, electronic, and transport properties. However, their syntheses remain challenging given the lack of reliable protocols for generating nonhexagonal rings during the in-plane tiling of carbon atoms. We report the bottom-up growth of an ultraflat biphenylene network with periodically arranged four-, six-, and eight-membered rings of sp2-hybridized carbon atoms through an on-surface interpolymer dehydrofluorination (HF-zipping) reaction. The characterization of this biphenylene network by scanning probe methods reveals that it is metallic rather than a dielectric. We expect the interpolymer HF-zipping method to complement the toolbox for the synthesis of other nonbenzenoid carbon allotropes.

Click Chemistry in Ultra-high Vacuum - Tetrazine Coupling with Methyl Enol Ether Covalently Linked to Si(001).

The additive-free tetrazine/enol ether click reaction was performed in ultra-high vacuum (UHV) with an enol ether group covalently linked to a silicon surface: Dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate molecules were coupled to the enol ether group of a functionalized cyclooctyne which was adsorbed on the silicon (001) surface via the strained triple bond of cyclooctyne. The reaction was observed at a substrate temperature of 380 K by means of X-ray photoelectron spectroscopy (XPS). A moderate energy barrier was deduced for this click reaction in vacuum by means of density functional theory based calculations, in good agreement with the experimental results. This UHV-compatible click reaction thus opens a new, flexible route for synthesizing covalently bound organic architectures.

Combined XPS and DFT investigation of the adsorption modes of methyl enol ether functionalized cyclooctyne on Si(001).

The reaction of methyl enol ether functionalized cyclooctyne on the silicon (001) surface was investigated by means of X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Three different groups of final states were identified; all of them bind on Si(001) via the strained triple bond of cyclooctyne but they differ in the configuration of the methyl enol ether group. The majority of molecules adsorbs without additional reaction of the enol ether group; the relative contribution of this configuration to the total coverage depends on substrate temperature and coverage. Further configurations include enol ether groups which reacted on the silicon surface either via ether cleavage or enol ether groups which transformed on the surface into a carbonyl group.

Synthesis of 6,13-difluoropentacene.

6,13-Difluoropentacene was synthesized from 1,4-difluorobenzene. Friedel-Crafts annulation of the latter with phthalic anhydride and subsequent reduction of the anthraquinone gave 1,4-difluoroanthracene. After ortho-lithiation and reaction with phthalic anhydride a carboxylic acid was obtained whose Friedel-Crafts acylation and subsequent reductive removal of the oxygen-functionalities resulted in the formation of the target compound. The HOMO-LUMO gap of 6,13-difluoropentacene was determined via UV-vis spectroscopy and compared to other fluorinated pentacenes.

Total Synthesis of (-)-Preussochromone A.

An enantioselective total synthesis of the natural product (-)-preussochromone A is reported. The tricyclic thiopyrane skeleton could be assembled via Lewis acid-mediated cycloisomerization of a precursor with a 2-thiochromenone substructure and an α-ketoester moiety. The chromenone core was synthesized by cyclization of a dithioketene acetal and oxidation to a 2-sulfonylchromenone to set up the subsequent thia-Michael-retro-Michael addition of an aliphatic thiol producing the highly oxidized side chain.

Unilaterally Fluorinated Acenes: Synthesis and Solid-State Properties.

The rapid development of organic electronics is closely related to the availability of molecular materials with specific electronic properties. Here, we introduce a novel synthetic route enabling a unilateral functionalization of acenes along their long side, which is demonstrated by the synthesis of 1,2,10,11,12,14-hexafluoropentacene (1) and the related 1,2,9,10,11-pentafluorotetracene (2). Quantum chemical DFT calculations in combination with optical and X-ray absorption spectroscopy data indicate that the single-molecule properties of 1 are a connecting link between the organic semiconductor model systems pentacene (PEN) and perfluoropentacene (PFP). In contrast, the crystal structure analysis reveals a different packing motif than for the parent molecules. This can be related to distinct F⋅⋅⋅H interactions identified in the corresponding Hirshfeld surface analysis and also affects solid-state properties such as the exciton binding energy and the sublimation enthalpy.

Understanding Substrate Selectivity of Phoslactomycin Polyketide Synthase by Using Reconstituted in Vitro Systems.

Polyketide synthases (PKSs) use simple extender units to synthesize complex natural products. A fundamental question is how different extender units are site-specifically incorporated into the growing polyketide. Here we established phoslactomycin (Pn) PKS, which incorporates malonyl- and ethylmalonyl-CoA, as an in vitro model to study substrate specificity. We combined up to six Pn PKS modules with different termination sites for the controlled release of tetra-, penta- and hexaketides, and challenged these systems with up to seven different extender units in competitive assays to test for the specificity of Pn modules. While malonyl-CoA modules of Pn PKS exclusively accept their natural substrate, the ethylmalonyl-CoA module PnC tolerates different α-substituted derivatives, but discriminates against malonyl-CoA. We show that the ratio of extender transacylation to hydrolysis controls incorporation in PnC, thus explaining site-specific selectivity and promiscuity in the natural context of Pn PKS.

Selective cytotoxic activity of isolated compounds from Globimetula dinklagei and Phragmanthera capitata (Loranthaceae).

This study aimed to evaluate the selective cytotoxicity of six natural compounds on four cancerous cells (MCF-7, HeLa, Caco-2 and A549) and two normal intestinal and lung cells (Hs1.Int and Wl-38) cells. We also attempted to analyze basically the structure-activity relationships and to understand the mechanism of action of active compounds using the Caspase-Glo® 3/7 kit. Globimetulin B (2) isolated from Globimetula dinklagei was significantly cytotoxic on cancerous cells with 50% inhibitory concentrations (IC50) ranging from 12.75 to 37.65 µM and the selectivity index (SI) values varying between 1.13 and 3.48 against both normal cells. The compound 3-O-ß-d-glucopyranosyl-28-hydroxy-α-amyrin (5) isolated from Phragmanthera capitata exhibited the highest cytotoxic activity on HeLa cells with the IC50 of 6.88 µM and the SI of 5.20 and 8.71 against Hs1.Int and Wl-38 cells, respectively. A hydroxyl group at C-3 of compounds was suggested as playing an important role in the cytotoxic activity. The induction of caspase-3 and -7 activity represents some proof that apoptosis has occurred in treated cells. Globimetulin B (2) selectively killed cancer cells with less toxicity to non-cancerous cells as compared to conventional doxorubicin therapy.

Copper-Free Click Reaction Sequence: A Chemoselective Layer-by-Layer Approach.

An additive-free chemoselective ligation of dual clickable building blocks is demonstrated. The challenge of balancing reactivity and stability was achieved by employing a small, electron-deficient tetrazine bearing an azido group and an enol ether functionalized cyclooctyne. The chemoselective sequence of strain-promoted azide-alkyne cycloaddition (SPAAC) and inverse-electron-demand Diels-Alder (IEDDA) reaction is demonstrated with a cholic acid derived triazide as a molecular surface model for layer-by-layer synthesis.

Synthesis of Naphthocyclobutenes from α-Naphthyl Acrylates by Visible-Light Energy-Transfer Catalysis.

Methyl (α-naphthyl) acrylates bearing an ortho-substituent with a hydrogen atom produce naphthocyclobutenes upon Ir(Fppy)3-mediated photosensitization. This reaction can be described as a carbon analogue of the Norrish-Yang reaction: upon triplet excitation of the acrylate a 1,5-HAT results in a 1,4-diradical which forms the cyclobutene. Diastereoselectivities up to >20:1 were observed for the ring-closure reaction.

Total Synthesis of (-)-Preussochromone D.

An efficient, stereoselective synthesis of the natural product (-)-preussochromone D is reported. The tricyclic skeleton was assembled by a diastereoselective intramolecular aldol addition of a chromanone to an α-ketoester. Further key steps are an asymmetric 1,4-addition of diisopropenyl zinc to a chromenone and an intermolecular diastereoselective aldol addition of methyl diazoacetate to an aldehyde. The diazo group was oxidized to generate the α-ketoester while oxidative side reactions at the chromanone could be prevented by the use of a difluoromethyl ether as a protecting group.

Total Synthesis of (+)-Nivetetracyclate A.

A stereoselective synthetic approach to the natural product (+)-nivetetracyclate A is reported. The tetracyclic skeleton was assembled in a convergent manner by an alkylation to a diarylmethane and subsequent Friedel-Crafts acylation. Further key steps are an asymmetric Sharpless epoxidation and the boron trifluoride-mediated diastereoselective rearrangement of an epoxy alcohol to a ß-hydroxy aldehyde. Optimized timing for the deprotection step and the chemo- and stereoselective Noyori -transfer hydrogenation of a 1,4-diketone allowed the late stage introduction of the C4 stereocenter.