Metal-Free Aryl Cross-Coupling Directed by Traceless Linkers.

The metal-free, highly selective synthesis of biaryls poses a major challenge in organic synthesis. The scope and mechanism of a promising new approach to (hetero)biaryls by the photochemical fusion of aryl substituents tethered to a traceless sulfonamide linker (photosplicing) are reported. Interrogating photosplicing with varying reaction conditions and comparison of diverse synthetic probes (40 examples, including a suite of heterocycles) showed that the reaction has a surprisingly broad scope and involves neither metals nor radicals. Quantum chemical calculations revealed that the C-C bond is formed by an intramolecular photochemical process that involves an excited singlet state and traversal of a five-membered transition state, and thus consistent ipso-ipso coupling results. These results demonstrate that photosplicing is a unique aryl cross-coupling method in the excited state that can be applied to synthesize a broad range of biaryls.

Autonomous Supramolecular Interface Self-Healing Monitored by Restoration of UV/Vis Absorption Spectra of Self-Assembled Thiazole Layers.

Longevity of complex organic devices critically depends on the supramolecular integrity of the constituting layers and interfaces. Because the latter are soft matter, they can structurally respond to perturbation of their supramolecular structure by relaxing back to a thermodynamically favorable state. To use this response for self-healing of optoelectronically active layers and particularly interfaces, the degraded dyes in these layers need to be exchanged with non-degraded ones. Here, we present a dye layer interfaced between a solid surface and a dye reservoir that autonomously self-heals after photo-degradation of single molecules to restore its optical function. Surface sensitive in situ photothermal deflection spectroscopy reveals that this supramolecular self-healing approach critically depends on the thermodynamic stability of the layer, the chemical change of the dye upon degradation, and the medium dissolving the degraded dye and providing the reservoir dyes. Hence, the interplay of these parameters is key to successfully using this supramolecular self-healing approach to thin layers and interfaces in organic device for increased sustainability of organic optoelectronics and related fields.

Assembly of T-Shaped Amphiphilic Thiazoles on the Air-Water Interface: Impact of Polar Chromophore Moieties, as Well as Dipolarity and π-Extension of the Chromophore on the Supramolecular Structure.

The supramolecular structure essentially determines the properties of organic thin films. In this work, we systematically investigate the influence of the chromophore on the supramolecular structure formation at air-water interfaces by means of the Langmuir-Blodgett technique. Therefore, we focus on the recently introduced class of double-anchor T-shaped amphiphilic dyes, namely, 4-hydroxy-thiazole chromophores that are centrally equipped with an amphiphilicity-inducing hexanoic acid. The thiazoles contain hydrophilic subphase-anchor groups in the 2-position (4- N, N-dimethylaminophenyl (Am), 2-pyridyl (Py), and 4-nitrophenyl (Ni)), whereas the chromophores are systematically extended in the 5-position with various substituents. The combination of the Langmuir technique with online fluorescence measurements revealed that the π-π interactions that are pronounced in the case of 4-methoxybiphenyl derivatives yield the most distinct supramolecular structures. Whereas in the case of Py and Ni derivatives ordered J-type supramolecular structures in microdomains are formed, the Am derivative forms ordered supramolecular structures that are more homogeneous, which are, however, not stabilized by J-type dipolar interactions. Because of the synergetic π-π and dipolar stabilizations, the Ni derivative bearing the 4-methoxybiphenyl unit forms exceptionally stable quasi-two-dimensional Langmuir monolayers reaching very high surface pressures beyond 60 mN/m without any sign of disturbance of the Langmuir monolayer.

Photoannealing of Merocyanine Aggregates.

In this work we elucidate the fundamental difference between aggregate formation of donor-π-acceptor merocyanines in their electronic ground and excited states. While increasing the π-bridge size favors formation of π-stacked aggregates in the dark, irradiation with visible light causes reorientation of the dyes to form prototype H-aggregates with compensating dipole moments. This photoannealing changes the supramolecular structure and its UV-vis spectroscopic properties dramatically, thus being of importance for the function of active layers composed of these dyes. Aggregates of the ground state dyes are bound cooperatively through ππ-London dispersion interactions and hydrogen bonds between the polar α-cyano-carboxylic acid groups. However, charge transfer upon photoexcitation leads to repulsion of the polar acid groups. Electronic excitation of the dyes approximately doubles the ground state dipole moment, thus driving molecular reorientation into prototype H-aggregate structures. We show that this photoinduced supramolecular rearrangement can disrupt the large polymeric aggregates formed in the dark. The photoinduced supramolecular structural changes reported in this work will influence the performance of optoelectronic devices composed of these structures and must be controlled to avoid morphological decomposition of active layers upon operation.

On the Control of Chromophore Orientation, Supramolecular Structure, and Thermodynamic Stability of an Amphiphilic Pyridyl-Thiazol upon Lateral Compression and Spacer Length Variation.

The supramolecular structure essentially determines the properties of organic thin films. Therefore, it is of utmost importance to understand the influence of molecular structure modifications on supramolecular structure formation. In this article, we demonstrate how to tune molecular orientations of amphiphilic 4-hydroxy thiazole derivatives by means of the Langmuir-Blodgett (LB) technique and how this depends on the length of an alkylic spacer between the thiazole chromophore and the polar anchor group. Therefore, we characterize their corresponding supramolecular structures, thermodynamic, absorption, and fluorescence properties. Particularly, the polarization-dependence of the fluorescence is analyzed to deduce molecular orientations and their possible changes after annealing, i.e., to characterize the thermodynamic stability of the individual solid state phases. Because the investigated thiazoles are amphiphilic, the different solid state phases can be formed and be controlled by means of the Langmuir-Blodgett (LB) technique. This technique also allows to deduce atomistic supramolecular structure motives of the individual solid phases and to characterize their thermodynamic stabilities. Utilizing the LB technique, we demonstrate that subtle molecular changes, like the variation in spacer length, can yield entirely different solid state phases with distinct supramolecular structures and properties.

Pushing to the low limits: tetraazaanthracenes with very low-lying LUMO levels and near-infrared absorption.

Here we propose the combination of the 4-alkoxythiazole donor motif with highly photostable tetraazaanthracenes as electron-acceptor units. The segregated frontier orbitals in these dyes afford optical band gaps of 1.4-1.1 eV. Cyclic voltammetry confirmed the very low-lying LUMO levels that are attributed to the highly electron-deficient tetraazaanthracene moiety.

Mechanistic investigations of the 2-coumaranone chemiluminescence.

2-Coumaranones are evolving as a new, efficient, versatile, and synthetically accessible platform for the next generation chemiluminescent probes. Despite the favorable quantum yields, the exact mechanism of their chemiluminescence remains elusive. Here, we analyze the details of the mechanism of the 2-coumaranone chemiluminescence using a combination of experimental and computational methods. By using EPR spectroscopy we show that superoxide radical anions are involved in the reactions, in support of the hypothesis that the mechanism includes a single electron transfer step. The decomposition of the high-energy intermediate, 1,2-dioxetanone, is described in the ground state and in the first three excited singlet states, and indicates that there is at least one conical intersection, which is crucial for generation of excited-state molecules. A peroxy anion that is generated was found to be able to undergo a side reaction that leads to the same (isolated) product as in the light-generating reaction. These results demonstrate the applicability of 2-coumaranones as a model system for several bioluminescence reactions and may lead to the design of new 2-coumaranone derivatives with superior emission characteristics for bioanalytical applications.

From Highly Fluorescent Donors to Strongly Absorbing Acceptors: The Tunable Properties of Fluorubines.

The synthesis and characterization of three novel fluorubine derivatives is reported via three to four simple reaction steps with isolatable intermediates. The functional dyes are characterized by their strong absorption peaks in the visible region and their high fluorescence quantum yields. A significant and useful feature is that the properties can be tuned over a wide range by changing the pH. Transformation of the dyes into protonated amidinium salts leads to narrower band gaps and to drastically lower LUMO energies. Further reduction of the pH results in the doubly protonated species with a high electron-deficiency and LUMO energies of -4.8 eV, bathochromic shifts, and a strong intensity increase of up to ε = 120 000 M-1 cm-1.

π-Extension of a 4-ethoxy-1,3-thiazole via aryl alkyne cross coupling: synthesis and exploration of the electronic structure.

A series of four donor aryl alkynyl substituted thiazole derivatives 3a-d and three similar aryl donor-acceptor systems 6a-c have been synthesized. All compounds bear different electron-donating groups in the 5-position of the thiazole core. The influence of both electron donor strength and the additional phenylethynyl unit on photophysical properties, i.e. UV/Vis absorption, fluorescence emission and fluorescence lifetime, has been evaluated. Additionally, theoretical calculations have been performed at the CAM-B3LYP/6-31+G(d,p) level and good agreement with the experimental data has been achieved. The new derivatives synthesized via palladium catalyzed cross coupling are characterised by moderately strong emission between 474 and 538 nm (ΦF = 0.35-0.39) and Stokes' shifts ranging from 0.54 to 0.79 eV (4392-6351 cm(-1)). The smaller chromophores of type 6 exhibit modest to high fluorescence emission (ΦF = 0.45-0.76) between 470 and 529 nm and their Stokes' shifts range from 0.59 to 0.65 eV (4765-5251 cm(-1)).

Stable and easily accessible functional dyes: dihydrotetraazaanthracenes as versatile precursors for higher acenes.

A series of new dihydrotetraazaanthracenes and one new dihydrotetraazatetracene as substances for applications in organoelectronic devices and as suitable building blocks for higher azaacenes was synthesised. The condensation of aromatic diamines with dichlorodicyanopyrazine led to these tricyclic/tetracyclic compounds. Syntheses of N-substituted phenylenediamines were developed to enable the introduction of multiple functional groups such as ester, amino, or nitro groups on the chromophoric system. Relationships between the structure and the spectroscopic properties could be derived from UV/Vis absorption and fluorescence spectroscopy, as well as by DFT and TD-DFT calculations of molecular and aggregate structures. The absorption spectra are dominated by π-π* transitions of the single molecules, whereas aggregation needs to be taken into account to obtain reasonable agreement between theory and experiment in certain cases. Single-crystal X-ray analyses were carried out to examine the morphology and solid packing effects. Finally, a dihydrotetraazaanthracene was used as a building-block to create a mesoionic octaazapentacene.

And yet they glow: thiazole based push-pull fluorophores containing nitro groups and the influence of regioisomerism.

Reported is a study on the influence of regioisomerism on the photophysical properties in 4-hydroxy-1,3-thiazole-based push-pull-chromophores/fluorophores to evaluate the molecular structure-property relationship as a basic foundation for future design strategies concerning this class of dyes. Surprisingly, the nitro groups used as acceptors do not act as a fluorescence quencher, instead the derivatives synthesized exhibit quantum yields of 37-40%. Two 4-ethoxy-1,3-thiazole derivatives which differ only in the positioning of their electron donating (methoxy) and electron withdrawing (nitro) groups have been synthesized and examined in terms of their photophysical properties, i.e. UV/Vis absorption and fluorescence emission spectra. Additionally, quantum chemical calculations have been performed to unravel the underlying fundamental transitions and to explain the experimental results.

A small azide-modified thiazole-based reporter molecule for fluorescence and mass spectrometric detection.

Molecular probes are widely used tools in chemical biology that allow tracing of bioactive metabolites and selective labeling of proteins and other biomacromolecules. A common structural motif for such probes consists of a reporter that can be attached by copper(I)-catalyzed 1,2,3-triazole formation between terminal alkynes and azides to a reactive headgroup. Here we introduce the synthesis and application of the new thiazole-based, azide-tagged reporter 4-(3-azidopropoxy)-5-(4-bromophenyl)-2-(pyridin-2-yl)thiazole for fluorescence, UV and mass spectrometry (MS) detection. This small fluorescent reporter bears a bromine functionalization facilitating the automated data mining of electrospray ionization MS runs by monitoring for its characteristic isotope signature. We demonstrate the universal utility of the reporter for the detection of an alkyne-modified small molecule by LC-MS and for the visualization of a model protein by in-gel fluorescence. The novel probe advantageously compares with commercially available azide-modified fluorophores and a brominated one. The ease of synthesis, small size, stability, and the universal detection possibilities make it an ideal reporter for activity-based protein profiling and functional metabolic profiling.

Modified bibenzimidazole ligands as spectator ligands in photoactive molecular functional Ru-polypyridine units? Implications from spectroscopy.

The photophysical properties of Ruthenium-bipyridine complexes bearing a bibenzimidazole ligand were investigated. The nitrogens on the bibenzimidazole-ligand were protected, by adding either a phenylene group or a 1,2-ethandiyl group, to remove the photophysical dependence of the complex on the protonation state of the bibenzimidazole ligand. This protection results in the bibenzimidazole ligand contributing to the MLCT transition, which is experimentally evidenced by (resonance) Raman scattering in concert with DFT calculations for a detailed mode assignment in the (resonance) Raman spectra.

Trapped in imidazole: how to accumulate multiple photoelectrons on a black-absorbing ruthenium complex.

Ruthenium dyes incorporating a 4H-imidazole chromophore as a ligand exhibit a spectrally broad absorption in the UV/Vis region. Furthermore, they show the ability to store two electrons within the 4H-imidazole ligand. These features render them promising molecular systems, for example, as inter- or intramolecular electron relays. To optimize the structures with respect to their electron-storage capability, it is crucial to understand the impact of structural changes accompanying photoinduced charge transfer in the electronic intermediates of multistep electron-transfer processes. The photophysical properties of these (reactive) intermediates might impact the function of the molecular systems quite substantially. However, the spectroscopic study of short-lived intermediates in stepwise multielectron-transfer processes is experimentally challenging. To this end, this contribution reports on the electrochemical generation of anions identical to intermediate structures and their spectroscopic characterization by in situ resonance Raman and UV/Vis spectroelectrochemistry and computational methods. Thereby, an efficient two-electron pathway to the 4H-imidazole electron-accepting ligand is identified.

Synthesis and biological activity of quaternary ammonium salt-type agents containing cholesterol and terpenes.

New quaternary ammonium salt-type compounds with lipophilic cholesterol and terpene moieties were synthesized. The compounds showed promising antibacterial and antimycobacterial activities. Those compounds containing the cholesterol moiety showed significant activity against Staphylococcus aureus, Staphylococcus epidermidis, and Enterococcus faecium. On the contrary, the antimycobacterial activity increased with the presence of the terpene unit in the molecule.

Evidence supporting a 1,2-dioxetanone as an intermediate in the benzofuran-2(3H)-one chemiluminescence.

The mechanism of the chemiluminescent reaction of ethyl (5-fluoro-2-oxo-2,3-dihydrobenzofuran-3-yl) carbamate (a 2-coumaranone derivative) with a base and molecular oxygen was investigated. New evidence from the reaction kinetics and absorption/emission profiles was obtained, supporting the existence of a 1,2-dioxetanone as an intermediate: (i) its characteristic activation parameters (ΔH(≠) = 7.2 ± 0.1 kcal mol(-1); ΔS(≠) = -45 ± 5 cal K(-1) mol(-1)) indicating a high degree of thermal instability and (ii) its bimolecular decomposition rate constant for the reaction with perylene. The newly developed methodology has been shown to be suitable for determining the reactivity of such thermally unstable peroxides, which are very difficult to prepare and isolate, using this alternative approach of in situ generation of a 1,2-dioxetanone.

Synthesis and optical properties of various thienyl derivatives of pyrene.

A series of various thienyl derivatives of pyrene were synthesized by Stille cross-coupling procedure. Their structures were characterized by (1)H NMR, (13)C NMR and elemental analysis. The spectroscopic characteristics were investigated by UV-vis absorption and fluorescence spectra. Based on quantum chemical calculations, the energy levels of investigated molecules with respect to the pyrene molecule were also discussed.

Total synthesis and detection of the bilirubin oxidation product (Z)-2-(3-ethenyl-4-methyl-5-oxo-1,5-dihydro-2H-pyrrol-2-ylidene)ethanamide (Z-BOX A).

The selective total synthesis of the pure Z-isomer of BOX A (8a), a product of oxidative heme degradation with significant physiological impact, was achieved in four to six steps starting from 3-bromo-4-methylfuran-2,5-dione (1). Z-BOX A forms a strong hydrogen bridge framework in the crystalline state. LC-MS techniques allow identification and characterization of isomeric forms of BOX A.

Superstructures of fluorescent cyclodextrin via click-reaction.

Mono-(6-azido-6-deoxy)-ß-cyclodextrin (CD) was covalently attached to an alkyne-modified 5-methyl-2-(pyridin-2-yl)thiazol-4-ol yielding a fluorophore containing CD in a click-type reaction. Intermolecular complexes were formed by poly(host-guest)-interactions. The supramolecular structures were characterized by (1)H NMR-ROESY spectroscopy, dynamic light scattering, UV-vis spectroscopy, fluorescence spectroscopy, and asymmetric flow field-flow fractionation. By adding potassium adamantane-1-carboxylate, the thiazol dye is displaced from the CD-cavity and the elongated noncovalent polymeric structures collapse.

2-Isopropyl-2-(6-meth-oxy-1,3-benzo-thia-zol-2-yl)-5,5-dimethyl-1,3-thia-zolidin-4-one.

The title compound, C16H20N2O2S2, crystallizes with two enanti-omers (A and B) in the asymmetric unit. The most noticeable difference between these two mol-ecules is the relative orientation of the benzo-thia-zole rings, with S-C-C-S torsion angles of -19.4 (2) (mol-ecule A) and 100.6 (1)° (mol-ecule B). The amide structure of the thia-zolidinone rings leads to inter-molecular hydrogen-bonded dimers of the R and S enanti-omers.