Werner Fudickar

Why triple bonds protect acenes from oxidation and decomposition.

An experimental and computational study on the impact of functional groups on the oxidation stability of higher acenes is presented. We synthesized anthracenes, tetracenes, and pentacenes with various substituents at the periphery, identified their photooxygenation products, and measured the kinetics. Furthermore, the products obtained from thermolysis and the kinetics of the thermolysis are investigated. Density functional theory is applied in order to predict reaction energies, frontier molecular orbital interactions, and radical stabilization energies. The combined results allow us to describe the mechanisms of the oxidations and the subsequent thermolysis. We found that the alkynyl group not only enhances the oxidation stability of acenes but also protects the resulting endoperoxides from thermal decomposition. Additionally, such substituents increase the regioselectivity of the photooxygenation of tetracenes and pentacenes. For the first time, we oxidized alkynylpentacenes by using chemically generated singlet oxygen ((1)O(2)) without irradiation and identified a 6,13-endoperoxide as the sole regioisomer. The bimolecular rate constant of this oxidation amounts to only 1 × 10(5) s(-1) M(-1). This unexpectedly slow reaction is a result of a physical deactivation of (1)O(2). In contrast to unsubstituted or aryl-substituted acenes, photooxygenation of alkynyl-substituted acenes proceeds most likely by a concerted mechanism, while the thermolysis is well explained by the formation of radical intermediates. Our results should be important for the future design of oxidation stable acene-based semiconductors.

Remote substituent effects on the photooxygenation of 9,10-diarylanthracenes: strong evidence for polar intermediates

Two different reaction pathways in the photooxygenation of 9,10-diarylanthracenes are identified, with strong evidence for polar (forward, singlet oxygen addition) and radical (backward, thermolysis) intermediates.

9,10‐Diarylanthracenes as Molecular Switches: Syntheses, Properties, Isomerisations and Their Reactions with Singlet Oxygen

A series of 9,10-diarylanthracenes with various substituents at the ortho positions have been synthesised by palladium-catalysed cross-coupling reactions. Such compounds exhibit interesting physical properties and can be applied as molecular switches. Despite the high steric demand of the substituents, products were formed in moderate-to-good yields. In some cases, microwave conditions further improved yields. Bis-coupling afforded two isomers (syn and anti) that do not interconvert at room temperature. These products were easily separated and their relative stereochemistries were unequivocally assigned by NMR spectroscopy and X-ray analysis. The syn and anti isomers exhibit different physical properties (e.g., melting points and solubilities) and interconversion by rotation around the aryl-aryl axis commences at <100 degrees C for fluoro-substituted diarylanthracenes and at >300 degrees C for alkyl- or alkoxy-substituted diarylanthracenes. The reactions with singlet oxygen were studied separately and revealed different reactivities and reaction pathways. The yields and reactivities depend on the size and electronic nature of the substituents. The anti isomers form the same 9,10-endoperoxides as the syn species, occasionally accompanied by unexpected 1,4-endoperoxides as byproducts. Thermolysis of the endoperoxides exclusively yielded the syn isomers. The interesting rotation around the aryl-aryl axis allows the application of 9,10-diarylanthracenes as molecular switches, which are triggered by light and air under mild conditions. Finally, the oxygenation and thermolysis sequence provides a simple, synthetic access to a single stereoisomer (syn) from an unselective coupling step.

Novel Anthracene Materials for Applications in Lithography and Reversible Photoswitching by Light and Air

Herein we demonstrate how the photoreaction between anthracenes and singlet oxygen ((1)O(2)) is employed for applications either as photoswitch or as photoresist. Thin films of the diaryl-alkyl anthracene 1 and the analogous oligomeric species 2 were irradiated under photomasks to generate pattern structures composed of 1/1-O(2) and 2/2-O(2). Kelvin probe force microscopy (KPFM) provided a powerful and nondestructive method to image the pattern information. The following studies based on AFM, KPFM and contact angle measurements unfold that the two species 1 and 2 underwent different progressions after the imaging step. Degrading is observed for the monomeric compound 1 and the pattern eventually becomes recognizable in topography. In the oxidized state (1-O(2)) the monomeric species remains physically stable. In consequence, the unreacted portion is removable and the remaining oxygenated form 1-O(2) is sufficiently stable to protect an underlying substrate (e.g., silver) from etching. Thus, the system 1/1-O(2) operates as photoresist. On the other hand, both states of the oligomer 2 remain stable. The film is stable up to temperatures >120 degrees C required to erase the pattern within acceptable time by cycloreversion. Anthracene 2 therefore acts as erasable and rewritable photochromic switch. The different behavior between 1 and 2 is explained by phase transitions which cause crystallization and finally ablation. Such transitions affect only the monomeric system 1/1-O(2) and not the oligomeric system 2/2-O(2). In conclusion, we designed two very similar materials based on diarylanthracenes, which can act either as a photoresist or as a rewritable photochromic switch.

Remarkable oxidation stability of glycals: Excellent substrates for cerium(IV)-mediated radical reactions

The remarkable stability of glycals under oxidative conditions becomes apparent by their redox data in solution, computed HOMO energies, and behavior on the addition of electrophilic radicals generated in the presence of cerium(IV) ammonium nitrate. Oxidation potentials up to 2.03 V vs ferrocene were obtained, which are exceptionally high for cyclic enol ethers but correlate nicely with the reaction times of the radical reactions. Protecting groups have a strong influence on the oxidation stability and HOMO energies of glycals as E(ox) is shifted from O-silyl over O-benzyl to O-acetyl by more than 500 mV. Interestingly, this effect must be transmitted through sigma-bonds, even up to the para-position of a benzoate group, as verified by a wide variation of remote substituents in the carbohydrate. Favorable interactions of the sigma*-orbital of the adjacent C-O bond with the HOMO of the double bond are proposed as a mechanistic rationale, which might be important for the redox behavior of other allylic systems. Finally, donors and acceptors in the 1-position exert the strongest influence on the oxidation stability, shifting the potentials by almost 1 V and resulting in different follow-up reactions of the cerium(IV)-mediated additions of malonates. It is the remarkable oxidation stability of glycals that makes them valuable building blocks in carbohydrate chemistry.

Role of Distance in Singlet Oxygen Applications: A Model System

Herein, we present a model system that allows the investigation of a directed intramolecular singlet oxygen ((1)O2) transfer. Furthermore, we show the influence of singlet oxygen lifetime and diffusion coefficient (D) on the preference of the intramolecular reaction over the intermolecular one in competition experiments. Finally, we demonstrate the distance dependence in quenching experiments, which enables us to draw conclusions about the role of singlet oxygen and (1)O2 carriers in photodynamic therapy.

Starre Lipidmembranen und Nanometerlücken – Motive zur Gestaltung molekularer Landschaften

Amphiphile Lipide assoziieren in Wasser spontan zu Micellen, Vesikeln, Monoschichten oder biologischen Membranen. Diese Aggregate sind weich und leicht verformbar. Sie verhalten sich wie komplexe Flussigkeiten, weil sie lediglich durch schwache, nichtgerichtete Krafte zusammengehalten werden. Das wichtigste Charakteristikum der fluiden Monoschichten ist ihre Fahigkeit, hydrophobe Fremdmolekule in Form frei beweglicher Monomere zu losen. Zur Fixierung der Komponenten von Reaktionsketten eignen sie sich nicht. Ersetzt man die Alkylketten durch starre Geruste oder verknupft man die Kopfgruppen uber intermolekulare Wechselwirkungen, so verfestigen sich die Aggregate und ihr fluider Losungsmittelcharakter geht verloren. Konstruktionsplane fur die Synkinese (Synthese nichtkovalenter Verbindungen) von chiralen Oberflachen und von Reaktionsraumen definierter Abmessungen konnen mit steifen Lipidmembranen realisiert werden. Monoschichten und Nanometerporen auf festen Substraten erhalten scharfe Kanten, und frei stehende Nanometersaulen auf glattem Untergrund zerfliesen nicht mehr. Funf Beispiele sollen illustrieren, welchen Nutzen starre molekulare Aggregate haben: 1) Kationische Domanen aus steifen Kantenamphiphilen in fluiden Membranen wirken als manipulierbare Ionenkanale. 2) Spharische Micellen, micellare helicale Fasern und vesikulare Rohrchen lassen sich in trockenem Zustand als stabile Materialien isolieren und lagern. Auf glatten Oberflachen bilden sie molekulare Landschaften. 3) α,ω-Diamid-Bolaamphiphile bilden auf glatten Oberflachen massive, wenige Nanometer dicke Wande, die von Aminen nicht durchdrungen werden konnen. Um Steroide und Porphyrine herum formen sie starre Nanometerhohlraume, deren hydrophobe Wande und wassergefullte Zentren funktionalisiert werden konnen. 4) Die Struktur steifer Oligophenylen- und Chinon-Monoschichten auf Elektroden andert sich drastisch und reversibel bei Potentialanderungen. 5) 1010 Porphyrinkegel auf einer 1 cm2 grosen Goldelektrode konnen individuell von AFM- und STM-Spitzen angesteuert und elektrochemisch, photochemisch und mechanisch untersucht werden. Kurz gesagt: Die steifen Mono- und Doppelschichten bieten eine vielfaltige Membranchemie, die die klassischen, fluiden Alkylamphiphile nicht aufweisen.

Synthesis of Pyridylanthracenes and their Reversible Reaction with Singlet Oxygen to Endoperoxides

The ortho, meta, and para isomers of 9,10-dipyridylanthracene 1 have been synthesized and converted into their endoperoxides 1-O2 upon oxidation with singlet oxygen. The kinetics of this reaction can be controlled by the substitution pattern and the solvent: in highly polar solvents, the meta isomer is the most reactive, whereas the ortho isomer is oxidized fastest in nonpolar solvents. Heating of the endoperoxides affords the parent anthracenes by release of singlet oxygen.

Intermediates in the Formation and Thermolysis of Peroxides from Oxidations with Singlet Oxygen

Herein we describe the recent mechanistic understandings of the singlet oxygen ene reaction to give hydroperoxides and the [4+2] cycloaddition affording endoperoxides. Both experimental findings and theoretical work conclude in the formation of intermediates structurally similar to perepoxides during the ene reaction. Such intermediates mainly control the regio- and stereoselectivities of this reaction class. For the [4+2] cycloaddition, both a synchronous concerted reaction (benzene, naphthalenes) and a stepwise reaction with a non-symmetric zwitterionic intermediate (larger acenes) have been found. The thermolysis of endoperoxides derived from acenes proceeds stepwise for anthracenes, but in a concerted manner for less stable adducts such as naphthalene.

Activation of Anthracene Endoperoxides in Leishmania and Impairment of Mitochondrial Functions

Leishmaniasis is a vector-borne disease caused by protozoal Leishmania. Because of resistance development against current drugs, new antileishmanial compounds are urgently needed. Endoperoxides (EPs) are successfully used in malaria therapy, and experimental evidence of their potential against leishmaniasis exists. Anthracene endoperoxides (AcEPs) have so far been only technically used and not explored for their leishmanicidal potential. This study verified the in vitro efficiency and mechanism of AcEPs against both Leishmania promastigotes and axenic amastigotes (L. tarentolae and L. donovani) as well as their toxicity in J774 macrophages. Additionally, the kinetics and radical products of AcEPs’ reaction with iron, the formation of radicals by AcEPs in Leishmania, as well as the resulting impairment of parasite mitochondrial functions were studied. Using electron paramagnetic resonance combined with spin trapping, photometry, and fluorescence-based oximetry, AcEPs were demonstrated to (i) show antileishmanial activity in vitro at IC50 values in a low micromolar range, (ii) exhibit host cell toxicity in J774 macrophages, (iii) react rapidly with iron (II) resulting in the formation of oxygen- and carbon-centered radicals, (iv) produce carbon-centered radicals which could secondarily trigger superoxide radical formation in Leishmania, and (v) impair mitochondrial functions in Leishmania during parasite killing. Overall, the data of different AcEPs demonstrate that their structures besides the peroxo bridge strongly influence their activity and mechanism of their antileishmanial action.