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.

Epoxidation reactions have been established as an important method for the formation of carbon-oxygen bonds. In 1980 Sharpless and Katsuki developed an enantioselective variant for allylic alcohols which is still one of the most elegant methods in asymmetric synthesis. ['] Over the last few years unfunctionalized olefins have also become the focal point of interest as substrates, yielding epoxides in high enantiomeric excess when the optically active (salen)manganese(III) complexes 1 designed byJacobsen and Katsuki are used."] The best selectivities are achieved with cis-alkenes; however, recently the epoxidation of olefins having tri-and tetrasubstituted double bonds has also been successful with enantiomeric excesses of over The oxidants of choice are iodosobenzene and sodium hypochlorite at room temperature as well as meta-chloroperbenzoic acid at -78 "C. The proposed mechanism of the JacobsenKatsuki epoxidation is based on investigations by Kochi et al. While the existence of analogous CrV -0x0 complexes has been established by X-ray crystallographic analysis, the more reactive MnV complexes could only be speculated on. Recently, however, Plattner et al. made an important contribution to the mechanistic understanding by proving such intermediates with electrospray tandem mass ~pectrometry.[~I

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.

Monolayers and thin films of diphenylanthracene react in the presence of a singlet oxygen sensitizer by irradiation on air to the corresponding endoperoxides and reconvert to the starting compounds upon heating. This reaction has been applied to create 2D fluorescent pattern structures. Motifs which are written by this technique can be erased and replaced by new images.

The generation of reactive singlet oxygen under mild conditions is of current interest in chemistry, biology, and medicine. We were able to release oxygen from dipyridylanthracene endoperoxides (EPOs) by using a simple chemical trigger at low temperature. Protonation and methylation of such EPOs strongly accelerated these reactions. Furthermore, the methyl pyridinium derivatives are water soluble and therefore serve as oxygen carriers in aqueous media. Methylation of the EPO of the ortho isomer affords the parent form directly without increasing the temperature under very mild conditions. This exceptional behavior is ascribed to the close contact between the nitrogen atom and the peroxo group. Singlet oxygen is released upon this reaction, and can be used to oxygenate an acceptor such as tetramethylethylene in the dark with no heating. Thus, a new chemical source of singlet oxygen has been found, which is triggered by a simple stimulus.

The ortho, meta, and para isomers of 9,10-dipyridylanthracene 1 have been synthesized and converted into their endoperoxides 1-O 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.

Singlet oxygen ( 1 O 2 ), as an important active reagent, has found wide applications in photodynamic therapy (PDT), synthetic chemistry and materials science. Organic conjugated aromatics serving as hosts to capture and release singlet oxygen have been systematically investigated over the last decades. Herein, we present a [6+6] organoplatinum(II) metallacycle by using ~180° dipyridylanthracene donor and ~120° Pt(II) acceptor as the building blocks, which enables the capture and release of singlet oxygen with relatively high photooxygenation and thermolysis rate constants. The photooxygenation of the metallacycle to the corresponding endoperoxide was performed by sensitized irradiation, and the resulting endoperoxide is stable at room temperature and can be stored under ambient condition over months. Upon simple heating the neat endoperoxide under inert atmosphere at 120 °C for 4 h, the resulting endoperoxide can be reconverted to the corresponding parent form and singlet oxygen. The photooxygenation and thermolysis products were characterized by NMR spectroscopy and ESI-TOF-MS analysis. Density functional theory calculations were conducted in order to reveal the frontier molecular orbital interactions and reactivity. This work provides a new material-platform for singlet oxygen related promising applications.

Singlet oxygen fuels anthracenes and forces an axial rotation of aryl substituents during the oxidation. The molecular switch can be synthesized in only one step from commercially available starting materials. Thermal cleavage of the resulting endoperoxides proceeds quantitatively and affords a simple 180° switch with oxygen as the only waste product. The initial trans state is attained by heating, and repetitive cycles are possible.