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.

Octaanionic meso-tetra(3,5-dicarboxylatophenyl) porphyrin 1 was adsorbed to gold electrodes at pH 12 and stayed there after repeated washing with 10-2 M KOH. The fluorescence on sputtered gold surfaces amounted to 10% of the intensity observed on an organic subphase. Addition of 10-6 M aqueous solutions of the manganese(III) complexes of an isomer mixture of tetracationic β-tetraethyl-β‘-tetrakis(1-methyl-4-pyridinium)- and meso-4-(1-methyl-4-pyridinium)phenyl porphyrins 2 and 4 at pH 12 quenched the fluorescence quantitatively. Visible spectroscopy proved that the amount of porphyrin 1 on the gold surface had not changed. The octaanionic porphyrin 1 was then embedded in a membrane by self-assembly of a bolaamphiphile containing two secondary amide groups. Two hydrogen bond chains rigidify such a monolayer. The emission of porphyrin 1 remained after the self-assembly process. 1 was now localized on the bottom of a rigid membrane gap. Its fluorescence was again quantitatively quenched by the tetracationic manganese(III) porphyrinate 2, which fit in with the membrane gap. A larger manganese(III) porphyrin with a phenyl spacer between the porphyrin and methyl pyridinium rings could not enter, and no quenching was observed. The same experiment with a more fluid membrane made of octadecanethiol showed no such discriminating effect. The entrapment of 1,2-trans-cyclohexanediol within the “immobile” water volume of the membrane gap is also reported. Water-soluble compounds have thus been separated within a 2 nm3 water volume from bulk water. So far, the membrane pores with a porphyrin bottom resemble natural enzyme clefts.

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.

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.

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.