H.-D. Brauer

Determination of rate constants of 1O2 consumption by 1O2 acceptors in weakly deactivating solvents

Abstract The rate constants k d obs of 1 O 2 consumption were measured directly in CS 2 for eight 1 O 2 acceptors using the time-resolved technique. The values obtained agree well with the values of k 1 obs in toluene and benzene which were estimated indirectly from the reactivity parameter β previously determined in self-sensitized or sensitized photo-oxygenation experiments. Only the measurements of the self-sensitized photo-oxygenation of 9,10-diphenyl-anthracene (DPA) and 9,10-dimethylanthracene (DMA) in CS 2 yield values of β which are consistent with a long lifetime of 1 O 2 of τ Δ = 34 ms in this solvent. The determination of β from measurements of the self-sensitized photo-oxygenation in CS 2 results in β values which are too high for the rest of the 1 O 2 acceptors, indicating that the lifetime of 1 O 2 in CS 2 is strongly reduced in these experiments. The same behaviour is observed for the self-sensitized photo-oxygenation of DPA and DMA in Freon 113.

A NEW REUSABLE CHEMICAL ACTINOMETER FOR UV IRRADIATION IN THE 248 - 334 nm RANGE

The highly reversible thermally stable photochromic system consisting of heterocoerdianthrone‐endoperoxide (HCDPO) and its parent compounds HCD and 02 is proposed as a new reusable liquid chemical actinometer in the UV region 248 ≤λ≤ 334 nm. The foremost advantageous features of this new system are: (1) high reproducibility and accuracy, (2) high sensitivity, (3) almost wavelength independent quantum yields, (4) no loss in accuracy even after 100 repeated actinometric cycles and (5) very easy handling and straightforward calculability of the radiation quantum flux.

CHEMICAL ACTINOMETRY IN THE VISIBLE (475 ‐ 610 nm) BY meso‐DIPHENYLHELIANTHRENE

Abstract The self‐sensitized photooxidation of meso‐diphenylhelianthrene to its endoperoxide was kincticallv examined. It is recommendable as a convenient, reliable, and wavelength independent actinometer for the visible wavelength range. The calibration values, the actinometric procedure and the evaluation method are given in detail.

Comparison of the photochemical and thermal rearrangement reaction of endoperoxides

Abstract The yields Qdec(S1) and Adec of the photochemical and thermal rearrangement reaction have been determined for 13 endoperoxides of the anthracene and tetracene series. A strong substituent effect operates on Qdec(S1) and Adec, which are closely correlated. The interpretation of our results leads to the postulate that the lowest excited singlet state of endoperoxides is deactivated exclusively by a chemical process, by OO bond breakage on the repulsive S1 potential surface. Re-formation of ground state endoperoxide occurs by subsequent OO bond formation. This conclusion is confirmed by the observation that even for photochromic endoperoxides with low Qdec(S1) neither fluorescence nor phosphorescence occur. The intermediate biradical, which is the common precursor in the photochemical and thermal rearrangements, is involved in a chemical equilibrium with ground state endoperoxide. For the photochromic endoperoxides, and most other endoperoxides, the rate constants of thermal cycloreversion and of thermal OO bond cleavage are of the same order of magnitude.

Die kinetik der selbst-sensibilisierten photooxidation des heterocoerdianthrons im bereich niedriger konzentrationen

The self-sensitized photooxidation of heterocoerdianthrone was investigated in air-saturated benzene and carbon disulfide as a function of substrate concentration at 25 °C. At very low concentrations the overall quantum yield was found to be independent of substrate concentration. This effect is interpreted in terms of a re-encounter of O2(1Δg) and ground state substrate molecules generated in the same triplet—triplet annihilation act. In both solvents the overall re-encounter probability is found to be 0.74. In benzene the overall quantum yield is independent of substrate concentration at concentrations <10−6 M, in carbon disulfide at concentrations <10−7 M. It can be shown that this results from the different life times of O2(1Δg) in the respective media.

Near-infrared phosphorescence emission of compounds with low-lying triplet states

Abstract A sensitive near-infrared emission spectrometer allowed us to detect corrected phosphorescence spectra of several singlet oxygen sensitizers in the wavelength range between 700 and 1400 nm. The compounds investigated in this work were tetracene,9,10-diphenyltetracene, 5,10,15,20-tetraphenylporphyrin, Zn-tetraphenylporphyrin, and the Si-naphthalocyanine bis(tri-n-butylsiloxy)silicon-2,3-naphthalocyanine in degassed solvent matrices at 77 K. Triplet-state energies and the room-temperature phosphorescence quantum yield of the silicon naphthalocyanine have been determined.

Die entwicklung eines neuen photochromen strukturprinzips basierend auf der reversiblen photo-oxidation

Zusammenfassung Die durch Photo-oxidation entstehenden Endoperoxide (POs) aromatischer Kohlenwasserstoffe (KWs) konnen thermisch aber auch photochemisch mit der Quantenausbeute Q 1 wider in die Edukte KW und O 2 gespalten werden. Beide Reaktionen konkurrieren mit den thermisch oder photochemisch induzierten Umlagerungsreaktionen der Endoperoxide, die mit den Asbeuten A dec beziehungsweise Q dec erfolgen. Das Verhaltnis Q 1 / Q dec bestimmt die Reversibilitat der potentiell photochromen Systeme Es kann eine enge Korrelation zwischen A dec und Q dec nachgewiesen werden. Weiterhin wird ein Zusammenhang zwischen Struktur und Grosse von A dec beziehungsweise Q dec fur bestimmte Endoperoxide gefunden. Basierend auf diesen Erkenntnissen und auf Literaturdaten wird ein Konzept fur die Struktur hochreversibler photochromer Systeme vorgeschlagen. Zur Prufung des Konzepts wurden zwei neue Kohlenwasserstoffe und deren Endoperoxide hergestellt. Eine Untersuchung der photochromen Eigenschaften der beiden neuen Systeme bestatigt das entwickelte Konzept.

Generation of 1O2 from oxygen quenching of the lowest excited singlet and triplet states of some aromatic compounds

Abstract Measurements of the overall quantum yield of the tetracene-, rubrene- and heterocoerdianthrone-sensitized photoperoxidation of 1,3-diphenyl-isobenzofuran and of the self-sensitized photoperoxidation of 9,10-dimethyl-anthracene, 9,10-diphenylanthracene, 1,4-dimethoxy-9,10-diphenylanthracene, benzodixanthene, anthradichromene, dimethylhomoocoerdianthrone and meso-diphenylhelianthrene as a function of dissolved oxygen are presented. The experimental results indicate that whenever the oxygen quenching of the S1 state of the compounds investigated leads directly to the formation of the T1 state molecular singlet oxygen in its 1Δg state is produced with an efficiency of about unity, if the S1-T1 splitting exceeds the excitation energy of O2(1Δg) at 7880 cn−1, i.e. the process S1 + 3O2 → T1 + 3O2 is negligible compared with the energy transfer process S1 + 3O2 → T1 + O2(1Δg). The results further imply that the S1 state quenching of some aromatic compounds involves induced internal conversion. If the energy of the T1 state exceeds the excitation energy of O2(1Δg) the oxygen quenching of T1 occurs also only via the energy transfer mechanism T1 + 3O2 → S0 + O2(1Δg).

Picosecond laser photolysis study of the photocycloreversion of heterocoerdianthrone endoperoxide

Abstract The formation of heterocoerdianthrone (HCD) following laser pulse photolysis of its endoperoxide (HCDPO) was observed by laser-induced fluorescence. The rise time of HCD was determined to be τ r = 40±10 ps. The lifetime of the S 3 (ππ * ) state of HCDPO, from which cycloreversion occurs, was estimated by steady-state fluorescence measurements to be τ

Self-sensitized photo-oxidation of aromatic compounds and photocycloreversion of endoperoxides: applications in chemical actinometry

Abstract The self-sensitized photo-oxidation of meso-diphenylhelianthrene is recommended for actinometric application in the 475 - 610 nm range. The photoreversible photocycloreversion of the endoperoxide of heterocoerdianthrone is excellently suited for reusable actinometry inthe 248 - 334 nm range. Both photochemical systems represent reliable, convenient and sensitive actinometric systems.