Goethe
Johann Wolfgang Goethe-Universität

Frankfurt am Main

Institut für Physikalische und Theoretische Chemie


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Photochemistry with Singlet Oxygen 

General Reaction Kinetics


Endoperoxides of Aromatic Compounds, a Particularly Interesting Class of Compounds

Endoperoxides (APOs) are formed during the self-sensitzed photooxygenation of aromatic compounds (A) by reaction of A with the metastable O2(1Dg). The photochemistry of APOs is very particular, see our most recent review Ref. 121. Reactions originating from upper excited states (one of the rare exceptions of Kasha's rule) are observed as well as two different competing reaction channels: (1) The irreversible rearrangement reaction occurs from the lowest excited singlet state S1. (2) The cycloreversion reaction reforming the aromatic compound and singlet oxygen occurs from upper excited singlet states Sn and triplet states Tn with n>=2. Refs. 17,19, 23,28,30,37-39,43-45,47-49,66,77,82,121.

Our time-resolved investigations of APOs with ps and fs time resolution demonstrated that the photocycloreversion is a very fast two step process. The first C-O bond breaks in less than 350 fs, the second bond breakage requires between 1.6 und 75 ps, depending on APO. Refs. 46,61,63,68,72,121.
 
 

Photochromic Systems, a Colourful Story with Application Perspectives

There are several very sensitive and highly revesible photochromic systems among the systems APO/A, which are suited for data storage, since the electronic spectra of A and APO and A are far apart (A absorbs in the VIS, APO only in the UV region) and since A strongly fluoresces, whereas APO is nonfluorescent. Refs. 16,27,30,45,67,88,121. A very particular photochromic system was developed by us, which can be read fadelessly or bleached, depending on the choice of the irradiation wavelength. Ref 30
 
 

Chemical Actinometers, Light Measuring Systems

Chemical Actinometers (CAs) are well-defined photochemical reaction systems, which are calibrated for the use as convenient and reliable measurement of a UV/VIS irradiation dose. On the basis of the clean photochemistry of some photooxygenation reactions we developed several very accurate and reliable CAs. Using a highly reversible photochromic system APO/A we introduced even a reusible CA. Refs.32,33,42,43,55,58,65,123
 
 

O2(1Dg) and O2(1Sg+) Singlet Oxygen Standards, Still Calibrating

Phenalenone is a sensitizer of singlet oxygen, which is distinguished by a quantum yield of intersystem crossing of unity, by an absorption extending from the UV to the VIS range, and by solubility in solvents of any polarity. We determined for phenalenone the quantum yield of sensitization of singlet oxygen in 13 solvents of very different polarity (from H2O to CCl4) using different techniques. The quantum yield of singlet oxygen sensitization is near unity in all these solvents. With these results we established phenalenone as the first universal standard for the quantitative determination of O2(1Dg) in solution. We could even determine by time-resolved emission spectroscopy at 1275nm in CCl4 an absolute value for the quantum yield of O2(1Sg+) sensitization by phenalenone. By that phenalenone may be used as first standard for the quantitative determination of O2(1Sg+) in CCl4, refs.84,86, and in other solvents as well, ref. 129.
 
 


General Reaction Kinetics

Kinetics of the Photoinduced Cleavage of (Coumarin-4-yl)methyl Esters

The photorelease of biomolecules from inactive precursors (caged compounds) is a powerful tool to study the fast kinetics or spatial heterogeneity of biochemical responses in cell or tissue culture. In caged compounds the biological recognition or activity has been disabled by chemical modification at an essential functionality by introducing a photosensitive protecting group (caging group) until light of a specific color uncages and activates the probe. (Coumarin-4-yl)methyl derivatives are newly developed caging groups and have been successfully applied to protect biological activity in a variety of functional groups. In close collaboration with the groups of Prof. Jürgen Bendig, Humboldt University Berlin, and Dr. Volker Hagen, Institute of Molecular Pharmacology Berlin, we investigated the photokinetics of a series of (coumarin-4-yl)methyl caged compounds, refs. 105,117. The combination of photochemical and fluorescence quantum yield measurements and time-resolved fluorescence measurements indicates a very fast bond cleavage. The formation of the highly fluorescent (6,7-dimethoxycoumarin-4-yl)methyl alcohol of the corresponding esters with diethyl phosphoric acid and methyl sulfuric acid occurs even already during an excitition pulse of 0.5 ns duration. Both caged compounds belong to the fastest phototriggers known, ref. 127. (Coumarin-4-yl)methyl caged compounds are excellent phototriggers with high application potential, refs. 125,128.

These studies have been financially supported by the Adolf Messer Stiftung.

Reaction Volume of Contact Complex Formation

Determination and Interpretation of activation volumes and reaction volumes using high pressure techniques and photoacoustic calorimetry. The reaction volume of contact complex formation is hereby of particular interest. This process is part of a general preequilibrium in any bimolecular reaction. Our investigations demonstrate that already during contact complex formation - in the absence of any bond formation -  a negative volume effect occurs: the reaction volume of contact complex formation, which amounts at least to -10 ml mol-1 in nonpolar solvents. This is of great importance for the interpretation of activation and reaction volumes. Refs. 9-11,14,24,40,54,57,74,95,101
 

These studies have been financially supported by the Deutschen Forschungsgemeinschaft.
 
 

Reaction Kinetics of the Enantioselectively catalyzed Reduction of Prochiral Ketones with Borane Reagents

The biological and pharmaceutical effect of chiral compounds is in general restricted to one enentiomeric form. Since in chemical synthesis usually only racemic mixtures are obtained from prochiral reactants, extensive and expensive procedures are necessary to separate the undesired enantiomer. Therefore, the preparation of enantiomeric pure compounds by means of enantioselectively catalazed procedures becomes more and more important. 

The enantioselective reduction of prochiral ketones with borane reagents catalyzed by oxazaborolidines (OABs), the so-called Corey-Bakshi-Shibata (CBS) reaction, has found great interest because of its high synthetic potential. Depending on ketone OAB, reductant, and procedure ee values up to 99 % have been found. A lot of studies was carried out to further improve the enatioselectivity: changes of the OAB substituents, of the borane reagents, the solvents, and the temperature and even quantum mechanical calcultions have been made. However, no kinetic studies had been performed. Thus, the rate determining steps of the catalyzed reduction and of the competing direct, uncatalyzed reductions were still unknown. 

To fill this gap, we set out for the first kinetic investigation of this interesting reaction. We studied the direct and the catalized reduction of the prochiral ketone pinakolone with borane-THF (B-T), borane-DMS (B-D) and catecholborane (CB) using four differently substituted OABs. We found surprising results.Already the uncatalyzed direct reductions are complex composed reactions. The principal reactions are still first order in ketone P and borane reagent with B-T and B-D. However, catecholborane the reduction is zero order in P and second order in CB! We could describe the kinetics of these reactions by numerical integration. 

The actual catalytic reduction competes with the direct reduction. The kinetics are similar with B-T and B-D. In the series of the four OABs the reaction order in P shifts gradually from first to zero order behaviour. The reaction order in OAB catalyst is always unity, the reaction order in borane reagent is smaller than 1. The whole catalytic reaction can be described by numerical integration for one of the OABs. The comparison of the four catalysts demonstrates that the rate determining step of the catalytic reaction is the hydride transfer in the ternary complex composed of OAP, borane and ketone confirming the respective hypothesis of Corey. The graduation of the reactivity of the catalysts correlates with the ee values. Refs. 98,99,104,107

These studies have been financially supported by the BMBF and the Adolf Messer Stiftung.

Procedures and Instruments: 

Stopped-Flow, Diode-Array-Spectrometer, NMR, 
formal kinetics, numerical integration.