Johann Wolfgang Goethe-Universität

Frankfurt am Main

Institut für Physikalische und Theoretische Chemie



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Photophysics and Photochemistry of Singlet Oxygen - a Very Special Area

Movie of a turning 3D Plot illustrating the dependence of the rate constants of sigma singlet oxygen (yellow), delta singlet oxygen (green) and triplet ground state oxygen            formation on the triplet energy and oxidation potential of the sensitizers. The surface represents the calculated dependence. Details see refs. 119, 120 and 133.

A Molecule With Triplet Ground State: O2(3Sg-)

Molecular oxygen in its 3Sg- ground state has triplet multiplicity but not singlet multiplicity unlike most natural compunds. Chemical reactions forming singlet molecules from triplet and singlet reactants are forbidden by Wigner's spin selection rule. Thus, the triplet multiplicity is the actual reason, why most reactions of oxygen with organic substances, although being exergonic, do not proceed at room temperature but upon heating or in the presence of catalysts. It is said that reactions of  organic compunds with oxygen are kinetically inhibited. This effect enables our life in an oxygen containig atmosphere. 

The First Excited State: O2(1Dg) Singlet Oxygen

The lowest excited singlet state of O2 lies by only 94 kJ mol-1 above the triplet ground state. This 1Dg  state is commonly populated by electronic energy transfer from photoexcited sensitizers. The transition to the 3Sg- ground state is strictly forbidden for the isolated O2(1Dg) molecule. The forbiddeness becomes weakened in collissions. Nevertheless, the O2(1Dg) singlet oxygen is a metstable species even in the condensed phase. Because of its singlet multiplicity no spin-forbiddeness exists for reactions of O2(1Dg). Therefore, and due to its excitation energy of  94 kJ mol-1 singlet oxygen is chemically extraordinary reactive. 

The present knowledge about physical mechanisms of generation and deactivation of singlet oxygen has recently been presented by us in a comprehensive review. Ref. 120 of our publication list. A review dealing specially with new developments in photosensitization of singlet oxygen appearrd in 2006. Ref. 133

Chemistry of Singlet Oxygen and Some of its Applications

The enormous reactivity of singlet oxygen finds applications in bleaching and disinfection reactions as well as in many chemical syntheses as for example in the reaction of singlet oxygen with aromatic compounds (A), which leads to the formation of endoperoxides (APO). Both the thermal and the photochemical cleveage of APOs leads back again to aromatic compound A and singlet oxygen. APOs can be part of highly reversible photochromic systems, some are outstanding chemical actinometers., ref. 123. One takes advantage of the toxicity of singlet oxygen in some medical applications as for example in the photodynamic therapy (PDT) of tumors. In PDT a suited photosensitizer is adminstered the patient. After 24 to 48 hors most of the sensitizer has disappeared rom the sound tissue. The sensitizer is still enriched in the tumor tissue after that time due to a retarded concentration decrease. The sensitizer is excited by irradiation with red laser light. This leads in the presence of oxygen by energy transfer to the formation of  O2(1Dg) in the tumor tissue inducing tumor necrosis. Porphin derivatives are mostly used as sensitizers in this treatment. However, there are efforts to develop alternative improved sensitizers for PDT. Refs. 69,70,76,120,133

Radiationless Deactivation of O2(1Dg) Singlet Oxygen

Three different radiationless deactivation processes compete with chemical reactions of O2(1Dg). 

e-v Deactivation, a Slow Process With an Enormous Variation of Rate Constants

The electronic excitation energy of O2(1Dg) is converted into vibrational energy of terminal bonds of deactivating collision partner. The rate constant of this spin-forbidden and thus relatively slow deactivation increases exponentially with theenergy of the stretching vibration of the deactivating bond in the series C-F to C-D, O-D, C-H and finally O-H. This particularity leads to an enormous solvent dependence of the O2(1Dg) lifetime. We found that the variation of lifetimes extends over six orders of magnitude! The shortes lifetime is observed with 3.1 ms in H2O the longest with 300 ms in perfluorodecaline. We developed a model, which quantitatively describes the e-v deactivation. Refs. 60,75,79,87,120

CT Quenching, the Second-Fastest Deactivation Process

The charge-transfer (CT) induced deactivation is observed with quenchers of low oxidation potential. Intermediately exciplexes are formed, which decay by internal conversion to unstable ground state complexes, as we could demonstrate. Refs. 112,113,115,120

Almost Diffusion-Controlled, Electronic Energy Transfer

Quenchers with very small triplet energy can deactivate by spin-allowed electronic energy transfer. This in principle fastest deactivation process proceeds almost diffusion-controlled, if the triplet energy of the quencher is smaller than the excitation energy of O2(1Dg). In a recently published paper we demonstrate that the excess energy dependence of the rate constants follows the same rules for the quenching of singlet oxygen by ground state carotenoids as for the quenching of triplets states by ground state oxygen. The deactivation proceeds via internal conversion of excited encounter complexes, ref. 122. Most plants protect themselves from the toxic effect of O2(1Dg) by synthesizing carotenes or lycopenes. Ref. 120.

The Second Excited State O2(1Sg+): Again a Singlet

Only little above the 1Dg excited singlet state lies the 1Sg+ singlet state of the oxygen molecule with excitation energy 157 kJ mol-1. The 1Sg+ state is very fast and completely deactivated to the metastable 1Dg state in collisions with other molecules, as we have found. The deactavation follows very similar rules as in the case of  O2(1Dg). The rate constants, however, are large by a factor of about 106 than in case of the e-v deactivation of  O2(1Dg) due to the omit of the spin-forbiddeness and the smaller excess energy. A model developed by us describes quantitatively the whole variaton of the rate constants of the e-v deactivation of  O2(1Dg) and O2(1Sg+) over more than ten orders of magnitudes. The ev-deactivation is so fast that the O2(1Sg+) lifetime reaches maximum values of about 150 ns only in perchlorinated solvents. In most liquids, however, the lifetime of O2(1Sg+) is significantly smaller than 1 ns. That is the reason why neither CT deactivation nor chemical reactions can compete with e-v deactivation: O2(1Sg+) is in contrast to O2(1Dg) chemically not reactive! Refs. 80,85,87,97,100,120

Phosphorescence of O2(1Dg) Singlet Oxygen, an Unusual Solvent Effect

The transition to the X3Sg- ground state is strictly forbidden for O2 in the a1Dg state. The radiative lifetime 1/ka-X amounts for the isolated O2(1Dg) molecule to about one hour (ka-X = 2.6E-4 s-1). The phosphorescence a -> X is the most forbidden transition of a molecule, according to Kasha.  This strict forbiddeness becomes weakened in collisions, so that the a -> X phosphorescence at 1275 nm can be observed in solution. The strength of that perturbation depends extremely on the solvent. The rate constant ka-X of the phosphorescence a -> X varies from 0.2 s-1 in H2O to about 3.0 s-1 in CH2I2! Ogilby found a strong non-linear correlation of ka-X with the solvent refractive index. Our investigations showed that this is not the consequence of an isotropic perturbation by a continuum solvent. Instead, the a -> X emission is induced by bimolecular collisions even in solution.  We demonstrated that the transtion moment Ma-X of the collision induced emission a -> X is directly proportional to the molecular polarizability of the collision partner of O2. These findings explain for the first time and quantitatively the strong and in some solvent mixtures strange looking solvent dependence of ka-X. Refs. 56,71,92,102,103,120

Fluorescence b -> a, a Transition Between Two Excited States and a Partner of the a -> X Phosphorescence

The radiative transition from the second excited b1Sg+ singlet to the a1Dg singlet state is also forbidden for the isolated O2 molecule. This b -> a transition, which occurs at 1935 nm, is strongly enhanced in collisions. Fink determined rate constants of the bimoleculat collision-induced fluorescence b -> a for a series of colliders in the gas phase, where the lifetime of O2(1Dg) is sufficient. Our analysis demonstrated that the transtion moment Mb-a of the collision induced fluorescence b -> a is again directly proportional to the molecular polarizability of the collision partner. Thus, the same kind of perturbance enhances the a -> X and b -> a emissions. According to Minaev this parallelity is a consequence of the strong spin-orbit-coupling of oxygen. Refs. 92,103,120

Sensitization of Singlet Oxygen by Triplet States, an Exothermic, Spin-Allowed but Still not Diffusion-Controlled Energy Transfer

Efficiencies SD of sensitization of O2(1Dg) have been determined for hundreds of sensitizers because of the importance of O2(1Dg) as chemical reagent. During these studies it could not be differentiated to which amount O2(1Dg) is directly formed or indirectly via the upper excited very short lived O2(1Sg+). This is the reason why no definite relation between SD and molecular parameters of the sensitizer was found. However, we developed recently a method for the seperate determination of the rate constants k1S, k1D, and k3S of the three deactivation channels leading to formation of O2(1Sg+), O2(1Dg), and O2(3Sg-) in the quenching of the triplet state T1 of the sensitizer  by O2. This method allows for the first time to reveal the rules which govern the competition of these three channels. We found that the excitation energy and the electronic configuration (np*, pp*) of the T1 state as well as the oxidation potential of the sensitizer and the polarity of the solvent decisivly determine this competition. Refs. 84,90,110,111,114,116,118-120,126,129,130,133
3D Plot of the rate constants of sensitization which depend in a well defined way on triplet state energy and oxidation potential of the sensitizer. Refs. 119,120,133.

These studies are financially supported by the Deutschen Forschungsgemeinschaft and recently also by the Adolf Messer Stiftung.

Procedures and Instruments:

Stationary and time-resolved emission in the UV/VIS (ns-ms) und NIR (ns-s) regions. Refs. 109,110.

Stationary and time-resolved absorption, iterative deconvolution, 

excimer laser, N2 laser, Nd:Yag laser with frequency tripling

IR-semiconductor detectors and photomultipliers, photoacoustic calorimetry