Johann Wolfgang Goethe-Universität
Frankfurt am Main
Institut für Physikalische und Theoretische Chemie
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.
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.
e-v Deactivation, a Slow Process With an Enormous Variation of Rate ConstantsThe 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 ProcessThe 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 TransferQuenchers 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.
Fluorescence b -> a, a Transition Between Two Excited States and a Partner of the a -> X PhosphorescenceThe 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 TransferEfficiencies 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.
Stationary and time-resolved absorption, iterative deconvolution,
excimer laser, N2 laser, Nd:Yag laser with frequency tripling,
IR-semiconductor detectors and photomultipliers, photoacoustic