For more than 70 years, researchers in several areas of science have been intrigued by the physical and chemical properties of the lowest excited states of molecular oxygen. With two singlet states lying close above its triplet ground state, the O2 molecule possesses a very unique configuration, which gives rise to a very rich and easily accessible chemistry, and also to a number of important photophysical interactions. In particular, photosensitized reactions of the first excited state, O2(∆g), play a key role in many natural photochemical and photobiological processes, such as photodegradation and aging processes including even photocarcinogenesis. Reactions of O2(∆g) are associated with significant applications in several fields, including organic synthesis, bleaching processes, and, most importantly, the photodynamic therapy of cancer, which has now obtained regulatory approval in most countries for the treatment of several types of tumors. The development of both applications and novel observation techniques has strongly accelerated during the past few years. Significant recent advances include, for example, the development of novel luminescent singlet oxygen probes,1-4 the time-resolved detection of O2(∆g) in a transmission microscope,5 the first time-resolved measurements of singlet oxygen luminescence in vivo,6 and the observation of oxygen quenching of triplet-excited single molecules.7 Experimental and theoretical studies on the mechanisms of photosensitized formation of excited O2 states and of their deactivation have been performed for almost 40 years. While most early liquid-phase studies were exclusively concerned with O2(∆g), recent technological advances also made possible time-resolved investigations of the second excited state, O2(Σg), which can be formed in competition with O2(∆g) in many cases. A significant number of * Corresponding author. Tel.: ++49 69 79829448. Fax: ++49 69 79829445. E-mail: R.Schmidt@chemie.uni-frankfurt.de. 1685 Chem. Rev. 2003, 103, 1685−1757

Physical mechanisms of generation and deactivation of singlet oxygen.

Singlet oxygen, O2(a1Δg), the lowest excited electronic state of molecular oxygen, has been known to the scientific community for ∼80 years. It has a characteristic chemistry that sets it apart from the triplet ground state of molecular oxygen, O2(X3Σ−g), and is important in fields that range from atmospheric chemistry and materials science to biology and medicine. For such a “mature citizen”, singlet oxygen nevertheless remains at the cutting-edge of modern science. In this critical review, recent work on singlet oxygen is summarized, focusing primarily on systems that involve light. It is clear that there is indeed still something new under the sun (243 references).

Singlet oxygen: there is indeed something new under the sun

Molecular oxygen forms a reversible charge transfer complex (CTC) with poly(3-alkylthiophenes). The complex is weakly bound but possesses a distinct absorption band in the visible region. The electronic properties of the neutral polymer are modulated by the CTC. The extent of modulation of these properties is proportional to oxygen pressure and is fully reversible. Analysis of poly(3-hexylthiophene) field effect transistors under increasing pressures of oxygen shows that the carrier concentration increases, conductivity increases, and the charge carrier mobility is lowered by the formation of the charge transfer complex. The CTC manifests itself as a fluorescence quencher of mobile polaronic excitons, and it explains why oxygen quenches luminescence with much greater efficiency than predicted by a collisional quenching model. Implications of the charge transfer complex on the photochemistry of poly(3-alkylthiophenes) and photosensitization of singlet oxygen are discussed.

Interaction of oxygen with conjugated polymers : charge transfer complex formation with poly(3-alkylthiophenes)

Polyatomic molecules radiationless transition rate statistical analysis, suggesting role of Franck-Condon principle in determination of nonradiative decay rates

Multiphonon Processes in the Nonradiative Decay of Large Molecules

Cette sonde est basee sur le quenching a fluorescence et consiste en 2 brins de 250 μm de fibre optique plastique se terminant par une section de polymere poreux de 3 mm de long et 0,6 de diametre

R. S. Mulliken

Papers

Molecular Complexes and their Spectra. XII. Ultraviolet Absorption Spectra Caused by the Interaction of Oxygen with Organic Molecules1