Signaling Recognition Events with Fluorescent Sensors and Switches.

2. Activatable Photosensitizer Design Considerations 2842 2.1. Activation Strategy 2842 2.2. Photosensitizer Selection 2844 2.3. Photosensitizer Conjugation 2845 3. Examples of Activatable Photosensitizers 2845 3.1. Environment-Activated Photosensitizers 2845 3.2. Enzyme-Activated Photosensitizers 2846 3.3. Nucleic Acid-Activated Photosensitizers 2852 3.4. Other Activation Mechanisms 2853 4. Conclusion and Outlook 2855 5. Abbreviations 2855 6. Acknowledgments 2855 7. References 2855

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Activatable photosensitizers for imaging and therapy

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

Methylene blue in photodynamic therapy: From basic mechanisms to clinical applications.

Publisher Summary This chapter discusses the reduction potentials involving inorganic free radicals in aqueous solution. Because free radicals are usually transients, knowing their thermodynamic properties is primarily useful in mechanistic studies. Thus, the useful redox couples associated with a given free radical correspond to plausible elementary steps in reaction mechanisms. All potentials are expressed against the normal hydrogen electrode (NHE). Apart from the NHE, the standard state for all solutes is the unit molar solution at 25 °C. This violates the usual convention for species, such as O 2 that occur as gases, but because the rates of bimolecular reactions in solution are significant, the unit molar standard state is most convenient. The emphasis is on electron transfer reactions in which no bonds are formed or broken, electron transfer reactions in which concerted electron transfer and bond cleavage could occur, and certain atom transfer reactions. The chapter also presents a tabulation of ∆ f G ° values for all the radicals. A common approach in estimating the thermochemistry of aqueous free radicals is to use gas-phase data with approximations of solvation energies.

Reduction potentials involving inorganic free radicals in aqueous solution

— The production of singlet oxygen by thiazine dye photosensitization, as measured by the rate of photooxidation of tryptophan, was found to be very sensitive to changes of pH in the range 5–9. For methylene blue in aerated solutions, the production of 1O2* is approximately five times more efficient in basic than in acidic medium. This was shown to be related to the pK's of the triplet dyes, by evaluating the yields of 1O2* from the lifetimes and the quenching rate constants for the two ionic species of sensitizer triplets measured by laser flash photolysis. Changes in the quenching rate constants of the thiazine triplet states can be correlated with the triplet energies.

pH dependence of singlet oxygen production in aqueous solutions using thiazine dyes as photosensitizers.

In a recent article (Bonneau et al., 1975), we reported the variation of photosensitized '02 production via methylene blue and thionine as a function of pH and showed this variation to be directly related to: (1) the energy difference between the triplet dye (donor) and singlet oxygen (acceptor) and (2) lifetimes of the triplet dye. At that time we also predicted that toluidine blue, a thiazine dye which has found important applications in photobiology as an effective electron acceptor (Hasegawa et al., 1969; Mathis, 1972) should also show a wide variation of photosensitized '0, production as a function of pH. If such a variation of photosensitized '02 production as a function of pH occurs for toluidine blue, we deemed it necessary to examine its extent and range of greatest sensitivity. As pointed out for methylene blue, a slight change of pH in the near neutral zone can significantly change the '0, production. Thus, depending on the exact pH of the solution and whether or not the solution is buffered, one could conceivably classify toluidine blue as a good or a poor sensitizer. One of the toluidine blue acid-base equilibria can be represented as in the triplet state as being equal to 7.6 f 0.5, using the flash photolysis technique. Using a laser flash photolysis technique previously described (Bonneau et al., 1975) we have measured, in both slightly acidic and basic media, the quenching rate constants by oxygen for toluidine blue triplet states. For non-buffered aqueous solutions of toluidine blue, concentration being 3 x M , the quenching rate constants for the triplet states are 2.9 x 10' mol-'s-' at pH 3 for the 3BH:' species and 3.3 x lo9 mol-' s' at pH 8 for the 'BH' species. Under the same experimental conditions the lifetimes of the triplet species in the absence of oxygen were measured as 5ps for 3BHif (PH 3) and 60ps for 3BH' (PH 8). From the rate constants it is possible to evaluate the quantum yield of '0, production, assuming, in analogy with other thiazine dyes studied, a simple energy transfer process as shown:

pH dependence of singlet oxygen production in aqueous solutions using toluidine blue as a photosensitizer.

Singlet-singlet absorption and intersystem crossing from the 1B3u− state of naphthalene

— The pH dependence of the apparent reactivity of thiazine dyes in their triplet states has been studied in aqueous solutions, using as electron donor HY-3, the trianionic species of ethylene diamine tetraacetic acid (EDTA), in the pH range 4–8. The pH dependence is found to be related to a change in the degree of protonation of the triplet excited dye. The apparent reactivity and lifetime of two differently protonated forms of thionine, azur B and methylene blue were determined by classical and dye-laser flash techniques, making it possible to evaluate the rate constant for electron abstraction of these molecules in their triplet states. It is found that: (a) protonation on the ring nitrogen increases the electron-abstraction rate constant of the triplet-state species about twenty-fold, and (b) methylation on the side amino groups decreases it.

MECHANISM OF PHOTOREDUCTION OF THIAZINE DYES BY EDTA STUDIED BY FLASH PHOTOLYSIS‐II. pH DEPENDENCE OF ELECTRON ABSTRACTION RATE CONSTANT OF THE DYES IN THEIR TRIPLET STATE

Jacques Joussot-Dubien

Papers

pH dependence of singlet oxygen production in aqueous solutions using thiazine dyes as photosensitizers.

pH dependence of singlet oxygen production in aqueous solutions using toluidine blue as a photosensitizer.

Singlet-singlet absorption and intersystem crossing from the 1B3u− state of naphthalene

MECHANISM OF PHOTOREDUCTION OF THIAZINE DYES BY EDTA STUDIED BY FLASH PHOTOLYSIS‐II. pH DEPENDENCE OF ELECTRON ABSTRACTION RATE CONSTANT OF THE DYES IN THEIR TRIPLET STATE

Multiplet structure of the emission bands of coronene and perylene in n-heptane single crystal