Quenching excited sensitizer

Zimmerman and co-workers were also able to obtain some information regarding the multiplicities of the excited states responsible for the initial /9-cleavage through quenching and sensitization studies. It was found that both trans-to-cis and cis-to-trans isomerizations could be sensitized by chlorobenzene under conditions where the latter absorbed over 95% of the light. The same product ratio was obtained under these conditions as in the direct irradiation of the ketones. With 1,3-cyclohexadiene or 2,5-dimethyl-2,4-hexadiene as quenchers nearly 90% of the reaction of the trans isomer could be quenched. Again the ratio of the quenched reaction products was the same as in the unquenched reaction. The reaction of the cis isomer, on the other hand, could not be quenched by 1,3-cyclohexadiene or 2,5-dimethyl-2,4-... 

Many compounds sensitize biomolecules to damage by UVA (320-380 nm) and visible light. Two general mechanisms of sensitization are encountered. The Type I mechanism involves electron or hydrogen transfer from the target molecule to the photosensitizer in its triplet state. If 02 is present, this can be reduced to 02 by the reduced sensitizer. In the Type II mechanism, the excited sensitizer is quenched by 02, which is excited to the singlet state (typically A"g) and attacks the target molecule. Photosensitization is exploited in photodynamic therapy (PDT) for the destruction of cancerous or other unwanted cells. 

A study has been made of the relative efficiencies with which various transition metal chelates quench triplet benzophenone.194 The chelates vary widely in efficiency, and no generalizations can be drawn except that in some cases triplet energy transfer to a coupled metal-ligand triplet energy level probably accounts for at least part of the quenching. Rare earth ions can quench excited triplets by energy transfer, since, as discussed earlier, sensitized fluorescence of the metal ion results. 

Among the 1,3-linked bichromophoric anthracenes listed in Table 3, 1,3-di-9-anthryl-l-propanone 21a, l,3-di-9-anthryl-l-butanone 21b, and l,3-di-9-anthryl-2-methyl-l-propanone 21c are exceptional because their photochemical isomerization by intramolecular 4n+4n cycloaddition to give 22 is characterized by high quantum yields, viz. 0.65, 0.40, and 0.72, respectively. For photochemical cycloadditions of linked anthracenes, the quantum yield of 0.72 is the highest ever observed. Oxygen quenching and sensitization experiments indicate that 21a, 21b, and 21c undergo the 4n+4n cycloaddition in the excited triplet state (see Section II.C). 

The quenching rate constants (kq) (7 j0 of the excited sensitizer by PVS° and DQS0 in the Si02 colloid are 1.5xl09 and 4xl08 M-1.s-1... 

A significant drawback of metals for photoelectrochemical applications lies in their ability to efficiently quench excited states via energy transfer processes, as discussed below. Direct detection of photosensitized electron transfer to or from a metal electrode surface has been observed [30]. However, unlike dye-sensitized semiconductor systems, little examination of the kinetics of such systems has yet been undertaken. 

The photoreduction of 2- and 3-chloroanisole in alcoholic solvents has been studied and is considered to be best accounted for by invoking methoxyphenyl radicals which abstract hydrogen atoms from the solvent378.4-Chloroanisole probably reacts partly via a homolytic cleavage, but in view of the results of quenching and sensitization experiments, another pathway consists of electron transfer from the solvent (ROH) to excited aryl halide, followed by dissociation of the radical anion into chloride ion and aryl radical. 

The photonucleophilic substitution of the monochloroanisoles in alcoholic solvents had been studied earlier378 and it was then concluded that 4-chloroanisole reacts via a radical anion formed by electron transfer from the solvent to an excited molecule, whereas 3-chloroanisole undergoes substitution via an aryl cation. In another study379, it was found, on the basis of quenching and sensitization experiments and on the basis of the ratios of... 

Comparison of Measured Lifetimes of the First Excited Singlet States of Various Donors in Aerated Solutions, td, with Those Estimated from Quenching and Sensitization Constants, Kt and K Obtained for Various Donor-Biacetyl Systems... 

The key step in these DCA-sensitized photooxygenations of enol ethers, involves a diffusion-controlled rate electron-transfer fluorescence quenching of the singlet excited sensitizer by the donors (Eox in the range 1.24-1.30 V vs SCE) with the generation of the radical ion pairs. Seemingly, formation of the ultimate products 59-63 could be easily rationalized on the basis of the classical Foote mechanism... 

In fact, the addition of 1,4-dimethoxybenzene (DMB) (0.13 equiv.), easier to oxidize than 21, appreciably quenches the reaction. The laser flash photolysis with a nitrogen-pulsed laser (337 nm) of a TPP+BF4 (1.6 x 10-4 M) dichloromethane solution, in the presence of 21 (6.2 x 10 2 M), conditions in which nearly 80% of the singlet excited sensitizer (Ered = 0.29 V vs SCE) is quenched by 21, gives a transient absorption (A 550 nm) assigned to a pyranil radical TPF. 

There are two possible excited state interfacial electron transfer processes that can occur from a molecular excited state, S, created at a metal surface (a) the metal accepts an electron from S to form S+ or (b) the metal donates an electron to S to form S . Neither of these processes has been directly observed. The two processes would be competitive and unless there is some preference, no net charge will cross the interface. In order to obtain a steady-state photoelectrochemical response, back interfacial electron transfer reactions of S+ (or S ) to yield ground-state products must also be eliminated. Energy transfer from an excited sensitizer to the metal is thermodynamically favorable and allowed by both Forster and Dexter mechanisms [20, 21]. There exists a theoretical [20] and experimental [21] literature describing energy transfer quenching of molecular excited states by metals. How-... 

Large geometrical distortion in the lowest excited states of Pt(gly)2 is also suggested by spectroscopic studies, which have indicated that similar square planar complexes have pseudotetra-hedral excited states (93a), by semi-empirical molecular orbital calculations (56), and also by recent quenching and sensitization studies of the cis trans reaction (Section lII-D-1). ... 

In fact, as shown by the quantum yields observed in fert-butyl alcohol, these reactions appear to be quite inefficient, e.g. formation of 5 and 6. From quenching and sensitization studies, it was shown that the triplet excited state is responsible for these rearrangements. The rate of rearrangement of triplet enone to product (or some precursor of product) was found to be about 10 s F... 

Yokota and Tanimoto (39) have worked out the expected fluorescence decay when both quenching and diffusion are active and the diffusion is not fast enough to maintain the initial distribution of excitation. In such a case the decay function of the excited sensitizers is given by (39)... 

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