Sensitizers, triplet

Attempts to sensitize the rearrangement with benzophenone, propiophenone, and chlorobenzene failed, as indicated in Table 8.1. Although the reaction could not be sensitized, triplet energy transfer was taking place inasmuch as compound (1) quenched the photoreduction of benzophenone without the formation of any new products (Table 8.2). 

Herkstroeter and Hammond found support for this postulate from a flash photolysis study. They were able to measure directly the rate of sensitizer quenching (energy transfer) by cis- and fra/w-stilbene. When a sensitizer triplet had insufficient excitation energy to promote fims-stilbene to its triplet state, the energy deficiency could be supplied as an activation energy. The decrease in transfer rate as a function of excitation energy of the sensitizer is given by... 

Significantly, there is no variation in the relative product distribution in this case as a function of the sensitizer triplet energy. 9-Anthraldehyde (Et = 42 kcal/mole) was ineffective as sensitizer for this reaction, presumably because its lowest triplet is lower in energy than that of cyclo-pentadiene.<23>... 

As with cyclopentadiene, the relative yields of products (19)—(21) were relatively insensitive to the sensitizer triplet energy(37) although an effect of temperature on dimer distribution has been noted.0,28 The direct photolysis of cyclohexadiene with wavelengths greater than 330 nm yielded products (19)—(21), although the product distribution in this case was more nearly statistical [(19) (20) (21) = 44% 2470 33%] 28 . 

The product distribution in this reaction was found to be dependent upon the triplet energy of the sensitizer. Variation of the relative amounts of cyclobutanes (32) and (33) to cyclohexene (34) with sensitizer triplet energy... 

As has already been mentioned (vide supra, p. 118) the population of the triplet state of a carbene depends on the effectiveness of the intersystem crossing steps kisc and k lse- These rates can be altered by dilution. Another technique which exclusively populates the triplet state of a carbene is sensitization, or energy transfer. A triplet sensitizer is required for this purpose — usually an aromatic ketone. In these ketones the intersystem crossing efficiency is almost 100%. Energy is then transferred from the sensitizer triplet to the diazoalkane, thus populating the triplet state of the latter. 

In cases of self-quenching, the acceptor and ground state sensitizer will compete for the sensitizer triplet as illustrated in the following scheme. 

Notice that this is the same as the mechanism written in Section II except that step (9), self-quenching, has been added. If we assume that the quantum yield of intersystem crossing for the sensitizer is unity and that all the sensitizer triplet is quenched either by the acceptor or by ground state sensitizer, then the following expression for 4>a, the actual quantum yield for reaction of the substrate triplet, can be derived ... 

The quantum yield for product formation from the sensitizer triplet, 0, is given by the expression ... 

Equation (15) permits us to examine the circumstances under which back transfer will become important. If we assume that energy transfer to the acceptor is the only mode by which the sensitizer triplet decays to the ground state (i.e., kd 0), Eq. (15) reduces to ... 

When a ketone is excited in hydrogen containing solvent, the solvent competes with acceptor for the sensitizer triplet. A higher acceptor concentration favors energy transfer (ke) over hydrogen abstraction (ka) as expressed by the following equation ... 

In the initial work135,136 on the sensitized isomerization of stilbene, it was found that quinone sensitizers gave photostationary states which were trans-rich relative to those predicted by energy transfer (Fig. 3). The original plot of sensitizer triplet energy versus isomer ratio of the photostationary state therefore showed several maxima and minima. Later work on the rates of energy transfer from various sensitizers to stilbene proved that the plot was in fact a smooth curve and that the photostationary states observed with quinones were not the true ones.137 Irradiation of benzene solutions of... 

The effect of singlet quenchers on the sensitizer triplet yield (Ot) is derived similarly. 

In the triplet counting technique the substance whose intersystem crossing yield is to be determined is used as a sensitizer for the photoisomerization of piperylene (1,3-pentadiene). Under conditions where essentially all of the sensitizer triplets are intercepted by diene, the quantum yield of isomerization is directly proportional to the intersystem crossing yield of the sensitizer. If the diene interferes with sensitizer singlets however, the yield of triplets will fall, resulting in an anomalously low value for 0,. 

Azides. In a study of the photosensitized decomposition of alkyl azides,202 an apparently linear relationship between sensitizer triplet energy and the log of the rate constant for energy transfer was obtained. Phenan-threne, however, gave a somewhat larger value than expected for this rate constant, which was determined from the variation in quantum yield of... 

The use of ketones with (ir, n) triplet states to sensitize photoreductions will be attended by chemical sensitization for example, with benzophenone in isopropyl alcohol and 0.5M acceptor, 1% of the sensitizer triplet will still abstract from the solvent even if the acceptor quenches at the diffusion controlled rate. Failure to determine that the quantum yield of reduction was greater than 0.01 might lead to the conclusion that triplet sensitization was occurring. 

Flash spectroscopic measurements of the rate constants for quenching of sensitizer triplets by the stilbenes have borne out the original interpretation of the photostationary state data very satis-... 

Figure 3-24 Simplified diagram of the experimental apparatus for the optically sensitized triplet generation/TR3 studies. Requisite time delays were obtained either by optical delay or by the two-laser experimental configuration. The optical delay line is not to scale its actual length was approximately 120 ft. (Reproduced with permission from Ref. 82. Copyright 1981 American Chemical Society.)...

An additional step in the cascade reaction scheme is the quenching of the sensitizer triplet state with relatively low-concentration radicals (Fig. 1.5) (Papper et al 1999, 2000 Papper and Likhtenshtein, 2001). The entire investigated reaction that is shown in Fig. 1.5 is the sequence of the four kinetic processes and serves as a basis for the spin-triplet-photochrome labeling technique. This technique combines the three types of biophysical probes stilbene photochrome probe, triplet probe and stable nitroxide-radical spin probe, which depresses the sensitiser exited triplet state. 

Due to the spin conserving rule, the rate constants of some excitonic interactions are subject to modulation by weak magnetic fields (less than IT). The magnetic field modulation of the singlet exciton fission rate [see Scheme (80)] can be experimentally observed as a magnetic field effect on prompt fluorescence [192,200,201,273-278]. The magnetic field sensitive triplet-triplet (80) and triplet-doublet (110) annihilation... 

Nonradiative Processes. The nonradiative pathways which depopulate are still a matter of some discussion. It has been pointed out that the short lifetime of precludes collisional quenching of this electronic state, and the failure to observe sensitized triplet emission from biacetyl (92) indicates that the... 

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