Photoexcited quinone

Such a rate constant behavior is ascribed to an inner-sphere ET process. Most importantly, there is unambiguous spectroscopic and kinetic evidence for the formation of encounter complexes [ArH, Q ] between the arene and the photoexcited (quinone) acceptor prior to electron transfer [54]. 

In contrast, electron transfers from unhindered (or partially hindered) donors such as hexamethylbenzene, mesitylene, di-ferr-butyltoluene, etc. to photoactivated quinones exhibit temperature-independent rate constants that are up to 100 times faster than predicted by Marcus theory, poorly correlated with the accompanying free-energy changes (see Figure 20A), and only weakly affected by solvent polarity and salt effects. Most importantly, there is unambiguous (NIR) spectroscopic and kinetic evidence for the pre-equilibrium formation K c) of long-lived encounter complexes (exciplexes) between arene donor (ArH) and photoexcited quinone acceptor (Q ) prior to electron transfer (A et) [20] (Eq. 95). 

Intuitively, suitable substituents will favor placing the spin on one set and the charge on the other. Indeed, solution phase, chemi-ionization via a photoexcited quinone of gem-diaryl-substituted methylenecyclopropane results in two, non-interconverting, different radical cations. For the 2,2-derivatives (11) the electron is lost from the ring. This results in... 

One may alternatively remove an electron from one of the n bonds of 3,3 -bicyclopropenyl, although in fact this process produces the same radical cation as before because of extensive through-bond interactions. Solution phase, photoexcited quinone-induced chemi-ionization of 3,3 -dimethyl-3,3 -bicyclopropenyl (22) results in the... 

The types of compounds presently known to donate a hydrogen atom to photoexcited quinones include aldehydes, primary and secondary alcohols, esters and lactones, ethers and thioethers, olefins having allylic hydrogen atoms, alkylbenzenes, benzene, and saturated hydrocarbons. This list is undoubtedly incomplete. Acetone, methyl ethyl ketone, acetic acid, and -butyl alcohol react extremely slowly. 

The irradiation of an alkane solution in acetonitrile with visible light in the presence of catalytic amounts of quinone and copper(II) acetate gives rise to the formation of almost pure alkyl hydroperoxide which is decomposed only very slowly under these conditions to produce the ketone and alcohol [64c,d]. It has been proposed [64d] that the first step of the reaction is a hydrogen atom abstraction from the alkane, RH, by a photoexcited quinone species to generate the alkyl radical R and semiquinone. The former is rapidly transformed into ROO and then alkyl hydroperoxide, while the latter is reoxidized by Cu(II) into the initial quinone (Scheme IX. 10). 

Shi, Y. Wan, P. Charge polarization in photoexcited alkoxy-suhstituted biphenyls formation of biphenyl quinone methides. J. Chem. Soc., Chem. Commun. 1995, 1217-1218. 

Various enol silyl ethers and quinones lead to the vividly colored [D, A] complexes described above and the electron-transfer activation within such a donor/acceptor pair can be achieved either via photoexcitation of charge-transfer absorption band (as described in the nitration of ESE with TNM) or via selective photoirradiation of either the separate donor or acceptor.41 (The difference arising in the ion-pair dynamics from varied modes of photoactivation of donor/acceptor pairs will be discussed in detail in a later section.) Thus, actinic irradiation with /.exc > 380 nm of a solution of chloranil and the prototypical cyclohexanone ESE leads to a mixture of cyclohexenone and/or an adduct depending on the reaction conditions summarized in Scheme 5. 

Photoinduced electron transfer (PET Scheme 6.2) is a mild and versatile method to generate radical ion pairs in solution," exploiting the substantially enhanced oxidizing or reducing power of acceptors or donors upon photoexcitation. The excited state can be quenched by electron transfer (Eq. 7) before (aromatic hydrocarbons) or after intersystem crossing to the triplet state (ketones, quinones). The resulting radical ion pairs have limited lifetimes they readily undergo intersystem ... 

Another variant of the photochemical reaction between A-hetero-cyclic o-quinones and olefins has been described by Mustafa et al.196 200 Stilbene reacts with CXLIX and with CL to give the photoproducts CLVIII and CLIX, respectively. Similar photoaddition products were obtained by the interaction of phenanthraquinone with a-stilbazole245 and with l,2-di-(4 -pyridyl)ethylene.241 Although the process has been suggested as involving diradicals, it is not clear whether the quinone or the olefin undergoes photoexcitation. 

Schuchmann MN, Bothe E, von Sonntag J, von Sonntag C(1998) Reaction of OH radicals with benzo-quinone in aqueous solutions. A pulse radiolysis study. J Chem Soc Perkin Trans 2 791-796 Schuchmann MN, Schuchmann H-P, Knolle W, von Sonntag J, Naumov S, Wang W-F, von Sonntag C (2000) Free-radical chemistry of thiourea in aqueous solution, induced by OH radical, H atom, cx-hydroxyalkyl radicals, photoexcited maleimide.and the solvated electron. Nukleonika 45 55-62... 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

The photochemist is rather familiar with the photoexcited triplet states and the associated intersystem crossing processes. It is well documented that the photoexcited triplet state plays an important role in organic photochemistry. It is thus conceivable that the electron spin polarization of the photoexcited triplet can be further transferred to a radical pair formed by the reactions of the triplet with a suitable substrate. Such a photoexcited triplet mechanism was first proposed by Wong and Wan in 1972 (135) to account for the "initial polarization" observed in the naphthosemiquinone radical formed in the photoreduction of the parent quinone in isopropanol. It was further considered that the triplet mechanism might also lead to CIDNP if such initially polarized radicals react rapidly to give products with nuclear spin polarization induced via the Overhauser mechanism. 

The formation of the hydrogen bond between the quinone oxygen atom and the amide proton in the reduced form (Fc—Q" ) is observed in photoinduced ET (133). Photoexcitation of the Q moiety in Fc—Q in deaerated PhCN with 388-nm femtosecond (150 fs width) laser light results in appearance of a new absorption... 

Figure 9. Inclusion of quinone substrates into the cyclodextrin moiety promotes the occurrence of an electron transfer process from the photoexcited porphyrin fragment to the quinone.

Reaction 1 represents formation of the geminate reaction products [the chlorophyll r-cation radical ( Chl+) and quinone radical anion ( Q )] within the bilayer by electron-transfer quenching of the photoexcited chlorophyll triplet state reaction... 

Fig. 19.20 Stereoview of the photosynlhelic reaction center. The photoexcited electron is transferred from the special pair to another molecule of bacteriochlorophyll (BCD. then to a molecule of bacteriopheophytin (BPh), then to a bound quinone (Q), all in a period of 250 ps. From the quinone it passes through the nonheme iron (Fe) to an unbound quinone (not shown) in a period of about 100 p,s. The electron is restored to the hole in the special pair via the chain of hemes (He I, etc.) in four cytochrome molecules, also extremely rapidly ( 270 ps). The special pair here is rotated 90° with respect to Fig. 19.19. [From Deisenhofer, J. Michel, H. Huber, R. Trends Biochem. Sci. 1985. 243-248. Reproduced with permission.]...

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