Triplet radical ions

On the other hand, oxidation of a DNA base by a triplet state of the an-thraquinone (AQ5"3) generates a contact ion pair in an overall triplet state, and back electron transfer from this species to form ground states is prohibited by spin conservation rules. Consequently, the lifetime of the triplet radical ion pair is long enough to permit the bimolecular reaction of AQ- with 02 to form superoxide (02 ) and regenerate the anthraquinone. 

The third intra-pair reaction to be discussed involves bond formation between radical anion and cation without intervening transfer both singlet and triplet radical ion pairs can couple. For example, the bifunctional radical cation 24 generates two chloranil adducts, most likely via zwitterions (e.g., 74 and 75 ), initiated by forming a C O bond. The CIDNP results indicate that 74 and 75 are formed from a singlet radical ion pair. Adduct 75 is a minor product, as the major spin density of 24 + is located in the allyl function which, therefore, is expected to be the principal site of coupling. 

In this context it is useful to remember that the concept of the possible recombination of triplet radical ion pairs is not an ad hoc assumption to rationalize certain Z - E isomerizations, although the CIDNP effects observed during an isomerization reaction played a key role in understanding this mechanism. Triplet recombination has been accepted in several donor-acceptor systems as the mechanism for the generation of fast (optically detected) triplets [169-171], and invoked for several other reaction types [172]. The CIDNP technique is a sensitive tool for the identification of this mechanism, for example, in the geometric isomerization of Z- and E-1,2-diphenylcyclopropane and in the valence isomerization of norbornadiene (vide infra). Most of these systems have in common that the triplet state can decay to more than one minimum on the potential surface of the parent molecule. 

Ti of 10-methylphenothiazine to dicyanobenzene, givining a triplet radical ion pair involing the lO-methylphenothiazine cation and dicyanobenzene anion radicals. In the latter reaction, the Si- Ti ISC of 10-methylphenothiazine is much faster than the reaction from Sj. 

Boxer s group [2] first made a ns-laser photolysis apparatus with a super-conducting magnet. The sample was excited at 532 or 600 nm with a frequency-doubled YAG pumped dye laser (8ns, fwhm) and was probed at 860 nm with a laser diode. The maximum field of their magnet was 5 T. With this apparatus, they measured the quantum yield of triplet states (detected optically in quinone-depleted photosynthetic reaction centers (RCs) from R. spheroids, R-26 mutant, as a function of applied magnetic strength and temperature. The reaction scheme for qinone-depleted RCs is shown in Fig. 12.1. Here, the singlet and triplet radical-ion pair (RIP) are represented by [D" A ] and [D A ], respectively, and the rate constants of the S-T conversion of RIP, the recombination from [D A ], and the recombination from [D A ] are denoted by hsT, ks, and kj, respectively. 

Alder reaction the major Diels-Alder product is the trans adduct rather than the cis adduct (Scheme 11). Such a stereochemistry indicates that the Diels-Alder reactions proceed by a stepwise mechanism rather than a concerted mechanism. The photo-induced electron transfer from Danishefsky s diene to Ceo gives the triplet radical ion pair. The triplet radical ion pair is then converted to the singlet radical ion pair to give a zwitterionic intermediate (or a diradical intermediate) in competition with the back electron transfer to the reactant pair. The bond formation occurs stepwise with no symmetry restriction for the bond formation. Thus, both trans and cis adducts were obtained as the final products [305]. 

Radical ion pairs are involved in the photochemical isomerism of P-ionone in the presence of pyrene. trans- -lonone is converted to its radical anion on irradiation in the presence of triphenylphosphine or triethylamine. CIDNP analysis has shown that the radical anions convert to neutral radicals by pro-tonation. Triphenylamine can be used as an electron donating sensitizer for the phototransformations of the enones (75)-(77). Triplet radical ion pairs are formed, which afford the triplet of the enone by back electron transfer, cis-Isomers of the enones are formed. The retinoid acitretin undergoes photo-... 

In a related study. Levin et have examined the relaxation of triplet exciplexes through electron transfer pathways. The magnetic field dependence of the radical ion pair recombination rate constants indicated that these reactions were predominantly of triplet radical ion pairs recombining through electron... 

Styrene derivatives are commonly used addends in the photocycloaddition studies of 1,4-quinones. With Z- and -anethole, 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), and 9,10-anthraquinone in acetonitrile solvent yield spiro-oxetanes in which the trans-isomer (e.g., 4 from naphthoquinone) predominates. The process has been studied in detail by CIDNP techniques from which it is deduced that product formation proceeds from triplet radical ion pairs to the triplet biradical, and that there is no significant contribution from direct conversion of exciplex intermediates into the biradicals. Spiro-oxetane formation between simple alkenes and BQ generally has low regioselectivity but this is markedly improved with alkylidene cyclohexanes (Figure 87.3) such that the major isomer can be used as a new access to useful synthetic building blocks. For the BQ/homobenzvalene 5 system, however, where the difference in stability between the intermediate biradicals can be expected to be considerably less, the selectivity ratio for the spiro-oxetanes 6 and 7 is reduced to 3 1, respectively, and the addition to NQ yields only the cyclobutane derivative 8. Quadricyclane and norbornadiene undergo the same photocycloaddition reaction to BQ, affording the oxolane 9 and the spiro-oxetane 10. Evidence from CIDNP... 

Cydopropylbenzene is a good electron donor and, not surprisingly, yields triplet radical ion pairs with photoexcited CA. In dichloromethane, these intermediates lead to the spiro-tetrahydrofuran adduct 64 in 68% yield, but in acetonitrile, this drops to 5%. Both NQ and 2,3-dichloro-NQ react similarly with cyclopropylbenzene on irradiation and again the yields of adducts corresponding to 64 are very dependent on solvent polarity. 

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