Triplet State Mechanism

A recently popular mechanism involves the intersystem crossing of the cis- or trons-stilbene singlet state, produced upon direct photolysis, to its corresponding triplet states, which would then undergo the type of reactions given in Eqs. (9.8)-(9.10) and (9.17)-(9.19)  

If we know the extinction coefficients for the two isomers at the wavelength of photolysis (3130), it should be possible to calculate the pss for the direct 

The small effect that has been observed can be quantitatively accounted for via Forster-type long-range singlet energy transfer, 

In addition, - it was noted that whereas the azulene effect on the sensitized reaction is sensitive to changes in solvent viscosity, the azulene effect on the direct photoreaction was independent of solvent viscosity, as would be predicted for Fdrster-type energy transfer. The inescapable conclusion is that cis-trans isomerization upon direct irradiation of stilbene takes place in the singlet manifold. 

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Ma J, Lin W, Wang W, Flan Z, Yao S, Lin N (2000) Triplet state mechanism for electron transfer oxidation of DNA. J Photochem Photobiol B Biol 57 76-81 Maeda M, Nushi K, Kawazoe Y (1974) Studies on chemical alterations of nucleic acids and their components VII. C-alkylation of purine bases through free radical process catalyzed by ferrous ion. Tetrahedron 30 2677-2682... 

The vexed question of the mechanism of the direct and sensitized cis-trans isomerization of stilbene has been the subject of recent papers.546 The studies of the two groups differ in the favoured mechanism for the direct cis-trans isomerization, one proposing the conventional triplet-state mechanism, the other plausibly invoking the intervention of a doubly excited singlet potential surface with a deep potential minimum at a 90° configuration. 

SCHEME 4 Proposed triplet state mechanisms for photo release of substrates from the p-hydroxyphenacyl protecting group. 

In the early 1990s, a new spin polarization mechanism was posPilated by Paul and co-workers to explain how polarization can be developed m transient radicals in the presence of excited triplet state molecules (Blattler et al [43], Blattler and Paul [44], Goudsmit et al [45]). While the earliest examples of the radical-triplet pair mechanism (RTPM) mvolved emissive polarizations similar in appearance to triplet mechanism polarizations, cases have since been discovered m which absorptive and multiplet polarizations are also generated by RTPM. 

The transfer of energy must proceed with net conservation of spin. In the usual case, the acceptor molecule is a ground-state singlet, and its reaction with the triplet state of the sensitizer will produce the triplet state of the acceptor. The mechanism for triplet photosensitization is outlined below ... 

I have elected to include a discussion of the variational principle and perturbational methods, although these are often covered in courses in elementary quantum mechanics. The properties of angular momentum coupling are used at the level of knowing the difference between a singlet and a triplet state. 1 do not believe that it is necessary to understand the details of vector coupling to understand the implications. 

The mechanism of the Patemo-Biichi reaction is not well understood, and while a general pathway has been proposed and widely aceepted, it is apparent that it does not represent the full scope of reactions. Biichi originally proposed that the reaction occurred by light catalyzed stimulation of the carbonyl moiety 1 into an excited singlet state 4. Inter-system crossing then led to a triplet state diradical 5 which could be quenched by olefinic radical acceptors. Intermediate diradical 6 has been quenched or trapped by other radical acceptors and is generally felt to be on the reaction path of the large majority of Patemo-Biichi reactions. Diradical 6 then recombines to form product oxetane 3. 

Semiempirical (PM3) and ab initio (6-3IG basis set) calculations are in agreement with the hypothesis described in Section I (99MI233 OOOJOC2494). In the case of the sensitized reaction, when the excited triplet state is populated, only the formation of the radical intermediate is allowed. This intermediate can evolve to the corresponding cyclopropenyl derivative or to the decomposition products. In a previously reported mechanism the decomposition products resulted from the excited cyclopropenyl derivative. In our hypothesis the formation of both the decomposition products and the cyclopropenyl derivatives can be considered as competitive reactions. 

Theoretical calculations explain the photochemical behavior of phenylthiazoles (Fig. 14) (99MI233). The RCRE mechanism cannot be invoked because the radical intermediates have higher energies than the corresponding triplet states. Furthermore, the formation of the Dewar isomer is favored in comparison with the formation of the zwitterionic intermediate. Nevertheless, the reaction conditions used by Kojima and Maeda could allow for an endothermic reaction giving this type of intermediate. The same results were obtained using 2,5-diphenylthiazole. 

Semiempirical calculations on 94 showed that it can isomerize only via the ICI mechanism in fact, the triplet state cannot evolve to give the corresponding biradical derivatives (Fig. 16) (99MI233). 

Semiempirical calculations on 4-methylisothiazole showed that the reaction can occur through an ICI mechanism with the formation of the Dewar isothiazole derivative (Fig. 18) (OOOUPl). In fact, the triplet state of the isothiazole cannot evolve to the biradical. The ZI mechanism can be excluded Only the intermediate 96 showed an acceptable energy however, it is a resonance stmeture of Dewar isothiazole. 

The results described above represent the first example of the FR mechanism (Scheme 1). Semiempirical calculations on this molecule showed that the intersystem crossing to the excited triplet state is favored The reaction cannot be sensitized by xanthone because the triplet state of 3,4-diphenyl-1,2,5-oxadiazole is lower than that of xanthone. The cleavage of the triplet state to the biradical is favored, considering the relative energy of this intermediate (Fig. 23) (OOOUPl). 

Alcohols can be added to certain double-bond compounds (cyclohexenes, cycloheptenes) photochemically ° in the presence of a photosensitizer such as benzene. The mechanism is electrophilic and Markovnikov orientation is found. The alkenes react in their first excited triplet states. ... 

Ordinary aldehydes and ketones can add to alkenes, under the influence of UV light, to give oxetanes. Quinones also react to give spirocyclic oxetanes. This reaction, called the Patemo-BUchi reaction,is similar to the photochemical dimerization of alkenes discussed at 15-61.In general, the mechanism consists of the addition of an excited state of the carbonyl compound to the ground state of the alkene. Both singlet (5i) and n,n triplet states have been shown to add to... 

The delocalization of excessive a- (or P-) spins and the bond polarization can take place among radical orbitals, p and q, and the central n (or o) and n (or o ) orbitals, resulting in the electron transferred configurations (T) and locally excited configurations (E), respectively (Fig. 5a). The delocalization-polarization mechanisms are different between singlet and triplet states, as addressed in the following subsections. 

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