Singlet State Mechanism

This mechanism is consistent with all of the experimental and theoretical information presented. The fact that no troiu-stilbene fluorescence is observed when cij stilbene is irradiated would indicate that the following reaction is not important, that is, the twist singlet is much lower in energy than is the trans singlet state  

Saltiel has pointed out that the experimental study of Lewis, Dalton, and Turro can be interpreted in terms of nonvertical singlet energy transfer to produce the twisted or phantom singlet state  

It is possible to estimate the rate of vertical singlet energy transfer (9.31) and (9.33) (when Forster-type energy transfer is negligible, spectral overlap integral is very small) from the relation 

If fluorescence and cis-trans isomerization (9.26)-(9.29) are the main competing reactions upon direct excitation, then inhibition of rotation about the central bond should produce an increase in the fluorescence quantum yield. The rigid systems (3) and (4) both have fluorescence quantum yields of 1.0 at room temperature. While the fluorescence of rrons-stilbene is a 

Fioure 9.7. Temperature dependence of p/ -ieaeo d t- o of /roiu stilbene in methylcyclohexane and isohexane. (Data from Ref. 38 see also Ref. 1.) 

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Di-7r-methane reactions have been reported for the aryl olefins (39).32 In all cases studied the product was the arylcyclopropane (40). The formation of the products was sensitive to the nature and position of the substituent on the aryl ring. These results, and the fact that the quantum yield for product formation is higher in cyclohexane than in acetonitrile, suggest that the biradical intermediate (41) might be important in the transformation. An excited singlet-state mechanism is proposed. 28... 

An intramolecular charge transfer excited state may be involved, and the inverse relationship between fluorescence and photoisomerisation suggests a singlet state mechanism. Irradiation of (Z)-urocanic acid, 3-(lH-imidazoI-4-yl)prop-2-enoic acid, in the presence of nitro blue tetrazolium and sodium azide promotes its photoisomerisation in a process which involves reversible addition of the azidyl radical to the double bond. ... 

Diphenylcyclobutene affords the methyl ethers (689) and (690) on photolysis in methanol, and the corresponding acetates are obtained if the reaction is carried out in acetic acid, evidently by a singlet-state mechanism proceeding via a cyclobutyl... 

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. 

It is evident from the exceptions noted that the mechanism proposed above does not fully capture the pathways open to the Patemo-Biichi reaction. A great deal of effort has been devoted to deconvoluting all of the possible variants of the reaction. Reactions via singlet state carbonyls, charge-transfer paths, pre-singlet exciplexes, and full electron transfer paths have all been proposed. Unfortunately, their influence on product... 

There are several data in agreement with a mechanism involving an excited singlet state (93JA7645 97JOC8325). Theoretical calculations support this... 

This behavior has been verified with bithiazole derivative 94 (Scheme 36). Neither acetophenone nor benzophenone sensitized the reaction, in agreement with an ICI mechanism via the excited singlet state (86TL6385 87JA938). 

The lack of any difference in the rate of isomerization between fluoro-sulfonic acid solutions of 34 which had been thoroughly degassed, and those which were saturated with oxygen, suggests that the reaction does not proceed via a triplet mechanism. In fluorosulfonic acid no unproton-ated acid is detected, ruling out the possibility of n,7r excitation. Thus, there is little doubt in this case that it is the Tr,Tr singlet state which is the reactive species. Experiments carried out with a variety of methyl-substituted protonated cydohexadienones have likewise ruled out the... 

It is possible that some of these photochemical cycloadditions take place by a lA + A] mechanism, which is of course allowed by orbital symmetry when and if they do, one of the molecules must be in the excited singlet state (5i) and the other in the ground state.The nonphotosensitized dimerizations of cis- and trans-2-butene are stereospecific,making it likely that the [n2s + n2s] mechanism is operating in these reactions. However, in most cases it is a triplet excited state that reacts with the ground-state molecule in these cases the diradical (or in certain... 

The orbital phase theory is applicable to the singlet diradicals [20]. The electron configuration of the singlet states of the cross- (TMM) and linear (BD) conjugate diradicals is shown in Scheme 9, where the mechanism of the delocalization of a and P spins between the radical centers through the double bond are separately illustrated by the arrows. The cyclic [-a-Tr-b-T -] interaction is readily seen to occur for the spin delocalizations. The p orbital a) in one radical center and the n orbital are occupied by a spins, and therefore, electron-donating orbitals. The p orbital (b) in the other radical center and the ii orbital are not occupied by a spins. 

The delocalization-polarization mechanism in the singlet state is more complicated than that in triplet. Similar to the triplet state, there also exists a cyclic - G- T - E- T - configuration or -7t-p-7t -q- (-o-p-o -q-) orbital interaction in the singlet (Fig. 6). In the singlet state, however, the radical orbital q is an electron-accepting orbital (A) for the a-spin electron (rather than the donating orbital in triplet). Thus, there is an additional path of a-spin electron delocalization, - G- T - Tj- T - or... 

Fig. 6a-c Through-bond interactions in the singlet state of 1,3-diradical, a Mechanism of electron delocalization and polarization of a-spin electrons, b Cyclic orbital interaction, c Orbital phase discontinuity (denoted by the dotted lines)... 

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