Exciplex mechanism

Intramolecular Folding The Excimer/Exciplex Mechanism and Dewar Isomerization (Butterfly Mechanism)... 

The most recent work210 on sensitized decomposition of peroxides dealt primarily with ketones (cf. Section III.C.3) although it was also demonstrated that anthracene could sensitize decomposition of benzoyl peroxide. In view of the earlier work206,209 and the fact that benzoyl peroxide quenches anthracene fluorescence at a rate of 1.0 x 1010M 1 sec-1 in benzene,211 a singlet exciplex mechanism is strongly implicated. 

I . Molecules with heavy atoms act as quenchers by an exciplex mechanism. 

The absorbances at both 500 and 610nm, which appeared first, became larger with increasing number of carbons, reached a maximum of Cj n, and then became smaller with further increases in the carbon number. Presumably this is due to crystallization (orientation) of the long alkyl chains. As shown in Figure 9.6, the dication and counteranion moieties in an anisotropic phase are situated next to each other at the site, and the distance between both ions is large compared with that in an isotropic (disordered) phase. Thus the electron transfer from counteranion to dication hardly takes place via the exciplex mechanism. 

In photolysis of ketones CIDNP studies have confirmed that the Norrish type I split occurs predominantly from a phototriplet state (32,38,118), although some of the reactions with aliphatic ketones exhibit polarization involving both the excited singlet and the triplet (47,118) states as well as the postulated exci-plex intermediates (71). An exciplex mechanism has also been postulated in the CIDNP observation of the photolysis of tri-fluoroacetophenone with dimethoxybenzene in acid solutions (117). 

In 1987, Comelisse and coworkers reported a semi-empirical calculation for the meta cycloaddition of benzene with ethylene, which impressively consolidates many experimental aspects of this reaction and flts well with the Mattay analysis. This calculation, performed at the CNDO/S and MNDO levels, supports an exciplex mechanism with charge polarization, in accord with Mattay s treatment and a related mechanism developed earlier in the Comelisse laboratories. According to this mechanism (equa-... 

Although not directly related to TET, Scaiano [63] had also obtained linear Arrhenius plots in the 227-313 K range and KIE of 3 for 2 -methylacetophenone photoenolization, and concluded that thermal activation was dominant for this reaction. However, it was later shown that tunnelling was the dominant mechanism for this and other related reactions [64], which clearly show nonlinear Arrhenius plots at lower temperatures and a KIE of 150 at 180K. Marcantonatos exciplex mechanism was also shown to be inconsistent with experimental data on uranyl ion quenching and the role of radiationless transitions was emphasized [65]. 

Recent studies by Schuster and co-workers8,19 have cast serious doubt on the Corey-de Mayo exciplex mechanism. On the basis of new kinetic data it was assumed that the enone triplet excited states react with alkenes to give 1,4-diradicals directly (k5) without the involvement of a discrete exciplex intermediate. 

Photoirradiation of benzene and its derivatives with alkenes give ortho-, meta-, para-cycloaddition products. In most cases, either meta- or orr/io-adducts are obtained as major products [5]. Bryce-Smith and Gilbert suggested a prefulvene type diradical intermediate mechanism (Path A) for the weta-adduct [6], whereas Morrison and Srinivasan groups [7, 8] proposed the exciplex mechanism (Path B) for the formation of these adducts (Scheme 9.1). The exciplex intermediates undergo photo-induced electron transfer processes between donor (D) and acceptor (A) to produce radical ion pairs as intermediates, stabilized by coulombic interactions to give adducts [9]. 

Another important photoreaction is the 1,3 cycloaddition of ethene to benzene. Although a 1,2 and a 1,4 photocycloaddition can also take place, the 1,3 photochemical addition is the most important, since it yields a variety of natural products depending on the substitution pattern of the alkene and arene. The reaction mechanism which was obtained by experimental results indicates an exciplex mechanism, whereas a SINDOl calculation favors the prefulvene mechanism.Since the experiments were done in solution and the calculations were performed in a vaccum, this is not at all surprising. According to the SINDOl calculations, there is a conical intersection near the prefulvene minimum, which is very flat. The semiempirical calculations are supported by MCSCF ab initio calculations, which show a remarkable similiarity in the important structures for both the exciplex and the prefulvene mechanisms.This demonstrates that semiempirical programs on the Cl level are well suited for photochemical problems. 

In the case of a triplet-state exciplex mechanism, agreement between and will occur only by coincidence, so that is compatible with a triplet-state reaction but not with a singlet-state process. 

This behavior is exemplified by the photodehalogenation of 4-chlorobiphenyl, which is convenient for study because its quantum yield of reduction in the absence of amines is very low (O < 0.001). In acetonitrile, the addition of 0.1 M amine increased 4>, to 0.07 (butylamine), 0.25 (dipropylamine), and 0.49 (triethylamine). Initially," the singlet exciplex mechanism was suggested with Et3N as the amine (Kj = 23 M" = 20 M ), but in later work on the same system was consistently found to be greater... 

These data were accepted for a long time as proper measures of alkene reactivity in photocycloadditions and were critical elements in the formulation of Corey s famous exciplex mechanism to explain enone photocycloadditions.However, as is now well known, product ratios in photochemical processes reflect relative quantum efficiencies for disappearance of starting materials and/or formation of products and rarely reflect relative rates of reaction of the photoexcited state, particularly when it is a triplet state. ° ... 

Weedon s data clearly demonstrate that not only is it not necessary to invoke exciplexes or oriented Jt-complexes in order to explain the regioselectivity in [2+2]-photocycloaddition of cyclopentenones and cyclohexenones to alkenes, but also that expectations based on the exciplex mechanism are contrary to experimental observations. It is clear that regioselectivity is entirely due to differences in behavior of the intermediate triplet 1,4-biradicals, not to their rates of formation, as was originally hypothesized. ... 

The Gorey-de Mayo exciplex mechanism " for photocycloaddition of enones to alkenes has provided a stimulus for workers in this field and undoubtedly has had heuristic value in accounting for experimental observations in a vast number of reactions. Nonetheless, the experimental findings discussed above make it clear that this mechanism for enone [2 2]-photocycloadditions has been discredited and should be abandoned. 

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