Exciplex formation approximation

Weller24 has estimated enthalpies of exciplex formation from the energy separation vg, — i>5 ax of the molecular 0"-0 and exciplex fluorescence maximum using the appropriate form of Eq. (27) with ER assumed to have the value found for pyrene despite the doubtful validity of this approximation the values listed for AHa in Table VI are sufficiently low to permit exciplex dissociation during its radiative lifetime and the total emission spectrum of these systems may be expected to vary with temperature in the manner described above for one-component systems. This has recently been confirmed by Knibbe, Rehm, and Weller30 who obtain the enthalpies and entropies of photoassociation of the donor-acceptor pairs listed in Table XI. From a detailed analysis of the fluorescence decay curves for the perylene-diethyl-aniline system in benzene, Ware and Richter34 find that... 

The theory of geminate recombination experienced a similar evolution from primitive exponential model and contact approximation [19,20], to distant recombination carried out by backward electron transfer [21], However, all these theories have an arbitrary parameter initial separation of reactants in a pair, / o. This uncertainty was eliminated by unified theory (UT) proposed in two articles published almost simultaneously [22,23], UT considers jointly the forward bimolecular electron transfer and subsequent geminate recombination of charged products carried out by backward electron or proton transfer. The forward transfer creates the initial condition for the backward one. This is the distribution of initial separations in the geminate ion pair/(ro), closely analyzed theoretically [24,25] and inspected experimentally [26,27], It was used to specify the geminate recombination kinetics accompanied by spin conversion and exciplex formation [28-31], These and other applications of UT have been covered in a review published in 2000 [32],... 

The quantum yield for the formation of the cycloaddition product has been found to be temperature dependent, increasing by a factor of approximately three as the temperature is lowered from 65 ( = 0.24) to 5°C ( = 0.69). Photolysis of mixtures of the olefin and f/my-stilbene in the presence of sensitizers yielded no cycloaddition product (42) but rather only m-stilbene. This suggests that the cycloadduct is produced via a singlet reaction. This conclusion is supported by the fact that tetramethylethylene quenches fluorescence from the /rans-stilbene singlet. A plot of l/ (42) vs. 1/[TME] (TME = tetramethylethylene) is linear. The slope of this plot yields rate constants for cycloadduct formation which show a negative temperature dependence. To account for this fact, a reversibly formed exciplex leading to (42) was proposed in the following mechanism<82) ... 

The regioselectivity of the addition with respect to the arene has been interpreted in terms of both meta addition of the alkene to the Sj arene via an exciplex followed by formation of the vinylcyclopropane system,and initial rearrangement of the arene to a prefulvene species which then undergoes concerted alkene addition (see below). The orientation of the substituents on the wcfa-cycloadducts from some systems requires that both mechanisms operate in competition. The reaction shows no apparent regioselectivity with simple alkenes thus 2-methylpropene gives both wc/a-cycloadduct isomers in approximatively equal amounts. ... 

When the structure of one of the two-component crystals was obtained and analyzed (15), it was concluded that the ground state orientation of the components would predict essentially no face discrimination and that approximately equimolar amounts of the enantiomeric dimers 4a and 4b should have been obtained. As a result, it was concluded that the thiophene compound must deform in the excited state to an arrangement that favors the formation of one of the products. This hypothesis was supported by theoretical calculations based on the assumption that the reaction proceeds through an intermediate exciplex (16) spectroscopic evidence for such a species was found in the case of diene 1 (17). 

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