Donor-acceptor pairs Exciplex

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... 

Detailed kinetic exploration of the relaxation of the initially formed radical ion pair would probably be of most utility in addressing the question of the degree of charge separation in exciplex or in radical ion pairs. Rentzepis and coworkers have recently found that in strongly interacting donor-acceptor pairs, for example, indene-chloranil, the initially formed ion pair relaxes within a few picoseconds to an equilibrated solvated complex (67). Time-resolved resonance Raman spectroscopy has also been used recently as a kinetic monitor for radical ion reactivity (68). 

An exciplex can exert a favorable effect on cycloaddition reactions in two ways. First, the prior association favors the bimolecular process. Second, exciplex formation will bias the system toward bond-formation of the donor-acceptor pair, and away from competing chemical processes, such as dimerization of the donor. 

In our simplified scheme, there are four possibilites to form cycloadduct from donor olefins and acceptor olefins (1) concerted reaction of the partners, (2) the partners form an exciplex, which collapses to cyclobutanes directly, (3) the exciplex collapses to tetramethylene diradical, which then close to cyclobutanes, and (4) direct formation of diradical from donor/acceptor pair and then cyclization. 

If variations in the distance between H O molecules and the donor-ac-ceptor system are taken to be characteristic for solvation changes and hence for variations in solvent polarity, the schematic diagram of Figure 5.39 is obtained which shows the solvent dependence of the fluorescence of donor-acceptor pairs. According to this diagram the energy of the exciplex fluorescence decreases with increasing solvent polarity. 

Photoelectron transfer oxidation of various phenols in the presence of 2-nitrofluorene has been examined in both acetonitrile and cyclohexane solution. Although no charge transfer donor-acceptor pairs are present in the ground state, a contact exciplex is apparent in cyclohexane, and in acetonitrile the anion radical of 2,6-dimethylphenol has been observed as the final product. Irradiation of... 

Donor-acceptor pairs. 123-24. 214, 216, 218, 286. See also Charge transfer Exciplex... 

The reactive species generated by the photoexcitation of organic molecules in the electron-donor-acceptor systems are well established in last three decades as shown in Scheme 1. The reactivity of an exciplex and radical ion species is discussed in the following sections. The structure-reactivity relationship for the exciplexes, which possess infinite lifetimes and often emit their own fluorescence, has been shown in some selected regioselective and stereoselective photocycloadditions. However, the exciplex emission is often absent or too weak to be identified although the exciplexes are postulated in many photocycloadditions [11,12], The different reactivities among the contact radical ion pairs (polar exciplexes), solvent-separated radical ion pairs, and free-radical ions as ionic species... 

The studies related to the interactions of electronically excited arene molecules with tertiary amines have provided a basis for the present understanding of exciplexes and radical ion-pair phenomena [41,82], PET reactions of amines yield planar amine radical cations (Eq.20) which are deprotonated to give a-amino radicals (Eq.21) and usually cross-coupling (Eq.22) between radical pairs of donor-acceptor terminates the photoreaction [32a, 83]. Mechanistic studies revealed contact ion pair (CIP) intermediate for these reactions [84, 85]. 

Weller and Zachariasse thoroughly investigated exciplex formation and luminescence for donor acceptor systems in THF [18]. A particularly interesting result from their work came from an examination of the temperature dependence of radiative charge recombination between 9,10-dimethylanthracene anion (DMA") and TPTA+ in THF [19]. They found that both exciplex emission and fluorescence from DMA were observed in solution at low temperature (ca. —50°C). As the solution temperature is raised, the excimer emission decreases in relative intensity, and at room temperature the emission is nearly completely DMA fluorescence. The monomer-to-exciplex emission intensity ratio as a function of temperature follows Arrhenius kinetic behavior and yields an activation barrier that is nearly the same as the energy gap between the exciplex and the DMA states. Thus, their model consisted of reaction of the solvent-separated ions to form an intimate emissive ion pair which could dissociate to yield the singlet anthracene derivative. 

As mentioned earlier in the discussion of exciplex formation, electron transfer between an excited state species and a ground state molecule (Equation 2.8 and Equation 2.9) is frequently observed in the photochemistry of systems containing an electron donor-acceptor combination. As a result, a pair of radical ions is formed that react with oxygen but with different rates. The reaction of ground state oxygen with radical anions occurs rapidly and yields superoxide anion (Equation 2.16). The superoxide then adds to the radical cation forming D02 (Equation 2.17). When D is an olefin, D02 is a dioxetan that is liable to cleave to yield ketones as products. 

We also note that when ion radical reactivity is slowed, ex-ciplex chemistry (which is completely different) becomes competitive. Thus, in the reaction with acenaphthene several products corresponding to the expected pattern for benzylation (see Scheme 12) are formed. However, here product II (which does not arise from secondary photocycloaddition from one of the previously mentioned products) Is also produced. This can be explained by an initial [2-f2] photocycloaddition between the two naphthalene rings which precedes alkylation (Scheme 3). Thus, when the donor is a naphthalene rather than a benzene derivative, the twofold effect of the diminished acidity of the benzylic position and of the increased 7i-donor-acceptor interaction makes the exciplex path sufficiently important to compete with the ion pair chemistry. Going further this way, i.e. toward more positive AG , cycloaddition via exciplex completely dominates, as indicated in Section 2. 

In polar solvents, such as acetonitrile, organic donor-acceptor systems such as those listed in Table 6.2 show only the fluorescence due to A no new fluorescence appears as in exciplex formation. Flash spectroscopy shows absorption spectra characteristic of the hydrocarbon radical anion and the amine radical cation. The product in these solvents is either an ion-pair or two free ions, stabilised no doubt by solvation, and the reaction is a complete transfer of an electron from one molecule to another, rather than exciplex formation. The reaction goes effectively to completion, and so (with only one fluorescence lifetime to be considered) the kinetic equations for the intensity and lifetime reduce to the simple Stem-Volmer forms (Equations (6.16) and (6.19)). The rate constants for the reactions of aromatic hydrocarbons with various amines in acetonitrile are found to be correlated with the free-... 

If the interaction between the donor and acceptor in the encounter pair is strong (Scheme 4.3), this encounter pair (DA) is called an exciplex (see Section 4.4). 

It should be noted that de-excitation of exciplexes can lead not only to fluorescence emission but also to ion pairs and subsequently free solvated ions. The latter process is favored in polar media. Exciplexes can be considered in some cases to be intermediate species in electron transfer from a donor to an acceptor (see Section 4.3). 

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