Exciplex intermediate

Studies with model compounds have demonstrated that photodehydrochlorination is sensitized by jj-cresol triplets via a charge-transfer exciplex intermediate in which the alkyl chloride is the electron acceptor (15). The detailed mechanism suggested for this process (15) is outlined in Equations 11 and 12. 

Photochemistry of amines and amino compounds folded conformations of the singlet exciplex intermediates in non-polar solvents31. 

W-Ethyl-A -(3-sulfopropyl)-m-anisidine, triglyceride determination, 736, 634 Ethynyl hydroperoxide, C-O distance, 102-3 ETR see Electron transfer reactions Exchange reactions, alcohol/ether, 157-8 Exciplex intermediate, ene reaction, 832-3, 835... 

In CT complexes such as that between furan and tetracyanoethylene split maxima are often observed in the electronic absorption spectrum, since inner, as well as the outermost, occupied orbitals of the (donor) heterocycle are involved. However, the absorption maxima run parallel with the first ionization constants and can be used to predict them (75JCS(F1)2045, 68MI31000). Again, furan is thought to quench the fluorescence of 1-cyanonaphthalene because it forms an exciplex intermediate with a fluorescence of its own to the red side of the original one the exciplex has CT character and the frequency of the new fluorescence correlates with the first ionization potentials in a series of furans (75CC203). 

The photoaddition of benzo[c]furan to a variety of alkenes has been described. With cycloheptatriene the [4 +4] adducts (320) and (321) and the [4 + 6] adducts (322) and (323) are obtained in addition to the photodimer (301) and the photooxidation product 1,2-dibenzoylbenzene. The [4 + 4] adducts may be formed in a concerted manner via an exciplex intermediate in a 7r-7r singlet state. The [4+6] adducts, formally symmetry forbidden,... 

The reaction of t-1 with dimethyl fumarate is proposed to occur via the weakly fluorescent singlet exciplex intermediate (76). Increasing the solvent polarity results in a decrease in both the exciplex fluorescence intensity and the cycloaddition quantum yield, presumably due to radical-ion pair formation. The low efficiency of cycloaddition from c and the absence of triplet cycloaddition indicate that a planar stilbene chromophore is necessary for exciplex formation (see also Sections V-B and C). 

Mechanistic investigations by Chapman and co-workers (99) indicated that these reactions occurred via a nonfluorescent singlet exciplex intermediate. While the rate constant for quenching of - -t5 by 2,3-dimethy 1-2-butene is slower than the rate of diffusion (Table 8), the limiting quantum yield for cycloaddition is 1.0. Thus, highly efficient exciplex cycloaddition may account for the absence of exciplex fluorescence, as in the case of t-1 photodimerization. Photochemical [2+2] cycloaddition reactions have also been observed to occur upon irradiation of the cyclic c-1 analogues diphenylcyclobutane (7) and diphenyl-vinylene carbonate (10) with 2,3-dimethyl-2-butene (96) however, the mechanistic aspects of these reactions have not been investigated. 

A) adducts are also known to form through exciplex intermediates. For example, photoexcited substituted anthracenes react with dienes, eq. 17 (54) ... 

On the other hand, the digermirane (18) reacts with Cgo to afford the 1,4-adduct (19, twist form) selectively15. The structure of the 1,4-adduct was confirmed by 13C-13C INADEQUATE (Incredible Natural Abundance Double Quantum Experiment) spectroscopy (equation 5). The rate of disappearance of Cgo was suppressed by addition of diazabicyclo[2.2.2]octane and 1,2,4,5-tetramethoxybenzene. Furthermore, the reaction was completely inhibited by addition of rubrene as triplet quencher. One plausible rationale for these observations is that an exciplex intermediate may be responsible for formation of the products (Scheme 6). 

A review of photo-cycloadditions of dienones and quinones has been published.41 The first example of a Lewis acid-catalysed 2 + 2-cycloaddition of styrene with naphthoquinone has been reported.42 FMO methods have been used to investigate the effect of substituents on the regiochemistry of the 2 + 2-photo-cycloaddition of a, fi-unsaturated carbonyl compounds with substituted alkenes.43 Evidence has been presented for the presence of a triplet exciplex intermediate in the photo-cycloaddition of 4,4-dimethylcyclohexenone to 1,1-diphenylethylene.44 The intramolecular 2-1-2-photo-cycloaddition of 2-acyloxy-3-hexenoylcyclohexenones (26) is highly diastereo-selective yielding the tricyclic adduct (27) (Scheme 10).45... 

Photoreaction of cyanonaphthalenes with tetramethylethylene leads to cyclobutane formation in nonpolar solvents via exciplex intermediates, whereas in polar media electron transfer occurs [115-117], 1-Azetine is formed in the photoreaction of highly electron-donating alkene with 1-cyanonaphthalene [118]. 

Caldwell, R.A., Hrncir, D.C., Munoz, T. Jr, and Unett, D.J. (1996) Photocycloaddition of 4,4-dimethylcyclohexenone to 1,1-diphenylethylene. Evidence for a triplet exciplex intermediate. Journal of the American Chemical Society, 118, 8741-8742. 

This mechanistic sequence (Sch. 4) wherein the triplet excited enone adds to the alkene, either via an exciplex intermediate or directly, to afford triplet 1,4-biradicals, which (after undergoing intersystem crossing) either cyclize to product(s) or revert to ground state reactants, is confirmed by both semi-empirical and ab-initio calculations [21-24], The origin of regioselectivity is supposed to stem from the primary binding step, the enone triplet being considered as a (nucleophilic) alkyl radical at C(3) linked to an (electrophilic) ot-acyl radical at C(2) [25], Thus additions of C(2) to the less substituted terminus of electron rich alkenes and of C(3) to the least substituted terminus of electron deficient alkenes should occur preferentially [26],... 

Using the temperature effect, it was proven that non-emitting exciplex intermediates were also involved in the cycloaddition. For example, in the reaction of naphthalene with diphenylacetylene, with increasing temperature both the quantum yields of photoaddition and the quenching of the naphthalene fluorescence by acetylene decrease by the same magnitude [65],... 

The above result unequivocally demonstrates that the seemingly weak solvation effect can play a decisive role, which controls and even switches the stereochemical outcome of the enantiodifferentiating photoisomerization. Temperature, pressure, and solvation, which function as environmental factors to control the enantiodifferentiation in the excited state, are all entropic in nature. Probably the key is the critical control of the weak interactions involved in the exciplex intermediate, as with the biological and supramolecular interactions in... 

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