Stilbene exciplex

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) ... 

Photoinduced intramolecular interaction of t-S and tertiary amine moieties linked with a polymethylene chain has also been studied24. The photoexcitation of fraws-stilbene in which a tertiary amine is attached to the ortho position with a (CH2)i-3 linker leads to fluorescent exciplexes by intramolecular electron transfer, and results in no more than trans-cis isomerization. The failure to give adducts from the intramolecular exciplexes could arise from the unfavourable exciplex geometry to undergo the necessary bond formation. 

Nitro substituted stilbenes have been used as models in stUbene cis trans-isomerization studies. It had been reported previously, that cis -v benzene solution, and a photostationary state of 99.5% trans isomer had been reached 132) ... 

According to the model for [2+2] cycloaddition shown in Fig. 2, it should be possible to reach the pericyclic intermediate upon irradiation of the cycloadduct. If a common intermediate is attained from the cycloaddition and cycloreversion processes, then the sum of the quantum yields for the two processes should equal unity. This has, in fact, been observed to be the case for several exciplex and anthracene excimer systems (49b,52). Stereospecific cycloreversion of stilbene dimers 11 and 12 to t-1 has been observed to occur upon 254 nm... 

Adams and Cherry (78) have investigated the effects of stilbene substitution on the behavior of their excited complexes with fumaronitrile and find that the rate constants for fluorescence and nonradiative decay are insensitive to substitution, but that the rate constant for intersystem crossing is increased by electron-donating substituents (lower stilbene oxidation potential). This trend is attributed to a decrease in the energy gap between the excited complex and locally excited 3t (Fig. 4). The observed energy gap dependence of the exciplex lifetime could also account for the absence of fluorescence (or cycloadduct formation, see Section IV-B) from the excited charge-transfer complexes of t-1 with stronger electron acceptors such as maleic anhydride (76) or tetracyanoethylene (85). 

FIGURE 4. Energy level diagram for the trans-stilbene locally excited singlet and triplet states, the trans-stilbene-fumaronitrile (FN) exciplex, and triplet fumaronitrile. 

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). 

FIGURE 5. Presumed geometry of the trans-stilbene chiral fumarate exciplex precursor of the p-truxinate cycloadduct. 

This is the only reported example of a chemically productive reaction between a stilbene excimer or exciplex with a third molecule. ... 

TABLE 11. trans-Stilbene Fluorescence Quenching and Exciplex Fluorescence Data for Amine Quenchers3... 

FIGURE 10. Relative quantum yields for exciplex fluorescence (filled symbols) and addition product formation (open symbols) versus solvent dielectric constant for trans-stilbene with di isopropyl methyl amine (0)> ethyldiisopropylamine (A), and triethylamine ( ) in hexane-ethyl acetate and ethyl acetate-acetonitrile mixed solvents. From ref. (114) with permission of the American Chemical Society. 

VII. REACTIONS OF SINGLET ARENES WITH STILBENES A. Singlet Quenching and Exciplex Formation... 

Cycloaddition is a singlet state reaction, triplet quenching yielding only stilbene isomerization. In the limit of high t-1 concentration, the quantum yield for the formation of 89 and 90 is 0.66 and no c-1 is formed. Nonradiative exciplex decay is proposed to occur by partitioning at the pericyclic minimum (Fig. 2) between products and reactants. In the limit of high c-1 concentration, 91 is formed with a quantum yield of 0.05 and the predominant exciplex decay pathway is dissociation to yield f-c, which decays to a mixture of t-1 and c-1. 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

The addition reactions of It with amines are also presumed to occur via exciplex or radical-ion pair intermediates however, exciplex fluorescence is observed only under conditions where chemical reactions do not occur. Transfer of hydrogen from the amine a-C-H (tertiary amine) or N-H (secondary amine) bond results in the formation of a radical pair which ultimately gives rise to stilbene amine adducts and other free-radical derived products. The radical-ion pairs can also be intercepted by external electrophiles and nucleophiles, leading to formation of radical-ion-derived products. 

It is hardly surprising that different chemical reactivity might be expected from the exciplex and the radical ion pair formed by complete electron exchange. Lewis observation (50) that in the excited state interaction of trans-stilbene with either electron-rich or electron-poor alkenes cycloaddition is more efficient from the relatively less polar exciplex than from radical ion pairs is typical for many such cycloadditions. 

In parallel to this example, Lewis and coworkers have also observed inverse reactivity for electron transfer and cycloaddition efficiency in the interaction of trans-stilbene with unsaturated esters (52). This result is understandable if the exciplex, rather than the radical ion pair, arranges the olefins in the correct geometry for cycloaddition. Analogous results have also been reported in the (2+2) cross photoreaction of cyano-substituted stilbenes with dienes, where all possible regioisomers were formed, eq. 16 (53a) ... 

That the cycloaddition occurs via the less highly polar exciplex is also supported by Kaupp s studies of photocycloaddition between trans-stilbene and cyclic unsaturated ethers (57). 

The proposition that locally excited triplet states can be formed from back electron transfer within a doublet-doublet radical ion pair has firm theoretical (88) and experimental support. For example, with time-resolved Resonance Raman spectroscopy, one can directly monitor the chemical fate of the exciplex, solvent separated ion pair, and doublet free radical ion pairs formed between stilbene and amines. As might be expected from the above discussion, adduct formation is observed from the exciplex or contact ion pairs, whereas enhanced intersystem crossing ensues from the solvent separated ion pairs, producing spectroscopically observable stilbene triplets. This back electron transfer process, eq. 30 (89),... 

Dorr, Lewis, and co-workers found evidence through quenching experiments and flash spectroscopy for a triplex in the system trans-stilbene — amine — benzene — [105]. They quenched singlet excited trans-stilbene with various mono- and diamines and found a steric effect on the quenching constant The a, co-diamines (dabco, diaminoethane, -propane and -butane) quenched the stilbene fluorescence more efficiently than the monoamines, depending on the chain length between the amino groups. This was ascribed to the formation of cyclic radical cations, with a N-N three electron a-bond. In this case, an exciplex between diamine and stilbene is formed. 

In nonpolar solvents, the cyclobutane 44 is the only product upon irradiation of trans-stilbene/dimethyl fumarate mixtures. Exciplex emission, the detection of a weak ground state complex, as well as the stereospedfity of the cyclobutane formation support the proposed mechanism outlined in Eq. (26). 

As mentioned above, in photoreactions, the intermediates are formed through the excitation of one partner or the excitation of CT complex. In the former case, the excited molecule interacts with the partner to form the exdplex, while the latter produces excited EDA complex or in short, excited complex . Actually, some studies have shown that exciplexes and excited complexes are identical species [32-35] 1) They have the same spectral character. For example, when 0.12 M Pms-stilbene and 0.43 M fumaronitrile are excited in the charge-transfer absorption band (360 nm), both the spectral distribution and lifetime of this emission are identical with those obtained for their exciplex [36]. 2) They undergo the same follow-up reactions. Lewis [35] reported a similar cycloaddition quantum yield of the two above processes in the photocycloaddition of stilbene with dimethyl fumarate. Accordingly, here we will use them interchangeably. 

Deuterium labelling studies have also been used to investigate the reaction of stilbenes and related compounds with amines (Lewis, 1979). It is known that tertiary amines form fluorescent exciplexes with stilbenes in nonpolar solvents and that polar solvents are necessary for chemical reaction to occur (Lewis and Ho, 1977). This suggests that radical ions are involved in product formation. When secondary amines are used, reaction occurs in solvents of widely differing polarity and this is presumably due to the acidity of the secondary N—H bond. N-deuteriated diethylamine reacts with 1,2-diphenyl-cyclobutene in benzene to give products [65], [66] and [67] incorporating deuterium (Scheme 6). For the reaction with unsymmetrically substituted... 

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