Stilbenes photoinduced electron transfer

It is now well established that the cation radicals of unsaturated and strained hydrocarbons undergo a variety of isomerization (e.g., Scheme 18) and cycloaddition reactions with much faster rates than those of the corresponding neutral molecules [162-165]. A cation radical chain mechanism analogous to Scheme 17 was reported for one-way photoisomerization of cis-stilbene (c-S) to truws-stilbene (f-S) via photoinduced electron transfer, as shown in Scheme 18 [166], Once c-S + is formed, it is known to isomerize to t-S + [167,168]. The free energy change of electron transfer... 

Tertiary amines have also been employed in electron transfer reactions with a variety of different acceptors, including enones, aromatic hydrocarbons, cyanoaro-matics, and stilbene derivatives. These reactions also provide convincing evidence for the intermediacy of aminoalkyl radicals. For example, the photoinduced electron transfer reactions of aromatic hydrocarbons, viz. naphthalene, with tertiary amines result in the reduction of the hydrocarbon as well as reductive coupling [183, 184]. Vinyl-dialkylamines can be envisaged as the complementary dehydrogenation products their formation was confirmed by CIDNP experiments [185]. 

Fig. 28 Photoinduced electron transfer studies carried out on 6-mer (not shown) and 7-mer hairpin DNA duplexes capped by a stilbene acceptor chromophore.31 167 168 The duplexes contain A-T base pairs and a single G-C base pair, whose position in the duplex is varied, (a) The photoinduced ET process is illustrated for 3G C, in which the G-C base pair is third removed from the stilbene (St) group. The stilbene fluorescence is quenched by electron transfer from the G donor, (b) Damping factors for charge separation, /3(CS), and subsequent charge recombination, /3(CR), for n G C, in which the G base is connected to the T-bearing strand, (c) Damping factors, /3(CS) and /3(CR), for n C G, in which the G base is connected to the A-bearing strand.

As discussed earlier, deprotonation of a-carbon forms a major reaction pathway for the disappearance of the amine radical cation. Studies of photoinduced electron-transfer reactions of tertiary amines by Lewis [7, 11] and by Mariano [5, 10] have contributed significantly towards our understanding of the factors that control this process. Lewis and coworkers used product-distribution ratios of stilbene-amine adducts to elucidate the stereoelectronic effects involved in the deprotonation process [5, 10, 121, 122]. In non-polar solvents, the singlet excited state of tran -stilbene forms non-reactive but fluorescent exciplexes with simple trialkylamines. Increasing solvent polarity brings about a decrease in the fluorescence intensity and an increase in adduct formation. For non-symmetrically substituted tertiary amines two types of stilbene-amine adduct can be formed, as is shown in Scheme 9, depending on whether the aminoalkyl radical adding to the stilbene radical is formed by de-... 

Besides photochemical cis z trans isomerization and physical quenching not involving a photoreaction, several processes may occur that do not lead to cis- and trans-stilbenes, for example, dimerization, rearrangement and subsequent cleavage, addition to some other molecule, and cyclization. Under suitable conditions (e.g., at low concentrations) several side routes may be hindered. On the other hand, in the presence of appropriate additives new reactions may be favored, such as photoinduced electron transfer, cycloaddition, dehydrogenation, oxetane formation, and oxygenation. 

The radical anions of tram- and c/s-stilbene can be distinguished by ESR [503]. A radical ion pair has been observed by this method using trial-kylamines in acetonitrile [497], Electron back transfer (i.e., a reaction of the radical anion of frans-stilbene with the radical cation of the donor) opens a new pathway for intersystem crossing to the tram triplet state. Time-resolved resonance Raman spectroscopy of photoinduced electron transfer from amines to frans-stilbene has been reported [497,504], In the photooxidation of frans-stilbene the radical cation has been observed by flash photolysis using cyanoanthracenes [505-507], The radical cations of cis-and frans-stilbene were also produced by electron transfer from biphenyl to excited 9,10-dicyanoanthracene and subsequent electron transfer from stilbene to the radical cation of biphenyl [508]. External magnetic field effects... 

The toluene-soluble fraction consists of a major product that has been identified as a CgQ-TEA monocycloadduct 7V-ethyl-rra/w-2, 5 -dimethyl-pyrrol-idino[3, 4 l,2][60]fullerene (14) by use of matrix-assisted laser desorption ionization mass spectroscopy and NMR methods [71]. It is interesting that the photochemical reaction actually results in the formation of a cycloadduct. This is unique to the fuilerene system because there have been no reports of cycloadducts in reactions involving nonfullerene acceptors [119-122], such as rra/is-stilbene. In the context of the classical photoinduced electron transfer-proton transfer mechanism [124], a two-step process for the formation of the cycloadduct has been proposed [71]. 

Klaukien, H. and Lehnig, M., H CIDNP study of the photoinduced electron transfer reaction between trans-stilbene and triethylamine,/. Photochem. PhotobioL A Chem, 84, 221, 1994. 

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. 

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