Indole triplet excited state

The photochemical addition of alkenes to the pyrrole ring of indole to give cyclobutane products was first reported by Julian and Foster in 1973 [43], They found that both electron-rich and electron-poor alkenes add to indole, but only if an acyl group is present on the indole nitrogen atom. With monosubstituted alkenes the reaction was regioselective and, with the exception of vinyl acetate, the exo stereoisomer was produced as either the major or the only product (Scheme 15). A mechanism involving the indole triplet excited state was suggested, based on the observation that the reaction could be sensitized by acetophenone and quenched by naphthalene. 

Undoubtedly, the most characteristic property of 1,2-dioxetanes and a-peroxylactones is the fact that they emit light on thermal decomposition. Since in liquid media in the presence of molecular oxygen triplet excited states are efficiently quenched, the observed direct chemiluminescence is ascribed to the fluorescence of the carbonyl product. This fluorescence occurs usually at 420 10nm and corresponds to n n excitation.The shortest wavelength emission has probably been observed for the indole-1,2-dioxetane (17) that occurs at 320 nm. ... 

Several reviews have been written concerning the phosphorescence of intrinsic indole probes [201] and the use of triplet excited states to probe proteins [202]. Triplet tryptophan can be used to probe proteins, but its photophysics is complex, because in a homogeneous solution, the decays of the excited triplet tryptophan or indole derivatives do not usually follow first-order kinetics, due to protonation, triplet-triplet annihilation, or self-quenching [203,204]. In ad-... 

The intermediate 37 can be viewed as arising from interaction of the triplet excited state of the indole with the alkene partner so as to form the most stable triplet 1,4-biradical intermediate possible. Alternatively, 37 may arise from attack of the 2-position of the triplet-excited indole on the less-substituted, and hence less sterically hindered terminus of the alkene. Either way, the preferential formation of 37 means that with monosubstituted alkenes, the photocycloaddition reaction with AT-acylin-doles is regioselective, while with unsymmetrically 1,2-disubstituted alkenes the reaction will be nonregioselective, as was seen for the reaction between N-benzoylindole and methyl 3-methyl-2-butenoate shown in Scheme 19. Should it be desired, the opposite regiochemistry to that normally obtained with monosubstituted alkenes can be induced by tethering the alkene to the indole [52] the reaction of 1,2-disubstituted alkenes can be made regioselective in the same way [53]. Examples of this are shown in Schemes 20 and 21. 

The C2-C3 bond of an acylindole can act as the alkene component and add to the triplet excited state of an acylindole to produce indole dimers. This was first reported by Hino and co-workers in 1977, who found that UV light irradiation of 1,3-diacetylindole yields a single dimer [55]. The structure was determined by X-ray crystallography to be the syn head-to-tail dimer 62 (Scheme 23). More recent work has shown that this stereochemistry of addition is exceptional and that other N-acylindoles give mixtures of the anti head-to-head and head-to-tail dimers 63 and 64 only (Scheme 24) [56]. 

Under acetone sensitization, 3-substituted indoles couple with 5-bro-mo-l,3-dimethyluracil at the indole 2-position (Scheme 29). It is proposed that this reaction proceeds via electron transfer from indole to the uracil triplet excited state since better electron donors than indole quench the reaction [14a, 64,65]. A similar reaction occurs when indole or 3-methylin-dole is photolyzed w ith 3,4-dibromo-AT-methylsuccinimide (Scheme 30) [65]. The quantum yield of this reaction is 0.14 in cyclohexane and 0.49 in diethyl ether, and drops to 0.02 in acetonitrile, which suggests that full electron transfer and radical ion-pair separation does not occur in this case. 

Photolysis Mechanism. The photophyslcs and photochemistry of the Indole ring have been comprehensively reviewed in Refs. 21-23. The unique properties of the Indole-rlng excited states, either the singlet or the triplet. Include effects by solvents, temperature, and polarity. 

While further investigating the origin of the regiochemical course of the photocycloaddition reactions between A/ -benzoylindoles and monosubstituted alkenes, Weedon and Hastings demonstrated that the reactions involve the triplet excited state of the indole derivatives (Scheme 4). The reactions then proceed through the formation of a 1,4-biradical intermediate 20, resulting from the bonding of the 2-position of the indole to the unsubstituted terminus of the aUcene, which is least able to support a radical center [18-20]. 

One of the most efficient enzymatic systems that generates triplet state acetone is the horse radish peroxidase(HRP)-catalyzed oxygenation of isobutyraldehyde (Eq. 79). Related enzymatic processes include the autoxidation of linear carbox-aldehydes, malonaldehyde, a-formylphenylacetic acid, indole-3-acetal-dehyde, indole-3-pyruvic acid, ° and indole-3-acetaldehyde. Consequently, there is no question about the existence of enzymatically generated electronic excitation in the cell however, what do these excited states do in the biological system ... 

Table 1 lists some of the photophysical properties of indole of relevance for planning photochemical reactions. The numbers should be taken as guidelines only, given the perturbations that can result from changing structure and medium. The data in Table 1 indicate that fluorescence and intersystem crossing of the singlet excited state are both fast, efficient processes. Consequently, both triplet photochemistry and fast singlet photochemistry can be expected. 

Shimizu et al report that while [2.2] paracyclophane (55) undergoes two-photon dissociation in low temperature matrices by way of the triplet state, in the gas phase, the efficient two-photon process proceeds via a hot molecule formed by internal conversion from the initially formed singlet excited state. The photocleavage of 2-nitrobenzyl ethers and ester has been widely reported and has now been evaluated as a deprotection methodology for indoles, benzimidazole, and 6-chlorouracil (Voelker et al). The mechanism of the cleavage of such compounds is considered to involve the o-quinonoid intermediate, but previously these had only been deduced from transient electronic spectra produced in flash photolysis experiments. Infrared spectral data from photochemical studies of 2-nitrobenzyl methyl ether in argon and nitrogen matrices have now been published which confirm that the intermediate does indeed have the o-quinonoid structure... 

Work on indole, tryptophan, etc. continues because of their relevance to the complex field of protein photophysics. Creed has produced reviews of the photophysics and photochemistry of near-u.v.-absorbing amino-acids, viz. tryptophan and its simple derivatives, tyrosine and its simple derivatives, and cysteine and its simple derivatives. The nature of the fluorescent state of methylated indole derivatives has been examined in detail by Meech et al. Another investigation on indole derivatives deals particularly with solvent and temperature effects. Fluorescence quenching of indole by dimethylfor-mamide has also been examined in detail. Fluorescence excitation spectra of indoles and van der Waals complexes by supersonic jets give microscopic solvent shifts of electronic origin and prominent vibrational excitation of L(, states. Conventional flash photolysis of 1-methylindole in water shows R, e p, and a triplet state to be formed. " Changes in the steady-state fluores-... 

The quenching of chemically excited triplet acetone by biologically important compounds such as indoles, tyrosine derivatives, quinones, riboflavin, and xanthene dyes has been studied. Quenching occurs by electron and Forster transfer processes. Triplet-state parameters have been presented for the... 

The majority of reversible photoinduced PT reactions takes place between a donor and an acceptor that are chemically different, being either different atoms or identical atoms in inequivalent positions of a molecule or a molecular assembly [12,13], Examples are systems in which the phenomenon was first discovered and which (sometimes in the form of related compounds and derivatives) continue to be the object of present work methyl-salicylate [6,7], n-salicylidene aniline [8], 3-hydroxyflavone [10], and dimers of 7-aza-indole [9], Other molecules include 2-(2 -hydro-xyphenyl)benzothiazole [163,164] and 2-(2 hydroxyphenyl)benzoxazole, which has been studied in detail by Grellmann and co-workers [165,166], and where in addition to the fast transfer in the excited singlet state, the slower transfer in the triplet manifold could be characterized by transient absorption measurements in solution at temperatures down to 55 K. 

It is well established that o grgen quenches both excited singlet and triplet states of aromatic compounds. Thus, indole derivatives in solution at room temperature display a ratio for the rate constant Icnk/I qp of approximately two (48, 49). However, the quenching of alkaline phosphatase phosphorescence by oxygen proceeds at a rate kqp = 1.2 x lO s (47). The magnitude of this rate... 

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