Triplet state photolysis

The photosensitized dimerization of isoprene in the presence of henzil has been investigated. Mixtures of substituted cyclobutanes, cyclohexenes, and cyclooctadienes were formed and identified (53). The reaction is beheved to proceed by formation of a reactive triplet intermediate. The energy for this triplet state presumably is obtained by interaction with the photoexcited henzil species. Under other conditions, photolysis results in the formation of a methylcydobutene (54,55). 

Photolysis of 3-phenyl-2,l-benzisoxazole in 48% HBr produced reduction and substitution products via a proposed triplet state nitrenium ion intermediate (71HCA2111). Photolytic decomposition of 5-bromo-3-phenyl-2,l-benzisoxazole in 48% HBr gave 2-amino-5-bromoacetophenone and 2-amino-3,5-dibromoacetophenone (Scheme 18). A nitrenium ion intermediate was also proposed for the photolytic decomposition of 3-phenyl-2,l-benzisoxazole in concentrated HCl (Scheme 19) (7IHCA2111). 

Nanosecond flash photolysis of 1,4-dinitro-naphthalene in aerated and deaerated solvents showed a transient species with absorption maximum at 545nm. The maximum of the transient absorption was independent of solvent polarity and its lifetime seemed to be a function of the hydrogen donor efficiency of the solvent. The transient absorption was attributed to the lowest excited triplet state of 1,4-dinitronaphthalene. Based on spectroscopic and kinetic evidence, the triplet state of 1,4-dinitronaphthalene behaved as an n - Tt state in nonpolar solvents,... 

In weaker acid systems, other reactions involving the triplet state supervene to the exclusion of dimerization. Photolysis of 85 in 3-3% sulfuric acid, 96-5% acetic acid, and 0-2% water gave as products tri-phenylmethane (93), 9-phenylfluorene (94), 6is-9-phenylfluorenyl peroxide (95) and benzophenone (96). When benzene was present, tetra-phenylmethane (97) was also formed in addition to the other products. When the triphenylmethyl cation is irradiated in 3-3% H2SO4, 80 1% HOAc, 16-4% toluene, and 0-2% H2O, the products observed were... 

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

Lu, C. Y. Lui, Y.Y. (2002). Electron transfer oxidation of tryptophan and tyrosine by triplet states and oxidized radicals of flavin sensitizers a laser flash photolysis study. Biochimica et Biophysica Acta (BBA) - General Subjects, Vol. 1571, No.l, (May 2002), pp. 71-76, ISSN 0304-4165... 

The comparatively small size of the simplest carbene (methylene) ensures that it has a definite mobility in frozen inert matrices, which leads to the formation of dimerization products under these conditions. It became possible only in 1981 to detect in the spectra of the diazomethane photolysis products bands at 1115 cm (Ar matrix) and 1109 cm (Xe matrix) which were attributed to the deformation vibration of methylene in its ground triplet state (Lee and Pimentel, 1981). 

The triplet state is usually the ground state for non-conjugated structures, but either species can be involved in reactions. The most common method for generating nitrene intermediates, analogous to formation of carbenes from diazo compounds, is by thermolysis or photolysis of azides.246... 

Herkstroeter and Hammond found support for this postulate from a flash photolysis study. They were able to measure directly the rate of sensitizer quenching (energy transfer) by cis- and fra/w-stilbene. When a sensitizer triplet had insufficient excitation energy to promote fims-stilbene to its triplet state, the energy deficiency could be supplied as an activation energy. The decrease in transfer rate as a function of excitation energy of the sensitizer is given by... 

The previous sections have shown that cis-trans isomerization of stilbene can take place via the lowest triplet state of stilbene. The question to be considered now is whether the isomerization upon direct photolysis takes place via the singlet state, the triplet state, or a vibrationally excited ground state.a 7 81 50)... 

A recently popular mechanism involves the intersystem crossing of the cis- or trons-stilbene singlet state, produced upon direct photolysis, to its corresponding triplet states, which would then undergo the type of reactions given in Eqs. (9.8M9.10) and (9.17M9.19) ... 

Dimers (73) and (74) were formed in approximately equal amounts in all cases, although, as in the cases of 2-cyclopentenone and 2-cyclohexenone, the relative amount of (72) (either cis-syn-cis or cis-anti-cis) was found to vary substantially with solvent polarity. As in 2-cyclopentenone, this increase in the rate of head-to-head dimerization was attributed to stabilization of the increase in dipole moment in going to the transition state leading to (72) in polar solvents. It is thought that the solvent effect in this case is not associated with the state of aggregation since a plot of Stem-Volmer plot and complete quenching with 0.2 M piperylene indicate that the reaction proceeds mainly from the triplet manifold. However, the rates of formation of head-to-head and head-to-tail dimers do not show the same relationship when sensitized by benzophenone as in the direct photolysis. This effect, when combined with different intercepts for head-to-head and head-to-tail dimerizations quenched by piperylene in the Stem-Volmer plot, indicates that two distinct excited triplet states are involved with differing efficiencies of population. The nature of these two triplets has not been disclosed. 

Irradiation of the cis isomer results in the isomerization with quantum yield, equal to 0.56. The fact that the quantum yields of cis and trans formation are similar and add to unity suggests the presence of a common intermediate, thought to be a twisted triplet state. Gas-phase photolysis of trans-... 

By studying the photolysis of sulfone (33) in the presence of sensitizers of varying triplet energy, it was found that the triplet state of (33) lies approximately in the range of 53.0-59.5 kcal/mole. 

Products from the photolysis of the cyclopropyl ketone (44) are dependent on the pH of the solvent.(42) In aqueous dioxane only the 2,3-diphenyl-phenol (45) is formed along with the photoacid (46). Bond cleavage at c takes place via both the singlet and triplet states, whereas bond cleavage at a with phenol formation takes place in the triplet state ... 

In addition, it was observed that the sensitized photolysis produced the same distribution of products with the same efficiency (fingerprint characteristic of the triplet state). From quenching studies the specific rate constant for the rearrangement could be obtained. Phenyl migration rearrangement is of intermediate efficiency, interposed between the more efficient and less efficient type A processes (Table 7.4). The type of mechanism proposed for this transformation is as follows ... 

Upon low conversion direct photolysis the cis isomer (10) gave only the cis isomer (12) and the trans isomer (11) gave only the trans isomer (13). The triplet sensitized reaction of (10) and (11) gave rise only to cis-trans isomerization. Thus the di-ir-methane photorearrangement from the triplet state cannot compete with triplet state deactivation via cis-trans isomerization (Zimmerman has termed this the free rotor effect). Several other examples of regio-specilicity and stereospecificity in di-w-methane photoreactions are as followsa8 a3) ... 

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