Energy transfer triplet sensitization

Recently, Bach and Schroder " reported the photocycloaddition of a-alkyl-substituted enecarbamates 41 to benzaldehyde affording 3-aminooxetanes 42 in moderate to good yields (46 to 71%). The a-phenyl-substituted enecarbamate did not lead to a photocycloaddition product presumably due to rapid energy transfer (triplet sensitization) from the electronically excited aldehyde. An increase in steric bulk of the alkyl substituent R shifted the diastereomeric ratio cis/trans in the direction of the thermodynamically more stable cis product. 

Figure 28-4 Schematic energy levels for cis- and fra/7S-1,2-diphenyl-ethene. The upward transitions are achieved by transfer of energy from triplet sensitizer. The downward transitions from the nonplanar stage marked with the rate constants k, and kc involve loss of thermal energy to the solvent, or phosphorescence. The lower energies assigned to the S0 and T, states of frans-1,2-diphenylethene relative to the So and T, states of the cis isomer reflect steric hindrance between the phenyl groups of the cis isomer.

A second method for observing phosphorescence spectra is to populate the triplet via energy transfer with sensitizers. With this sensitized phosphorescence a very inefficient intersystem crossing step can be overcome. The bicyclic azo compound 8 a and its 3,6-dimethylanalog 8 b appeared ideal for such a study since they show little photochemistry from the triplet state. However this investigation was not successful 23) (see also 24>). The triplet energy of the 3( n ) and 3( wr ) state of pyridazine 25a> and benzo[c]cinnoline 12 25b> could be determined from the crystal absorption spectra at 4.2 K. 

The energies of the triplet states of sensitizers TX (ITX) and of Pis TPO (BAPO) are close to each other ( 260 kJ/mol), allowing for slightly exothermic or thermoneutral T-T energy transfer from sensitizer to PI. Direct photolysis of phosphine oxides results in a well-documented initial strong absorptive (A) pattern of ESR spectra (see Fig. 12.2). Sensitization by TX or ITX of the photolysis of phosphine oxides leads evidently to the same radicals, but an initial polarization pattern is quite different, namely, emission/absorption (E/A) pattern (see Fig. 12.3). 

Long triplet lifetime to increase the probability of energy transfer between sensitizer and reactant. 

Tatikolov, A.S., Dmitrenko, O., and Terenetskaya, L.P., Triplet—triplet energy transfer in sensitized c s-tra s-isomerization of previtamin D, Chem. Phys. Reports, 14, 461—466,1995. 

The energy of the triplet state of the sensitizer ( Sens ) must be greater than that of the reactant. If this condition is not met, the energy transfer becomes endothermic and cannot compete with other transformations of Sens. ... 

The common case, and the one that will be emphasized here, is triplet sensitization. In this case, the intersystem crossing of the sensitizer must be faster than energy transfer to the reactant or solvent from the singlet excited state. 

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

Attempts to sensitize the rearrangement with benzophenone, propiophenone, and chlorobenzene failed, as indicated in Table 8.1. Although the reaction could not be sensitized, triplet energy transfer was taking place inasmuch as compound (1) quenched the photoreduction of benzophenone without the formation of any new products (Table 8.2). 

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... 

As the triplet energy of the sensitizer becomes less than that necessary to excite the trans-diene triplet, energy transfer to the cis triplet becomes increasingly important and the product derived from this state (vinylcyclohexene) increases proportionally. This explanation is summarized in Figure 10.1.(11>14)... 

Another explanation has been offered to explain the large proportion of cyclobutane derivatives produced by low-energy sensitizers, especially for the anthracene derivatives.<17) This is that energy transfer to diene occurs from the second excited triplet state of the sensitizer rather than the first. Experiments using a large number of anthracene derivatives as sensitizers... 

Aryl ketones are often used to effect cis and tram isomerization of olefins.(118-ia0) Although this, in some cases, can be viewed as an energy transfer process where the ketone triplet transfers its energy to the olefin, which then isomerizes, the failure of noncarbonyl sensitizers of comparable triplet energy to isomerize the olefins suggests that a process other than energy transfer may be involved. Schenck and Steinmetz<118) suggested that isomerization results from decomposition of a biradical carbonyl-olefin adduct similar to that involved in oxetane formation ... 

Thus Saltiel has concluded that the small increase in [T]S/[C], in going from benzene to acetone indicates that a mixed mechanism is operative for acetone-sensitized isomerization, that is, both triplet energy transfer and, to a minor extent, Schenck intermediates are involved. When acetophenone or benzophenone is used as a sensitizer the pss is close to the thermodynamic... 

A photosensitized dimerization of an isolated olefin, norbomene, has been reported by Scharf and Korte.<3) Irradiation in acetone or in the presence of acetophenone (Et = 74 kcal/mole) produced dimers (5) and (6) as major products. However, benzophenone (Et = 69 kcal/mole) failed to sensitize the reaction to (5) and (6), but in ether solution led to the quantitative formation of benzpinacol and in benzene to the oxetane (7) in 80% yield. Sensitizers of intermediate energy, such as xanthone (Et — 72 kcal/mole), demonstrated a competition between energy transfer to form triplet norbomene and cycloaddition to form the oxetane ... 

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