Triplet-Sensitized Isomerization

A method for the determination of intersystem crossing quantum yields involving triplet-sensitized isomerization has been developed by Lamola and Hammond.<55) In this method the compound whose intersystem crossing... 

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) ... 

Kuki, M., Y. Koyama, and H. Nagae. 1991. Triplet-sensitized and thermal isomerization of all-frans, 7-cis, 9-cis, 13-cis, and 15-cis isomers of (3-carotene Configurational dependence of the quantum yield of isomerization via the T1 state. J. Phys. Chem., 95 7171-7180. 

By-products of the direct excitation of 104 were the isomeric cis- and main products of the triplet-sensitized excitation of 104 in the presence of ketones. Acid-catalyzed prototropic rearrangement of a colorless precursor (141) was suggested to account for formation of 142. ... 

It has been assumed so far that the sensitizer acts by an energy-transfer mechanism, but in some cases other modes of interaction may occur. It is possible that electron transfer takes place to give the radical anion or the radical cation of the alkene, which is the species that subsequently isomerizes. This is likely to be the case in the chlorophyll-sensitized isomerization of vitamin A acetate, which is used commercially to obtain the required all-trans isomer 12.8) from the mixture of Isomers resulting from the synthesis. Unlike triplet-sensitized reactions, electron-transfer isomerizations frequently lead to a predominance of the most thermodynamically stable isomer. 

Triplet-sensitized isomerization can also be achieved, in which case the relative values of triplet energies are the most important factor. 

Continuous processes if photochemical (and thermal) reactors are installed in series. Such an arrangement could be useful in the production of previtamin D (PD), where tachysterol (T) produced as a secondary product (Figure 21) may be recovered by a subsequent triplet sensitized cis-trans isomerization [2, 3, 75-77]. 

The stilbenes have played a crucial role in the development of modern photochemistry. Direct or triplet sensitized irradiation of trans-stilbene (t-1) in dilute solution results in isomerization to cis-stilbene (c-1) as the exclusive uni-molecular photochemical reaction (1-3). Direct irradiation of c-1 results in isomerization to both t-1 and trans-4a,4b-di-hydrophenanthrene (2), which revert to c-1 both thermally and photochemically and can be trapped by oxidants such as iodine or oxygen to yield phenanthrene (3) (4-6). Triplet sensitized irradiation of c-1 yields only t-1. These unimolecular isomerization pathways are summarized in eq. 1. 

The pioneering investigations of Hammond et al. (7) in the 1960s on the triplet-sensitized cis-trans isomerization of the stilbenes provided much of the basis for our current understanding of triplet-triplet energy transfer. The detailed... 

Quantum yields for isomerization of t-1 to c-1 via intersystem crossing are similar to those obtained in triplet sensitization, indicating that t is formed with near unit efficiency. Similar results were reported for the interaction of t with inorganic anion electron donors (127b). Rate constants for intersystem crossing of the t-1-ethyldiisopropylamine and triethylamine exciplexes obtained from the measured exciplex lifetimes are 5 x 107 s"1 (116), similar to the values for the exciplexes of pyrene (128) and naphthalene (121) with triethylamine. Larger intersystem crossing rate constants are observed for the exciplexes of t-1 with the bicyclic amines l-azabicyclo[2.2.2]-... 

In comparison to the conventional two-way isomerization (Z E) of stilbene and other aryl alkenes 12 (Scheme 5), a novel one-way isomerization (Z - E) of C=C double bonds was achieved upon replacing a phenyl group of stilbene by a 2-anthryl group9a,c. Tokumaru and coworkers found in isomerization studies on substituted anthracenes9 that substitution at the C=C bond resulted in complete isomerization of the Z-isomer to the corresponding /i -isomer upon irradiation, via a quantum chain process. Interestingly, the isomerization takes place as an adiabatic process in the triplet manifold on both direct and triplet sensitized irradiations. 

The fact that the intraligand transition is lowest in energy in ClRe(CO)3-(fnms-4-styrylpyridine)2 also allows a direct assessment of the effect of the coordination on the decay properties of the intraligand excited state. Data for both the photoisomerization by direct irradiation, Table 19, and the triplet sensitized isomerization, Table 20, of ClRe(CO)3(fra s-4-styrylpyridine)2 and frans-styrylpyridine... 

Table 20. Triplet sensitized isomerization of f/vwis-styrylpyridines and XRe(CO)3 (frans-sty rylpyridine)21)...

The study of the direct irradiation and triplet sensitized isomerization of styryl-ferrocene, reaction (46), is noteworthy.134) For either direct or benzophenone sensitized photoisomerization the photostationary state is exclusively the trans isomer. 

However, while 313 and 366 nm direct irradiation yields a cis- trans quantum yield of-0.005, the benzophenone triplet sensitized and the low energy direct irradiation yield is only —0.001. These results support the notion that two reactive excited states are involved one associated with the styrylferrocene which leads to the more efficient isomerization and a ferrocene-type excited state at lower energy which has a smaller tendency to undergo cis -trans isomerization. It is interesting to note that the 100% trans photostationary state is likely very close to the amount expected at thermal equilibrium. Such a result would be consistent with an intramolecular CT transition to yield a radical-anion ferricinium complex as in reaction (47). The radical anion could yield cis trans isomerization just as in the Ru(II)-styrylpyridine complexes. The wavelength dependence of the isomerization of the styrylferrocene is consistent with (47) in that upper excited states are required for the photooxidation of ferrocene to ferricinium in CC14.135 ... 

It is known that simple p,y-unsaturated ketones 69 do not undergo ODPM rearrangement by the acetophenone-sensitization. Instead, under these conditions 69 undergoes alkene E/Z isomerization. However, when thioxanthone is used as a sensitizer, the [ly-unsaturated ketones 69 react to form ODPM rearrangement photoproducts 70 (Scheme 4.28) [41 ]. This result shows clearly that the energy of the triplet sensitizer plays an important role in determining the outcome of the triplet ODPM rearrangement of p,y-unsaturated ketones. 

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