Sensitization triplet

Triplet sensitization is associated with exothermic energy transfer from the triplet excited state of the sensitizer to the ground state of the reactant. The majority of sensitization processes in organic resist systems is based on the reaction 

Reiser, Photoreactive Polymers The Science and Technology of Resists, p. 84, John Wiley Sons, Hoboken, NJ (1989). 

Merle Aubry, D. A. Holden, and J.E. Guillet, Photophysics and photochemistry of naphthyl ester polymers in solution, Macromolecules 13, 1138 (1980). 

Given that the overall spin of the system is conserved, the transfer process is spin-allowed and occurs efficiently, provided only that transfer is exothermic, that is, x(sensitizer) x(reactant). It must he mentioned that in triplet sensitization, only the triplet state of the reactants is populated. Consequently, the photoreaction occurs exclusively from the triplet state and may proceed hy a different route from the singlet state reaction.  

Reiser has identified a number of attributes that candidates for triplet sensitizers must possess, as follows  

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Figure Bl.16.2. X-band TREPR spectra obtained at 0.1 ps after 308 mn photolysis of a fliiorinated peroxide dimer in Freon 113 at room temperature. Part A is the A/E RPM spectrum obtained upon direct photolysis part B is the E/A RPM spectrum obtained upon triplet sensitization of this reaction using benzophenone.

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. 

Similarly quantitative yields of (141) are obtained from (140) in benzene solution by triplet sensitization using acetophenone. 

Oxadiazole was obtained through the first excited singlet state. When the reaction was carried out in the presence of a triplet sensitizer, 99 was not detected but the quinazolinone 100 was obtained (Scheme 41) [91JCS(P2)187]. Compound 99 cannot be obtained via the Dewar isomer. The author supposed the formation... 

In addition, there are many surface modification processes that use triplet sensitizers to permit oxidation reactions. In a typical process, polyisocyanate is applied on a polyolefin together with a sensitizer such as benzo-phenone and then irradiated with UV light. As shown in Eq. (15) the sensitizer has an oxidizing effect to produce hydroxyl groups over the polymer surface. These hydroxyl groups finally react with isocyanate to provide a functional polymer [56,57]. 

An essentially quantitative yield (98%) of the thermally stable hexafluoro-2-aza-bicyclo[3.2.0]hepta-3,6-diene 18 is obtained from the small-scale photolysis of the hexafluoro-l//-azepine 17.230 Hexafluoro-l//-azepine-l-carbonitrileand-l-carboxamidebehave similarly. On scaling up the process for the l-(ethoxycarbonyl) derivative 17, however, a much lower yield (59%) of bicycle 18 resulted, although this was improved to 81% by carrying out the photolysis in hexane or rm-butyl alcohol in the presence of a triplet sensitizer, e. g. ben-zophenone. 

While di-i-butyl (34) and dicumyl hyponitrites (35) have proved convenient sources of Tbutoxy and cumyloxy radicals respectively in the laboratory,71 72 115"117 the utilization of hyponitrites as initiators of polymerization has been limited by difficulties in synthesis and commercial availability. Dialkyl hyponitrites (16) show only weak absorption at A>290 ntn and their photochemistry is largely a neglected area. The triplet sensitized decomposition of these materials has been investigated by Mendenhall et a .11 s... 

Polymers are often side products. Photochemical addition of aldehyde to conjugated C=C units can be efficient when a triplet sensitizer (p. 316) such as benzophenone is... 

During the last decade, Bentrude et al. [65] has shown that the triplet-sensitized photorearrangement of allylphosphites and analogs (Scheme 28) is a powerful method for preparation of alkylphosphonates from phosphites. Moderate to high yields were observed when triphenylene was used as photosensitizer [66,67]. 

Scheme 28 Triplet-sensitized photorearrangement of allylphosphites. Reprinted with permission from [67]. Copyright 2000 American Chemical Society...

The triplet-sensitized photorearrangement of allylphosphites [65- 67] offers a rapid and efficient access (60-70% yields) to oligonucleotides, which are key molecules in the antisense approach to antiviral and antitumor therapy (Scheme 40) [39,40,73]. 

Scheme 40 Triplet-sensitized photorearrangement of allylphosphites applied to the preparation of dinucleosides...

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... 

When Hammond and co-workers(59) found that the intersystem crossing quantum yield for aromatic ketones was unity (see Chapter 3) it was a short but very important step to realize that these compounds should be ideal triplet sensitizers. Thus one can excite the triplet state of molecules that otherwise would be formed inefficiently, if at all, by intersystem crossing. This idea resulted in a number of papers in the early 1960 s from the Hammond group on this topic. It is not possible in this short section to survey this area, but a few of the early studies are indicated by the following reactions ... 

The decarbonylation of dibenzyl ketone has been shown to result from the carbonyl triplet state by its ability to be quenched by 1,3-cyclohexadiene or l,3-pentadiene.<66) Using 1,3-cyclohexadiene as quencher, photodimers of the cyclohexadiene were obtained. Since these are formed only by triplet sensitization,<66) the quenching of ketone triplet states, rather than their excited singlets, was assured. Further evidence for a triplet reaction follows from the fact that decarbonylation could be sensitized by acetone under conditions where the sensitizer absorbed 93% of the light. 

It has been proposed that when a triplet sensitizer has insufficient exdta-tation energy to promote an acceptor to its triplet state, this deficiency can be supplied as an activation energy ... 

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. 

Finally, it is worth noting that, in response to triplet sensitization, simple acyclic allylic chlorides may be converted into the corresponding chlorocyclopropanes (101). Perhaps this reaction can also occur during the photodegradation of PVC. 

The Diels-Alder reaction of isopyrazoles 365 with MTAD gives azoalkanes 366. Direct as well as triplet-sensitized (benzophenone) photolysis of these compounds leads to the corresponding housanes (bicyclo[2.1.0]pentanes) 367. Under acidic conditions, the housanes rearrange to the corresponding bicyclic products 368 <1995JOC308,... 

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