7050
Chemically Produced Excited States. Energy Transfer, Photochemical Reactions, and Light Emission Emil H. White,* Peter D. Wildes, Jacek Wiecko, Harold Doshan, and C. C. Wei Contribution f r o m the Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218. ReceivedApril21,1973
Abstract: Electronically excited states formed in chemical reactions have been used to sensitize several photochemical reactions (“Photochemistry without Light”). The principal source of excited-state energy was trimethyl-l,2-dioxetane which cleaves thermally to acetone and acetaldehyde and yields excited-state products with reasonable efficiency. Photoreactive acceptors used were stilbene, acenaphthalene, 4,4-diphenyl-2,5-cyclohexadienone, and santonin. The results are discussed in terms of triplet-triplet energy transfer from the initially formed excited carbonyl compound to the reactive species. Variations in the apparent chemical yield of excited states observed with different acceptors are discussed with regard to possible quenching modes and energetics of the reactions. Chemiluminescence was observed from thermal decomposition of trimethyl-l,2-dioxetane in the presence of the fluorescent lanthanide complex, europium tris(thenoy1trifluoroacetonate)-1 ,lo-phenanthroline. The formation of excited states in the reaction of oxalate esters with hydrogen peroxide was also examined. In contrast to previous reports, we found no evidence of a stable intermediate in the reaction of oxalate esters with hydrogen peroxide.
E
lectronically excited states of molecules are usually produced from ground states by light absorption. They possess not only an enhanced general reactivity (photochemistry) compared to the ground state, but also the ability to emit light (fluorescence and phosphorescence). Excited states can also be formed directly in chemical reactions. When prepared in this way they have usually been detected through their light emission properties (chemiluminescence and bioluminescence). We have recently shown that chemically produced excited states can also be detected through their chemical reactions (photochemistry without light) ; the full account of these and related results is the subject of this paper. A lively interest in this area of research has led to several recent publications by other workers. Although a large number of chemiluminescent reactions are known, in only a few cases have the reactions been demonstrated to be reasonably efficient in the production of excited states. While the “photochemical” reactions of such excited states could be determined, the approach would not be general since X and Y would have to be “tailor-made” to generate the desired A* excited state. X
+ Y -+-A* --+A + hv
(1)
Generality can be introduced through the use of energy transfer. la Z+Y---tD*
LAD+**
(2)
(1) (a) E. H. White, J. Wiecko, and D. R. Roswell, J. Amer. Chem. Soc., 91, 5194 (1969); (b) E. H. White, J. Wiecko, and C. C. Wei, ibid., 92,2167 (1970); (c) E. H. White and C. C.Wei, Biochem. Biophys. Res. Commim., 39, 1219(1970). (2) (a) P. D. Bartlett and A. P. Schaap, J . Amer. Chem. Soc., 92, 3223 (1970); (b) S. Mazur and C. S. Foote, ibid., 92, 3225 (1970); (c) T. Wilson and A. P. Schaap, ibzd., 93, 4126 (1971); (d) A. P. Schaap, Tefrahedron Left., 1757 (1971); (e) N. J. Turro and P. Lechtken, J . Amer. Chem. Soc., 94, 2886 (1972); (f) A. A. Lamola, Biochem. Biophj’s. Res. Commun., 40, 304 (1971); (g) H. Gusten and E. F. Ullman, Chem. Commun., 28 (1970); (h) N. J. Turro and P. Lechtken, Tetrahedrori L e f t . ,565 (1973). (3) (a) I C2H,0H > n-C4H90H. In less reactive media, competing isomerization of 20 to aqueous CHICN the protoproducts 24,28, and 29 occurred; in nonhydroxylic media these were the principal photoproducts. Deuterium labeling studies involving irradiation of 20 in methanol-0-d revealed that the ethers 26 and 27 were formed without significant deuterium incorporation, whereas formation of the hydrocarbon products 24 and 25 was accompanied by extensive deuterium incorporation. These results are interpreted in terms of nucleophilic trapping of the Rydberg excited state 21a or the radical-cation intermediate 21b. The mechanistic ramifications of this interpretation are discussed.
-
revious studies in these laboratories4 and others5 have shown that on sensitized irradiation in hydroxylic media cyclohexanes, -heptenes, and -octenes afford a mixture of photoproducts which result from an initial light-induced protonation of the olefin ( c f . lb-d
P
(1) Presented in part as a plenary lecture at the Third International Symposium on Photochemistry, St. Moritz, Switzerland, July 1970; see Pure Appl. Chem., 24, 585 (1970). (2) Alfred P. Sloan Research Fellow. Send correspondence to this author at the University of North Carolina. (3) National Science Foundation Undergraduate Research Participant. (4) (a) P. J. Kropp and H. J. Krauss, J . Amer. Chem. Soc., 89, 5199 (1967); (b) P. J. Kropp, ibid., 91, 5783 (1969). (5) (a) For reviews, see J. A. Marshall, Accounts Chem. Res., 2, 3 3 (1969); (b) J. A. Marshall, Science, 170, 137 (1970).
-,2b-d).
In striking contrast, cyclopentenes and other highly constrained cyclic olefins exhibit radical behavior on irradiation under similar c ~ n d i t i o n s , ~ ~ , ~ whereas large-ring cyclic and acyclic olefins exhibit only cis e trans isomerization. Exocyclic olefins, such as the methylenecycloalkanes 3, exhibit no observable photobehavior, although they probably also undergo an undetected cis s trans isomerization under these conditions. The unique behavior of cyclohexenes, -heptenes, and -0ctenes leading to photoprotonation has been attributed4,jto initial cis trans isomeri( 6 ) R. R. Sauers, W. Schinski, and M. M. Mason, Tetrahedron Lett., 4763 (1967).
Journal of the American Chemical Society i 95:21 / October 17, 1973
