J . Am. Chem. SOC.1980, 102, 6051-6055
605 1
Quenching of the Fluorescence of 2-Norbornanone and Derivatives by Electron-Rich and Electron-Poor Ethylenes Nicholas J. Turro* and George L. Farrington Contribution from the Chemistry Department, Columbia Uniuersity, New York, New York 10027. Received February 15, 1980
Abstract: The rate constants for fluorescence quenching of a series of substituted 2-norbornanones by trans-dicyanoethylene (t-DCE) and cis-diethoxyethylene (c-DEE) have been determined. The effects of ketone structure on the n,r* singlet reactivity for the two olefins are discussed. The rate of quenching by t-DCE was reduced by substituents which block access to the exo face of the carbonyl, whereas quenching by c-DEE was most strongly influenced by substituents which increased the steric hindrance in the carbonyl plane near the oxygen atom. The differing structure-reactivity correlations provide evidence for two distinct quenching mechanisms consistent with a simple model for the spatial distribution of electron density in the n , r * state. The electron-poor olefin t-DCE interacts preferentially with the nucleophilic r* system of the n , r * excited carbonyl, while the electron-rich olefin c-DEE interacts with the half-filled electrophilic n orbital on the carbonyl oxygen. The variation of t-DCE fluorescence quenching rates with ketone substitution follows a pattern similar to that observed for the ground-state reduction of 2-norbornanones by sodium borohydride. The results are discussed briefly in terms of elementary orbital interactions between an n,r* state and an electron-poor or an electron-rich ethylene.
Introduction Oxetane formation via the [2 + 21 photocycloaddition of carbonyl compounds to olefins and other unsaturated substrates (the Paterno-Buchi reaction, eq 1 ) has been the subject of ex-
tensive syntheticl.2 mechanistic ~ t u d i e s ~and - ~ theoretical analyses.6-8 Photocycloadditions of alkyl and aryl aldehydes and ketones with olefins and 1,3-dienes have been the most widely investigated.’-* The scope of the reaction and its synthetic utility have been discussed in several reviews.* More recent studies have broadened the range of reactants to include photocycloadditions between alkyl ketones and electronrich and electron-poor olefins, and have sought to characterize the excited states involved and the detailed sequence of events following the ketone n,a* state-olefin e n c o ~ n t e r .Product-dis~ tribution and quantum-yield studies together with insights into the steric and electronic requirements for the initial interaction obtained from quenching studies have helped to define the factors governing bond formation. Steric interactions between quenchers and ketone n,a* excited states have also been found to have an important influence on the rate of quenching.] Therefore, steric effects provide another probe for the stereochemical requirements for exciplex formation. For example, Barltrop and Carless9 have found that a-methyl( I ) Turro, N. J. “Modern Molecular Photochemistry”; W. A. Benjamin/Commings: Menlo Park, Calif., 1978; Chapter 11. (2) (a) Arnold, D. R. Adv. Photochem. 1966, 6, 301. (b) Muller, L. L.; Hammer, J. “1,2-Cycloaddition Reactions”; Interscience: New York, 1960. (c) Turro, N. J.; Dalton, J. C.; Dawes, K.; Farrington, G.; Hautala, R.: Morton, D.: Niemczyk, M.; Schore, N. Acc. Chem. Res. 1972, 5, 92. (3) (a) Buchi, G.; Inman, C. G.: Lipinski, E. S. J . Am. Chem. SOC.1954, 76, 4327. (b) Yang, N. C.; Nussim, M.; Jorgenson, M. J.: Murov, S. Tetrahedron Left. 1964, 3657. (c) Yang, N. C.; Cohen, J. I.: Shani, A. J . Am. Chem. SOC.1968, 90, 3264. (4) (a) Turro, N.J.: Wriede, P. A. J . Am. Chem. SOC.1968, 90, 6863. (b) Ibid. 1970, 92, 320. (c) Turro, N. J.: Wriede, P. A,; Dalton, J. C. Ibid. 1968, 90, 3214. ( 5 ) (a) Kasha M. In “Light and Life”, McElroy, W. B., Glass, B., Eds.; Johns Hopkins Press: Baltimore, 1961; p 31. (b) Zimmerman, H. E. Adu, Photochem. 1963, l ? 183. ( 6 ) (a) Epiotis, N. D. J . Am. Chem. SOC.1972, 94, 1941. (b) Ibid. 1972, 94, 1946. (7) Herndon, W. C.; Giles, W. B. Mol. Phorochem. 1970, 2, 277. (8) Dauben, W. G.; Salem, L.: Turro, N. J. Acc. Chem. Res. 1975, 8, 41.
0002-7863/80/1502-605 l$Ol .OO/O
acrylonitrile quenches acetone fluorescence at approximately the same rate as acrylonitrile, but crotonitrile, which has a P-methyl group, quenches an order of magnitude more slowly. In studies of acetone and norcaphor fluorescence quenching by electron-rich olefins, plots of In [kq[/(kdiff - k f)] vs. olefin ionization potentials were found to exhibit virtually t i e same slope but with different intercepts.1° It was suggested that the nature of the charge-transfer interaction in the exciplex is similar for acetone and norcamphor, but that there is a difference in the environment of interaction which might be attributed to the amount of steric hindrance presented by the ketone to the approach of the olefins.
Steric Approach Control vs. Product Development Control Steric approach control in the sodium borohydride reduction of a series of methyl-substituted 2-norbornanones has been demonstrated.” For example, it has been shown that NaBH4 attacks the carbonyl a system of norcamphor (1) preferentially from the less sterically hindered exo face, and the thermodynamically less stable endo alcohol is the major product.llb Blocking of the exo face by the introduction of a syn 7-methyl group in 7,7-dimethylnorcamphor (3) slows down the overall rate of reduction and results in favored attack from the endo face of the carbonyl. W e selected a series of appropriately substituted 2-norbornanones as model systems for the investigation of the preferred approach of electron-rich and of electron-poor ethylenes in the quenching of the n,r* singlet states of alkanones. Spectroscopic and Dynamic Properties of 2-Norbornanones The absorption and fluorescence spectra of norcamphor are representative of the series of 2-norbornanones studied in this investigation.I2 The absorption spectrum of norcamphor consists of a broad envelope maximizing a t 289 nm in acetonitrile solution ( t 24.7) and exhibiting some vibrational structure. The emission spectrum is broad and unstructured, maximizing a t 410 nm. The absorption and emission maxima for the other 2-norbornanones in the series fall within a relatively narrow wavelength range:,,A 288-301 nm for absorption and 395-412 nm for emission. The (9) (a) Barltrop, J. A.; Carless, H. A. J. J . Am. Chem. SOC.1972, 94, 1951. (b) Barltrop, J. A,: Carless, H. A. J. Tetrahedron Letr. 1968, 3901. (10) (a) Schore, N. E.: Turro, N.J. J . Am. Chem. SOC.1975, 97, 2482.
(b) Yang, N . C.; Hui, M. H.: Schold, D. M.: Turro, N. J.; Hautala, R. R.: Dawes, K.; Dalton, J. C. J . Am. Chem. SOC.1977, 99, 3023. (11) (a) House, H. 0. “Modern Synthetic Reactions”: W. A. Benjamin: New York, 1965: Chapter 2, and references cited therein. (b) Brown, H. C.; Muzzio, J. J . Am. Chem. SOC.1966, 88, 281 1 . (12) Waddell, W. H.: Turro, N.J.; Farrington, G. Mol. Photochem. 1976, 7 , 475.
0 1980 American Chemical Society
6052
J. Am. Chem. SOC.,Vol. 102, No. 19, 1980
Turro and Farrington
Table I. Fluorescence Quenching of 2-Norbornanonesby t-DCE and c-DEE
29
7.1
5.7
5.1
1.2
28
6.9
5.9
4.7
1.2
5.0
7.3
5.0
1.0
1.5
5.6
2.6
4.7
1.2
0.55
14
2.7
4.8
2.9
0.56
14
5.1
6.0
2.3
0.85
21
2.6
4.4
4.8
0.59
4.7
0.6
(c)
(C)
(C)
5.2
0.4
2.9
1.8
0.14
1.5
95% exo stereochemistry by GLC analysis. Subsequent epimerization by refluxing with 3% NaOMe/MeOH resulted in a mixture of isomers in a 55:45 endo:exo ratio, and separation was achieved by preparative GLC (20 ft X 1/4 in 10% FFAP on Chromosorb P, CT = 170 “C). 1-Methylnorcampbor(5) was prepared from norcamphor by the method of Bartlett and Singer.24 3,3-Dimethylcamphor (10)was prepared by treating a benzene solution of camphor with NaNH, and methyl iodide according to the method of Haller and B a ~ e r . ~ ~ Carvonecamphor (12)was prepared via the photolysis of a solution of carvone in 4:l dioxane/H20 by a modification of the method of Meinwald and Schneider.26 Spiro[cyclopropane-l,7’-norbornan]-2’-one(2) and 7,7-dimethylnorcamphor (3) were synthesized as described by Turro and Farringt~n.~’ General Procedure for Stern-Volmer Quenching Measurements. A 0.1 M solution of ketone in acetonitrile was prepared and 2 mL pipetted into each of two 1-cm2 quartz fluorescence cells fitted with Teflon caps. Aliquots of solid t-DCE or liquid c-DEE were added to one cell and the peak height of the fluorescence A, was recorded on a Perkin-Elmer MPF-2A spectrophotometer with excitation at 313 nm. The other cell, to which no quencher was added, served as a reference standard, and the peak height for the sample solution at each quencher concentration was compared with that of the standard to compensate for any fluctuations in the excitation source intensity. At least five determinations of sample peak height vs. standard were taken at each quencher concentration, and the variation was found to be f2%. The initial sample/standard peak height ratio was defined as ,@: and subsequent sample/standard ratios provided the values of @f from which plots of $p/@fvs. quencher concentration could be derived. The absorbance of all solutions was checked periodically with a Gilford UV spectrophotometer to determine if any competitive absorption due to the quencher was occurring, but in no case did the quencher cause the absorbance of the solution to increase. Procedure for Relative Quantum Yield Determinations. A merry-goround apparatus was used to ensure the simultaneous irradiation of several samples with the same light intensity over a period of time. The apparatus consisted of an electronically driven turntable which revolved around a light source; holes in the turntable rim were designed to accommodate test tubes containing the samples to be irradiated. The entire turntable was placed in a large water bath which maintained a constant temperature (25 f 2 “C) during the irradiation. The light source, a Hanovia Type L-679A-36 450-W medium-pressure mercury arc lamp, was contained in a water-cooled quartz reactor fitted with a Pyrex filter sleeve; the reactor was surrounded by a Pyrex immersion well jacket containing a 313-nm filter solution (0.002 M KZCrO4in a 1% aqueous solution of K2C03). The samples to be irradiated were placed in 10 X 75 mm Pyrex test tubes (Corning) which had been cleaned with Chromerge solution, washed with aqueous NH,OH, thoroughly rinsed with distilled water, and oven dried. The test tubes were stoppered with serum caps. Solutions of norcamphor, 7,7-dimethylnorcamphor, and spiro[cyclopropane-l,7’-norbornan]-2’-onein purified acetonitrile were prepared and the optical densities were measured. Acknowledgments. The authors thank the Air Force Office of Scientific Research and the National Science Foundation for their generous support of this research. (23) Corey, E. J.; Hartman, R.; Vatakencherry, P. A. J . Am. Chem. SOC.
1962, 84, 2611.
(24) Bartlett, P. D.; Singer, G.D. J. Am. Chem. Sor. 1965, 87, 1297. (25) Haller, A.; Bauer, E. Ann. Chim. (Paris) 1917, 8, 125. (26) (a) Niemczyk, M. P. Ph.D. Thesis, Columbia University, 1972. (b) Meinwald, J.; Schneider, R. A. J . Am. Chem. Sor. 1965, 87, 5218. (27) Turro, N. J.; Farrington, G. L. J . Am. Chem. Soc., following paper in this issue.
