5328
J . Phys. Chem. 1985, 89, 5328-5330
Photochemistry of the o-Nltrobenryi System in Solution: Evidence for Singlet State Intramolecular Hydrogen Abstractiont R. W. Yip,* D. K. Sharma,* Division of Chemistry, NRCC, Ottawa, Ontario, Canada, K1A OR6, and Canadian Picosecond Laser Flash Photolysis Centre, Concordia University, Montreal, Canada, H3G 1M8
R. Giasson,s and D. Gravel* Departement de Chimie, UniversitP de MontrPal, C.P. 6210, Montreal, Quebec, Canada, H3C 3Vl (Received: August 5, 1985)
Several oxygen-substituted o-nitrobenzylic compounds in THF solution were studied by picosecond transient spectroscopy. The o-quinonoid intermediate resulting from intramolecular H abstraction was observed in those compounds which undergo efficient reaction. In the case of phenyl o-nitrobenzyl ether and p-cyanophenyl o-nitrobenzyl ether, both the triplet excited state and the o-quinonoid were observed at early times. For these two compounds, the o-quinonoid intermediate is formed from the singlet excited state at a rate much faster than triplet decay.
Introduction The recent identification of the triplet state of nitrobenzene and alkyl nitrobenzenes in solution by picosecond laser absorption spectroscopy' provides a potential13 important probe for the more complex intramolecular photochefiical reactivity of the o-nitrobenzyl systems which are of interest in the development of photochromic compounds,2 photolabile protective group^,^ and phot o r e s i s t ~ . ~Nitrobenzenes are known to react intermolecularly from the triplet excited state to give hydrogen abstraction? addition to olefins,6 and photoredox.' However, for intramolecular hydrogen abstraction from the o-benzylic or homobenzylic position, the spin multiplicity of the reactive excited state is not known* and, because of the high rates of intramolecular reactions, participation by the singlet excited state cannot be immediately dismissed. In addition to the question of the spin of the electronically excited state, it was the purpose of this work to look for intermediates in the rearrangement (see Scheme I for a representation of the generally proposed m e c h a n i ~ m ~and, ~ , ~ from ) rate measurements, to obtain some information on the influence of structure on the course and efficiency of the reaction. Toward these objectives, we report, herein, transient absorption spectroscopic measurements, at picosecond times, of a series of oxygen-substituted o-nitrobenzyl systems (Figure 1) in tetrahydrofuran (THF) solution which include the substituted o-nitrobenzyl acetals and alcohols known to undergo efficient photorearrangement. lo,'
Results and Discussion The technique used was identical with that described earlier.' Briefly, third harmonic Nd:YAG laser pulses at 355 nm of 3040-ps durations were used for excitation and continuum pulses with a useful window between 425 and 675 nm were used to probe the transient absorption. The triplet quencher, 1,3-~yclohexadiene, was obtained from Aldrich Chemical Co., Milwaukee, and was redistilled. The time-resolved absorption spectrum from the glycol ONPEG or the acetal ONBAA in T H F or 1,3-cyclohexadiene (see Figure 1 for structures) showed a single peak a t ca. 435 nm (Figure 2). The ca. 435-nm absorption band persists beyond 10 ns, the maximum delay time of our apparatus. That the absorption band is not due to a stable photoproduct was shown in a separate experiment in which a 1 X M solution of ONBAA in T H F was irradiated to 50% decomposition. In this steady-state ex-
'NRCC No. 24938. Presented, in part, at the 68th Canadian Chemical Conference, Kingston, Ontario, June 2-5, 1985. Canadian Picosecond Laser Flash Photolysis Centre, Concordia University, Montreal. $Holder of an NSERC scholarship 1983-86. f
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periment where the irradiated sample was monitored by an UVvisible absorption spectrometer, no absorption in the 435-nm region was observed. The ca. 435-nm band observed on picosecond excitation is located in the same wavelength region as the short-wavelength absorption band of the triplet state of nitrobenzenes] but is several times more intense. However, the absence of a second absorption band located toward 700 nm which is characteristic of the triplet state of the nitrobenzenes' demonstrates that the transient absorption from these two o-nitrobenzylic compounds is not due to the triplet state but probably, in view of the rapid buildup, due to an early intermediate resulting from reaction of the excited state(s). The acetophenone acetal ONAA which lacks an o-benzylic H and the acetate ONBA which does contain an o-benzylic H both afforded a transient absorption spectrum characteristic of the triplet statel (Figure 3). The striking difference in the transient ( I ) Yip, R. W.; Sharma, D. K.; Giasson, R.; Gravel, D. J . Phys. Chem. 1984, 88, 5770.
(2) Dessauer, R.; Paris, J. P. In "Advances in Photochemistry", Noyes, W. A,, Jr., Hammond, G. S., Pitts, J. N., Ed.; Interscience: New York, 1963; Vol. 1, p 275. (3)-(a) Htbert, J.; Gravel, D. Can. J . Chem. 1974, 52, 187. (b) Gravel, D.; Hebert, J.; Thoraval, D. Can. J . Chem. 1983, 61, 400. For excellent reviews see Amit, B.; Zehavi, U.; Patchornik, A. Isr. J. Chem. 1974, 12, 103. Pillai, W. N. R. Synthesis 1980, 1. (4) Reichmanis, E.; Gooden, R.; Wilkins, Jr., C. W.; Schonhorn, H. J. Polym. Sci., Chem. Ed. 1983, 21, 1075. (5) Hurley, R.; Testa, A. C. J. Am. Chem. SOC.1968, 90, 1949. (6) Charlton, J. L.; Liao, C. C.; de Mayo, P. J. Am. Chem. SOC.1971,93, 2463. (7) Wan, P.; Yates, K. J. Org. Chem. 1983, 48, 136. (8) Dopp, D. Top. Curr. Chem. 1975, 55, 49. (9) (a) Berson, J. A,; Brown, E. J . Am. Chem. SOC.1955, 77, 447. (b) Leighton, P. A,; Lucy, F. A. J. Chem. Phys. 1934, 2, 756. (e) de Mayo, P.; Reid, S. T. Q. Rev. Chem. SOC.1961, 15, 393. (d) George, M. V.; Scaiano, J. C. J . Phys. Chem. 1980.84, 492. (IO) Barzynski, H.; Sanger, D. Angew. Makromol. Chem. 1981, 93, 131, ( 1 1) Giasson, R.; Gravel, D., unpublished results.
0022-3654/85/2089-5328$01.50/0 Published 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985 5329
Letters
15
1
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ONBA
ONPEG
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ONBAA
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Figure 1. Structures of the o-nitrobenzyl compounds studied: o-nitrobenzyl acetate (ONBA); o-nitrophenyl ethylene glycol (ONPEG); onitrobenzaldehyde acetal (ONBAA); o-nitroacetophenone acetal (ONAA); o-nitrobenzyl phenyl ether (ONBPE); o-nitrobenzyl p-cyanophenyl ether (ONBCPE).
-
0
425
525
475
WAVELENGTH,
575
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675
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Figure 3. Transient absorption of the o-nitroacetophenone acetal ONAA in THF: 2-mmpath length cell; solute concentration, 0.02 M; 0, 45 ps; 0,75 ps; V, 425 ps. Probe delays are for 435 nm. Probe delays at 650 nm are 25 ps less than the corresponding delays at 435 nm.
0
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475
525 WAVELENGTH,
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Figure 2. Transient absorption of the o-nitrobenzaldehyde acetal O N BAA in THF: 2-mm-path length cell; solute concentration, 0.014 M; 0, 25 ps; 0 , 45 ps; A, 75 ps; V, 525 ps. Probe delays are for 435 nm.
absorption spectra between the acetate ONBA, on one hand, and the glycol ONPEG or the acetal ONBAA, on the other, all containing ortho benzylic H, is at first sight puzzling but is consistent with quantum yield and rate data which show that the acetate is relatively unreactive. The quantum yields of reaction = 0.05 reported for the o-nitrobenzyl acetate ONBA in dioxane) and o-nitrobenzyl alcohol" (anitrosocpd = 0.93 in dioxane) indicate that the alcohol reacts 19 times more efficiently than the acetate. Also, relative to o-nitrotoluene, the rate of disappearance of the benzaldehyde acetal ONBAA is 280, while that of the acetate ONBA is 12" which show that the aldehyde acetal ONBAA is 23 times more reactive than the acetate. Regarding the assignment of the transient absorption at ca. 435 nm from ONBAA and ONPEG, the following points may be noted. (1) A benzylic H ortho to the nitro group in the molecule is required and suggests that the transient is derived from reaction(s) involving the benzylic H. (2) The absorption is rapidly formed and thus suggests that the transient is an early intermediate in the reaction. (3) The appearance of the transient correlates with the photochemical reactivity of the o-nitrobenzyl compound. (4) The o-quinonoid isomer of o-nitrotolueneI2 and of 2,6-din i t r ~ t o l u e n e ~ is ~ -known '~ to absorb at 390 and 410 nm, respectively. The isomers from more highly substituted o-nitrobenzyl compounds absorb at longer wavelengthsI6 than those of the onitrotoluenes. From t h e above evidence, we therefore assign the (12) Wettermark, G. J . Phys. Chem. 1962, 66, 2560. (13) Wettermark, G.; Ricci, R. J . Chem. Phys. 1963, 39, 1218. (14) Langmuir, M. E.; Dogliotti, L.; Black, E. D.; Wettermark, G. J . Am. Chem. SOC.1969, 91, 2204. (15) Craig, B. B.; Atherton, S . J. SPIE, Int. Soc. Opr. Eng., Proc. 1984, 482, 96. (16) Margerum, J. D.; Miller, L. J.; Saito, E.; Brown, M. S . ; Mosher, H. S . J . Phys. Chem. 1962, 66, 2434.
425
475
525
575
625
675
WAVELENGTH, n m
Figure 4. Transient absorption of the o-nitrobenzyl phenyl ether ONBPE in THF: 2-mm path length cell; solute concentration, 0.02 M; 0 , 25 ps; 0,45 ps; A, 75 ps; v,525 ps. Probe delays are for 435 nm. Probe delays at 650 nm are 25 ps less than the corresponding delays at 435 nm.
ca. 435-nm absorption from the aldehyde acetal ONBAA and the glycol ONPEG to the o-quinonoid intermediate resulting from hydrogen abstraction by the o-nitro group (Scheme I). The absence of a reactive intermediate transient absorption from o-nitrobenzyl acetate (other than the triplet excited state) and the known low quantum yield and rate for reaction'0," suggest that the photoisomerization is quite sensitive to a polar substituent effect at the benzylic carbon. In an effort to test this hypothesis, we synthesized the phenyl and p-cyanophenyl ethers of o-nitrobenzyl alcohol, ONBPE and ONBCPE, respectively (Figure 1). Excitation of the phenyl and p-cyanophenyl ethers in tetrahydrofuran or acetonitrile resulted in transient absorption due to both the o-quinonoid and the triplet intermediates (Figure 4) with triplet lifetimes of 118 33 and 395 f 64 ps, respectively, in tetrahydrofuran. In the 625-650-nm wavelength region where absorption is due only to the triplet state, the transient absorption from the p-cyanophenyl ether was 1.5 times stronger than that from the phenyl ether. The amount of triplet absorption was sufficient for confirmation of the triplet assignment bands by diene quenching. For both ethers, t h e lifetime of t h e long-wavelength bands were decreased by the addition of 1,3-cyclohexadiene (ET = ca. 52 kcal mol-' 17) from which the quenching rate constants of (3 1) X lo9 and (6 f 4) X lo9 M-I s-l can be calculated for the phenyl and p-cyanophenyl ether, respectively. The transient absorption at ca.435 nm from the ethers, ONBPE and ONBCPE, is comprised of overlapping absorption by the
*
*
(17) Murov, S.L. 'Handbook of Photochemistry"; Marcel Dekker: New York, 1973.
J . Phys. Chem. 1985,89, 5330-5332
5330
quinonoid and the triplet. In the case of the phenyl ether ONBPE most of the absorption is probably due to the quinonoid intermediate unless there is a dramatic change in ca. 1:l relative intensities of the short and long wavelength transitions from the triplet state of that compound compared with those observed for nitrobenzene,’ o-nitroacetophenone acetal ONAA, and o-nitrobenzyl alcohol acetate ONBA. The significantly longer triplet decay times of the ethers compared with that of the very rapid rise of the 435-nm band show that the strong absorption at 435 nm observed at the end of the excitation pulse does not originate from the triplet excited state nor, therefore, from the triplet biradical (Scheme I) resulting from intramolecular hydrogen abstraction. As further confirmation, the ethers were flashed in the presence of olefins. Despite the large reductions in lifetimes of the triplet upon the addition of 1,3-cyclohexadiene in T H F solution (from 400 to 80 ps at 1.75 M cyclohexadiene), the absorption at 435 nm showed no decrease commensurate with quenching of its precursor during the accelerated decay of the triplet absorption. In these quenching studies, the maximum concentration of 1,3cyclohexadiene was limited by the presence of a detectable amount of a short-lived transient with a very broad spectrum which overlapped with the o-quinonoid intermediate at 435 nm. The small amount’* and short lifetime of this additional transient did not affect our qualitative conclusion regarding the absence of quenching of the o-quinonoid intermediate. In addition, we synthesized S-methyl-5-(hydroxymethyl)- 1,3-~yclohexadienefor quenching (2.0 M olefin) of the triplet from the p-cyanophenyl ether in acetonitrile solution. In this experiment, we observed a slight decrease (AOD = 0.05) in the total absorption at 435 nm during the accelerated decay of the triplet which is consistent with the decay of a small amount of overlapping triplet absorption at 435 nm as evidence by a similar decrease in the amount of triplet absorption in the 625-650-nm region. The olefin quenching experiments rule out the possibility that initial absorption at ca. 435 nm after excitation is due exclusively to the triplet and that the quinonoid absorption observed after decay of the triplet (as monitored in the 650-nm region) is very rapidly formed from the (18) AOD ca. 0.04 at 50 ps with 5.25 M 1,3-cyclohexadiene;not detectable below ca. 2 M 1,3-cyclohexadiene.
triplet during the decay process. We therefore conclude that the absorption at 435 nm observed at the end of the laser pulse consists mainly of the o-quinonoid intermediate arising exclusively from the singlet excited state and a small amount of overlapping triplet absorption. In the absence of quencher we note, for the p cyanophenyi ether ONBCPE, that the absorption at 435 nm does not decrease during the time that the triplet band at ca. 675 nm decays. This observation is consistent with either a slower triplet contribution to the formation of the quinonoid or the growth of another species (or both). Further studies are required to more clearly delineate the role of the triplet state in the intramolecular reaction. For the acetal from benzaldehyde and for o-nitrophenyl ethylene glycol where the triplet was not detected by the ca. 35-ps time resolution of our apparatus, we cannot exclude the possibility that the some or even all of the quinonoid observed immediately after the excitation originates from a short-lived triplet excited state. In those cases where both triplet excited state and the quinonoid intermediate were observed, the results clearly demonstrate that reaction from the singlet excited state is much more rapid than that from the triplet excited state owing to differences in the intrinsic reactivities of the two excited states. In contrast with the singlet excited state, the quinonoid intermediate is formed from the triplet excited state indirectly via a (triplet) biradical intermediate. Collapse of this biradical might not be rapid and can further retard the overall rate of formation of the quinonoid intermediate from the triplet excited state. In conclusion, for those o-nitrobenzylic compounds which undergo intramolecular reactions, the o-quinonoid intermediate and, in the case of the phenyl o-nitrobenzyl ether and p-cyanophenyl o-nitrobenzyl ether, the triplet excited state has been observed. In the latter case where both triplet and quinonoid are observed, the o-quinonoid intermediate is formed from the singlet excited state at a much more rapid rate than that for triplet decay. Electron-donating substituents at the benzylic carbon accelerate the intramolecular reaction to the o-quinonoid.
Acknowledgment. We thank N . Serpone and C. H. Langford of the Canadian Picosecond Laser Flash Photolysis Centre, and the Department of Chemistry, Concordia University for use of the facilities. We also thank the Natural Sciences and Engineering Research Council and Le MinistCre de I’Education du Quibec for financial assistance to D.G. and scholarships to R.G.
A Low-Temperature Emission Study of the Transfer of Triplet Excitation Energy in Dimethylbenzophenone Crystals Daniel J. Graham Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506-6045 (Received: July 19, 1985; In Final Form: September 24, 1985)
A low-temperature emission study of dimethylbenzophenone and naphthalene-dimethylbenzophenone crystals is presented. The phosphorescence quenching by the guest impurity is shown to be quite different from that observed in naphthalenebenzophenone systems. The differences are discussed qualitatively in terms of crystal structure and electron-phonon coupling.
Introduction Energy transfer properties of organic crystals have been studied An important emphasis has been on the ability of crystal imperfections and impurities to trap excitons. Work focusing on the triplet state was pioneered by Hochstrasser and
others some years a g ~ . ~ The - ~ studies centering on naphthalene-benzophenone (NAP-BZP) mixed crystals demonstrated (among other things) that impurities quench the host luminescence with remarkable e f f i ~ i e n c y . ~ - ~ It has been an aim of this laboratory to understand how molecular substituents, in certain cases, strongly influence the en-
(1) D. P. Craig and S. H. Walmsley, “Excitons in Molecular Crystals”, W. A. Benjamin, New York, 1968. (2) M. Pope and C. E. Swenberg, ‘Electronic Processes in Organic Crystals”, Oxford University Press, Oxford, 1982.
(3) R. M. Hochstrasser, J . Chem. Phys., 39,3153 (1963). (4) R. M. Hochstrasser, J . Chem. Phys., 40, 1038 (1964). ( 5 ) R. M. Hochstrasser, J . Chem. Phys., 40, 1041 (1964). ( 6 ) L. M. Peter and G. Vaubel, Phys. Status Solidi B, 58, 593 (1973).
0022-3654/85/2089-5330$01.50/00 1985 American Chemical Society
