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(2) Present address: Department of Chemistry, Mississippi State University, Mississippi State, Miss. 39762. (3) E. A. V. Ebsworth, R. Mould, R. Taylor, G. R. Wilkinson, and L. A. Woodward, Trans. Faraday Soc., 58, 1062 (1962). (4) D. R. Jenkins, R. Kewley, and T. M. Sugden, Trans. Faraday Soc., 58, 1284 (1962). (5) C. Gliewell, A. G. Robiette, and G. M. Sheldrick, Chem. Phys. Lett., 16, 526 (1972). (6) S.J. Cyvin, J. Brunvoll, and A. G. Ribiette, Chem. Phys. Lett., 11, 263 (1971). (7) A. G. Robiette, private communication. (8) K.-F. Dossel and A. G. Robiette, Z. Naturforsch. A, 32, 462 (1977); K.-F. Dossel and D. H. Sutter, ibkl., 32, 473 (1977); J. Chem. Phys., 59. 1028 (1973). (9) L. A. Carrdra, R.’O. Carter, J. R. Durig, R. C. Lord, and C. C. Millionis, J . Chem. Phys., 59, 1028 (1973). (10) J. R. Durig, M. J. Flanagan, and V. F. Kalasinsky, J . Chem. Phys., 66, 2775 (1977).
Sheng
(11) IUPAC, “Tables of Wavenumbers for the Calibration of Infrared Spectrometers”, Butterworths, Washington, D.C., 1961. (12) E. A. V. Ebsworth and M. J. Mays, J. Chem. Soc., 4844 (1962). (13) K. S. Kalasinsky, J. R. Durig, and V. F. Kalasinsky, Thirtysecond Symposium on Molecular Spectroscopy, Ohio State University, Columbus, Ohio, 1977, paper FA8, in press. (14) E. C. Tuazon and W. G. Fateley, J. Chem. Phys., 53, 3178 (1970). (15) F. N. Masri, J . Chem. Phys., 57, 2472 (1972). (16) F. A. Miller and W. E. White, Z. Elecktrochem., 64, 701 (1960). (17) G. Herzberg, “Infrared and Raman Spectra”, van Nostrand, Princeton, N.J., 1945, pp 267-268. (18) K. Hedberg, J . Am. Chem. Soc., 77, 6491 (1955). (19) J. R. Durig and P. J. Cooper, J . Mol. Sfrucf.,41, 183 (1977). (20) J. R. Durig, K. S.Kalasinsky, and V. F. Kalasinsky, J. Mol. Strucf., 35, 201 (1976). (21) J. A. Duckett, A. G. Robiette, and I.M. Mills, J . Mol. Spectrosc., 62, 34 (1976).
Laser Fluorescence of the Cyclohexadienyl Radical in the Irradiated Benzene Crystal S. J. Sheng Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received October 13, 1977) Publication costs assisted by the U.S. Department of Energy
Fluorescence of the cyclohexadienyl radical at 4.2 K is observed by site selective laser excitation. The visible transition previously observed in absorption is confirmed to be the lowest doublet-doublet transition. This is a weak but dipole-allowed transition. The present emission and emission excitation studies of cyclohexadienyl and perdeuterated cyclohexadienyl radicals allow us to make the vibrational analysis.
I. Introduction The cyclohexadienyl radical, formed either by hydrogen atom addition to benzene’-3 or by abstraction from cycl~hexadienes,~ is one of the most important reaction intermediates in radiation chemistry. Since its discovery, there have been extensive ESR ~ t u d i e s ~and - ~ optical absorption studies in the UV Recently Shida and Hanazaki have found both experimentally and theoretically a weak absorption in the visible region? The purpose of this work is to report for the first time a fluorescence study of the cyclohexadienyl radical trapped in benzene crystals. The vibrational analyses of the ground and first excited states are presented and the fundamentals are assigned. The key for the success of the fluorescence study is dye laser selective excitation. Our earlier attempts using a broad lamp excitation proved futile. 11. Experimental Section Spectral grade benzene purchased from Mallinckrodt Chemical Works and perdeuterated benzene from Merck Sharp and Dohme Inc. were degassed and sealed in Supersil quartz tubes. The benzene crystal was grown using the Bridgman method by lowering the sample tube slowly into a liquid nitrogen dewar. The cyclohexadienyl radical was produced and trapped upon the irradiation of the benzene polycrystals at 77 K by using a 9-MeV electron beam from an ARC0 LP-7 linear accelerator. The conM with centration of the radical was estimated to be a dose of lo7 rd. The sample was then cooled to 4.2 K for spectroscopic studies. A Molectron DL 300 dye laser The research described herein was supported by the Division of Physical Research of the U.S. Department of Energy. This is Document NO,NDRL-1817 from the Notre Dame Radiation Laboratory . 0022-3654/78/2082-0442$0 1.OO/O
pumped by a UV 1000 nitrogen laser was used to excite the radicals. The emission at a 90’ angle was focussed on the entrance slit of the SPEX 1402 0.85 m Czerny-Turner double monochromator equipped with a RCA C31034 photomultiplier tube. The pulse signal was detected and amplified by using a Hewlett Packard 185B sampling scope and the output was displayed on a Honeywell Electronik 194 strip chart recorder. The fluorescence decay was measured by synchronizing the time scan with a Northern Scientific NS-544 signal averager.
111, ~~~~l~~and ~ i ~ ~ The laser fluorescence spectra of CsH7. and C6D7radicals obtained by the single vibronic level excitation are shown in Figure 1. The laser frequency is tuned to resonantly excite the v16 vibration level (1474 cm-’ for C6H7and 1425 cm-’ for C,D,-) of the first excited doublet state. Each spectrum consists of a group of sharp lines with associated diffuse side bands. These diffuse side bands are a common feature of the spectra of radicals produced by ionizing radiation in the solid phase at low temperature. One typical example can be found in the irradiated naphthalene crystal.1° The amount of diffuse side bands can be reduced by annealing the crystal and can thus be attributed to lattice defects.ll The radicals in the environments of various lattice defects contribute to the diffuseness. The present site selective excitation is an attractive way to obtain a sharp fluorescence spectrum for the cyclohexadienyl Fdical. This also eliminates to a great extent the interfering fluorescence of other species. The following discussion will be based w o n the sharpened spectrum. The origin of the cyclohexadienyl radical spectrum is located at 5586 f 1A, in good agreement with the previous 77 K absorption s t u d i e ~ ~and 9 ~ Our independent sharp absorption lines a t 4.2 K. The deuterium 0 1978 American
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The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
Fluorescence Study of the Cyclohexadienyl Radical
443
TABLE I: Main Fundamentals in cm-I Appearing in the Fluorescence and Absorption of Cyclohexadienyl Radicals
FI uorescence 0
C,H,.
CAD,.
552(556) 527 1169(1177) 877 1566(1566) 1 5 2 3
Assignment C,H,
Ra
Ground 1.05 1.33 1.03
C,D,
Ra
577 867 1559
1.05 1.36 1.02
State 606 1178 u , ~ 1585 uI8
vI7
Excited State 5500
5700
5900
6100
6300
833(835) 978(981) 930 1128(1132) 838 1397(1401) 1474(1477) 1425
1.05 1.35
u,,
1.03
uL6
a R is the vibrational frequency ratio of C,H, vs. C,D, *. *The values in parentheses are obtained from spectral analysis of C,H, * in the 1,4-cyclohexadiene molecular aggregate.
I
c
5500
5700
5900
6100
6300
A , i
Flgure 1. The fluorescence spectra of cyclohexadienyl and perdeuterated cyclohexadienyl radicals in benzene and deuterated benzene polycrystals at 4.2 K.
shift of the electronic origin is 139 ern-'. The assignment of the cyclohexadienyl radical origin is based upon the following observations. The mirror image relationship between the absorption and emission peaks indicates that the 5586-fi peak is the origin of a radiation produced species. Each vibrational mode appearing and assigned in the ground and excited states (vide infra) has an identical isotope shift in frequency. Most importantly, the fluorescence and absorption studies of the irradiated 1,4-cyclohexadienemolecular aggregate in the present work establish an origin located at 5516.5 A. Clearly this indicates that the assignment of the 5586-A peak as the origin in the cyclohexadienyl radical formed by hydrogen addition to benzene is consistent with the assignment of the 5516.5-A peak as the origin i y the cyclohexadienyl radical formed by hydrogen abstraction to 1,4-cyclohexadiene. The small origin shift of 256 em-I represents the environment effect. The vibrational frequency and structure of the radical in two different host matrices are identical. In the early development of MO theory, LonguetHiggins, Pople,15and Dewar16 have already recognized that there should be two concommitant spectral transitions for the odd alternant P radicals, an intense one in the UV region and a weak one in the low energy side. Numerous radicals, allyl, benzyl, and triphenyl methyl radicals, to name a few, have been found to follow this general prediction. The present studies establish an additional example of five P electrons which have a yeak electronic transition a t 5586 A and a strong transition around 316 nm. The vibrational structure in the fluorescence spectra of cyclohexadienyl radicals is quite simple. The main fundamentals appearing in the fluorescence of C6H7-are 552, 762, 1169, and 1566 cm-l. Only one combination mode, 552 + 1566 cm-l, can be identified in the 0-3000-~rn-~ region. The corresponding fundamentals in the C6D7.are listed in Table I together with the isotope shifts. Based upon' the isotope shifts 552, 1169, and 1566 cm-I are , v16 eZgmodes of tentatively assigned to the v18, ~ 1 7 and
4600
48W
5000
5200
6400
5600
,A
Figure 2. The laser excitation spectrum of the cyclohexadienyl radical at 4.2 K. Three dyes are used in the scan of this spectrum. The intersecting points are indicated by the vertical dotted line. The spectrum is normalized according to the dye profile, but the variation of power output from dye to dye is not considered.
benzene, respectively. The relevant fundamental frequencies as well as the isotope shifts of benzene are also included in Table I for comparison. The nomenclature of the vibrations is borrowed from that of benzene." The excitation spectrum of the C6H7. monitoring the 552-cm-l vibronic band is shown in Figure 2. This spectrum is almost identical with the absorption spectrum published previously.8 The vibrational fundamentals 1128 and 1474 em-I (838 and 1425 cm-I for C6D7.)are assigned 7 of the first excited doublet state, respectively. to ~ 1 and The assignment is again based upon the deuterium shift (see Table I). In this respect we disagree with the previous assignment,8 which is based upon the comparison of the vibrational frequencies in the excited state of the C6H7with the fundamentals in the ground state of benzene. In the following we wish to discuss the symmetry aspects of the lowest electronic transition of the open shell cyclohexadienyl radicals. The radical has Czusymmetry with the twofold 2 axis lying in the molecular plane and bisecting the CHz group. The symmetry representations of the ground and first excited states8 are Bz and Az, respectively. The lowest electronic transition is dipole-allowed and should be polarized along the X axis, which lies in the molecular plane and is perpendicular to the Cz axis. Experimentally the 0,O band is indeed the most intense band and its intensity might be even proportionally larger than that shown in Figure 1 because of possible reabsorption. Three strong vibronic bands appearing in the fluorescence are correlated with the eZgmodes (vIB, ~17,and v16) of benzene. Examining the normal vibrations of these modes, we find they belong to the bl representation in CZu symmetry. Therefore vibronic activity might play a role in the lowest doublet-doublet transition of the cyclohexadienyl radicals. These vibronic bands are expected
444
A. J. Elliot and J. K. S. Wan
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
to be polarized along the C2 axis. A photoselection polarization rnea~urernent'~ was attempted in this work by studying the cyclohexadienyl radical in the polycrystalline benzene a t 4.2 K and EPA glass a t 77 K. At best, we can only conclude that all four bands are polarized in the plane. Finally, the results of fluorescence lifetime measurement are reported. The lifetime of the first excited state of the cyclohexadienyl radical is very short. Since the fluorescence decay is almost coincident with the laser pulse (fwhh = 8 ns), a computer program was written to deconvolute the time intensity profile of the emission by using an iterative pr0~edure.l~ The lifetime of the cyclohexadienyl radical, obtained in this manner is 2 f 1ns. We have also determined the lifetime of the deuterated analogue to be 6 f 1 ns. It is interesting to note that the decay rate of the excited CsH7. is three times faster than that of CsD7. due to nonradiative processes.
Acknowledgment. The author thanks Dr. Gordon Hug for helpful discussions.
References a n d Notes (1) S.Gordon, A. R. Van Dyken, and T. F. Doumani, J. Phys. Chem., 62, 20 (1958). (2) V. A. Tolkachev, Y. N. Molin, I. I. Tchkheidze, N. Y. Buben, and V. V. Voevodsky, Dokl. Akad. Nauk USSR, 141, 911 (1961). (3) H. Fischer, J . Chem. Phys., 37, 1094 (1962). (4) R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 38, 773 (1963). (5) M. C. Sauer, Jr., and B. Ward, J . Phys. Chem., 71, 3971 (1967). (6) B. D. Michael and E. J. Hart, J . Phys. Chem., 74, 2878 (1970). (7) T. Shida and W. H. Hamill, J . Am. Chem. Soc., 88, 3689 (1966). (8) T. Shida and I.Hanazaki, Bull. Chem. SOC.Jpn., 43, 646 (1970). (9) J. E. Jordan, D. W. Pratt, and D. E. Wood, J. Am. Chem. Soc., 96, 5588 (1974). (10) K. Nakagawa and N. Itoh, Chem. Pbys., 16, 461 (1976). (11) C. W. Jacobsen, G. H. Hong, and S. J. Sheng, to be submitted for publication. (12) G. Herzberg, "Infrared and Raman Spectra", Van Nostrand, Princeton, N.J., 1945, p 362. (13) A. C. Albrecht, J . Mol. Spectrosc., 6, 84 (1961). (14) A. E. McKinnon, A. G. Szabo, and D. R. Miller, J . Phys. Chem., 81, 1564 (1977). (15) H. C. Longuet-Higgins and J. A. Pople, Proc. Phys. SOC. (London), 68, 591 (1955). (16) M. J. S.Dewar and H. C. Longuet-Higgins, Proc. Pbys. SOC.(London), 67, 795 (1954).
A CIDEP Study of the Photoreduction of Quinones in the Presence of Phenols and 2-Propanol A. John Elllot and Jeffrey K.
S. Wan*
Department of Chemlstty, Queen's University, Kingston, Ontario, Canada K7L 3N6 (Received September 26, 1977) Publication costs assisted by the National Research Council of Canada
A method is described on how to evaluate the triplet quenching (chemical abstraction) rate constants from the CIDEP observed when quinones are photoreduced in the presence of a hydrogen donor. The technique, which utilizes an intermittent light source in conjunction with a 100-kHz ESR spectrometer, has been used to determine the relative triplet quenching rate constants for pentachlorophenol, 2-methylphenol, phenol, and 2-propanol with a number of p-quinones. In general 2-propanol was found to be about two orders of magnitude less effective a quencher than the phenols.
In roduc ion To establish the role of the photochemical triplet mechanism in CIDE(N)P, phenols and alcohols have been used extensively in this laboratory as hydrogen donors for the photoreduction of quinones.1-6 However, little is known about the relative efficiences of these two classes of compounds to chemically quench the quinone triplet state. Phenols were assumed to have quenching rate constants ( h ) about four to five orders of magnitude greater than dcohols. This assumption was based on a few, not totally related, studies: those where triplet duroquinone was quenched by durohydroquinone (a phydroxyphenol) and by a l c o h ~ l s ;and ~ - ~one where triplet biacetyl was quenched by phenol and 2-propan01.l~ Obviously laser flash photolysis would be the first experimental method chosen to determine the relative (or absolute) k,'s. However, while duroquinone is quite amenable to this t e ~ h n i q u e ,this ~ - ~is not the case for other less substituted p-benzoq~inones.~ Possible reasons for this are that their triplet lifetimes are too short and/or their triplet extinction coefficients may be less than that of duroquinone. Now that the mechanisms which give rise to the CIDEP observed when quinones are photoreduced are well understood,ll we can estimate the relative k,'s for a series of hydrogen donors by following the dependence of the initial 0022-365417812082-0444$0 1.OO/O
(triplet) polarization on the concentration of the hydrogen donors. Furthermore, if the triplet spin-lattice relaxation time (3T1)is known, we can estimate the absolute k,. Ideally these experiments should be carried out using a rapid response time ESR spectrometer ( N 1 p s ) in conjunction with a nanosecond laser hash light source.12 However, because of the large bandwidth required to obtain this response time, the signal-to-noise ratio is often poor which makes finding the signals and recording their time profile d i f f i ~ u l t .In ~ this paper we will describe how, under favorable conditions, a conventional ESR spectrometer using 100-kHz modulation (time response of -500 p s ) with its greatly enhanced signal-to-noise characteristics can be employed in conjunction with a rotating sector chopped light source to give the same information as the rapid response arrangement. With this experimental technique we have determined the relative h,'s for 2-propanol, phenol, 2-methylphenol, and pentachlorophenol quenching the triplet state of a series of benzoquinones and naphthaquinones to produce semiquinone radicals. As the 3T1for the duroquinone triplet is known,13an estimate of the kq's has been made. Furthermore, we have also shown that if EA (radical pair) and initial (triplet mechanism) polarizations1' are present together in an ESR spectrum, then the initial polarization can be separated by adding the time profiles of peaks @ 1978 American Chemical Society
