2966
Asok Chakrabarti and Noboru Hirota
(5) A. Zambelli, private communication. (6) N.G. McCrum, B. E. Read, and G. Williams, "Anelastic and Dielectric Effects in Polymeric Solids", Wiley, New York, N.Y., 1967. (7) F. C. Stehling and L. Mandelkern, Macromolecules, 2, 242 (1970). (8) A. Valvassori and V. Zamboni, XXlll JUPAC Conference, Boston, Mass., July 1971.
(9) S. Fiugge, "Encyclopedia of Physics", Vol. li, Springer-Verlag, Berlin, 1958. (10) K . Baba and R. S. Stein, Bull. Am. Phys. SOC.,Ser. 2, 17, 223 (1972). (11) K. Dusek and W. Prins, Adv. Polym. Sci., 6, 1 (1969). (12) A. Greene, K . J. Smith, and A. Ciferri, Trans. Faraday SOC., 61, 2772 (1965).
Optically Detected Magnetic Resonance and Spectroscopic Studies of the Lowest Excited Triplet States of Xanthone and Related Molecules in Crystalline Systems Asok Chakrabarti Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794
and Noboru Hirota" Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794 and Department of Chemistry, Faculty of Science, Kyoto University, Kyoto, Japan 606 (Received March 3, 1976; Revised Manuscript Received August 17, 1976)
The magnetic and dynamic properties of the lowest excited triplet (TI)state of xanthone trap and xanthone in diphenylmethane and 9,lO-dihydroanthracene host crystals were investigated a t 1.2 K using the optically detected magnetic resonance (ODMR) technique and phosphorescence emission and excitation spectroscopy. I t is unambiguously shown that the TI state of xanthone studied here is %7r* state, but in xanthone and diphenylmethane crystals there are two triplet species with different magnetic properties. The total and radiative decay rates from spin sublevels and the sublevel phosphorescence spectra were obtained. The results were discussed in terms of the possible mechanisms of radiative and radiationless transitions. I t is sugI R R * ('Bz) states and bl vigested that the main radiative mechanism is spin-orbit vibronic involving h*, brations. The relationship between the present results and those reported by other workers is discussed.
Introduction
The spectroscopic, photochemical, and magnetic properties of the lowest excited triplet states of aromatic carbonyls have been the topics of much interest in recent y e a r ~ . l Optically -~~ detected magnetic resonance (ODMR) studies a t zero field have revealed a number of interesting features concerning the magnetic and dynamic properties of the TI states of these r n o l e ~ u l e s . It ~ was ~ ~ shown ~ ~ ~ ~that ~ ~the ~ -radiative ~~ properties of the %T* aromatic carbonyls depend on many factors such as molecular structure, energy separation between 3n7r* and %* states (mrr~), and the nature of environment^.^^ The main radiative mechanism differs markedly depending on these factors within %R* aromatic carbonyls.45Here, we have investigated the TI state of xanthone and related molecules in order to clarify the main radiative mechanism and to obtain more information about the relationships between structures and radiative properties of aromatic carbonyls. The main motives of the present study are the following. (I) In case of ~ i aromatic ~ ~ carbonyls * of the benzaldehyde type with small LETT it was found that the vibronic and configurational mixing between 3 n ~ and * 37r7r* states is the important source of radiative activity.45However, different from benzaldehydes the carbonyl group is rigidly held to the planar a-electron system in xanthone. This may make the radiative property of the :%m* state xanthone very different * Address correspondence to this author a t the Department of Chemistry, Faculty of Science,Kyoto University. The Journal of Physical Chemistry, Vol. 80, No. 27, 1976
from those of the other %a* carbonyls. Comparison between the radiative property of xanthone with those of the other aromatic carbonyls appears to be useful in understanding structure-dynamic property relationship. (2) It is known that xanthone shows dual phosphorescence emissions in rigid glass a t 77 K.47 The nature of the phosphorescing species as well as the origin of the dual emission is still not well established despite wide interest in this phenomenon.fi~10~20,30,47-52 It was recently suggested that the TI state of xanthone in 3-methylpentane a t 2 K might have a very distorted structure52 but definitive evidence seems to be lacking. It was hoped that the zero field ODMR studies would help to answer some of these questions. In this work we have made detailed ODMR studies of xanthone in various single crystal systems. The So Ti absorption studies were also made to supplement ODMR studies.
-
Experimental Section (1) Sample Preparations. Xanthone was studied in pure crystals (traps), diphenylmethane (DPM), and 9,lO-dihydroanthracene (DHA) hosts. Benzophenone and 4-aminobenzophenone were studied in a DPM host. The 4,4'-dimethoxybenzophenone trap was also studied. Molecular structure and the axis systems used here are given in Figure 1. All chemicals except diphenylmethane were recrystallized and zone refined extensively prior to use. Diphenylmethane was purified by repeated distillation.
2967
Spectroscopic Studies of Xanthone Excited Triplet States
B
A
I
'
1
:
I
!
I : , I I
x -+y
sweeping the microwave repetitively over the entire region of resonance frequencies.53~60~6' We swept microwave repetitively at the rate of 3 s-l and detected the produced changes with a lock-in amplifier. The change in phosphorescence intensity ( U,,)produced by the rapid passage of microwave transition between i and j spin sublevels under steady state condition is given by61
AI,, = A(k,' - k,')(n, - n,) Y
1,where hIr and h,'
L - - X
Figure 1. Molecular structure, axis system, and energies of the triplet sublevels.
The single crystals of these systems were grown by the standard Bridgnian method. The guest concentration in the initial host-guest mixture was normally about 1%but the actual concentrations in the mixed crystals were considered to be very low, because the solubility of xanthone in DHA and DPM host is rather low. (2) ODMR Experiments. The experimental setup for the present zero field ODMR experiment is essentially the same as that previously reported.53 A crystalline sample held in a microwave helix at liquid helium temperature was excited by the light from an Osram 500w/2 high-pressure mercury arc C0S04 solution, and a lamp, filtered through a NiS04 Corning uv transmitting filter. The microwave was supplied to the helix through a coaxial cable with a Hewlett-Packard 8690 sweeper. The desired phosphorescence peak was selected with an Engis 60-cm monochromator. We have made the following three types of ODMR experiments a t 1.5-1.2 K. (A) Steady State Zero Field ODMR Measurements. The phosphorescence emission was obtained with an ordinary phosphoroscope using a sector which chops the emission at 720 Hz. The signal was detected with an EM1 6256B photomultiplier and a PAR HR8 lock-in amplifier. The microwave was repeatedly swept through the frequencies corresponding to the zero field transitions and the changes in the phosphorescence emission were stored and accumulated on a Varian C-1024. (B) Microwave Induced Delayed Phosphorescence ( M I D P ) Experiments. The total decay rate ( k , ) , relative radiative decay rate (klr),and populating rate ( P , )of each spin sublevel were determined a t 1.2 K by the standard MIDP techn i q ~ e . The " ~ vibronic ~ ~ ~ level dependence of kIrwas studied by determining hlr at selected vibronic bands. The chopper module was replaced by a shutter module which opens and closes excitation and emission lights synchronously. The experimental procedures and the methods of data analyses are similar to those given in the 1iteraturej3-j6 and are not repeated here. (C) Microwave Modulated and Sublevel Phosphorescence Spectra. In order to study the radiative mechanisms in detail it is desirable to obtain phosphorescence spectra from different sublevels separately. We have attempted to do this in the following way. (1)Rapid Passage Microwave Modulated Phosphorescence Spectra. The commonly used method to obtain the vibronic band dependence of the phosphorescence spectra is AM modulated PMDR m e t h ~ d . ~ However, ~ - S ~ when the line width of the resonance signal is rather broad, AM modulation can cause the microwave transition of only a small fraction of triplet state molecules. In such systems, much larger changes in phosphorescence intensities can be obtained by rapidly
+
(1)
are the radiative decay rates, and n, and n, are the populations of the sublevels i and j a t the time of microwave sweep. Since we only detect AIlj,the spectrum obtained gives k,' - k,' as a function of emission wavelength. When k,' > k,, k , follow the general decay aromatic carbonyls so far investigated. As pattern of the seen from the values given in Table 111, k,/k, >> k,'/kyr. Therefore, k , should have a large nonradiative decay component as in many other %r* aromatic carbonyls with relatively large LETT." In the present xanthone AETr is rather large and the mixing with the h n * state is not likely to be important in determining k,. Therefore, as in the other %T* aromatic carbonyl with relatively large AETT,spin-orbit mixing with the lna* state is considered as the main cause for the nonradiative decay from the z sublevel.4fiHowever, k , obtained for xanthone here is much smaller than those obtained for substituted benzaldehydes with the similar value of AEsT. In the case of parasubstituted benzaldehyde we have obtained k , = 50 s-' for the systems with L U ~=~2500 T cm-l. Rather small value of k , in xanthone may also be due to the small value of spin-orbit 'n**). matrix element, 'G = (3~n*13f,,,I The value of h , is similar to those of many other %T* aromatic carbonyls. The mixing with the lm*, ('TU*) state is considered to be the main cause of the y sublevel decay as in other aromatic carbonyls.
2971
Spectroscopic Studies of Xanthone Excited Triplet States TABLE I V ZFS and Decay Properties of the Related Molecules voo,
Xanthone trap (B) 4-Aminobenzophenone (DPM) 4,4'-Dimethoxybenzophenone Benzophenoneb (DPM)
ZFS,cm-'
0... -.1
D
24 882 21 690 24470 23 780
-0.1103 -0.0704 -0.0965" -0.1410
,
E
+0.0186 +0.0304 +0.0321" t0.0426
Total decay rate, s-' . x Y Z 0.45 1.7 51 45
1.75
16.0 17
42
310 750
Character 3PP* 3PP*
3na* (?) 3na*
Taken from the data by Batley,and Bramley (ref 39). ZFS for benzophenone obtained in DPM are somewhat different from 3,156 (1969);Winscon and Maki, Chem. Phys. Lett., 12,264 those obtained in other hosts; Chan and Schmidt, Symp. Faraday SOC., (1971);Shain and Sharnoff, J . Chem. Phys., 59,2335 (1973); Hochstrasser, Scott, and Zewail (ref 40). The value of k , is, on the other hand, considerably smaller than those of substituted benzaldehydes. This observation is consistent with the planar structures of xanthone. (C) SI- T1 Intersystem Crossing. In the case of xanthone in diphenylmethane and 9,lO-dihydroanthracene the populating rates represent the relative populating rates via Si T1intersystem crossings. In these systems the general selection rule for the intersystem crossing in 3mr*aromatic carbonyls, P , >> p,, p,, also holds. (D) Radiative Properties. While the pattern of the total decay rates from the sublevels of xanthone is similar to those of other 3aa* aromatic carbonyls, the radiative properties are quite different from the 3 ~ aaromatic * carbonyls of benzaldehyde type. First, the intensity of the 0-0 band is weak. Second, the carbonyl stretching mode is not the prominent vibrational band. In the following we analyze the obtained data based on the Czu symmetry. The direct spin-orbit coupling scheme under Czu symmetry is given in Table V. (1)Radiative Decay from the z Sublevel. There has been much discussion about the radiative mechanisms of the 3 ~ a * aromatic ~ a r b o n y l s . l - 3 ~ 9 ~ ~ ~ ~ ~ ~ - The ' 8 , 4 2suggested , 4 4 ~ 4 ~ main mechanisms include spin-orbit mixing with the lna* state9J4 and direct configurational and vibronic mixing with the 3na* state.14-16~,23~42,45 In the case of aromatic carbonyl of the benzaldehyde type, the following three mechanisms were found to be important, although the relative importance of the different mechanisms depends considerably on the system:42,45(a) Spin-orbit mixing with 1na* state +
(b) Direct configurational mixing with 3 n ~ state *
(c) Vibronic mixing with h a * state
so
vibr
:%a*(TlZ) c) 3na*(T2Z).c)h * ( l A 1 ) Although part of the z sublevel radiative decay may be due to the above mechanisms, xanthone in the present study is unique in that none of the above mechanisms are the main one for the following reasons. First, the z sublevel emits only weakly a t the 0-0 bands. Hence mechanism a or b cannot be the main one. The importance of mechanism a depends on AEsT, lG, and the oscillator strength for the Ins* SOtransition. The importance of mechanism b depends on AETT and the deviation from the planarity which allows the direct mixing of 3na* and 3aa* states. The relatively large AETT, the rigid planar structure of xanthone, and the small value of ' G are probably factors which are not in favor of mechanisms a and b. Although the major part of the z sublevel emission is vi+
TABLE V Routes of Direct Spin-Orbit Coupling ( Czv Symmetry)
Triplet states
Spin states and total symmetry
Mixing singlet state and state symmetry
''aa*('A1)
TPAd Ty(3B1) Tx(3B2 )
'nr* ('Az) ' U T * ( 'B1) 'nu*('Bz)
:'nn*(3A2)
TA3Ad T,(W TA3Bi)
'ST*('Ai) 'nu*('Bz) 'a~*('B1)
bronic (Herzberg-Teller mechanisms) mechanism c also cannot be the main one here, since this mechanism is incompatible with the phosphorescence polarization data by Pownall and H ~ b e rThese . ~ ~ authors found that the polarization of the phosphorescence of the long lived (-120 ms) xanthone triplet state is negatively polarized with respect to the So laa*(lAl) absorption a t the vibronic bands indicating that the lan*(lA1) state is not the main mixing singlet state. Their work suggests that the main mixing singlet state is likely to be bra*(lBz) state. If this is the case the possible vibronic spin-orbit mechanism should be
+
The vibrations most active in the vibronic mixing are 290 and 670 cm-l. If the above mechanism is correct these vibrations should be bl vibrations. Thus the radiative property of xanthone is strikingly different from those of many other 3aa*aromatic carbonyls in that both vibronic and configurational mixings between states are rather ineffective in producing 3na* and :'m* radiative activity of the z sublevel. This is probably because in xanthone the C=O group is rigidly held to the planar ring system and the carbon atoms next to the carbonyl group are fixed to rings. ( 2 ) y Sublevel Emissions. The y sublevel emission of the 3aa*aromatic carbonyl was ascribed to the direct mixing with the l u a ( l ~ u * )
so
3aa,*(3B1) f) lua*,'TCT*(~B~) Strong 0-0 and 1660-cm-l bands of the y sublevel emission of xanthone indicate the importance of the direct mechanism and are consistent with the above mechanism. The fact that hrr/hyr 1 a t the 0-0 band indicates that direct spin-orbit mixing with the l ~ a * ( ~ a u state * ) is a t least as effective as that with the lnA* state in producing radiative decay a t the 0-0 band in spite of the large energy difference between 3aa*and l u a * ( l a u * ) states.
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The Journal of Physical Chemistry, Vol. 80, No. 27, 1976
2972
Asok Chakrabarti and Noboru Hirota
(3) x Sublevel Emission. The emission from the x sublevel is the weakest. h,'/h.' was found to be -30. This value is similar to that obtained in the 37r7r* azaaromatics such as quinoxaline studied by Anteunis et al.72The small value of h,' is consistent with the planar structure of xanthone in the 37r7r* state. (E) Connection with the Other Related Work. I t is known that xanthone exhibits dual p h o ~ p h o r e s c e n c eone : ~ ~ shortlived with the lifetime of -25 ms and the other long-lived with an -120 ms lifetime. The short-lived one has been considered as arising from the 3n7r* state, while the long-lived one appears to originate from the 37r7r* state. However, it was suggested that the dual emission in rigid media may arise from two different conformations of the 3 n ~ state.52 * The long-lived ( T ~ 120 ms) species in 3-MP was ascribed to 3n7r* state xanthone which has a distorted structure. As we have shown here the long-lived xanthone triplet state observed by Pownall et al. in 3-MP a t 2 K is likely to be similar in character to 37r7r* state xanthone studied here and there seems little doubt about the 3 7 r ~ *assignment to the long-lived xanthone. Our ODMR data also do not support the suggestion that long-lived xanthone has a very distorted structure. The short-lived xanthone species ( T 25 ms) has a strong emission a t the 0-0 band with positive polarization with respect to So 3xa*(1A1)absorption. The phosphorescence spectrum is also characterized by a strong progression of the C=O stretching frequency. These properties have been generally thought of as indications of a 3na*nature.lS2 However, it has been shown that these properties also appear in the 3 ~ 7 r * carbonyls, if the energy separation between them is small and direct mixing between 3n7r* and 37r7r* states due to distortion from planarity is the main source of the radiative a ~ t i v i t y . ~ 6 In our xanthone systems the energy separation between the singlet and triplet n7r* states was found to be -1000 cm-l. On the other hand, in the short-lived xanthone triplet state in 3-MP the separation between the TI state and Inn* was estimated to be 1144 cm-l 47 which is still -150 cm-l larger than the 1n7r*-3na* separation determined in our system. Hence, it is possible that the short-lived species is also a 37ra*state with very small AETT(on the order of 100 cm-l). In the 3a7r* states with such a small AETr the radiative properties may resemble those of the h x * states as in the case of numerous %R* aromatic carbonyls of the benzaldehyde type with relatively small a T T . 4 2 ' 4 6Since the carbonyl group is rigidly held to the planar molecule in xanthone distortion from planarity in 3na* and 3aa* states would be smaller than in benzaldehyde. Nevertheless such a mixing may be significant when AETT is very small, A lifetime of 25 ms is also consistent with the 37r7~*assignment of the short-lived species. I t was found that the total decay rate of a series of 37r7r* aromatic carbonyl for is approximately proportional to Since A E ~ T the short-lived species is about half that for the long-lived xanthone studied here, their lifetime is predicted to be about 25 ms, if a similar correlation between the total decay rate and AEST holds for xanthone. In summary all spectroscopic data on the short-lived xanthone can be rationalized on the basis of the dominant 3x7r* character, although we have no direct evidence in favor of such an assignment. When we compare -25-ms lifetime of xanthone with those of 3na* benzophenone (-5 ms)l and anthrone (2 ms)l3 the lifetime of short-lived xanthone also appears to be in favor of a 37r7r* assignment.
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Acknoulledgment. This work was supported in parts by a NSF Institutional Science Development grant. We thank E. The Journal of Physical Chemistry, Vol. 80,
No. 27, 1976
T. Harrigan for his help in some measurements. N.H. thanks Professor L. Goodman and his group a t Rutgers University for stimulating and useful discussions. The authors are grateful to the referees for their helpful suggestions. References and Notes (1) S. P. McGlynn, T. Azumi, and M. Kinoshita, "Molecular Spectroscopy of the Triplet States", Prentice-Hall, Englewood Cliffs, N.J., 1969, Chapter 6. (2) R. S.Becker, "Theory and Interpretations of Fluorescence and Phosphorescence", Wiley, New York, N.Y., 1969, Chapter 12. (3) L. Goodman, "Excited States", Vol. 1. E. C. Lim, Ed., Academic Press, New York, N.Y., Chapter 6. (4) G. Porter and P. Suppan, Trans. Faraday SOC., 60, 1664 (1965). (5) R. Shimada and L. Goodman, J. Chem. Phys., 43,2027 (1966). (6) N. C. Yang and S.L. Murov, J. Chem. Phys., 43, 4358 (1966). (71 N. C. Yang, D. S. McClure, S.L. Murov. J. J. Houser. and R. Dusenberv. J. Am. Chem. Soc.. 89.5466 (19671. (8) R. M. Hochstrasser'andT. S. L k , J. Chem. Phys., 49, 4928 (1968). (9) T. Takemura and H. Baba, Bull Chem. SOC.Jpn., 42, 2756 (1969). (10) Y. Kanda, J. Stanislaus, and E. C. Lim, J. Am. Chem. SOC., 91, 1085 (1969). (11) R. M. Hochstrasser and C. A. Marzzacco, "Molecular Luminescence", E. C. Lim, Ed., W. A . Benjamin, New York, N.Y., p 631. (12) S. Dym, R. M. Hochstrasser, and M. Shafer, J. Chem. Phys., 48, 684 (1968). (13) S.Dym and R. M. Hochstrasser, J. Chem. Phys., 51, 2458 (1969). (14) W. A. Case and D. R. Kearns, J. Chem. Phys., 52, 2175 (1970). (15) (a) R. Zwarich and L. Goodman, Chem. Phys. Lett., 7, 609 (1970); (b) M. Koyanagi, R. Zwarich, and L. Goodman, J. Chem. Phys., 56,3044 (1972); (c) M. Koyanagi and L. Goodman, [bid., 55, 2959 (1971); 57, 1809 (1972); (d) L. Goodman and M. Koyanagi, Mol. Photochem., 4, 369 (1972). (16) Y. H. Li and E. C. Lim, Chem. Phys. Lett., 7, 15 (1970). (17) G. Fisher, Mol. Cryst., 11, 85 (1970). (18) Y. Tanimoto, H. Kobayashi, S.Negakura, and T. Azumi, Chem. Phys. Lett., 16, 10 (1972). (19) E. Migirdicyan, Chem. Phys. Left., 12, 413 (1973). (20) M. E. Long and E. C. Lim. Chem. Phys. Lett., 20, 413 (1973). (21) L. Goodman and M. Koyanagi, Chem. Phys. Lett., 21, 1 (1973). (22) 0.R. Loufty and J. M. Morris, Chem. Phys. Lett., 22, 584 (1973). (23) c. R. Jones, D. R. Kearns, and R. M. Wing, J. Chem. Phys., 58, 1370 (1973). (24) S. W. Mao and N. Hirota, Mol. Phys., 27, 327 (1974). (25) N. Kanamaru, M. E. Long, and E. C. Lim, Chem. Phys. Lett., 26, 1 (1974). (26) H. Hayashi and S. Nagakura, Chem. Phys. Lett., 18, 63 (1973); Mol. Phys., 27, 969 (1974). (27) S. J. Sheng and D. M. Hanson, J. Chem. Phys., 60, 368 (1974). (28) E . J. Baum, J. K. Wan, and J. N. Pitts, J. Am. Chem. SOC.,88, 2652 (1966). (29) N. C. Yang and R. L. Dusenbery, J. Am. Chem. SOC., 90, 5899 (1968). (30) P. J. Wagner, M. J. May, A. Haug, and P. R. Graber, J. Am. Chem. SOC.,92, 5269 (1970). (31) P. J. Wagner, A. E. Kemppainen, and H. N. Schott, J. Am. Chem. Soc., 95, 5604 (1973). (32) A. A . Lamola, J, Chem. Phys., 47, 4810 (1967). (33) M. Sharnoff, Mol. Cryst., 5, 297 (1969). (34) N. Hirota, Chem. Phys. Lett., 4, 305 (1969). (35) A. Nishimura and D. Tinti, Chem. Phys. Lett., 13, 278 (1972). (36) A . Nishimura and J. S. Vincent, Chem. Phys. Lett., 13, 89 (1972). (37) T. H. Cheng and N. Hirota, Chem. Phys. Lett., 13, 194 (1972); 14, 415 ( 1972). (38) C. R. Jones, F. Rappano, A. H. Maki, and D. R. Kearns, Chem. Phys. Lett., 13, 521 (1972). (39) M. Batley and R . Bramley, Chem. Phys. Lett., 15, 337 (1972). (40) R. M. Hochstrasser, G. R. Scott, and A. H. Zewail. J. Chem. Phys., 58,873 (1973). (41) C. R. Jones, D. R. Kearns. and A. H. Maki, J. Chem. Phys., 59, 873 (1973). (42) T. H. Cheng and N. Hirota, Mol. Phys., 27, 281 (1974). (43) S. W. Mao and N. Hirota, Mol. Phys., 27, 309 (1974). (44) M. A. Souto, P. J. Wagner, and M. A. El-Sayed, Chem. Phys., 6, 204 (1974). (45) E. T. Harrigan and N. Hirota, Chem. Phys. Left., 27, 405 (1974); Mol. Phys., 31, 681 (1976). (46) E. T. Harrigan and N. Hirota, Mol. Phys., 31, 663 (1976). (47) H. J. Pownali and J. R. Huber. J. Am. Chem. SOC., 93, 6429 (1971). (48) P. Gacoin and Y. Mayer, C. R. Acad. Sci. Paris, 267, 149 (1968). (49) R. N. Griffin, Photochem. Photobiol., 7, 157 (1968). (50) N. Y. C. Chu and D. R . Kearns, J. Am. Chem. Soc., 94, 2619 (1972). (51) Y. Tanimoto, N. Hirota, and S. Nagakura, Bull. Chem. SOC.Jpn., 48, 41 (1975). (52) H. J. Pownail, R. E. Conners, and J. R . Huber, Chem. Phys. Lett., 22, 403 (1973). (53) T. H. Cheng and N. Hirota, J. Chem. Phys., 56, 5019 (1972). (54) J. Schmidt, D. A. Antheunis, and J. H. van der Waals, Mol. Phys., 22, 1 (1971). (55) J . Schmidt, Thesis, Leiden. 1972.
2973
ESR Studies on Autoxidation of Tetralin A. L. Kwiram, Phys. Chem., Ser. One., 1972-1973, 4 (1972). M. A. El-Sayed, D. V. Owens, and D. S.Tinti, Chem. Phys. Lett., 6, 395 (1970). C. R. Chen and M. A. El-Sayed, Chem. Phys. Lett., 10,307 (1971). A. H. Francis and C. B. Harris, J. Chem. Phys., 57, 1050 (1972). C. B. Harris, J. Chem. Phys., 54, 972 (1972). D. S. Schweitzer, J. Zuclich, and A. H. Maki, Mol. Phys., 25, 193 (1973). The attempts to obtain sublevel phosphorescence spectra of other systems have been reported recently. (a) S. Yamauchi and T. Azumi, Chem. Phys. Lett.. 21, 603 (1973); (b) N. Nishi and M. Kinoshita, ibid., 27, 342 (1974). W. Rothman, A. Case, and D. R. Kearns, J. Chem. Phys., 43, 1067
(1965). (64) N. Hirota, J. Chem. Phys., 44, 2199 (1966). (65) N. Hirota. unpublished observation. (66) The line width for this transition was found to be broad (-0.1 GHz). Accordingly this transition frequency has considerable uncertainty. (67) R. M. Hochstrasser and G. J. Small, J. Chem. Phys., 45, 2270 (1966); 48, 3612 (1968). (68) R . E. Gerkin and A. M. Winer, J. Chem. Phys., 47, 504 (1967). (69) E. T. Harrigan and N. Hirota, J. Chem. Phys., 49, 2301 (1968). (70) G. Fisher, Mol. Cryst., 11, 85 (1976). (71) H. Hayashi and S. Nagakura, Mol. Phys., 24, 801 (1972). (72) D. A. Antheunis, J. Schmidt, and J. H. van der Waals, Mol. Phys., 27, 1571 (1974).
Electron Spin Resonance and Kinetic Studies on the Liquid-Phase Autoxidation of Tetralin with Lead Dioxide Shun-ichi Fukuzumi and Yoshio Ono" Department of Chemical Engineering, Tokyo lnstitute of Technology, Ookayama, Meguro-ku, Tokyo, Japan (Received August 16, 1976) Publication costs assisted by the Tokyo lnstitute of Technology
The autoxidation of tetralin with lead dioxide has been studied a t 291-323 K. The application of electron spin resonance (ESR) to this reaction has enabled the tetralin peroxy radical (a chain propagating agent) to be detected during the reaction. By measuring simultaneously the concentration of tetralin peroxy radical and the rate of oxygen absorption, the rate expressions for both the radical concentration and oxygen absorption have been determined under the same conditions. A reaction mechanism has been proposed such that the chain initiation involves the formation of the peroxy radical from the hydroperoxide over the catalyst surface and their desorption into the homogeneous phase, where the chain propagation and the chain termination proceed. The simultaneous measurements of the radical concentration and the rate of oxygen absorption have made the accurate determination of the absolute rate constants of the propagation and the termination possible.
Introduction The oxidation of organic compounds is usually carried out in the gaseous phase with heterogeneous catalysts or in the liquid phase with homogeneous catalysts. Recently, however, the autoxidation of liquid hydrocarbons with heterogeneous catalysts has been studied by several investigators.l-ll In our previous works,!)-ll we have studied the autoxidation of cumene as well as the decomposition of cumene hydroperoxide with lead dioxide and the application of electron spin resonance (ESR) to these reactions has enabled cumylperoxy radical (a chain propagating agent) to be detected in the reactant solutions. The ESR and kinetic results have lead to the conclusion that the autoxidation of cumene with lead dioxide is a radical chain reaction in which the chain initiation involves the formation of cumylperoxy radicals by the decomposition of the hydroperoxide over the catalyst surface and their desorption into the homogeneous phase, where the chain propagation and the chain termination proceed.9 The termination process has been considered to be the first-order decay of cumylperoxy radicals.1° However, with regard to primary or secondary hydrocarbons, the autoxidation with heterogeneous catalysts has not been studied as extensively as that of the tertiaries, such as
cumene, although the autoxidation with homogeneous catalysts has been studied extensively, especially in the case of tetralin,'2-16 which is a typical secondary hydrocarbon. The autoxidation of tetralin with manganese dioxide has been considered to be a radical chain reaction,8 but the reaction mechanism has not been elucidated in detail and the presence of peroxy radical has not been confirmed so far either. In the present work, we have studied the autoxidation of tetralin with lead dioxide by measuring both the concentration of tetralin peroxy radical with ESR and the rate of oxygen absorption and by examining the product distribution. A reaction mechanism, composed of elementary reactions analogous to those proposed in the autoxidation of cumene with lead dioxide9 except for the termination step, will be proposed. The absolute rate constants and the Arrhenius parameters for the propagation and the termination reactions can be determined directly by measuring the radical concentration and the rate of oxygen absorption simultaneously. Experimental Section Materials. Tetralin (reagent grade), obtained from commercial sources, was distilled and percolated through an activated aluminum column prior to use. The surface area of The Journal of Physical Chemistry, Vol. 80, No. 27, 1976
