2324
J . Phys. Chem. 1986, 90, 2324-2330
SPECTROSCOPY AND STRUCTURE A Kinetic Study of Dual Phosphorescence of Halogenated Benzenes in Rigid Glass Solution Takeshi Takemura,* Yukio Yamada, Minoru Sugawara, and Hiroaki Baba Division of Chemistry, Research Institute of Applied Electricity, Hokkaido University, Sapporo 060, Japan (Received: May 29, 1985; In Final Form: December 19, 1985)
The phosphorescence spectra, quantum yields, and decays of chlorobenzene (CB), p-dichlorobenzene (DCB), and p-dibromobenzene (DBB) have been measured in 2-methylpentane rigid glass solution as a function of temperature in the range 7C-100 K. The phosphorescence characteristics change markedly with temperature above 85 K. The results are reasonably interpreted in terms of the view that the halogenated benzenes exhibit dual phosphorescence originating from two low-lying triplet states, 3 ( ~ T, * ) and 3(n,u * ) . The rate constants and activation energies were determined for dissociative processes and mutual conversion processes in or between the two triplet states, the activation energies ranging from 3 to 6 kcal/mol. It is shown that the dissociative processes are of prime importance at higher temperatures in CB and DCB and at any temperature in DBB.
Introduction
The phosphorescence spectra of halogenated benzenes in rigid glass solution are in general very different from the spectra of benzene, toluene, and phenol, in contrast to the phosphorescence spectra of halogenated naphthalenes which are similar to the spectrum of na~hthalene.'-~In a previous paper,l we presented the following results of our study on the phosphorescence of chlorobenzene (CB), p-dichlorobenzene (DCB), and p-dibromobenzene (DBB) in rigid glass solution at a temperature of 77 K: (1) Any of these compounds exhibits dual phosphorescence consisting of a slow-decaying component with a maximum near 400 nm (slow phosphorescence) and a fast-decaying component with a maximum near 500 nm (fast phosphorescence). (2) For CB and DBB the fast phosphorescence is predominant, while for DCB the slow phosphorescence is predominant. (3) The radiative lifetime of the slow phosphorescence for each compound is close to the one expected from that of the corresponding halogenated whereas the radiative lifetime of the fast phosphorescence is considerably shorter than expected. (4) The slow phosphorescence is due to the transition 3(n,n*) So, while the fast one is considered to originate from the , state lying closely to the 3 ( ~ n*) , state.s-10 predissociative 3 ( ~ a*)
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( I ) Takemura, T.; Yamada, Y.; Baba, H. Chem. Phys. 1982, 69, 171. (2) Ichimura, T.; Hikida, T.; Mori, Y. J . Phys. Chem. 1975, 79, 291. (3) Lim, E. C.: Chakrabarti, S. K. Mol. Phys. 1967, 13, 293. (4) Castro, G.; Hochstrasser, R. M. J . Chem. Phys. 1966, 45, 4352. ( 5 ) McClure, D. S.J . Chem. Phys. 1948, 17, 905. (6) Gilmore, E. H.: Gibson, G. E.; McClure, D. S . J . Chem. Phys. 1952, 20, 829. (7) McGlynn, S. P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy of the Triplet State; Prentice-Hall: Englewood Cliffs, NJ, 1969. (8) In ref 1, we assigned the triplet state responsible for the fast phosphorescence to "u, x * ) without distinction between 3(u,a*) and 3 ( ~ u, * ) . In this paper, however, we regard the phosphorescent state in question as 3(a, u * ) by reference to the results of ab initio calculations of the low-lying triplet states of CB (Nagaoka, S.; Takemura, T.; Baba, H.; Koga, N.; Morokuma, K. J . Phys. Chem., in press). ( 9 ) (a) Niizuma, S.; Kwan, L.; Hirota, N. Mol. Phys. 1978,35, 1029. (b) Shinohara, H.; Hirota, N. J . Chem. Phys. 1980, 72, 4445. (IO) Igarashi, H.; Sasaki, A.; Kaya, K. Abstracts of Papers, Annual Symposium on Molecular Structure, Sapporo, Japan, Aug. 1977; Chemical Society of Japan: Tokyo, 1977; p 493.
0022-3654/86/2090-2324$01.50/0
( 5 ) The equilibrium C-X bond length (X being a halogen atom) is considered to be significantly greater in the 3(n,a*) state than in the ground state, So; this accounts for the observation that the 3(n,u * ) So phosphorescence spectrum has its maximum at an anomalously long wavelength (near 500 nm). In the present study, the kinetic behavior of the phosphorescent 3 ( x , n*) and 3 ( ~ ,u*) states of CB, DCB, and DBB has been investigated with a view of gaining a deeper understanding of the dual phosphorescence inherent in these halogenated benzenes. The phosphorescence quantum yields and decays were measured in 2-methylpentane rigid glass solution at different temperatures between 70 and 100 K, and thereby the rate constants and activation energies were determined for the dissociative processes and mutual conversion processes in or between the two triplet states concerned. The resulting kinetic data suggest that the dissociative processes are of prime importance at higher temperatures. For the sake of comparison, the phosphorescence properties of some halogenated naphthalenes such as 1,4-dibromonaphthalene have also been studied.
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Experimental Section Chlorobenzene, p-dichlorobenzene, and p-dibromobenzene (Wako Pure Chemical Co.) were purified by zone refining, repeated vacuum sublimation, or fractional distillation. 2Methylpentane (Tokyo Kasei Co.) was passed through a column of activated alumina and shaken with concentrated sulfuric acid. It was then washed with water, dried with phoshorus pentoxide, and finally purified by fractional distillation. 1,4-Dibromonaphthalene was purified by vacuum sublimation. Emission and excitation spectra under photostationary conditions were measured by means of the photon-counting technique, using a high-sensitivity emission spectrophotometer described in a previous paper." The phosphorescence decays (or lifetimes) were measured with a spectrophosphorimeter constructed in our laboratory.12 It consists of a xenon flash lamp (providing flashes with a duration of 3 p s ) and excitation and emission grating monochromators of a Hitachi MPF-2 fluorescence spectropho( 1 1) Takemura, T.; Aikawa, M.; Baba, H.; Shindo, Y . J . Am. Chem. SOC. 1976, 98, 2205. ( 1 2 ) Takemura, T.; Uchida, K.; Fugita, M.; Shindo, Y . ;Suzuki, N.; Baba, H. Chem. Phys. Lett. 1980, 73, 12.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 2325
Halogenated Benzenes in Rigid Glass Solution
TABLE I: Kinetic Data for Various Processes Related to the Two Low-Lying Triplet States of Halogenated Benzenes‘ AE, kcal mol-’ Bi at 90 K AEr, compd e,, s-l er, s-1 , :e S-I A,, S-1 kcal mol-’ ep, S-1 Ar, s - l kcal mol-l from b1 from @2 1600 ( k 2 d ) 50 ( k 1 0 ) 6 X 10’’ 5.5 ( k l z ) 200 ( k 2 0 ) 4 x 10” ( k 2 d ) 3.5 ( k 2 d ) 3.9 ( k 2 d ) CB 330 ( k 1 2 ) DCBb 160 ( k 2 d
+
k21)
(k12)
1000 ( k l d )
30 (kio) 4 X lOI5 (k2d
1.6 x 105
DBB
5.5
(k2d
6.3 x 10’’
4- k 2 1 )
+ k21)
’ + k12 >> k2d + k21
(4)
kzl. Thus, for CB at high temperatures, it follows from inequality 7 that k2d
>> k21
(18)
This relation is also considered to be applicable to DBB. As has been shown previously, for DCB at high temperatures, the phosphorescence decay curve observed a t any wavelength X in the range 400-500 nm is biexponential. From eq 11, therefore CfaGa(1) + CfbGb(X)
>0
(19)
Using eq 10-2 and 10-4 and recalling that X >.Y in this case (see eq 16), we get the relation
> k12 In the wavelength range concerned Ga(X) > bo,as will be seen later, so that X >> kI2. Thus, for DCB at high ( x - es)(Ga.(X)/Gb(X) + b O / a O i
temperatures, we have the relation kld
>> k12
(20)
We have already summarized in Table I the values of the constants ,e: A,, and AEi associated with 0, for CB, DCB, and DBB. On the basis of the information obtained from the preceding discussion, we can assign these constant values to the rate constants of the processes occurring in or from states 1 and 2; the results are given in parentheses in Table I. It will be shown later that kld > kzl (Le., K2 = 0) exists, eq 21 is obviously reduced to
+
Hence, we obtain Of - Y > k I 2(Le., K1 = 0), eq 22 becomes 42 =
1 - P + KIP
k20.r
(22’)
On the basis of the relations among the various rate constants described before, one can present the following statements. (1) For CB, at low temperatures it follows from inequalities 4 and 5 that K1 = 0 and K2 = 0, and hence 41 and 42are given by eq 21’ and 22’, respectively. At high temperatures, is given by eq 21’, since K2 = 0 on account of relation 18; and 42 is proportional to for the following reason. In eq 22, Kl is approximately equal to k12/(kld+ kI2)at high temperatures. If the activation energies associated with kld and k12are not appreciably different from each other, K1 is expected to be virtually independent of temperature. Also, p can be assumed to be independent of temperature. Furthermore, Y - K1kzl= k2d in the present case. It then follows that 42 0: l/kzd. On the other hand, it is evident from what has already been stated that Zp at 480 nm for CB can be regarded as proportional to &. These considerations are in conformity with the observation that in Figure 3 the plot of log ( l/Zp) against 1 / T for the phosphorescence at 480 nm gives a straight line at high temperatures. (2) For DCB, at low temperatures and d2 are given by eq 21’ and 22’, respectively, as in the case of CB. At high temperatures, 41 is proportional to I l k l d and 42is given by eq 22’, since K1 = 0 on account of relation 20. The proportionality 41 a l / k l d can be derived in a way similar to the one in which the proportionality 420: l/kZ was derived for CB, and is in conformity with the data for 410 nm in Figure 7. It is to be noted that in eq 21 K2(1 - p ) = 0, because the value of p for DCB is near unity as will be shown later. It may be easily found that at high temperatures 42 is inversely proportional to k2d kzl. (3) For DBB, a t any temperature c $ ~ is proportional to l/k2d. This proportionality is derived in the same way as that for CB. Thus, from the log (l/Zp) vs. l/Tplots in the high-temperature range (see Figures 3, 7, and lo), the activation energies (AE)can be obtained for some of the nonradiative processes on the basis of the following assumptions: For CB, Zp at 480 nm is proportional to d2; for DCB, Zp at 410 nm and Zpat 490 nm are proportional to d1and 42,respectively; for DBB, Zp at 500 nm is proportional to 41~.In Table I are summarized the values of AE obtained in this way; the rate constants to which the AE values are to be assigned are given in parentheses. The values of AEi (i = s or f) obtained from the decay constants (e,) are seen to agree fairly well with the corresponding AE values from the phosphorescence intensities (Zp). It is to be noted here that Ipat 410 nm for CB (Figure 3) seems to be attributable mainly to d2 owing to the predominance of b over a phosphorescence. Evaluation of Other Quantities. We here evaluate p and k20,r/k10,rfor CB and DCB. According to eq 21 and 22, the quantum yield ratio of b to a phosphorescence is given by
+
2330 The Journal of Physical Chemistry, Vol. 90, No. 11, 1986
Takemura et al.
1 - P + KIP x ho,r -(23) P + K2(1 - P ) y kl0J For CB, according to eq 12 and 14, X = 6, and Y = Or at both
assuming the following relations: k10,r/k20,r = 1/30, p = 0.6, Fa(410 nm)/Fb(410nm) = 4, and F,(480 nm)/Fb(480 nm) = 1/4. The constant C in eq 28 is so chosen that the calculated and observed Zp values may agree with each other at 77 K. The resulting log ( l/Zp) vs. 1 / T curves for CB are shown in Figure low and high temperatures. At 77 K, since K 1 = K2 = 0, eq 23 3 with broken lines. It may be said that the calculated curves may be written as are in agreement with the observed within the limit of error of (24) 42/41 = 1(1 - ~ ) / ~ 1 ( 6 s / 6 f ) ( k 2 0 , r / k l 0 , r ) the experiments and kinetic analyses. For DCB, one may consider that at any temperature klo kld It has been found experimentally that = 5 and Os/Of = 0.25 >> k l z . We assume that k10,r/k20,r= 1/30, p = 1, 1 - p K l p at 77 K.' On the other hand, at 90 K the quantum yield ratio = lo-', Fa(410 nm)/Fb(410 nm) = 4, and F,(490 nm)/Fb(490 is nm) = 1/4. Furthermore, since kzois not experimentally available, tentatively that kzo = 100 s-I. Thus, log (l/Zp) vs. 1 / T 42/41 = 1(1 - P + ~ ' P ~ / P ~ ~ ~ , / ~ f ~ ~ ~ 2 O(25) , r / ~ I O we , r assume ~ curves of DCB can be obtained for h = 410 and 490 nm, with since K2 = 0 on account of eq 18. It is to be noted that K 1 = the results shown in Figure 7. The calculated curves agree, kl2/(k]d + k I 2 )and 0 Q K l S 1. The values of and OS/Of although not satisfactorily well, with the observed. have been found to be 10 and 0.2, respectively, at 90 K. If p and k20,r/k10,r are assumed to be independent of temperature, it can Concluding Remarks be shown from eq 24 and 25 and the data given above that p 2 As was described in the Introduction of this paper, the view 0.6 and k20,r/k10,r 2 30, the equal signs holding when K , = 1. we advanced on the basis of the results of our previous study' is According to our experimental results, the smallest value of Of for that in halogenated benzenes the two low-lying triplet states, 3(r, CB at low temperatures is known to be 2 X lo2 s-I. Therefore, R*) and 3 ( r u , * ) , which are supposed to be responsible for the k20,rfor CB has to be smaller than 2 X lo2 s-]. Moreover, we dual phosphorescence, are located close in energy to each other, obtained klo,,= 6 s-I for DCB at 77 K.' Since the value of k I o , , with the equilibrium C-X bond length being significantly greater for CB is considered to be similar to that for DCB, we may in 3(r,u * ) than in 3 ( x ,r*) and SO(see Figure 11). The observed estimate that k20,r/k10,r i33. Thus, for CB kinetic behavior of the triplet states of the halogenated benzenes concerned in the present study is consistently and satisfactorily P = 0.6 k20,r/k10,r r=z 30 (26) explained by, and hence supports, our view mentioned above. This in turn suggests that at high temperatures K , = 1 and hence The activation energies evaluated for various processes are kld 0.3 at 77 K, so that (X/Y)(k20,r/klo,r) R 10 at 77 K. Then, it follows from eq 23 that singlet (7, r*) states through direct spin-orbit coupling and, 1- p +K g < since in general p + K2(1 - p ) S 1. Hence, therefore, it may have a much larger radiative rate constant than at 77 K the 3(a,r*) state. It is to be emphasized here that for DCB the ratio of the p=l K,=O (27) phosphorescence quantum yield from the 3 ( ru, * ) state to that , state increases considerably with increasing from the 3 ( rr*) It has been found that, at 90 K, = 1 and X / Y = 6,/6, = temperature. As a result, the spectrum of the former phos6. Substituting these values into eq 23, we again obtain eq 27. phorescence appears distinctly in the long-wavelength region The fact that K I = 0 at 90 K means that k]d >> k l z in agreement (Figure 6), in agreement with what is predicted from our potential with eq 20. The relation a. >> bo, used before for DCB, follows energy scheme (Figure 1l), that is, an unusually small vertical from p i= 1. transition energy from 3 ( ru, * ) to So. A Model Calculation of Zp as a Function of T. In order to Of the rate constants for the various processes associated with ascertain the validity of the foregoing discussion, we have made the two triplet states concerned, those for the dissociative processes, a model calculation of the relative phosphorescence intensity Zp kZdfor CB or DBB and k]d for DCB, have been found to be of as a function of T, using the various kinetic data given in Table prime importance at high temperatures (Table I). This observation I except the AE values from $I and I $ ~ . It is evident that the is naturally related to the exceptionally large photochemical rerelative phosphorescence intensity at A, Zp(X), under photoactivity of halogenated benzenes.15 It may be noted, however, stationary conditions can be expressed in the form that according to the results of the present study, k2d of CB seems Ip(X) = C[Fa(X)41 + Fb(X)'$ZI (28) to play only a minor role in the decay of the 3(r,a*) state at such a low temperature as 77 K, in contrast to our assumption in a where C is an arbitrary constant. We use here the original exprevious paper.' pressions of & and 42 given by eq 21 and 22. Registry No. CB, 108-90-7; DCB, 106-46-7; DBB, 106-37-6. For CB, from our previous descriptions and the data in Table I, one may suppose that at any temperature klo4- k12>> kld and k20 + k 2 d >> 4,. Then, on the basis of eq 21,22, and 28,Zp(X) (15) Nagaoka, S.; Takemura, T.; Baba, H. Bull. Chem. SOC.Jpn. 1985, can be calculated as a function of Tfor X = 410 and 480 nm by 58, 2082 and references cited therein.
_42 -$1
+ +
