J. Phys. Chem. 1983, 8 7 , 813-816
The Ost~ald-Wagner-Prager"-'~ theory of Liesegang ring formation postulates that nucleation is discontinuous in space and determines the ring locations. This is contrary to our findings. Furthermore, the theory does not apply to the case of no imposed spatial gradients. A theory of structure formation has been proposed which is based on a chemical instability'jJJ4J5due to autocatalytic colloidal growth, after nucleation, coupled with diffusion. The theory applies to cases with, and without, initial concen-
813
tration gradients. The basic assumptions of the theory need substantial further development so that they can be tested against the many details of the experiments reported. Acknowledgment. This work was supported in part by the National Science Foundation and by the Air Force Office of Scientific Research. Registry No. Pb12, 10101-63-0.
Temperature Dependence of the Nonradiative Relaxation Process of the Lowest Excited Singlet States of Meso-Substituted Bromoanthracenes
Masanao Tanaka, Ikuro Tanaka, Department of Chemlstw, Tokyo Institute of Technology, Ohokayama, Meguro, Tokyo 152, Japan
Shigeyoshi Tal, Kumao Hamanoue, Department of Chemistv, Kyoto Institute of Technoiogy, Matsugasaki, Sakyo, Kyoto 606, Japan
Minoru Sumitani, and Keitaro Yoshlhara Institute for Molecular Science, Mycda#l, Okazaki 444, Japan (Received: March 1, 1982; In Final Form: October 4, 1982)
Direct measurements of the temperature dependence of the fluorescence lifetimes of 9-bromoanthracene (BA) and 9,lO-dibromoanthracene(DBA) have allowed the determination of the rate constant for intersystem crossing (isc) in the form hisc = Aisc exp(-hE/kT). This isc is attributed to that from the lowest excited singlet state S1to an adjacent higher excited triplet state T,. The values of AI%,i.e., 600-710 cm-' for BA and 1100 cm-l for DBA, are about 300 cm-' larger than those deduced from the T, T1fluorescence (TTF) measurement by Gillispie and Lim (Chem. Phys. Lett., 63,355 (1979)). The values of the S1 decay constants consistent with those of the buildup times for triplet-triplet absorptions lead us to the suggestion that the T, lifetimes are tens of picoseconds, and this estimate gives good agreement between calculated and experimental am of DBA.
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Introduction Recently, the study of the nonradiative process of the fluorescing state in anthracene derivatives has received considerable attention from both experimental and theoretical points of view.' Since the rate of intersystem crossing (isc) is proportional to the vibrational overlap factor whose magnitude increases rapidly with decreasing energy gap between the two interacting states,2the large S1-T1 separation of anthracenes makes the direct isc to T1 of minute importance because of the extremely unfavorable vibrational overlap factor for such a process. As a result, the number and the order of the higher triplet state(s) lying near or very slightly above the lowest excited
singlet state may be expected to be important for isc. The existence of such higher triplet states has been supported by the following experimental results: (1) Rather long buildup times of the triplet-triplet absorption (72-86 ps) for nonfluorescent nitroanthracenes (9-nitro-, 9-benzoyl-lO-nitro-,and 9-cyano-10-nitroanthracene) lead us to the conclusion that the observed buildup times do not reflect the lifetimes of the singlet states but might represent the rates of nonradiative process in the triplet manifold, and that the indirect isc S1(mr*) T,(nrr*) Tl(m*)is the most important process to populate T1.3 (2) The T2(3B1,)state is found to lie below S1 by 600, '600, and 900 cm-' for anthracene,4v52-meth~lanthracene,~ and 1,5-
(1) J. B. Birks, "Photophysics of Aromatic Molecules", Wiley-Interscience, New York, 1970, Chapters 4 and 5. (2) G. W. Robinson and R. P. Frosh, J. Chem. Phys., 37,1962 (1962); 38, 1187 (1963).
(3) K. Hamanoue, S.Hirayama, T. Nakayama, and H. Teranishi, J. Phys. Chem., 84,2074 (1980). (4) R. E.Kellogg, J . Chem. Phys., 44,411 (1966). ( 5 ) Y. H.Meyer, R. Astier, and J. M. Leclercq, J. Chem. Phys., 56,801 (1972).
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0022-365418312087-0813$0 1.50lO 0 1983 American Chemical Society
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014
Tanaka et ai.
The Journal of Physical Chemistry, Vol. 87,No. 5, 1983
dichloroanthracene,Brespectively. (3) The position of the substitution controls the fluorescing behavior of the anthracenes, and the temperature dependence of the fluorescence yield can be explained in terms of the thermally activated isc through the T, (n 1 2) state assumed to be slightly higher in energy than S1.4*7-15(4) From the measurements of the fluorescence lifetimes and the second emission maxima in solutions, it is concluded that the solvent dependence of the nonradiative transition rates of the lowest excited singlet states in 9-bromoanthracene (BA) and 9,lO-dibromoanthracene (DBA) is due to the different extent to which S1and T, are stabilized by the solvent. The ratios of the solvent stabilization energies of the T, state to that of the S1 state are 0.4316for BA and 0.3117-0.3916for DBA. According to the T, T1 fluorescence measurements of Gillispie and Lim,14J5the energy differences between T, and S1,i.e., AE = E(T,) -E(&), are -300 cm-' for BA and 800 cm-' for DBA. These values are much smaller than those deduced from the temperature dependence of the S1 So fluorescence quantum yields by Kearvell and Wilkinson,l' i.e., -1100 cm-' for BA and -1600 cm-l for DBA. Although the values of 1600 cm-' for DBA are in the same range as those which were obtained by a single photon counting method in the temperature range 273-314 K,17we think that the activation energy should be deduced over a much wider temperature range by using the directly measured radiative lifetime. In this paper the direct observation of the S1 So fluorescence lifetimes of BA and DBA in 3-methylpentane and methylcyclohexaneover the temperature range 77-296 K is reported, and more reliable activation energies for isc are obtained.
0A in 3-MP at 296K 7
x
c
E
c
8 @A in 3-MP at 195K
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-
-
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Experimental Section 9-Bromoanthracene (EP grade) was purchased from Tokyo Kasei Kogyo Co., Ltd., and 9,lO-dibromoanthracene was synthesized by bromination of anthracene.l* After repeated crystallization from methanol, both samples were purified by vacuum sublimation. G.R. grade 3-methylpentane (Wako) and spectral grade methylcyclohexane (Eastman) were used as solvents without further purification. The sample solutions were degassed by several freeze-pump-thaw cycles. The excitation of 9-bromoanthracene (BA) and 9,lOdibromoanthracene (DBA) in 3-methylpentane (3-MP) were carried out by using the third harmonic (355 nm) with a pulse width of 15 ps from a mode-locked Nd3+:YAGlaser. In the higher temperature region (296147 K for BA and 296 K for DBA), the decay of SI So fluorescence was measured by a Hamamatsu (2-979 streak camera with an S-20 cathode. The streak image was detected by a T V
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(6)J. P.Roberta and R. S. Dixon, J. Phys. Chem., 76, 845 (1971). (7)E. J. Bowen and J. Sahu, J.Phys. Chem., 63,4 (1959). (8)W. R. Ware and B. A. Baldwin, J . Chem. Phys., 43,1194 (1965). (9)R.G. Bennett and P. J. McCartin, J. Chem. Phys., 44,1969(1966). (10)E.C.Lim, J. D. Laposa, and J. M. H. Yu, J. Mol. Spectrosc., 19, 412 (1966). (11)A. Kearvell and F. Wilkinson, "Transitions Non Radiatives Das les MolCcules", Paris, 1969; J . Chim.Phys., 20, 125 (1970). (12)T. F. Hunter and R. F. Wyatt, Chem. Phys. Lett., 6,221(1970). (13)R. P.Widman and J. R. Huber, J.Phys. Chem., 76,1524(1972). (14)G.D.Gillispie and E. C. Lim, J. Chem. Phys., 65, 2022 (1976). (15)G. D.Gillispie and E. C. Lim, Chem. Phys. Lett., 63,355(1979). (16)K.Hamanoue, T. Hidaka, K. Nakajima, T. Nakayama, H. Teranishi, M. Sumitani,and K. Yoshihara, paper presented at the 43rd Spring Meeting of the Chemical Society of Japan, Tokyo, Japan, 1981,Vol. 1, p 556,to be published. (17)Kam-Chu Wu and W. R. Ware, J. Am. Chem. SOC.,101,5906 (1979). (18)H. Gieman, Ed., 'Organic Syntheses", Wiley, New York, 1967,p 207.
0
400
1200
800
1600
Timelps
Figure 1. Fluorescence decay curves of BA in W P , [BA] = 3 X lo4 M: (-) observed curves by a streak camera; (. -) simulated curves based on a slngle exponential decay with k = 7.94 X los (a) and 1.78 X I O * s-' (b). h
dl
BA in MCH at 175K
.i'I c
I
0
...*
.
.
.
I
.
50 039 ns /channel
.
.
~
.
-.-
1 )O
(cha inel 1
Flgufe 2. Fluorescence decay curves of BA in MCH, [BA] = 3 X lo5 M: (. .) observed points by a conventional single-photon counting method; (-) convoluted curve from exciting light pulse (broken line) based on a single exponential decay of fluorescence with k = 2.94 x 108 s-1.
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camera and treated by a Hamamatsu C-1098 microprocessor. In the lower temperature region (138-77 K for BA and 265-77 K for DBA) the fluorescence decay was observed by using a Hamamatsu R1328U-02 biplanar photodiode and a Tektronix 7104 oscilloscope. In methylcyclohexane (MCH), we have also measured the fluorescence decay of BA, using the second harmonic (347.2 nm) with a pulse width of 50 ps from a mode-locked ruby laser in the temperature range 296-180 K. In the lower temperature range 175-77 K, a conventional single-photon counting instrument was used for the fluorescence lifetime measurements. The details of the picosecond ruby laser photolysis system have been described e1~ewhere.l~ Results The typical fluorescence decays of BA are shown in Figures 1 and 2. The smooth curves (dotted lines) in Figure 1are calculated by a least-squares method assuming a single exponential decay of fluorescence. In the case of a single photon-counting measurements (Figure 21, the broken line shows the pulse profile of the exciting light and the solid line is the curve convoluted from the exciting light (19)H. Shizuka, K. Tsutsumi, H. Takeuchi, and I. Tanaka, Chem. Phys., 59, 183 (1981).
The Journal of Physical Chemistry, Vol. 87, No. 5, 1983
Nonradiative Relaxation of Meso-Substltuted Bromoanthracenes
815
TABLE I : Frequency Factors Ai,, and Activation Energies AE in Various Solvents AT( oI1 /
solvent"
AiSc/lO1's - ~
3-MP MCH bromobenzene benzene 95% ethanol hexane
1.47 + 0.01 0.93 + 0.01 6.1 % 0.1 4 . 7 + 0.1 5.8 + 0.07 6.8 + 0.1
AEexPt/Cm-l
temp/K
method
AEC~:,P/~
cm-l
cm
BA 6 0 0 + 10 77, 104-296 k 480 1080 7 1 0 + 30 77,110-296 k 430 1110 1350 t 35 QF 0 1350 283-333 1270 1220 r 30 283-333 @F 150 1 1 1 0 + 20 283-333 @F 430 1110 1040 1 0 1 0 + 20 288-333 @F 53 0 DBA 3-MP 1.39 5 0.01 1 1 0 0 %20 77,113-296 k 530 1430 bromobenzene 9 . 0 % 0.4 1 7 5 0 %75 283-333 @F 0 1750 benzene 10.1 5 0.4 1 7 1 0 + 70 283-333 QF 170 1650 95% ethanol 9.1 % 3.5 1 5 9 0 t 50 283-333 @F 450 1480 1560 + 50 283-333 @F 500 1450 hexane 9.9 f 3.5 " The values of Aisc and AEexpt except for those in 3-MP and MCH are given in ref 11. Spectral shifts of the second emission band relative to that in bromobenzene. All spectra were taken with a Shimazu R F 502 fluorescence spectrometer. Calculated values by eq 4 with bromobenzene as a reference solvent.
Generally, the total rate of the S1 decay process is expressed as 12 = k,
L
I
0 0
0
0
0
. Q
0
00
...
Temperature ( K )
Figure 3. Variation of fluorescence lifetimes of BA and DBA with temperature.
pulse. All experimental fluorescence decays could be fitted with high precision to a single exponential decay function, that is, the average deviation for fitting to the experimental data was within f0.2% and the standard deviations of the best fit rate constants (k) were between 2 and 5%. In Figure 3, we show the fluorescence lifetimes of BA and DBA (T = k-l) as a function of temperature. The fluorescence lifetimes at temperatures between 77 and 100 K are nearly constant and become shorter as the temperature increases. Although the fluorescence lifetime of DBA at room temperature (T = 1.11 ns in 3-MP) is consistent with those observed by Wu and Ware" and Cherkasov et al.,20the values for BA (T N 0.13 ns in 3-MP and 0.28 ns in MCH) are an order of magnitude shorter than those observed by Cherkasov et al.% and Dreeskamp and Pabst.21 This discrepancy is probably due to uncertainty of their single-photon counting method in the subnanosecond region, because the fluorescence lifetimes of BA in acetonitrile are 0.16 ns by the present Nd3+:YAG laser photolysis and 0.5 ns by our single-photon counting method, the latter value being the lowest measurable value which the apparatus can give. (Although the fluorescence lifetime of BA (0.13 ns) in 3-MP by Nd3+:YAG laser photolysis is different from that (0.28 ns) in MCH by ruby laser photolysis, this discrepancy is due to the different methods of the measurements. Because in our Nd3+:YAG laser photolysis,ls we have obtained that the value in MCH is 0.20 ns.) (20) A. S. Cherkasov, V. A. Molchanov, T. M. Vember, and K. G. Voldaikina, Soviet Phys. Dokl., 1, 427 (1956). (21)H.Dreeskamp and J. Pabst, Chem. Phys. Lett., 61, 262 (1979).
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+ ki, + kist
-
(1)
where k,, kic, and kisc are the rate constants for the S1 So fluorescence, the S1 So internal conversion, and the S1 T, intersystem crossing, respectively. In the case of meso-substituted anthracenes, the internal conversion is an unimportant mode of decays1' Since k, has very little temperature dependence, almost all the temperature dependence of k is due to that of kist. Thus, kisc can be expressed as
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kisc = his:
+ Ais, exp(-AE/kT)
(2)
where kis: is independent of T , and the temperature-dependent component is described by an Arrhenius term of the frequency factor Ai, and activation energy AE. At 77 K the quantum yield of fluorescence of the meso-substituted anthracenes has been shown to be unity,l0J1and no triplet-triplet absorption was detected,'O i.e., his: N 0. Then, one can assume that k - k, = Aisc exp(-AE/kT) (3) From the fluorescence lifetimes at 77 K, the radiative lifetimes ( T = ~ k;l) were determined to be 12.8 and 10.2 ns for BA in 3-MP and MCH, respectively, and 12.5 ns for DBA in 3-MP. Using a Hitachi MPF-4 fluorescence spectrometer equipped with a time-resolved photometry accessory, we have also measured the radiative lifetimes in EPA (ether/isopentane/ethanol= 5:5:2 in volume ratio) to be 12.8 ns for BA and 11.8 ns for DBA. All these values except for that of BA in MCH are consistent with those estimated from the absorption spectra in alcohol solution, Le., 13.8 ns for BA and 12.5 ns for DBA.22 Arrhenius plots of eq 3 are shown in Figure 4, which give good straight lines down to 100 K for BA. For DBA, the experimental points deviate from the straight line at temperatures below 164 K. The values of hE and In Ai, obtained by a least-squares fit to eq 3 always give a standard deviation of less than 4%. Table I gives the values of Ab, and A E together with the literature values" (if we assume T~ of BA in MCH to be 12.8 or 13.8 ns, the corresponding AE is 680 f 30 cm-').
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Discussion As can be seen from Table I, Arrhenius-type fits to the isc rate yield activation energies of 600-710 cm-' for BA (22) See also ref 1, p 121.
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Tanaka et at.
The Journal of Physical Chemistty, Vol. 87, No. 5, 1983
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states. On the basis of their suggestion, the difference of 300 cm-l may be attributed partially to the fact that two Stokes shift are involved in the determination of the T, level compared to one for the S1 level. The remaining difference may reflect the symmetry and/or level density restrictions on the ability of thermally activated S1levels above the origin of T, to intersystem cross to the triplet manifold. Let us next try to analyze their results concerning the quantum yields of triplet-triplet fluorescence (TTF) in anthracenes, Le., @mF= 3 x lo-' for BA and 1 x lo4 for DBA, estimated to be accurate to within a factor of 2. If the TTF is actually assigned as T,(3B1,) T1(3Bz,),its quantum yield is given by
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Figure 4. Plots of In k vs. I/Tfor BA and DBA: k, are 7.81 X lo7 9 (in MCH) for BA, and 8.00 X lo7 s-' for DBA (in 3-MP) and 9.80 X 10 in 3-MP.
and 1100 cm-' for DBA. The difference in activation energies between BA and DBA indicates that the thermal activation of isc is a consequence of the lowering of &('La) below T, when the conjugation along the short axis is extended by substitution at these positions. The values of Aiecare two orders of magnitude higher than those in 9-phenylanthracene, 9-methylanthracene, and anthracene." This may be due to the heavy atom effect of Br. Wu and Ware" have succeeded in correlating the rate of the S1 T, isc in DBA as a function of the second emission band ~ ( 0 , l ) .By assuming the Arrhenius parameter Ai, is constant through the series of solvents, they gave the following relation between activation energy and 8(0,1) (CY - l)[P(O,l) - Do(0,l)l = AE - A & (4)
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where to(O,l) and AEo refer to a reference solution and CY is the ratio of the solvent stabilization energies of the T, state to that of the S1 state, Le., a = 0.31. Since we have also found that the same correlations exist with a = 0.43 in BA and cr = 0.39 in DBA,16the estimated difference in AE of BA between the solvents of MCH and 3-MP should be 30 cm-l, which is smaller than the observed difference 100 cm-'. Thus it is concluded that this discrepancy in AE of BA is due to the difference in the measurements. From the measurements of the temperature dependence of the fluorescence quantum yield in the temperature range 293-310 K, Kearvell and Wilkinson" obtained the frequency factors Ai, and activation energies AEBqtas shown in Table I. We also show AE&s which were deduced from eq 4, with bromobenzene as a reference solvent. Although the solvent effect on AE,, of Kearvell and Wilkinson can be explained in terms of the correlation of eq 4, the agreement between deduced values and our experimental values is not good. This could be due to the difficulty in the measurement of quantum yield and errors arising from the estimation of the radiative lifetime. Since we have measured the fluorescence decay directly and over much wider temperature range than them, and have observed directly the radiative lifetime at 77 K, our results are thought to be more reliable. According to the T,(3B,,) T1(3Bz,) fluorescence measurements of Gillispie and Lim,14J5the energy differences between the T, and S1 states are 300 cm-' (590-670 cm-') for BA and 800 cm-l (1140 cm-') for DBA in heptane, where the numbers in parentheses are the corresponding values deduced from eq 4 using our values as the references. Since they have observed the S1 So fluorescence, the TI Sophosphorescence, and the T, T, fluorescence spectra, their energy difference might correspond to the energy gaps between the T, and S1
-
-
-
--
@TTF
-
= @isckrT,-T17T,
(5)
where cPkc is the quantum yield of S1 T, isc, IZ'T,-T~ is the radiative decay rate constant from TJ3B1,) to T1(3Bh), and TT. is the T, lifetime. A t room temperature in 3-MP, aimcan be estimated to be 0.99 for BA and 0.91 for DBA from our measurements of the S1 Sofluorescence decay. Although the photochemical sensitization studies of Liu and c o - ~ o r k e r have s ~ ~ suggested ca. 200 ps for the T, lifetimes of several meso-anthracenes, the time evolutions of the T, T1 absorptions of BA and DBA can be fitted with high precision to a single exponential function with buildup times of 150 ps for BA and 1.0 ns for DBA in acetonitrile.I6 Since these values correspond to the decay times of the S1 states, i.e., 160 ps for BA and 1.27 ns for DBA in acetonitrile, one can say that the rate constant for internal conversion in the triplet manifold is very large compared with that of isc. Thus we would like to estimate N 10 ps for the T, lifetime given by Kokubun et al.,24who S1 inhave determined the quantum yields of the T, tersystem crossing of anthracene, 9-methylanthracene, 9-phenylanthracene, 9,10-dichloroanthracene,and 9,lOdibromoanthracene by a double excitation method. The choice of parameters of apiec, TT,, and krTn-T, (=2 X lo5-6 X lo5 s-1)5v6 gives = 2 X 104-6 X lo4 for BA and DBA, giving good agreement of calculated and experimental am of DBA. However, we still cannot explain the observed of the other anthracenes, that is, the discrepancy between calculated and experimental am are on the order of 10 to 100. Although some possible reasons for this discrepancy are discussed by Gillispie and Lim,15 we are still unable to check the experimental result of am in detail because of insufficient experimental data, but qualitative agreement is obtained for DBA. It will be possible to analyze the experimental a m quantitatively, after more precise information on the T, lifetimes and the order of the higher triplet states lying near the lowest excited singlet state are obtained.
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Acknowledgment. The authors express their sincere thanks to Professors Yuji Mori and Kinichi Obi, Tokyo Institute of Technology, Professor Mamoru Jinguji, Yamanashi Medical College, and Professor Hiroshi Teranishi, Dr. Toshihiro Nakayama, and Mr. Toshiharu Hidaka, Kyoto Institute of Technology, for their valuable help and discussion. Registry No. 9,10-Dibromoanthracene, 523-27-3. ~~~
~
~~~
~
~~
(23)R.S. H.Liu and P. E. Kellog, J.Am. Chem. SOC.,91,250(1969); R.S.H.Liu and J. R. Ed", ibid., 91,1492(1969);R. 0.Campbell and R. S. H. Liu, ibid., 95,6560(1973);C. C. Ladwig and R. S. H. Liu, ibid., 96,6210 (1974). (24)S. Kobayashi, K. Kikuchi, and H. Kokubun, Chem. Phys., 27,399 (1978).
