Fluorescent Study of Dichlorobenzenes
cm3/mol) and the Ag,P2O7 crystal (114.1 ~ m ~ / m o lSuch ).~~ dense packing of constituents is one of the structural features of the present glasses. Acknowledgment. The authors thank the personnel of the Radiation Center of Osaka Prefecture for permitting the use of the DS-701 IR spectrophotometer. They thank also Dr. S. Yamanaka for valuable discussions, especially on TLC. This work was supported by a Grant-in-Aid for Developmental Scientific Research from the Ministry of Education of Japan, and by the Asahi Glass Foundation for Contribution to Industrial Technology. Miniprint Matericul Available: Full-size photocopies of Table I1 (1page). Ordering information is available on any current masthead page.
References and Notes (1) T. Minami, Y. Takumie, and M. Tanaka, J . Electrochem. SOC.,124, 1659 (1977).
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(2) H. Rawson, "Inorganic Glass-Forming System", Academic Press, London 1967. (3) H. Kawazoe, M. Ikeda, and T. Kanazawa, Yogyo Kyokai Shi, 77, 23 (1969). (4) J. R. Van Wazer, "Phosphorus and Its Compounds", Vol. 1, Interscience, New York, 1958, Chapter 12. (5) S. Utsumi and M. Shima, Nippon Kagaku Zasshi, 81, 1626 (1960). (6) T. Yamabe, T. Iida, and N. Takai, Bull. Chem. SOC.Jpn., 41, 1959 (1968). (7) C. S. Hanes and F. A. Isherwood, Nature(London),164, 1107 (1949); E. Karl-Kroupa, Anal. Chem., 28, 1091 (1956). (8) D. E. C. Corbridge, Top. Phosphorus Chem., 6, 235 (1969). (9) D. E. C. Corbridge and E. J. Lowe, J . Chem. SOC.,493 (1954). (10) W. Bues and H. W. Gehrke, 2.Anorg. Allg. Chem., 288, 291, 307 (1956). (11) See paragraph at end of text regarding miniprint material. (12) T. Minami, H. Nambu, and M. Tanaka, J . Am. Ceram. SOC.,60, 283, 467 (1977). (13) T. Minami, T. Katsuda, and M. Tanaka, J . Non-Cryst. Solids, 29, 389 (1978). (14) D. A. Johnson, "Some Thermodynamic Aspects of Inorganic Chemistry", Cambridge University Press, London, 1968, pp 37, 41. (15) R. C. Weast, Ed., "Handbook of Chemistry and Physics", 54th ed, The Chemical Rubber Co., Cleveland, Ohio, 1973, pp F-194, 8-134-5.
Fluorescence Quantum Yields and Lifetimes of Dichlorobenzenes Akira Shimoda, Takumi Hikida, and Yuji Morl" Department of Chemistry, Tokyo Institute of Technology, Ohokayama, Meguroku, Tokyo, Japan (Received December 1 I , 1978)
The fluorescence spectra, the fluorescence quantum yields, and the fluorescence lifetimes of 0-,rn-, and p-dichlorobenzene vapor were measured at various excitation wavelengths. By 0-0 excitation, the collision-free fluorescence quantum yields and lifetimes are determined as follows: 0.042, 1.7 ns for p-dichlorobenzene; 0.016, 1.3 ns for m-dichlorobenzene;and 0.007,1.4 ns for o-dichlorobenzene. It is noted that the nonradiative transition rate is slow and the radiative transition rate is fast in p-dichlorobenzene when the excess vibrational energy is small. The difference in the fluorescence quantum yields and lifetimes of three isomers vanishes when the excess vibrational energy is more than 2000 cm-I.
Introduction The improvement of the technique for measuring a weak fluorescence enables us to study the collision-free fluorescence quantum yields and lifetimes of single vibronic levels of isolated molecules. Benzene has been the most extensively studied mo1ecule.l Deuterated benzenes have also been studied in detail and the experimental results are explained ,as the number of deuteration. The fluorescence lifetime in p-C6H4D2was slightly longer than that of 172-C&D2,2 implying that para-disubstituted benzene has less effective acceptors of excess energy in nonradiative transitions. In substituted benzenes with one or two halogen atoms electronic transitions of low energy become allowed by symmetry and radiative lifetimes decreases as the electronic wave functions are changed by substitution. The fluorescence quantum yield of the vibrationless level of p-C6H4F2is exceptionally large,314 indicating a small nonradiative transition rate compared with radiative transition. It has also been reDorted that the self-quenching cross section of p-C6H4F2is very large (nu2 N 400
A').!
Substitution 'of chlorine atoms into benzene induces fast nonradiative transitions due to increased spin-orbit interactions. Since the C-C1 bond dissociation energy is smaller than the excitation energy to the lowest excited singlet state, nonradiative processes must involve C-C1 bond cleavage as well as intersystem crossing or internal conversion. Weakness of the fluorescence intensity has hampered detailed studies on the dynamics of electron'
0022-3654/79/2083-1309$01 .OO/O
ically excited chlorine substituted benzenes though these molecules are of strong interest as an example of a system where various fast nonradiative processes occur competitively. A rather large quantum yield for photodecomposition and a very small quantum yield for fluorescence have been reported indicating that nonradiative transitions are much faster than fluorescence and that, with a large probability, excited molecules undergo dissociation into phenyl radicals and chlorine atom^.^^^ In this paper we will report the fluorescence spectra, fluorescence quantum yields, and fluorescence lifetimes of 0-,rn-, and p-C6H4C12 under collision-free condition. The pressure effects of ethane are also studied.
Experimental Section UV absorption spectra of dichlorobenzene vapor have been assigned in detail.8t9 Infrared and Raman spectra of dichlorobenzenes in the liquid and solid phases have also been extensively studied.lGl3 Although there are some ambiguities in the vibrational assignments, we choose the symmetry axis and the mode number as used commonly.loJ4 The vibrational assignments in the ground and excited states are shown in Table I. The excitation wavelength in this study is selected at the absorption peak^.^^^ When dichlorobenzenes are excited in this wavelength region, C-C1 bonds dissociate with very large quantum ~ i e 1 d s . l ~ The apparatus and techniques employed in the measurements of fluorescence spectra, quantum yields, and 0 1979 American Chemical Society
1310
The Journal of Physical Chemistry, Vol. 83, No. 10, 1979
A. Shimoda, T. Hikida, and Y. Mori
TABLE I: Vibrational Frequencies (cm-' ) of Dichlorobenzenes in Ground and Excited States
c2 ,(metal
D2h (para)
-
vib 1
sYm a,
gr
1096 307 0 747 626 328 3065 1574 125 1090
ex 1051
C,,(ortho)
SYm
gr
ex
a,
1124 3074 663 784 399 425 1576 778 999
1104
125 528136 244 1461/66
lifetimes have been described A slight change is made for the measurement of weak emission, that is, a bull-horn-shaped cell made of brass with two fused silica windows. Thus, the emission from the quartz window is minimized. The emission quantum yields are determined relative to that of na~htha1ene.l~ Lifetimes are measured by a single photon counting system.16 A coaxial self-triggered light pulser with tungsten electrodes (6 mm in diameter) is used. Larger electrodes are for stable long time operation (-10 h). The light pulses dispersed through a 0.5-m monochromator (Jasco CT50) with a bandwidth 2.8-3.5 nm excite the sample. The fluorescence is detected by a photomultiplier (HTV-R106) through a pyrex filter. The exciting light pulse has a width of about 4 ns and the pulse repetition rate is 3 X lo3 pulses/s. The fluorescence decay is well deconvoluted assuming a single exponential decay. Lifetimes as short as 0.9 ns can be measured with an accuracy of -0.1 ns. 0,m, and p-dichlorobenzenes are obtained commercially. p-Dichlorobenzene is recrystallized from an ethanol solution and sublimated repeatedly under vacuum. 0- and m-Dichlorobenzenes are distilled several times. Ethane is degassed a t 77 K. The samples are handled in a greaseless, Hg-free vacuum line. Through all experimental runs the vapor pressures of 0-,m-, and p-dichlorobenzenes are kept constant at their vapor pressures at 0 " C (0.2,0.4, and 0.2 torr, respectively).
Results Fluorescence Spectra. A low resolution fluorescence spectrum of p-dichlorobenzene (0.2 torr) excited at 279.7 nm (0-0 transition) is shown in Figure 1. Although the excitation bandwidth of 3.5 nm may excite some molecules to the levels with excess vibrational energy, the fluorescence spectrum shown in Figure 1may be considered as the emission spectrum from the lowest vibrational level of the excited state lBsU.Some vibrational structure can be seen, Since the pressure of p-dichlorobenzene is 0.2 torr and the lifetime is short (-1.7 ns) as will be seen later, the excited molecules are under collision-free conditions. The vibrational structure is diminished when the molecules are excited in the presence of added gases, or are excited a t shorter wavelengths. The fluorescence spectra of 0- and m-dichlorobenzenes excited with 0-0transitions also have vibrational structures as shown in Figures 2 and 3. The effects of excitation wavelengths and of foreign gas on the spectrum are similar to those in case of p-dichlorobenzene. The vibrational structures appearing in Figures 1-3 are assigned and summarized in Table I. Fluorescence Quantum Yield. The fluorescence quantum yields of dichlorobenzenes at various excitation wavelengths are shown in Table I1 along with the effects of the pressure of the added gas ethane. The fluorescence quantum yield of p-dichlorobenzene exhibits the highest
a1 a, a1
378
960
b, a1 bl a, b; bl
gr
ex
1129 3070 660 740 480 429 1575 752 1038
1089
b
610 307 1486 958
1 tI
310
300
290
280 nrn
Flgure 1. Fluorescence spectrum of pdichlorobenzene vapor (0.2 torr) excited at 279.7 nm which corresponds to the 0-0 transition. The e, l;,f, transitions are as follows: a, (sa)!; b, 1:; c, (sa);; d, (6a);l: (6a)il;. The arrow indicates scattered excitation light. The resolutions of the excitation and detection monochromators are 3.5 and 2.0 nm, respectively.
, I 340
320
300
280 nrn
Figure 2. Fluorescence spectrum of m-dichlorobenzene vapor (0.4 torr) excited at 276.3 nm (0-0transition\ transitions are as follows: a, (ea)!; b, l;,(sa):, c, (6a):lt c, (6a),I,; d, 1:. The arrow indicates scattered excitation light. The resolutions of the excitation and detection monochromators are 3.5 and 2.0 nm, respectively.
p
value (4.2 X when it is excited at the 0-0 transition. The value with excitation a t the 0-0 transition of m-diand that of o-dichlorobenzene chlorobenzene is 1.6 X is 7.3 X These values decrease with an increase in the excitation energy (p-dichlorobenzene decreases most
The Journal of Physical Chemistty, Vo/. 83, No. 10, 1979
Fluorescent Study of Dichlorobenzenes T A B L E 11: A, nm
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Fluorescence Quantum Yields and Lifetimes of Dichlorobenzenes
--
@*(x transtn
@Ax l o 3 )
r f , ns
rr, ns
rnr, ns
50
l o 3 )at ethane pressure (torr) 100
200
3 00
400
33.9 16.2 11.2 5.8 3.0 2.2
31.2 16.3 12.2 3.1 2.1
30.7 16.5 12.5 5.9 3.1 2.1
13.2 5.7 4.0 4.6 2.5 2.3
13.2 5.7 4.2 5.0 2.7 2.4
13.0 5.4 4.3 5.3 2.9 2.7
6.8 5.6 3.8 2.8 2.5 2.3
6.7 5.5 3.9 3.1 2.6 2.4
6.5 5.6 4.1 3.3 2.7 2.6
p-Dichlorobenzene (0.2 torr Bandpass, 35 A )
279.7 271.7 266.4 261.3 253.7 250.3 245.0 240.0 235.0 230.0 225.0 220.0
0- 0 1: ( 68 1: 1: (8a)AlA
1,'
42.2 16.5 9.9 4.5 2.6 2.2 2.0 1.7 0.2 0.05 0.02 0.02
1.7 1.7 1.3 0.9
40 102 126 191
1.8 1.7 1.3 0.9
38.0
37.9 16.5 11.8 6.1 2.9 2.1
10.9 5.2 2.8 2.1
m-Dichlorobenzene (0.4 torr Bandpass, 28 A )
276.3 269.0 262.2 255.7 250.0 245.0 240.0 235.0 230.0 225.0 220.0
16.1 5.8 4.0 4.3 2.3 2.1 2.0
1.3 1.1 1.0
83 173 264
1.3 1.1 1.0
14.7 5.8 3.9
13.8 5.8 4.0 4.3 2.3 2.1
1.8 0.25 0.04 0.04 o-Dichlorobenzene (0.2 torr Bandpass, 35 A )
275.9 268.8 261.2 254.2 250.0 245.0 240.0 235.0 230.0 225.0
0-0 1% 12;:t: 12:; 1:
7.3 5.7 3.5 2.5 2.1 2.0
1.4 1.3 1.0
189 228 283
6.8 5.3 3.6 2.7 2.3 2.1
1.4 1.3 1.0
2.0 1.2 0.3 0.07
40
t
P
101
0
a
Flgure 3. Fluorescence spectrum of odichlorobenzene vapor (0.2 torr) excited at 275.9 nm (0-0 transition). The transitions are as follows: a, (Sa)? b, l:, (6a):; c, (6a);l: d, 1:. The arrow indicates scattered excitation light. The resolutions of the excitation and detection monochromators are 3.5 and 2.0 nm, respectively.
Flgure 4. Fluorescence quantum yield vs. excess vibrational energy for (0)~ - C ~ H & I Z (0) , m-C6H& and (A)O-C6H&.
rapidly), and nearly coincide when the excess vibrational energy is about 2000 cm-'. With further increase in the excitation energy till about 6000 cm-l the decreasing trends are similar and rather slow. Under excitation with excess energy more than 601)o cm-l the molecules are brought into the Szstate and the fluorescence quantum yields decrease to very low values,
At the lowest excitation energy, the fluorescence quantum yields of dichlorobenzenes decrease with an increase in the pressure of ethane. At higher excitation energy the fluorescence quantum yields of dichlorobenzenes tend to increase as the ethane pressure increases. Lifetimes. The fluorescence lifetimes of three dichlorobenzenes are listed in Table 11, where those in the
0
4000 8000 A E ( cm-' )
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The Journal of Physical Chemistry, Vol. 83,
.l 'i h
I
.
0' 0
No. 10, 1979
1000
2000 A E ( cm-1)
I
3000
Figure 5. Nonradiative lifetime vs. excess vibrational energy for (0) p-C&Clp, (0) m-CeH&&, and (A) o-CaH,+Ci,.
presence of ethane are also shown. The radiative lifetimes (TJ and nonradiative lifetimes ( 7 3 in Table I1 are calculated using the observed fluorescence lifetimes and quantum yields. The lifetimes of dichlorobenzenes decrease with an increase in the excitation energy. The radiative lifetimes increase as the excitation energy increases. The nonradiative transition lifetimes, however, decrease with an increase in the excitation energy and this decreasing trend is remarkable in p-dichlorobenzene as has been reported for p-difluor~benzene,~,~
Discussion In p-dichlorobenzene, the radiative transition is faster than those in 0- and m-dichlorobenzenes, By reduction of molecular symmetry from DBh in benzene to D2,, in p-dichlorobenzene or Clv in 0- and m-dichlorobenzenes by the substitution of hydrogen atoms with two chlorine atoms, the S1 So transitions in dichlorobenzenes are electronically allowed. For disubstituted benzenes, the radiative lifetimes of the S1 So transitions are generally shorter in para-disubstituted benzene^.^^^ The radiative lifetime in p-difluorobenzene (11.3 or 16.1 ns)334is shorter than that in m-difluorobenzene (29.3 ns). The oscillator strengths of dichlorobenzenes have been calculated from their absorption spectra to be 0.0040, 0.0041, and 0.0068 for 0-, m-, and p-dichlorobenzenes, respectively.18 These values lead to radiative lifetimes for 0 - , m-, and p-dichlorobenzene of 290, 280, and 170 ns, respectively. The values in Table I1 are in qualitative agreement with the tendency of calculated radiative lifetimes, though the observed values of radiative transition rates are few times shorter than those calculated from absorption spectra. The difference between the calculated lifetimes from absorption spectra and the observed radiative lifetimes is also acknowledged in difluor~benzenes.~,~ The cause of this difference is not clear but could be due to a change in the molecular structure by excitation. The larger fluorescence quantum yield in p-dichlorobenzene is attributed to the faster radiative transition and to the slower nonradiative transition than 0- or m-dichlorobenzene. The nonradiative transition rate of p-dichlorobenzene excited a t its 0-0 absorption band is much slower than those of 0- and m- dichlorobenzene and the difference decreases with an increase in the excitation energy. The nonradiative transition rate of p-difluorobenzene is also much slower than that of m-difluor~benzene.~J~ This could be attributed to the slower S1 T1intersystem crossing rate in para-disubstituted benzenes with halogen atoms. The substitution of hydrogen atoms by chlorine atoms induces strong spin-orbit interactions in molecules. The higher molecular symmetry D2,, of p-dichlorobenzene, however, would restrict the symmetry species of vibrational levels in the triplet manifold coupled with the lower vi-
-
-
-
A. Shimoda, T. Hikida, and Y. Mori
brational levels of S1. In 0- or m-dichlorobenzene this restriction seems to be removed to some extent by the reduction of symmetry. With an increase in the vibrational energy in S1, the prepared fluorescent state contains various modes of vibrations, since fluorescence spectra have no vibrational structures. Especially in p-dichlorobenzene, the vibrational excitation in the S1state reduces the symmetry restriction of S-T coupling. Therefore, an increase in the vibrational energy in SI leads to an increase in the number of vibronic levels in the triplet manifold which can be coupled with the SI vibronic levels, because of symmetry and also Franck-Condon factors. The increased nonradiative transition rate in dichlorobenzenes, especially in p-dichlorobenzene, decreases the difference in the nonradiative transition rates of three isomers of dichlorobenzenes. The nonradiative transition rates in dichlorobenzenes are similar when the molecules are excited with an excess energy of about 2000 cm-l as shown in Figure 3. Increase in the excitation energy with excess energy more than 2000 cm-' above the 0-0 transition decreases the nonradiative lifetimes with nearly the same rate for three isomers of dichlorobenzene. The electronic interaction between the singlet and the triplet states is determined mostly by the interaction in chlorine atoms and seems to have similar magnitude among the dichlorobenzenes. Franck-Condon factors for singlet-triplet crossing and the level density in the triplet manifold in the region of higher excitation energy cannot have very different values among the dichlorobenzenes. The structureless spectrum in the presence of a foreign gas can be explained by the random distribution of vibrational energy due to collision with foreign gas molecules. The collision-induced vibrational relaxation in prepared SI states results in a slight increase in the fluorescence quantum yield a t higher excitation, while a t lower excitation the collisional vibration relaxation leads to a slight decrease in the fluorescence quantum yield. As the pressure of ethane increased, the distribution of the emitting states gradually approaches thermal equilibrium by vibration relaxation. Collisional quenching of electronic excitation seems to be very small. Collision with the foreign gas molecules can induce radiationless transitions to nonemitting levels. However, since the intramolecular nonradiative transitions in dichlorobenzene are fast, the collision-induced interaction with the nonemitting levels can contribute only slightly to the rate of nonradiative transition. Thus, only vibrational relaxation could be detected without strong electronic quenching by collision with a third body.
References and Notes (1) K. G. Spears and S. A. Rice, J. Cbem. Pbys., 55, 5561 (1971). (2) C. Guttman and S. A. Rice, J. Cbem. Pbys., 61, 651 (1974). (3) C. Guttman and S. A. Rice, J . Cbem. Pbys., 61, 661 (1974). (4) L. J. Volk and E. C. Lee, J. Cbem. Pbys., 67, 236 (1977). (5) L. J. Volk and E. C. Lee, J . Cbem. Pbys., 67, 242 (1977). (6) T. Ichimura and Y. Mori, J . Cbem. Phys., 58, 288 (1973). (7) T. Ichimura, T. Hlkda, and Y. Mori, J. Cbem. f'bys., 63, 1445 (1975). (8) H. Sponer, Rev. Mod. Pbys., 14, 224 (1942). (9) T. Anno and I. Matubara, J . Cbem. Pbys., 23, 796 (1955). (IO) J. R. Scherer and J. C. Evans, Specfrocbim. Acta, 19, 1739 (1963). (11) K. M. M. Kruse, J . Cbem. fbys., 25, 591 (1956). (12) M. C. Tobin, J . Pbys. Cbem., 61, 1392 (1957). (13) R. M. Hexter and D. A. Dows, J. Cbem. Pbys., 25, 504 (1956). (14) G. Vargnyi, "Vibrational Spectra of Benzene Derivatives", Academic Press, New York, 1969. (15) To be published. (Decomposition quantum yields at 253.7nm are 0.95,0.92,and 0.60for p-, m-, and odichlorobenzene, respectively.) (16) S. Yagi, T. Hikida, and Y. Mori, Cbem. Pbys. Left., 56, 113 (1978). (17) J. C.Hsieh, C. S. Huang, and E. L. Lim, J. Cbern. Pbys., 60, 4345 (1974). (18) H. Sponer, J . Cbem. Pbys., 22, 234 (1954). (19) H. G. Rockley and D. Phllips, J. Pbys. Cbem., 76, 7 (1974).
