RADIATIVEAND RADIATIONLESS PROCESSES IN AROMATIC MOLECULES
693
Radiative and Radiationless Processes in Aromatic Molecules.
Coronene
and Benzcoronenel by William R. Dawson and John L. Kropp Chemical Sciences Department, T RW Systems, Redondo Beach, California
(Received August 9,1808)
The lifetimes of fluorescence and phosphorescence as well as the quantum yields of fluorescence, phosphorescence, and triplet formation have been measured a t temperatures between - 196 and 23" for samples of coronene-hlz, coronene-dlz, and benzcoronene in poly(methy1 methacrylate). The rate constants for radiationless and radiative deactivation of the lowest excited singlet and triplet states S1 and T I have been calculated from the lifetimes and quantum yields. The rate constants for radiative deactivation of both Sl and T1 are constant between - 196 and 23" for coronene-h12, coronene-42, and benzcoronene. Rates of intersystem crossing from S1 to TI are also insensitive to variation of temperature. The radiative rate constants for depopulation of T I are the same for coronene-h12and coronene-d12but the radiative rate constant for depopulation of SIis 7 7 , higher for coronene-h12 than for coronene-dll. No significant radiationless deactivation of S1 directly to the ground state has been found for coronene-h12 or coronene-dlz between -196 and 23". However, in the case of benzcoronene direct radiationless deactivation of 81 to the ground level does occur a t 23' but not a t -196'. This is associated with the decrease of the fluorescence lifetime of benzocoronene with increasing temperature. An activation energy of 514 cm-1 for radiationless deactivation of SIcan be obtained from the fluorescence lifetimes.
Introduction Processes by which molecules relax from their excited states are of great interest. Models and theories to explain the radiative and radiationless internal conversion processes in aromatic molecules have been postulatedUzd4 Accurate specific rates of the various processes may be useful in testing these various theories of radiationless transitions. What are needed to calculate the various specific rates are measurements of the fluorescence yield, @F, the phosphorescence yield, QP, the triplet yield, @T> the fluorescence lifetime, TF> and the phosphorescence lifetime, 7p. We have developed techniques for measuring these parameters as a function of temperature and thus determining the variation of specific rates as temperature is varied. Initially, coronene-hl?, coronene-dlz, and benscoronene dissolved in poly(methy1 methacrylate) (PMM) have been studied. We had originally studied delayed fluorescence in coronene-hlz and coronene-dlz.K I n that work we assumed that @F was constant with temperature. It was desirable to check this assumption experimentally. We were also interested in studying radiationless processes as a function of temperature. Coronene has a long singlet and triplet lifetime and a large phosphorescence yield, hence it is a convenient compound for these studies also. Benzcoronene was also studied to compare results with those obtained for coronene. PMM was chosen as a matrix since it has n o first-order phase transitions between - 196 and 100'. Thus it presents a relatively stable environment to the aromatic hydrocarbon.
Determination of Rate Constants Figure 1 shows the energy level diagram for a typical hydrocarbon. The paths of radiative and radiationless
deactivation of the lowest excited singlet SI and the lowest triplet T1 are given together with the rate constants, I n Figure 1, kl is the rate constant for fluorescence, kz that for deactivation of SI directly to ground, and k3 that for intersystem crossing from SI to TI; ka is the rate constant for triplet emission and kg the rate constant for radiationless deactivation of the triplet. The following equations give the relation of the measured parameters to the rate constants determined. @F @F
= (hl =
+ k2 + k3)-l;
TP = (k4
@T = k 3 T F ;
k1T.F;
@P
=
+
kb)-'
k4GTT.P
The individual rate constants can be separated from the above relations so that they can be evaluated from measured parametem6 kl =
(PF7F-l
Icz = (1 -
ka kq = IC5
@F
1/7F,O
(1)
- @T)TF-~
(2)
=
@pq~-lrp-l
= (1
(3)
= TF@T-~
(4)
= l/rp,o
- @pQT-')rp-1
(5)
where rF,o and T P , ~are the radiative lifetimes, respectively, of phosphorescence a n d fluorescence. By using these equations together with the observed (1) Supported by the Office of Naval Research under Contract N00014-67-CO327. (2)(a) G. W. Robinson and R. P. Frosch, J. Chem. Phys., 38, 1187 (1963); (b) M. Gouterman, ibid., 36, 2846 (1952). (3) E. F. McCoy and I. G. Ross, Aust. J. Chem., 15, 573 (1962); G. R . Hunt, E. F. McCoy, and I. G. Ross, ibid., 15, 591(1962). (4) 8. H. Lin, J. Chem. Phys., 44,3759 (1966). (5) J. L. Kropp and W. R. Dawson, J. Phys. Chem., 71,4499 (1967). (6) M. W. Windsor and W. R. Dawson, Mol. Cryst. 4,253 (1968). Volume 73, Number
S
March 1969
WILLIAMR. DAWSON AND JOHNL. KROPP
694
Figure 1. State diagram of aromatic molecules and the first-order rate constants for deactivation of the lowest excited singlet and triplet states, SIand TI.
values for the parameters, the various rate constants can be determined.
Experimental Section Materials and Xanaple Preparation. The purity of coronene-hlz and of coronene-drz used were the same as beforeS6 Benzocoronene was used as received from Rutgerswerke. Pn!tRf samples were prepared as previously described6 Measurement of @F, (Pp,rp, and T F . The method used for determining the fluorescence yield @F and phosphorescence yield @P between - 196 and 23" is a modification of the technique used for measuring $F and @p, of aromatic compounds in PRII\I a t room temperature and described in detail elsewheree6 The essential features of this method are summarized here. Luminescence, emitted from the front face of a PMM sample upon which the excitation light impinges, is detected with an EM1 9558B multiplier phototube. $F and @P are determined from two measurements : (1)the equilibrium phototube output due to fluorescence and phosphorescence detected together with the excitation light on, and (2) the output due to phosphorescence detected immediately after the excitation is shut off. A PMM sample containing 1.0 X M pyrene is used as a luminescence standard at 23" ; Melhuish determined that (PF for pyrene in PR4M has a value of 0.61.' The above method must be modified to permit measurements below room temperature. Figure 2 shows the cell used. The outer cell consists of a Pyrex top, R, and a square-section quartz bottom, A, 1.5 in. on a side. Tube J is joined to part B at the top and to a copper block E, at the bottom by a Pyrex-Kovar seal D. Tube J serves as a reservoir for liquid nitrogen or other coolant. The copper block contains a hole with a 0.50-in. aperture F in the front face. The Ph4M sample G is placed in the copper block and a back plate, M, is screwed on to hold it in place. The copper block is then put reproducibly into the bottom section A by mating B and A. The two sections are mated at a flat, ground seal and are clamped together reproducibly The Journal of Physical Chemistru
Figure 2. Front and side views of the cell for measurement of fluorescence and phosphorescence yields between, - 198 and 23".
with a yoke. The copper block is positioned so that the front edge of the PMM sample is less than 2 mm from the quartz surface. This arrangement eliminates scattering of excitation and fluorescence or phosphorescence light by the corners of the quartz tube. The PnfM sample is cooled by pouring liquid nitrogen through the tube J into the reservoir in the copper block. The temperature of the sample is determined by using a copper-constantan thermocouple wire L whose junction is in the copper block. The entire apparatus is evacuated through K to maintain a low temperature. A nichrome heating wire H attached to the outside of the quartz bottom prevents frosting of A. The cell in Figure 2 was made to be compatible with the apparatus for measuring luminescence yields. Care was taken that the PMM samples were not exposed to oxygen from the time they were being prepared until the measurements were completed. The PMM samples in their original evacuated tube and all cell components were put into a nitrogen-filled glove bag. The PMM samples were then broken from the tube and quickly ground and polished. They were put into the copper block and the entire cell assembled in the glove box. This prevented absorption of atmospheric oxygen by the PMRf samples. Oxygen results in triplet quenching and reduced phosphorescence yield. The cell was constantly evacuated during the measurement by attaching it to a vacuum pump at K. Values of @F and @p are determined from comparison of the luminescence signals from the pyrene standard at 23" to those of PnlM samples containing coronene or benzcoronene at temperatures between - 196 and 23". (7) W. H.
Melhuish, J. Opt. SOC.Arnw., 54, 183 (1984).
RADIATIVE AND RADIATIONLESS PROCESSES IN AROMATIC MOLECULES These luminescence signals are corrected for the variation of the EM1 9558B phototube sensitivity with wavelength, The method used for calibrating the relative response of the phototube between 3000 and 7000 A has been described elsewherea8 The experimentally determined response curve decreased more rapidly beyond 4000 A than is reported for the 5-20 photo ~ u r f a c e . ~The wavelengths of phosphorescence and fluorescence is also needed to assign detector response factors. The fluorescence and phosphorescence spectra of coronene have been publishede6 The fluorescence of benzcoronene in P M h l is between 4300 and 5000 A and the phosphorescence is between 6000 and 6500 Values of the triplet lifetime T P were obtained from oscilloscope traces showing the decay of phosphorescence after the excitation light is shut off. Values of TF were determined using a TRW nanosecond spectral source.'O Lifetimes were determined from oscilloscope traces using a Tetronix Model Type 564 storage oscilloscope. The sample was placed in a square-section quartz dewar surrounded by a liquid temperature bath. For temperatures between - 196 and 23O, 3-methylpentane was used as a thermal bath. Above room temperature water was used. Measurement of @T. A method has been described for measuring the triplet yield, @T, of compounds in EPA at -196" from separate measurements of the excitation coefficient of triplet absorption ET and of ET~PT." The ET spectrum is obtained from a detailed correlation of the measured spectrum between 3200 and 7000 of the extinction coefficient of singlet absorption, ES, with the measured spectrum of the change in absorbance, AA , arising from T-T absorption associated with the excitation of molecules to the triplet state. The value of ET@T for a wavelength, A, is obtained from measurement of the initial rate of increase of AA at X resulting from absorption of excitation photons using eq 6.
695
E . --
A.
AA/I
=
~OOOET@TTP[~- exp( -t/.p)]
(6)
where I is the excitation intensity and t is the time after turning on the excitation beam. The method of measuring @T must be modified for measurements of compounds in PMM for two reasons. First, P M M absorbs below 3400 A and care must be taken that this absorption does not give rise to large error in determination of @T. Second, the PMM is a solid throughout the entire temperature range. EPA is liquid a t 23" and could be poured into the cell for measurement of ET and ET@T. Thus, for the measurements of ~ T @ Tand ET in EPA the same cell can be filled with EPA and ferrioxalate actinomer solutions. However, for solid PMM samples a new holder shown in Figure 3 had to be designed. The modified cell and sample holder, C, are shown in Figure 3. The quartz cell, B, has 4 cm long evacuated end sections which protrude from the ends of a urethane
Figure 3. Front and side views of cell and sample holder used for measurement of the triplet yield at - 196 and 23'.
foam box, A. One end of the cell is cemented to the box; the other end is fitted to the box with a sliding seal of silicone casting resin. The 0.5 cm long cylindrical PMPM sample, E, with parallel faces is cut, ground, and polished under nitrogen in a glove bag and inserted into C which has a 0.50-cm2aperture D. The holder is connected to a quartz tube, F, with a 19/38 T inner joint. This assembly is inserted into the quartz cell in the nitrogen atmosphere. The joint on the mouth of the cell allows reproducible positioning of the aperture in the center of the cell. The cell was connected to a vacuum pump and evacuated before the temperature of the PMM sample was lowered to -196" by filling the urethane foam box with liquid nitrogen. Samples of 3.5 X M coronene and 7.0 X M benzcoronene in P M h l were used for measurement of ET. The 20% contraction that occurs during polymerization of methyl methacrylate was accounted for in calculating concentrations. No further correction is necessary a t low temperatures since PMM contracts less than 3% upon cooling. The absorption spectra of the specimens kept under vacuum in the cell were measured at -196 and 23" using a Cary 14 spectrophotometer. The absorption spectra of a 0.50-cm sample of pure PMM was also measured at these temperatures as a background reference. The E S spectrum at each temperature was determined from the absorption spectra of the samples. The AA spectra of these samples were then measured between 3300 and 7000 A with the same PMM specimens at temperatures of -196 and 23" using the cell shown in Figure 3, and the optical assembly described elsewhere.l1 These measurements did not have to be corrected for light absorption by the PMM since the attenuation by a 0.50-cm path of PMM of light wavelengths longer than 3300 is less than 2%, but in other (8) W. R. Dawson and J. L. Kropp,J. O p t . SOC.Amer.. 55,822 (1965). (9) J. Sharpe, I R E Trans. Nuclear Sci., NS-7, 44 (1960). (10) TRW Instruments (Model 31A Manosecond Spectral Source), (11) W. R.Dawson, J. O p t . SOC.Amer., 58, 222 (1968). Volume 78*Number 8 March
ioeo
WILLIAMR. DAWSONAND JOHNL. KROPP
696 cases where it may be necessary to measure AA spectra the sample must be shortened and the below 3200 concentration of the aromatic compound increased sufficiently so that light absorption by the PRIM is negligible at wavelengths studied below 3200
A,
A.
6000A'
5000AO
40COAO
7000A'
20,000
30
28
26
24
22
3 .-1 v110 Crvl 1
20
18
16
14
Figure 4. Triplet absorption of coronene-dlz in PMM: solid line, 23'; dotted line, - 196'.
The ET spectra of benzcoronene and coronene-cllz in PMRl are shown in Figures 4 and 5. The ET spectra of coronene-cllzand coronene-hlz are identical to within the experimental error. There is also little change in the ET spectra of coronene-hlz, coronene-cllz,and benzcoronene between 23 and -196". This contrasts with the behavior of the singlet absorption spectra in PMM samples, which become sharper and usually shift t o the red as the result of a decrease of temperature. 5000d
6000n' I
25,000
I
20,000
15,000
ET 10,000
5000
,
~ 1 1 0 ~
Figure 5. Triplet absorption of benzcoronene in PMM; solid line, 23'; dotted line, - 196".
Values of ETCDT were also measured a t - 196 and 23" with PlCllM samples in the Dewar cell shown in Figure 3. The rate of buildup of triplet absorption upon turning on a beam of 3130-A excitation light was measured. The wavelengths of T-T absorption moniThe Journal of Physical Chemistry
tored were 4813 A for coronene and 5680 hi for benzcoronene samples. The concentrations of the benzcoronene and coronene in the samples used in these measureand LOX M , respectively, ments were 3 X which was sufficient to absorb the 3131-hi excitation light completely. Deterioration of samples containing coronene-hlz and coronene-cllz in PMM at 23" by the 3131-hi excitation light was indicated by the decreasing rates of triplet buildup occurring during successive intervals of excitation. However, an accurate value of ET@T could be obtained if the rate of buildup of the triplet absorption of a coronene sample is measured during the initial exposure to excitation light before appreciable deterioration occurs. The intensity of the excitation light I was measured by ferrioxalate actinometry.12 The actinometer solution (0.15 M ferrioxalate) in a 1 mm thick quartz absorption cell was placed in a separate holder similar to C, also having a 0.50-cm2 aperture and a 19/ 38 T joint. This holder was positioned in the cell in Figure 3 so that the aperture was centered in the same place as that of the PMM sample. Consequently, the same flux of light which excited triplet absorption was absorbed by the actinometer solution. Estimates of Error. Values of T P are accurate to within 5% as determined from the linearity of the semilog plot of the decay of OD with time and the reproducibility of values of the lifetime from sample to sample. Using the same criteria to determine the errors in TF gives an accuracy of about 5% for those fluorescence lifetimes which are much longer than 100 nsec, as in this work. Values of @F and @p depend upon the value of @F used for the reference pyrene sample of 0.61. Assuming this value to be correct the values of @F are reproducible to within 10% and those of @P to within 20%. The larger errors in CDP arise because the signal due to phosphorescence is smaller. Errors in values of CDT have been detailed elsewhere." There is an additional source of error in plastic samples in that a t room temperature there is deterioration under uv excitation, However, this can be counteracted by using fresh samples. We estimate that errors in @T are about 10%. As a consequence of these precision limits, the errors in lcl, kz, and k3, are less than 1201, while uncertainty in values of lc4 and k5 are about 20%. However, the accuracy of the ratio of values of any of these rate constants for a compound taken at two temperatures will be greater than the accuracy of the value of the rate constant itself.
Results The fluorescence lifetimes of coronene-hlz and coronene-dlz in PMM as a function of temperature are given (12) C. G. Hatohard and C . A. Parker, Proc. Roy. Soc., A235, 518 (1966).
RADIATIVE AND RADIATIONLESS PROCESSES IN AROMATIC MOLECULES I
697
I
1
I
1
1
a
0.95
A
1100
@L
350
0.26
0
b ?P,O A
r
i
8
6
-200
b 0
-100
I
I
300
65
bI
A
1; i
+loo
4
r 0.40 5 0 op 0.30
20 -200
!
1
in Figures 6a and 7a, respectively. The lifetime of coronene-hlzin PMM is constant from 77 to 340°K with a value of 320 15 nsec. The lifetime of coronene-dlz is also constant over the same temperature range with a value of 355 rt 15 nsec. The solid lines in Figures 6a and 7a represent the averaged values of the lifetime data. The fluorescence yields of these two compounds are likewise shown in Figures 6a and 7a and are constant with temperature with average values of 0.27 for coronene-hlz and 0.28 for coronene-dlz. There is no change of @T with concentration between 3 X and 1x i o - 3 ~ . The phosphorescence yields and phosphorescence lifetimes are shown for coronene-hlz and coronene-d12in Figure 6b and 7b, respectively. Again, there is no change in @P as the concentration is varied from 3 X to 1 X M . The triplet yield was determined a t - 196 and 23" and is 0.68 and 0.64, respectively, for coronene-hlz. The values of @T for coronene-dlz are 0.67 and 0.66 for these same temperatures. The results for benzcoronene differ from coronene in that the values of the fluorescence yield @F and TF, as well as the phosphorescence yield and r p vary with temperature. The variations of T F and @F with temperature are shown in Figure 8a. The curve drawn in Figure 8a is given by eq 7. The parameters given in eq 7 represent a least-square fit of the fluorescence lifetime data. The corresponding data for @p and r p are shown in Figure 8b. The value of @T at room temperature for benzcoronene is 0.58. At 77°K the value of +T has
I
0
-100
t50
T lDEG C )
T (DEG C)
Figure 6. Variation of luminescence lifetimes and yields with temperature for coronene-hla in PMhI: a. 0, fluorescence lifetime, T F , and A, radiative lifetime of fluorescence, T F , O , left-hand ordinate; 0 , fluorescence yield, OF,right-hand ordinate. b. 0 , phosphorescence lifetime, TP, and A, radiative lifetime of phosphorescence, T P , ~ left-hand , ordinate; 0, phosporescence yield, @P, right-hand ordinate.
0.24
I
I
Figure 7. Variation of luminescence lifetimes and yields with temperature for coronene-dla in PMM: a. 0, fluorescence lifetime, T F , and A, radiative lifetime of fluorescence, T F , O , left-hand ordinate; 0 , fluorescence yield, @F, right-hand ordinate. b. 0 , phosphorescence lifetime, T P , and A, radiative lifetime of phosphorescence, T P , O , left-hand ordinate; 0, phosphorescence yield, Qp, right-hand ordinate.
*
1050i
40.95
0.40
0.35
3
0.30
rP, 0
2.5 -200
I
I
*A
I
-100
0
.04 +loo
T iDEG C )
Figure 8. Variation of luminescence lifetimes and yields with temperature for benzcoronene in PMM: a. 0, fluorescence lifetime, T F , and A, radiative lifetime of fluorescence, T F , O , left-hand ordinate; 0 , fluorescence yield, OF, right-hand ordinate. b. 0 , phosphorescence lifetime, T P , and A, radiative lifetime of phosphorescence, TP,O,left-hand ordinate; 0, phosphorescence yield, OP,right-hand ordinate. Volume 73,Number 3 March 1969
698
WILLIAMR. DAWSONAND JOHN L. KROPP ~~~~
Table I : Rate Constants for Depopulation of SIand TI in Coronene and Benzcoronene Coronenehta
Process, 8eo-t
kl
kz ka k4 k6 a
Since + E
+ +T
77OK
Coronene-dit
296'K
8 . 4 X lo6 8.4 X 106 1 . 6 X 106 2.8 X 106 2 . 1 x 105 2.0 x 106 0.017 0.018 0.086 0.141 = 1.05, is considered t o be zero.
77OK
7.8 X 106 1 . 4 X 106
1.9 x 105 0.017 0.012
increased t o 0.66. The value of @T of benzcoronene in EI'A glass at -196" is 0.64, which is very similar to that in PAlM. Thus the triplet yield of benzcoronene is more sensitive t o change of temperature than to change of solvent.
Discussion Using eq 1-5 we can calculate the rate constants for the various processes that depopulate singlet and triplet states in coronene-hl2, coronene-dlz, and benzcoronene. The rate constants for these compounds at -196 and 23" are tabulated in Table I. The values of kl were calculated at -196, -130, 80, -37, 2, and 23" using eq 1 and values of TF taken from curves fitted to TF values in Figures 6a, 7a, and 8a and the values @F measured at each temperature. The reciprocal of values of kl are presented as T F , in ~ Figures 6a, 7a, and 8a for the compounds studied. As can be seen, there is no change in k1 at any temperature for any compound. Some of the values determined for coronene and for coronene-dlz were estimated earlier.6 The values of @F and TF at 23" are slightly revised from our earlier work. The values of @F for coronene-hlz and coronenedlz have been increased by 20% from the earlier values. The reason for this difference is the use in this work of a better phototube calibration factor. I n earlier calculations the published wavelength sensitivity curve for the EM1 9558B multiplier phototube (S-20 response) was used. The present values of luminescence yields were calculated using a phototube that has been calibrated in these laboratories. As described in the experimental section this calibration curve is different than the published one. This results in the increase in the present values of fluorescence and phosphorescence yields. Similarly, the previous value of @F for benzcoronene at 23" of 0.2811is now determined t o be 0.33. The present value of T F of coronene-hlz in PRIM is also 6% higher than the previous value of 300 nsec.6 We believe these latter values t o be more accurate. The value of T F for coronene-dl2 has not previously been measured. I n our previous work it was assumed that TF was the same for coronene-hlz and coronene-dl2. Using these values kl is now measured to be 8.4 X lo6 sec-l for coronene-dl2 compared t o 7 X lo6 sec-' previously. Average values for T F between -196 and 60" for The Journal of Physical Chemistry
Benzcoronene--
296OK
7.8 X 106 1.7 X 106 1 . 9 x 106 0.017 0.033
77OK
9.0
x
106
a 1 . 5 X 108 0.025 0.21
296O
9.6 X lo6 2.6 X 106 1.7 X 106 0.025 0.32
coronene-hlz and coronene-dlz of 320 and 355 nsec, respectively, differ slightly from each other. This difference is outside the 90% confidence limits established for each value by taking the average of values at all temperatures. Values of $F of coronene-dlz are 4% larger than those of coronene-hlz and kl of coronene-hlz is approximately 7% larger than kl of coronene-dl2. Similarly, values of k8 for coronene-hlz are approximately 7 % higher than corresponding values for coronene4 2 . This indicates that the rates of both fluorescence and intersystem crossing are decreased in coronene-hlz compared t o coronene-dlz. The value of ICz is always less than 10% of the total decay rate from SI for coronene-hlz and coronene-dlz. In view of the experimental errors and the indirect method of determining kz as a small difference between large numbers, the probable error in our values of k z is such that we cannot rule out the possibility that k z may actually be zero. This is in agreement with previous results. Limlareported that k2 is zero within experimental error for benzene in an EPA glass at - 196". Similarly, Medinger and Wilkinson14 and Parker and Joyce16found that kz is zero for several aromatic compounds in fluid solvents a t room temperature. However, in PMM at 23", it appears that kz is not zero for many aromatic compounds such as benzcoronene. The rate constants derived for 1,2-benzcoroneneshow a different behavior with changing temperature than do those of coronene. The fluorescence lifetime and fluorescence yield of this compound are temperature dependent and decrease by about 20% from 77°K to room temperature. It can be seen from Figure 8a that values of T F and @F can be normalized to nearly fit the same curve indicating that kl for benzcoronene is temperature independent; kl has a constant value of 9.1 X lo6sec-l within 7% from - 196 t o 23'. This is shown by values of T F , ~shown in Figure 8a. The decrease in @T from -196 to 23" is only 12%. However, ks is still constant a t these two temperatures to within 12%. The value of @F @F = 1.05 at 77°K. This gives a negative value for kz. The best we can do is assume that kz is 0 at - 196", but the fact that values of TF, @F, and @T
+
(13) E.C.Lim, J. Chem. Phys., 36, 3497 (1962). (14) T. Medinger and F. Wilkinson, Trans. Faraday Soc., 61, 620 (1965). (15) C.A. Parker and T. A.Joyce, i b i d . , 62,2786 (1966).
RADIATIVE AND RADIATIONLESS PROCESSES IN AROMATIC MOLECULES all decrease between 12 and 20% as the temperature increases from -196 to 23", besides showing that the rate constants for fluorescence and triplet formation remain nearly constant with temperatures, implies that the temperature sensitive process observed is the Sl-So quenching of the excited singlet to ground. At room temperature the value of kz is estimated to be 2.6 X lo6 see-'. As in the case of coronene, this is not a reliable value due to experimental uncertainty and it is still quite small. However, with benzcoronene the parallel temperature dependence of the measured parameters is strong evidence that some radiationless deactivation of S1 to So occurs a t 23" and that kz increases with increasing temperature. The phosphorescence lifetime and yields all vary with temperature as shown in Figures 6b, 7b, and 8b. However, the value of k4 (Table I) is independent of temperature for both coronene-hlz, and benzcoronene. Values of k4 are the same within our experimental error for coronene-h12 and coronene-d12, but the value of kd for benzcoronene is 50% greater than that for coronene. Siebrand16 in his theoretical treatment has concluded that most hydrocarbons (including coronene) should have values of T P , ~= 2 0 4 0 sec ( k p = 0.050-0.025 sec-l), but experimental data here show that the value of k p can be much less than 0.025 sec-l and is only 0.017 sec-' for coronene. The value of ICs, is reduced upon deuteration in coronene-dlz compared to coronene-hlz. At -196" kS for
699
coronene-h12is 7 times that of coronene-dlz. However, a t room temperature this factor has decreased to 4. The value of k6 increases between -196 and 23" by a factor of 1.6 in coronene-h12,but by a factor of 2.2 in coronene-dl2. These results indicate a greater temperature sensitivity for radiationless depopulation of triplet coronene-dlz than coronene-hlz. This has been qualitatively noted previously.6 It is interesting that even for coronene-dlz at - 196" the radiationless quenching of the triplet plays an important role in deactivation of the triplet state. Radiationless quenching of the triplet stiIl accounts for 40% of the energy loss in coronene-dlz where H vibrations are removed. Benzcoronene fluorescence lifetime and yield are temperature dependent. Thelifetime data can be fitted to eq 7. The best fit of the T F data to the plot is given
- (l/n)
( ~ / T F )
=
A exp(-AB/RT)
(7)
in Figure 8a. The parameters that best fit the data are A = 7.6 X lo6sec-I and AE = 514 cm-l. This is a low activation energy. It is important to determine whether these temperature-dependent activation energies are intermolecular effects or dependent upon solvent properties. However, more work is needed in other plastic hosts to determine this. Such work is now in progress and will be reported elsewhere.l7 (16) W. Siebrand, J. Chem. Phys., 46,440 (1967). (17) J. L. Kropp and W. R. Dawson, t o be published.
Volume '7'8,Number 8 March 1060
