1747
RADIATIVE AND RAD~ATIONLESS PROCESSES IN AROMATIC MOLECULES
Radiative and Radiationless Processes in Aromatic Molecules. Pyrene’
by J. L. Kropp, W. R. Dawson, and M. W. Windsor Chemical Sciences Department, Systems Group of T R W , Inc., Redondo Beach, California (Received November 27, 1968)
90278
The fluorescence yield @F, triplet yield @T, and fluorescence lifetime T F for pyrene-hloand pyrene-dlo have been measured as a function of temperature. The phosphorescence yield Qp and phosphorescence lifetime r p have 0.8, @T 0.2, and (@F @T) ,- 1 for both pyrene-hlo and pyrene-dlo. also been determined. At - 196’ @F However, in pyrene-dlo the value of @T increases with increasing temperature, indicating that the rate constant for intersystem crossing has a temperature-dependent part. At room temperature (% %) is less than unity for both pyrene-hlo and pyrene-dlo, indicating that internal conversion from the lowest excited singlet directly into the ground state is occurring. This process is totally temperature dependent and is negligible at - 196’. The variation of fluorescence lifetime and triplet yield with temperature for pyrene-hlo is attributable in toto to this temperature-sensitive internal conversion; in pyrene-dlo both internal conversion and intersystem crossing are temperature dependent and contribute t o the observed variations. N
N
+
+
Introduction Properties of the excited singlet and triplet states of pyrene have been studied by a large number of workers. The fluorescence ~ i e l d ~ and - ~ lifetime,6 emission excimer formaspectra,6 T-T absorption tion,10 and delayed fluorescence have all been studied. l1 The data on pyrene have mostly been obtained in liquid solution or in a rigid glass, although the fluorescence yield3 and T-T absorption9 have been obtained in plastics. In liquid solutions the measurement of rate constants is complicated by viscosity effects and diff usion-controlled bimolecular processes such as the formation of excimers. A system minimizing such side effects would aid in understanding internal conversion in pyrene. The use of plastic media,12 together with techniques developed recently in our laboratory,13-15 makes it possible to measure the fluorescence yield, @F, the phosphorescence yield, @P, the efficiency of triplet formation, @T, the fluorescence lifetime, T F , and the phosphorescence lifetime, r p , all as a function of temperature and in a matrix that remains rigid over the entire temperature range studied. The study of the behavior of these parameters in pyrene with temperature may elucidate the mechanisms of deactivation of pyrene’s first excited singlet and triplet states. We have measured the values of $F, $T, $p, r p , and T F for pyrene-hlo and pyrene-dlo in poly(methy1 methacrylate) (PMM) at various temperatures between -196 and 23’. The values of these parameters can be related to the rate constants for depopulation of the excited singlet and triplet states of pyrene by the relation~~~ kl
k2
=
= a.FrF-1
[I - (@F f @T)]TF-l ka = a T 7 F - l
kq
k5
=
=
@p@T-’TP-l
(1 -
@p@T-l)TP-l
where kl is the radiative rate constant for emission from the first excited singlet SI, to the ground state, So, and kl = T F , ~ - ~(where T F , is ~ the radiative fluorescence lifet,ime); kz is the rate constant for the radiationless process from S1 to So; k3 is the rate constant for the process of intersystem crossing from SI to TI where T1 is the lowest triplet state; ka is TP,O-’ (where T P , ~is the radiative phosphorescence lifetime) and is the radiative rate constant for emission from TI to SO;and kg is the rate constant for radiationless deactivation from T1 to So. These definitions are summarized in Figure 1.
(1) Work performed under the Office of Naval Research, Contract No. 00014-67-CO327, Task No. NR 051-488. (2) W. R.Dawson and M.W. Windsor, J . Phys. Chem., 72, 3251 (1968). (3) W. H. Melhuish, J . Opt. SOC.Amer., 54, 183 (1964). (4) T. Medinger and F. Wilkinson, Trans. Faraday SOC.,62, 1785 (1966). (5) B. Stevens, M. F. Thomaz, and J. Jones, J . Chem. Phys., 46, 405 (1967). (6) J. L. Kropp, unpublished data, this laboratory. (7) D. P. Craig and I. G. Ross, J . Chem. SOC.,1589 (1954). (8) G. Porter and M.W.Windsor, Proc. Roy. Soc., Ser. A , 245, 238 (1958). (9) M.W. Windsor and J. R. Novak, “The Triplet State,” A. Zahlan, Ed., Cambridge University Press, New York, N. Y . , 1967, p 229. (10) J. B. Birks and L. G. Christophorou, Spectrochim. Acta, 19, 401 (1963). (11) J. B. Birks, D. J. Dyson, and I. H. Munro, Proc. Roy. SOC., Ser. A , 215, 575 (1963). (12) M. W. Windsor in “Physics and Chemistry of the Ormnio Solid State,” Vol. 2, D. Fox, M. M. Labes, and A. Weissberger, Ed., Interscience Publishers, New York, N. Y., 1965, p 369. (13) W. R. Dawson, J . Opt. SOC.Amer., 58, 222 (1968). (14) M. W. Windsor and W. R. Dawson, Mol. Cryst., 4, 253 (1968) ; “Organic Scintillators,” D. L. Horrocks, Ed., Gordon and Breach, New York, N. Y., 1968, p 253. (16) W. R. Dawson and J. L. Kropp, J . Phys. Chem., 73, 693 (1969).
Volume 79, Number 6
June 1969
J. L. KROPP,W. R. DAWSON, AXD M. W. WIWDSOR
1748
-
x
I
T
f
1
I
,
,
Figure 1. Schematic energy level diagram for pyrene.
Results The fluorescence lifetime and fluorescence yields of pyrene-hlo and pyrene-dlo were measured at various The fluorestemperatures between -196 and f23". cence lifetime data was fitted to the equation
,
,
0
+50
jo.40
Figure 2. Variation of lifetimes and yields with temperature for pyrene-hlo in PMM: fluorescence lifetime T F , 0, and radiative lifetime TF,O) left-hand ordinate; fluorescence yield @F, e, right-hand ordinate.
g,
data are given for pyrene-dlo in Figure 3. The parameters found for least-squares fit to eq 1 are given in Table I for pyrene-hlo and pyrene-dlo. Table I: Values of Parameters for Eq 1 for Pyrene-hlo and Pyrene-dlo A,
AB,
sec -1
om-'
6 . 9 X lo6 8 . 0 X 10"
475 f:50 430i50
TF,O-', 8ec -1
Pyrene-hlo Pyrene-dlo
1.94 X 10" 1.87 X lo6
Values of the triplet yield, (PT, and the phosphorescence yield, @P, for pyrene-hlo and for pyrene-dlo in PMM, together with the lifetime of the triplet state, r p , determined by the decay of T-T absorption are given in Table I1 for room temperature, -80, and Table 11: Values of QF, QT) Qp, and TP for Pyrene-hlo and Pyrene-dlo in PMM at Three Temperatures - 196'
Temp
Pyrene-hlo
TF,
nsec
@F
QT @Pa
(1)
where T F is the fluorescence lifetime at temperature T; r 0 is the fluorescence lifetime a t 77°K; A is the preexponential factor, and AE is the activation energy of the temperature-dependent process. The fluorescence lifetime of pyrene-hlo between -196 and +23", together with the curve given by the least-squares fit of the data, is given in Figure 2. Values of @F taken as a function of temperature are also given. The scales in Figure 2 are adjusted so that the values of T F and @F a t -196" lie at the same point on the graph. Similar T h e Journal of Physical Chemistry
I
T (OCI
Experimental Section The apparatuses used for measuring the various yields and lifetimes in PMM as a function of temperature are described e1se~here.I~We used Melhuish's M value3 of 0.61 for the fluorescence yield of pyrene-hlo in PMM at 23" as a standard and obtained @F values at other temperatures relative to this value. The method of preparation of PMM samples has also been given before.ls Pyrene-hlo was obtained from Rutgerswerke and was not further purified. Pyrenedlo was obtained from Merck Sharp and Dohme Ltd. of Canada and was purified by passing it in benzene solution through a column of basic alumina. For the measurement of ET%, @F, @p, and lifetimes, samples of M were pyrene in PiLIRII at a concentration of M were used to used. Concentrations of about measure ET. The mercury excitation light was filtered by passage through a Corning 7-54 filter and a 1-cm path of a solution containing 350 g of Tu'iS04.7H20 per liter of aqueous solution. This filter combination passes only light of wavelength shorter than 340 nm. The value of @T at -80" was determined in the same apparatus described previously. l 5 However, Dry Ice instead of liquid nitrogen was the coolant.
- l/ro = A exp(-AE/RT)
,
- 100
-200
1/TF
1.0
L
t
1
TP,
Pyrene-dlo
sec
TF) nsec
QF QT
- 800
$23'
515 0.78 0.22 0.0024 0.59
470 0.72
...
400 0.61 0.18 0.0017 0.45
535 0.79 0,150
452 0.69 0.157 0.013 3.4
354 0.53 0.25 0.012 2.4
QP
0.010
TP, sec
3.7
... 0.0024
a The absolute values of @p for pyrene-hlo and pyrene-dlo are probably accurate to only one significant figure. However, the ratio of these quantities is much more accurate. Thus the value of Qp for pyrene-hlo a t 23" is less than at lower temperatures, and the second figure is given to indicate this.
1749
RADIATIVE AND RADIATIONLESS PROCESSES IN AROMATIC MOLECULES
700P-+-
400 nm
500 nm
1.0
600
3#80
9
4oo
m
t1
3, 60
3.40
1
1
I
-100
I
I
I
I -50 0 +50 * T('C) Figure 3. Variation of lifetimes and yields with temperature for pyrene-dlo in PMM:, fluorescence lifetime TF, 0, and radiative lifetime T F , ~ 9, , left-hand ordinate; fluorescence yield @F, 0 , right-hand ordinate.
-150
- 196". Values of @F and T F a t these temperatures are also included in the table. These are the first values of @F, @P, @T, and T F reported in PMM for pyrene and pyrene-dlo. However, Kellogg and Schwenker16reported values of 0.5 and 0.4 sec for T P of pyrene-hloin PMM at -196 and +23", respectively, and 3.2 and 2.5 sec for rP for pyrene-dlo in PMM a t the same temperatures. Our values are in agreement with theirs. The T-T absorption spectrum of pyrene-d1o in P M M in the uv and visible region was also determined a t -196, -80, and $23". The spectra a t -196 and +23" are given in Figure 4. This figure shows that ET is independent of temperature in PMM. The values of eT(max) at 413 nm are approximately 40,000 at all three temperatures in good agreement with Brinen's value of 43,OOOI' obtained in 2-methyltetrahydrofuran at -196" but lower than the value of 60,000 obtained in our laboratory in an EPA glass.18 Discussion The rate constants for activation and deactivation of pyrene-hlo and pyrene-dlo can be derived using the equations given in the Introduction. The first-order rate constants so determined at three different temperatures are given in Table 111. The values of T F , ~(k1-l) are also given as a function of temperature in Figures 2 and 3. These values were obtained by combining measured values of @F with values of TF calculated for the required temperature from eq 1, using the parameters in Table I. The values of kl from both Table I11 and Figures 2 and 3 show that k1 is constant with temperature and has an average value of 1.52 X 106 sec-' for pyrene-hlo and 1.50 X lo6 see-I for pyrene-dlo. The former value is the same as that determined by Birks, Dyson, and ?c/Iunroll for pyrene in cyclohexane (1.5 X lo6see-I).
Figure 4. Triplet-triplet absorption spectrum of pyrene-dlo in PMM: solid line, -23'; dotted line, -196'.
Values of ET for pyrene-hlo were not measured. The given in Table I1 for pyrene-hlo a t -196 and 23" were obtained by measuring ET@T at these temperatures and assuming that the value of ET for pyrene-hlo is the same as that for pyrene-dlo.lD The value of @T for pyrene-dlo clearly increases with rising temperature. This is reflected in the behavior of k3 for pyrene-dlo which increases with increasing temperature from 3.0 X lo5 sec-I a t -196" to 7.1 X 106 sec-I a t 23". Such enhancement of the population of the triplet state with temperature has been observed in substituted anthracenesz0and has been suggested to occur in p y ~ - e n e . ~It , ~ is attributed to a thermally activated intersystem crossing from S1 to a triplet level Tz lying somewhat above S1 in energy (ks(T)in Figure 1). From studies of the fluorescence lifetime of pyrene in ethanol as a function of temperature, Stevens, Thomaz, and Jones (STJ)5 calculate that @F at -196" is unity, implying that @T = 0 at this temperature. They suggested that a t higher temperature the rate of @T's
(16) R. E. Kellogg and R. P. Schwenker, J . Chem. Phys., 41, 2860 (1964). (17) J. S, Brinen, ibid., 49, 586 (1968). (18) W. R. Dawson and M. W. Windsor, unpublished data, this laboratory. (19) Coronene-h1z and coronene-dlz have identical fT spectra in PMM (see ref 15). (20) R. G. Bennett and P. J. McCartin, J. Chem. Phys., 44, 1969 (1966). Volume 78, Number 6 June 1969
1750
J. L. KROPP,W. R. DAWSON, AND M. W. WINDSOR
Table 111: Rate Constants for Deactivation of Pyrene in PMM Rate constant, m c -1
kl k2 kS k4
ka
-Pyrene-ko-
Pyrenedl- 800
7
- 196'
- 800
1.51 X lo6 0.0 4 . 3 x 106 0.018 1.7
1.53 X lo6
... ... ... ...
+23'
1.52 X lo8 5.9 x 105 4 . 5 x 105 0.021 2.2
intersystem crossing is temperature dependent with an activation energy of 800 cm-'. From their observations of the T-T absorption spectrum of pyrene-dlo in epoxy plastic a t 23", Windsor and NovakQlocated a triplet level, 1300 cm-l (new measurements give a value of 950 50 cm-I), above S1 and believe this to be the second excited triplet. They also studied the T-T absorption of pyrene-dlo in EPA at -196" and observed triplet formation, indicating that the value of OT is definitely not zero but might be as low as a few per cent. They concluded that a slow temperature-independent crossing from S1 to T1 (ka in Figure l), populates the triplet state at -196", but that the thermally activated process from SI to Tzis dominant at 23". The present direct measurements of OT and calculations of k3 confirm that such an activated process does occur for pyrene in PMM. The intersystem crossing process identified by k3 is not the only temperature-dependent process deactivating the S1 state of pyrene. At room temperature our results show that the sum (@F OPT)is significantly less than unity for both pyrene-hlo (0.79) and pyrene-dlo (0.78). Thus slightly over 20% of the total quanta originally in SI are dissipated radiationlessly via process kz without going through state TI. Similar results have been reported previously for several other aromatic moleculesa21 This conclusion and our values of OPTdisagree with results for pyrene-hlo in ethanol reported by and by Medinger and WilkinParker and Joyce (PJ)22 son (MW).4 Both sets of authors concluded that in OT) = 1.0 a t room temperature, but their system (@F ~ reported whereas MW obtained a value of 0.38 for O T PJ a figure of 0.28. Both values are considerably higher than our value of 0.18 obtained in a rigid plastic medium. At 196" the values of (OF OT) are very close to unity. For pyrene-dlo the sum is 0.94, while for pyrenehlo it is 1.00. Therefore, it follows that, for both protonated and deuterated pyrene at - 196", k2 is close to zero. In fact, for pyrene-hlo we calculate kz to be zero. I n the case of pyrene-dlo, kz is small and, since it is calculated as the difference [l - ((ap OT)],it may well be zero allowing for our experimental uncertainty. Comparing the values of kz at -196" and at 23", we conclude that the radiationless deactivation of the singlet (S1) directly to ground accounts for a significant part of
*
+
+
-
+
+
The Journal of Physical Chemistry
- 196'
1.48 X lo6 1 x 105 3 . 0 X lo6 0.018 0.25
1 . 5 1 x lo6 3 . 4 x 105 3 . 4 x 105 0.025 0.27
+23O
1.50 X lo6 6 . 5 X lo6 7 . 1 X lo6 0.020 0.39
the temperature-dependent intramolecular fluorescence quenching in both pyrene-hl, and pyrene-dlo. We conclude then that two radiationless processes, intersystem crossing with rate constant k3, and internal conversion S1+w+So with rate constant ICz, compete with the radiation of fluorescence kl in deactivating the S1 state. Further, one of these, k3, has a temperatureindependent and a temperature-dependent part while kz is almost totally temperature dependent. Table I11 shows that the radiative process kl is independent of temperature. Thus the observed variation in TF with temperature is attributable in part to changes in kz and in part to changes in k3, each with its own activation energy. The experimental accuracy of our T F us. temperature data is not high enough to permit unambiguous resolution into a two-component decay curve. We can, however, fit the data, to a single exponential with a AE value of 475 cm-l for pyrene-hlo and 430 cm-1 for pyrene-dlo (Table I). These values must be related to the variation with temperature of the sum of the rate constants ( k z ka). From the values of kz and ka for pyrene-dlo given in Table 111, we can calculate separate activation energies for the temperature-dependent components of S1w+Sa internal conversion and S1m+TZ crossing. We obtain AE = 320 cm-l for the kz process and AE = 900 cm-l for the kS process in pyrene-dlo. The latter value is in agreement with the more recent value of 775 50 cm-l for the SITzenergy gap for pyrene-dlo in Pi\lb/I plastic obtained recentlyz3 in our laboratory. The 320-cm-l value is similar to that observed for the thermal quenching of fluorescence in many other aromatic molecules by Kropp and DawsonZ1and attributed by them to quenching via low-frequency vibrations of the aromatic molecule itself. Unlike these other molecules, however, pyrene-dlo exhibits in addition, a significant variation in the rate of intersystem crossing with temperature, which accounts for the somewhat higher value of AE as determined from the T F us. temperature data.
+
*
(21) J. L. Kropp and W. R. Dawson, Proceedings of Conference on Molecular Luminescence, Loyola University, Chicago, Ill., Aug 1968, W. A. Benjamin, New York, N. Y . , 1969. (22) C. A. Parker and T. Joyce, Trans. Faraday Soc., 62, 2785 (1966). (23) J. R. Novak and M. W. Windsor, unpublished data, thia laboratory.
RADIATIVE AND RADIATIONLESS PROCESSES
IN
AROMATIC MOLECULES
Using the above data we can estimate how the relative contributions of internal conversion from Si into So and intersystem crossing from SI to Tz change as the temperature is varied. The observed values of kz can be separated into a temperature-independent part, kzO, which we assume to be equal to the value a t - 196" and a temperature-dependent part, k z ( T ) = ( k , - kzo). The same can be done for k3. Values of kz(!!') and k 3 ( T ) obtained from the data in Table I11 for pyrene-dlo show that for this compound it is the increase in lit(!!') which provides the larger contribution to the reduction in fluorescence lifetime, cF, between - 196 and +23". At -80", k z ( T ) is six times greater than k 3 ( T ) . The disparity between k,(!!') and ks(!!') lessens with rise in temperature, and at room temperature the two contributions are comparable (5.5 X lo6 sec-' for k~ and 4.1 X lo5sec-l for k3). The behavior of the triplet yield with temperature is different for pyrene-hlo and pyrene-dlo (Table 11). While @T increases with temperature for pyrene-dlo, it decreases from 0.22 to 0.18 for pyrene-hlo. Correspondingly, the calculated values of k3 for pyrene-hlo a t -196" and at +23" are the same (Table 111). At - 196", k3 for pyrene-hlo is greater than for pyrene-dlo, while at +23" the total rate of triplet formation is greater in pyrene-dlo than in pyrene-hlo because of temperature-dependent crossing present in the deuterated compound. On the other hand, the values of are comparable a t room temperature for the two compounds. Because the value of kz at -196" is close to zero for both pyrene-hlo and pyrene-dlo, we cannot establish the existence of an isotope effect upon kz. The lack of temperature dependence of k g for pyrenehlo demonstrates that the variation of CF with temperature is largely due to variations in kp, the rate constant for internal conversion to the ground state. This result disagrees with that of STJ5 who attributed the temperature dependence of CF for pyrene-hlo in ethanol entirely to variations in k3, the rate constant for intersystem crossing. However, we have observed that the chemical nature of the environment exerts a significant effectupon the values of @T and CF for pyrene-hlo. This implies that the various rate constants for depopulation of SI will vary from solvent to solvent. Data for pyrene-hlo and pyrene-dlo, given in Table 11, show the respective values of @F and CF are comparable a t -196". However, a t room temperature the values of @F and CF for pyrene-hlo are about 10% higher than those for pyrene- IO. Dawson and Windsor, obtained similar results for protonated and deuterated pyrene in ethanol solution a t room temperature. Their value for the fluorescence yield of pyrene-hlo at 23" is 0.53 and that for pyrene-dlo is 0.44. To explain this difference, they advanced the hypothesis that theT, triplet level is lower in pyrene-dlo than in pyrene-hlo, thus reducing the SIT, gap and causing the triplet yield for pyrene-d1o to be higher a t room temperature than that for pyrene-
1751
hlo, Because the value for k3 in pyrene-hlo determined in the present work is temperature insensitive, our data imply that the SITz energy gap in pyrene-hlo i s sufficiently large that at room temperature crossing from S1 to T, is negligible. This implies that the Tz triplet level in pyrene-hlo must lie higher above SI than in pyrene-dlo. These new data provide further support for the mechanism suggested previously by Dawson and Windsor. Recent measurements in our laboratory of @T in EPA give values of about 0.05 for pyrene-hlo and about 0.04 for pyrene-dlo.l8 These values are substantially lower than the values we obtain in PMM and indicate that the magnitude of the temperature-independent component of the intersystem crossing rate constant k3 may be quite susceptible to changes in the chemical environment surrounding the pyrene. Corresponding to this the lifetimes of pyrene-hlo and pyrene-dlo vary depending on the solvent glass in which they are dissolved. At this time the reasons for the different values of @T and CF in various low-temperature glasses are not known. Values of k h for pyrene-hlo and pyrene-dlo given in Table I11 vary between 0.018 and 0.025 sec-I. However, the variation is within the relatively lower accuracy of the data, and there is no evidence of any large temperature effect upon kq, nor are there any significant differences in kq between protonated and deuterated pyrene. Thus we conclude that the radiative lifetime C P , ~for the triplet state of pyrene is independent of temperature and compound deuteration. The value lies between 40 and 55 sec. This value is similar to values of CP ,o determined for coronene-hlz, coronene-d12, and benzcoronene.16 On the other hand, as expected, the rate constant k5 for the radiationless decativation of T1 is dependent on both compound deuteration and temperature.
Conclusions (1) The rate constant kl for pyrene-hlo and pyrenedlo is independent of temperature between -196 and +23" and does not change upon deuteration. ( 2 ) At - 196" for both pyrene-hlo and pyrene-dlo internal conversion to the ground state ( k z ) is unimportant and may be entirely absent. However, the rate of this process increases with temperature, and at 23" it competes significantly with fluorescence emission and intersystem crossing. (3) At -196", for both pyrene-hi0 and pyrene-dlo, intersystem crossing ( k 3 ) is a significant process. Respectively, 22 and 15% of the molecules in S1 go over to the triplet manifold. This result is in disagreement with the conclusion of STJ5that for pyrene-hloin ethanol intersystem crossing is negligible at - 196". However, whereas for pyrene-hlo the intersystem crossing rate is unchanged by an increase in temperature, for pyreneIO the rate a t 23" is more than double that at -196", going from 3.0 X lo5to 7.1 X lo5sec-'. (4) At 23" intersystem crossing (k3) and direct inVolume 73, Number 6 June 1969
1752
WILLIAMR. DAWSON AND JOHN L. KROPP
ternal conversion into the ground state (k2) have similar rates. With deuteration kz increases slightly (from 5.9 to 6.5 X lo6 aec-l) and ka more substantially (from 4.5 to 7.1 X lo6 sec-I). (5) The radiative lifetime of the lowest triplet state T P , (k4-l) ~ is independent of temperature and compound deuteration and lies between 40 and 55 sec. Radiationless quenching of TI (kh) shows a five- to sevenfold reduction upon deuteration and is somewhat less important a t lower temperatures. Even for pyrene-dlo at - 196" the nonradiative decay of T1 is 14 times faster than the emission of phosphorescence. Thus the observed triplet lifetime of 3.7 sec is controlled almost completely by the nonradiative process. In the absence of such quenching the triplet lifetime would lie in the range 40-55 sec. Note Added in Proof. Recently, Jones and SiegelZ4 have published the results of a similar study of pyrene in
PMM. In their work they found that t p ~and T F varied with temperature for both pyrene-$0 and pyrene-dlo. They attributed the entire temperature variation to a change in k3, the rate constant for intersystem crossing. Our results do not agree with this interpretation, especially for pyrene-hlo. Also, their values of T F at - 196" of 425 and 434 nsec for pyrene-hlo and pyrene-dlo, respectively, are considerably less than our values of 515 and 535 nsec. As mentioned above, we have data suggesting that the chemical environment may affect the emission properties of pyrene. Thus, the differences between our results and those of Jones and Siegel could be due to different methods of sample preparation. Further work will be necessary to explain these differences.
(24) P. F. Jones and S. Siegel, Chem. Phys. Lett., 2, 486 (1968).
Radiationless Deactivation and Anomalous Fluorescence of Singlet 1,12-Benzperylene' by William R. Dawson and John L. Kropp Chemical Sciences Department, Systems Group of TRW Inc., Redondo Beach, California 90378 (Received December 6 , 1968)
Values of the rate constants for processes that deactivate the lowest excited singlet state of 1,la-benzperylene have been determined between -196 and 23" from measurements of the quantum yields of triplet formation, QT, fluorescence, QF, and the fluorescence lifetime, TF. The rate constants for intersystem crossing to the triplet state are nearly the same at -196 and 23". The rate constant for direct radiationless deactivation to the lowest singlet is probably zero at - 196" but increases to 6 X 106 sec-' at 23". This is similar to results obtained for other aromatic hydrocarbons. However, the fluorescence of 1,lkbenzperylene is atypical. An increase in the rate constant of fluorescence, kl, from -196 to 23" is observed resulting from an increase in QF with temperature while rF decreases over the same temperature range. The increase in IC1 with increasing temperature is attributed to a temperature-dependent emission from the second excited singlet state. Since the second excited singlet state of 1,12-benzperylene lies only 1275-1400 om-l above the ~ L Istate, , there is significant thermal population of the lL, state of 23". The observation of a temperature-dependent antiStokes fluorescence supports this interpretation. The effect of solvent upon the energy gap between the lL, and lLb states and the fluorescence properties of l,l2-benzperylene are discussed.
Introduction We have been investigating the mechanisms and rate constants for depopulation of the lowest excited singlet (S1) and triplet (TI) states of various aromatic molecules in poly(methy1 methacrylate) (PMM) and have found that for most molecules the fluorescence yield, @F, and the fluorescence lifetime, T F , decrease in the same proportion with increasing temperature so that kl, the rate constant for fluorescence from S1 to So is independent of The Journal of Physical Chemistry
temperature from - 196 to 23".2 More complete data on coronene-hl2, coronene-dl2, and benzcoronene3a and (1) Supported by ONR under Contract N00014-67-C0327, Task NO. NR061-488. (2) J. L. Kropp and W.R. Dawson, Proceedings of International Conference of Molecular Luminescence, Loyola University, Chicago, Ill., 1968, W. A. Benjamin, New York, N. Y., 1969. (3) (a) W. R. Dawson and J. L. Kropp, J . Phys. Chem., 73, 693 (1969); (b) J. L. Kropp, W.R. Dawson, and M. W. Windsor, ibid., 73, 1747 (1969).
