468
J. Phys. Chem. 1982, 86, 468-472
Time-Resolved Anisotropy of Delayed Luminescence. 1,2-Benzanthracene in Polystyrene Matrices R. D. Burkhart' and A. A. Abla Department of Chemlstry, Unlverdty of Nevada, Reno, Nevada 89557 (Received: August 25, 1981)
The polarization of phosphorescence, delayed fluorescence, and delayed excimer fluorescence following pulsed UV laser excitation has been determined at different delay times following the excitation pulse. The absolute values of the polarization of phosphorescence and delayed fluorescence are found to increase for delay times between 10 ps and 40 ms. The polarization of delayed excimer fluorescence is negligibly small over this same time range and shows no time dependence. The time-dependent polarizations are thought to be a result of heterogeneously distributed solute molecules in which rapid randomization of the emission dipole occurs by energy transfer in regions of high concentration and the fact that, in these same regions, triplet decay is faster than average because of enhanced rates of triplet-triplet annihilation. The long-time-delayed fluorescence polarization, in some samples, is as high as 0.3, indicating that the emitting species is, at most, one migratory step away from the original site of excitation. This result is in accord with earlier findings that delayed fluorescence emission, at delay times longer than 10 ms, involves the interaction of a mobile triplet exciton with a triplet trapped at a site composed of a molecular pair. The lack of significant polarization of the delayed excimer fluorescence indicates that excimer sites are populated primarily as recipients in energy-transfer processes as opposed to direct photoexcitation.
Introduction Light emission originating from the triplet state of 1,2benzanthracene (BNZ) consists of annihilation (P-type), delayed fluorescence (DF), delayed excimer fluorescence (DEF), and phosphorescence. The respective wavelengths for A- me 410,520, and 605 nm.1-3 At room temperature, solutions of BNZ in a rigid polystyrene matrix, at concentrations less than about 0.2 M, have DF and phosphorescence decays that obey single exponential behavior yielding 7DF = 95 ma and T~ = 225 m ~ . ~ In homogeneous solutions it is expected that TDF = l/z T~ if the excitation intensity is sufficiently low and if the delayed fluorescence is of the annihilation types4 For these rigid polymer solutions it is consistently observed that TDF is less than one-half 7 . In addition, it has been noted that the ratio of delayed huorescence to phosphorescence intensities (IDF/Ip)is proportional to the square of the solute concentration, even after correcting for differences in the intensity of absorbed radiation. I t is well recognized that a heterogeneous distribution of solute molecules accounts for many of the differences in luminescence kinetics found for rigid solutions as opposed to fluid ones.5 Indeed, for the present system of 1,2-benzanthracene in polystyrene, it has been proposed that DF emission results from an interaction between a migrating triplet exciton and an immobilized triplet. Because of the square law relationship between IDF/lp and the solute concentration, it was proposed that the immobilized triplets consisted of pair species in sufficiently close proximity so that transfer of triplet energy between the two members of the pair was highly probable compared with transfer to a next nearest n e i g h b ~ r . ~ Thus,for this particular system, some indication of order in the heterogeneous solute distribution is seen, and this order may have an influence not only on luminescence (1) Wyrsch, D.; Labhart, H. Chem. Phys. Lett. 1971,8, 217. (2) Nickel, B. Chem. Phys. Lett. 1974, 27, 84. (3) Burkhart, R. D. Chem. Phys. 1980,46, 11. (4) Birks, J. B. "Photophysics of Aromatic Molecules"; Wiley: New York, 1970; p 385. (5) Naqvi, K. R. Chem. Phys. Lett. 1968, 1, 497. 0022-3654/82/2086-0468$01 .25/0
decay kinetics but also on energy migration kinetics. I t seemed worthwhile, therefore, to pursue the matter one step further to see whether this proposed mechanism for delayed fluorescence production could stand the test of an independent experimental approach. Studies of fluorescence depolarization and the relation between the rate of depolarization and the rate of energy transfer have received some attention in the literature.6 It is clear that a transition dipole, initially oriented vertically in the laboratory frame, will approach a random orientation as the singlet exciton migrates from one molecule to the next, even in the absence of molecular rotation or translation. A similar expectation would hold for phosphorescence even though the mechanism of energy migration for singlets and triplets is quite different. The maximum luminescence polarization obtainable depends upon the angle between absorption and emission oscillators and for the collinear case is 0 ~ 5 .It~ has been pointed out by Krishna and Goodmad and by El-Sayedg that the phosphorescence of fused-ring aromatic hydrocarbons should be polarized at 90° to the ?r ?r* absorption oscillator, the fluorescence, of course, being collinear with absorption. Thus, for 1,Zbenzanthracene prompt fluorescence and delayed fluorescence should exhibit a maximum polarization of 0.5 while phosphorescence would have a maximum value of -0.33.7 If we let Po symbolize the polarization in the absence of any energytransfer steps and let P symbolize the polarization after ii transfer steps, it has been shown that6
-
(1/P - 1/) = (l/Po- Y3)[l + (3rt/2) sin2 01
(1)
where 0 is the angle between emission oscillators of the donor and acceptor species. EisenthallO has demonstrated that the phosphorescence of phenanthracene-dlo is negatively polarized when sensitized by certain aromatic ketones whose excited states are produced by photoselection. (6) Weber, G. Trans. Faraday SOC.1964,50, 552. (7) Perrin, F. Ann. Phys. (Paris) 1929, 12, 169. (8) Krishna, V. G.; Goodman, L. J. Chem. Phys. 1962,37,912. (9) El-Sayed, M. A. Nature (London) 1963, 197,481. (10) Eisenthal, K. B. J. Chem. Phys. 1969,50, 1689.
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 4, 7982 469
Flgure 1. Experimental apparatus for time-resotved o p h l anisotropy: (C) mechanicel chopper, (L) lenses, (F) W-transmitting fitter, (S) sample contalner, (P) polarizer.
This can only occur if the probability of energy transfer is dependent upon relative orientation of donor and acceptor molecules. It is not presently known whether an angular dependence exists for triplet transfer between like molecules. One can, however, write P = Po/[1 sii(1- Po/3)] (2)
+
where s = 1for the extreme case of no angular dependence of the transfer probability and s = 0.6 if the transfer probability varies as cos2 8, which would be the opposite extreme for a transfer due to coupled oscillators. The actual case of an electron-exchange mechanism occurring a t short range is probably somewhere between these extremes. Equation 2 assumes, of course, that no molecular rotation has taken place within the duration of the measurement. Values of ii calculated from eq 2 turn out to be rather valuable in interpreting the observed time dependence of the delayed luminescence polarization. They are particularly useful in connection with delayed fluorescence polarizations as well as those of delayed excimer fluorescence and have helped to reinforce earlier conclusions about the mechanisms of these processes which were based upon rate measurements alone.
Experimental Section Since the experimental results obtained in this study were quite different from original expectations, several alternate experimental arrangements were employed as a safeguard against artifacts arising from equipment. The two systems which proved to give the most reliable results utilized UV laser excitation, and a schematic diagram of these systems is shown in Figure 1. In one set of experiments (the original ones) a Chromatix CMX-4 pulsed dye laser was used. The dye was rhodamine 6G in 1:l methanol-water solution. The 598-nm emission was passed through a frequency-doubling crystal yielding a mixture of 299- and 598-nm light, the UV portion being polarized in the vertical plane. In most experiments the slowest pulse frequency of 5 Hz was used. If slower frequencies were needed, a mechanical chopper was employed driven by synchronous motors and phased to admit a fixed fraction of the laser pulses. A second system was assembled which was essentially as shown in Figure 1 but utilizing a flash lamp pumped NdYAG laser (Quanta Ray Model DCR). Multiples of the fundamental 1066-nm emission were obtained by using a crystal harmonic generator. The desired laser emission at 355 nm was isolated with a prism separator but was also passed through a filter to remove any scattered 1066- or 533-nm emission. In most experiments with this system an additional filter was inserted between the emitting
sample and the polarizer. This was especially desirable with very dilute samples since excitation light reaching the polarizer was capable of exciting coating material on the polarizer films. It was found that this material was phosphorescent and its emission interfered with the sample emission. When a color filter appropriate for the emission being studied was used, this extraneous source of phosphorescence was effectively eliminated. A Tektronix Model TM 503 function generator was used to trigger the laser externally at frequencies ranging from 0.1 to 10 Hz. The optical components in the system of Figure 1include fused-silica lenses in both excitation and emission beams. Following the excitation lens the W-transmitting filter was inserted into the beam at an angle of 45' to the beam direction. The small amount of light reflected from the filter surface was sent to an auxiliary photomultiplier tube, thus providing a trigger signal which was used to activate a delay circuit in a Tektronix Model 465 oscilloscope. After a preselected delay time the signal averager (Nicolet Model 1072) was triggered. The photomultiplier voltage existing at 1ps after this second trigger is the one which is read by the signal averager. The sample emission was passed through a polarizer (Polaroid Corp.) and was then focused onto the entrance slit of the monochromator using another fused-quartz lens. The lenses and the windows were tested to be certain that they did not themselves emit any luminescence. The monochromator is a Spex Model 1670 instrument driven by a Spex Model 1673 variable-speed stepping motor. Sample emission was detected by an EM1 Model 9789 photomultiplier tube using a Brandenburg Model 472R power supply. In the kinetics mode the trigger signal was used to initiate a sweep of 256 channels of the signal averager, the dwell time per channel having been previously selected as well as the fiied emission wavelength. The measurement of time-resolved optical anisotropy was carried out at the fixed wavelength corresponding to phosphorescence or delayed fluorescence. The emission signal with the polarizer oriented vertically (Iv)or horizontally (Ih) with respect to the laboratory frame was measured by using delay times after the excitation pulse which varied from a few microseconds to several hundred milliseconds. The 1,Zben~anthracenetriplets have a lifetime of 225 ms under the prevailing conditions, and it was necessary to provide a time lapse between successive excitation pulses long enough for the triplets to decay through several lifetimes. This was accomplished by using the rotating sector arrangement or by external triggering of the laser. For each intensity measurement at least 50 separate pulses were used and the average was recorded. The bias of the light collection system for vertical vs. horizontally polarized emission was tested by using light from an incandescent source passed through a frosted plate of fused quartz. A negligible correction was found at the wavelengths used. Since the excitation light is vertically polarized, the formula for calculating the polarization is p = ( I v - I h ) / (1" + I h ) (3) In addition to these two experimental arrangements employing laser excitation, a conventional system using a xenon-mercury arc lamp and out-of-plane excitation and emission choppers was also employed. The details describing this apparatus may be found in earlier publications from this laboratory." Since both conventional and pulsed laser excitation was used in this work, a few words may be said about the (11) Burkhart, R. D.;AvilBs, R. G. J. Phys. Chem. 1979, 50, 1897.
470
3E0
Burkhart and Abia
The Journal of Physical Chemistry, Vol. 86, No. 4, 1982
q00
us0
h00
WRVELENETH
hC0
< NM >
600
610
Figure 2. Delayed luminescence of a 0.033 M solution of 1,2-benzanthracene in polystyrene viewed through vertical (V) and horizontal (H) orientations of the emission polarizer. Excitation light is vertlcalty Polarized.
advantages and limitations of the laser systems. For time-resolved experiments using the customary dual chopper arrangement, there is an inevitable “dark time” between the cessation of the excitation pulse and the s t a r t of an emission pulse. With mechanical choppers or shutters it is difficult to reduce this time below the millisecond range. With a monochromatic laser source, however, the emission chopper is unnecessary as long as the emission wavelength differs sufficiently from the excitation wavelength or multiples thereof. In the present case, scattered light at 598 nm from the dye laser did interfere somewhat with the 1,Zbenzanthracene phosphorescence which has a A, a t 605 nm. By moving the emission monochromator to 608 nm, however, this overlap was effectively removed except for the two samples having the smallest solute concentrations and hence the smallest phosphorescence intensities. No problems of this sort were encountered with the Nd:YAG laser since all excitation lines are well removed from the relevant emissions of 1,2-benzanthracene. In these experiments it was feasible to examine luminescence signals at times no less than 10 ps after the excitation pulse. The purification of materials and the techniques used in sample preparation are identical with those described earlier.3
Experimental Results Figure 2 provides an overview of the way in which the three emission bands are polarized for these solutions of 1,2-benzanthracene. The results are depicted for a polystyrene matrix containing 1,Zbenzanthracene at a concentration of 0.033 M. Clearly the delayed fluorescence band (410 nm) is positively polarized, the phosphorescence band (605 nm) is negatively polarized, and the delayed excimer fluorescence band (520 nm) shows very little polarization at all. The signs of the delayed fluorescence and phosphorescence polarizations are entirely consistent with earlier observations of Krishna and Goodmane in their study of fused-ring aromatic compounds. To the best of our knowledge, this is the first report of a polarization study of delayed excimer fluorescence. When our attention was turned to the time dependence of these polarizations, a very surprising observation was made. Originally it was the dye laser experiments which indicated that the phosphorescence polarizations increased in absolute value with increasing time following the excitation pulse. For example a 0.066 M solution of 1,2benzanthracene in polystyrene yielded polarizations of 0.00 at 0.8 ms, -0.01 at 4 ms, and -0.27 at 20 ms following the
Flgwe 3. Time dependence of delayed luminescence polarizations of a rigid sokrtkn of 1,2-benzanttuacene folkwing an exdtabbn p u b from the Nd:YAG laser at 355 nm. Closed circles are delayed fluorescence, crosses are delayed excimer fluorescence, and open circles are phosphorescence.
TABLE I: Phosphorescence Polarizations for Various Concentrations of l,2-Benzanthracene in Polystyrene and Various Delay Times Following the Excitation Pulse from a Nd:YAG Laser at 355 nm polarizations delay 0.009 M time, ms
____
0.050 0.200 1.00 2.00 5.00 10.00 20.00
-0.09 -0.13 -0.12 -0.1 5 -0.20
0.033
0.075
0.136
0.22
M
M
M
-0.04
-0.06
-0.06 -0.08
-0.06
-0.08 -0.09 -0.12
M -0.06 -0.08
-0.08
-0.09 -0.11
-0.18
-0.15 -0.19 -0.25
-0.08
-0.09
-0.12 -0.17 -0.20 -0.24
-0.09 -0.15 -0.28
excitation pulse. The same sample yielded delayed fluorescence polarizations of 0.04, 0.14, and 0.27 at the same three delay times. Since the 299-nm excitation light used in the dye laser experiments was also absorbed by the polarizer film,it was thought that spurious emission from the polarizer itself may be affecting these results. It happens that the third harmonic 355-nm excitation light from a Nd:YAG laser corresponds to an absorption minimum of the polarizer film. Furthermore, insertion of a color filter between the sample and the emission polarizer protects the latter from the excitation light while allowing the sample emission to pass. In Figure 3 are results of experiments in which polarizations of delayed fluorescence, delayed excimer fluorescence, and phosphorescence have been measured between 0.2 and 20.0 ms following an excitation pulse from the Nd:YAG laser. Here, for a 0.075 M solution, one sees monotonically varying delayed fluorescence and phosphorescence polarizations which increase in absolute magnitude with increasing time following the excitation pulse. It is worth noting that the pulse duration of the Nd:YAG laser is only a few nanoseconds and that the time scale of our polarization measurements is well beyond this range. The apparent regularity and symmetry of the results shown in Figure 3 is certainly satisfying. As a further aid in their interpretation, it was decided to test the effect of varying solute concentration. Tables I and I1 summarize phosphorescence and delayed fluorescence polarizations, respectively, for several different samples in the concentration range from 0.009 to 0.22 M. A table of delayed excimer fluorescence polarizations is not included simply because they are all negligibly different from zero and, for
The Journal of Physical Chemistry, Vol. 86, No. 4, 1982 471
TimeResolved Anisotropy of Delayed Luminescence
TABLE 11: Delayed Fluorescence Polarizations for Various Concentrations of 1,2-Benzanthracene in Polystyrene and Various Delay Times Following the Excitation Pulse from a Nd:YAG Laser at 355 nm polarizations delay time, ms
0.033
M
1.00
0.11 0.14 0.17
2.00 5.00 10.00 20.00
0.17 0.18 0.18
0.050 0.200
0.075
M 0.11 0.15 0.19 0.20 0.22 0.22 0.26
0.136 M
0.22
0.14
0.12 0.16 0.19 0.20 0.24 0.23 0.24
0.16 0.21
0.24 0.25 0.27
M
each sample, no time dependence of the polarization is observed. Delayed fluorescence results from the 0.009 M solution are not reported here because the sample emissions are too weak to provide reliable data. In general, it was possible to obtain a reproducibility of fO.01 unit in the polarization values although, for the longtime values a t 20 ms, the signal intensity has declined considerably and f0.02 units would be a more accurate estimate of the reproducibility. Even though the time-resolved delayed fluorescence and phosphorescence polarizations show the same general trends, some interesting differences in behavior are noted. For example, the phosphorescence polarizations in some samples approach very closely the theoretical limit of -0.33, whereas the delayed fluorescence polarizations achieve, at best, only about 50% of the theoretical limit of 0.5. Another interesting observation is that the change in delayed fluorescence polarizations over the time range from 50 ps to 20 ms is only slightly greater than twofold for one sample. For phosphorescence, the same time range yields about a fourfold change in polarization. In a few experiments the delayed fluorescence polarization was investigated at a delay time of 10 ps following an excitation pulse. Polarizations of 0.04, -0.01, and 0.04 were obtained for concentrations of 0.033,0.075, and 0.22 M, respectively. From these measurements it is clear that the processes responsible for the enhancement of delayed fluorescence polarization are operative on the microsecond time scale.
Discussion Since both delayed fluorescence and phosphorescence polarizations are very close to zero at delay times in the low microsecond range, the species emitting at these short delay times are primarily those which have become electronically excited by means other than direct light absorption. Of course, the most likely method of indirect excitation is by energy transfer. The rate of energy transfer is strongly dependent on the average distance of intermolecular separation for both singlet and triplet states. Thus, it seems likely that the majority of the emitting species in the microsecond time scale are in regions of relatively high concentration. The rate of triplet-triplet annihilations leading to delayed fluorescence is also strongly dependent on intermolecular separation distances. For this reason, triplets which happen to be in regions of relatively high solute concentration will undergo such reactions with a larger rate constant than those in more dilute regions. Furthermore, triplet concentrations are, of course, enhanced in these high solute regions and, because of the bimolecular nature of the annihilation process, a further decrease in triplet lifetimes in such regions is expected. Thus, on the basis of this model of nonuniform solute distribution, there
emerges a pattern of rapid scrambling of the emission dipole in those same regions of the solution where triplet lifetimes will be the shortest, that is, in the concentrated regions. It is therefore proposed that nonuniformity of the solute distribution is primarily responsible for the observed results. Our earlier studies on the kinetics of triplet decay in these rigid solutions clearly indicated the existence of nonexponential phosphorescence and delayed fluorescence behavior in the early portions of the decay period. The present results suggest that mixed fiist- and second-order processes are the likely cause of this behavior. They further suggest that species emitting at long times, i.e., 10 ms or more, are relatively isolated having undergone little or no energy transfer during their entire lifetime. Of course, this cannot be true for delayed fluorescence processes since they require at least one energy-transfer step in order to occur at all. The sharply contrasting behavior of the delayed fluorescence polarizations compared with delayed excimer fluorescence polarizations is very revealing in terms of earlier proposed mechanisms for these processes. The excimer is certainly the simplest to understand since it has no ground-state counterpart and thus cannot be directly excited. Since the solution is rigid, however, there must exist paired solute molecules having a relative orientation favorable for excimer formation if electronic excitation of one of the partners occurs. Thus, in this rather indirect manner, it is possible to achieve photoselection of the excimer. Let us symbolize the ground-state molecular pair by lE0 and use lE*and 3E*to symbolize singlet and triplet excimers, respectively. The following processes may be considered for production of 3E*:
-
+ hu + lE0 + lE0
lE0 S,
T,
-+
'E*
(4)
lE* 3E*
(6)
(7) These include direct photoexcitation, energy transfer from mobile singlet excitons, and energy transfer from mobile triplet excitons, where T, and S, are intended to symbolize mobile triplet and singlet excitons, respectively. Delayed excimer fluorescence would occur as a result of
T,
+ 3E*
-+
lE* + lS0
(8)
-- -
where lS0 is a ground-state molecule. The present results clearly rule out the sequence 4 7 8 since the emitting species would then be identical with the photoexcited lE* produced by direct excitation and would have a transition dipole with a fixed orientation relative to that of excitation. The sequences 5 7 8 or 6 8 would both give rise to negligible polarization; however, if process 5 is important, one would expect to observe prompt excimer fluorescence from these solutions. As reported e ~ r l i e r , ~ such an emission has not been observed and so a triplet migration sequence involving process 6 followed by process 8 is the only one which fits the available data. Concerning the delayed fluorescence Polarization, several mechanisms for this type of emission may also be considered. Probably the simplest would just involve the interaction of two freely migrating triplet excitons. That is +
T,
+ T,
-+
lS* + lS0
where lS* is the electronically excited singlet. No polarization of 's*would be expected, and processes of this sort are probably responsible for the bulk of the delayed
472
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The Journal of Physical Chemistry, Vol. 86, No. 4, 1982
fluorescence signal observed in the low microsecond time domain. It might be argued that significant polarization could result from reaction 9 if photoselected T, were initially formed as neighbors or next nearest neighbors. Taking account of the molar absorbancy index of 1,2-benzanthracene a t the exciting wavelength, the sample dimensions, the excitation pulse energy, and the quantum yield of triplet formation, the probability of pairwise triplet formation is only about 1in lo4for a 0.1 M solution. The probability of the pairwise occurrence of photoselected species is, of course, even smaller so it is reasonable to conclude that many migratory steps are necessary to achieve reactions such as reaction 9 and that negligibly small polarizations of the emission are expected. It has been demonstrated in earlier work that the delayed fluorescence emitted from these rigid solutions is of the annihilation type. To account for the significant polarizations observed at long delay times, one of the triplets involved in the annihilation must have been a photoselected one. This directly excited triplet is, therefore, trapped, and the annihilation process must occur between a migrating triplet and this trapped species. It was suggested previously that molecular pairs with separation distances significantly smaller than the average could be sites for triplet trapping. These paired species would not necessarily have the correct relative orientation to form an excimer but would simply be sufficiently close together so that the probability of a triplet exciton migrating between the two members of the pair is much greater than that for migration to a next nearest neighbor. Let us identify the two members of such a pair species as lSO(l) and 'S0(2) while their corresponding triplet states are 3S*(l) and 3S*(2). If we let T, represent a mobile triplet exciton at a next-nearest-neighbor site, then energy transfer to the paired species would occur by T, ~ ( i )is0 + 3s*(1) (104
T,
+ +~
-
-
( 2 )
is0
+ 3s*(2)
(lob)
and subsequent energy-transfer steps of the type 3s*(1) + ~ ( 2 ) 1so(i)+ 3 ~ 2 ) (11) would be more probable than transfer back to the next
nearest neighbor. If either member of the pair had already been directly photoexcited to the triplet state before the transfer from T, occurred, then 3s*(1) + 3s*(2) 3s*(1)
+ 3s*(2)
-
-
~ ( 1+ )~ ( 2 )
+
~ ( 1 )~ ( 2 )
(12) (13)
would take place, leading to delayed fluorescence. In this way either the emitting species is the photoselected molecule or else it is one step removed as is required by the polarization data.
Conclusions The time dependence of the optical anisotropy associated with the triplet state of 1,Qbenzanthracene in a polystyrene matrix is dominated by the nonuniform distribution of solute molecules. In regions of high concentration, depolarization is fast on the microsecond time scale and the contribution of triplet-triplet annihilations ensures that triplet decay is also fast in these regions. Thus, the triplets remaining at times greater than about 10 ms following the excitation pulse are relatively isolated and retain their polarization. The large DF polarizations found at long times imply that at least one partner in the annihilation reaction has remained immobile during its lifetime or has moved, at most, one migratory step; the other partner is necessarily a molecule representing the terminal point of an excitonic migration of undetermined length. Since the delayed excimer fluorescence exhibits no polarization, it is probable that the excimer sites are excited to their triplet state primarily by acting as the terminus of triplet migration sequence. A second migratory sequence terminating at the same excimer site leads to the observed nonpolarized delayed excimer fluorescence. Acknowledgment. This work was partially supported by a grant from the Research Advisory Board of the University of Nevada, Reno. The lasers were obtained from the San Francisco Laser Center supported by NSF Grant No. CHE 79-16250 to the University of California, Berkeley, and to Stanford University. We also thank Dr. D. A. Lightner for use of the polarizers, Dr. K. R. Naqvi for helpful advice, and Sheila Koenig for technical assistance.
