Langmuir 1992,8, 469-474
469
Triplet-Triplet Annihilation of Pyrene Derivatives as Mobility Probes in Sodium 1,4-Bis(2-ethylhexyl)sulfosuccinate/Water/Isooctane Reversed Micelles1 C. Bohne? E. B. Abuin? and J. C. S ~ a i a n o * * ~ Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR6 Received September 6, 1991 Triplet-triplet annihilation (TTA) of pyrene probe moleculeswas employed as a probe to study mobility of sodium 1,4-bis(2-ethylhexyl)sulfosuccinatereversed micelles in the microsecond time domain. Interand intramicellar processes can be differentiated by using different probe-to-micelleconcentrationratios. TTA for micelles containing only one pyrene molecule occurs with a rate constant of about one-third of the value for the diffusion-controlled limit, and it is ascribed to an intermicellar process in which the collision of triplets does not require the contact of water coresof the two micelles. The delayed fluorescence excimer-to-monomer (EIM) intensity ratio resulting from T T A is much lower than the one observed in homogeneous solution, indicating that the microenvironment for triplet-triplet encounter is more rigid when compared to solutions. T T A from micelles containing more than one probe (multiple occupancy) show initially a higher E/M intensity ratio for the delayed emission than the one observed for intermicellar processes,indicating that when two pyrenes are in the same micelle a more favorable geometry for excimer formation exists in the triplet-triplet encounter complex. A decrease with time of the E/M intensity ratio is observed under multiple occupancy conditions.
Introduction Reversed micelles in organic solvents are droplets of water surrounded by a layer of surfactant molecules, such as AOT (sodium 1,4-bis(2-ethylhexyl)sulfosuccinate).The single-phase microemulsion is stable for certain wateroil-surfactant compositions over a range of temperatures.5 The size of these micelles increases as a function of the water-to-surfactant molar ratio (R = [HzOl/[AOT]). At low (0.1 M) AOT concentrations and at a given R , the shape and size of each micelle can be considered to be independent on the surfactant concentratione6 The water core is surrounded by a negatively charged interface. The negatively charged moiety of 1-pyreneaulfonate(PSA)-one of the probe moleculesemployed in the present study--will be exposed to the water core, but the pyrene chromophore wilI be located at the micellar i n t e r f a ~ e Oxygen . ~ ~ ~ quenching experiments established8 that with increasing R (R 5 30)there is amoderate displacement of the probe molecule to more polar environments, but this displacement occurs within the interface. The second probe used, l-pyrenedodecanoic acid (PDA) will probably be solubilized in a similar environment as PSA, although due to the longer hydrocarbon chain the chromophore could be located further into the isooctane phase. Dynamic processes of AOT reversed micelles involve different processes, such as the collision between two micelles in which only interface interaction occurs, collisions *Address correspondence to J. C. Scaiano, Department of Chemistry, University of Ottawa, Ottawa, Ontario, CanadaKlN 6N5. (1) NRCC 33259. (2) Present address: Department of Chemistry, Univepity of Victoria, Victoria, Canada V8W 3P6. (3) Permanent address: Departmento de Quimica, Facultad de Ciencia, Universidad de Santiago, Chile. (4) Present address: Department of Chemistry,University of Ottawa, Ottawa, Canada K1N 6N5. (5) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J. Chem. SOC., Faraday Trans. 1 1987,83, 985-1006. (6) Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979,83,480. (7) Verbeeck, A.; De Schryver, F. C. Langmuir 1987,3,494-500. (8) Saez, M.; Abuin, E. A.; Lissi, E. A. Langmuir 1989,5, 942-947.
Scheme I
collision complex
dimer
+
J
m cluster
with water core mixing that correspond to the formation of dimers, and finally cluster formation (Scheme I). Conflicting numbers were reported for the exchange rate constant of probe molecules between micelles. Stoppedflow experiments, in which intermicellar electron transfer, proton transfer, and metal-ligand complexation were monitored, and laser flash photolysis experimenta, in which intermicellar triplet quenching was monitored, led to exchange rate constants of ca. lo7 M-' s-l. This value is 2-3 orders of magnitude lower than for the diffusioncontrolled limit of micelle-micelle encounters, indicating that water pool mixing does not occur at every encounter.699JO On the other hand, exchange rate constants in the order of lo9 M-ls-l were obtained from the analysis of fluorescence decay measurements of probes located either in the water core or at the interface that are quenched by molecules believed to be solubilized in the aqueous (9) Atik, S.S.;Thomas, J. K. J. Am. Chem. SOC.1981,103,3543-3550. (10) Atik, S. S.;Thomas, J. K. Chem. Phys. Lett. 1981, 79, 351-354.
0743-7463/92/2408-0469$03.QQIQ 0 1992 American Chemical Society
470 Langmuir, Vol. 8, No. 2, 1992
core.7J1 In a recent paper,12 in which phosphorescence and fluorescence quenching experiments were reported, it was suggested that the fast processes observed in the earlier reports7J by analyzing fluorescencequenching data were due to quenching to the fluorescent probe molecule within clusters formed by individual micelles. It was suggested that these clusters facilitate material exchange between discrete micelles.12 We decided to investigate probe mobility in reversed micellar solution by following the triplet-triplet annihilation (TTA) of PSA and PDA. In a recent paper13 we suggested that pyrene TTA could be a useful probe in the study of mobility in organized systems. Quenching experiments normally involve two moleculeswith different structures; thus, the solubilization sites of the excited probe and the quencher can be very different. In contrast, in the T T A process two excited triplet molecules are involved and thus are located on average in the same type of site in the reversed micelle. In addition, excited triplet states are longer lived than singlets, and thus it is possible to monitor directly bimolecular processes that occur in the microsecond time domain. For some molecules, such as pyrene and several derivatives, the TTA process leads to delayed fluorescence (DF). Under pulsed laser excitation it was recently reported13 that the DF shows a contribution of excimer emission while the prompt fluorescence, which for the purpose of this paper corresponds to the emission from singlet states formed directly in the excitation pulse, shows only monomer emission. This is due to the fact that in the TTA process two pyrene molecules are always present in the solvent cage that leads to the formation of the excited singlet responsible for DF, whereas the pyrene concentration is too low for a singlet state excited by the laser pulse to encounter a ground-state molecule during the excited singlet state lifetime. The excimer-to-monomer (EIM)intensity ratio of the DF is constant over time after the decay of the prompt fluorescence (-3.0 ps for most pyrene derivatives), suggesting that only one luminescent process, in this case TTA, is operating. The decay of the DF shows a significant contribution from second-order TTA, and the TTA rate constant can be obtained from the analysis of the decay profile. The TTA rate constant in homogeneous solvents with different viscosities approaches the diffusion-controlled limit when spin statistics is taken into account.13 A higher EIM intensity ratio of the delayed emission relative to the prompt fluorescence had been previously observed by several researcher^,^^-^^ but under their (11) Lang, J.; Jada, A.; Malliaris, A. J. Phys. Chem. 1988, 92, 19461953. (12) Jbhannsson, R.; Almgren, M.; Alsins, J. J.Phys. Chem. 1991,95, 3819-3823. (13) Bohne, C.; Abuin, E.; Scaiano, J. C. J. Am. Chem. SOC.1990,112, 4226-4231. (14) Tanaka, C.;Tanaka, J.; Hutton, E.; Stevens, B. Nature 1963,198, 1192. (15) Parker, C. A.; Hatchard, C. G. Trans. Faraday Soc. 1963,59,284295. (16) Birks, J. B.; Moore, G. F.; Munro, I. H. Spectrochim. Acta 1966, 22,323-331. (17) Parker, C. A. Spettrochim. Acta 1966, 22, 1677-1678. (18) Moore, G. F.; Munro, I. H. Spectrochim. Acta 1967,23A, 12911298. (19) Razi Naqvi, K. Chem. Phys. Lett. 1968,1, 561-562. (20) Birks, J. B.; Srinivasan, B. N.; McGlynn, S. P. J. Mol. Spectrosc. 1968,27, 266-284. (21) Birks, J. B. Chem. Phys. Lett. 1968,2, 417-419. (22) Stevens, B. Chem. Phys. Lett. 1969,3, 233-236. (23) Razi Naqvi, K. Chem. Phys. Lett. 1970, 5, 288-290. (24) Van Willigen, H. Chem. Phys. Lett. 1975, 33, 540-544. (25) Spichtig, J.; Bulska, H.; Labhart, H. Chem. Phys. 1976,15,279293.
Bohne et al. experimental conditions the singlets formed by direct excitation led to significant excimer formation, reflecting the relatively high ground-state concentration. Under the conditions of those earlier reports the decay approached fmt-order kinetics due to low triplet concentrations. Under the conditions of laser excitation employed here and in recent work13low pyrene concentrations can be used, thus avoiding the detection of excimer emission in the prompt fluorescence. Further, the triplet concentration generated is sufficiently high for second-order processes to dominate the triplet decay. It was also shown that the DF of several pyrenes have different EIM ratios for their delayed luminescence. This effect is due to different interaction modes of the molecules in the triplet-triplet encounter complex. In addition, in the case of PDA in homogeneous solvents the EIM intensity ratio is solvent dependent. A higher excimer content is observed in nonpolar solvents. This effect was attributed to carboxylic association assisting excimer f0rmati0n.l~ The detection of DF of pyrene in vesicles has been described earlier,29but no kinetic data were obtained. In the present work we report that T T A is a very good probe for processes in the microsecond time domain, and the analysis of the EIM intensity ratio of the DF makes it possible to differentiate between inter- and intramicellar processes. Experimental Methods AOT was purified by standard procedures.% 1-Pyrenedodecanoic acid (PDA), 1-pyrenesulfonate (sodium salt, PSA) and 1,3,6,9-pyrenetetrasulfonate(sodiumsalt, PTS) from Molecular Probesand isooctanefrom BDH (Omnisolv)were used as received. The water employed was conductivity grade and had been purified by a Milli-Q water purification system from Millipore. Absorption spectra were measured on a HP-8451Adiode array spectrophotometer. The laser system employed has been previously de~cribed.3~-33 Absorption spectra and luminescencedata were acquired with an EG & G gated and intensified optical multichannel analyzer (OMA). The delay after the laser pulse can be adjusted in 10-ns steps up to 1.0 ms, whereas the gate for data collection is either fixed at 20 ns or adjusted in 10-ne incrementa between 120 ns and 1 ms, although the longest gate width employed was lo00 ns. AOT solutions containing PSA and PTS were prepared by injection from aqueous stock solutions of known concentrations. After injection of the pyrene probe the samples were shaken for at least 30 s. PDA was introduced by injection from stock solutions of the probe in isooctane. Aliquots of water (pH 8.0) were subsequently added to obtain the pools at the desired R value. These solutions were cloudy after extensive sonication and were passed through Millipore filters before use. The concentration of the pyrene probes was such as to obtain absorbances between 0.1 and 0.6 at 337 nm for a 7-mm optical path. All sampleswere deaerated by nitrogen bubbling for 15 min, and the experimentswere performed in 3 X 7 mm2or 7 X 7 1111112 cells. The probes were excited with a Molectron UV-24nitrogen laeer at 337.1 nm (-8 ns, 700 ps). This observation parallels results in homogeneous solution13where no T T A was observed due to charge repulsion. An inspection of the transient absorption spectra indicates that for PSA and PDA the radical cation is also (34) For the purpose of this paper prompt fluorescence refers to the emission of singlets formed by direct laser excitation.
formed (half-life at ca. 3.0 ps). The formation of radical cations was previously observed for pyrenes in acetonitrile and hydroxylic s~lvents.'~ Reversed micelles could increase the yield of cation formation as it was shown that electrons are efficiently captured by the water This transient is associated with a fluorescent emission with lower EIM intensity ratios than the one that corresponds to the delayed fluorescence. All studies in the present work were performed after the decay of the emission related to the cation. The amount of radical cations at a given laser dose is higher for PSA than PDA, suggesting that the former is located closer to the water core. The time evolution of the DF contains information on the TTA rate constant. Emission spectra were collected at a constant gate with different delays after the laser pulse. The gate width was set between 500 and 1000 ns to maximize the precision of the experimental data at long delays. Monomer and excimer emissions were integrated between 370 and 410nm and 500 and 600 nm, respectively. The time evolution of the delayed fluorescence is given by
where [To]is the initial triplet concentration, lzl is the first-order decay rate constant, 2 k m ~is the TTA rate constant, and a is an experimental parameter that incorporates emission quantum yields, quantum yield of single formation in the TTA process, and the OMA sen~itivity.'~The triplet concentrations were obtained from triplet-triplet absorption data collected at a delay (ca. 4.0 ps) where prompt fluorescence does not distort the absorption signal and the concentration values were extrapolated to zero time. A molar absorptivity of 25 OOO M-l cm-l and an optical path length of 2.3 mm were used to calculate the c~ncentration.~'The experimental data were fitted,as previously described,13to eq 1by a computer u floated. program in which values for a,kl, and 2 k ~ were The same values for the TTA rate constant were obtained when the monomer or the excimer emissionswere analyzed. The 2 k m values ~ are below the diffusion-controlled limit (Table I).38 In homogeneous solution the observed TTA rate constants are close to the diffusion-controlled limit. For this reason, the decrease observed in the AOT system is due to a lower encounter frequency of the probes than the encounter frequency of the micelles. T o analyze for intra- and intermicellar probe mobility, we studied the TTA process for micelles containing more than one probe molecule. This can be achieved by using low micelle and high probe concentrations and by adjusting the water content a t constant AOT and pyrene concentrations. The different conditions employed are shown in Table 11. For none of these conditions was there any excimer fluorescence observed for the prompt emission. Under conditions of occupancy higher than one the EIM intensityratioof the DF is not constant over time; a higher (35) Thomas, J. K.; Grieser, F.; Wong, M. Ber. Bunsen-Ges. Phys. Chem. 1978,82,937-949. (36)Bakale, G.; Beck, G.; Thomas, J. K. J. Phys. Chem. 1981, 85, 1062-1064. (37) Although the cell is 3 mm wide, the effective optical path length is 2.3 mm due to the angle of incidence of the laser beam.18 (38) The diffusion-controlled collision rate constant for AOT micelles wasestimatedtobe1.7 X lO"JM-1s-1inheptane~asolventwithaviscosity slightly smaller (0.42 cP) than isooctane (0.5 cP). Taking into account ~ the TTA process,13the diffusion-controlled a spin-statisticsfactor of 6 /for limit of 2kWA is 9 X 1Og M-l s-l.
Bohne et al.
472 Langmuir, Vol. 8, No. 2,1992 Table 11. Fraction of Pyrene Occupied Micelles with More Than One Probe Molecule (fp),' Observed Rate Constants for the E/M Intensity Ratio Decay (kobs),b and Fraction of Triplet Pyrene Occupied Micelles with More Than One Triplet Molecule (fT)' LAOTI, conditions a b C
d e f g h
M 0.052 0.051 0.051 0.052 0.051 0.051 0.096 0.174
[PSAI,
kob
pM
R
fp
49 48 48 98 97 96 48 48
24.3 34.4 41.9 24.3 34.3 41.9 40.8 40.9
0.185 0.315 0.566 0.342 0.546 0.840 0.229 0.142
X
lo+, s-l
f~
0.076 2.4 f 0.6 (2) 0.136 1.6 f 0.3 (3) 0.262 0.106 2.0 f 0.4 (2) 0.184 1.4 (1) 0.352 2.4(1) 0.097 0.060
Values correspond to f>l = (1 - fo - fi)/(l- fo), fo and f i being the fractions of empty and singly occupied micelles, respectively, calculated assuming a Poisson distribution. Aggregation numbers were obtained from ref 52. Errors given as standard deviations; numbers of determinations in parenthesis. The fraction of micelles containing a certain number of triplet molecules was calculated assuming a Poisson distribution. The triplet concentration was assumed to be 10 p M for a-c, g, and h and 14 p M for d-f.
0.8 '.O
0.2
' i w
'"1 ;i
c
0.8
I1
0.6 0.4
0.2
10
I
I
I
I
I
10
20
30
40
50
II
Time, ~.ls
Figure 2. Temporal dependence of EIM. (I)50 gM PSA in 0.05 M AOT with R = 24.3 (A), 34.4 (B),and 41.9 (C). (11) 100 g M PSA in 0.05 M AOT with R = 24.3 (A), 34.3 (B),and 41.9 (C).
EIM value is observed at short delays (Figure 2).39 There is a distinct correlation between the high EIM intensity ratio and the fraction of micelles that contain more than one probe molecule (compare Figure 2 with conditions in Table 11). The EIM intensity ratio decays to a constant value which is dependent on the R value and the probe concentration. Slightly higher final EIM ratios are observed for higher R and probe concentration values. Apparent first-order rate constants (k&) were estimated from logarithmic plots and are shown in Table 11. These rate constants do not depend on the fraction of micelles occupied with more than one pyrene molecule. It was recently shown12 that quenching processes can occur within AOT clusters as a result of intracluster probe exchange. Cluster formation is favored when the composition is close to that for phase transition. Thus, it will be more prominent at high AOT concentrations and higher temperatures. It was also shown12that the clusters are bigger when dodecane rather than isooctane was employed as the organic solvent. We measured the TTA of PSA in AOTI dodecanelwater reversed micelles. No evolution of ~~~
the EIM intensity ratio was observed.4O Considering that cluster formation would be less favorable in isooctane and at lower AOT concentrations,12we conclude that clusters are not responsible for the EIM evolution observed in isooctane.
~~
(39)Under these experimental conditions eq 1cannot be employed as
the EIM ratio changes with time and intra- and intermicellar process contribute to the triplet decay.
Discussion Our observations can be readily divided into two groups: (a) at low occupancy (51) we observe that EIM is independent of delay time once the changes due to prompt phenomena and radical ion involvement are complete, and (b) when a significant fraction of the micelles has multiple pyrene occupancy (this occurs at the higher R values), a time dependence of EIM is observed. The details of our discussion will be divided along these lines; however,some general characteristics are worth presenting before. One- and two-photon ionization is a rather common phenomenon in the photochemistry of pyrene under various experimental c ~ n d i t i o n s ; ~the l - ~reversed ~ micellar systems discussed herein are no exception.35 Ion recombination contributes to the emission, and it is thus necessary to wait typically 3 ps for ion contributions to be sufficiently small as not to interfere with the measurements of delayed fluorescence. While the concentration of triplet states exceedsthat of single states after a couple of hundred nanoseconds, the luminescence is dominated by prompt fluorescence for several lifetimes, given the weak nature of DF emission. The delay mentioned above in connection with ionic phenomena is also sufficient for prompt fluorescence to decay below an acceptable level. Our experiments do not yield any information about the transient events that occur over the first 3 ps, although extrapolating the DF over this period is straightforward. It is interesting to note that even under conditions of multiple occupancy, the emission during the first 2.0 ps shows no excimer emission. This clearly shows that for both PSA and PDA no singlet-state-ground-stateinteractions take place during the singlet lifetime. Thus, we can place a conservative upper limit of 5 X 106 s-l ( R 1 25) for the rate constant for the interaction of an excited and a ground-state pyrene when they share the same micelle. This number should be compared with rate constants of 9 X lo6, 2.8 X lo7, and 5 X lo7 s-l for pyrene excimer formation in cetyltrioxyethylene sulfate micelles," for 1bromonaphthalene triplet-triplet annihilation in cetyltrimethylammonium bromide micelles,45and for radicalradical reactions in triplet-derived radical pairs including the thiophenoxy radical46in SDS micelles, respectively. These micelles are not of the same size as the AOT micelles studied here. (a) Systems with R < 12 under Single Occupancy Conditions. Naturally under these conditions DF arises exclusively from processes that involve intermicellar interactions. The TTArate constants obtained are around one-third of the value expected for a diffusion-controlled reaction.38 This is perhaps not surprising considering that reversed micelles are large aggregates and the probe molecule covers only a small fraction of the surface area of the micelle. The fact that the values for PSA and PDA are quite similar suggests that any differences in their (40)Conditions employed [PSA] = 50 pM,[AOT] = 0.053. EIM = 0.50at R = 10.4,and E / M = 0.71 at R = 15.5. (41)Piciulo, P. L.;Thomas, J. K. J. Chem. Phys. 1977,68,3260-3264. (42)Jones, C. M.;Asher, S. A. J. Chem. Phys. 1988,89,2649-2661. (43)Iu, K.K.;Thomas, J. K. J. Phys. Chem. 1991,95,506-509. (44)Infelta, P. P.;Gritzel, M. J. Chem. Phys. 1979,70, 179-186. (45)Rothenberger, G.;Infelta, P. P.; Gritzel, M. J.Phys. Chem. 1981, 85,1850-1856. (46)Bohne, C.;Alnajjar, M. S.; Griller, D.; Scaiano, J. C. J.Am. Chem. SOC.1991,113, 1444-1445.
Triplet-Triplet Annihilation of Pyrene Derivatives
Langmuir, Vol. 8, No. 2, 1992 473
occupancy. We initially thought that triplet self-quenchlocations at the micellar interface are not sufficient to ing could account for this observation. However, it soon influence the dynamics of the TTA process. became clear (Table 11) that increases in occupancy do A low and constant EIM intensity ratio for the delayed not have any significant effect on the observed rate for fluorescence was observed under conditions of single EIMdecay. This seems to rule out quenching by groundoccupancy. The EIM value is in fact smaller than those state pyrenes as a likely explanation. Similarly, the same observed in any homogeneoussolvents where the molecules experimental results lead us to discard the possibility of can explore all conformations. This suggests that the energy transfer from a triplet in a singly occupied micelle sandwich conformation (in which the two pyrenes align to a ground-state pyrene in a multiply occupied micelle face-to-face) which is required for excimer formation already containing a triplet. cannot be readily achieved as a result of intermicellar TTA. We also thought that a relaxation of the Poisson The results obtained under high occupancy conditions distribution of probes could account for the evolution of (vide infra) demonstrate that high EIM ratios are indeed EIM. We note that this distribution applies independently observed when the optimum conformation is achieved by to the triplets and ground states. Since the former are two molecules present in the same micelle. Why then is produced at the expense of the latter, the initial (i.e., it that during TTA this conformation cannot be achieved? immediately after the laser pulse) distribution will not We believe that the only reasonable conclusion is that follow Poisson statistics for either electronic state. Analintermicellar TTA occurs when the two micelles collide, ysis of these data47led us to conclude that the changes but that this process can occur without exchange of the expected from this relaxation would be too small to explaln pyrenes and without communication of water pools. Thus, the time dependence observed for EIM. the two triplet states undergo TTA while remaining From the data discussed above and our understanding identified with the original micelle in an encounter that of transient phenomena at low occupancy, we believe that probably involves a “sticky” collision where the micellar the evolution of EIM cannot involve diffusional encounters pair remains as such for a sufficiently long time for regions between AOT micelles. The inescapable conclusionis that of the micellar surfaces to come into contact. Given the the process must involve intraaggregate processes that known formation of relatively stable micellar clusters, it would be largely independent of the overall micellar is hardly surprising that collisions can be of an associative concentration. We were surprised that an intraaggregate nature. Quite clearly, only a minor fraction of these process could take several microseconds. Note that our encounters leads to mixing of the water pools; if we use results require an intraaggregate (not necessarily intrathe low excimer contribution to the DF as a reference, we micellar) process. In relation with this, we note that Almwould estimate that the rate constant for water pool gren et al.12 have recently quantified the aggregation exchange is at least about 100 times slower (13 X lo7M-l (cluster formation) of AOT reversed micelles. They have s-l) than the values obtained for TTA. shown that aggregation is more efficient in dodecane than I t is interesting to note that studies of the kinetics of in isooctane. The fact that we do not detect EIM evolution intermicellar processes will be expected to lead to very in the dodecaneIA0Tlwater system leads us to conclude different rate constants depending on whether the probe that cluster formation cannot explain the EIM evolution. reaction selected requires communication between the The arguments above leave slow intramicellar TTA as water pools or if interface-interface interaction is suffithe most likely explanation. The fraction of micelles with cient. Studies where the two molecules involved in the or more triplets is presented in triplets that contain two probing reaction reside in different micellar regions will Table II.48 The increase of the EIM delayed intensity clearly be more difficult to interpret than processes such fluorescence ratio (Figure 2) can be correlated with the as TTA where probing involves two identical molecules in increase of the fraction of micelles with two or more the same electronic state. We feel that a better undert r i ~ l e t s . 4One ~ possibility is that the slow kinetics reflect standing of these two types of interactions (pool-pool vs a slower encounter rate (-3.6 X lo5s-lI5O in the reversed interfaceinterface) may lead to easier interpretation of micelles than that observed in normal micelles ((0.9-5) X earlier data, perhaps some of the dramatic discrepancies in the reported kinetics mentioned in our I n t r ~ d u c t i o n ~ J ~ “ ~lo7 ~ s-1).4446 Although reversed micelles are bigger than normal ones, the decrease seems too large to be explained can be reconciled if probe location and the need for pool a slower encounter rate. Another possible reason for by exchange are taken into consideration. the slow kinetics may be related to the need for spin (b) Experiments Involving Multiple Probe Occuevolution when the triplet-triplet pair has a quintet conpancy (High R). The key characteristic that differenfiguration. Thus, these encounters can have a singlet, tiates transient phenomena under these conditions is the evolution of EIM with time even after the prompt and ion (47)The equilibrium distribution of triplets was calculated from the probability of triplets and nonexcited ground states being independently phenomena mentioned above are over. Figure 2 illustrates distributedover themicellepopulationaccordingto aPoissondistribution. these results. Typical lifetimes for the evolution of EIM For example with an initial triplet concentration of 10 pM,the fraction are ca. 5 ps. We note that these lifetimes are much shorter of micelles containing a triplet and no “companion” increases from 0.230 than the triplet decay observed for low occupancy con(initial) to 0.319(equilibrium) in the case of experimental conditions c and from 0.057 to 0.097 in the case of conditions f. The fraction of empty ditions. As illustrated by a comparison between Figure micelles (not containing any pyrene molecule) for conditions c is 0.31 2 and Table I1 there is a direct correlation between the whereas for f it is 0.070. Taking into account that more triplets would degree of multiple occupancy and the initial magnitude tend to redistribute for conditions f and that the fraction of micelles without any pyrene is much smaller for this condition, the value of kob of the EIM intensity ratio. It should be reemphasized should be smaller for f than for c if redistribution of triplets was the cause that singlets produced by direct excitation do not lead to for the EIM decrease. any excimer emission; clearly they are unable to move (48) A Poisson distribution was assumed. Although it is not strictly correct, as the initial distribution does not follow a Poisson distribution, sufficiently fast within the micelle to meet the other the numbers indicate the increase of the fraction of micelles with more (ground-state) pyrenes present in the micelle. than two triplets. (49)Note that in Figure 2 EiMcorrespondsto the excimer-to-monomer The fact that EIM values decrease over a few microintensity ratio and not to absolute intensity values. The intensity of the seconds and then fall in line with the low EIM values that delayed fluorescence decays over a much longer time domain than shown characterize single occupancysystems (videsupra) suggests in this figure. (50) Due to spin statistics only 5/9 of the encounters lead to TTA. the depletion of triplets in the micelles with multiple
474 Langmuir, Vol. 8,No. 2,1992
triplet, or quintet overall configuration. In homogeneous systems it is generally accepted that quintet encounters are dissociative and their only effect is an effective reduction of the TTA rate constant. It is conceivable that in reversed micelles the singlet and triplet encounters decay rapidly, while the quintet encounters-which would then account for the EIM evolution-must undergo spin evolution (intersystem crossing) before they can annihilate and as a result yield delayed fluorescence. Similar spin evolution is common in triplet radical pairs in micellesS5l We are not aware of any direct measurements of the dynamics of quintet pair evolution in micellar systems with which we could compare our data, and at this point this interpretation, while favored by us, is nontheless speculative.
Conclusions Our results indicate that T T A is an efficient process in AOT reversed micelles, occurringwith rate constants which are typically about one-third of the rates for diffusioncontrolled processes. Delayed fluorescence is readily observed as a result of TTA, but the values of E / M are quite small compared with homogeneous solution values. We conclude that TTA can occur between two triplets in different micelles without material exchange and/or communication of the water pools. Not surprisingly, two (51) Bittl, R.; Schulten, K.;Turro, N. J. J.ChemPhys. 1990,93,82608269. (52) Maitra, A. J. Phys. Chem. 1984,88, 5122-5125.
Bohne et al.
pyrenes that remain associated with the interfaces of two different micelles are unable to achieve the sandwich conformation required for excimer formation; as a result excimer emission is weak in this type of TTA. This type of micellar interaction is very fast, but should be clearly distinguished from processes that cannot occur unless there is mixing of the water pools; presumably this must be a requirement for other processes, such as reactions leading to bond formation. We feel that establishing whether a certain process can result from interface-interface interaction or whether it requires pool-pool mixing holds the key to understanding the dramatic differences between reported rates of intermicellar interactions. In this sense probing reactions where the participants are identical (as in TTA) greatly facilitate the interpretation of the data. Finally, multiple occupancy leads to a time-dependent evolution of the EIM ratio during the early stages of the triplet decay. After ruling out other possible explanations we speculate that this evolution may reflect the dynamics of spin evolution in intramicellar quintet encounters between two triplets.
Acknowledgment. Thanks are due to Mr. S. E. Sugamori and G. Charette for technical assistance. C.B. thanks the CNPq (Brazil) for a postdoctoral fellowship, and E.B.A. thanks the Guggenheim Foundation for a fellowship. Registry No. AOT, 577-11-7;PSA, 59323-54-5;PTS, 5957210-0;PDA, 69168-45-2;isooctane, 540-84-1.
