J . Phys. Chem. 1984, 88, 959-964
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Time and Temperature Dependences of the Delayed Fluorescence in Isotopically Mixed Naphthalene Crystals Ph. Pee, Y. Rebizre, F. Dupuy, R. Brown, Ph. Kottis,* Centre de Physique MolPculaire Optique et Hertzienne, UniversitP de Bordeaux I and C.N.R.S.. F 33405 Talence, France
and J. P. Lemaistre Centre de MZcanique Ondulatoire AppliquPe (C.N.R.S.),F 75019 Paris, France (Received:April 6, 1983)
An investigation of the delayed fluorescence intensity of isotopically mixed naphthalene crystals is presented for various concentrationsof traps (0.6% IC I15%) and for a large domain of temperatures (1.6 K I T 5 30 K). The time dependence of the delayed fluorescence shows, at all concentrations, a sharp buildup to a maximum, 1-10 ps after the laser pulse, followed by an exponential or nonexponential decay, depending on the trap concentration. As for the temperature effect, the delayed fluorescence is temperature independent in the range 1.6 K IT I5 K; at higher temperatures, it shows thresholds of strong enhancement and strong quenching. We discuss competing radiative and nonradiative processes accounting for the features of the delayed fluorescence efficiency in conjunction with the topology of the traps, the laser excitation intensity, and the time after the excitation.
I. Introduction Delayed fluorescence in pure and mixed crystals has been investigated both experimentally and theoretically by many authors.’” The assignment of this emission to the bimolecular annihilation of triplet-state excitons makes it a sensor of the dynamics of exciton interactions in disordered systems (Le. the trap topology). In a recent paper,’ we reported observations on the trap phosphorescence of a deuterated naphthalene crystal (Nd,) containing various concentrations of isotopic traps (Nh,). In this previous work, we analyzed the phosphorescence emission at low temperatures after nonselective (lamp) and selective (laser) excitations. This study allowed us to relate the temperature and concentration dependences of the phosphorescence to the topology of the traps, with emphasis on the important role of local inhomogeneities in the trap distribution. What is specific to the triplet-state topology is that these local inhomogeneities are amplified by the short range of coupling between triplet traps. Clusters of sites behave as centers for local processes of trapping and fusion (the so-called dynamical clusters’*) and as conducting sites for long-range processes (intercluster transfer via the crystal exciton band, for instance). In the present paper, we study the complementary aspect of the exciton dynamics, Le. the exciton annihilation. The analyses of the time, the temperature, the excitation intensity, and the trap concentration dependences of the delayed fluorescence provide (1) A. Suna, Phys. Reu. B: Solid Srare, 1, 1716 (1970). (2) S. E. Webber and C. E. Swenberg, Chem. Phys., 49, 231 (1980). (3) V. M. Kenkre and Y . M. Wang, Chem. Phys. Lett., 87, 263 (1982). (4) V. M. Kenkre, Phys. Reu. B Condensed Mafter, 22, 2089 (1982). (5) H. Sternlicht, G. C. Nieman, and G. W. Robinson, J . Chem. Phys., 38, 1326 (1963). (6) P. W. Klymko and R. Kopelman, J . Chem. Phys., 86, 3686 (1982). (7) R. Brown, J. P. Lemaistre, J. Megel, Ph. Pb,F. Dupuy, and Ph. Kottis, J . Chem. Phys., 76, 5719 (1982). (8) We have adopted hercthe usual assumption that triplet-state interactions are limited to the (a$) plane, which is valid for naphthalene. (9) We think that the actual radiative preparation of the distribution of pairs of excitons arid its relation to the distribution of trap pairs should be more elaborated. In addition, many exciton correlations should be considered both at low temperatures and at high temperatures where the reservoir of isolated excitons, proportional to Io, intervenes. (10) We checked that there were no biphotonic monomolecular contributions. (1 1) IbF(C)is related to R(C) defined in section IIIB by IbF(C) = $,(C) R(C) exp(AEE,/kBT1),where TI = 6 K is the temperature at the maximum. (12) R. Kopelman, E. M. Monberg, and F. W. Ochs, Chem. Phys., 19,413 (1977). (13) T. Holstein, S. K. Lyo, and R. Orbach in “Topics in Applied Physics”, Vol. 49, W. M. Yen and P. M. Selzer, Eds., Springer-Verlag, Heidelberg, West Germany, 1981.
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useful information on the time-dependent distribution of distances between excitons pz(R,t). (In rather simplified language, p2(R,t) may be called the biexcitonic density at distance R . ) Cooperating or competing processes modifying p2(R,t)such as laser excitation, fusion, thermally assisted transfer, and supertrapping are pointed out, and their ability to induce or to quench the delayed fluorescence channel is discussed. In what follows, we shall present the experimental conditions in section I1 and the experimental data in section 111. The discussion and conclusion are presented in section IV. 11. Experimental Conditions
Experiments were done on host crystals of deuterated naphthalene (Nd,) containing protonated naphthalene traps (Nh,). Trap molar concentrations were checked by mass spectroscopy and ranged from 0.6% to 15%. Monocrystals were prepared by the Bridgeman method from zone-refined materials (> 100 passes). Laser excitation of the triplet exciton band of the host crystal Nds or selective excitation of the triplet trap states Nhs was provided by a tunable dye laser (Molectron DL 400) pumped at 20 Hz by a nitrogen laser (Molectron UV 1000). Temporal studies were done with a PAR 162-163 boxcar. The decay of the signal was recorded first with the laser tuned to an absorption frequency of the crystal and then with the laser tuned off resonance by 2 A. The difference between the two signals eliminates the laser light scattered by the crystal. Delayed fluorescence intensities depend strongly on temperature. In view of the low intensities at some temperatures, the temperature dependence was determined by using a chopper to observe delayed fluorescence during a 12.5-ms interval at either short or long times after each excitation (cf. Figure 1). Note that the short time interval includes the laser flash. The ratio between the intensities at short and at long times is in good agreement with the value deduced from the temporal studies on integrating the delayed fluorescence signal.I0 In both the temporal and the temperature studies, signals were detected either by a Hamamatsu R446 photomultiplier placed behind an MTO 325a filter, passing all of the delayed fluorescence spectrum, or by an EM1 6256A photomultiplier placed behind a 2.6 A/mm Jobin-Yvon spectrometer with the slits set at 200
w. 111. Experimental Results
In this section we shall present the experimental results on the time, temperature, and excitation intensity dependences of the delayed fluorescence at trap concentrations ranging from 0.6% to 15%. 0 1984 American Chemical Society
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obrarratinn
Figure 1. Experimental conditions of excitation and of observation of the temperature dependence of the delayed fluorescence intensity at short (1) or long (2) times.
I
I
Figure 3. The normalized decay of the delayed fluorescence of a crystal containing 9% traps, after excitation of the host triplet band at 25 and 5 K.
li Figure 4. Logarithmic plot of the delayed fluorescence intensity at different trap concentrations, following excitation of the host triplet band.
I
,
0
2
Cm15%
I
1
-1
I
1
Figure 2. Normalized decay of the delayed fluorescence of isotopically mixed naphthalene crystals at different guest concentrations, following laser excitation of the host band at 25 K. -2
I
A. Time Dependence. Figure 2 shows the buildup and the decay (over the domain 300 ns-25 ws) of the delayed fluorescence z D F ( t )
at 25 K after a laser excitation in the triplet exciton band of the crystal (Nd,). The integrated signal (over 10 ns) has been recorded at four trap concentrations: C = 2%, 3%, 9%, and 15%. The buildup and the maximum are strongly concentration dependent and show that we are dealing with trap fusion processes. The rise time becomes shorter as the concentration increases. We notice that the maximum of IDF(t) is sharp at t = 1.5 ws for C = 15% while it is flat and spreads from 1.5 to 10 ps for C = 2%. The position and the shape of the maximum of IDF(t) @e. at short times) do not depend significantly on the temperature as illustrated
Figure 5. Log-log plot of the delayed fluorescence intensity of a crystal containing C = 3% traps. 1, is the maximum intensity reached at t = 6.5 ps. Under our experimental conditions, we found the decay to be nonexponential, being ZDF(t) a e~p(-AtO.~~), where A = 3 for t expressed in ps.
in Figure 3 where I D F ( t ) has been recorded at two temperatures (5 and 25 K) for C = 9%. Following the buildup, a nonexponential decay is observed at concentrations ranging from 0.6% to 10%
Delayed Fluorescence in Napthalene Crystals
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 961
1R
L
(a.u.)
100
Figure 7. Relative importance of the thermal contribution to the delayed fluorescence intensity in the regions I and I1 at different trap concentrations.
TABLE I: Variation of the Power Dependence of IDF(C,7') on I,, the Laser Intensity (IDF(C,7') (cf. Section IIIC)
=e)
a,long times
T, K
Figure 6. Partial quantum yields of delayed fluorescence, as a function of concentration and temperature, observed at short times (full circles), PDF(C,T)= So4mIDF(C,T,t)dt, and at long time (open circles), = ~lJm,27'57sIDF(C,T,t) dt. They correspond to excitation of the host triplet band in the conditions of Figure 1.
(cf. Figures 4 and 5 ) . For higher concentrations the decay becomes exponential with a lifetime of about 1.5 ~s for C = 15% (cf. Figure 4). The intensity was too weak at long times to observe the tail of the decay at C = 15%. Furthermore, Figure 3 shows that the decay becomes faster as the temperature decreases. B . Temperature and Concentration Dependences. Figure 6 shows the temperature dependence (1.6 K IT I35 K) of the delayed fluorescence intensity ZDF(C,T) for trap concentrations C = 0.6%, 3%, and 9%. The integrated intensity over a few milliseconds has been monitored at short and at long times after the laser excitation (cf. section I1 and Figure 1); therefore, in what follows, we indicate them as PDF(C,7') and PDF( C, T), respectively. Notice once more that the annihilation we observe seems to be specific to the trap distribution since the temperature dependence of ZDF(C,7') is roughly the same either on exciting the crystal exciton band or on directly exciting the traps. In the latter case the intensity is weaker (by a factor of for C = 3%). In Figure 6 we may distinguish four temperature domains for discussing the annihilation efficiency in terms of exciton interactions and motions in the trap topology: (i) In the low-temperature domain (region I), ZDF(C,T) is temperature independent, increasing with C. In this region we shall denote ZDF(C,T) = Z,,(C). A consistently similar temperature-independent behavior has been reported for exciton transfer probed with the supertrap phosphorescence emission.' (ii) When the temperature is raised (region 11) ZDF(C,T) shows a sharp increase from Z,,(C) to a maximum value Zmax(C)at T I = 6 K, this increase being much more pronounced for PDF(C,T) than for PDF(C,T). In Figure 7, we show the concentration dependence of this thermal contribution by plotting R(C) = IPmax(C) - Po(C)l/P0(C). At low concentrations the thermal contribution
1.6 3.8 4.6 12 18.5 20 20.9 21.8 23 26 31.6
0.6%
3%
a,short times
0.6%
2.19
1.76
2.14 2.01
1.7 1.60
2.02 1.53 1.60 1.49
1.67 1.81 1.78
1.74
1.78 1.73
1.40 1.59
1.27 1.17
1.06 0.92 1.43
3% 1.57
1.02
1.46 1.33
1.41 1.37
1.52
to PDF(C,Ti) is very large ( R = 200 for C = 0.6%); it becomes negligible for high concentrations ( R 0 for C > 7%). Its exponential decay with increasing C suggests the crossing (for T = 6 K) of a percolation threshold at C N 7% (see section IV). (iii) In region 111, ZDF(C,T) shows a different behavior with temperature, according to whether it is observed at short or at long times: PDF(C, 7') decreases with increasing temperature, indicating quenching of the annihilation by transfer in the traps and subsequent supertrapping in a time scale of milliseconds;PDF( C, T ) increases with increasing temperature, indicating that supertrapping is not efficient in a time scale of microseconds. (iv) Beyond 18 K the delayed fluorescence ZDF(C,T) rises strongly with temperature. It shows one maximum at 26 K for PDF(C,T) and another one at 22 K for IIDF(C,T). Both maxima are consistent with an activation energy of -90 crn-l, which corresponds to the trapcrystal energy gap. In region IV we observe the dominant role played by the exciton band of the crystal whose thermal activation may enhance the annihilation efficiency by a factor of 30. As it has been observed for region 111, PDF (C,T) is (at T > 22 K) strongly reduced by further quenching processes associated with trapping in the crystal exciton band, which is known to be very efficient. Furthermore, these processes are efficient enough (at T > 25 K) to quench even PDF( T), i.e. in the microsecond time scale. C. Excitation Intensity Dependence. The excitation intensity dependence of the delayed fluorescence (ZDF( C, 7')) evolves in a complex manner, owing to the various cooperating or competing processes that are active between the laser pulse and the obser-
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vation. Our data obey a simple empirical relation between the excitation intensity Io and the delayed fluorescence: IDF(C,n0: Z& where a is a parameter varying widely with the temperature (cf. Table I). For instance, IIDF(C,T), for which any radiative process inducing annihilation is absent, shows a quadratic variation ( a = 2 ) at low temperatures (region I) evolving to a linear variation (a = 1) at higher temperatures (region IV) (cf. Figure 6 and Table I). IV. Discussion The analysis of the delayed fluorescence channel at very low temperatures in a disordered two-dimensional distribution of triplet excitons should be straightforward, once the reduced density of pairs of excitons p2 (R,t) is known. The latter function embodies the radiative preparation of pairs of excitons at a distance R and subsequent processes such as annihilation and transfer-induced processes in the topology of traps. Thus, we may write for the time dependence of the delayed fluorescence intensity
with A ( R ) the annihilation rate at distance R: A ( R ) = 27r/hIT(R)12p
(2)
V ( R ) is the annihilation matrix element at distance R and p is the density of vibronic levels at twice the energy of the triplet state. In eq 1, we have neglected convolution with the decay of the fluorescent state, so that it is valid for times longer than the singlet lifetime. The evolution of p2(R,t) will be very complicated in general. Our experimental data suggest three contributions: the spontaneous annihilation of coherent excitons, (dp2/dtJo;an incoherent contribution, (dp2/dtJ,,,due to trap-to-trap transfer assisted by phonon scattering; and an incoherent channel at high temperatures, (dp2/dtJCr,due to thermal activation via the host crystal band:
dt
10.5
A
c.
10%
cr
Qualitatively, the first channel may explain the temperature-independent delayed fluorescence of region I, Figure 6, with p2 given by p2(R,t) = p2(R,0)exp(-A(R)t), p2(R,0)being the initial density of pairs at distance R. This density obviously depends on Zo, the excitation intensity. The transfer channel accounts for phononassisted exciton redistribution in the traps at temperatures lower than the temperature of activation to the host band. We assign this channel to the thermal contribution in regions I1 and I11 of Figure 6. The “crystal” channel accounts for triplet exciton redistribution via thermal activation to the host crystal band. It is dominant in region IV of Figure 6. Indeed, its specific effect is to couple the reservoir of isolated 1 s), where the majority triplet excitons with long lifetimes ( T of the excitation of the sample is stored, to a small number of pairs of sites with short lifetimes due to annihilation ( T 1 MUS). The integration of the motion of p2 is a difficult problem. A detailed description of the three contributions to p2(R,t) is in preparation, based on a two-dimensionals model of triplet intera c t i o n ~ .However, ~~ our experimental data obtained for a large domain of temperatures (1.6 K IT I30 K) and times (100 ns It I25 ms) allow us to discuss limiting cases where only one or two channels in eq 3 contribute to the delayed fluorescence. These limiting cases allow a very interesting, although qualitative, discussion of the main features of ID,(c,r)and IDF(t) and of their dependence on the trap topology. A. Time Dependence of the Delayed Fluorescence. Let us assume that the laser excitation of the triplet traps, via the crystal exciton band followed by a very fast trapping, prepares a distribution p2(R,t) of exciton pairs. At very low temperatures, we observed, depending only on the traps concentration, a constant delayed fluorescence level (region I in Figure 6 ) . According to
-
-
Figure 8._Histogram of the separation between nearest-neighbor traps in the (a‘$) plane of naphthalene obtained by a Monte Carlo calculation. The step of the histogram is 1.698 A (one-third of the distance between translationally inequivalent neighbors). Each histogram is the average of 50 grids of 10000 sites.
the model of eq 3, we assume this emission to originate from a topology of exciton pairs undergoing direct annihilation, exciting the fluorescence with characteristic decay times ( 2 ) . Now, considering the data displayed in Figures 2, 3, and 6 (region I), we may resume conclusions drawn in section 111: (i) Exciton annihilation is temperature independent at low temperatures. (ii) Exciton annihilation occurs in the traps. (iii) At short times (t < l o M S ) , the annihilation is predominantly temperature and transfer independent, and this seems to be the case even at temperatures as high as 30 K. These conclusions suggest that the coherent channel in eq 3 may allow one to describe the behavior of the delayed fluorescence at low temperatures or, in the time scale of microseconds, at higher temperatures. As a preliminary of such a description, we show in Figure 8 calculations of trap pair distributions that may be paralleled to the behavior of ZDF(t) as reflecting delayed processes of annihilation ( 2 ) in various pairs of excitons. Indeed, to a slow buildup, a flat maximum and to a nonexponential decay of IDF(t) at low concentrations (2% IC I 9%) (cf. Figures 2-5) correspond a large dispersion of pair distributions, hence a large dispersion of annihilation rates. The calculated pair dispersion and the value of the distance ro(C) around which it occurs strongly depend on the trap concentration
The Journal of Physical Chemistry, Vol. 88. No. 5, 1984 963
Delayed Fluorescence in Napthalene Crystals (cf. Figure 8). Furthermore, to the sharp buildup and maximum of IDF(t) at C = 15% and to its exponential fast decay correspond a very narrow pair distribution with a maximum around a shorter distance ro(C),which should lead to a unique and fast annihilation rate ( 2 ) . B . Temperature Independence ofthe Excitons Annihilation. The variation with temperature of the partial quantum yields PDF( C,T) and &F( C, T ) brings information on temperature-induced bimolecular processes that affect exciton dynamics in the traps (in the time scale ps It Ims). According to the model of eq 3, ZDF(C,T)in the region 1.6 K I T I 5 K originates from direct annihilation in various exciton pairs: IsDF(C,T) reflects exciton airs with short distances and short lifetimes ( 7 1 ps), while DF(C,T) reflects weakly bound pairs of excitons with longer lifetimes ( T 10 ms). The ratio PDF/?DF is about 10 in region I of Figure 6; it shows the importance of exciton inhomogeneities in the sample. When the trap concentration increases from 1% to 1076, the increase of the quantities PDF(C,T) and &F(C,T) is quite well accounted for by a corresponding statistical narrowing of the pair dispersion and by an increase of their radiative efficiency. As for region I1 in Figure 6, it is characterized by a sudden thermal contribution to IIDF(C,T), which we assume to be a thermal activation of transfers in the traps, i.e. contribution of channel “tt” in eq 3. We may account empirically for this variation as follows:
-
s
-
where the second term accounts for trap-to-trap thermally activated transfers.” From the analysis of our experimental data we found AEa N 30 cm-’, which appears as an activation energy. However, we have shown in experiments probing transfers in the traps that, for each trap concentration C, there exists a temperature threshold Tp(C)of dynamical percolationI2, with typical variation of the transfer efficiency as T crosses Tp(C) to higher values. Along the same lines, we may account for the following important features observed for the delayed fluorescence: (i) A sudden increase of ZDF(C,T) as T crosses Tp 5 K; for C = 0.6%,this increase multiplies PDF(C,T) by 100 and I“DF(C,T)by 5. (ii) With increasing concentration from 1% to lo%, the thermal contribution diminishes and becomes unobservable at 9%. Indeed, for low trap concentrations, we have in the sample a very large number of pairs of excitons with large distances, which do not contribute to &F(C,T) for T < Tp. When T crosses the corresponding threshold value, these pairs of excitons percolate to fusion, and that explains the strong and sudden increase of &(C,T). For obvious reasons, this thermal contribution is proportionally less important for PDF(C,T), which is concerned with a small number of pairs of excitons. This situation of a thermally or concentration-induced percolative transition may also be viewed from the standpoint of the microscopic transport equation written on the trap site populations:
-
(5) where kji(T) is the transfer rate between the slightly nonresonant traps i and j . “Raman type” twephonon-scattering processes result in a dependence of transfer rates kji on T.13 Equation 5 exhibits a percolative transition to long-range exciton transport when a sufficiently large number of rates kij(T) are faster than 1/fT, the molecular decay rate. This can be achieved either by increasing the temperature at fixed concentration or by increasing the concentration at fixed temperature. We suggest that this may account for the difference between IDF( T ) in regions I and I1 at low and high trap concentrations. At the higher concentrations, the shorter distances determine a sufficient number of fast transfer rates for ( 5 ) to be above or near the percolation threshold even at low temperatures in region I. Increasing the rates by increasing the temperature can have no further effect on the yield of delayed fluorescence until the values of kij(T ) are strong enough to determine very long-range exciton transport to super/traps. Quenching of the delayed fluorescence would than be expected.
In the temperature region 111, the variation of PDF(C,T) and & ( C , q reflects typical transfer-activated supertrapping that quenches slow annihilation and competes with fast annihilation. At the request of one of the reviewers, we noticed that the crystals contain traces of 2-methylnaphthalene, a supertrap for both the triplet and the singlet states. Supertrap fluorescence at high temperatures is negligible compared to trap fluorescence, and any supertrap phosphorescence was below the level of detection. Previous work7 shows that, when a voluntary concentration (3 X M) of supertraps is added to the crystals, supertrap fluorescence and phosphorescence dominate the trap emissions at the same temperatures. The depression of the delayed fluorescence in region I11 is then much more pronounced. No other supertraps were detected down to 5000 A. In this case, the three channels of eq 3 are effective so that quantitative analysis is not possible in the present stage of our calculations. In the temperature region IV, we observe a typical case where channel “ct” in eq 3 dominates the exciton redistribution, because it injects, via the crystal exciton band, the reservoir of isolated triplet excitons in the radiatively active pairs. The quantum yield PDF(C,T), especially, increases at least by 1 order of magnitude. This increase of the fast annihilation channel reflects the dynamic properties of a small number of close pairs of excitons (e.g. at distance R o ) that behave as exciton traps: owing to their fast annihilation rate A ( R , ) , they may emit many photons if the transfer rate via the crystal exciton band, Le., ka( T))is fast enough so that A(R,J >> k,, >> At, where At indicates the integration time for FDF(C,T) (cf. Figure 1). These pairs appear as a selective and efficient radiative channel via which all the excitons in the crystal should leak out, unless efficient supertrapping in the time scale of approximately microseconds is activated and that seems to be the case for T > 25 K. To resume the above comments, we may say that according to the importance of microscopic inhomogeneities in the trap distribution and to the excitation intensity Io, the delayed fluorescence in mixed crystals originates from four subdomains: (i) Areas where exciton pairs, proportional to Io2,undergo direct annihilation by an Auger-like mechanism. (ii) Areas where excitons pairs, depending on the temperature, undergo such an efficient transfer in traps that annihilation (fast) may be considered as a percolative local transition (so that annihilation is not affected by further increase of temperature). (iii) Areas where excitons are so distant that annihilation must be preceded by a succession of exciton transfers in the traps assisted by two-phonon-scattering mechanisms. (iv) Areas where reside isolated excitons proportional to the excitation intentiy Io. They communicate with radiatively active pairs only by thermal activation of the exciton crystal band. Further analysis of these four channels, contributing to the delayed fluorescence, needs a more sophisticated elaboration on excitoncorrelated dynamics. Such an approach is in preparation for a two-dimensional lattice. V. Final Remarks The present investigation of delayed fluorescence in isotopically mixed naphthalene crystals leads us to make the following general remarks: (i) Delayed fluorescence is due mainly to trap-trap exciton annihilation and not to mixed host-trap annihilation. As it has been pointed out for exciton transfer, the results on delayed fluorescence strongly reflect the inhomogeneous distribution of traps in the sample. Even at the highest experimental temperatures, where redistribution of trap excitons occurs via the crystal exciton band, the buildup and the decay of delayed fluorescence reflect the inhomogeneous distribution of trap-to-trap distances. (ii) The low-temperature thresholds observed at low trap concentration, and absent at higher concentrations, are comprehensible in view of the percolative transition to extended states (but at what scale?), which must exist for short-range triplet-state interactions. A detailed quantitative study of the temperature dependence of triplet dynamics is now in p r 0 g r e ~ s . I ~ (14) R. Brown, F. Dupuy, and Ph. Pbe, Ph. Kottis, in preparation.
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(iii) However, phonon-scattering processes must tend to zero with T,whereas both delayed fluorescence in this paper and transfer to supertraps in a previous paper7 tend toward concentration-dependent finite limits at the very lowest temperatures reached, T 1.6 K. This leads us to reexamine whether emissions at very low temperatures may be explained by microscopic quantum delocalized states. At any event, whether the low-tem-
-
perature results be best described by coherent states (direct annihilation) or by a master equation for site populations, there must be percolative behavior due to the short range of triplet-state interactions and to the strong microscopic inhomogeneities in mixed crystals. Registry No. Naphthalene-d8,1146-65-2; naphthalene, 91-20-3.
Photoprocesses on Colloidal Clay Systems. 3. Interaction of Dodecanol and I t s Micelles with Colloidal Montmorillonite R. A. DellaCuardia and J. K. Thomas* Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: April 18, 1983; In Final Form: July 21, 1983)
The fluorescent probe pyrene and 1-dodecanol have been incorporated in the interlamellar spaces of the clay mineral montmorillonite. This powder has been suspended in aqueous solution to form colloidal clay particles containing these molecules. Upon suspension, a fraction of the pyrene and dodecanol molecules form micelles that incorporate pyrene. The critical micelle concentration (cmc) of these micelles is approximately 3 X lo4 M. They render a nonpolar environment, and fluorescence quenching studies with hydrophobic and hydrophilic molecules show the expected trends of enhanced quenching rates in the former case and reduced rates in the latter. Quenching studies with the suspension containing pyrene in the interlamellar spaces of the clay and in the dodecanol micelles indicate that the pyrene excimer exists only in the micelles but not in the clay particles. The results show that the diffusion of molecules within the domain of the montmorilloniteparticles is significantly reduced compared to that of homogeneous aqueous solution. The system models natural conditions when clay colloids coexist with organic micelles. Fluorescence techniques enable kinetics in both particles to be observed independently in addition to ions interacting between the two colloids.
Introduction There is currently a large interest among photochemists in interfacial photochemistry. Colloidal systems, Le., micelles, microemulsions, vesicles, and semiconductors, are of interest because of their ability to promote specific photochemical reactions and inhibit others.’+ Recently, work has appeared that involves the photochemistry that occurs on the surface of colloidal clay As a continuatbn of this work, results are presented for colloids of montmorillonite particles that have pyrene and 1-dodecanol incorporated in their interlamellar spaces. The luminescence quenching techniques employed in this work have been especially useful in characterizing the nature of these systems and explaining their catalytic effects. The clay mineral used in this work was montmorillonite. It is an aluminosilicate containing aluminum in an octahedral configuration sharing oxygen atoms with silicon, which is in a tetrahedral configuration. Montmorillonite is referred to as a 2:l layered mineral because its aluminum shares oxygen atoms with silicon on either side of it. There then occurs an expandable layer into which water, organic molecules, or cations may be intercalated. The mineral possesses a periodic negative charge along its structure due to the isomorphous replacement of aluminum for ferrous or magnesium ions. The small size of these atoms permits them to take the place of the Si and AI atoms. The replacement of an atom of higher positive valence for one of lower valence (1) Fendler, J. H.; Fendler, E. J. “Catalysis in Micellar and Macromolecular Systems”; Academic Press: New York, 1975. (2) Turro, N. J.; Braun, A.; Gratzel, M. Angew. Chem. 1980, 80, 675. (3) Thomas, J. K. Chem. Rev. 1980,80, 283. (4) Bard, A. J. J . Phys. Chem. 1982, 86, 172. (5) Nozik, A. Annu. Reu. Phys. Chem. 1978, 29, 189. (6) Fendler, J. H. J . Phys. Chem. 1980, 84, 1485. (7) DellaGuardia, R. A.; Thomas, J. K. J . Phys. Chem. 1983, 87, 990. (8) DellaGuardia, R. A.; Thomas, J. K. J . Phys. Chem. 1983, 87, 3550.
0022-365418412088-0964$01.50/0
results in a net negative charge. This excess of negative charge is balanced by the adsorption of cations on the mineral’s surface. In the presence of water, these charge-balancing cations may be exchanged with other cations available in solution. Several studies have investigated the adsorption and nature of primary alcohols in montmorillonite powders.”6 The alcohols can be adsorbed from the vapor phase, pure liquid, aqueous solution, or in the presence of another s ~ l v e n t .The ~ results obtained from these clays depend on several factors, especially on the counterion that is adsorbed on the clay’s surface and the method of preparation. All of these previous investigations involved powders, and no studies have been undertaken on the colloidal nature of these “organoclays”. It was found in this work that a fraction of the dodecanol is released into the aqueous phase upon suspension of these particles and that the dodecanol molecules subsequently form micelles. The fluorescent probe pyrene was also incorporated into the interlamellar spaces with the dodecanol. It is also released from the clay and solubilized in the dodecanol micelles. The resulting suspension contains pyrene solubilized in two distinct environments: cosolubilized with dodecanol in the interlamellar spaces of the montmorillonite particles and in the hydrophobic core of the dodecanol micelles in the aqueous phase. The colloidal properties of this system are unique in that selected photochemical reactions can be carried out either on the surface of the clay particles or in the dodecanol micelles or simultaneously in both systems. This work represents, to our knowledge, the first (9) Theng, B. K. G.‘The Chemistry of Clay-Organic Reactions”; Adam Higler: London, 1978. (1 0) German, W. L.; Harding, D. A. Clay Miner. 1969, 8, 2 13. (11) Dowdy, R. H.; Mortland, M. M. Clays Clay Miner. 1967, 15, 259. (12) Brindley, G. W.; Ray, S.Am. Mineral. 1964, 49, 106. (13) German, W. L.; Harding, D. A. Clay Miner. 1971, 9, 167. (14) MacEwan, D. M. C. Trans. Faraday SOC.1948, 44, 349. (15) Greene-Kelly,R. Trans. Faraday SOC.1955, 51, 412. (16) Brindley, G.W.; Hoffman, R. W. Clays Clay Miner. 1962, 9, 546.
0 1984 American Chemical Society
