J. Phys. Chem. 1982, 86, 1636-1641
1698
torsion. In this case a two-dimensional potential function would be necessary to accurately describe the barriers, since one rotor is asymmetric we are not able to perform these calculations.
Acknowledgment. We thank W. Jones for her assistance in recording preliminary infrared data and Dr. P. H. Hackett for discussions concerning the relevance of his MPIRD studies on C2F51.
Quenching of Pyrene Fluorescence by Alkyl Iodides in Sodium Dodecyl Sulfate Micelles J.-E. Lofroth' Department of Physlcal Chemlsby, Chalmers Unhwslly of Technology end Unlverslty of Oateborg, 8 4 12 96 Gijteborg, Sweden
and M. Ahngren Department of Physlcal Chemisby 1, Chemical CentersP.O.B. 740, 5-220 07 Lund 7, Sweden (Recelved: November 4, 1981)
A time resolved fluorescence study with the single photon counting method has been undertaken to estimate rates of entry and exit of alkyl iodides of different chain length into and out from micelles. The system studied was pyrene in sodium dodecyl sulfate (SDS); the pyrene fluorescence quenched by ethyl, butyl, hexyl, octyl, decyl, and dodecyl iodide. The entrance rate constants were found to depend only slightly on the number of carbon atoms in the chain of the quenchers, from 9.7 X lo9 M-'s-l to 6.6 X lo9 M-' s-l for ethyl iodide and octyl iodide, respectively. Intramicellar quenching was described by first-order rate constants varying from 2.9 x IOs s-l to 5.9 X lo6 s-l in the series from ethyl to octyl iodide, then decreasing to 4.5 X lo6 s-' for decyl and dodecyl iodide. A more pronounced dependence on the chain length was found for the exit rate constants, 8.3 X 106s-l for ethyl iodide and 0.4 X 106 s-l for Odyl iodide. In cyclohexane the quenching of pyrene fluorescence with these quenchers could not be described by a Stern-Volmer equation, but it showed a positive deviation.
Introduction In a previous report,' we presented results from a study of micellar aggregates with a method based on fluorescence quenching. With the chosen pair of fluorescent species and quencher the size of different types of aggregates and the rate constants describing the quenching were determined. One of the basic assumptions in that study was that both the fluorescent molecule and the quencher stay in or on a micelle for a time much longer than the unquenched lifetime of the excited species. If this is not true, but the quenchers are free to move between the micelles and the water region during the excited lifetime, it is, at least in principle, possible to obtain information about the rate parameters describing the entrance and exit processes. The fluorescence decay for the micelle bound excited species after a 6 pulse excitation has been derived by several authors and is given by F(t) = F(0) exp(-At + B(e-ct - 1)) (1) where the parameters A, B, and C contain the information of interest. (The same decay law described the situation in our previous report but with different meaning of the parameters.) In the analysis section this equation will be discussed in detail and it will be shown how experiments at different quencher concentrations and micelle concentrations could give the rate parameters for the system. This paper presents measurements of the fluorescence decay of pyrene, P, in sodium dodecyl sulfate, SDS, mi(1)M.Almgen and J.-E. Lofroth, J. Colloid Interface Sci., 81,486 (1981). (2)(a) P.P.Infelta, M. Grfitzel, and J. K. Thomas, J. Phys. Chem., 78,190 (1974); (b) M.Tachiya, Chem. Phys. Lett., 33, 289 (1975);(c) P.P.Infelta, ibid., 61,88(1979);(d) J. C.Dederen, M. van der Auweraer, and F. C. de Schryver, ibid., 68,451 (1979);(e) M.D. Hatlee and J. J. Kozak, J . Chem. Phys., 72,4358(1980); (f) M.van der Auweraer, J. C. Dederen, E. Gelad6, and F. C. de Schryver, ibid., 74,1140 (1981). 0022-365418212086-1636$01.25/0
celles in water in the presence of ethyl, butyl, hexyl, octyl, decyl, and dodecyl iodide, EI, BI, HI, 01, DI, DDI, at different SDS and alkyl iodide, AI, concentrations.
Experimental Section 1. Materials. Sodium dodecyl sulfate (BDH, specially pure) was used as supplied. A preparation twice recrystallized from methanol did not show any significant differences. Pyrene (Aldrich, 99%) was twice recrystallized from ethanol. The alkyl iodides were purchased from Fluka (EI, 99.5% and HI, 98%), Merck (BI, 97%, 01,95%, and DDI, 97%), or Koch-Light (DI, 98%). All solutions were made with distilled water, run through a millipore system. 2. Sample Preparation. Stock solutions of SDS, P in SDS, and AI in SDS were prepared, the pyrene solution by stirring a mixture of P and SDS in water for several days, the AI solutions by injecting the iodie with microsyringes in SDS stock solutions. Appropriate volumes of these solutibns were then mixed and diluted to the mark in 10-mL volumetric flasks. As the alkyl iodides, specially EI, vaporize easily no deoxygenating of the samples could be done. Thus all measurements reported below were made on air-equilibrated solutions. The concentration of pyrene was kept low enough to avoid excimer formation. 3. Steady-State and Time-Resolved Fluorescence Studies. Absorption spectra were recorded with a Beckman Acta I11 UV visible spectrophotometer and emission spectra with an Aminco SPF 500 corrected spectra spectrofluorometer before and after each time resolved experiment. As the concentration of pyrene was kept constant for each micelle and AI concentration the relative heights of the peaks in the emission spectra for an unquenched and quenched solution at fixed micelle concentration could be used to determine the relative fluorescence intensities. In repeated experiments, each time on freshly 0 1982 American Chemlcai Society
Fluorescence Studtes of Mtcelles
The Journal of Physical Chemisw, Vol. 86, No. 9, 1982 1637 nsoc
0
I
500
1000
'
I
I
the following rate constants are introduced: l / r Ois the reciprocal lifetime of unquenched pyrene fluorescence, s-'; k , is the entry of a quencher molecule into a micelle, M-' s-l; k- is the exit of one quencher molecule from a micelle, s-l; k , is the quenching of an excited pyrene molecule by one quencher molecule in the micelle, s-l. The rates of the last two processes are assumed to depend on the number i of quencher molecules in the micelle and are given by ik-[Pi*] and ik [Pi*], respective, where [Pi*] is the concentration of micehes with i quenchers and one excited probe. Thus the rate equation for this situation is given by --= d[Pi*l dt
1
A
100
200
( 1 /+ ~ k,[Q,] ~
channel number
Flguro 1. Decay profiles for pyrene fluorescence quenching by butyl iodide in SDS micelles (6.220 ns channel-').
prepared solutions, the measured intensities deviated at most by 2%. This indicates that the solutions did not change in composition during a time-resolved experiment and that all AI was dissolved. The fluorescence decay of pyrene was followed at 380 nm (BalzersR-UV 11nm interference filter) with the single photon counting technique; the instrument is described el~ewhere.~The lamp was a gated flashlamp from PRA (Model 510 B), equipped with a new body of MACOR,4 fded with H2 at 0.5 atm pressure, and run at 30 kHz. The FWHM value of the pulse as measured by the photomultiplier (RCA 8850 with energy discrimination against multiple photon events) was 2 ns when the fluorescent solution was replaced by a Ludox scattering solution. The excitation light was selected by a monochromator (JobinYvon H10 vv)and set at 335 f 8 nm. With these settings and on the time scale used,the observed fluorescence signal could be regarded as the true fluorescence decay after subtraction of the ba~kground.~The position of the peak of the Ludox signal was chosen as zero time channel as discussed in ref 1. The data were analyzed by a fitting procedure using a modified nonlinear Levenberg-Marquardt algorithm! Both the x: and a plot of the weighted residuals, WR,were used as criteria for a good fit. x: and WR are defined as
where N = number of channels (= A - B + 1where A and B are defined in Figure l),v = number of estimated parameters, Ft = calculated intensity in channel i and Fi" = the observed number of counts in channel i. A RUNS
test' was also done to judge the fit if the x,2 value was higher than we normally accepted. All measurements were made at 25.0 "C.
Analysis To describe the decay of excited pyrene in a micelle containing i quenchers and at most one pyrene molecule, (3) J.-E. L6froth, S. BergstrBm, and D. Biddle, in preparation. (4) R. L. Lyke and W. W. Ware, Reu. Sci. Instrum., 48, 321 (1977).
(5)A. E.W. Knight and B. K. Selinger,A u t . J. Chem., 26,l (1973). (6)International Mathematical & Statistical Libraries, Inc., Houston, TX, routine ZXSSQ. (7)N. R. Draper and H. Smith, "AppliedRegression Analyeis", Wiley, New York, 1966, pp 95-99.
+ ik- + ik,)[Pi*] (i + 1) k-[Pi+~l- k+[QJ[P*i-1]
256
i = 0, 1, 2, ...; [P*-,]
0
where [Q,] is the quencher concentration in water. The distribution of the quenchers is by these assumptions Poissonian.2bpb If the concentration of fluorescent molecules in the micelles and the intensity of the exciting light are kept low, as they are in our experiments, the above assumptions lead to the following decay law for the resulting total fluorescence signal 2a-f
F(t) = F(0) exp(-At
+ B(e-ct - 1))
(14
where
k,k,
and K = k+/k-; C = k , + k- [Qt] = total concentration of quencher in the solution; and [MI = micelle concentration. If t a then F(t) = F(0)e-At-B and if t 0 then F(t) = F(0)e-At. If it is possible to identify the steady-state part, i.e., the long time behavior of the fluorescence signal, the dependence of A on [Qt] can be determined for a fixed micelle concentration. From the slope of the A vs. [Qt] plot a can be calculated at this concentration. (70 can be determined in a separate experiment without quencher.) It is also possible to estimate B and the constant C for each [Qt] by holding the found A fixed in a fitting over the whole curve. With different [QJ the /3 parameter can then be calculated at the actual detergent concentration from the slope of a B vs. [QJ plot. By rearranging eq l b and ICit is seen that
-
-
1
n b
1 = -[MI a k-k,
fl
+k+kq b
and (3)
With the help of these equations and experiments at different micelle concentrations analyzed in the same way as above at each quencher concentration, it is possible to extract all of the rate constants k-, k,, and k,. As there is probably more confidence in the slopes than in the in(8) (a) M. Almpen, F. Grieser, and J. K. Thomas, J.Am. Chem. SOC., 101,279(1979);(b) M.Almgren, F. Grieser, and J. K. Thomas, ibid., 101, 2021 (1979).
1638
Lofroth and Almgren
The Journal of Physical Chemistry, Vol. 86, No. 9, 1982
't /
I
!BIj %lO3/M
Figure 2. Values of A and
[a,] from Table I plotted according to eq
lb.
tercepts of the experimental 1/a vs. [MI and 1/p vs. [MI plots, we suggest that the final estimates of the rate parameters are made according to k- = k, =
(slope slope l / a
C (slope
1/WZ
[El: x l O Y M
Figure 3. Values of 8 and IC.
[a,] from Table I plotted according to eq
I
(4)
(5)
The entry rate constant k+ is more difficult to estimate since it can be deduced only with help of the intercepts slope 1 / a k , = k- intercept 1/ a (64 or slope 1/@ k , = k- intercept 1/p
(6b)
However, if there is reason to believe that k- k , then k- = C (94 k , = l/(slope l / a ) (9b) Results and Discussion h u l t s are presented for pyrene fluorescence quenching by six different alkyl iodides. The fluorescence decay of pyrene in the presence of different amounts of butyl iodide at fixed detergent concentration, 0.040 M, is shown in Figure 1. The decay curves have been normalized to the same peak height from approximately 40 OOO. From the limiting slopes at long time the rate constant A is calculated by computer fitting to a straight line for each
micelle] a 103/M
cx
[Qlx -
A x 1031~ i 0 - 6 / ~ - 1 0.0
0.5 1 .o 1.5
*
2.0 2.5 For B
B
6.13 6.71 0.481 7.17 0.756 8.97 1.127 9.17 1.314 10.00 1.822 and C estimation only.
10-6/~" 4.86 6.23 6.17 6.50 6.57
xu2 1.10 0.93" 1.28" 1.32" 1.100 1.220
quencher concentration and the result is shown in Figure 2. The behavior is as predicted by eq l b with a = 1.55 X lo9 M-'s-l and intercept = 6.13 X lo6 s-l. For each quencher concentration parameters B and C were estimated by fitting to all the corresponding curve in Figure 1with zero time as indicated and the found A fiied. Figure 3 is a B vs. [Qt] plot for this detergent concentration and it verifies eq ICwith p = 171.8 M-l. Values of A, B , and C estimated from these fits are summarized in Table I.
Fluorescence Studies of Micelles
The Journal of Physical Chemistry, Vol. 86,No. 9, 1982 1839
4
7
0.4
0.3
2 5
0.2
0.1
0 0
Flgure 5. Values of l l p and [MI plotted according to eq 3. The point 1//3 = 1.39 X M has been obtained from experiments giving Figure 3.
TABLE 11: Parameters Describing the Dependence of A, B, and C on Micelle Concentration for Different w e n c h e r s
cx quencher
10-6/s-'
E1 BI HI 01 DI DDI
12 i: 2 6.1 r 0.8 6.4 r 0.6 5.6 * 0.1 4.5 4.6
slope int 1/a x CY x 106/s 10'O/M s
0.500 0.926 1.602 2.398
4.292 1.473 1.548 1.442
slope l/p
int l/p x 103/M
17.397 1.590 1.429 0.904
4.105 0.600 0.134 0.292
By repeated experiments at different detergent concentrations, we finally can plot 1 / avs. [MI, Figure 4, and l//3 vs. [MI, Figure 5, for the butyl iodide quenching experiments. The micelle concentration has been calculated as [detergent] - [free monomer] = average aggregation number where we have chosen [free monomer] = cmc (critical M and average agmicelle concentration) = 8.2 X gregation number = 62.9 To obtain Figures 4 and 5, experiments were run at least twice for each quencher concentration and for each micelle concentration over normally five to six different quencher concentrations. The dependence of a and 0 on micelle concentration were established by measurements over at least four different detergent concentrations. Slopes, intercepts, and C values calculated in this way for the six alkyl iodides are given in Table 11. No dependence of A on [Qt] could be detected for decyliodide, but Figure l for this quencher would be a family of curves with equal slopes in the long time domain. The C values quoted for DI and DDI were obtained by assuming A = l/s0(= 6.13 x lo69-l) in the fitting procedure and experiments were made at only one micelle concentration. Estimated B values were in good agreement with ii (f [Qt]/[M]) for DI up to ii = 1 and [MI = 1 X M, but deviated more at higher quencher concentrations. These solutions at high concentrations, 1 < ii < 2.7, also showed (9) N. J. Turro and A. Yekta, J.Am. Chem. SOC.,100,5951 (1978), and references therein.
0.5 n
1.0
Flgure 8. Luminescence quenching of pyrene by octyl iodide for different detergent concentrations. Continuous curve is the integrated form of eq 1 with assumptions k - > k - .
an aging effect in that a precipitate was formed during the day. This must be interpreted as evidence for saturation at only slightly more than one decyl iodide per micelle. A similar conclusion is drawn for DDI where the aging effect was even more pronounced. In Table I11 the final estimates of k-, k,, and k, are presented using eq 4, 5, 6a, and 6b. I t is seen that there is a trend in 12, if eq 6a is used but not if we use eq 6b. From Table I1 we find for octyl iodide a slope 1//3 < 1, which is physically impossible (if the micelle size is constant) since it implies that either k- or k, is negative. Thus we cannot use both the slope and the intercept from the 1/0 vs. [MI plot to calculate the rate constants. For decyl iodide no quencher concentration dependence was found in A. We may therefore assume that k- k-. The intensities are presented as In Fo/F vs. ri where Fo and F are the intensities without and in the presence of quenchemg The agreement is acceptable, again confirming the assumption for octyl iodide. For ethyl iodide we may check the results by instead assuming k- >> k, and calculate k- and k, by eq 9a and 9b. The results are also given in Table IIIb together with k, from eq 6a. For methylene iodide in SDS micelles k- and 12, have been estimated to 9.5 X lo6 s-l and 25 X lo9 M-' s-l, respectively, by Infelta et a1.2a Methylene iodide and ethyl iodide should indeed have similar values of these parameters. The trend in k, from eq 6a is also in accord with a close to diffusion-controlled entry process for the solubilization in micelles, as found for some arenes in SDS.& Also the entry of the micelle monomer itself seems to be close to diffusion controlled.1° (10) E. A. G. Aniansson, S.N. Wall, M. Almgren, H. Hoffmann, I. Kielmann,W. Ulbricht, R. h a , J. Lang, and C. Tondre,J.Phys. Chem., 80, 905 (1976).
1640
Lofroth and Almgren
The Journal of phvsical Chemistry, Vol. 86, No. 9, 1982
Final Estimates of Rate Constants k - , k , , and k , , and the Distribution Constant K = k + / k - ( M - ' )
TABLE 111:
I
quencher (a1
k- x
k, x
S''
S-'
8.3 1.4 0.75 0.4
2.9 4.8 5.4 5.9 4.5 4.6 2.o 5.6
E1 BI
HI 01
DI DDI
12 0.4
E1
01
(b)
k, x M-Is-',
Ba
ca
M-1
KIM",
s-l
eq 6a
eq 6b
9.7 8.8 7.8 6.6
35.2 3.7 8.O 1.2
1.2x 6.3x 10.4x 16.5x
eq 6b
4.2x 103 2.6x 103 10.7x 103 3.0 x 103
103 103 103 103
1.2x 103 17.2x 103
for 0 1 , HI, and BI with Different
X"'
K/M-',
eq 6a
14 6.9
TABLE IV: Estimated Values of B, C, and aq x x 10'
k, x
z
Bb
r m ; [ SDS] = 0.07 M X" '
Cb
z
aa
01
0.81 1.61 2.42 3.22 4.84
0.474 0.870 1.246 1.568 2.211
5.44 5.51 5.48 5.64 5.88
1.25 1.16 1.17 1.33 1.04
1.89 0.84 0.06 0.51 0.08
0.81 1.61 2.42 3.22 4.03
0.307 0.653 0.938 1.269 1.586
8.25 6.55 6.56 6.32 6.54
1.28 1.26 1.13 0.95
0.41 1.14 1.31 1.28 0.81
0.81
0.332 0.565 0.718 0.886 1.311 1.418
5.36 5.30 5.70 6.41 6.49 6.92
0.96 1.11 1.25 0.98 1.07 1.10
0.47 0.07 1.39 1.08 1.77 0.24
0.61 1.130 1.633 2.064 2.905
4.14 4.25 4.24 4.37 4.61
1.26 1.11 1.09 1.21 0.93
1.66 0.58 0.06 1.40 0.39
75 70 67 64 60
0.470 0.995 1.446 1.998 2.446
4.90 4.40 4.40 4.15 4.40
1.14 0.87 1.12 1.06 0.86
0.29 0.02 0.40 1.58 2.42
58 62 60 62 61
0.715 1.408 1.928 2.358 3.353 3.678 = 163 ns, C in
2.64 2.39 2.47 2.75 3.00 3.15
1.oo 1.20 1.34 1.09 1.19 1.22
0.73 0.86 0.50 1.85 1.32 0.88
88 87 80 73 69 65
HI 1.00
BI
1.61
2.42 3.22 4.84 5.64 a
With
from fit at long times, C in l o 6 s - ' .
With T -
=
T,
l o 6s - ' .
The results by Aniansson et al.l0also make it possible to predict the behavior of the rate constant k-. It was found that the logarithm of the exit rate constant decreased linearity with the number of carbon atoms in the chain. The change was in accord with a linear increase in the free energy of transfer of a monomer tail from micelles to water, corresponding to a "Hydrophobic effect" of about 1kT per CH2 group. The effect on the value of the rate constant do thus amount to a decrease by a factor of about 7 on addition of two CH2 groups. In view of this the observed differences in k- for BI, HI, and 01are impossibly small. In the analysis of the data we have aasumed that the size of the micelles is unaffected by the addition of pyrene and alkyl iodide. This could be a severe fault as recently shown by Offen et a1.l1 They measured with the method of quasielastic light scattering the influence of added small molecules, among them pyrene and hexane, on the average hydrodynamic radius of several micellar aggregates. Hexane was found to have a stabilizing effect on smaller micelles. The radius of gyration decreased from 106 A at [hexane] = 1.2 X 10" M to 34 A at 9.5 X M in dodecyltrimethyl bromide micelles ([detergent] = 0.035 M) at high ionic strength. Similar effects have been found when alkanes were added to different micelle solutions.12 Stabilization of small aggregates upon alkyliodide addition will raise the micelle concentration and if we use a smaller micelle concentration than the true concentration in the 1/a vs. [MI and 1//3vs. [MI plots the slopes and the in-
tercepts will be influenced leading to wrong estimation of the rate parameters. Pyrene, on the other hand, was shown in Offen's work to increase the micelle size. It has been found that pyrene prefers large, rodlike CTA (cetyltrimethylammonium) bromide micelles to small globular CTA chloride mi~e1les.l~Addition of both pyrene and alkyl iodide could then create a situation where the probe molecules tend to avoid each other: the alkyl iodides would be found preferentially in the small aggregates and pyrene in the larger micelles. Since the study by Offen et al. dealt with micelle solutions at high salt concentrations where the micelles are rodlike, and fairly high concentrations of solubilizates were used, the results are not directly applicable to our solutions. However, the possibility of induced changes in micelle properties and of strongly nonPoissonian distributions should be investigated. The influence of broad micelle size distributions on estimated parameters was studied both theoretically and in computer-simulated fluorescence decay e~periments.'~ Decays corresponding to the situation k- > k- were generated and analyzed with eq l a where now
(11)H.W.Offen and D. R. Dawson, J.Colloid Interface Sci., 80,118 (1980). (12)E.Vikingstad, J. Colloid Interface Sci., 68,287 (1979).
(13)M.Almgren, J.-E. Lofroth, and R. Rydholm, Chem. Phys. Let 63,265 (1979). (14)M.Almgren and J.-E. Lofroth, J . Chem. Phys., in press.
A =
1/70;
B = [Qt]/[M]; C = k,
(10)
In this case it was shown in ref 14 that the variation of estimated B values with x , the ratio of micelle bound quenchers to micelle bound detergent monomers, assuming a Gaussian micelle size distribution, should follow
a, = a,
- (a*/2)x
Fluorescence Studies of Mlcelles
The JWtnal of phvsical Chemistty, VOI. 86, NO. 9, 1982 1841
where d, is the calculated aggregation number from quenching experiments (= B / x ) , 6, is the weight average aggregation number, and u is the standard deviation of the Gaussian distribution. It was also found that the presence of polydispersity introduced trends in the estimated k, values. The presence of polydispersity in the SDS-pyrene-allryl iodide systems studied in this work will influence the decay of the excited state. In Table IV we present values of B and C estimated from the decay of excited pyrene quenched by 01, HI, or BI in SDS micelles, [SDS] = 0.070 M. Results from estimations with two different models are given. Those marked a have been obtained as discussed above in the Analysis section with A = l/r, fixed in the fitting procedure. In those designated b we held A = l/ro = 1/163 (ns)-’ flxed and estimated B and C. Thus in the b analysis we assume the quenchers to stay in or on the micelle during the lifetime of the excited state. The aggregation numbers, d, have been calculated as dp = B / x , with b estimated B values, as discussed in connection with eq 10. As judged by the x: and 2 values the b analysis is as acceptable as a. The trends in d, for 01are in line with a p~lydispersity.’~ This might have been introduced by the presence of pyrene and/or 01 as discussed above. The absence of variation in 6, for HI can be the result of opposing effects: (a) polydispersity, which would have given higher d, values for the lower quencher concentrations, and (b)correlations between A B, and C in the model, which result in compensating errors in the estimations. The parameter values yielding the lowest x: value does not need to be most reliable in a physical sense. For example, the estimated C value, 4.90 X lo6 s-l, for the lowest quencher concentration falls outside the range of the others and is too high. This has been compensated for in the B value which then is too low, resulting in a too small 6, value for this quencher concentration. This kind of correlation is likely to be the major effect behind the results for BI. In this case a usage of 7, = 163 ns, which in this case certainly is wrong model, in the fitting procedure results in too small C values and too high B values, the compensatingeffect cooperating with an eventually present polydispersity. Thus an acceptable x> value and also an acceptable z value from the RUNS test, can be obtained for an incorrect model. The pyrene fluorescence was also quenched by oxygen in our experiments. We have not considered this as a problem since the lifetime of pyrene in the excited state is long enough so that is experiences an average distribution of oxygen. This is obvious given the values of k-, k,, and k+ measured by Turro et al.lS for oxygen quenching of 1,5-dimethylnaphthalene fluorescence in SDS micelles. They obtained for these parameters 5.3 X lo7s-l, 5.3 X lo7 s-l, and 1.4 X 1O’O M-l s-l. Entrance and exit rates of
oxygen in SDS micelles are thus much higher than the rates for the alkyl iodides. Preliminary measurements have also been undertaken on the quenching of pyrene fluorescence by alkyliodides in a homogenous solvent, cyclohexane. The results show strong positive deviations from the Stern-Volmer equation in the concentration range 0-1 M used for the alkyl iodides, even a t very low AI concentrations (
