11911
COMMUNICATIONS TO THE EDITOR
Comparison of Micellar Effects on Singlet Excited States of Anthracene and Perylene’ Publication costs assisted by Meilon Institute
Sir: Recent fluorescence depolarization studies by Shinitzky, et al., made use of aromatic hydrocarbon fluorescent probes to measure the microviscosity in the interiors of certain cationic micellar systems? Almgren has examined the sensitized fluorescence of napthalene in sodium phenylundecanoate, a n anionic surfactant, and determined that napthalene lies at the micelle core in his system.3 We have begun to investigate the behavior of excited states of condensed aromatic hydrocarbons in micellar systems, and our preliminary findings indicate that the locations of these hydrocarbons depend both on the substrate and surfactant used. Further, i t may be seen that in certain systems the excited substrate can provide useful data concerning the nature of the micellar surface. In these experiments we have compared the influences of cationic hexadecyltrimethylammonium bromide (CTAB), CH3(CH2)15(CH3)3N+Br-, and the corresponding chloride (CTACl), upon the fluorescence lifetimes ( 7 f ) of anthracene and perylene in aqueous systems. Fluorescence lifetime measurements were carried out by the conventional method of pulse fluorimetry utilizing a nanosecond spark, pulse sampling oscilloscope, and PDP-8 computer for data control and analysis.4 It was found that in water solutions perylene ( M ) has a very short lifetime (7f < 0.5 nsec) and exhibits a very low intensity of emission. However, addition of either CTAB or CTACl a t concentrations corresponding to the region of the micelle concentration (cmc) or above is accompanied by a sharp increase in 7f and emission intensity. In CTAB solutions, the fluorescence lifetime plateaus just above the cmc (9 x 10-4 M ) a t a value of 5.0 0.2 nsec, in agreement with a n earlier finding of Shinitzky, et al., for solubilized perylene? In direct contrast, however, similar addition of CTAB to an aqueous solution of anthracene (-3 X 10-6 M , 7f = 2.3 f 0.2 nsec) sharply quenches the anthracene fluorescence in the cmc region. These effects are illustrated in Figure 1. Substitution of CTACl for CATB produces very little decrease in the anthracene fluorescence lifetime compared to water solutions. Measurements of 7f anthracene were then carried out in systems of mixed CTAB and CTACl micelles maintaining the total surfactant concentration a t 0.01 M. From a plot of 7f us. the concentration of CTAB, [CTAB], it is seen that 7 r may be related to [CTAB] by the pseudo-first-order quenching relationship given in Figure 2. This expression assumes no substantial changes in structure upon the formation of the mixed micelles which might effect the location of anthracene within the system. In aqueous solution Br- has been found to be an inefficient quencher of these excited singlets, kg < 10s M-1 sec-I. Hence for rf to be affected to the extent observed by
i o
0
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20
30
40
[CTAB], M x IO4
Figure 1. Effect of CTAB micelle formation on the fluorescence M anM perylene ( B ) and -3 X lifetimes of -1 X thracene (0) in aqueous solution.
*
0.002
0.004
0.006
0,008
[CTAB], M
Figure 2. Fluorescence lifetime of -3 X M anthracene as a function of increasing CTAB concentration, [CTAB], in a CTACICTAB mixed micelle system. Total surfactant concentration is maintained at 0.01 M.
the presence of B r , the anthracene must be solubilized at the micelle surface, readily accessible to the high concentration of Br- ions in the Stern layer. By contrast, the perylene must as previously suggested2 reside in the interior of the micelle, well removed from the B r ions. Given the high sensitivity of anthracene fluorescence to the surface environment, the singlet state should provide (1) Supported in part by the U.S. Atomic Energy Commission. (2) M. Shinitzky, A.-C. Dianoux, C. Gitler, and G. Weber, Biochemistry, 10, 2106 (1971). (3) M. Almgren. Photochem. Photobioi., 15, 297 (1972). (4) L. K. Patterson,to be published.
The Journal of Physical Chemistry, Voi. 77, No. 9, 1973
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Communications to the Editor 0.01 I
0.02 I
0.03
004
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005 [Br-]
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which was applied to the mixed micelle data, one may construct quenching curves for both sets of experimental data. The CTACl + Br- case may be seen to give the better fit to the proposed equilibrium. It must be noted that the model reflected in eq 2 represents considerable simplification of the real system, ignoring as it does structural changes and substrate distribution changes, that may occur in the micelle a t high concentrations of added electrolyte? The large deviation in the CTAB C1- case may be due to just such changes in micelle structure. Meguro and Kondo measured the rates of increase in specific conductance with concentration above the cmc in dodecylpyridinium halides and found the value for the chloride to be greater than that for the bromide by a factor of about 2 indicating for that case a relatively stronger binding of Br-.6 In the pyridinium systems it has been shown that some portion of the binding is due to chargetransfer p h e n ~ m e n a .With ~ this taken into account the conductance studies do indicate trends in relative stability of halide binding qualitatively consistent with findings presented here. It has been suggested that the binding of conjugated molecules at the CTAB micellar surface may be explained in terms of an interaction between the a system of the molecule and the positively charged surface.* However, all the factors determining the selectivity by which some aromatic hydrocarbons are solubilized a t the surface of CTAX while others are found in the interior are not fully understood and are the focus of further investigation.
+
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I 0.2
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[Cl-1
[X?, M
Figure 3. Effects of Br- added to 0.01 M CTACl (0) and CIadded to 0.01 M CTAB ( 0 ) upon Tf of solubilized anthracene. The upper horizontal scale applies to the CTACI-Br- system while the lower is for the CTAB-CI- system. The curves represented by the dotted lines were calculated from eq 1 in Figure 2 and an equilibrium constant of 10 for eq 2.
a good probe for stability of counterion binding in the Stern layer when other competing ions are introduced into the system. Substitution of a nonquenching ion for Br- at the surface should, then, be accompanied by increasing 7 f . Accordingly, T f a n t h r a c e n e was measured in 0.01 M CTAB solutions as a function of added NaC1. The results are given in Figure 3. It is apparent that C1- rather inefficiently replaces Br- at the micelle surface. The inverse experiment, beginning with 0.01 M CTACl and determining 7f anthracene with increasing KBr confirms that Br- is much more strongly bonded at the positively charged surface or in the Stern layer (Figure 3). Assuming reversibility and an equilibrium constant of 10 for the system CTACl
+
Br-
CTAB
+
C1-
(2)
one may calculate concentrations of [CTAB] produced either by the addition of B r to CTACl or C1- to CTAB. By substituting [CTAB] calculated in this way into eq 1
The Journal of Physical Chemistry, Vol. 77, No. 9, 1973
(5) J . Cohen and T. Vassiliades, J. Phys. Chem., 65,1774 (1961) (6) K. Meguro and T. Kondo. Nippon Kagaku Zasshi, 80,818 (1959). (7) P Mukerjee and A. Ray, J. Phys. Chem., 70, 2150 (1966). (8) J . H. Fendler and L. K. Patterson, J. Phys. Chem., 74, 4608 (1970).
Radiation Research Laboratories and Center for Special Studies Meilon Institute of Science Carnegie-Mellon University Pittsburgh, Pennsylvania 15213 Received October 30. 1972
Larry K. Patterson* Eric Vieil
