1094
J . Phys. Chem. 1984, 88, 1094-1098
a pretreated surface. For 3, complete reorientation appears to be somewhat delayed, although no pronounced effect is apparent for @ below 2 X lo4 M. In contrast. the adsorption profile for 4 is virtually independent of pretreatment at all concentrations studied. These results suggest mechanistic differences in the formation of adsorbed layers of $-oriented species. For 1-3, adsorption on clean electrodes at high concentration probably leads directly to $-orientations: a mechanism which stipulates adsorption of flat adsorbed species as an intermediate step seems unlikely because of retardation effects when the electrode is purposely pretreated with a full or fractional monolayer of *-bonded intermediates. For 4, on the other hand, the clean and pretreated electrode adsorption profiles are indistinguishable from one another; hence, for this compound it is almost certain that formation of the $-orientation involves adsorption of *-bonded species as an intermediate step. This deviant behavior of 4 is attributed to steric hindrance by the methyl groups, and it may be mentioned that the fully methylated derivative, durohydroquinone, shows no tendency at all toward reorientation up to the solubility limite3s4 When the electrode was precoated with aromatic at concentrations in the transition region (where was intermediate between the values for flat and edge orientations), the resulting adsorption profiles were essentially identical with those for clean electrodes (at @ > CJ, Figure 2. This suggests that in the transition region the adsorbed monolayer consists of “tilted” molecules rather than a mixture of flat and edge oriented species. The retardation effects observed for 1-3 indicate that reorientation of a layer of flat adsorbed intermediates to edgewise structures is a much less facile process than direct adsorption in the vertical orientation. The fact that the retardation is independent of whether initial pretreatment consisted of a full or fractional monolayer provides evidence that (i) adsorption on a sparsely precoated electrode at high concentration involves com-
pletion of the flat oriented monolayer prior to formatiod of the v2-oriented layer; and (ii) reorientation of nearly isolated flat adsorbed intermediates is at least as difficult as that of closepacked molecules. These findings are consistent with the general observation that the strength of adsorption of an individual species (in a given orientation) tends to decrease as the adsorbate population is i n ~ r e a s e d that ; ~ ~ is, disruption of the original mode of attachment of isolated molecules would be more difficult than that of close-packed, less strongly bound species. The barrier to flat-to-vertical reorientation observed in this work may account for at least some of the differences between results from various studies on the adsorption of aromatic molecules from aqueous solution^.^-^^^^^ However, the lack of evidence for vertically oriented species in vacuum studies remains. It is likely that this is due to differences in adsorbate concentrations. In the solution s t ~ d i e s ~packing - ’ ~ ~ ~densities ~ indicative of $-orientations were obtained at C? > M. For qualitative comparison with gas-metal systems, this concentration may be expressed in terms of sample pressiure P by the ideal gas law P = (n/V)RTi= @RT, where n is the sample mole number, V the volume, R the gas M constant, and T the absolute temperature. At 25 “C, translates to about 18 torr. The cited UHV studies which showed only *-bonded species’9-22~25~29 utilized pressures near torr. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Air Force Office of Scientific Research for support of this research. Registry No. 1, 123-31-9; 2,571-60-8; 3,608-43-5; 4,615-90-7; Pt, 7440-06-4. (54)Wieckowski, A.;Rosasco, S. D.; Schardt, B. C.; Stickney, J. L.; Hubbard, A. T. Inorg. Chem., in press. (55) Somorjai,G.A.“Chemistry in Two Dimensions: Surfaces”; Cornel1 University Press: New York, 1981.
Eiectrochromism in Viscous Systems. Excited-State Properties of all-trans-Retinal Ake Davidsson,* Department of Inorganic Chemistry 1, University of Lund, Chemical Centre, S-220 07 Lund, Sweden
and Lennart B.
Johansson
Department of Physical Chemistry, University of Umeb, S-901 87 UmeB, Sweden (Received: June 13, 1983)
-
A model for electric-field-inducedabsorption spectra in viscous systems is presented in this work. This model has been applied
in the investigations of the lBU+ So transition of all-trans-retinal in polyethylene and polypropylene matrices. From the anomalous electric-field dependence of the electrochromism, we conclude that retinal interacts with its surroundings in a way explained by a proposed model. Excited-state properties of all-trans-retinal have been evaluated. The all-trans-retinal molecule has a differencein the dipole moment between the ground state and the lBIUstate of about 15 D, and the corresponding transition moment is directed along this dipole moment change. A very large polarizability change of about 1000 A3 upon excitation is also found. In order to explain the electrochromismof all-trans-retinal, both first- and second-orderperturbations of the electric transition dipole moment are necessary.
Introduction When an electric field is applied to a system of chromophores, the electronic charge distribution, energy levels, and population of molecules will be affected. This will in turn affect the absorption coefficient of the chromophores. The difference in absorption obtained with and without an applied electric field is called the electrochromism (EC). The measurement of field-induced spectral changes provides a useful way for determining excited-state properties like dipole moments, polarizabilities, parameters describing intermolecular interactions, and the orientation of the 0022-3654/84/2088-lO94$01.50/0
transition dipole moments. The theoretical basis necessary for such an analysis has been developed by Liptay’ and Lin.2 Most investigations of chromophoric molecules in condensed media have hitherto been made in nonpolar liquids.’~~ Polymers like polyethylene and polypropylene can also be used as matrices, which has been shown r e ~ e n t l y . ~One advantage with this technique (1) W. Liptay, Ber. Phys. Chem., 80,207 (1976). (2) S.H.Lin, J . Chem. Phys., 62,4500 (1975). (3) A. Davidsson, Chem. Phys., 35,413 (1978).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 6,1984 1095
Electrochromism in Viscous Systems
stands for the space average over the Eulerian angles Cl. In the presence of an external electric field, F , the wave functions la) and Ib) are perturbated. The electronic transition dipole moment will therefore change so that
'1,
fi(a,b,QML) = fio(a,b,QML)+ F@(a,b,QML) + Pb'(a,b,flML) (2) is the unperturbed transition dipole moment tensor. D'and DZare the perturbations due to the first andjhe second order, respectively. Analytical expressions of D' and DZcan be obtained from ref 2. The energy difference between the levels corresponding to the states la) and Ib) is according to
0 01
- c
t
+L
ov
10 V D C
6
1 minute
h AW = -$A&(b,a,QML) - y2FASU(b,a,QML)F
TIME
lb
Figure 1. (a) Time dependence of the absorption of all-trans-retinal at 350 nm in the presence of a static electric field (F = 1.7 X lo* V/m) and when the field is switched off. (b) Electric-field dependence of h A X ' O ( 4 3 2 ) : (A)modulated ac electric field; (0) modulated dc electric field.
is that the applied electric field can be about 2 orders of magnitude larger than that applicable on a nonpolar liquid, without electric breakdown. Another advantage is that the orientation of nonpolar molecules by the electric field can be neglected. For polar molecules like retinal, the situation is a bit more complicated. From our experiments we can conclude that in an electric field the retinal molecules align in the polymer matrix. The alignment is not described by the same Boltzmann distribution that one normally can use in gases and liquids like hexane.' This interpretation is strongly supported by the observation of a nonlinearity between the electrochromism and the square of the electric field strength shown in Figure lb. A modification of the model normally used in the analysis of EC obtained in polymers3 is therefore necessary. Such a model that takes into account the local anisotropy around a molecule in the matrix is presented here. Although all-trans-retinal is a very well-studied molecule both experimentally and theoretically, very few investigations have been concerned with its excited-state properties. Mathies and Stryer4 have studied retinal in p-dioxane and found large values of the dipole moment in the excited state. In this paper we have determined the changes in dipole moment and polarizability between the ground and the excited states of retinal in polyethylene and polypropylene matrices. Basic Model The absorption of polarized light of the frequency w by a system of chromophores can generally be written ~ ( w= ) constant (6D(a,b,QML)6Q(QML) ~ ( w ) ) (1) Here 6 is a unit vector along the electric field of the light. D denotes the transition dipole moment t e n ~ o rgenerated ,~ from the direct product of the dipole moment for an electronic transition from a state la) to a state Ib). B(w) is the spectral band shape of the transition and Q(QML) the orientation angular distribution of the molecules (M) with respect to a laboratory system (L). ( )
A i represents the difference in the electric dipole moments of the states Ib) and la), and Atu is the corresponding difference in the polarizability. The energy difference, hAw, causes a shift of the band-shape function B"(w). If the shift, Au, is small and if we can assume that @(w) is uniformly shifted, the perturbated band shape is given by the Taylor expansion
Let us assume that each chromophoric molecule solubilized in a polymeric matrix is located in a cavity and that the cavities have a random angular distribution. The local environment around a molecule in the cavity is anisotropic. In the presence of the external electric field the chromophores tend to align, due to the interactitn between the field and the permanent electric dipole moment R(a) and/or the anisotropic polarizability &(a). However, this orientation may be reduced by the interaction with the cavity so that the total orientational potential becomes
where 0 Iq and p 5 1 . The orientational distribution is then given by the Boltzmann distribution Q(QML)= C ~ x P ( - U ( Q M L ) / W
(1976). (5) L. B. A. Johansson and G. Lindblom, Q. Reu. Biophys., 13, 63, (1980). (6) S.Hotchandani, P. Paquin, and R. M. Leblanc, Can. J . Chem., 56, 1985 (1978).
(6)
By a Taylor expansion of Q(QML) with respect to the second and the third term in eq 5, we obtain Q~QML) =
[+
C 1
(kn7.
'/' -R(a)@
-A(a)FlT
) + / . i ; p-@&(a)F T
]
(7)
The absorption for a system of chromophores solubilized in a polymer matrix in the presence of an external field can be derived by combining eq 1-4 and 7. The equation relevant for the EC studies with a modulated ac field is then
-A-4x(h) - A(X)((3 cos' x - 1)X + r) + F2
+ A(h))((cos2 x - 2 ) Tr A 5 + (1 -
L lOhc( 1% 3 COS' x)(fiA5fi
+ 2A(X)
(
aZA(X) aA(x) A'+ Z ) + v) + + 2xax iohZc2 ax2
)
((2 - cos2 x)lAd12 + (3 cos2 x - 1)IfiAAI') (sa)
7
V = -y3E(AB(b,a) &a)) -
( (sD'(a,b)G)(Ai?(b,a)i) )
Do(a,b) q ((GD'(a,b)G)&a)?)
(4) R. Mathies and L. Stryer, Proc. Natl. Acad. Sci. U.S.A.,13, 2169
(3)
x = k-T
Do(a,b)
1 P +-(3$2(a)fi 30 k T 2
(8b) - Tr 5 ( a ) )
+
I 30( 2, k T (31fi2(a)12- Ig(a)IZ) (8c)
1096 The Journal of Physical Chemistry, Vol. 88, No. 6, 1984 7 ((i?D’(a,b)b) $(a)i) y=kT Do(a,b)
Davidson and Johansson
1p
+ -Tr E(a) + 6kT
1.o
( (i?D2(a,b)E))
(84 l ? z=-(fiAd(b,a) 15 k T
Q
- AA(b,a) &a))
0.5
+
((i?d’(a,b)d)(Al?(b,a)fi) Do(a,b) Do(a,b) = (bDO(a,b)S)
@e)
0
(8f)
Here we have neglected terms containing F3, P,..., etc. fi and f a r e the unit vectors along the transition dipole and the applied electric field, respectively, and x denotes the angle between the electric-field vectors of the applied field and the light. A is the wavelength of light, h is Planck‘s constant, and c is the velocity of light. V, X , Y, and Z are further discussed in the last section of this paper. In order to extract molecular information from eq 8a, a system of four equations can be generated, namely, AAgoO(A# A,,), AA900(Amax), AA54.70(A#A,,), and AA54.70(A,). ,A, refers to the maximum of A(A) where dA/dA = 0. From a least-squares fitting to the experimental values of AA9O0(A) and A P I ~ ~ . ~ O ( Awith ) X and_Yas parameters, we get the follo-wing molecular quantities: IAR(b,a)12,T r AE(b,a) - 3 / 5 V , IfiAR(b,a)I2, and PAg(b,a)fi + 2 - 1/5V. Experimental Section The EC measurements were performed on a circular dichroism spectrometer (Jasco 540) which is modified as described elsewhere.3 The electric field is generated by a high-voltage source and controlled by the reference frequency (370 Hz) of the spectrometer. The modulated ac field, shown in Figure lb, was generated by a high-voltage source that every second period changes the polarity of the electric field. A more detailed instrumental description can be found in ref 3. The measurements of the dielectric permittivities were made with a General Radio Type 1656 impedance bridge. all-trans-Retinal from Serva, Sigma, and Eastman Kodak was used. N o significant spectroscopic difference between the different preparations was observed in our experiments. A slight change in the absorption spectrum of retinal due to the formation of 9-cis-retinalio was observed in polymer films that had been exposed to daylight for about 2 days. The polymers used as matrices were polypropylene, PP (Homo, Perstorp AB), and linear low-density polyethylene, PE (DFDS0600, Unifos Kemi AB). The polymers were 25, 32, 45,75, and 100 Fm in thickness. The Lorentz field approximation has been adopted in the evaluation of the molecular electronic state parameters. Results If a static dc field is applied to a polymer matrix containing solubilized all-trans-retinal, the absorbance of the system changes with time in a characteristic and reproducible way, as is shown in Figure l a , A prompt drop in absorbance is observed when the field is applied, followed by a much slower decrease to a steady-state absorbance which is reached within about 2 min. When the field is switched off, the absorbance initially changes very rapidly and is then again followed by a slow relaxation. This relaxation is somewhat longer than the one observed in the presence of the field. These observations can be explained by assuming that the chromophores are located in cavities. The local anisotropic motion of a retinal molecule in the cavity is rapid while the motion of the cavity as whole is much slower. Therefore, the instantaneous change in absorbance is due to the local rapid reorientation in the cavities plus the intrinsic electrochroism. The slow relaxation processes with and without the electric field are caused by the reorientation of the cavities. If the modulated dc field shown in Figure l b is applied to the system, the time average of this field is nonzero and the cavities may align. Such an alignment would cause terms with a linear electric-field
250
300
350
LOO
L50
WAVELENGTH /nm
Figure 2. Absorption and EC spectra of all-trans-retinal in PE: (0)
calculated AAgoo(X);
(0) calculated A k 4 . ’ O ( X ) .
dependency in eq 8. A cavity alignment is supported by nonlinear behavior shown in Figure 1b. On the other hand, the application of a modulated ac field (185 Hz) will eliminate with slow orientation of the cavities. The electrochroism obtained with this field will therefore only contain the local orientation of the chromophore and its intrinsic EC. According to eq 8 the electrochromism for such a system will be directly proportional to the square of the applied electric field. This is also found experimentally as can be seen in Figure lb. The absorption and EC spectra of all-trans-retinal in polyethylene (PE) are shown in Figure 2. The circles and the squares illustrate the typical fitting between calculated and experimental EC spectra. The molecular electronic state properties presented in Table I are mean values of 15 different experiments in which the alltrans-retinal concentration was varied between 0.25 and 9 mM. The values are reduced by about 3% on going from the lowest to the highest concentration. Could this dependence be due to the variation in the dielectric permittivity and thereby a variation in the local electric field (cf. eq 8)? The dielectric permittivity of polymer films containing different retinal concentrations has been investigated. The permittivity is found to be the same as that of an empty film within 2% and can therefore not be the explanation. The linearity of Beer’s law has also been tested. We could not find any deviation for concentrations up to 6 mM in ethanol or 2.5 mM in isooctane. These observations are in agreement with the results of Bauer and Carl,’ who have measured the dielectric permittivities of all-trans-retinal in different solvents. Neither can the Onsager reaction-field model,* with a reasonable cavity r a d i ~ s give , ~ any explanation to the dependence on the concentration. At present we cannot give a reasonable quantitative explanation to the small variation in the electronic state parameters with concentration. The variation is included within the limits of error given in Table I. In Figure 3 EC spectra of all-trans-retinal in PP and P E are presented. The absorption spectra have maxima at 375 nm in P E and 378 nm in PP. The full width at half-maximum are 74 nm in P E and 79 nm in PP. No EC analysis has been done of the weak absorption band located around 250 nm, since the absorption data here are very unreliable. The EC spectra (cf. Figure 3) of thee 250-nm band show large similarities with the analyzed (7) P.-J. Bauer and P. Carl, J. Am. Chem. Soc., 99, 6850 (1977). (8) L. Onsager, J . Am. Chem. Soc., 58, 1486 (1936). (9) B.Linder in “Advances in Chemical Physics”, Vol. 12, I. 0. Hirschfelder, Ed., Interscience, New York, 1967.
The Journal ofPhysical Chemistry, Vol. 88, No. 6,I984
Electrochromism in Viscous Systems
1097
TABLE I: Evaluated Electronic State Properties of all-trans-Retinal in Polyethylene and Polypropylene IAR I, D
PE PP
@A&,
15.5 i 1 15.5 i 1
D
Tr A:-
"5
V, A 3
2900 i- 300 3300 i 700
15.6 t 1.5 15.6 i 1.5
$A$'+
Z-
' I 5V , A 3
X,(A/V)2
2800 i 300 2100 + 500
y, ( A m ' -18.6 -16.6
73 L 12 114 t 28
*3 f
-
5
+
rn
0
"a
3-
2 1-
0-1
-
-2 -3 -
A L50
LOO 350 300 250 WAVELENGTH/
nm
Figure 3. EC spectra in PP (above) and PE (below): (-) AA54.70; (- --) AAWO.
EC spectra. An interesting observation is the likely vibrational progression (1300 f 200 cm-I) described in the 375-nm band of all-tram-retinal in PE. To our knowledge a vibrational structure has never been observed in the absorption spectra of all-transretinal in any solvent at room temperature or at low temperature. Although the absorption spectrum does not directly reveal any vibrational structure, its EC spectrum may do that, which is clearly demonstrated by L h 2
Discussion The EC analysis, in this work, is based on the assumption that a single electronic transition is involved in the absorption. The 375-nm band of all-trans-retinal has been assigned to the lB,+ So(nn*)transition. However, Birge et al.1° have shown by So transition, symtwo-photon spectroscopy that also the IA, metry forbidden in one-photon spectroscopy,contributes with about 10% to the absorption band at 375 nm. This transition should be centered about 2400 cm-' lower in energy than the main lBu+ Sotransition. A mixing of the lBU+and the IA; states by the applied electric field might then introduce complications in the analysis of the EC data. We find, however, that the experimental data can be fitted to our EC model. Notice also that the absorption and the EC spectra (cf. Figure 3) of retinal in the two matrices are quite different. The contribution from the !A; So transition is therefore assumed to be negligible and/or within the experimental uncertainty. Another explanation may be that the excited values may be close to those of the IBu+ So transition. According to Onsager,s the local field at a molecule in a continuous dielectric with a dielectric permittivity t is composed of a cavity field, (3t/(2c l))Fappl,due to the induced polarization of the medium, and a reaction field, due to mutual interaction between the molecule and its surroundings. This model for approximation of the local electric field introduces in its most simplified shape, Le. for cavities with spherical symmetry, one further parameter, namely the cavity r a d i ~ s .The ~ contributions to the evaluated electronic-state quantities from the cavity field and the ration field compensate each other to some extent." In this way +-
+ -
-
-
-
+
(10) R. R. Birge, J. A. Bennett, L. M. Hubbard, H. L. Fang, B. M. Pierce, D. S.Kliger, and G . E. Lerot, J . Am. Chem. SOC.,104, 2519 (1982).
the Lorentz field approximation, ((e 2)/3)Fappl,becomes a good approximation of the local field in nonpolar media without any specification of the local environment around the molecule. all-trans-Retinal in p-dioxane has previously been investigated with electrochromism by Mathies and StryerS4Both the experimental procedure and the analysis are different from ours. The molecula,r quantities that can be compared with theirs are lARl and IpARI, which both seem to be in agreement with our findings. They have, however, used a cavity field as an approximation of the external applied electric field while we perform the Lorentz field approximation. With the cavity-field approximation our values of IARI and IfiARl become about 20% larger than theirs. In the comparison of different electronic state properties, it should be noticed that the evaluated properties include the molecule and its local environment, which is in this case polymers of alkanes relative to p-dioxane. One should therefore keep in mind that the excited-state properties are estimated within the uncertainties introduced by the performed local-field approximation. An interesting observation is that although both the absorption and the EC spectra of all-trans-retinal in PE and PP are different, the excited-state properties in Table I are very similar. The X and Y values include different terms of ground electronic state properties and terms due to the perturbation of the transition dipole moment. For a negligible orientation of the retinal molecules in the applied electric field, 7 = p = 0, and X would be zero, which is not the case. In the other extreme when 7 = p = 1, i.e. for an ordinary Boltzmann distribution, X would be very similar in both matrices. We find quite different X values in PE and PP, and thus the chromophore-polymer interactions are different. The negative values of Y found in both PE and PP implies that th_e terms containing D' and D2 cannot be neglected, since Tr &, lR(a)I2,p , and 7 are positive quantities (cf. eq 8d). The polarizability terms include the unknown quantities V and Z . From a linear combination of the experimentally determined quantities Tr A& - 3/5Vand fiA&fi Z - '/SF', we find that Tr A& = 3000 f 900 and 4500 f 1900 A3 in PE and PP, respectively. It has then been assumed that the transition dipole is parallel with AaZr,that Aaxx= Aayy,and that Tr A& is large compared to Aa,. The terms containing 17 in Z and V include the dipole moment of the ground state. The dipole moment is found to be about 5 D.'0*'3 By inserting this value in Z and V, one finds that the contributions of these terms to the polarizability are within the experimental errors. These findings show that there is a remarkably large polarizability of all-trans-retinal in the excited lBU+state. The presence of an lA; state lying close to the 'B,+ state can, however, explain the large polarizability provided that the transition moment between these states is allowed. The perturbations of the transition moment of the IBu+ Sotransition depend on the energy separation, AE, between all electronic states essentially as2
+
-
Dl(a,b) D2(a,b)
a
l/hE
(9a)
(l/AE)*
(9b)
0~
Hence, in analogy with the explanation of the large polarizability, the D' and D2 terms may also be large. This is consistent with the discussion above of their contributions to X and Y. The large shifts observed for the 'BU+ So transition of alltrans-retinal in different solvents could be explained by the large polarizability. If we consider eq 8a and regard F as a local electric field caused by the surrounding, this could correspond to a random local electric field for x = 54.7'. A large negative value of the
-
(11) W. Liptay, Z . Naturforsch., A, 20A, 272 (1965). (12) R. R . Birge, Annu. Reu. Biophys. Bioeng., 10, 315 (1981). (13) A. B. Myersand R. R. Birge, J . Am. Chem. Soc., 103, 1881 (1981).
1098
J. Phys. Chem. 1984, 88, 1098-1101
second-order perturbation of ihe transition moment would decrease the absorption band, the lARI2would broaden the band through the second derivative, and a positive Tr AS would shift the band maximum to lower energy with increasing local electric field. In nonplar media the local fields are mainly due to induced dipoles, It is reasonable to think that the local fields increase in nonpolar media on lowering the temperature. Thus, the absorption of
all-trans-retinal would decrease and shift to lower energy at lower temperatures, which is in accordance with the results of Birge.12
Acknowledgment. Simon Danielsson is acknowledged for construction of the high-voltage source, and the Swedish Natural Science Council for financial support. Registry No. all-trans-Retinal, 116-31-4.
Study of the Solubilization of Aromatic Hydrocarbons by Aqueous Micellar Solutions Panagiotis Lianos, Department of Physics, University of Crete, Iraklion, Greece
Marie-Laure Viriot, G.R.A.P.P.,LA 328 du CNRS, 54042 Nancy-Cedex, France
and Raoul Zana* Centre de Recherches sur les MacromolBcules, CNRS, 67083 Strasbourg- Cedex, France (Received: July 8, 1983)
Photophysical studies involving measurements of microenvironment polarity, pyrene fluorescence lifetime, and efficiency of excimer formation by pyrene and dipyrenylpropane as well as the measurement of the time required for solubilizingpyrene by various micellar solutions have confirmed the existence of a weak interaction between neutral arenes and the quaternary ammonium head groups of some micelle-forming surfactants. Several aspects of the rate of pyrene solubilization by micellar solutions have been explained in a consistent manner in terms of micelle formation-breakdown kinetics.
Introduction Fluorescence probing of micelles has proved to be a powerful tool in the study of several aspects concerning both structure and dynamics of micelles as well as interactions between micelles.' Pyrene and its derivatives are probably the most frequently used probes. There exist more or less standard methods of solubilizing pyrene and other similar aromatic, nonpolar probes in micelles.2 All users of these methods must have been aware of a varying behavior of different micelle-forming surfactants in solubilizing pyrene. However, there is no published systematic account concerning the conditions of solubilization of this probe and the origin of the observed differences. Likewise, the literature reveals that many authors concluded that the site of solubilization of aromatic probes is close to the micelle surface, in the so-called palisade layer, without offering much explanation for the choice of such a site, and differences in site depending on the probe and ~ u r f a c t a n t . ' ~ .One ~ notable exception is the study of Almgren et aL4 These authors suggested that a weak interaction between pyrene and the quaternary ammonium group is probably responsible for the large differences between the maximum solubilities of pyrene (and other arenes) in anionic micelles and micelles of surfactants with a quaternary ammonium head group. This interaction also explains differences in the probe solubilization sites.4 The present study which has been performed on widely (1) (1) (a) Dorrance, R. C.; Hunter, T. F. J . Chem. SOC., Furuday Trans. 1 1972,68, 1312. (b) Thomas, J. K.Chem. Rev. 1980,80,283 and references therein. (c) Waka, Y.;Hamamoto, K.;Mataga, N. Chem. Phys. Lett. 1978, 53, 242. (d) Moroi, Y.J . Phys. Chem. 1980, 84, 2186. (e) Fendler, H. J. Zbid. 1980, 84, 1485. (2) (a) Infelta, P. P. Gratzel, M. J. Chem. Phys. 1979, 70, 179. (b) Almgren, M. J . Am. Chem. SOC.1980, 102,7882. (3) (a) Thomas, J. K. Acc. Chem. Res. 1977, 10, 133 and references therein. (b) Mukergee, P.; Cardinal, J. R. J. Phys. Chem. 1978, 82, 1620. (c) Pyter, R. A.; Ramachandran, C.; Mukerjee, P. Zbid. 1982, 86, 3206. Ramachandran, C.; Pyter, P.; Mukerjee, P. Zbid. 1982, 86, 3198. (4) Almgren,M.; Grieser, F.; Thomas, J. K. J. Am. Chem. SOC.1979,101, 279.
differing surfactants leads to the same conclusion from a very different approach, namely, the rate of intramicellar excimer formation by the micelle-solubilized probes and the rate of solubilization of pyrene by micellar solutions. It also reveals some interesting features of the kinetics of solubilization of pyrene (and most likely of other arenes) by micellar solutions. Experimental Section Pyrene, dipyrenylpropane (DPyP), sodium dodecyl sulfate (SDS), tetradecyltrimethylammonium bromide (TTAB), hexadecyltrimethylammonium bromide (CTAB), dodecyldimethylammonium propanesulfonate (DDAPS), and alkyltrimethylammonium hydroxides (dodecyl-, DTAOH; tetradecyl-, TTAOH; and hexadecyl-, CTAOH) were the same as used Sodium tetradecyl sulfate (STS) and hexadecyl sulfate (SHS) were purchased from Merck (99% pure) and used as received. Tetradecylpyridinium bromide (TPB) was purchased from Fluka and purified by treatment with charcoal and repeated recrystallizations in ethyl acetate or ethyl acetate + alcohol. Hexadecyltrimethylammonium chloride (CTAC) was obtained from CTAB by means of ion-exchanged resin (Merck, type 111). Sodium dodecanoate (SD) was prepared as an aqueous solution by stoichiometric neutralization of dodecanoic acid by carbonate-free sodium hydroxide. The spectrofluorimeter and the fluorescence lifetime apparatus were the same as previously des~ribed.~ The quantities of interest in the present study are the values of the micelle aggregation number n, the rate constant ko for the decay of fluorescence of micelle-solubilized monomeric pyrene, the rate constant kE for intramicellar excimer formation, the ratio 11/13of the intensities of the first to the third vibronic peaks in the fluorescence spectrum ( 5 ) Lianos, P.; Lang J.; Strazielle, C.; Zana, R. J . Phys. Chem. 1982,86, 1019. (6) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1981, 84, 100. (7) Lianos, P.; Zana, R. J . Phys. Chem. 1983, 87, 1289.
0022-3654/84/2088-1098$01.50/00 1984 American Chemical Society
