3684
J. Phys. Chem. 1981, 85, 3684-3689
reaction between ZnTPPS- and Co'I'TMPyP is the most favored, both in AE (- 1.3 V) and in the charge (5- with 3+), and yet the rate constant is 1.4 X lo9 M-l s-l, probably lower than the diffusion-controlled limit. ComTAPP, with the same charge but somewhat lower potential, is reduced by ZnTPPS- more slowly by a factor of 2.5. CoI'ITPPS, which is the least favored in terms of potential and is further inhibited by a charge of 5-, is reduced with a rate constant of only 2 X lo' M-' s-l. Even though this value is two orders of magnitude lower than that observed for Co'I'TMPyP, since the difference is partly owing to the effect of redox potential, the rate inhibition ascribable to the charge would be less than a factor of 10. This estimate su ests that the reacting species (ZnTPPS)5-. and Co TPPS(OH)2" are not affected by their full charge, but rather by only 1 or 2 negative charges, i.e., only the charges close to the area of interaction. The effect of the overall charge is thus exhibited mainly as an effect on the redox potential and only to a small extent as an electrostatic effect on the rate constant, in agreement with a previous sugge~ti0n.l~ The rates of electron transfer from (ZnTAPP)-. and (ZnTMPyP)-. to the three Co"'P do not show wide variations because in these cases the effect of charge is reversed and is partially offset by the effect of redox potentials. A comparison of the absorption at 730 nm observed following the reduction of CoIIITMPyP by (CH&2O- and by (ZnTPPS); indicates a difference in mechanism. While (CH3)2CO-was found to reduce both the metal center and the ligand, as discussed above, the electron donated by
Yi
(ZnTPPS)-. goes predominantly to the Co"' center, with little formation of a ligand anion radical absorbing at 730 nm. This finding indicates that reduction by (ZnTPPS);, which is thermodynamically less favorable than reduction by (CH3)&O-, is also more selective and directs the electron preferentially to the Co center. It may also indicate that the electron is donated to the Co mainly via the axial OH group rather than through the porphyrin ligand of C O ~ T M P ~ P ( O AHsimilar )~ suggestion was advanced for other reductions of this compound.35 Electron Transfer from Co"TTPS to Z n p . ZnP+ radicals were obtained by the reaction of ZnP with Br2- in neutral solutions as described above. Co"TPPS was prepared from CoII'TPPS by quantitative reduction with sodium dithionite in the absence of oxygen and was transferred anaerobically into the ZnP solution. By monitoring the decay of the Z I P absorption at -700 nm as a function of [Co'ITPPS] the second-order rate constants were derived (Table 111). Here again, the effect of the multiple charge is relatively small, as observed with ZnP- + Co'"P, and the rate constants are of the same order of magnitude, indicating similar mechanisms. Acknowledgment. I thank Drs. G. Ferraudi and P. Hambright for helpful discussions. Supplementary Material Available: Figures 1, 6, and 8 are available as supplementary material (3 pages). Ordering information is given on any current masthead page. (35)Pasternack, R.F.;Sutin, N. Inorg. Chem. 1974,13, 1956.
Ionization Potentials of Polyacene Molecules in Micellar Systems or in Liquid Homogeneous Solutions A. Bemas,*
D. Grand, S. Hautecloque, and A. Chambaudet
ERA 718,Unlversit.4 Paris-Sud, Ba^t. 350, 9 I405 Orsay. France (Received: March 19, I98 1; In Final Form: July 1, I98 I)
It has been emphasized previously that photoionizablehydrophobic solutes confined in anionic aqueous micelles constitute promising systems for photosensitized water splitting and solar energy conversion and storage. In the present study, the one-photonionization of perylene (Pe) and tetracene (Tet) in anionic lauryl sulfate (NaLS) or neutral Triton X 100 (TX 100) micelles has been compared with the photoionization process in homogeneous solutions (tetramethylsilane(Me,Si) or methanol). The photoionization yield as a function of the exciting photon frequency has been measured by photocurrent recording (in Me,Si) or by scavenging the hydrated electrons ' ) . The shape of the photoionization efficiency curves by various neutral or charged scavengers (NzO,Nos-, H in the threshold energy region is found to differ markedly for Pe or Tet in NaLS micelles on one hand and neutral micelles or homogeneous solutions on the other. It is suggested that such a difference illustrates the reduced recombination of geminate ion pairs in the former case. The lowering of the ionization threshold energy of Pe and Tet in micelles ( A I M ) relative to their gas-phase ionization potential amounts to 2.3-2.35 eV for both solutes. AIMi is interpreted in terms of the parameters which have been currently considered to govern the optical ionization of impurity molecules in condensed media. In such a framework the electric field gradient at the micelle-water interface does not seem to contribute significantly to the observed A I M i value.
Introduction Micelles, which are among the simplest organized systems, have received much attention during the past decade or so, mainly along three lines of interest. (i) Because of the possibility of organizing reactants at a molecular level and of a strained geometry for the excited sequestered chromophore, miscellaneous photoreactions have been shown to occur with a higher efficiency in micelles than in homogeneous solutions. (ii) Ionic micelles with their 0022-3654/8 1/2085-3684$0 1.25/0
electrical double layer have been considered as models of biological aggregates, particularly biological membranes. (iii) Associated with photoionization of solutes located in the hydrophobic core of anionic micelles, high yields of hydrated electrons have been reported,l" the neutraliza(1) S. C. Wallace, M. Gratzel, and J. K. Thomas, Chem. Phys. Lett., 23, 359 (1973). (2) M. Gratzel and J. K. Thomas, J. Phys. Chem., 78,2248 (1974).
0 1981 American Chemical Society
Ionization Potentials of Polyacene Molecules
tion back-reaction being inhibited by the electrostatic barrier at the lipid-water interface. The latter observation has appeared promising from the viewpoint of photosensitized water dissociation and solar energy conversion and storage. Hence, even though the structural features and dynamics of micellar assemblies are still much debated: photochemical applications of micellar systems have recently multiplied.’ We will focus our attention on item iii and more particularly on the problem of the energy required to produce separated ion pairs in micellar systems. It has been claimed recently that aminoperylene in anionic micelle solutions can be photoionized by 530-nm laser pulses in a monophotonic process by green-light photons,8i9 the substantial correlative decrease in ionization potential compared with gas-phase values being ascribed to high polarization energy terms.* However, the linear dependence of the ionization yield on the laser energy observed in such micellar systems can also be interpreted on the basis of a reduced efficiency for the geminate ion recombination back-reaction, leading to an apparent change in the order of the laser photoionization process from biphotonic to monophotonic.lOJ1 The present study consists of attempting to evaluate the first ionization potential of substituted benzene (tetramethylbenzidine) or polyacene molecules (perylene and tetracene) dissolved in either anionic (NaLS) or neutral (Triton X 100) micelles and, in the case of anionic surfactant, to examine the role of the electric double layer on Iw The chromophores have been excited under conditions where biphotonic processes can be totally disregardedlZJ3 and the associated difficulties of interpretation avoided. Meanwhile a similar attempt for pyrene in NaLS micelles has been reported,14using time-resolvedtunable laser spectroscopy. The authors conclude that the mechanism for two-photon ionization is the same in micellar aqueous solutions and in pure methanol but that the apparent ionization threshold is significantly reduced for the pyrene-NaLS system. The ionization-potential values found respectively for pyrene, perylene, and tetracene dissolved in micelles or homogeneous solutions will be compared and discussed. Experimental Section Three solutes have been selected for their pronounced hydrophobic character associated with relatively low gaseous-phase ionization potentials: 3,3’,5,5’-tetramethylbenzidine (TMB) from Merck, high-purity perylene (Pe) from Schuchardt, and high-purity tetracene (Tet) from Eastman Kodak. They were used as supplied. Tetramethylsilane (Me4Si) from Fluka (Puriss.) was purified over a mixture of molecular sieves preheated at 400 “C and distilled under vacuum. (3) S. A. Alkaitis, G. Beck, and M. Griitzel, J.Am. Chem. SOC.,97,5723 (1975). (4) S. A. Alkaitis, M. Gratzel, and A. Henglein, Ber. Bunsenges. Phys. Chem., 79, 541 (1975). ( 5 ) S. A. Alkaitis and M. Gratzel. J. Am. Chem. SOC.. 98.3549 (1976). (6) F. M. Meneer. Acc. Chem. Res.. 12. 111 (1979). ’ (7) N. J. Turr; hi. Gratzel, and A. M.Braun,‘Angew.Chem., Int. Ed. Engl., 19, 675 (1980). (8) J. K. Thomas and P. L. Piciulo, J. Am. Chem. SOC.,100, 3239
.~.
(1978). . .,.
(9) J. K. Thomas and P. L. Piciulo, J. Am. Chem. SOC.,101, 2502 (1979). (10) G. E. Hall, J. Am. Chem. SOC.,100, 8263 (1978). (11) G. E. Hall, Private communication. (12) E. Amouyal, A. Bernas, and D. Grand, Photochem.Photobiol., 29, 1071 (1979). (13) A. Bernas, D. Grand, and E. Amouyal, J. Phys. Chem., 84,1259 (1980). (14) S. C. Wallace, G. E. Hall, and G. Kenney-Wallace, Chem. Phys., 49, 279 (1980).
The Journal of Physical Chemistw, Vol. 85,No. 24, 198 1 3685
Methanol (MeOH) was doubly distilled under N2 atmosphere, first over 2,4-dinitrophenylhydrazineand concentrated H2S04 and then over a mixture of clean, dry magnesium and iodine to remove traces of water. Anionic or nonionic surfactants were introduced in deionized distilled water at concentrations above the critical micellar concentration (cmc), i.e., 0.1 M for anionic lauryl sulfate (NaLS from Merck) and 4.5 X lo9 M for neutral Triton X 100 (TX 100, Merck, for gas chromatography). The solutes were dispersed in the micelles by magnetic stirring overnight at room temperature and, in the case of Pe and Tet, in NaLS by warming at 40 “C. The solutions were then filtered, and the concentrations of solubilizates determined by spectrophotometric measurements assuming that the variation of the solute extinction coefficient on going from a homogeneous to a micellar solution can be neglected. The estimated concentrations were thus 4 X 104-1 X 10” M for Pe, 1 X lo+ M for Tet, and 2 X 10“-6.5 X M for TMB. Hence, for NaLS for which the aggregation number is around 60,15a Poisson distribution law7 indicates that most of the micelles are empty and less than 10% contain one solute molecule. Deoxygenation of the solutions was achieved by pumping over a primary vacuum which has proved not to destroy the micellar assemblies. Continuous illuminations with monochromatic light (bandwith N 10 nm) were performed with a xenon source (Osram XBO 2500 W) fitted to a Bausch and Lomb monochromator. At 265 nm, the light intensity was 7.2 X 1013photons cm-3 s-l, conditions which prevent biphotonic processes.16 During the irradiations, solutions were magnetically stirred. The electron photoejection process was displayed by using various electron scavengers or by measuring photocurrents. The neutral scavenger N20 (2 X M) was used for methanol solutions but rejected in the case of micellar systems. The photoproduced N2 was measured by gas chromatography, corrections for residual atmospheric Nz being applied. An anionic scavenger such as NO3- (N03Li,2 X 10“‘ M) was used both in MeOH and in micellar solutions. In the latter case, the scavenger is definitely located in the aqueous phase. Its reduction leading to NOz- has been measured by the Schinn te~hnique.’~NO, is also known to oxidatively quench triplet states of aromatic molecules, leading to NOz- formation, i.e. NO3- + S* S+ + NO$-
--
N032-+ H20 NO2
+S
+
NO2 + 20H-
S+
+ NOz-
In our experiments, these reactions can be disregarded; excitation in the entire absorption spectrum of perylene does not lead to NO, for wavelengths greater than 270 nm. A cationic scavenger such as H+ (HC104,7 X 10-5-7X M) is also located in the aqueous phase, either close to the periphery of the negatively charged NaLS micelles or in the bulk of the solution. The production of Hz upon electron capture was measured with a calibrated McLeod gauge. (15) J. H. Fendler and E. J. Fendler, “Catalysis in Micellar and Macromolecular Systems”, Academic Press, New York, 1975, p 20. (16) U. Lachish, A. Schafferman, and G. Stein, J. Chem. Phys., 64, 4205 (1976). (17) M. B. Schinn, Ind. Eng. Chem., Anal. Ed., 13, 33 (1941).
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The Journal of Physlcal Chemistry, Vol. 65, No. 24, 1961
Bernas et ai.
4 Photocurrent
O.9L0.i)
0.0
CI I
1
( a r b i t r a r y units) h
/ I
Flgure 1. Tetramethyibenzidine In 0.1 M NaLS solutions. Absorption spectra of the foliowlng: (a) cation TMB' obtained at A, , 320 nm, [TMB] = 2 X 10" M-l; (b) cation TMB+ and dimer T M B P obtained M-l; (c) cation TMB+ and dlmer at Ayk330 nm, [TMB] = 6.5 X TMB, obtained at i,,,, 320 nm, [TMB] = 6.5 X lo-' M-'.
Careful attention was paid to the absorption spectra overlap of solutes, scavengers, or surfactants and, when needed, the necessary absorption corrections have been made. Photoconductivity measurements were performed on degassed Me4Si solutions of Pe and Tet, the applied electric field being -15.6 kV cm-'. The photocurrents recorded in the photoionization threshold region were of the order of 10-13-10-14A.
1.1
1 5 1 I
l
l
1
1 .
200 210 220 230 2 4 0 2 5 0 260 270
hnm)
Figure 2. 9,- vs. excitation wavelength: (a) peryiene (8 X 10" M-l) in Me& solutions by photocurrent; (b) perylene (6 X IO-' M-') In MeOH solutlons with (+) N03Li (2 X M-I) and (A)N,O (2 X lo-* M-') as ea; scavenger.
Tetramethylbenzidine. For TMB as a solute, recourse to an electron scavenger was unnecessary. In effect, it has been shown previously that the lifetime of the TMB+ cation radical in NaLS systems is remarkably long.5 The photoionization yield has thus been followed at 461 nm, the maximum of the main absorption band of TMB+. Photoionization is detected for excitation wavelengths equal to and shorter than -360 nm, that is, electron ejection occurs as soon as the TMB molecule absorbs light. No photoionization threshold proper could thus be evidenced for the TMB-NaLS system. Another point worth mentioning is the appearanceunder certain conditions of TMB concentration and excitation light frequency-of extra bands located at 665 and 375 nm. The intensity of such bands is found to increase M with TMB initial concentration above Co = 4 X (for C < Co, the extrabands are absent) and, when the light frequency increases, that is when the photoionization efficiency grows larger (Figure 1, curves a-c). Such observations are consistent with the attribution of the 665- and 375-nm bands to the dication TMB,2+ which has been observed in low-temperature matrice@ as a result of the equilibrium 2TMB+ P TMB;+ With respect to the pyrene excimer emission reported for pyrene in sodium hexadecyl trioxyethylene sulfate (CToES) micelle^,'^ the dication formation is favored by the exceedingly long life of the monocation. The observation of a dication formation might result from intermicellar exchange or might be indicative that a distribution function other than a Poisson law is obeyed.z0
Perylene. The photoionization of Pe has been studied in a more detailed way, electron ejection from Pe molecules confined in anionic (NaLS) or neutral (TX 100) micelles being compared with the photoionization of Pe in Me4Si or methanol homogeneous solutions. Pe-Me4Si. MelSi constitutes a useful solvent for photoionization studies. In effect, high-sensitivity conductivity measurements can be performed. The system leads to high electron mobilities and free-ion yields, and the ground-state energy Vo of the ejected quasi-free electron has been previously determinedVz1 The photoionization efficiency curve of Pe in Me4Si is shown in Figure 2a, where the photocurrent intensity is plotted as a function of the exciting-light wavelength. An ionization threshold energy Iliqis observed at 5.25 eV, which, combined with the known Vo value of Me4Si (Vo= -0.60 eV), allows an estimation of the "effective" radius of the Pe cation to be used in the calculation of a polarization energy term. This point will be considered further in the Discussion. Pe-MeOH. Figure 2b represents the photoionization efficiency curves for Pe dissolved in methanol obtained from photoelectron scavenging by either N20 or NO3-. In spite of the two different scavenger concentrations and the different detection sensitivities (3 X lOla molecules for N20 and -2 X 1015molecules for NO3-), the two curves are practically identical, showing an ionization threshold at 4.85 eV. The Iliqvalue of Pe is thus significantly smaller (-0.4 eV) for polar MeOH than for nonpolar Me4Si, in accordance with what has been repeatedly noted for solid or liquid solutions. A difference of 0.35 eV in Ili was also found for indole as a solute in liquid Me4Si o r h e O H at room temperature.13 A constant Iliqdifference, irrespective of the ionized solute, is consistent with the observation previously reported for hydrocarbon liquidsz1that the shift in Ztiq of a given solute in various solvents is essentially determined
(18)K. Takemoto, H. Matsusaka, S. Nakayama, K. Suzuki, and Y. Ooshika, Bull. Chem. SOC.Jpn., 41,764 (1968). (19) P. P. Infelta and M. Gratzel, J. Chem. Phys.,70,179 (1979).
(20)T. F.Hunter, Chem. Phys.Lett., 75,152 (1980). (21)R. A. Holroyd, S. Tames, and A. Kennedy, J.Phys.Chem., 79, 2857 (1975).
Results
6oi
Ionization Potentials of Polyacene Molecules FLuarescence ( a r b i t r a r y u fintensity i,
The Journal of Physical Chemistry, Vol. 85,No. 24, 198 1 3887
~
t
Ft
Flgure 3. Perylene emission spectra (a) in nhexane, (b) in 0.1 M NaLS solutlon, and (c) in methanol.
t
@&rbitrary u n i t s )
,,t
:;I 0.5
23 0
Flgure 4. 4 e- vs. excitation wavelength; perylene 4 X 10" M-' in 0.1 M NaLS solution with (a) N03Li (2 X M-') as scavenger and (b) CI0,H (7 X M-') as scavenger; (c) perylene in MeOH, as curve b, Figure 2; (d) perylene (2.5 X lo-' M-l in neutral Triton X 100 4NOBLi(2 X M-').
by that of the solvent Vovalue. Considering that, in methanol also, the photoejected electron is initially in a quasi-free delocalized state, the observed difference in Zbq would reflect the difference in the Vo values of the two solvents. The present results would thus confirm the value Vo= -1.0 f 0.1 eV previously derived for methan01.l~ Pe in Anionic or Neutral Micelles. From the highly hydrophobic character of Pe, one can reasonably infer that Pe molecules are indeed confined within the micelles. The question arises however as to the exact average location of the solute, Le., inside the hydrocarbon core or close to the polar heads of the ionic surfactant chains. Fluorescence spectra may provide an answer, particularly when the vibronic structures of the emission spectrum are strongly environment sensitive, as exemplified by pyrene molecules. The situation is less clear however in the case of Pe (Figure 3). With respect to the emission spectrum recorded from n-hexane solutions (spectrum a), the fluorescence spectrum relative to NaLS micelles (spectrum b) is shifted to the red (-7 nm) whereas it is slightly shifted to the blue (-3 nm) for polar MeOH solutions (spectrum c). On the other hand, the shoulder located at 443 nm in spectrum a is absent in b as well as in spectrum c, suggesting that the solubilizate molecules are somewhat affected by the polar heads of the surfactant. The photoionization efficiency curves for the system Pe-NaLS are displayed in Figure 4. The use of the neutral NzO scavenger is precluded for micellar systems since N 2 0 is soluble both in the lipid and in the aqueous phase.
41.5
J/
Flgure 5. Dependence of the photoelectron yield wlth the hoton energy for perylene solutions: (a) in NaLS, a (hu - Iilq); (b) In MeOH, log 4 e(hu - IiiJ
The photoionization curve a was obtained from Nosaq as an electron scavenger and curve b with H+ag. In the HCIOl concentration range employed, no effect of the scavenger concentration was noticeable. The experimental points of curve a appear more scattered, but the difference in the two ionization threshold values remains within the experimental error. The local distribution and concentration of these positively or negatively charged scavengers might be quite different at the periphery of the micelles. The fact that quite similar curves are however observed suggests that the photoelectrons tunnel across the electric double layer and that the scavenging reaction occurs mainly in the bulk of the aqueous solution. The ZMi value obtained, 4.60 eV, is significantly lower than Zliq in methanol (4.85 eV) or Zliq in MelSi (5.25 eV), but another striking difference is the shape of curves a and b as compared with the ionization curve relative to homogeneous solutions-MeOH, for example (curve c). On the other hand, when Pe is dissolved in neutral surfactant micelles, it gives rise to a photoionization efficiency curve such as d (Figure 4) which leads to a threshold energy equal to that obtained from NaLS anionic micelles but to a shape analogous to that observed from homogeneous methanol solutions. Figure 5 illustrates that different threshold laws apply to anionic micelles on one hand (curve a) and neutral micelles and homogeneous solutions on the other (curve b) . In the latter case, a logarithmic relationship log pe hv - Ziiq (1) appears to be valid in an energy range extending over a few tenths of an electron volt above the threshold, as previously noted also for aqueous indole photoi~nization.'~ For the Pe-NaLS system, curve b (N03-a ) and curve c (H', ) of Figure 4 are now well represente2 by a single curve ?curve a, Figure 5) when plotted as co, =
K(hv - Z,iq)l
(2)
Thus, perylene photoionization in anionic micelles appears to obey a linear relationship, at least over a limited energy range in the threshold region. This is reminiscent of a situation encountered in gas-phase photoionizations. The implications of such a remark will be discussed below. Tetracene. Curve a of Figure 6 represents the photoionization efficiency, derived from photocurrent mea-
3688
The Journal of Physical Chemistry, Vol. 85, No. 24, 1981
Bernas et al.
TABLE I: Photoionization Threshold Energy Values for Perylene and Tetracene in Homogeneous Solutions or NaLS Micelles Pe Tet
6.9 6.6
5.25 4.95
2.95 3.1
t @ g ( a r b i t r o r y units)
3 t
210
\
2 50
300 A h )
Figure 6. @e- vs. excitation wavelength for tetracene: (a) photocurrent in Me$i solution of tetracene (4 X 10“ M-’); (b) tet;acene (3 X 10“ M-’) in 0.1 M NaLS solution N03Li (2 X 10- M- ).
+
surements as a function of the exciting-light wavelength for tetracene in Me4Si solutions. For tetracene in NaLS micelles, NO3, was used as the electron scavenger and curve b was obtained. Two features are again striking: the ionization threshold relative to NaLS micelles is shifted to the red (-0.70 eV) as compared with Me4Si solutions; and the shape of the Tet-NaLS photoionization curve appears markedly different from that determined from homogeneous Me4% solutions. In the Tet-NaLS case, however, the (pe data are less reliable than for the Pe-NaLS system because of a simultaneous photodegradation of tetracene.
Discussion The difference in the shape of the ionization efficiency curves for NaLS micelle solutions on one hand, and for neutral micellar systems or homogeneous solutions on the other, might reflect the increased probability for the photoelectron to escape from geminate recombination in the former case. Such an interpretation stems from the remark that a threshold law such as expressed by relation 2 is close to what has been theoretically predicted and experimentally observed for gas-phase electron impact ionizations:22 cr
a
(E-
with n N 1. The same type of threshold behavior is also encountered and has been analyzed for gas-phase photoionization~.~~ If such an interpretation of curve b in Figure 5 is correct, it seems in turn to imply that the shape of the photoionization efficiency curves currently obtained from homogeneous solutions is also related to an increased probability of electron escape as the exciting photon energy is increased and not to an increase in the density of accepting levels. (22)L.G. Christophorou, “Atomic and Molecular Radiation Physics”, Wiley-Interscience, 1971. (23)P. M. Guyon and J. Berkowitz, J.Chem. Phys., 54,1814 (1971).
4.85 4.55
4.5 5-4.60 4.25
4.50 4.23
2.3-2.35 2.35
Finally, the difference in shape of curves a (or b) and c in Figure 4, i.e., in ionization yields at various excitation wavelengths, corroborates and extends a previous conclusion drawn from laser experiments* at a fixed light frequency that, for a given solute, higher ionization yields are reached from anionic micellar systems compared with neutral or cationic ones. As noted above, the ionization threshold values determined for Pe or Tet in micellar systems (IMJ are ca. 0.7 or 0.3 eV smaller than those observed in Me4Sior methanol solutions, respectively. As to the decrease AIw = I - Iw, it amounts to 2.3-2.35 eV for perylene and 2.35 eV !or tetracene (last column of Table I). It is remarkable that the AIMivalue reported for the two laser beam excitation of pyrene in NaLS micelle~ is ~also ~ very similar: 2.3 eV. In the present study (D2 lamp with S04C0 filters) a crude estimate for pyrene has also indicated 5.4 < IMi< 5.5 eV. As pointed out earlier, the optical ionization potential of an impurity molecule in a condensed phase-liquid or solid rare gases24or liquid hydrocarbons21-is related to the ionization potential Ig of the isolated molecule by eq 3 Iliq
= Ig
+ P+ + v,
(3)
where P+ represents the adiabaticub electronic polarization of the medium by the positive ion and V, the conduction band edge energy of the solvent. Such a relationship expresses only that, compared with gas-phase ionization conditions, and on a time scale commensurate with electronic excitation processes, the “final” energy is V, for the quasi-free delocalized electron and Ptfor the cation-medium system. An independent determination of Iliqand V , and a knowledge of Ig from the literature provides P+ by difference. On the other hand, P+ may be calculated from an expression analogous to Born’s equation: -e 2 P+ = - ( 1 - X o ; l ) (4) 2rt where r+ is an effective ionic radius and Eo,, the solvent optical dielectric constant. Hence, the cation effective radius can be evaluated. The data relative to Me4Si lead to rt = 2.95 A for Pe and 3.1 A for Tet. Knowing that the solute cation remains confined in the hydrocarbon interior of the micelle whereas the photoelectron diffuses to the aqueous phase, it is now interesting to compare the experimentally determined I M values with calculated ones based on P+ terms corresponding to the lipid phase, P+,HC, and a Vovalue relative to water. The mere significance of “V;’ in polar solvents has been questioned and d i s c ~ s s e d .The ~ ~ ~crux ~ ~of the matter(24)(a) B.Raz and J. Jortner, Chem. Phys. Lett., 4,155 (1969); (b) I. Messing and J. Jortner, Chem. Phys., 24, 183 (1977). (25)A. Henglein, Can. J. Chem., 55,2112 (1977). (26)C.E.Krohn and J. C. Thompson, Phys. Reu. B, 20,4365(1979).
J. Phys. Chem. 1981, 85,3689-3694
whether the ejected electron is initially in a quasi-free delocalized state or immediately localized in clusters of appropriate structure-will not be considered here. A pragmatic approach is to use the photoelectron fundamental energy determined for water from either photoelectr~chemical~~ or photoionizationz8experiments, irrespective of its physical meaning: bottom of a conduction band or depth of unrelaxed traps. These two sets of determinations lead to a common average value ‘‘V; = -1.2 eV. Using such “V;’ values and calculating P+sIcterms from ~ d the r+ values previously derived, one estimates I ~ i , d = 4.50 f 0.10 eV for perylene and IMi,cdcd = 4.23 f 0.10 eV for tetracene. The agreement between calculated and experimental values is striking for the two solutes (Table I). Similarly, the values IMi= 5.4 eV and AIMi= 2.3 eV reported for pyrene in NaLS micelle^'^ can be readily accounted for on the basis of “VO)’water and a P+,Hcterm amounting to -1.1 eV, on the plausible assumption that for pyrene r+ N 3 A. Thus, the electric field gradient at the micelle surface which certainly inhibits the ion-pair recombination does not appear to alter significantly the energetics of the charge separation process. A different c o n c l ~ s i o nwas ~ ~derived (27) J. K.Sass and H. Gerischer in “Photoemiasion and the Electronic Properties of Surfaces”,B. Feuerbacher et al., Eds., Wiley-Interscience, New York, 1978. (28) D.Grand, A. Bemas, and E. Amouyal, Chem. Phys. 44,73 (1979).
3889
by reference to the Iliqvalue for pyrene in methanol which was found to exceed I M i by -1.2 ev. Iliq,MeOH = 6.6 eV appears however surprisingly high; if expressed by relationship 3 with “VO))Me0H = -1.0 e V 3 (obtained from two different solutes and for one solute with two different electron scavengers),it would lead to P+ = -0.1 eV, hence, to r+ N 30 A. Concerning the photoionization of pyrene, another difficulty is also to reconcile the reported ionization threshold values I U q , M e o ~= 6.6 eV and Iliq,n pentane = 4.80 eV,29which reveal an unexpected trend on going from a polar to a nonpolar solvent. In conclusion, continuous low-intensity irradiations indicate that the lowering, relative to the gas phase, of the ionization potential of aromatic solutes located in micelles (AI,) is not anomalously high when compared to homogeneous solutions. Hence, the interfacial electric potential appears not to affect appreciably the magnitude of IMi. The relatively low I M i values determined for pyrene,14 as well as for perylene and tetracene, result, in our opinion, from the mere fact that one combines hydrophobic polyacenes with low gas-phase ionization potentials with the low“V,,” value of water. Acknowledgment. We thank A. Petit for his valuable technical assistance. (29) K.Siomos and L. G. Christophorou, Chem. Phys. Lett., 72, 43 (1980).
Transfer Free Energies of p-Alkyl-Substituted Benzene Derivatives, Benzene, and Toluene from Water to Cationic and Anionic Micelles and to n-Heptane Cella Hlrose and LUISSepGlveda’ Department of Chemlstty, Faculty of Sciences, Universlty of Chile, Las Palmeras 3425, Casilla 653, Santiago, Chlle (Recelved:April 15, 1981; In Flnal Form: July 20, 1981)
Transfer free energies of benzene, toluene, and a series of p-alkyl-substitutedphenols, phenoxide ions, benzoic acids, benzoate ions, anilines, and anilinium ions from water to cetyltrimethylammonium bromide (CTAB) and to sodium lauryl sulfate (LS) micelles have been determined by absorbance and ultrafiltration methods. All solutes interact more strongly with CTAB than with LS micelles. Replacement of a proton in the benzene molecule by a hydrophilic group enhances its interaction with micelles. Transfer free energies for the same solutes from water to n-heptane were also determined. Comparison of the two free-energy values suggests that, while benzene and toluene are located in the micellar core, the other solutes are not. The contributions of hydrophilic and hydrophobic moieties of the solutes to the transfer free energies from water to micelles and to n-heptane have been calculated. Hydrophobic moieties do not contribute equally to the total transfer free energies in every series studied.
Introduction In a recent paper, Bunton and Sep6lveda’ showed the hydrophobic and Coulombic interactions of p-alkylphenols and p-alkylphenoxide ions with cetyltrimethylammonium bromide (CTAB) micelles. The free energies of transfer (Apto) of substrates from water to micelles were obtained from the binding constants of the substrates to micelles. It also was shown that the contribution to the free energy of transfer from water to CTAB micelles per CH2group of the p-alkyl chain was about half the contribution of a (1) Bunton, C. A.; Sepdlveda L. J. Phys. Chem. 1979,83,680. 0022-3654/81/2085-3689$01.25/0
CH2 group for transfer from water to hydrocarbon2 and that the main driving force for the binding of phenols and phenoxide ions to CTAB micelles came from the interaction of the cationic head group with the benzene moiety. This behavior cannot be explained without more information about this and similar systems, and in the present work we extended the earlier measurements to anionic micelles of sodium lauryl sulfate (LS) and to such solutes as benzoic acids, benzoates, anilines, protonated anilines, benzene, and toluene. In addition, we also measured the (2) Tanford, C. “The Hydrophobic Effect”; Wiley-Interscience: New York, 1973.
0 1981 American Chemical Society
