Langmuir 1985, 1, 544-548
544
surface tension of the free air/water interface fell to 40-45 mN m-l, as against 64 mN m-l for the PK 15 layers which had not been partially dried. This shows clearly the damaging effect of partial air drying and suggests that the yLv at the free surface is close to that at the contacting air bubble. The estimated ycv values of 60 mN m-l and 29 mN m-l perhaps hold at different stages, the cell being hydrophilic before drying and hydrophobic later. This suggests that the conformation and orientation of cell surface molecules at the cell/vapor interface differ from those at the cell/ PBS interface. If so, the interpretation of the ycv values is further complicated since the basic equation in the estimation procedure postulates the substrate to have the same surface structure a t the substrate/vapor and substrate/liquid i n t e r f a c e ~ . ' ~Differences * ~ ~ ~ ~ ~ in the conformation and orientation of surface polymeric chains a t a substrate/air interface and a substrate/water interface have been proposed in the case of polymeric hydro gel^^^ and sulfonated p ~ l y e t h y l e n e . ~The ~ differences in the contact angles, 15' vs. 2 2 O , between the sessile drop and adhering bubble methods can be ascribed both to the differences in cell surface configuration and yLvvalues in the two situations. The foregoing results indicated that the sessile drop method with its attendant air-drying can cause severe cell damage so that interpretation of the measured contact angles is almost impossible. A seemingly reasonable ycv
value (68 mN m-l) was obtained if one followed current practice,9J2J3J9assuming that y~~ is equal to yLvo. However, it is clear from the foregoing results that the ycv value thus obtained is of little physical or biological meaning. For this reason, methods that do not involve cell drying and that minimize cell damage, such as the adhering bubble method, are far more suitable for contact angle determinations in biological systems. Recently, an immiscible phase method developed by AlbertsonZ0has also been used to obtain contact angles on cell layer^.^^^^ Present results show that it is difficult to eliminate surface tension reduction for a water drop contacting a biological surface and the relation yLv< yLvowill hold for most cases. Consequently, the simultaneousdetermination of both contact angle and surface tension under these experimental conditions is a necessary extension to current practice. Finally, it is noted that apart from the problem of applicability of Young's equation to biological ~ y s t e m sthe ,~ complexity of the experimental variables and the application of equilibrium concepts to analyze nonequilibrium cell surface properties raise formidable questions about the quantitative meaning of the estimated interfacial tensions.
Acknowledgment. We thank T. Masuoka for his assistance and Dr. A. Sonada and Dr. M. Iwakura for the PK-15 and E. coli cells. Thanks are also due to Dr. K. Tsuda for his continuous interest.
Heat of Immersion of ZnO in Organic Liquids. 2. Immersion in Benzene, Cyclohexene, and Cyclohexane Yasuharu Suda and Tetsuo Morimoto* Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama 700, Japan Received January 28, 1985. I n Final Form: June 3, 1985 The heat of immersion of ZnO samples, controlled in the amount of surface hydroxyls, was measured in three organic liquids, benzene, cyclohexene, and cyclohexane, to study the effects of the *-electrons in organic molecules and the amount of surface hydroxyls on the interaction energy. On every sample with different amounts of surface hydroxyls, the net heat of adsorption depends on the nature of immersiond liquids and increases in the order, cyclohexane < cyclohexene < benzene. On the other hand, it increases in every liquid with decreasing concentration of surface hydroxyls of the sample, very slowly and almost linearly over the concentration range of more than 3 OH nm-2 and sharply over the range of smaller concentration of hydroxyls. Among various kinds of interaction energies the dispersion energy is predominant and remains almost unchanged with varying surface field strength, and the induced-dipole effect as a matter of course increases with increasing field strength. In the cases of cyclohexene and benzene, an additional effect of *-electrons increases linearly with surface field strength. This suggests that charge-transfer complexes are formed between the dehydroxylated sites of ZnO and the *-electron molecules.
Introduction Surface hydroxyls have a large influence on the surface property of metal oxides. From the heat-of-immersion measurement of ZnO in linear aliphatic organic liquids with different dipole moments, it was found that the surface field strength changes drastically in the presence of surface hydroxyls, or it decreases linearly with increasing concentration of the hydroxyls.' Such data enable us to analyze individual contributions in the interaction energy
between a surface and a molecule, which is composed of three kinds of contributions due to permanent dipole, induced polarization, and dispersion force^.^-^ Benzene derivatives form various charge-transfer complexes, as the derivatives have different degrees of electron-donating (2) Chesaick, J. J.; Zettlemoyer, A. C.; Hedey, F. H.; Young,G. J. Can.
J. Chem. 1966,33, 251.
(3) Dear, D. J. A.; Eley, D. D.; Johnson, B. C. Trans. Faraday SOC.
----. --. (4) Cochrane, H.; Rudham, R. Trans. Faraday S O ~1965, . 61, 2246. 1963. 59. 713.
(1) Morimoto, T.; Suda, Y. Langmuir 1986, 1 , 239.
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(5) Lavelle, J. A.; Zettlemoyer, A. C. J. Phys. Chem. 1967, 71,414.
0 1985 American Chemical Society
Langmuir, Vol. 1, No. 5, 1985 545
Heat of Immersion of ZnO in Organic Liquids
Table I. Monolayer Capacity of Organic Molecules/(molecules n d ) degassing temp/OC specific surface area (m2g-l) benzene cyclohexene cyclohexane
25 2.82 1.97 1.94 1.99
100 2.89 1.98 1.96 1.98
200 2.90 2.00 1.99 1.96
300 3.13 2.05 2.08 1.91
400 3.23 2.10 2.16 1.87
500 3.19 2.13 2.21 1.84
600 3.26 2.14 2.23 1.83
ability or electron acceptability.s However, few examples have been reported on the formation of charge-transfer complexes in the systems of benzene derivatives and metal oxide surfaces.'-13 Kiselev et al. reported the interaction of *-electrons in benzene with the hydrogen atom in surface hydroxyls on silica? Morimoto and Naono" measured the heat of adsorption of benzene on silica gel from the cyclohexane solution. The present work has been undertaken to investigate the effect of n-electrons on the interaction of benzene and cyclohexene on ZnO, by measuring the heat of immersion of the differently dehydroxylated ZnO samples in benzene, cyclohexene, and cyclohexane. Experimental Section Materials. The original sample of ZnO was the same as used in the previous work,' which was provided by Sakai Chemical Co. and had been made by burning Zn of 99.99% purity in the air. It has been found that the sample is of the wurtzite structure with the well-developed (1010)plane.15 After the pretreatment at 600 OC in a vacuum of 1 X lo4 Pa for 4 h, the sample was exposed to saturated water vapor at room temperature for 5 h to permit hydration. Then it was subjected to degassing at various temperatures from 25 to 600 OC for 4 h, which produced the samples having different concentration of surface hydroxyls. The surface water content remaining on the samples was measured by the successiveignition loss method.le The surface area of the sample once treated at 600 OC was measured by applyingthe BET method to the N2 adsorption data and found to be 3.26 m2g-'. Benzene from Kanto Chemical Co. and cyclohexaneand cyclohexene from Nakarai Chemical Co. were all predried by activated molecular sieve 4A. Then the first two liquids were distilled in the presence of metallic sodium and the third was distilled in the absence of it. All the solventswere stored in contact with activated molecular sieve 4A. Heat-of-Immersion Measurement. The heat of immersion was measured at 28 & 0.1 OC by using an adiabatic calorimeter equipped with a thermistor of 10 kQ resistance as a temperature-sensing element." In order to avoid the effect of a trace of water on the heat-of-immersiondata, the immersionalliquids were used in contact with molecular sieve 4A. Results a n d Discussion The surface hydroxyl content of ZnO measured by the successive ignition loss method is plotted against the ~
Mulliken, R. S.; Pereon, W. B. 'Molecular Complexes";Wiley-Interscience: New York, 1969. (7) Basila, M. R. J. Chem. Phys. 1961,35, 1151. (8)Whalen, J. W. J. Phys. Chem. 1962,66, 511. (9)Galkin, G.A.;Kiaelev, A. V.; Ligin, V. I. Trcms. Faraduy SOC.1964, (6)
60, 431.
(10)Cusumano,J. A.;Low,M. J. D. J. Phys. Chem. 1970, 74,1950. (11)Cusumano, J. A.;Low, M. J. D. J. Colloid Znterfuce Sci. 1972,38, 245. (12)Anderson, J. H.; Lombardi,J.; Hair, M. L. J. Colloid Interface Sci. 1976, 60,619. (13)Pohla, W. J. Chem. SOC.,Faruduy f i u ~ 1. 1982, 78,2101. (14)Morimoto, T.;Naono, H. Bull. Cham. SOC.Jpn. 1972,46,700. (16)Morimoto, T.;Nyao, M. J. Phy8. Cham. 1974, 78, 1116. (16)Morimoto,. T..:Shomi,. K.:. Tanaka. H.Bull. C h m . Sac. Jon. 1984. 37,392. (17) Nagao, M.; Morimoto, T. J. Phys. Chem. 1969, 73,3809. (18)Results to be published in detail. (19)'Kagaku-binran Kisohen 11"; Nihonkagakukai, Ed.; Maruzen: Tokyo, 1984;Vol. 3.
Degassing temperature, T
Figure 1. Relation between water content of ZnO and degassing temperature.
%i 3
100
I
0
300 400 500 200 Degassing temperature , 'C
100
600
Figure 2. Relation between heat of immersion of ZnO and cyclohexene; ( 0 )cydegassing temperature: (m) benzene; (0) clohexane. evacuation temperature of the fully hydroxylated sample in Figure 1. It is seen from Fig. 1that the concentration of surface hydroxyls decreases with rising temperature of evacuation, sharply around 300 "C, and becomes very small after the treatment at 600 "C. The heat of immersion of ZnO in the three immersional liquids is given in Figure 2 as a function of degassing temperature of the hydroxylated sample. On the samples degassed at temperatures between 25 and 200 "C where most of the ZnO surface is covered with hydroxyls, the heat of immersion is about 250 erg being very close to each other, regardless of the nature of liquids used. When the degassing temperature exceeds 300 "C, the immersional heat increases considerably with increasing temperature of degassing, and it becomes greater in the order benzene > cyclohexene > cyclohexane. For cyclohexene the heat value increases steeply around 500 "C. In Table I, the monolayer capacity rm of three organic molecules, which was obtained from the adsorption isotherms measured on the same systems, is listed together with the specific surface area of the sample calculated from the N2adsorption data. A small jump in the surface area value can be observed near the 300 "C treatment. This
546 Langmuir, Vol. 1, No. 5, 1985
Z
: t
I
Suda and Morimoto
I
0.5
LZ
Surface hydroxyl content , OH'S nm'2
Figure 3. Relation between net heat of adsorption of organic molecules and water content of ZnO (m) benzene; (0)cyclohexene; ( 0 )cyclohexane.
may be due to the opening of small depressions which have been covered by chemisorbed water. For the calculation of rm,the surface area of the bare surface, Le., of 600 "C treated sample, was employed. The data in Table I show that the change in rm with the degassing temperature of ZnO depends on the nature of adsorbates; that is, rm of benzene and cyclohexene increases, but that of cyclohexane decreases. Furthermore, on the 25 "C treated surface the number of adsorbed cyclohexene molecules is slightly smaller than that of benzene, but the situation is reversed when the sample is treated at temperatures higher than 200 "C. By subtracting the surface enthalpy of immersional liquids from the heat-of-immersion value, one can obtain the net heat of adsorption per unit area, and next the net heat of adsorption per molecule can be obtained by introducing the data in Table I. The heat value thus calculated is illustrated in Figure 3 as a function of the amount of surface hydroxyls. Figure 3 shows that the net heat of adsorption depends on the nature of immersional liquids and increases in the order benzene > cyclohexene > cyclohexane, over the whole range of surface hydroxyl content. Moreover, the net heat of adsorption of every molecule is the largest on the bare surface of ZnO, and it decreases with increasing concentration of surface hydroxyls, sharply until the concentration attains 2-3 OH nm-2 and then slowly until the full hydroxylation is reached. In the previous paper, it has been found that the field strength of the fully hydroxylated surface of ZnO is very small, whereas it increases remarkably and linearly when the dehydroxylation proceeds.l Therefore, it is reasonable to consider that an increase in the net heat of adsorption of every organic molecule with the progress in the dehydroxylation of the surface (Figure 3) is ascribed to the increase in the surface field strength. Moreover, the fact that the net heats of adsorption of different molecules are very s i m i i to each other on the fuUy hydroxylated surface suggests that the dispersion force Se the predominant factor among various interaction forces acting between the surface and each molecule, because of a small field strength of the surface. When a partial dehydroxylation of ZnO occurs, the surface field strength is enhanced because of the exposure of higher valence ions, Zn2+and 02-.'Thus, the interaction
Figure 4. Adsorption models of organic molecules on (1010)plane of ZnO (a) benzene on hydroxylated ZnO; (b) benzene on partially dehydroxylated surface; (c) benzene on dehydroxylated surface; (d) cyclohexene on dehydroxylated surface.
$ 1 E 2.0
I
0
2 Number of
I
I
4
6
n -electrons Figure 5. Relation between net heat of adsorption and number of ?r-electrons. ZnO treated at various temperatures: ( 0 )25; (0) 100;(m) 200; (0)300; (A)400;(A)500; ( 0 )600 O C .
energy between the surface and an adsorbed molecule will increases owing to the induced polarization effect. With the cyclohexene molecule, the velectrons are located so that the molecule has a small dipole moment, which will result in an additional enhancement in the interaction energy, compared with that of cyclohexane molecule. If we aseume the flat adsorption, the occupied area of the benzene molecule can be calculated from the molecular model to be 0.426 nm2. Thus, the data in Table I suggest that the flat adsorption of the molecule takes place on the surface of ZnO. Figure 4 demonstrates the adsorption models of benzene and cyclohexene on the well-developed (lOl0)plane of ZnO. Since the cross-sectional area of each molecule used here is almost the same, the molecules will be adsorbed flatwise on the surface of ZnO as suggested from Table I. When the surface is partially dehydroxylated, these molecules may be adsorbed in such a tilted form as illustrated in Figure 4b. Further dehydroxylation
Langmuir, Vol. 1, No. 5, 1985 547
Heat of Immersion of ZnO in Organic Liquids
Table 11. Effect of r-Electrons on Heat of Adsorption/(lO-lserg molecule-l) 100, 200, 300, 400, 500, 3.68 x 103 5.00 x 104 1.33 x 105 2.98 x 105 3.08 x 105 E,+ E , Ed E,+ E, Ed E,+ E , Ed E,+ E , Ed E,+ E, Ed
degassing temp/'C, F/(statvolt cm-') ~~~
a/cm3 benzene 6.35 X cyclohexene 7.80 X cyclohexane 9.25 X Mdl U d 2
0.430 12.7 X 0.626
X X
(=E,,) (=E,,)
9.73 0.794 X lo-' 10.4 9.20 2.64 X lo-' 9.45 9.06 1.16 X lo-' 9.04 0.67 1.36 0.14 0.41
of the surface will again lead to the flat adsorption of the molecule as shown in Figure 4c, which will result in the generation of a larger heat of adsorption. A steep increase in the heat-of-adsorption curve of every molecule in Figure 3 at a concentration less than 3 OH nm-2 may correspond to the commencement of this type of adsorption. Figure 5 shows the relationship between the net heat of adsorption per molecule and the number of a-electrons in the molecules. It is seen from Figure 5 that the larger the number of a-electrons in a molecule, the larger the net heat of adsorption, and that a linear relationship holds between them on the samples treated at temperatures from 25 to 300 "C. When the treatment temperature becomes higher than 400 O C , the heat of adsorption of cyclohexene deviates to a higher level from the straight line connecting the heat values of benzene and cyclohexane, probably because of the polar adsorption of the cyclohexene molecule as stated above. On the other hand, the adsorption data in Table I reveal that the number of cyclohexene molecules adsorbed on ZnO exceeds that of benzene when the pretreatment temperature of the sample is raised. These facta may be favorable to the postulate of the tilted adsorption of the cyclohexene molecule on the dehydroxylated site (Figure 4d). Evaluation of the Effect of a-Electrons on Adsorption Energy. As illustrated in Figure 5, the a-electrons have a large effect on the adsorption energy of molecules. A few reports have been concerned with the interaction between the hydrogen atom in surface hydroxyls and the a-electrons in the benzene mo1ecule,'-l3 but we have no knowledge about the interaction energy of the bare surface of metal oxides with the a-electrons of organic molecules. The data in Figure 5 clearly indicate that the surface of ZnO interacts with the a-electrons of the molecules used, in addition to the other kinds of contributions. For the interaction energy between a surface and a molecule, eq 1 is generally u ~ e d . ~Here, - ~ Ed Etotd
= Ed + E, + E ,
(1)
denotes the adsorption energy which comes from the dispersion force. The term E,, is the interaction energy between the electrostatic field strength F of the surface and the dipole moment ~1 of the adsorbed molecule, which is given in the form
E,, = -PF
(2)
E , is the contribution due to the induced dipole moment in the adsorbed molecule, and is expressed by E , = - aF/2
(3)
For the adsorption of benzene or cyclohexane, the second term in eq 1is zero, because both kinds of molecules have no dipole moment. We can calculate the term E , by combining the F value obtained in the previous work' and eq 3 and then estimate the term Ed by subtracting E, from the measured adsorption energy in Figure 5. For the analysis of the adsorption energy of cyclohexene, the effect of the dipole moment should be taken into consideration.
0.562 1.13 0.818
2.82 4.45 4.11
11.2 9.22 8.78 2.42 0.44
11.5 8.46 6.48 5.02 1.98
2.97 4.72 4.33
14.5 11.7 8.14 6.36 3.52
600, 3.38 x 105
E,+E,
Ed
3.63 5.58 5.28
17.8 12.4 8.59 9.21 3.83
I
5 6-
-: e
2 1, -i 0
r
w
0
1 2 3 Electrostatic field strength, 105statvolt cm-'
Figure 6. Plot of E, against electrostatic field strength: (w) benzene; (0) cyclohexene.
The values of calculated (E, + E,) and Ed are listed in Table 11. For the calculation of E,, the polarizability a perpendicular to the molecular plane was introduced,20 since the flat adsorption occurs with these molecules. As stated above, E,, in the term ( E , + E,) is zero for the interaction of surfaces with benzene or cyclohexane, while E,, is comparable to E , for the interaction with cyclohexene; e.g., E,, and E , are 1.66 X and 9.75 X erg molecule-', respectively, for the 200 OC treated sample, and 1.02 X and 3.70 X erg molecule-' for the 500 "C treated sample. The data in Table I1 show that the Ed values of cyclohexane are almost constant over the whole range of degassing temperature of ZnO and always smaller than those of benzene and cyclohexene on every surface. We can postulate from the above discussion that the difference between the Ed value of benzene or cyclohexene and that of cyclohexane is attributed to the contribution of the surface--electron interaction. The difference thus calculated, E,, can be found to increase with rising F value of the sample, as shown in Figure 6. Taking account of both a fairly large electron-donating character of the benzene molecule and a large polarizing effect on the bare ZnO surface, it may be considered that the contribution of a-electrons is a kind of electrostatic interaction acting between the Zn2+ion and the a-electrons. A linear relationship holds between the E , value and the field strength F , and in addition the straight line is composed of two parts. Actually, the slope of the straight line in Figure 6 has the dimension of dipole moment, as can easily be understood from the comparison with eq 2. The dipole moment calculated from the straight line is 1.47 and 10.25 D for benzene adsorption on weakly and strongly dehydroxylated surfaces, respectively. Similar consideration can be made on the adsorption of cyclo(20) Landolt-Bomstein "Zahlenwerte und Funktionen aus Physik, Chemie, Astronomie, Geophysik, und Technik"; Springer-Verlag: Berlin, 1951; Aufl. 6, Band 1, Teil 3.
548
Langmuir 1985, 1, 548-552
hexene, though the slope is smaller than in the case of the benzene molecule (Figure 6). The dipole moment of surface complexes formed by the adsorption of cyclohexene can be computed to be 0.62 and 5.58 D on weakly and strongly dehydroxylated surfaces, respectively. In conclusion, these considerations suggest that charge-transfer complexes are formed on the bare surface of ZnO, corre-
sponding to each value of the dipole moments calculated above.
Acknowledgment. We thank Professor Mahiko Nagao for his valuable d~scussionin this research. Registry No. ZnO, 1314-13-2;benzene, 71-43-2; cyclohexene, 110-83-8;cyclohexane, 110-82-7.
Luminescence Quenching of Ruthenium(11) Photosensitizers by Cu2+in Triton Surfactant Media Seth W. Snyder, Douglas E. Raines, P. T. Rieger, and J. N. Demas* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901
B. A. DeGraff* Department of Chemistry, James Madison University, Harrisonburg, Virginia 22807 Received February 15, 1985. I n Final Form: June 3, 1985 The interactions of several luminescent a-diimine ruthenium(I1) photosensitizers with Triton X-100 and X-114 micelles were studied by using excited-state lifetime measurements and quenching by Cu2+as probes. Binding of the photosensitizerto the micelles exerts an enormous shielding effect, and quenching of the sensitizer by Cu2+is suppressed by a factor of 120 in Triton X-100 and about 10 in Triton X-114. This suppression is attributed to the fact that the sensitizer binds to the surface of the dry hydrocarbon core, and the poly(ethy1ene oxide) sheath of the micelle acts as a membrane that blocks penetration of the hydrophilic quencher Cu2+.Because Cu2+can only quench unshielded free sensitizer, multiple exponential decays are observed when populations of bound and unbound sensitizers are similar and exchange of the sensitizer is low compared to the excited-state lifetime. This model is shown to be consistent with the known photosensitizer-micelle equilibrium constants and bulk Cu2+Stern-Volmer quenching constants.
Introduction The photochemistry and photophysics of transitionmetal complexes in organized assemblies are currently being actively studied.' The majority of studies have involved ionic micelles since they have shown promise as a media for charge separation in solar energy conversion schemesa2 We are currently engaged in a systematic examination of the interactions of a-diimine ruthenium(I1) photosensitizers with ionic3i4and nonionick7 surfactants. While there are numerous kinetic studies on the quenching of Ru(I1) photosensitizers by Cu2+and other cations in the presence of ionic surfactants? there has been a paucity of such studies involving nonionic surfactants. In this paper we report the results of quenching experiments in nonionic Triton surfactants. We show that while binding of the photosensitizers to the micelles has (1)For recent reviews, see: (a) Gratzel, M. Acc. Chem. Res. 1981,14, 376. (b) Turro, N. J.; GrHtzel, M.; Brawn, A. M. Angew. Chem., Int. Ed. Engl. 1980,19,675. (c) Yekta, A.; Aikawa, M.; Turro, N. J. Chem. Phys. Rev. 1978,4, Lett. 1979,63,543. (d) Kalyanasundaram, K. Chem. SOC. 453. (e) Thomas, J. K. Acc. Chem. Res. 1977,10, 133. (2) For examples, see: (a) Whitten, D. G.; et al. In Hautala, R. R.; King, R. B.; Kutal, C. Sol. Energy: Chem. Conuers. Storage, [Symp.] 1979, 117. (b) Thomas, J. K. Ibid. 1979, 141. (3) Hauenstein, B. L.; Dressick, W. J.; Buell, S. L.; Demas, J. N.; DeGraff, B. A. J. Am. Chem. SOC.1983,105,4251. (4) Buell, S. L. Ph.D. Thesis, University of Virginia, Charlottesville, 1983. ( 5 ) Mandal, K.; Hauenstein, B. L.; Demas, J. N.; DeGraff, B. A. J. Phys. Chem. 1983,87, 328. (6) Hauenstein, B. L.; Dressick, W. J.; Gilbert, T. B.; Demas, J. N.; DeGraff, B. A. J. Phys. Chem. 1984,88, 1902. (7) Dressick, W. J.; Hauenstein, B. L.; Gilbert, T. B.; Demas, J. N.; DeGraff, B. A. J. Phys. Chem. 1984,88, 3337. (8) (a) Atherton, S. J.; Baxendale, J. H.; Hoey, B. M. J. Chem. SOC., Faraday Truns. 1 1982, 78,2167. (b) Ziemiecki, H.; Cherry, W. R. J.Am. Chem. SOC.1981,103, 4479.
0743-7463/85/2401-0548$01.50/0
Table I. Binding Constants to Triton Micelles and Stern-Volmer Quenching Constants with Cu2+for Ru(I1) Photosensitizers Ru(I1) KDM, icmc, KSV, I," TD, TDM, mM-' mM M-' M ps complex ps Triton X-100 (phen)? 0.923b 0.20c 42 0.12 (Me-phen)?+ 1.37 2.05 18.3 0.8 67 0.12 (5,6-Mezphen)?+ 1.83 2.76 62.2 0.474 96 0.12 (4,7-Mezphen):+ 1.72b 2.85* 52.5b 1.55b 133 0.18
(phen),Phzphenz+3.Mb 4.6gb 333b Triton X-114 (phen)? 0.923b 2.9 (5,6-Mezphen):' (4,7-Mezphen):+
1.89 1.55
2.97 2.87
123 73.1
0.315b
0 0.497 0.711
62.6 0.12 42 96 133
0.12 0.12
0.12
Ionic strength. Reference 5. Reference 20.
relatively little effect on their luminescent properties, the accessibility of the hydrophilic metal cation quenchers to the sensitizers is radically affected. We have examined the effect of micelle structure, including the thickness of the hydrophilic ethylene oxide outer shell, on quenching behavior. The model developed should be appropriate for a variety of photosensitizer-quencher-urfactant systems.
Experimental Section The ligands and their abbreviations are 1,lO-phenanthroline (phen), 5-methyl-1,lO-phenanthroline (Me-phen),5,g-dimethyl1,lO-phenanthroline (5,6-Mezphen), 4,7-dimethyl-l,10phenanthroline (4,7-Mezphen), and 4,7-diphenyl-1,10phenanthroline (Phzphen). RuCl, and all ligands were purchased from GFS Chemical Co. The syntheses are described elsewhere? (9) Hauenstein, B. L.; Mandal, K.; Demas, J. N.; DeGraff, B. A. Inorg. Chem. 1984,23, 1101.
0 1985 American Chemical Society
