J . Phys. Chem. 1989, 93, 6751-6754 perhaps providing some insight into the nature of metalsupport interactions.
Acknowledgment. This work was supported in part by the US. National Science Foundation Solid State Chemistry Program (grant DMR 86-15206). We thank B. Phillips for the use of his electric field gradient calculation program and for obtaining
6751
powder X-ray diffraction spectra, C. Weiss for assistance with the synthesis of crystalline boehmite, Dr. A. Irwin for the surface areameasuremeni, and Dr. A. Thompson and Dr. G. Turner for helpful discussions, Registry NO. A1203, 1344-28-1; AlO(OH), 24623-77-6; Al(OH)I, 21645-51-2; I7O, 13968-48-4; boehmite, 1318-23-6; bayerite, 20257-20-9.
Orientation of Caroviologens in Model Membranes L. B.-A. Johansson,*’tM. Blanchard-Desce,t M. Almgren,s and J.-M. Lehnt Department of Physical Chemistry, University of UmeP, S-901 87 Umei, Sweden, Chimie des Interactions MolCculaires, CoIl?ge de France, 11 Place Marcelin Berthelot, 75005 Paris, France, and The Institute of Physical Chemistry, University of Uppsala, P.O. Box 532, S-751 21 Uppsala, Sweden (Received: December 15, 1988)
The bis(4-pyridinium) polyenes, called caroviologens, are of potential interest as devices for electron transfer by a molecular wire-type process. The orientation of three caroviologens (denoted by 12+, 22+,and 32+)solubilized in five different lipid model membranes was studied by means of polarized electronic absorption spectroscopy. The model membranes were macroscopically aligned, and the dichroic ratio of the electronic spectrum was determined in the wavelength region 350-700 nm. It is found that the orientation of the caroviologens depends on the length of the polyene chain as well as the thickness of the lipid bilayer. If the long axis of the caroviologen exceeds or matches the thickness of a lipid bilayer, the long axis of the molecule is oriented preferentially perpendicular to the membrane.
Introduction Molecular wires that would allow an electron flow between different molecules are of great interest in the design of molecular devices.’-3 The caroviologens, which are potential candidates for such molecular wires, combine the properties of the carotenoids and of the viologens: The pyridinium group of the viologens is water soluble and electroactive for electron exchange, while the nonpolar conjugated polyene chain conducts electrons efficiently. The solubility properties are typical of amphiphile molecules, and the caroviologens should dissolve in lipid bilayers. Depending on the orientation of the caroviologen in a membrane it may more or less efficiently serve as a conductor of electrons across or along the bilayer. This work aims at investigating the orientational dependence of the caroviologens on their length and on the thickness of the bilayer. Three caroviologens denoted by 12+, 22+,and 32+ (see Figure 1 ) were studied. The thickness of the lipid model membranes was varied from about 20 to 40 A. Materials and Methods The bis(4-pyridinium) polyenes 12+, 22+,and 32+(see Figure 1) were synthesized as described in ref 1. The lipids 1-oleoyl monoglyceride and 1,2-dioleoyl-sn-glycero(3)phosphocholine (DOPC) were purchased from Aktieselskabet Grindstedvaerket, Grindstedt, Denmark, and Avanti Polar Lipids, Inc., respectively. 1-Octyl monoglyceride was synthesized at Syntestjanst, Chemical Centre in Lund, Sweden. Penta(ethy1ene glycol) mono-n-dodecyl ether (C12EOS)was obtained from Nikko Chemicals Ltd., Tokyo, Japan. Sodium octanoate from BDH was purified by filtering a solution of the amphiphile in methanol and active coal. The lamellar liquid crystals were prepared as described in ref 4 with a molar ratio of lipid/chromophore varying between lo3 and 104. At molar ratios of about lo2, the systems contain crystals and are then most likely saturated with respect to the caroviologens. The lamellar phases were composed of sodium octanoate/]-decanol/water in the amounts l l .9/28.1/60.0 wt %, octyl monoglyceride/water in the amounts 70.0/30.0 wt %, C,,EO,/ ‘University of Umeb. *ColEgede France. )University of Uppsala.
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water in the amounts 69.0/31.0 wt %, oleoyl monoglyceride/water in the amounts 90.0/10.0 wt %, and DOPC/water in the amounts 80.0/20.0 wt %. The refractive index values of the lamellar phases containing sodium octanoate, octyl monoglyceride, CI2EOs,oleoyl monoglyceride, and DOPC are 1-40, 1.42, 1.43, 1.47, and 1.47, respectively, as obtained with an Abbe refractometer. The macroscopical alignment of the lamellar phases was checked by observing the samples when placed between two crossed polarizers. The dichroic ratio of the electronic spectra were recorded on a Varian Cary 219 absorption spectrometer supplemented with sheet polarizers (Model HNP‘B, Polarizers, Ltd.). The samples were thermostated within 293 f 1 K.
Theoretical Prerequisites Lamellar liquid crystals containing small amounts of chromophores can be macroscopically aligned between quartz plates.e6 The lamellae align with their planes parallel to the quartz plates. These systems are uniaxially anisotropic, and the optical axis and the director of the membranes coincide with the normal to the quartz plates. By shining linearly polarized light with its polarization at the angles w and 90’ (I) relative to the optical axis, the absorbances A , and A , were measured. From the dichroic ratio D = A J A , , information about the molecular orientation of the chromophores can be calculated from6-’ 3 s cos2 w D=1+ ( I - S)nZ
S=
(3COS2P-1) 2
(2)
(1) Arrhenius, T. S.; Blanchard-Desce, M.; Dvolaitzky, M.; Lehn, J.-M.; Malthete, J. Proc. Natl. Acad. Sei. U.S.A. 1986, 83, 5355. (2) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89. (3) Stiegman, A. E.;Miskowski, W. M.; Perry, J. W.; Coulter, D. R.J . Am. Chem. Soc. 1987, 209, 5884. Effenberger, F.; Schlosser, H.; BBuerle, P.; Maier, S.; Port, H.; Wolf, H. C. Angew. Chem., Int. Ed. Engl., 1988, 27, 281. Blanchard-Desce, M.; Ledoux, I.; Lehn, J.-M.; Malthete, J.; Zyss, J. J . Chem. SOC.,Chem. Commun. 1988, 736. See also references cited in ref 1. (4) De Vries, J. J.; Berendsen, H. J. C. Nature 1969, 221, 1139. (5) Lindblom, G.Acta Chem. Scand. 1972, 26, 1745. (6) Johansson, L. B.-A; Lindblom, G.;Wieslander, A.; Arvidson, G. FEBS Lett. 1981, 128, 97.
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6752 The Journal of Physical Chemistry, Vol. 93, No. 18, 1989
Johannson et al.
TABLE I: Dichroic Ratio ( D ) Determined at w = 45O, Order Parameter (S),and Wavelength of the Absorption Maximum (kx) for 12+, 22+, and 3*+ When Solubilized in Different Model Membranes carovioloaen system sodium octanoate/ 1-decanol/water 1 -octyl monoglyceride/water penta(ethy1ene glycol) mono-n-dodecyl ether/water I -oleoyl monogl yceride/water 1,2-dioleoyl-sn-glycero(3)phosphocholine(DOPC)/water ethanol
12+
estd bilayer thickness, A
D
24a 23b 33’ 34c 40d
2.20 1.63 1.31 1.09 0.79
S A, nm 0.63 533 0.47 537 0.31 533 0.12 542 -0.46 535 557
32+
22+
D 2.15 1.66 2.25‘ 1.P
1.15
S A, nm 0.62 540 0.48 545 0.75 505 0.7 520 0.18 540 560
D
,A, nm 1.89 0.56 507 1.50 0.42 506 not measured 1.7( 0.6 507 1.45 0.39 505 530 S
+
‘From X-ray.* *Estimated from 0.15 0.1265N (A) in ref 11. N is the number of C atoms in the alkyl chains. CFromX-ray.I3 dFrom X-ray.I2 eEstimated from the dichroic ratio at the red edge of the absorption spectrum.
12+
22+
32’ R = C H , and X = I -
Figure 1. Structural formulas of three bis(4-pyridinium) polyenes or caroviologens denoted by 12+, 22+, and 32+. The schematic shows the orientation of th_e electronic transition dipole moment @’) with respect to the normal (N) or the symmetry axis of a lipid bilayer.
where n denotes the average refractive index of the lamellar phase,’ the order parameter S describes the average orientation ( ( ...) ) of the electronic transition dipole moment with respect to the director, and /3 is the an_glebetween the transition dipole moment @’)and the director (N) of the bilayer, as is shown in Figure 1. Since the transition dipole moment has a fixed direction in the chromophore, S yields information about the orientation of this molecular axis. The order parameter is a number between -l/z and 1 for all orientational distributions of uniaxial symmetry. In particular, the limits of -1/2 and +1 correspond to all molecules being oriented with their transition dipole moments perpendicular ( p = n/2) and parallel (B = 0), respectively, to the director of the lipid bilayer.
+
Results and Discussion The caroviologens, 12+,22+,and 32+,solubilized in five lamellar (7)Johansson, L. B.-A.; Davidsson, A. J . Chem. SOC.,Faraday Trans. 1 1985, 81, 1375.
liquid crystals composed of different amphiphilic molecules were studied by means of polarized absorption spectroscopy (see Table I). The anionic detergent sodium octanoate, 1-decanol, and water form a lamellar liquid crystal with a bilayer thickness of 24 A.* The nonionic 1-octyl monoglyceride and water form bilayers that have about the same thickness. The bilayers of penta(ethy1ene glycol) mono-n-dodecyl ether and water are ca. 30 A. The lipid bilayer of 1-oleoyl monoglyceride and that formed by the zwitterionic 1,2-dioleoyl-sn-glycero(3)phosphocholine(DOPC) have a thickness of 34 and 40 A, respectively. The electronic absorption spectra of the caroviologens are broad and structureless with absorption maxima around 550 nm as can be seen in Figure 2. The dichroic ratio of 12+,22+,and 32+solubilized in the octyl monoglyceride and sodium octanoateln-decanol systems is constant at wavelengths between 500 and 600 nm. This shows that the main absorption band contains one direction of the electronic transition dipole moment. The strong transition is most likely polarized perpendicular to the C2-axis and along the conjugated m y s t e m of the chromophore. In all of the systems studied here, the dichroic ratio varies with wavelength in the region 350-450 nm, which means that two or more electronic transitions contribute to the absorption. The absorption spectra of 12+show an absorption peak at about 430 nm, which is most distinct in the lamellar phases built up with DOPC and the oleoyl monoglyceride, respectively. The magnitude and/or shift of this peak seems to depend on the composition of the lamellar phase. In the bilayers of DOPC, the dichroic ratio D > 1 for the 430-nm band, while D < 1 for the strong band centered at 530 nm. This suggests that the peak at 430 nm is due to a weak electronic transition that is polarized perpendicular to the long axis of the molecule. In the spectra of 22+,several weak shoulders appear below 500 nm while minor spectral changes are observed for 32+,as is shown in parts b and c, respectively, of Figure 2. The shapes of the absorption spectra of 22+and 32+were found to change gradually with time so that the absorption increases at shorter wavelengths but decreases in the red part of the spectrum. The spectral changes of 22+and 32+in the bilayers composed of sodium octanoate/ldecanol and octyl monoglyceride are much slower as compared to those of the longer lipids. One possibility would be that trans-cis isomerization is taking place with time similar to that observed in carotenoids?JO Another possibility is the formation of dimers, trimers, etc., which enables strong intermolecular interactions between the caroviologens (see ref 1 and papers cited therein). This mechanism may explain the observed spectral changes and
(8) Ekwall, P.; Mandell, L.; Fontell, K. Acta Chem. Scand. 1968, 22, 1543. (9) Donahue, J. M.; Waddell, W. H. Photochem. Photobiol. 1984,40,339. (10) Carotenoids; Isler, O., Ed.;Birkhiuser Verlag: Basel, Switzerland, and Stuttgart, West Germany, 1971. (1 1) Tanford, C. The Hydrophobic Effect; Wiley-Interscience: New York, 1980. (12) Lis, L. J.; Mc Alister, M.; Fuller, N.; Rand, R. P. Biophys. J . 1982, 37, 657. (13) Lindblom, G.; Larsson, K.; Johansson, L. B.-A.; Fontell, K.; Forsen, S . J . Am. Chem. SOC.1979, 101, 5465.
The Journal of Physical Chemistry, Vol. 93, No. 18, 1989 6753
Orientation of Caroviologens in Model Membranes a
1‘’
--
. . . . ..
22’
2.0 1.5
n* 1.0
350
400
450
550
500
600
350
650
400
450
Wavelength inml
500
550
600
650
Wavelength lnml
C
-- 2.0 b 1.5
3’
*
t 0
1.0
E
e
g
n
360
400
450
500
550
800
650
Wavelength lnml
Figure 2. (a) Normalized absorption spectra and the dichroic ratio [D4s0(X)]of 12+ solubilized in lamellar liquid crystalline phases of the following compositions: sodium octanoate/l-decanol/water(-), I-octyl monoglyceride (--), penta(ethy1ene glycol) mono-n-dodecyl ether I-oleoyl monoglyceride (-) and 1,2-dioleoyl-sn-glycero(3)phosphocholine (DOPC, -). (b) Normalized absorption spectra and the dichroic ratio [D450(X)]of 22+ solubilized in lamellar liquid crystalline phases of the following compositions: sodium octanoate/ 1-decanol/water (-), 1-octyl monoglyceride (-), penta(ethy1ene glycol) mono-ndodecyl ether (-), l-oleoyl monoglyceride (-) and 1,2-dioleoyl-sn-glycero(3)phosphocholine(DOF‘C, -). (c) Normalized absorption spectra and the dichroic ratio [D450(X)]of f2+ solubilized in lamellar liquid crystalline phases of the following compositions: sodium 1-oleoyl monoglyceride (-) and octanoate/ 1-decanol/water (-), 1-octyl monoglyceride (--), penta(ethy1ene glcyol) mono-n-dodecyl ether 1,2-dioleoyl-sn-glycero(3)phosphocholine (DOPC, -).
-
-
implies that the rate of aggregation of 22+and 32+occurs on the time scale of days. In ethanol, none of the caroviologens showed any significant spectral change with time during 1 week. The absorption spectra are however red shifted about 20 nm, as compared to the spectra observed in the model membranes. Qualitatively, the shifts could be explained by a lower polarity in the membranes. It appears that these spectral shifts do not correlate with a presumed isomerization of Z2+ and 32+since 12+ remains stable. The dichroic ratios of both Z2+ and 32+decrease monotonously with decreasing wavelength, as can be seen from Figure 2b,c. This shows that two or more different absorbing species must be present. From the dichroic ratio and eq 1, we obtain values on the order parameter ranging between 0.4 and 0.6 for the three caroviologens solubilized in the octyl monoglyceride and the sodium octanoate/l-decanol systems (see Table I). These high S values strongly suggest that the chromophores are oriented with their long axis preferentially parallel to the C,-axis or the director of the bilayers.
(-a),
-
(-a),
The lengths of the 12+, 22+,and 32+molecules are 29, 34, and 36 A, respectively, and exceed the thinnest bilayer, which is about 20 A. Hence, in the octyl monoglyceride and sodium octanoateln-decanol systems they must simultaneously reside partly in the bilayer and water regions. Previously, the order parameter was determined for alltrans-retinal solubilized in the same kind of lipid bilayers as is used in the present work, Le., the octyl and oleoyl monoglycerides and DOPC.6 The all-trans-retinal with its polar carbonyl head group and conjugated hydrocarbon chain resembles half of a caroviologen. A comparison of the orientation of all-tramretinal and the caroviologens shows that the molecular long axis of the caroviologens has nearly twice as large an order as that of retinal. This is a reasonable finding if the caroviologene molecules are “anchored” simultaneously a t both sides of the lipid bilayer. The order parameter of 12+ in the lamellar phase of DOPC is -0.46, which is close to the theoretical minimum of S = -1/2. Hence, the long axis of nearly all 12+ molecules is oriented parallel to the bilayer surface. This could also be expected since the bilayer
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J . Phys. Chem. 1989, 93, 6154-6157
thickness exceeds the length of 12+ by a factor of 1.4. It is of interest to determine whether the 12+ molecules reside in the interior or at the lipid-water interface. A localization close to or at the lipid-water interface in lipid bilayers of DOPC is strongly supported by the order parameter of about -'I2,the poor solubility in alkanes, and the presence of cationic head groups. The order parameter of 12+, 22+, and 32+in DOPC increases as -0.46,0.1 8, and 0.39, respectively. A similar pattern is also observed in the &EO5 and oleoyl monoglyceride systems. Hence, an increase of the caroviologens' length favors an orientation perpendicular to the bilayer surface. In conclusion, the orientation of the shortest caroviologen, 12+,
varies depending on the bilayer thickness, from being preferentially perpendicular to the lipid bilayer to an almost complete orientation in the plane of the bilayer. The orientations of 22+ and 32+are preferentially perpendicular to the bilayers and appear to be less sensitive to the bilayer thickness.
Acknowledgment. This work was supported by the Swedish Natural Science Research Council. Registry No. 12+, 79296-94-9; 22+, 105256-30-2; 32+,105256-32-4; DOPC,4235-95-4; CI2EO5,3055-95-6; 1-oleoylmonoglyceride, 111-03-5; 1-octyl monoglyceride, 10438-94-5; sodium octanoate, 1984-06- 1; n-decanol, 1 12-30-1.
Thermal Decomposttion of Chemisorbed Azomethane on Pd( 111) Luke Hanley, Xingcai Guo, and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: December 27, 1988)
The adsorption and thermal decomposition of azomethane (CH3N=NCH3) on Pd( 11 1) under ultrahigh-vacuum conditions was studied by temperature-programmed desorption and work function measurements. Adsorbed azomethane is stable on the surface until N-N bond scission occurs at -250 K. Above 280 K, further decomposition forms H(a) and HCN(a) which subsequently desorb at higher temperatures. No C-N bond dissociation is observed on Pd(ll1). Thus, both N-N and C-H bond cleavages are observed for chemisorbed azomethane on Pd( 11 l), while only C-N bond cleavage is observed in the gas-phase thermal decomposition of azomethane.
Introduction The photochemistry of molecules adsorbed on single-crystal surfaces has become the focus of several recent experiments.' For a molecule to be considered in surface photochemioal studies, both its gas-phase photolysis and its surface thermochemistry and kinetics must be well characterized. Azomethane fits the first criterion, since it is known to photolyze under irradiation with UV or visible light.2 The adsorption behavior of azomethane has only been briefly studied on Pt( 1 11),3and no studies have been made of azomethane on Pd surfaces. We have therefore examined its adsorption and thermal decomposition on Pd( 111). The results presented here also serve as a preface to photochemical studies which are presently under way. Azomethane, CH3N=NCH3, was shown by dipole moment experiments4 to be entirely in the trans configuration in the gas phase. Electron diffraction studies5 verified this geometry and also measured the interatomic distances and angles. In the gas phase, azomethane thermally decomposes above -500 K to N2 and CH3 radicals; the latter then scavenge hydrogen or methyl groups from other azomethane molecules to form methane and ethane.6 One would expect azomethane to bond to a transition-metal surface through the N-N r-bond.' Adsorption of the (1) See, for example: Costello, S. A.; Roop, B.; Liu, Z.-M.; White, J. M. J . Phys. Chem. 1988, 92, 1015). Harrison, I.; Polanyi, J. C.; Young, P. A. J. Chem. Phys. 1988,89, 1475. Marsh, E. P.;Schneider, M. R.; Gilton, T. L.; Tabares, F. L.; Meier, W.; Cowin, J. P. Phys. Reo. Letf. 1988, 60, 2551. Grassian, V. H.; Pimentel, G. C. J . Chem. Phys. 1988, 88, 4484. Celii, F. G.; Whitmore, P. M.; Janda, K. C. Chem. Phys. Lett. 1987, 138, 257. Germer, T. A.; Ho, W. J. Chem. Phys. 1988.89, 562. (2) Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wiley: New York,
1967; pp 450-463. (3) Berlowitz, P.; Yang, B. L.; Butt, J. B.; Kung, H. H. Surf. Sci. 1986, 171, 69. (4) West, W.; Killingsworth, R. B. J . Chem. Phys. 1938, 6, 1. (5) Boersch, H. Monarsh. 1935, 65, 311. (6) Forst, W. J . Chem. Phys. 1966, 44, 2349, and references within. (7) Albert, M. R.; Yates, J. T., Jr. The Surface Scientist's Guide To Organometallic Chemistry; ACS: Washington, DC, 1987; p 53.
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molecule in this fashion would be expected to alter the surfacecatalyzed thermal chemistry significantly from the gaseous thermal decomposition. As will be shown below, thermal decomposition on the surface does differ fundamentally from that in the gas phase.
Experimental Details The experimental apparatus has been described in detail previously*and will only be briefly discussed here. The stainless steel Torr) was ultrahigh-vacuum system (base pressure 1 X equipped for Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), work function measurements (Ac#J, by Kelvin probe), and temperature-programmed desorption (TPD) with a differentially pumped, apertured quadrupole mass spectrometer. The Pd(l11) surface was dosed with azomethane either by a calibrated and collimated effusive molecular beam doserg (during TPD and AES) or by system doses (during Ad measurements). System doses were calibrated with a nude Bayard Alpert ionization gauge, assuming the ionization efficiency of azomethane is equal to that of N2. For TPD measurements, the crystal was resistively heated a t a rate of 2 K/s and cooled by liquid nitrogen to 87 K. The Kelvin probe was calibrated by measuring the known Ad for a saturation coverage of C O on clean Pd( 111),I0 and the error in Ad was within 0.07eV. The Pd( 111) was oriented by Laue X-ray diffraction, polished, and cleaned in vacuo by Ar+ sputtering and oxygen treatments, as previously detailed."J2 (8) Gates, S.M.; Russell, J. N., Jr.; Yates, J. T., Jr. Surf. Sci. 1984, 146, 199. (9) Bozack, M. J.; Muehthoff, L.; Russell, J. N., Jr.; Choyke, W. J.; Yates, J. T., Jr. J. Vac. Sci. Technol. 1987, AS, 1. Winkler, A.; Yates, J. T., Jr. J . Vac. Sci. Technol. 1988, A6, 2929. (10) Ertl, G.; Koch, J. 2.Naturforsh. 1970, 25A, 1906. (1 1) Grunze, M.; Ruppender, H.; Elshazly, 0. J . Vac. Sci. Techno/. 1988, A6, 1266. Musket, R. G.; McLean, W.; Colmenares, C. A.; Makowiecki, D. M.; Siekhaus, W. J. Appl. Surf Sci. 1982, 10, 143. (12) Guo, X.;Hoffman, A,; Yates, J. T., Jr. Surf. Sci. 1988, 203, L672.
0 1989 American Chemical Society
