Langmuir 1997, 13, 4877-4881
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Incorporation of a Ruthenium-Rhodium Dyad in Monolayers at the Gas/Water Interface Heinz Huesmann,*,† Daniela B. Spohn,† Maria T. Indelli,‡ Maria A. Rampi,‡ and Dietmar Mo¨bius† Max-Planck-Institut fu¨ r biophysikalische Chemie, D-37070 Go¨ ttingen, Germany, and Dipartimento di Chimica, Universita` di Ferrara, Via L. Borsari, 46, I-44100 Ferrara, Italy Received March 10, 1997. In Final Form: June 11, 1997X Organized monolayers with the water soluble metal complex dyad RhIII(dc-bipy)2-(Mebipy-CH2-CH2Mebipy)-RuII(phen)25+ have been formed at the air/water-interface by cospreading with stearic acid as lipid anchor. From the surface pressure/area isotherms it is concluded that the dyad is kept at the air/ water interface by interaction with the stearic acid. According to the reflection spectra of the mixed monolayers of the dyad cospread with stearic acid, no loss of the dyad into the aqueous subphase is observed with increasing surface pressure. A structural model with two coexisting phases is proposed, one of which consists of stearic acid with dense-packed dyad molecules underneath, the second phase being composed of stearic acid and a multilayer of dyad underneath.
1. Introduction Covalently linked donor-acceptor systems as a class of supermolecules are of particular interest for the study of vectorial photoinduced electron transfer processes in order to construct devices for the conversion and storage of solar energy.1-5 Particularly suited are metal complex dyads due to the defined photophysical properties where the energetic states can be tuned precisely by the choice of metal ion and ligands. Their use in devices of molecular dimensions requires their incorporation with unambiguous orientation into assemblies that provide an adequate environment for controlled electronic interactions. Previous studies of the Ru-Rh dyad discussed here (Figure 1) have shown that it is possible to organize the dyad in monolayers at the air/water interface by cospreading with either negatively or positively charged lipids.6 This particular characteristic is due to the four carboxylic residues on the Rh(III) subunit which provide a range of charge for this subunit from +3 to -1 depending on the degree of protonation of the carboxylic groups. In contrast, the Ru(II) subunit is permanently positively charged. Consequently, the orientation of the dyad can be controlled by the appropriate lipid anchor. The Ru(II) subunit is linked to a negatively charged lipid. In the case of a positively charged lipid like protonated eicosylamine, the Rh(III) subunit is linked to the anchor by interaction with the deprotonated carboxylic residues. 2. Materials and Methods The metal complex dyad RhIII(dc-bipy)2-(Mebipy-CH2-CH2Mebipy)-RuII(phen)25+(PF6-)5 (dc-bipy ) 4,4′-dicarboxy-2,2′bipyridine; Mebipy ) 4-methyl-2,2′-bipyridine; phen ) 4,7dimethyl-1,10-phenanthroline) was synthesized and isolated by †
Max-Planck-Institut fu¨r biophysikalische Chemie. ‡ Universita ` di Ferrara. X Abstract published in Advance ACS Abstracts, August 1, 1997. (1) Closs, G. L.; Miller, J. R. Science 1989, 240, 440. (2) Gust, D.; Moore, T. A. Top. Current Chem. 1991, 159, 103. (3) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis Horvood: New York, 1991. (4) Warman, J. M.; Smit, K. J.; Jonker, S. A.; Verhoeven, J. W.; Oevering, H.; Kroon, J.; Paddon-Row, M. N.; Oliver, A. M. Chem. Phys. 1993, 170, 359. (5) Fujihira, M. In New Developments in Construction and Functions of Organic Thin Films; Kajiyama, T., Aizawa, M., Eds.; Elsevier: Amsterdam, 1996; Vol. 4, p 181. (6) Huesmann, H.; Bignozzi, C. A.; Indelli, M. T.; Pavanin, L.; Rampi, M. A.; Mo¨bius, D. Thin Solid Films 1996, 284-285, 62.
S0743-7463(97)00257-6 CCC: $14.00
Figure 1. Chemical structure of the dyad RhIII(dc-bipy)2(Mebipy-CH2-CH2-Mebipy)-RuII(phen)25+. Indelli et al.7 and used without further purification. Stearic acid (C18) was purchased from Sigma Chemicals. The dyad and lipid were dissolved in a mixture of methanol and chloroform (1:3, v/v). The method of cospreading8 has been used to form organized monolayers of metal complex dyads at the air/water interface.6,9 Water from a Milli-Q system, Millipore Corp., was used as subphase for a circular trough (light reflection) and rectangular (7) Indelli, M. T.; Bignozzi, C. A.; Harriman, A.; Schoonover, R. J.; Scandola, F. J. Am. Chem. Soc. 1994, 116, 3768. (8) Hada, H.; Hanawa, R.; Haraguchi, A.; Yonezawa, Y. J. Phys. Chem. 1985, 89, 560. (9) Rampi, M. A.; Bignozzi, C. A.; Caruso, P. L.; Mo¨bius, D.; Scandola, F. In Modular chemistry; Michl, J., Ed.; NATO ASI series; in press.
© 1997 American Chemical Society
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Figure 3. Reflection spectra of a mixed monolayer of dyad/ stearic acid (fD ) 0.25) formed by cospreading on water at 20 °C at different surface pressures. Inset top right: absorption spectrum of the dyad dissolved in CH3OH/CH3CN (1:3) at room temperature. Figure 2. Surface pressure π/area per lipid molecule Am,L isotherms (full lines) and surface potential ∆V/area per lipid molecule Am,L isotherms (dotted lines) of mixed dyad:stearic acid monolayers of different molar fractions fD of the dyad on pure water at 20°C formed by cospreading (a, 0.00; b, 0.11; c, 0.16; d, 0.25; e, 0.33; f, 0.5). troughs (isotherms, Brewster angle microscopy) made from PTFE furnished with poly(ethylene oxide) barriers. The surface pressure π was measured by a filter paper Wilhelmy balance and the surface potential ∆V with a vibrating plate condenser.10,11 Resonant light reflection spectra at constant surface pressure were measured under normal incidence according to Gru¨niger et al.12 Brewster angle microscopy13 (BAM) was performed with a BAM2, NanoFilm Technology GmbH, using the 514 nm line of an Ar/Kr laser (Coherent INNOVA70 Spectrum) as light source.
3. Results 3.1. Surface Pressure/Area and Surface Potential/ Area Isotherms. The surface pressure/area (π/A) isotherms of the monolayers of dyad/stearic acid measured for different molar fractions of the dyad, fD (see Figure 2), show a significant expansion of the area per lipid molecule, Am,L, at low surface pressures indicating a strong interaction of the dyad with the stearic acid due to its net positive charge of 5+ (if the four carboxylic groups are protonated). For molar fractions of the dyad, fD g 0.25, a phase transition can be observed in a range of surface pressure from 5 to 10 mN/m. Upon increase of the surface pressure the isotherms merge with that of the pure matrix at about 45 mN/m. The surface potential/area isotherms of the mixed dyad/stearic acid monolayers (see Figure 2, dotted lines) show a rather abrupt increase upon compression at areas larger than those of surface pressure onset. The surface potential values go through a maximum and decrease upon further compression. The maximum is observed at the area of surface pressure onset. It is notable that the pure dyad, i.e., in the absence of stearic acid, does not form a stable monolayer. 3.2. Reflection Spectra. The reflection spectra of the complex monolayer of dyad cospread with stearic acid (dyad/stearic acid 1:3) measured at different surface pressures show that the dyad is present at the air/water (10) Kinloch, C. D.; McMullen, A. I. Rev. Sci. Instrum. 1959, 36, 347. (11) Kuhn, H.; Mo¨bius, D.; Bu¨cher, H. In Physical Methods of Chemistry; Weissberger, A., Rossiter, B., Eds.; John Wiley & Sons: New York, 1972; Vol. 1 Part 3B; p 656. (12) Gru¨niger, H.; Mo¨bius, D.; Meyer, H. J. Chem. Phys. 1983, 79, 3701. (13) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590.
interface (see Figure 3). By comparison with the spectrum in solution (see inset Figure 3) the different electronic transitions of the dyad in the complex monolayer can be assigned.7 The broad band at about λ ) 450 nm represents the metal-to-ligand charge-transfer transition (MLCT) of the Ru(II) subunit. At higher energy, the ligand-centered (LC) transitions of the Rh(III) subunit (302, 313 nm) and Ru(II) subunit (280, 295 nm) occur as small peaks in the shoulder of the strong phenanthroline band with maximum at about 270 nm. No saturation of the reflection spectrum is observed with increasing surface pressure indicating that the surface density of the dyad increases with compression of the monolayer. The merging of the isotherms of the monolayers of dyad cospread with stearic acid at high surface pressure is not due to a loss of dyad into the subphase. 3.3. Monolayer Morphology. Figure 4 shows Brewster angle microscopy images of a complex monolayer consisting of the dyad cospread with stearic acid, molar fraction of the dyad fD ) 0.25, at a surface pressure of π ) 0 mN/m (a), π ) 4 mN/m (b), and π ) 16.5 mN/m (c) on pure water. The monolayer shows at π ) 0 mN/m (before reaching the surface pressure onset upon compression) the coexistence of three phases as characterized by different brightness. The darkest area (phase I) is assigned to stearic acid in a gas-analogue phase while the phase II (gray) appears with bright spots (phase III). It is notable that phase III is only present in the phase II region. The contrast arises from different monolayer thickness and/or refractive index of the phases. With increasing surface pressure (π ) 4 mN/m, Figure 4b) phase I disappears and the number of phase III domains increases. These bright domains become dense-packed (π ) 16.5 mN/m, Figure 4c) and clearly exhibit an optical anisotropy (see magnification in top left corner of Figure 4c) similar to observations with liquid crystalline phases of dense-packed hydrocarbon chains.14 From this optical anisotropy it can be concluded that the molecules forming this phase are packed in a kind of long-range order. The limited lateral resolution of the instrument precludes a more detailed analysis of the inner structure of these round domains. At a surface pressure of π ) 16.5 mN/m (cf. Figure 4c) the domains appear densely packed but distinct with lateral dimensions in the micrometer (14) Overbeck, G. A.; Ho¨nig, D.; Mo¨bius, D. Thin Solid Films 1994, 242, 213.
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Figure 5. Area per lipid molecule Am,L (left, open squares) and the product of Am,L∆V (right full diamonds) at surface pressure onset versus mixing ratio r, data taken from Figure 2. The dotted line shows the linear fit according to eq 3, the dashed line according to eq 6. For further details, see text.
coexisting with free stearic acid molecules. The area of the monolayer at surface pressure onset πon (when |∂π/∂A| > 0 upon compression) characterizes the situation when the molecules come into contact. The mean area of the monolayer at πon is given by the sum of the area of its components
A(πon) ) NL,freeAL + NDALD
(1)
with NL,free being the number of free lipid molecules and AL as the area per free lipid molecule. Further, we consider that all dyad molecules form the supramolecular cluster and, therefore, that NLD is equal to ND. Here, we disregard the fact that small round domains are detectable already before the surface pressure onset (see Figure 4a) which are tentatively assigned to a phase of clusters LD of dyad and stearic acid with an organization different from that of the surrounding monolayer with smaller brightness. Consequently, the area per spread lipid molecule Am,L(πon) is
Am,L(πon) ) Figure 4. BAM images of a mixed monolayer consisting of dyad cospread with stearic acid (fD ) 0.25) on pure water: (a) π ) 0 mN/m; (b) π ) 4 mN/m; (c) π ) 16.5 mN/m.
range. The size of the domains remains approximately constant during compression; however, their density increases. 4. Discussion The dyad is anchored to the air/water interface in the complex monolayer by interaction with the stearic acid. In order to understand the observed expansion of the monolayer with respect to the pure stearic acid, we assume the formation of a phase with supramolecular clusters (LD) consisting of one dyad and nL molecules of stearic acid sitting on top of the positively charged Ru(II) subunit
NL,freeAL NDALD + NL NL
(2)
with NL being the number of spread lipid molecules. The ratio ND/NL is equal to the mixing ratio r of the components, and according to the assumption that no free dyad is present in the complex monolayer, eq 2 becomes
Am,L(πon) ) AL + r(ALD - nLAL)
(3)
The areas Am,L(πon) taken from Figure 2 are plotted in Figure 5 (open squares) against the ratio r of dyad:stearic acid in the spreading solution. With exception of the value for the ratio r ) 1, Am,L(πon) follows a linear relationship with the ratio, and a slope of the straight line of 0.96 nm2 is obtained ignoring the value for r ) 1 for a linear regression (correlation coefficient ) 0.993). According to eq 3, we calculate nL ) 2.25 considering the area per
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supramolecular cluster, ALD, to be equal to the crosssectional area per dyad for a nearly vertical orientation in the supramolecular cluster LD (ALD ) AD ) 1.5 nm2) as given by the three-dimensional model of the dyad. Alternatively, a number of nL ) 2 could be taken as reasonable since the Ru(II) subunit carries two positive charges, and then the area AD ) 1.4 nm2 is obtained from the slope. The molecules of stearic acid sitting atop of the dyad do not form an ordered liquid-crystalline phase due to the large area available so that the lipid molecules cannot come close enough for effective van der Waals interaction. For r ) 1, the area per stearic acid at surface pressure onset Am,L(πon) deviates from the observed linearity indicating that the amount of stearic acid is insufficient to anchor all dyad molecules at the air/water interface. Therefore, the dyad seems to be partially lost into the subphase in this case. It should be possible to rationalize the surface potential ∆V at surface pressure onset within the frame of this model. Then the measured surface potential is the weighted average of the surface potentials of the two phases, i.e., the free lipid, ∆VL, and the supramolecular cluster, ∆VLD, according to
∆V ) aL∆VL + aLD∆VLD
(4)
where aL and aLD are the area fractions of the two phases. According to eq 3
∆V )
AL r ∆VL + (A - nLAL)∆VLD Am,L Am,L LD
Am,L∆V ) AL∆VL + r(ALD - nLAL)∆VLD
(5) (6)
The values of the product Am,L‚∆V plotted in Figure 5 (full diamonds) against r follow a linear relationship except the value for r ) 1 as in the plot for the area at surface pressure onset, AmL(πon). This confirms the simple model describing the monolayer organization of different dyad/ stearic acid mixtures at surface pressure onset. The dotted line represents a linear regression with a correlation coefficient of 0.990. From the slope, the potential ∆VLD ) 0.550 V is obtained with ALD ) 1.5 nm2, AL ) 0.24 nm2, and nL ) 2.25. The fact that ∆VLD is much more positive than ∆VL ) 0.240 V (calculated from the ordinate value and AL ) 0.24 nm2) confirms an orientation of the dyad with the Ru(II) subunit pointing up and the Rh(III) subunit down. However, a quantitative interpretation of ∆VLD is impossible since too many parameters are involved. Upon further compression of the complex monolayers, the dyad molecules can no longer be accommodated at the air/water interface directly below the lipid anchor. The excess dyad is either lost into the aqueous subphase or may be forming a second layer underneath the dyad monolayer that is directly in contact with the anchor. In the following we concentrate on the complex monolayer of dyad cospread with stearic acid, molar ratio in the spreading solution 1:3 (fD ) 0.25; corresponding to curve d in Figure 2). The resonant light reflection follows the absorption spectrum and is proportional to the surface density under certain conditions.12 The reflection ∆R at normal incidence at λ ) 450 nm can be used to calculate the surface density of the dyad. At λ ) 450 nm only the Ru(II) subunit causes resonant light reflection due to the MLCT transition. The surface density, σdyad, of the dyad can be expressed as12
σ∆R )
∆R constant forientdyadRi1/2
(7)
Figure 6. Surface densities σ of the dyad in a mixed monolayer dyad:stearic acid ) 1:3, calculated from surface pressure/area isotherms (full symbols) and reflection ∆R at 450 nm (open symbols), respectively, for different surface pressures π.
where forient is a factor taking the particular orientation of the transition moment at the interface into account, σ∆R is the surface density of the dyad, and dyad(450 nm) ) 17670 L mol-1 cm-1 the extinction coefficient in solution. Ri is the reflectivity of the clean air/water interface and can be calculated from the refractive indices of water and air. The transition moment of the MLCT in the Ru(II) subunit is spatially symmetrically distributed, and therefore, the orientation factor is equal to unity (forient(450 nm) ) 1) for all orientations of the dyad (constant ) 3.82 × 10-7 nm2 mol cm/L-1). In Figure 6 the surface density resulting from the light reflection of the dyad at λ ) 450 nm and the surface density calculated as the reciprocal area per dyad from the π/A isotherms, Am,D ) rAm,L
σπ/A ) 1/Am,D
(8)
are plotted against the surface pressure. The surface density σ∆R calculated from the light reflection (full circles) is in reasonable agreement with the surface density of the dyad calculated from the isotherms (open squares). The difference between the two data sets could be partially due to some relaxation during the spectral measurements causing a decrease of the initial surface pressures. Therefore, no dyad is lost in the aqueous subphase during compression, and despite its solubility in water the dyad remains at the interface. This poses the problem of accommodating the dyad in the monolayer with fD ) 0.25. The following model is suggested: The two phases coexist at a surface pressure of π g 4 mN/m corresponding to the BAM images in parts b and c of Figure 4: (1) a monolayer of dense-packed dyad (MD) underneath a layer of stearic acid and (2) a phase consisting of the dyad forming a multilayer (XD) underneath a layer of stearic acid molecules in order to accommodate all dyad molecules at the interface. The term monolayer is still used here for the complex film although, strictly speaking, lipid and dyad molecules, respectively, are arranged in different planes. The BAM images of Figure 4b and Figure 4c clearly show that at a surface pressure higher than 4 mN/m two
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phases coexist. The optical anisotropy in the round domains might be explained either by long range order of the lipid or by a particular packing of the dyad molecules underneath lipid. The mean area per lipid molecule of the monolayer consisting of dyad and stearic acid is too large to assign the anisotropy of the domains only to the lipid. The dyad layer underneath the matrix seems to be densely packed and might cause the anisotropy. A direct assignment of the MD and XD phase to the domains and interdomain area, respectively, on the basis of the results presented here is not possible. However, it seems reasonable to assume that a multilayer of dyad is less ordered than a densely-packed layer, and therefore the MD phase consisting of a densely packed single layer of dyad with long range order underneath the lipid layer is tentatively assigned to the domains. Additional methods of investigation are required to arrive at an unambiguous assignment.
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5. Conclusion The Ru-Rh dyad is incorporated in mixed monolayers with stearic acid by cospreading. No loss of the dyad into the aqueous subphase upon compression of the monolayer is observed. The complex monolayers with different molar ratios of dyad/stearic acid can be interpreted at surface pressure onset by coexistence of free stearic acid and a supramolecular cluster consisting of dyad and of lipid (1: 2). Upon further compression, the dyad may be accommodated in the complex monolayer by formation of a phase with a multilayer of dyad underneath the lipid monolayer. Acknowledgment. This work was supported by the European network “Amphiphile monolayers at fluid interfaces and on solid support” (Contract No. ERBCHBGCT930477). LA970257J
