7494
J. Phys. Chem. B 1997, 101, 7494-7504
Two-Dimensional Emission Quenching and Charge Separation Using a Ru(II)-Photosensitizer Assembled with Membrane-Bound Acceptors Leif Hammarstro1 m,*,† Thomas Norrby,‡ Gunnar Stenhagen,§ Jerker Ma˚ rtensson,§ Bjo1 rn A° kermark,‡ and Mats Almgren† Department of Physical Chemistry, Uppsala UniVersity, Box 532, S-751 21 Uppsala, Sweden; Department of Organic Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden; and Department of Organic Chemistry, Chalmers UniVersity of Technology, S-412 96 Go¨ teborg, Sweden ReceiVed: March 27, 1997; In Final Form: June 30, 1997X
Novel syntheses of the bipyridine ligand 1, dcHb (dcHb ) 4,4′-dicarboxy-2,2′-bipyridine), by anionic oxidation of 4,4′-dimethyl-2′,2-bipyridine (dmb) using molecular oxygen (4 atm), and of the sensitizer precursor 4, tris(4,4′-diethoxycarbonyl-2,2′-bipyridine)ruthenium(II) bis(triflate), from a chloride-free Ru(II) precursor 3b, RuII(DMSO)4(triflate)2-n(EtOH)n (n ) 0-2, DMSO ) dimethyl sulfoxide, triflate ) OSO2CF3) are reported. The anionic sensitizer Ru(dcb)34- (5) was shown to bind to vesicles of lecithin when these were made positively charged by cationic bipyridinium electron acceptors. With cetylmethylviologen (CMV2+) as quencher, the time-resolved decay of the Ru(dcb)34- emission followed a model for diffusion-controlled quenching in two dimensions. However, the diffusion coefficient obtained from a fit to the data was very small, (6 ( 2) × 10-11 m2 s-1, comparable to values for amphiphiles in bilayers, even though Ru(dcb)34- diffuses in the water region at the vesicle surface. The quantum yield of primary charge separation was 0.06 ( 0.02, which is significant, if not high, despite the large elecrostatic attraction between the reactants. Attempts were made to increase the charge separation yield by the use of a monocationic acceptor. Possible extensions of the system are discussed, such as charge separation across the vesicle membrane and the covalent linking of a donor to the sensitizer.
Introduction In attempts to achieve efficient photochemical charge separation, as well as to understand fundamental properties of photoinduced electron transfer, several supermolecules have been synthesized, having photosensitizer molecules covalently linked to electron acceptors and/or donors.1 Recently, there has also been an increased interest in systems where the reactants are assembled by weaker forces, such as electrostatic attraction or hydrogen bonds.2 We are currently constructing Ru(II)-polypyridine sensitizers covalently linked to a series of electron donors,3 aiming eventually at a catalytic donor function. In the present study, we have investigated the possibility of assembling anionic sensitizer-donor molecules of this kind, and cationic bipyridinium electron acceptors that are hydrophobically bound to the surface of lipid vesicles, by using the electrostatic attraction between the reactants (Figure 1).4 Some favorable properties were expected to follow from this organization of the reactants, which is why we wanted to examine this system in some detail. First, the electrostatic attraction to the bipyridinium-containing vesicles may prove to be a versatile and synthetically simple way of adding an acceptor functionality to such sensitizer-donor supermolecules. Second, one may use acceptors with low opposite charge, since the sensitizer would be attracted by the collective electric field from several acceptors at the vesicle surface. Thus, one may reduce the pairwise attraction between the reactants, which lowers the cage-escape efficiency after photoinduced charge separation in bimolecular reactions,5 and still assemble the reactants efficiently. Third, with the sensitizer diffusing in the water region around the vesicle, the quenching was expected to be more efficient than when both the Ru complex and the acceptors are amphiphilic. In the latter case, quenching was found to be rather inefficient,6 presumably due X
Abstract published in AdVance ACS Abstracts, August 15, 1997.
S1089-5647(97)01080-8 CCC: $14.00
Figure 1. Schematic structure of the system investigated, where the hydrophilic Ru(dcb)34- is attracted to the surface of the vesicles containing the cationic acceptor cetylmethylviologen (CMV2+).
to the slow diffusion of amphiphilic molecules in lipid bilayers.7 Finally, the electron transfer from the donors to the photooxidized Ru is expected to be slow in the donor-sensitizer molecules mentioned above, and subsequent reactions, e.g., oxidation of water, will be inhibited if the reductive equivalent and the donor are not kept apart. Therefore, the vesicle membranes may be © 1997 American Chemical Society
Emission Quenching and Charge Separation of use not only to assemble the reactants but also to further stabilize the charge separation products by transmembrane electron transfer. We have previously studied the transmembrane transfer of electrons from external electron donors to secondary acceptors in the vesicle interior, using the amphiphilic bipyridinium acceptors cetylmethylviologen (N-hexadecyl-N′-methyl-4,4′bipyridinium, CMV2+) and cetylbipyridine (N-hexadecyl-4,4′bipyridinium, CB+) as mediators.8 This reaction was in both cases shown to proceed via the transmembrane diffusion of the uncharged, reduced forms of the mediators, i.e., CMV0 and CB0. While CB0 was formed by direct one-electron reduction of CB+ by the external reductants, CMV0 was formed in an indirect way, via a disproportionation equilibrium: 2V+ S V2+ + V0. The uncharged forms of the mediators diffused rapidly through the vesicle membrane (τ ) 0.7 ms for CB0;8b a similar value was inferred for CMV0) and reacted with the secondary acceptors. In the present system, reduced acceptors formed by photoinduced electron transfer from the Ru sensitizer could possibly be withdrawn from the external interface of the vesicle to the interior, where their electrochemical energy might be utilized in secondary reactions. The oxidized Ru complex left at the interface may be reduced again by an electron donor, and thus go through cycles of excitation and electron transfer to acceptor molecules at the vesicle surface. The donor-sensitizeracceptor-membrane assembly we aim at would in this way exhibit some analogies with a natural photosynthetic reaction center,9 featuring a cyclic function of the sensitizer, transmembrane electron transfer, and a membrane-bound acceptor pool, although there are obviously important differences as well. In the present study, we focus on the quenching behavior of the anionic Ru complexes and vesicle-bound bipyridinium acceptors and on the organization of the reactants, as well as on the efficiency of the primary charge separation. Therefore, the donor could be excluded, and the simple anionic sensitizer ruthenium tris(4,4′-dicarboxylato-2,2′-bipyridine) (Ru(dcb)34-) was used as a model of the donor-sensitizer molecules. Some possible extensions and limitations are discussed. Results and Discussion Syntheses. The dicarboxylic acid ligand 1, 4,4′-dicarboxy2,2′-bipyridine (dcHb), has been prepared earlier from 4,4′dimethyl-2,2′-bipyridine (dmb) by oxidation using potassium permanganate10 or by chromic acid11 which require laborious isolation and purification methods. We here report a novel, speedy, and transition-metal-free procedure for the synthesis of 1 (Scheme 1), adopted from an earlier method reported for the preparation of monocarboxylic acids from picolines,12 by anionic oxidation of dmb with molecular oxygen (O2) at 4 atm pressure. The oxygen atmosphere is obtained in a standard Fisher-Porter 500 mL reaction flask tested to 15 atm. CAUTION: Always observe proper protective measures (e.g., eye protection, shield) when working with oxygen under pressure. The synthesis reported here of the octahedral ruthenium(II) complex (point group symmetry13 D3) 4, tris(4,4′-diethoxycarbonyl-2,2′-bipyridine)ruthenium(II) bis(triflate) is by a novel method based on the ruthenium(II) precursor 3a, RuCl2(DMSO)4, readily available from RuCl3‚3H2O and dimethyl sulfoxide (DMSO). In our hands the procedure by Wilkinson et al.14 most likely yielded a mixture of trans- (minor) and cis-dichloro (major) species. Alessio et al.15 more recently have reinvestigated the RuCl2(DMSO)4 system; and this precursor in the synthesis of ruthenium(II) complexes has found recent use, e.g., by Bossmann et al.16 The complexation reaction with the rather
J. Phys. Chem. B, Vol. 101, No. 38, 1997 7495 SCHEME 1
electron-poor ligand 2, 4,4′-diethoxycarbonyl-2,2′-bipyridine (diester) was facilitated by exchange of the rather strongly bound chloride ligands for ethanol or triflate ions. Precipitation of the chloride ions as AgCl(s) was achieved by refluxing of the RuCl2(DMSO)4 species 3a with 2.05 equiv of silver triflate in ethanol (12-15 h), yielding 3b, RuII(DMSO)4(triflate)2-n(EtOH)n (n ) 0-2). Slow addition of the chloride-depleted 3b to a refluxing ethanolic solution of 2 yielded 4. The formation of the product was monitored by the appearance of the deep red color of 4, and by thin-layer chromatography (TLC). It is very important to fully remove all chloride ions, since they tend to form byproducts, e.g., the blue 7, trans-dichloroRuII(diester)2 (point group symmetry D4h); green 8, cis-dichloroRuII(diester)2 (point group symmetry C2) (Figures 2 and 3) which are very stable kinetically; extended refluxing (5-7 days) of a reaction solution containing 7 and 8 does not lead to conversion into 4 to any useful extent. Complexes displaying trans-bipyridineruthenium coordination geometry are very rarely encountered, e.g. Meyer et al.17 A small amount of a pink species 9 (not shown) containing 12 protons which are inequivalent in NMR (point group symmetry C1) was sometimes also formed. By NMR and UV-vis (Figure 3) we have indications that this is yet another Ru(II) species complex composed of two molecules of diester 2 and two different anionic ligands. Finally, a lateeluting dark purple species 10 was observed after chromatography, which we ascribe to species 9 where one or several of the ester groups have been hydrolyzed (UV-vis and NMR spectra in Supporting Information). Basic Characteristics of Ru(dcb)84-. The absorption spectrum of Ru(dcb)34- in water shows a strong band in the visible region, with a maximum at 466 nm, characteristic of the lowest metal-to-ligand charge-transfer (MLCT) transition in polypyridine complexes of Ru(II).18 The emission from the
7496 J. Phys. Chem. B, Vol. 101, No. 38, 1997
Hammarstro¨m et al. attained much faster than the rate of decay of the excited species.21 In the present study, however, one may conclude that only the fully dissociated species has to be considered, since the experiments were in all but a few cases performed at pH ) 11. Emission Quenching by MV2+. As for a large number of Ru(II)-polypyridine complexes, the excited state of Ru(dcb)34was quenched by methylviologen (MV2+).5b,c,18b,22 The reduction of the emission intensity under steady-state illumination is shown in Figure 4. For a purely dynamic quenching process, the emission intensity is given by the well-known Stern-Volmer equation:23
I0/I ) 1 + KSV[Q]
(1)
where I0 and I are the emission intensities in the absence and presence of quencher, respectively; KSV ) τ0kq, where τ0 is the unquenched lifetime, kq is the second-order rate constant for the quenching reaction, and [Q] is the concentration of quencher. In the present system, the charged ground-state reactants form ion pairs to some extent. If the emission intensity from excited Ru(dcb)34- within ion pairs is negligible due to rapid static quenching, eq 1 may be rewritten to include a combination of dynamic and static quenching:24 Figure 2. trans- (7) and cis- (8) dichlororuthenium(II)(4,4′-diethoxycarbonyl-2,2′-bipyridine)2 byproducts which have been separated and identified. H-6 and H-6′ (H-5′, H-3′, -R′) denotes the two magnetically different pyridine moieties of each 4,4′-diethoxycarbonyl-2,2′-bipyridine ligand (pyridine or pyridine′), with regard to the assignment of the 1H NMR spectra in the experimental part.
lowest MLCT state is centered around 645 nm (corrected) and was found to decay with a lifetime of 690 ns in N2-purged water at 293 K.19 The dissociation behavior of the corresponding acid form Ru(dHcb)32+ has been studied by Park and co-workers.20 They reported the observation of two reversible deprotonation steps: first two protons dissociate independently, forming a neutral complex, and then, at higher pH the remaining four protons dissociate independently of each other, giving the Ru(dcb)34species. According to their data, the pH values where 50% of the protons in each of the two steps had dissociated were pH50% ) 0.7 and 3.7. The corresponding values for the excited state were pH*50% ) 1.9 and 4.1. These values were calculated with the implicit assumption that the dissociation equilibrium was
I0/I ) (1 + KSV[Q])(1 + KA[Q])
(2a)
[(I0/I) - 1]/[Q] ) KSV + KA + KSVKA[Q]
(2b)
where KA is the ion-pair formation constant. Equation 2a predicts an upward curvature in the I0/I vs [Q] plot, due to static quenching, and this is also seen in Figure 4. The inset shows the data plotted according to the linearized eq 2b, and from the values of the slope and intercept of the fitted line, one obtains the two solutions KSV ) 1.23 × 104 M-1 and KA ) 9 × 102 M-1, or vice versa. Independent measurements of the emission lifetime vs [Q] resulted in single-exponential decay curves, since all emission emanated from the Ru(dcb)34- that were not statically quenched. The lifetime data was plotted according to
τ0/τ ) 1 + KSV[Q]
(3)
(not shown) and gave a value KSV ) 1.22 × 104 M-1, corresponding to kq ) 2.57 × 1010 M-1 s-1 (τ0 ) 475 ns in air-saturated water).
Figure 3. UV-visible spectra in CH2Cl2 of 4, [RuII(diester)3]2+(OTf)2; 7, [trans-Cl2RuII(diester)2]; 8, [cis-Cl2RuII(diester)2]; 9, RuII(diester)2(Cl)(X) (a pink species, not completely identified). The spectra have been normalized for comparison, and the ligand-centered π-π* absorption below 300 nm is shown smaller than the actual amplitude.
Emission Quenching and Charge Separation
J. Phys. Chem. B, Vol. 101, No. 38, 1997 7497 TABLE 1: Values for the Diffusion Coefficient Obtained from a Fit to Eq 4 for Different Vesicle Preparationsa [Q] × 10-16 b (m-2) 1.1 2.8 3.8 4.2 5.5 6.0 6.7 8.1 10.0 11.6
D (m2 s-1)c preparation no. I II III IV 7.6 5.3 6.3 6.6 5.0 4.4 4.5 7.0 6.3 5.6
σtotd (mC m-2)
σqe (mC m-2)
38 48 12 49 60 47 21 47 32 49
3.5 9 12 13 18 19 21 26 32 37
a
Figure 4. Quenching of the emission intensity from Ru(dcb)34- by MV2+, determined by steady-state fluorimetry. The data are plotted according to eq 2a, and the line is a least-squares linear fit to the data for low concentrations. Inset: The same data plotted according to eq 2b, and the line is a least-squares linear fit to the data. Conditions: air-saturated water at 20 °C. [Ru(dcb)34-] ) 7 µM, pH ) 11 ([NaOH] ) 1 mM).
Figure 5. Quenching of the emission intensity from Ru(dcb)34- by CMV2+ in lecithin vesicles, determined by steady-state fluorimetry. Conditions as in Figure 4, with [lecithin] ) 3.0 mM. In this figure, [CMV2+] ) 140 µM corresponds to [Q] ) 1.0 × 1017 m-2.
To conclude, these experiments show that the quenching of excited Ru(dcb)34- by MV2+ is diffusion controlled (kq ) 2.6 × 1010 M-1 s-1), and that ion pairs are formed between the reactants in the ground state, with a formation constant KA ) 9 × 102 M-1 (in water at 293 K, ionic strength ) 2 × 10-3 M). This information is needed for further discussion (see below). Quenching by CMV2+ in Vesicles. The amphiphilic viologen N-cetyl-N′-methylviologen (CMV2+) binds completely to vesicles of egg lecithin,8a with the viologen headgroup exposed to the water phase and the cetyl chain in the hydrocarbon region of the vesicle bilayer. Upon addition of CMV2+ to a solution of Ru(dcb)34- and uncharged (zwitterionic) vesicles of egg lecithin, the emission from the Ru-complex was considerably quenched, but the ratio I0/I did not follow a simple dependence on [Q] (Figure 5). One may have expected that Ru(dcb)34would form ion pairs with one (or two) CMV2+ molecules at the vesicle surface. However, with a static quenching within such ion-pairs, this would lead to a linear (or quadratic) dependence of I0/I on [Q]. The data in Figure 5 instead suggest a cooperative effect, where the attraction of Ru(dcb)34- to the vesicle was determined, e.g., by the surface charge density of the vesicles. Numerical calculations using the Poisson-Boltzmann equation25 suggest that the binding of the tetravalent Ru(dcb)34- to
Note that the average value of D varies somewhat with vesicle preparation, but there is no detectable covariation of the D and [Q] values. b Surface concentration of the quencher CMV2+ (molecules/ m2). c Lateral diffusion coefficient obtained from a fit to eq 4. d Surface charge density of the vesicles. e Surface charge density contribution from the quencher only.
the charged vesicles is strong, even at relatively low surface concentrations of viologen. Although the Poisson-Boltzmann equation may not be very accurate for multivalent ions, the fraction of Ru(dcb)34- that is further away than a molecular diameter (≈1 nm) from the vesicle surface is expected to be very small. Therefore, the observed reduction of the emission intensity as the concentration of viologen increased should be due to an increasingly efficient quenching of Ru complexes already bound to the vesicle surface and does not reflect an increasing fraction of bound complex. Time-resolved emission studies were performed in order to investigate the localization of Ru(dcb)34- and the quenching behavior. The concentration of CMV2+ in the outer monolayer of the vesicle membrane was varied from 0 to 10 mol %. The surface charge density σtot was in most cases kept around 40 mC/m2 or above (see Table 1) by addition of N-cetylpyridinium (CPy+). This corresponds to at least one unit charge per six lipids and was done in order to ensure a high degree of Ru(dcb)34- binding. Separate experiments showed that CPy+ did not quench the Ru(dcb)34- emission. Some representative decay curves are shown in Figure 6. It is clear that the emission decay cannot be described by a simple mono- or biexponential function, as would have been expected for a system where Ru(dcb)34- formed simple ion pairs. Not even a fit to three exponentials gave satisfactory results. Instead, the curves were fitted to a kinetic model for diffusion controlled quenching in two dimensions:
I(t)/I0(t) ) (1 - R)exp{-(t/τ0) - 7.44[Q]r(tD)1/2 2.28[Q]Dt} + R exp{-t/τfree} (4) where τ0 is the unquenched emission lifetime, [Q] the surface concentration of quencher (molecules per unit area), r the interaction distance, and D the lateral diffusion coefficient. The second term was included to describe the emission decay of those Ru(dcb)34- that were not bound to the vesicles, with lifetime τfree, and R is a factor (0-1) weighing the contributions from the two exponential functions. This equation (apart from the second exponential function) was derived by Owen,26 and the numerical factors were modified according to Caruso et al.27 The fitted curves are also shown in Figure 6, and the quality of the fit was good, as judged from the χ2 values and the residual plots. Separate experiments on a longer time scale than in Figure 3 established that the value τfree ) 590 ns was independent of [Q], and τfree was therefore constrained to this
7498 J. Phys. Chem. B, Vol. 101, No. 38, 1997
Figure 6. Representative results from time-resolved single-photoncounting experiments on the emission quenching of Ru(dcb)34- by CMV2+ in lecithin vesicles. The solid lines are fits to eq 4, and a representative residual is shown (inset). The dotted lines indicate the parts of the decay curves represented by the second exponential term in eq 4, i.e., a single-exponential decay with lifetime ) τfree. Thus, the difference between the solid and dotted lines are the contribution from the two-dimensional quenching at the vesicle surface. In the figure, curves with different values of R were chosen so as to obtain a clear presentation with a good separation between the curves. Note though that there was no clear dependence of R on [Q] for the entire set of data in Table 1. Conditions: N2-purged water at 20 °C, pH ) 11 ([NaOH] ) 1 mM), [Ru(dcb)34-] ) 25-30 µM, [lecithin] ) 6.5-7.0 mM; (from top) [Q] ) 2.8 × 1016 m-2, 4.2 × 1016 m-2, and 11.6 × 1016 m-2. The surface charge density (σ) was kept approximately constant by addition of CPy+ (see Table 1).
value in the fitting procedure. The reduction of τfree compared to τ0 ()690 ns) is probably due to the encounter between free Ru(dcb)34- and vesicles. In the fitting process, the values of [Q] and r )1.1 nm28 were also kept constant. A 10% change in either of these parameters resulted in only a 10% difference in the value of D obtained. The quenching by MV2+ was clearly diffusion controlled (see above), and it is likely that this is the case also for the vesicle-bound CMV+, as required by the model behind eq 4. The values of the diffusion coefficient D obtained from the fits are given in Table 1. The independence of the D values on [Q] gives important support for the correctness of the physical model chosen for the quenching process. For example, the results of Medhage and Almgren29 show that an attempt to fit the data from a three-dimensional diffusion-controlled quenching reaction to a two-dimensional model results in too high values of D that also vary with the concentration of quencher, in contrast to the present results. The D value was also independent of σtot, as expected from the model. It appears therefore that the major fraction of the Ru(dcb)34- molecules are bound as counterions at the vesicle surface and that this fraction is quenched by two-dimensional, diffusional encounter with CMV2+. The average value of D varied between the different vesicle preparations, and this type of variation is not uncommon in quantitative kinetic studies in vesicular systems.8 The origin of these variations could be slight variations in the vesicle characteristics between the preparations, e.g., differences in size (and hence in radius of bilayer curvature) or other properties that affect the lipid packing in the vesicle bilayer. The data for the first 15 ns after the excitation showed a decay that was more rapid than predicted by eq 4, and these data were excluded from the fits. We ascribe this to quenching of excited Ru(dcb)34- that were within, or close to, reaction distance with a quencher molecule already at the moment of excitation. The model behind eq 4 does not include this contribution but
Hammarstro¨m et al. assumes an infinite rate of reaction at distances equal to the reaction distance.26 Nevertheless, the quenching process is expected to be well within the diffusion-controlled limit, since the diffusion coefficients were much smaller than those for free molecules in water. To determine if there was any quenching on a time scale too short to be resolved by the single-photon-counting equipment (static quenching), comparison was made between the intensity values obtained in Figure 5 with those obtained by integrating the time-resolved curves in Figure 6. For the highest value of [Q] ()11.6 m-2) used in the latter experiments, I/I0 ) 0.07, while in the steady-state measurements I/I0 ) 0.03 at the same [Q] (the steady-state value was corrected for the different τ0 with and without oxygen in this comparison). This would indicate a small fraction of static quenching. However, also a very small increase in R in the time-resolved measurements, due to the irreversible photoreaction (see below), would account for the difference between the values from steady-state and timeresolved experiments. Thus, even though we cannot completely exclude the presence of static quenching its contribution must be smaller than 5% of I0 even at the highest quencher concentrations. An unspecified, irreversible reaction occurred during the timeresolved emission experiments, induced by the excitation light (λ ) 327 nm), leading to a continuously increasing emission intensity during the data collection. This was also observed when the samples were excited by the excimer/dye laser system (λ ) 466 nm). At both excitation wavelengths, prolonged excitation resulted in a blue coloration of the solution, characteristic of viologen radicals.30 This was most evident when micelles of pure CMV2+ (6 mM, which is above the cmc31) instead of vesicles with CMV2+, was used to bind the Ru complexes. The accumulation of viologen radicals indicates that a component of the system acts as a sacrificial donor, preventing the recombination of viologen radicals with the oxidized Ru complexes. It seemed that the increase in emission intensity was due to an increasing fraction of Ru complexes that are not bound to the vesicles. One possible reason would be a photoinduced decarboxylation of Ru(dcb)34-, as a consequence of irreversible electron transfer from the carboxylate groups; either to the oxidized Ru in the same complex (the oxidation is metal-centered18), in analogy with, e.g., oxalate,32a or to CMV2+, although 466 nm is well outside the previously reported viologen-carboxylate charge-transfer bands.32 This would also explain the accumulation of viologen radicals. However, we have not made any direct attempts to determine the mechanism responsible for the irreversible, photoinduced changes observed in the Ru(dcb)34-/CMV2+ system, and any conclusion must therefore await further studies of the topic. The irreversible photoinduced reaction would explain why the fraction of unbound complexes is higher than expected from the Poisson-Boltzmann calculations; R ) 0.05-0.2 for curves with σtot > 30 mC/m, and somewhat higher for the lower values of σtot. The escape of some Ru complexes from the vesicle surface during the measurements does not seem to change the time behavior of the emission decay from that predicted by eq 4. This is also expected if the Ru complexes may be regarded as either free or bound to the vesicle surface. Only the relative amplitudes of the two exponential terms in eq 4 would change during the measurements. The value of R would depend on several independent parameters, as, e.g., the number of photons absorbed, and consequently no clear dependence on [Q] or σtot can be observed. Finally, it must be noted that the value of D, i.e., the sum of the lateral diffusion coefficients for Ru(dcb)34- and CMV2+, is
Emission Quenching and Charge Separation
J. Phys. Chem. B, Vol. 101, No. 38, 1997 7499
of the same order of magnitude as experimental values for lipids and other amphiphilic molecules incorporated in liquid-crystalline bilayers or monolayers.7 These values are 2 orders of magnitude lower than the D values in water for free molecules of the same size as Ru(dcb)34-. Although the quenching is more efficient than in previous experiments with an amphiphilic Ru complex,6 we expected an even higher efficiency, since Ru(dcb)34- is diffusing in the water region around the vesicle. The reason for the low value of D is not clear, but because of its importance we take the liberty to speculate on its origin. One could conceive that the Ru(dcb)34- would bind rather tightly to the cationic amphiphiles and thus follow their lateral movements in the bilayer. However, binding to CPy+ cannot explain the apparent low value of D, since the same values were obtained also when no CPy+ was present (see Table 1). Nor can Ru(dcb)34- be bound to the quencher molecules, since this would result in “ion-pair quenching”, and the emission decay curves would not follow the predictions of eq 4, as discussed above. Another possible explanation is that the encounter between the reactants is in some way hindered by the lipid headgroups. One may envisage that the reactants are bound at different depths in the bilayer, that their encounter rate is limited by diffusional fluctuations of the headgroups, or that they are forced to attain an orientation relative to each other in which quenching is relatively unfavorable. To some extent, this effect could be accounted for within the two-dimensional model, by assuming a less than diffusion-controlled reaction rate. It would lead to low apparent values of D. Medhage and Almgren29 have studied the consequences of a decrease in reaction rate upon encounter for a reaction in 0-3 dimensions. From their results it is clear that for reactions in two dimensions that are not diffusion controlled the values of D obtained in the data analysis, with an erroneous assumption of diffusion control, are indeed lower than the true ones. However, it is uncertain if D values can be obtained that are 2 orders of magnitude lower that those corresponding to diffusion control, without a clear deviation of the emission time behavior from that predicted by eq 4. Further experiments on similar systems are necessary in order to determine the origin of the low D values presently obtained. Charge Separation Yield. The mechanism for the quenching of Ru(dcb)34- by viologen is, as for many other Rupolypyridine complexes,22 electron transfer from the excited Ru complex to the quencher. Flash-photolysis experiments were made in order to measure the yield of charge separation, ΦET, defined according to + 1 [CMV ]max ΦET ) f [*Ru(dcb) 4-] 3
(5)
t)0
where f is the fraction of excited Ru(dcb)34- that is quenched, [*Ru(dcb)34-]t)0 is the concentration of excited Ru-complexes immediately after the excitation pulse, and [CMV+]max is the maximum concentration of charge separation products (measured just after complete excited-state decay but before significant recombination). For a solution with 29 µM Ru(dcb)34-, 390 µM CMV2+, and 7.30 mM lecithin ([Q] ) 1.1 × 1017 m-2), the values [*Ru(dcb)34-]t)0 ) 3.3 ( 0.3 µM and [CMV+]max ) 0.2 ( 0.05 µM were obtained from the flash-photolysis measurements (see Experimental Section, curves not shown). Inserted in eq 5, with f ) 0.93 ( 0.03, these values give ΦET ) 0.04-0.09 (I ) 2 × 10-3 M). This may be compared with the value ΦET ) 0.38 for the couple Ru(bpy)32+-MV2+ (in water, I ) 0.01 M).28 Some estimates using conventional models of bimolecular electron-transfer processes serve to show that the value of ΦET
obtained is reasonable. For a photoinduced electron-transfer reaction, where the rate of reformation of the excited precursor complex is negligible, ΦET ) k′-d/∑ki, where k′-d is the rate constant for separation of the products from the solvent cage, and ∑ki is the sum of the rate constants for all reactions of the products in the cage.5 In the present case, ∑ki ) k′-d + kBET, where the latter is the rate constant for back electron transfer to re-form the ground-state reactants. In the present system, as opposed to Ru(bpy)32+/MV2+, there is an electrostatic attraction between the products within the solvent cage, and apparently a relatively low value of the diffusion coefficients (see above). Consequently, the value of k′-d is expected to be much lower, and therefore, the value of kBET must also be lower than for Ru(bpy)32+/MV2+ in order for ΦET to reach the observed value. Indeed, on energetic ground the back electron transfer is expected to be in the Marcus inverted region33,34 for both systems, and the value of ∆G° is about 0.2 eV35 more negative for the Ru(dcb)33-/CMV+, which is expected to give a significantly lower rate of back electron transfer than for Ru(bpy)33+/MV+. The reorganization energy may also be lower due to the less polar environment at the vesicle, as compared to the homogeneous Ru(bpy)32+/MV2+ system, which is also expected to reduce the rate of backelectron transfer in the inverted region. Finally, the slightly larger reaction distance in the present system28b may reduce the electronic coupling and contribute to the decrease in kBET. Thus, it seems reasonable that the value of kBET is relatively low in the present system, which compensates to a great extent the likewise low value of k′-d, so that ΦET reaches a detectable level. The transmembrane electron transfer via the rate-determining disproportionation of viologen radicals (see Introduction) occurs on the time scale of 1s at low surface concentrations.5 Therefore, it cannot compete with the recombination of the reduced acceptor and the oxidized sensitizer, which was found to occur on the time scale of 1 ms for one reactant pair per vesicle. To overcome this problem, one may add an electron donor (see Introduction) and/or use a single-electron acceptor that may diffuse through the membrane without the need for further reaction, e.g., cetylbipyridine (CB+, see Introduction). Quenching by CB+ in Vesicles. Attempts were also made to quench the excited Ru(dcb)34- by cetylbipyridine (CB+) in vesicles. The methylbipyridine (MB+) was found to quench the emission from Ru(bpy)32+ by the same mechanism as MV2+, i.e., by oxidative electron transfer.6 Also, it has been shown that CB+ binds completely to lecithin vesicles and that the reduced form (CB0) transfers electrons through the bilayer of lecithin vesicles with a lifetime of 0.7 ms.8b Therefore, if CB+ would accept an electron from the excited Ru(dcb)34-, the system could be used to achieve a photoinduced charge separation across the vesicle membrane, with a membrane-bound acceptor pool and a secondary electron acceptor added to the vesicle interior. The advantages with CB+ as compared to CMV2+ are that with the former there will be no electrostatic attraction between the charge separation products within the solvent cage. Also, CB+ only has to be reduced in one step before it can diffuse across the membrane, and may therefore compete with the diffusion-controlled recombination with the sensitizer. Unfortunately, addition of CB+ up to 10 mol % of the lipids in the external monolayer of lecithin vesicles did not quench the Ru(dcb)34- emission at pH g 5.0 (where the quencher is in the unprotonated form CB+). At higher concentrations of CB+ (≈15 mol %), the vesicles were unstable and precipitated within
7500 J. Phys. Chem. B, Vol. 101, No. 38, 1997 some minutes upon addition of Ru(dcb)34-. This was the case both when CB+ was added before and after vesicle preparation. The reason for the lack of quenching by CB+ and MB+ is likely their relatively low reduction potentials. The reduction potential of CB+ and MB+ depends on pH, due to a protonation equilibrium at the nonalkylated nitrogen, and the pKa values for MB+ are 3.5 and 10.5 for the oxidized and reduced forms, respectively.36 Although the potentials of the excited Ru(dcb)34(-0.70 V vs NHE20) and the quenchers should be equal at pH ≈ 7.5 these potentials involve protonation of the reduced acceptor. In the quenching reaction, however, at pH g 5 the protonation is much slower than the rate of dissociation of the reactant encounter/successor complex. Therefore, quenching is not exergonic unless MB+ or CB+ are protonated already in the oxidized form (E° ) -0.45 vs NHE30). This is in agreement with results from earlier experiments with Ru(bpy)32+, which is a stronger photoreductant: the rate constant for quenching by HMB2+/MB+ in homogeneous solution was equal to that of MV2+ at pH < 3, but smaller and constant at pH > 4.5.6 Possible Development of the System. It seems that the system studied provides the desired organization of the reactants at the vesicle surface, although the quenching was much slower than expected. The large attractive field from the charged acceptor/vesicle assembly should allow the use of Ru complexes with a somewhat lower net negative charge than Ru(dcb)34-, which would lend some freedom in the modification of the complexes. Despite the relatively large electrostatic attraction between the charge separation products within the solvent cage, a measurable yield of charge separation was obtained, and a reduction of the acceptor and sensitizer charge should give some improvement. Also, if electron transfer from a donor to the oxidized sensitizer could compete with the cage recombination, one could obtain a higher charge separation yield. Thus, by using the donor-sensitizer molecules discussed in the Introduction, one may realize a system with a donor-sensitizer-acceptor pool that performs photoinduced charge separation in a lipid membrane. The problem with the irreversible photoinduced reaction could possibly be solved by using negatively charged substituents other than carboxylic groups. To also achieve electron transfer across the vesicle membrane, a monocationic acceptor should preferably be used, which becomes uncharged after reduction by a single electron. In this way, it could transverse the membrane directly, in contrast to the viologens (see Introduction), and perhaps avoid recombination with the sensitizer. This acceptor could be a cetylbipyridine that is modified by electron-withdrawing substituents, so that it may quench Ru(dcb)34- by electron transfer. However, the substituent will make the molecule more polar, which will reduce the rate of its transmembrane diffusion.8b,37 This process will thus be less able to compete with recombination with the oxidized sensitizer. With some fine-tuning of the acceptor properties, it may be possible to optimize the quenching and charge separation processes. Conclusions The photosensitizer Ru(dcb)34- was shown to bind strongly to the surface of lipid vesicles when the latter were positively charged by incorporation of amphiphilic bipyridinium acceptors. The quenching of the excited sensitizer by the viologen CMV2+ followed a model for diffusional quenching in two dimensions. The value of the diffusion coefficient obtained is of the same order of magnitude as for amphiphilic reactants bound in bilayers. This is much smaller than the value expected, since Ru(dcb)34- diffuses in the water region around the vesicle interface.
Hammarstro¨m et al. The charge separation yield was significant, if not high, and by development of the system it may be improved. The system thus realizes photoinduced charge separation using sensitizers preassembled with a membrane-bound acceptor pool. From previous studies, it is clear that an extension to include charge separation across the lipid membrane is possible. The system may thus prove to be a convenient way to add an acceptor function to sensitizer-donor molecules. Experimental Section Methods. The 1H NMR and 13C NMR spectra of the ligands 1 and 2 and complexes 4-10 were recorded in deuterated chloroform, methanol, dimethyl sulfoxide (DMSO), or deuterium oxide made alkaline with NaOH (using a solvent suppression routine decoupling the solvent hydroxy resonance at 4.75 ppm), on Bruker AM 400 (400 MHz proton, 100 MHz carbon) or DMX 500 (500 MHz proton, 125 MHz carbon) instruments. Chemical shifts (δ) are reported in parts per million (ppm) downfield from TMS (tetramethylsilane). IR spectra were recorded on an ATI Mattson Infinity Series FT-IR spectrometer as KBr disks. Melting points were obtained on a Bu¨chi apparatus and are uncorrected. Analytical TLC was performed on precoated aluminum oxide gel 60 F254 plates (neutral, Merck) with UV detection. Aluminum oxide gel (neutral, activated, 60 mesh, 95+%, Alfa) was used for preparative column chromatography of the diester 2 and complexes 4 and 7-10. The electronic absorption spectra were recorded on a Varian Cary 5E UV-vis-NIR spectrophotometer at 25.0 °C ((0.1 °C). Steady-state emission spectra were recorded on a Perkin-Elmer LS-5 luminescence spectrometer or a SPEX Fluorolog 2 spectrofluorimeter. The electrospray ionization mass spectrometry (ESI-MS) experiments were performed on a ZabSpec mass spectrometer (VG Analytical, Fisons instrument). Electrospray conditions were as follows: needle potential, 3 kV; acceleration voltage, 4 kV; bath and nebulizing gas, nitrogen. Liquid flow was 50 µL/min using a syringe pump (Phoenix 20, Carlo Erba, Fisons instrument). Solvent compositions were 50% acetonitrile-50% water containing 1% acetic acid or 100% acetonitrile. Accurate mass measurements were obtained by the use of polyethylene glycol (PEG) as an internal standard. Small unilamellar vesicles were prepared by sonication with a tip. A small sample of lecithin dissolved in chloroform was transferred to a glass tube. The chloroform was removed by overnight storage at low pressure, and 5 mL of water or buffer was added. The lecithin solution was sonicated at room temperature, using a Soniprep 150 from MSE Scientific Instruments, Crawley, UK. This procedure has been demonstrated to give a relatively monodisperse sample of unilamellar vesicles with an average radius around 15 nm.8 The surface concentration of quencher, [Q], was calculated using a value of 0.71 nm2 for the surface area of the lecithin.25 The headgroup area of the quenchers was relatively small, and they were present at a mole fraction less than 0.1. Thus any possible expansion of the vesicle surface due to the addition of the quenchers was neglected in the calculation of [Q]. All photophysical and photochemical measurements were performed in milli-Q water at pH ) 11 (1 mM NaOH), at 20 ( 2 °C. Some experiments with MB+ and CB+ were performed in 2 mM phosphate buffer at different pH values. Time-resolved emission experiments were performed using single-photon counting; a mode-locked Nd:YAG laser was used to pump a DCM dye laser. The output from the dye laser was frequency doubled to 327 nm and used to excite the samples. The instrumental response function fwhm ≈ 400 ps, as determined with a scattering sample. The emission was
Emission Quenching and Charge Separation observed around 610 nm using an interference filter with 10 nm bandwidth. Measurements of ΦET was conducted with a conventional flash-photolysis setup. An ELI-94 excimer laser, operating with XeCl (λ ) 308 nm) was used to pump a LT-1113 dye laser (both from the Estonian Academy of Sciences) that excited the sample (λexc ) 466 nm,
