The Journal of
Physical Chemistry VOLUME 98, NUMBER 31, AUGUST 4,1994
Q Copyright 1994 by the American Chemical Society
LETTERS Photosensitization of Colloidal SnOz by Ruthenium(11) Polypyridine Dissolved in a Supported Surfactant Bilayer William E. Ford' and Michael A. J. Rodgers' Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403 Received: March 18, 1994; In Final Form: June 4, 1994' Negatively charged SnO2 particles (ca. 4-nm diameter) serve as supports for bilayers composed of
didodecyldimethylammonium ion in aqueous sols. An amphiphilic Ru(I1) polypyridine complex incorporated into the bilayer photosensitizes electron injection into the conduction band of the Sn02, but only the population of complex dissolved in inner monolayer, in contact with the support, is active. The high quantum efficiency of charge transfer together with the relatively slow rate of recombination between the conduction band electron and the Ru(II1) polypyridine complex makes this medium promising for studies of electron-transfer processes in surfactant bilayer assemblies.
Introduction The design of molecular systems that are capable of selfassembly into spatially organized and functional supramolecular devices is a rapidly developing area of research that draws on expertise from many fields of science.' One of the earliest and most widely studied supramolecularsystems is the synthetic lipid bilayer. Interest in bilayers as models for investigation of mechanismsof processes mediated by biological membranesarose soon after the discovery that lipids extracted from natural sources assemble spontaneously into bilayer structures under appropriate conditiom2 One process of particular interest is the mediation of vectorial electrontransport through membranes. Light-driven electron transport across lipid bilayer membranes is fundamental to photosynthesis and could be the basis for artificial photosynthetic and other molecular photonic devices.'~~ The mechanism of electron transport through lipid membranes is still a subject of intense research effort despite investigations spanning the past two decades. A generalfeature of model systems for suchinvestigationsisasymmetryof themembranewith respect to the locations of the electron donor and acceptor molecules, which can be dissolved in either the lipid or aqueous phases.3 The approach that we are taking is to use colloidal redox-active Abstract published in Aduance ACS Abstracts. July 15, 1994.
inorganicparticles as supports for surfactant bilayers containing the photosensitizer? The inorganic particles are hydrous metal oxides which are either negatively or positivelycharged,depending on the state of ionization of the surface, and the charge of the surfactant is opposite to that of the particle, so that the surfactant in the inner monolayer is also the counterion. The electron donating or accepting character of the inorganic support makes the bilayer intrinsically asymmetric with respect to redox reactions. The supported bilayers system with which this report deals consists of a colloidalsemiconductor, SnO2, as the inorganic phase, didodecyldimethylammonium ion (DDMA+) as the surfactant, and an amphiphilicanalogueof Ru(bpy)s2+(bpy a 2,2'-bipyridine) as the photosensitizer. The SnO2 particles have an average diameter of ca. 4 nm and are negatively charged due to ionization of the Sn-OH groups at the surface. Photosensitization of colloidal SnO2 by R~(bpy)3~+ occurs by injection of an electron from the metal-to-ligand charge-transfer (MLCT) excited state (*) of the complex into the conduction band of SnO2 (eq l).5,6
+
Ru(bpy)32+ hv (532 nm)
-
*Ru(bpy)F
-+
R u ( b p ~ ) ~+~e,,-+ (Sn02) (1)
Recombination between the resultant Ru(II1) complex, R~(bpy)~3+, and conduction band electron, e,b-(SnOz), restores
0022-3654/94/209~-14~5S04.50/0 Q 1994 American Chemical Society
7416 The Journal of Physical Chemistry, Vol. 98, No. 31, 1994
Letters
the system to its initial state (eq 2). Electron injection is rapid
ca
0.03
5 0.02 n
2
compared to recombination and competes effectively with the intrinsic decay of *R~(bpy)~2+, so that the quantum efficiency of formation of R ~ ( b p y ) ~ 3and + eCb-(SnO2)approaches unity.6 The amphiphilic photosensitizer Ru(bpy)2L2+ (L 5-hexadecamido-1,lO-phenanthroline) has spectroscopic and redox properties738 that closely resemble those of Ru(bpy)32+ and is readily incorporated into molecular assemblies with DDMA+.
0.00
Experimental Section Colloidal Sn02 containing 17 g of SnOz per 100 g of sol was purchased from Alfa and characterized as described earlier.6The concentration of particles was calculated assuming a diameter of 4 nm and density of 6.95 g/cm3. The sol is basic, and the particles are negatively charged, with K+ as counterion. The average number of K+ ions per particle was determined to be 195 f 30 by gravimetric analysis with tetraphenylborate ion after precipitating the SnO2 with acetic acid. DDMABr was purchased from Eastman and was recrystallized twice from ethyl acetate. [Ru(bpy)2L]Clz was synthesized as outlined previously.s Dispersions of DDMABr in water were prepared by dissolving 0.0145 f 0.0003 g of the solid in 150 r L of ethanol and then adding 1.00 mL of water with vigorous stirring. To 0.20-mL aliquots of this dispersion were added 0-20-rL portions of the colloidal Sn02to give the desired average number of surfactant molecules per particle. The resultant suspensions were heated to near the boiling point briefly and then immersed in an ultrasonic bath (Bransonic, Model 1200) for ca. 60 s before diluting them with 2.00 mL of water. Generally, [Ru(bpy)zL]Clz was added to the solution of DDMABr in ethanol before dispersing the surfactant in water, but in some cases the DDMABrSnO2 suspensions werediluted withan aqueous solutionof [Ru(bpy)zL]Cl2 instead of pure water. Nanosecond laser flash photolysis (ca. 7 4 s pulse width) and steady-state luminescence experiments were performed with excitation wavelengths of 532 and 452 nm, respectively, as described earlier.6 Air-saturated samples were used since dissolved 02 does not detectably react with ecb-(Sn0z).6
Results The addition of colloidal S n 0 2 to dispersions of DDMA+ ( B r salt) in water results in the formation of a precipitate, but warming the mixture causes the precipitate to disperse into an opalescent sol if the amount of SnO2 relative to DDMA+ is not too high.9 This behavior is unaffected by the inclusion of Ru(bpy)zL2+at a level of ca. 1 molecule per 100 DDMA+ molecules. A quantitative measure of light scattering by the sol is the slope of the line obtained by plotting the optical density due to scattered light versus (1/X)4, where X is the wavelength of light, according to the Raleigh relationship.10 The inset in Figure 1 shows the expected linear dependence in the wavelength range 900-600 nm, where absorption by Ru(bpy)2LZ+is negligible. Figure 1 shows the dependence of the slope of the Raleigh plot on the average number of DDMA+ molecules per SnO2 particle in samples B-G containing a fixed concentration of DDMA+ and increasing number of Sn02 particles. When the number of surfactant molecules per particle falls below ca. 200, the precipitate fails to disperse completely upon warming, and the resultant sample (G) is relatively turbid. Light scattering by sols containing Sn02 alone increases smoothlywith concentration and is relatively small (Figure 1). Time-resolvedluminescencedata for samplesA and F prepared without and with SnO2, respectively, are shown in Figure 2. The average number of DDMA+ molecules per Sn02 particle in sample F is 210, which is close to the value where light scattering
0.01
8 0
2
4
6
8
V."
150
300
450
600
750
900
Avg. Number of DDMA' Molecules per Particle
Figure 1. Dependence of the turbidity of samples B-G on the average number of DDMA+ molecules per SnOz particles is shown by the clmed circles (solid curve). The measure of turbidity was obtained from the slopesofplotsofopticaldensity(l-cmpathImgth)~trsus(l/h)~according to Rayleigh's equation as shown in the inset for samples A and F. The concentrationsof DDMA+ and Ru(bpy)zL2+were 2.4 0.1 mM and 24 h 3 pM, respectively. The Rayleigh plot slopes of samples containing the same concentrations of colloidal SnOz as samples E 4 but without DDMA+and Ru(bpy)zL2+are shown by the open circles (dashed curve).
-5
g
-lo
-
Y
4
-15
-20
-
--
-40 0 40 80 120 160200
TIME (ns) -25 -0.5
0.5
2.5
1.5
3.5
4.5
TIME (I4 Figure 2. Time-resolved luminescence data for samples A (open circles) and F (closed circles) obtained at a monitoring wavelength of 630 nm. The solid lines are fits of the data to single-exponential decays having lifetimes of 555 3 ns in A and 549 5 ns in F. The data points for sample F at times earlier than 110 ns after laser excitation were not included in the fit due to contribution by a more rapidly decaying component (see text). As shown in the inset, the rapid lumincscence component in F decays primarily within the laser pulse (dotted line) and is not resolved.
*
rises sharply (Figure 1). The luminescence data reflect the kinetic behavior of the MLCT excited state of Ru(bpy)ZL2+, *Ru(bpy)2L2+. The decay in sample A is exponential with a lifetime of 555 3 ns, whereas two populations of *Ru(bpy)ZLz+ exist in F. One population in F, which constitutes 37 f 3% of the total, is unquenched and decays with thesame lifetime as that in sample A. The remaining population of *Ru(bpy)zL2+ in F decays primarily within the duration of the laser pulse and is unresolved (Figure 2, inset). Steady-state luminescence measurements show the same trend: the luminescence intensity of sample F is 36 f 4% of the intensity of sample A. Time-resolvedabsorption data for these samples were obtained with a monitoring wavelength of 452 nm, where the production of either *Ru(bpy)zLZ+ or Ru(bpy)2L3+results in a net negative absorbancechange, M,due to bleaching of the MLCT absorption band of Ru(bpy)ZLZ+. Data for samples A and F are shown in Figure 3. The transient produced in sample A decays exponentially and has the same lifetime, r * , as the luminescence decay (Figure 2), so that it is due solely to *Ru(bpy)2L*+.l1 The absorbance change in sample F shows a transient component
*
Letters
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7417
-
-0.02
+
-0.04
B
-
-0.06
-
-0.08
-0.10
'
-2
of samples A (no SnO2) and B-G, whose compositions are given in Figure 1. Distinct quenched and unquenched populations of *Ru(bpy)2L2+were observed in each of the samples (B-G) that contained SnO2, and Ru(bpy)2L3+was produced in these cases. The ordinate in the plot in Figure 4 is the ratio C+/(C+ C*), where the concentrations are the values obtained by fitting the time-resolved absorption data for these samples to eq 3. The values plotted are the averages obtained with the parameter @ equal to either 0.35 or 0.40.16 The values of A€*and A@ used to calculate C* and Cy from AA values were -1.2 X 104 and -1.4 X 104 M-l cm-l, re~pectively.1~ The abscissa in Figure 4 is (1 - L/L(A)), where L is the luminescence intensity of the sample and L(A) is the intensity of sample A. The abscissa is thus the fraction of *Ru(bpy)zL2+in the sample that is quenched by SnO2. The values of (1 - L/L(A)) measured by both the time-resolved and steady-state methods are plotted. This plot shows that there is good correspondence between the values determined by either method of measuring L. The data in Figure 4 were used to estimate the quantum efficiency of electron injection, @+. Although the quenching of *Ru(bpy)2L2+by SnO2 occurs by electron transfer, the recombination reaction (e.g., eq 2) could be rapid on the nanosecond time scale, so that theobserved @+ would be less than 1. Denoting the concentration of quenched *Ru(bpy)2L2+ that does not generate observable Ru(bpy)2L3+as 0,then @+ = C y / ( @ Cy). Also, the fraction of *Ru(bpy)zLZ+ in the sample that is quenched, (1 - L/L(A)), is equal to the ratio (@ e)/(@ C+ C). These relationships lead to eq 4, in which only the
I 2
10
6
14
18
TIME (ps) Figure 3. Time-resolved absorption data for samples A (open circles) and F (closed circles) obtained at a monitoring wavelength of 452 nm. The solid line in A is a fit of the data to a single-exponentialdecay having a lifetime T* = 573 f 1 ns and initial intensity I P A C * = -0,0988 f O.ooo4. The solid line in F is a fit of the data to eq 3 obtained with the value of T* fixed at a value of 550 ns, and the residuals are plotted in the inset. Thevaluesoftheotherparametersobtainedare IC*Ae* =-0.0335 f 0.0015, IC+At+ = -0.0735 h 0.0037, TK = 3.05 f 0.43 ps, and @ =
0.354 f 0.014. ".O
+
+
0.6
d / ( d+ @+e) = 1 - L/L(A)
0.0
0.0
0.4
0.2
0.6
0.8
1 - L/L(A)
Figure 4. Plot of the data for samples A-G according to eq 4. The luminescence results plotted along the abscissaare basedon either steadystate (open circles) or time-resolved (closed circles) measurements. The bars associated with the points along the ordinate denote the values of cC/(C++ C)that were obtained from fits of time-resolved absorption data to eq 3 with @fixedat a value of either 0.35 or 0.40. The two lines graphed are fits of the data to eq 4 for values obtained with 6 = 0.35 (dashed line) or @ = 0.40 (solid line). The values of a+associated with these lines are 1.04 f 0.04 and 0.83 f 0.03, respectively.
attributed to unquenched *Ru(bpy)2L2+as well as a longer-lived transient attributed to Ru(bpy)zL3+ resulting from electron injection (e.g., eq 1). Assuming that the decay of Ru(bpy)2L3+ obeys the Kohlrausch relaxation function,12J3the absorption data for F were fit14to eq 3, where 1 is the path length (1.0 cm), C AA(t)
= lC*Ac* exp(-t/r*)
+ l d A ~ e+x p [ - ( t / ~ ~ ) @ ](3)
and C+ represent the initial concentrations of the unquenched *Ru(bpy)2L2+and Ru(bpy)2L3+,respectively, and A€* and A€+ are the extinctioncoefficientchangesassociated with the formation of these two species. The decay of * R ~ ( b p y ) ~ is L characterized ~+ by thelifetime T * , and thedecay of Ru(bpy)zL3+by the parameters T K and @ that are associated with the Kohlrausch function. The inset in Figure 3 is an example of residuals showing that the transient absorbance change in F conforms to eq 3. The time required for the concentration of Ru(bpy)zL3+ to decay to l / e of its initial value is T K = 3.1 f 0.4 ps.15 The data plotted in Figure 4 represent a compilation of the time-resolved and steady-state photophysical characterizations
+ +
(4)
value of @+ is unknown. The lines drawn in Figure 4 are leastsquares fits of the data to eq 4 obtained with @ equal to either 0.35 or 0.40. The value of @+ obtained from these fits is either 1.04 f 0.04 or 0.83 f 0.03,respectively. Thus, it can be concluded that the quenching of *Ru(bpy)2L2+by SnO2 generates Ru(bpy)zL3+ with close to unit efficiency. An additional experiment involved mixing an aqueous solution of Ru(bpy)2L2+with a sol of DDMA+-Sn02 prepared without Ru(bpy)zL2+. The final composition of the sample was the same as that of F, and it was examined within 5 min of mixing. The transient behavior was identical to that observed for sample F (Figures 2 and 3). In contrast, the DDMA+Sn02 sol did not quench photoexcited Ru(bpy)32+,which is not an amphiphilic molecule. Thus, the quenching by DDMA+Sn02 depends on the amphiphilic character of the photosensitizer and is not simply a counterion exchange reaction occurring at the Sn02 particle surface.
Discussion Aqueous dispersions of DDMABr contain positively charged bilayer vesicles, which may coexist with micelles and other kinds of aggregatesa20The precipitate that is formed upon simple mixing of sols of SnO2 and DDMABr is likely to be an agglomerate of negatively and positively charged particles. The peptization that occurs upon warming apparently involves a redistribution of the surfactant molecules so that they coat the Sn02 particles.4V2l Peptization is incomplete, however, when there are fewer than an average of ca. 200 DDMA+ molecules per particle.22 This number is close to the number of K+ ions originally associated with each particle, so that peptization may occur when the DDMA+ concentration is high enough for the coated particles to bear a net positive charge. The value of 200 molecules per particle correspondsto a SnO2 surface area available per DDMA+ of ca. 0.25nm2.Z4 The cross-sectionalarea occupied by DDMA+ molecules in monolayers and bilayers is ca. 0.6-0.7 nm2.4.20Thus, the DDMA+-coated SnO2 particles are likely to have a double layer of surfactant and carry a net positive charge. The small
7418 The Journal of Physical Chemistry, Vol. 98, No. 31, 1994
diameter of the Sn02 particles severely constrains how the DDMA+ molecules can assemble on the surface, so that only a loosely packed double layer of the surfactant may be possible.2s The photophysical behavior of Ru(bpy)2L2+in these samples is consistent with this description of bilayer deposition on the particles. Two distinct populations of photoexcited complex exist in the samples containing SnO2: one which is rapidly quenched via electron transfer to the inorganic phase and the other which is unquenched. The quenched chromophores are identified as those adjacent to the SnO2 surface, i.e., dissolved in the inner monolayer of DDMA+-coatedSn02 particles. The unquenched chromophores can be attributed to those dissolved in the outer monolayer of the coated particles, adjacent to the aqueous phase, as well as those dissolved in DDMA+ vesicles that are not associated with SnO2 particles or dissolved in the aqueous phase.% Samples with a relatively high average number of surfactant molecules per SnOzparticle (e.g.,E D ) are most likely to contain "empty" DDMA+ vesicles as well as coated SnO2 particles. As the average number of surfactant molecules per particle decreases, more of the DDMA+ molecules become bound to SnOz particles, so that the proportion of the quenched population relative to the total increases (Figure 4).
Conclusion Our results demonstrate two functions of colloidal SnO2 particles in these samples: (1) they serve as supports for bilayers composed of DDMA+ and Ru(bpy)2L2+, and (2) they act as reservoirs for electrons injected by photoexcited Ru(bpy)2LZ+ molecules whose chromophores are in contact with the particle surface. The DDMA+-coatedSnO2particles are supramolecular assemblies into which photoactive and redox-active molecules can be readily incorporated. The ease of preparation of these assemblies and their suitability for examination by absorption and luminescencespectroscopicmethods should make them useful for investigations of a variety of bilayer-mediated chargeseparation processes. Acknowledgment. Support for this project was provided by NSF Grant CHE-9208551 and by the Center for Photochemical Sciences and Bowling Green State University. References and Notes (1) (a) Ahlers, M.; Muller, W.; Reichert, A,; Ringsdorf, H.; Venzmer, J. Angew. Chem., In?. Ed. Engl. 1990,29, 1269. (b) Balzani, V.;Scandola, F. Supramolecular Photochemistry; Horwood: Chichester, 1991. (c) Whitesides, G. M.; Mathias, J. p.; Seto, C. T. Science 1991, 254, 1312. (d) Kunitake, T. Angew. Chem., In?. Ed. Engl. 1992, 31, 709. (e) Lehn, J.-M. Science 1993, 260, 1762. (2) (a) Tien, H. T. Bilayer Lipid Membranes (ELM): Theory and Practice:Marcel Dekker: New York. 1974. (b) Jain. M. K.: Wagner. R. C. Introduction to Biological Membranes;John Wiley & Sons: New qork; 1980. (3) (a) Calvin, M. J. Membr. Sci. 1987, 33, 137. (b) Hurst, J. K. In Kinetics and Catalysis in Microheterogeneous Systems; Gritzel, M., Kalyanasundaram, K., Eds.; Marcel Dekker: New York, 1991; pp 183-226. (c) Lymar, S.V.; Parmon, V. N.;Zamaraev, K. I. Top. Curr. Chem. 1991,159, 1. (d) Tollin, G. In Chlorophylls;Scheer, H., Ed.; CRC Press: Boca Raton, FL, 1991; Section 1 , pp 317-337. (e) Robinson, J. N.;Cole-Hamilton, D. J. Chem. Soc. Rev. 1991, 20, 49. (4) Recent referencts dealing with surfactant bilayers on colloidal inorganic particles include: (a) Esumi, K.; Sakamoto, Y.; Meguro, K. J.
Letters Colloid InterfaceSci. 1990,134,283, (b) DeCuyper, M.; Joniau,M. Lungmuir 1991, 7, 647. (c) Capovilla, L.; Labbe, P.; Reverdy, G. Lungmuir 1991, 7 , 2000. (d) SWerlind, E.; Bjbrling, M.; Stilbs, P. Lungmuir 1994, 10, 890. ( 5 ) Mulvaney, P.; Grieser, F.; Meisel, D. Lungmuir 1990, 6, 567. (6) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1994, 98,3822. (7) Schanze, K. S.;Sauer, K. J . Am. Chem. Soc. 1988, 110, 1180. (8) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1992, 96,2917. (9) The temperature required for this change is in the range 5&55 OC, but the samples were heated to near the boiling point to ensure equilibration. (IO) Kerker, M. The Scattering of Light and Other Electromagnetic Radiation; Academic Press: New York, 1969; pp 325-328. (1 1) Thevalue of T* determined by monitoring lumincacunceisconsistently 4 & l%lowerthan thevaluedetermined byabsorbancechangeforRu(bpy)Lz+ as well as Ru(bpy),2+. (12) (a) Plonka, A. Time-Dependent Reactivity of Species In Condensed Media; Springer-Verlag: Berlin, 1986. (b) Drake, J. M.; Klafter, J.; Lcvitz, P. Science 1991, 251, 1574. (13) Conformity of the kinetics of recombination (e.g., eq 2) to the Kohlrausch function was shown previously with Ru-polypyridine complexes that were either electrostatically or covalently attached to Sn02 particles.6 (14) Nonlinear regression using the Marquardt-Levenberg algorithm was performed with Sigmaplot. (15) TheKolhrausch-typedecaybehaviorindicatesthattherecombination betweeen Ru(bpy)zL'+ and %-@no2) is characterized by a distribution of rateconstantswhmepeakvalueisr~--' = (3.3 t 0 . 5 ) X I O 5 r 1 . Theparameter fl is inversely related to the width of the distribution; thevalue of 0.35 0.02 is indicative of a relatively broad distribution.12 (16) The averagevalue of T* for samples BG determined by time-resolved luminescence was 530 f 20 ns. A 4% greater value of T * , 550 M,was used in fitting the absorption data for these samples to eq 3.11 Fits to eq 3 for samples EG had local minima for values of 4 in the range 0.35-0.40, the corresponding values of TK were 3.7 & 0.8 p. The data for samples B-D were assumed to have 4 values in this range because no minima were found. (17) The uncertainty in both values is t O . l X 104 M-1 cm-1, The value of A€* for Ru(bpy)2Lzt in aqueous dispersions of DDMABr was determined by comparing M at 452 nm to the value in an optically matched solution of Ru(bpy)32+ in water. The value of At* for the latter was taken as -1.0 X 104 M-1 cm-1.18 The value of A& for the formation of Ru(bpy)zLS+ was obtained by oxidatively quenching *Ru(bpy)2L2+with methyl viologen in water and comparing the hA values at 452 and 395 nm of the products with those of Ru(bpy)2L,++ Q-(SnOz). Theextinctioncoefficients of themethylviologen radical at these two wavelengths were taken as 800 and 4.2 X 104 M-1 cm-1, respectively,l9andthecontribution of--(SnO2) was assumed tobe negligible. (18) Yoshimura, A.; Hoffman, M. Z.; Sun, H. J . Photochem. Photobiol. A: Chem. 1993, 70, 29. (19) Watanabe, T.; Honda, K. J. Phys. Chem. 1982,86, 2617. (20) (a) Okahata, Y.; Shimizu, A. Lungmuir 1989, 5, 954. (b) Miller, D. D.; Magid, L. J.; Evans, D. F. J. Phys. Chem. 1990,94,5921. (c) Dubois, M.; Zemb, Th. Lungmuir 1991,7,1352. (d) Okuyama, K.; How, K.; Maki, N.; Hamatsu, H. Thin Solid Films 1991, 203, 161. (e) Menger, F. M.; Balachander, N. J. Am. Chem. Soc. 1992,114, 5862. (21) Heating sols of DDMABr without SnOz to 55 O C causes a reversible change in light scattering from opalescent to clear. This change is not due to the phase transition from solid to liquid crystalline state of the DDMA+ bilayer, however, which occurs below 20 OC." (22) Samples prepared with ca. 100 DDMA+ molecules per particle fail to peptize at all in water but are completely dispersible in chloroform. This behavior indicates that the particles possessa monolayerof DDMA+molecules whose dodecyl chains are oriented toward the solvent phase.(23) Hu, N.; Rusling, J. F. Anal. Chem. 1991,63, 2163. (24) If the concentration of DDMABr in the aqueous phase in equilibrium with the particles is assumed to be the critical micelle concentration, 0.16 mM," then the calculated surface area per molecule is ca. 0.26 nm2. (25) Israelachvili, J. N.; Marpdja, S.;Horn, R. G. Q. Rev. Biophys. 1980, 13, 121. (26) Theemissionlifetimeof *Ru(bpy)2Lz+inwater,519k5ns,isshorter than that in aqueous dispersions of DDMABr, 550 & 10 ns. The fact that the emission decay in the latter samples is exponential implies that either the complex is predominantly dissolved in the micellar phase or else its rate of exchange between the micellar and aqueous phases is rapid compared to the rate of emission decay.
