J. Phys. Chem. 1995, 99, 5139-5145
5139
Photosensitization of Colloidal SnO2 in an Asymmetric Supported Bilayer Composed of Two Types of Surfactants William E. Ford* and Michael A. J. Rodgers" Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403 Received: November 10, 1994; In Final Form: January 26, 1 9 9 9
Surfactant bilayers composed of a cationic surfactant, didodecyldimethylammonium ion (DDMA'), and a zwitterionic one, 1,2-dioctanoyl-sn-glycero-l-phosphocholine, deposit onto negatively charged colloidal (ca. 4 nm diameter) SnO2 particles such that the DDMA+ molecules are predominantly located in the inner monolayer, Le., serving as counterions to the particle surface charges. The asymmetric configuration of the constituents of the bilayer is revealed by the photophysical properties of an amphiphilic ruthenium(I1) polypyridine complex, Ru(bpy)2L2+(bpy 2,2'-bipyridine, L 5-hexadecamido-1,lO-phenanthroline). The excited state of this photosensitizer, *Ru(bpy)2L2+,is quenched to an extent of >95% via electron transfer to the SnC& particle conduction band, indicating that almost all of the photosensitizer molecules are dissolved in the monolayer adjacent to the SnO2 surface. The conduction band electron produced via this reaction is shown to reduce l,l'-di-n-hexadecyl-4,4'-bipyridinium ion, C16V2+,that is also incorporated into the supported bilayer, so that the net transfer of an electron from *Ru(bpy)2L2+to c16v2+is mediated by the SnO2 particle. The overall efficiency for the reduction of C16V2+by *Ru(bpy)2L2+is 0.3 f0.1. The trapping of the conduction band electron by C16V2+has the effect of lengthening the lifetime of Ru(bpy)2L3+ by an order of magnitude over the lifetime observed without ClaV2+.
Introduction We recently demonstrated that colloidal Sn02 particles serve as supports for surfactant bilayers composed of didodecyldimethylammonium ion (DDMA+).' An amphiphilic Ru(II) polypyridine complex, Ru(bpy)2L2+(bpy = 2,2'-bipyridine, L = 5-hexadecamido-1,lO-phenanthroline),is readily incorporated into these bilayers and photosensitizes electron injection into the conduction band of the SnO2 substrate (eq 1). This process Ru(bpy),L2+
+ hv (532 nm) - * R ~ ( b p y ) ~ L-~ +
R U ( ~ P Y ) , L+ ~ +e,,-(sno,)
(1)
generates a strong oxidant, Ru(bpy)2L3+(E0(3+/2+) = +1.25 V vs NHE),2 and a relatively strong reductant, e,b-(Sn02) (E" = -0.65 V at pH 9-10),3 at the bilayer-particle interface. These species decay by recombining (eq 2) but are long-lived Ru(bpy),L3+
+ eCb-(SnO2)- R~(bpy)~L'+
(2)
enough to be involved in secondary electron transfer processes. By incorporating oxidants or reductants that are situated at the bilayer-water interface in this system, it should be possible to investigate charge separation across the supported bilayer. Such investigations are complicated by the fact that a substantial fraction (> 30%) of the photoexcited Ru(bpy)2L2+ molecules do not react via eq 1, but decay via intrinsic nonradiative and radiative pathways instead (z 550 ns).' These unquenched chromophores are 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 SnOz particles or dissolved in the aqueous phase.' Ideally, 100%of the *Ru(bpy)2L2+would react via eq 1. This condition not only would result in a greater overall quantum efficiency for the generation of Ru(bpy)2L3+ and e,b-(Sn02) but also would simplify analysis of the kinetics of @Abstractpublished in Advance ACS Abstracts, March 15, 1995.
0022-3654/95/2099-5139$09.00/0
redox reactions involving these species, including recombination (eq 2), by eliminating unquenched *Ru(bpy)2L2+as an underlying transient. Supported bilayers that are asymmetric with respect to the surfactants in the opposing monolayers can be readily prepared in a stepwise layer-by-layer manner. Esumi et aL4 and Capovilla et aL5 have reported a wide variety of such systems. Asymmetric bilayers are of interest as media for photoinduced charge separation because of the possibility of putting amphiphilic electron donor molecules in one monolayer and amphiphilic acceptor molecules in the other.6 This effectively fixes the positions of the donor and acceptor groups transversely within the membrane, thereby imparting it with rectifying properties. Such arrangements are technically difficult to achieve with vesicles but have been widely studied in Langmuir-Blodgett fiims.7 This paper describes our effort to substitute DDMA+ in the outer monolayer of the supported bilayer system examined previously with 1,2-dioctanoyl-sn-glycero1-phosphocholine (diCgPC). This surfactant is zwitterionic* and so should have little tendency to replace the DDMA+ in the inner monolayer. As in the previous experiments, the mixed-surfactant supported bilayer suspensions are opalescent and amenable to characterization by laser flash photolysis. As intended, the use of the second surfactant has largely eliminated the unquenched population of *Ru(bpy)2L2+ in the bilayer. We also show that e,b-(SnO;?) photogenerated in this system reduces an amphiphilic viologen which is also incorporated into the bilayer, thereby extending the lifetime of the charge-separated state.
Experimental Section Materials. Colloidal SnO2 containing 17 g of SnO2 per 100 g of sol was purchased from Alfa and characterized as described ear1ier.I~~ The 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. DDMABr was purchased from Eastman and was
0 1995 American Chemical Society
Ford and Rodgers
5140 J. Phys. Chem., Vol. 99, No. 14, 1995
recrystallized twice from ethyl acetate. DiCsPC from Sigma (approximately 99%) was assumed to be anhydrous but may contain up to ca. 5% HZO.~"[Ru(bpy)zL]Clz was synthesized as outlined previously.I0 [Ru(bpy)3]Clz*HzOwas purchased from Aldrich and used as received. 1,l'-Di-n-hexadecyl-4,4'bipyridinium dichloride (hexadecylviologen dichloride, C I 6VC12) was prepared by Dr. Yohmei Okuno and was available from previous studies." Water was distilled and then passed through a SybrodBamstead NANOpure II purification system (resistivity 17 MQ or greater). The other compounds and solvents were reagent grade materials from commercial sources. Measurements. All spectroscopic measurements were conducted at the ambient temperature, 23 f 2 "C, with 1 cm2 quartz cuvettes. The ground-state absorption spectra were measured with a diode array spectrophotometer (Perkin-Elmer, Lambda Array 3840) operated in the high-resolution mode (0.25 nm). Steady-state emission (uncorrected) and excitation measurements were with a Perkin-Elmer LS-5B spectrofluorimeter. The laser flash photolysis experiments were carried out using the frequencydoubled output (532 nm) of a Q-switched Nd:YAG laser (Continuum, YG660) as the excitation source (ca. 7 ns pulse width), as described earlier? An optically matched solution of [Ru(bpy)3]C12 in water was used for actinometry.I2 Airsaturated samples were used since dissolved 0 2 does not detectably react with ecb- ( S ~ O Z ) . ~ Solution of [Ru(bpy)zL]Clz in HzO. A stock solution of [Ru(bpy)zL]Clzin methanol (ca. 0.02M) was diluted with water to give a final concentration of [Ru(bpy)zL]Clz of 25-30 pM. The actual concentration was determined spectrophotometrically based on extinction coefficient at 452 nm of 1.4 x lo4 M-' cm-I. Sol of DDMABr and Cl6VClZ in € 3 2 0 . DDMABr (0.01484 f O.OOOO4 g) and c16vc12 (O.OOO79 f 0.00004 g) were dissolved in 150 p L of 95% ethanol, with warming. This solution was stirred (vortex mixer) while 1.00 mL of HzO was added, resulting in an opalescent sol. The sample eventually becomes turbid and solid appears, but heating (>55 "C) and sonicating (Branson 1200 ultrasonic bath) restores it to the opalescent state. A 0.20 mL sample of this sol was diluted with 2.00mL of the solution of [Ru(bpy)zL]Clz in Hz0 for one of the laser flash photolysis experiments described below. For each molecule of [Ru(bpy)zL]Clzin this sample, there were on average 83 molecules of DDMABr and 3.0molecules of cl6VClZ. Supported Bilayers of DDMABr, c16vc12, and [Ru(bpy)zL]Clz on Colloidal SnOz. To 0.20 mL of the sol of DDMABr and c16vc12 in H20 was added 16.0 p L of colloidal SnO2, giving a precipitate. Heating and sonicating this mixture caused the precipitate to disperse completely, resulting in an opalescent sol. This sol was diluted with 2.00 mL of the solution of [Ru(bpy)2L]Clz in H20 for one of the experiments described below. For each molecule of [Ru(bpy)2L]Clz in this sample, there were on average 83 molecules of DDMABr, 3.0 molecules of c16vc12, and 0.35particles of SnO2. Supported Bilayers of DDMABr, DiCsPC, and [Ru(bpy)zL]Clz on Colloidal SnOz. DDMABr (0.001 61 f 0.00004 g) and diCsPC (0.00497 f0.00004 g) were dissolved in ca. 100p L of 95% ethanol and then the solvent was removed by blowing with a stream of N2. To the resultant clear film was added 0.20mL of H20. Stirring and sonicating this sample gave an opalescent sol. A 20.0 pL sample of colloidal SnOz was added, resulting in the formation of a precipitate. The precipitate dispersed completely upon stimng and sonicating the sample, giving an opalescent sol. This sol was diluted with 2.00 mL of the solution of [Ru(bpy)zL]Cl2 in HzO for one of
0.65
1
0.021
-...-. 0.00 400
500
600
7M)
WAVELENGTH (nm)
Figure 1. Absorption spectrum of Ru(bpy)2L2+ (25 pM) in the DDMA+-diCsPC-SnOz sol. The dashed line is the contribution of light scattering to the observed spectrum. Inset: plot of the optical according to density in the wavelength range 600-900 nm vs Rayleigh's equation for light scattering by small particles. The slope of the line obtained by linear regression is 2.4 x lo9nm4. The negative intercept is an artifact arising from inappropriate base line correction.
the experiments described below. For each molecule of [Ru(bpy)zL]Cl2in this sample, there were on average 63 molecules of DDMABr, 175 molecules of dicspc, and 0.53particles of SnOz. Supported Bilayers of DDMABr, DiCsPC, c16vc12, and [Ru(bpy)zL]Clz on Colloidal SnOz. A 0.125 mL sample of the sol of DDMABr and cl6vc12 in H20 was added to diCsPC (0.00405 f 0.00004g), together with 75 pL of H20. An opalescent sol was obtained upon mixing. To this sol was added 20.0 pL of colloidal Sn02. The resultant precipitate was dispersed by heating and sonicating the sample. This sol was diluted with 2.00mL of the solution of [Ru(bpy)zL]Clz in H20 for one of the experiments described below. For each molecule of [Ru(bpy)2L]Cl2 in this sample, there were on average 62 molecules of DDMABr, 143 molecules of diCsPC, 2.2 molecules of c16vc12, and 0.53 particles of Sn02.
Results Chloroform-DispersibleParticles. The precipitate that is obtained when colloidal ,51102 is added to an aqueous sol of DDMA+ (Br- salt) is completely dispersible in water when the average number of DDMA+ molecules per Sn02 particle is greater than ca. 190 but fails to do so below that ratio. The solids that fail to disperse can be isolated by centrifuging the samples and decanting off the supernatant aqueous phase. When the amount of DDMA+ is kept constant and the amount of Sn02 is increased, the volume of the solid so obtained increases as the average number of DDMA+ molecules per particle decreases from 185 to 90. The precipitates obtained with ratios between 150 and 90 disperse completely into chloroform containing ca. 10 vol % of ethanol to yield opalescent sols, demonstrating that the particles are hydr~phobic.'~ Water-DispersibleParticles. When colloidal SnO2 is added to an aqueous sol containing both DDMA' (Br- salt) and dicsPC, the resultant precipitate is completely dispersible in water even though the average number of DDMA+ molecules per SnO2 particle is 110-120, i e . , conditions under which the precipitate fails to disperse in the absence of diCsPC. Figure 1 shows the absorption spectrum of one of these mixed surfactant sols which also contains Ru(bpy)2L2+. The inset in this figure is a plot of the absorbance data according to the Rayleigh equationI4 over the wavelength range 600-900 nm, where
J. Phys. Chem., Vol. 99, No. 14, 1995 5141
Photosensitization of Colloidal SnO2 0
0 -6 -12 -18
-24
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-50
150
50
250
-30
-0.5
0
4.08
-5
0.5
1.5
2.5
3.5
4.5
TIME (ps) DDMA+-Sn02 (open circles) and DDMA+-diCsPC-SnOZ (filled circles) sols obtained at a monitoring wavelength of 610 nm. Inset: data from the same samples comparing the decay profiles at early times to the laser pulse profile (dotted line).
absorption by Ru(bpy)2L2+is negligible. The slope of this plot, 2.4 x lo9 nm4, falls within the range of values ((1-3) x lo9 nm4) observed earlier' in DDMA+-$n02 supported bilayer sols and indicates that the particle sizes in both systems are similar. The SnO2 sol alone does not absorb above 400 nm.'* Time-resolved luminescence data for Ru(bpy)2L2+ in the DDMA+-diCgPC-Sn02 mixed surfactant sol are compared to data for a DDMAf-Sn02 sol in Figure 2. The results for the mixed surfactant system show that '95% of the total population of *Ru(bpy)2L2+is quenched by the SnO2, predominantly within the laser pulse (ca. 7 ns); 4 f 1% of the *Ru(bpy)2L2+decays with a lifetime in the 500-600 ns range, which is characteristic of its behavior in aqueous media without Sn02.I In contrast, earlier work showed that 66 f 4% of the *Ru(bpy)2L2+ in the DDMA+-Sn02 sol is quenched by the Sn02, the remaining 34% being unquenched.' The transient absorption data for these same two sols are compared in Figure 3. The absorbance changes, AA, were monitored at 452 nm, where bleaching of the ground-state MLCT absorption band of Ru(bpy)2L2+ occurs during the formation of either *Ru(bpy)2L2+or Ru(bpy)zL3+.I Comparison of the absorption data (Figure 3) to the luminescence data (Figure 2) shows that a longer-lived transient than *Ru(bpy)2L2+ is produced in both sols. The longer-lived component is due to the formation of Ru(bpy)2L3+as a result of electron injection into the conduction band of the SnO2 particle, and its decay is via the recombination reaction. As in earlier work,' the kinetics of recombination between e,b-(SnOz) and Ru(bpy)2L3+ in the DDMAf-diCgPC-Sn02 sol are well described by eq 3,which where 0 < ,8 5 1
5
15
45
25
35
45
TIME (vs) Figure 3. Time-resolved absorption data for the DDMA+-Sn02 (open
Figure 2. Time-resolved luminescence from Ru(bpy)2L2+ in the
AA(t) = AA, exp[-(r/qf],
I
-0.001 1
350
TIME (ns)
(3)
is the Kohlrausch decay function.I6 This function models a system that relaxes with a distribution of exponential decay times whose peak value is t ~ The . parameter3! , is inversely related to the width of the distribution. The parameter AAo in eq 3 is the initial absorbance change. The inset in Figure 3 shows the residuals of a least-squares fit" of the data for the DDMA+diCsPC-SnO2 sol to eq 3. Analysis of data recorded over 3 decades in time shows that the observed transient is due almost entirely to Ru(bpy)2L3+; i.e., little contribution is made by unquenched *Ru(bpy)2L2+in this system. The average values of the parameters obtained are AAo = -0.105 f 0.008, ZK =
circles) and DDMA+-diCsPC-Sn02 (filled circles) sols obtained at a monitoring wavelength of 452 nm. The bleaching recovery observed at this wavelength is due to the decay of both *Ru(bpy)2L2+and Ru(bpy)2L3+. Inset: residuals of the nonlinear least-squares fit of the data for the DDMA+-diCEPC-Sn02 sol to eq 3 with the following parameters: AAo = -0.105, t~ = 2.03 p, and /3 = 0.255; *Ru(bpy)2L2+ does not contribute significantly to the observed decay in this sample.
2.3 f 0.5 ,us, and j3 = 0.26 f 0.03. Actinometry with aqueous R~(bpy)3~+ as standardI2shows that the efficiency of formation of Ru(bpy)2L3+from *Ru(bpy)2L2+in the DDMA+-diCgPCSnO2 sol is close to 1. Thus, as in the DDMA+-Sn02 sol,' quenching of *Ru(bpy)2L2+by the Sn02 generates Ru(bpy)2L3+ with near unit efficiency. On a per-photon-absorbed basis, the yield of Ru(bpy)2L3+ is higher in the mixed-surfactant sol, however, because a greater percentage of the total population of *Ru(bpy)2L2+ is quenched by the SnOz. Photosensitized Reduction of Hexadecylviologen. Hexadecylviologen (Cl sV2') was incorporated into the aqueous bilayer assemblies described above as an electron acceptor. Time-resolved absorption data for bilayers composed of DDMA+, Ru(bpy)2L2+,and c,6v2+ supported on Sn02 particles in water are compared to data for bilayers prepared without SnO2 in Figure 4. The data for the sol containing SnO2 show that a transient absorbing at 399 nm is produced within ca. 100 ns after laser excitation (Figure 4a, filled circles). The difference absorption spectrum that is associated with this transient is shown in Figure 5 (open circles). The maximum occurs near 399 nm and is consistent with the transient being the semireduced species, c16v'+.'8The slower rising component of the absorbance increase at 399 nm observed in Figure 4a may be the result of a superposition of a bleaching recovery of a weakly quenched population of *Ru(bpy)2L2+on top of the signal due to the formation of c16v" (see below). In contrast to the sol containing SnO2, a net bleaching is observed at 399 nm in the sol without SnO2 (Figure 4a, open circles). The recovery of this bleaching is not exponential, but a large proportion of it decays nearly exponentially with a lifetime in the 500-600 ns range. Luminescence measurements (not shown) demonstrate that this transient is due to *Ru(bpy)2L2+,whose difference absorption spectrum has a net bleaching at 399 nm (A€ = -2800 f 300 M-' cm-I). Apparently, a significant proportion of *Ru(bpy)2L2+ in the sample is weakly quenched by c16v2+because its lifetime is close to the one observed in sols of Ru(bpy)2L2+and DDMA+ alone (see above). The more rapidly decaying portion of the bleaching signal at 399 nm is likely due to electron transfer quenching of *Ru(bpy)2L2+by C16v2+,since the luminescence measurements also indicate that quenching occurs in that time
5142 J. Phys. Chem., Vol. 99, No. 14, 1995
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1-
0.03
AA
0.01
-0.01
AA o ' o l o ~ 0.0051
k
-0.005 0
3
2
1
4
5
TIME (Ps)
Figure 4. Time-resolved absorption data for the DDMA+-C16V2+ (open circles) and DDMA+-C16V2+-Sn02 (filled circles) sols obtained at monitoring wavelengths of (a) 399 nm and (b) 602 nm.
-
0.05
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-
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-
AA
I
0.00 380
385
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WAVELENGTH (nm)
Figure 5. Difference absorption spectra of the transients produced in the DDMA+-C16V2+-Sn02 (open circles) and DDMA+-C16V2+diC&'C-Sn02 (filled circles) sols. The two spectra were recorded with delay times (following laser excitation) of 1.6 ps and 150 ns, respectively.
domain. The absorption data clearly show that electron transfer directly from *Ru(bpy)2L2+ to c16v2' produces negligible concentrations of long-lived c16v'+, however, since the absorbance change at 399 nm returns to the base line within 2 ps. Time-resolved absorption data obtained at 602 nm for these two samples described above are shown in Figure 4b. This wavelength was selected because it is the maximum of a second absorption band in the spectra of viologen radicals.I8 The data for both samples show the formation of transients absorbing at 602 nm within the laser pulse, but the transient in the sample with Sn02 (filled circles) is much longer-lived than in the sample without SnOz (open circles). The decay of the 602 nm transient in the sample containing SnO2 parallels that observed at 399 nm after ca. 2 ps. The ratio between the absorbance change at
602 nm and that at 399 nm, 0.42 f 0.02, is about 20% greater than the ratio expected,I9 taking into account the negative contribution to A6 at 399 nm that is made by Ru(bpy)2L3+,but this difference may reflect a broadening of the absorption spectrum of the viologen radical relative to its solution spectrum. The results obtained at 602 nm thus support the assignment of the transient absorbing maximally at 399 nm as being c16v'+.20 The shorter-lived transient absorbing at 602 nm in the sample without SnO2 could be due to *Ru(bpy)2L2+,whose difference absorption spectrum shows a relatively weak absorbance increase at this wavelength (A6 = 900 f 300 M-I cm-I), a n d or the Ru(bpy)2L3+-C~6V'+radical pair. In any case, the fact that the transient in the sample without SnO2 is much shorterlived than in the sample with SnO2 corroborates the fact that the quantum yield of long-lived C16v'+ via direct electron transfer from *Ru(bpy)2L2+is negligible. The supported bilayer system described above had DDMABr as the sole surfactant, so that 59 f 6% of the total population of *Ru(bpy)2L2+was quenched by the SnO2. That percentage increased to 98 f 3% in the mixed-surfactant sol containing c16v+.As in the single-surfactant sol, the formation of Cl6v'+ was evidenced by the appearance of a transient absorbing maximally near 399 nm (Figure 5 , filled circles). The data in Figure 6 compare the kinetic behaviors of the absorbance changes at 399 and 452 nm in the DDMA+-C16V2+-diCsPCSn02 system on two different time scales. In Figure 6a, the rapid rise in absorbance at 399 nm parallels the bleaching at 452 nm, showing that the reduction of c16v2+to cl6v'+and oxidation of *Ru(bpy)2L2+to Ru(bpy)2L3+ are nearly simultaneous within the resolution of the apparatus (ca. 7 ns). The data in Figure 6b, obtained on the longer time scale, show that the two transients decay differently. The absorbance at 399 nm returns to the preflash base line more rapidly than the bleaching at 452 nm, indicating that the oxidation of cl6v'+is more rapid than the reduction of Ru(bpy)2L3+. The faster decay of cl6v'+ is expected since the sample was air-saturated; dissolved 0 2 can oxidize C]6v'+ to c16v2+while being unreactive toward Ru(bpy)2L3+. This reaction leaves an excess of Ru(bpy)2L3+ over C16v" and results in a net bleaching at both 399 and 452 nm after ca. 400 ps (Figure 6b).21 The data in Figure 6 , as well as measurements made at an intermediate time scale (not shown), were analyzed to estimate the rate of recombination between c16v'+and Ru(bpy)2L3+as well as the efficiency of formation of c16v'+ from *Ru(bpy)2L2+. The kinetics of the decay of AA at 452 nm fit reasonably well to the Kohlrausch decay function (eq 3) when the fit is limited to t L 1 ps after the laser pulse. The values of the parameters obtained at AAo = -0.044 f 0.011, t~ = 34 f 8 ps, and p = 0.29 f 0.08. The /3 value is the same, within uncertainty, as that obtained in the mixed-surfactantsol without c16v2+, while the ZK value is larger by an order of magnitude. The efficiency of formation of c16v'+from *Ru(bpy)2L2+ is estimated from the absorbance change at 399 nm as 0.22 f 0.05, when the extinction coefficient of cl6v'+at 399 nm is assumed to be the same as that of methylviologen radical at 395 nm in aqueous solution, 4.2 x 104 M-I cm-'.Is Altematively, the efficiency can be estimated by calculating the yield of Ru(bpy)2L3+from the absorbance change at 452 nm obtained from the fit to eq 3 mentioned above. The efficiency of formation of this long-lived Ru(bpy)2L3+ from *Ru(bpy)2L2+ is 0.35 f 0.16. Assuming that the concentrations of Ru(bpy)2L3+and c16v'+are the same, these calculations point to a value of 0.3 f 0.1 for the efficiency of formation of C16v". The determination of the efficiency of formation of cl6v" is complicated by the fact that c16v2+decomposes during the
J. Phys. Chem., Vol. 99, No. 14, 1995 5143
Photosensitization of Colloidal SnOz
1
0.06
Exc.
Em.
I
i -0.02
1
1 \
J
tJ452nm
WAVELENGTHWTI)
I 0
1 0 0 m 3 0 0 4 w 5 0 0
TIME (ns) 0.048
WAVELENGTH (nm)
0.038
Figure 7. Steady-state excitation and emission spectra of the DDMA+C16V2+-diC8PC-Sn02 sol recorded after the laser flash photolysis experiment. Inset: fluorescence excitation and emission spectra of the 3,4-dihydro- 1,l’-dimethyl-3-oxo-4,4‘-bipyridinium cation in HzO (ref
0.024 0.012
22). 0.0M) -0.012
’
-0.024 -50
I 50
150
250
350
450
TME (PI Figure 6. Time-resolved absorption data for the DDMA+-CI~V”diCgPC-SnOz sol obtained at monitoring wavelengths of 399 nm (filled circles) and 452 nm (open circles). Panels a and b show the data recorded on two different time scales. The absorbance data at 399 and 452 nm primarily reflect the formation and decay of c1,5v’+and Ru(bpy)2L3+, respectively.
flash photolysis experiment. Evidence for decomposition is the fact that the maximal absorbance change at 399 nm induced by laser pulse excitation decreases during the course of the experiment even though the initial concentration of Ru(bpy)2L3+ produced remains constant. Further evidence for degradation is the green luminescence that the sample has under long wavelength ultraviolet (black) light after the flash photolysis experiment; this luminescence is absent in the sample prior to the experiment. Comparison of the ground-state absorption spectra of the sample before and after laser flash photolysis reveals the formation of a new absorption band maximum near 420 nm (data not shown). Figure 7 shows the steady-state excitation and emission spectra obtained after laser flash photolysis. The excitation and emission maxima are near 435 and 535 nm, respectively. These spectra closely resemble those reported for the 3 ,Cdihydro- l,l’-dimethy1-3-0~0-4,4’-bipyridinium cation, which is one of the two oxidation products obtained when methylviologen is oxidized by two electron equivalents in aqueous solution (see Figure 7, inset).22
Discussion Evidence for Supported Bilayers Composed of DDMA’ and DiCaC. The results described above indicate that colloidal SnO2 particles serve as supports for two kinds of surfactants, a cationic one (DDMA+) and a zwitterionic one (diCsPC). Similarly, Esumi et aL4 and Capovilla el aL5 demonstrated that mixed surfactant bilayers form on diverse support materials such as Fe2O3, TiO2, A1203, and laponite clay. In these examples, the surfactant molecules apparently are segregated so that the two monolayers of the bilayer are composed predominantly of
one type of surfactant or the other. Such asymmetric bilayer arrangements exist because the polar headgroup of one of the surfactants has a high affinity for the support while the other surfactant does not. In the present case, the affinity of DDMA’ for the Sn02 particle is largely electrostatic in nature.23 The displacement of K+ by DDMA+ as counterion to the negatively charged particle is driven largely by additional free energy decreases due to self-association of the surfactant molecule^.^^ The diC8PC molecules, being zwitterionic, cannot substitute as counterions and so should have a lower affinity than DDMA+ for the surface of SnO2. The average number of DDMA+ molecules per SnOz particle used in these experiments, 110-120, is about half the value required to produce supported bilayers composed solely of this surfactant.’ The fact that the precipitate obtained with this surfactant-to-particle ratio is dispersible in chloroform is good evidence that the surfactant molecules are adsorbed to the particle and oriented so that their hydrocarbon chains extend away from the surface.I3 The fact that the particles disperse readily in water when diC8PC is included with DDMA’ indicates that the mixed-surfactant system contains supported bilayers, so that polar surfactant headgroups are on the exterior in contact with the aqueous phase. Since the DDMA+ molecules most likely constitute the inner surfactant layer, it follows that the diC8PC molecules constitute the outer layer.24 Figure 8 is an idealized cartoon representation of the asymmetric bilayer. Photosensitized Electron Injection by Ru(bpy)2L2+. The photophysical properties Ru(bpy)2L2+in the DDMA+-diCgPC-Sn02 system are consistent with the asymmetric supported bilayer arrangement described above. The fact that ’95% of the *Ru(bpy)2L2+is quenched by the SnO2 support shows that almost all of these molecules are in contact with the SnOz surface.26 Thus, the distribution of Ru(bpy)2L2+ apparently parallels that of DDMA+ in the sol. This distribution exists despite the fact that the Ru(bpy)2L2+was initially dissolved in the aqueous phase extemal to the surfactant-coatedparticles (see Experimental Section), so that diffusion of the photosensitizer molecules through the bilayer took place prior to the laser flash photolysis measurements. Our previous study of the DDMA’-SnO2 system indicates that redistribution of the Ru(bpy)2L2+ molecules is complete within 5 min of mixing but does not occur with R~(bpy)3~+, which lacks amphiphilic character.’ This
5144 J. Phys. Chem., Vol. 99, No. 14, 1995
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H20
Figure 8. Cartoon representation of the mixed-surfactant bilayer on a colloidal SnOz particle (cross-sectional view).
behavior may reflect a dynamic exchange of the surfactant molecules between the bilayer, micellar, and aqueous phases, but the exact mechanism by which Ru(bpy)2L2+diffuses across the bilayer is yet to be determined. As in our earlier studies,'*9 the kinetic profile of the recombination between Ru(bpy)2L3+and e,b-(SnO2) conforms, to a good approximation, with the Kohlrausch decay function. The two parameters that characterize the decay function, ZK and p, are, within uncertainty, the same in both the DDMA+-Sn02 and DDMA+-diCgPC-Sn02 systems. The absence of a significant unquenched population of *Ru(bpy)2L2+in the latter system facilitates kinetic analysis of the recombination reaction as well as secondary electron transfer reactions such as the reduction of viologens by e,b-(SnOz). Reduction of Hexadecylviologen by e,b-(SnOz). Electron transfer between methylviologen (MV2+) and aqueous colloidal Sn02 was studied by Mulvaney et who showed that the reduction of MV2+ by e,b-(SnO;?) can occur when the pH is above 3.27 Assuming that the reduction potentials of c16v2+ and MV2+ are equivalent, and ignoring effects of bilayer adsorption on the energetics, the free energy change for the reduction of c16v2+by e,b-(SnO2) at the ambient pH of 9-10 is ca. -0.2 eV. The results obtained with both the DMMA+-C16V2+-Sn02 and the DDMA+-C16V2+-diCsPC-Sn02 systems show that SnO2 mediates the reduction of C16V2+ by *Ru(bpy)2L2+via e,b-(SnOz) as intermediate rather than direct electron transfer from *Ru(bpy)2L2+to the viologen. These results are in accord with the free energy considerations above. Reduction of the viologen via direct excitation of the conduction band of SnO2 can be ruled out as a possibility because the particles are transparent toward 532 nm light.I5 The rate of formation of Cl6V'' is 10' s-I or greater in these systems. Similar results were obtained by Frei et a1.,28who studied the photoreduction of MV2+ by phenylfluorone mediated by colloidal TiO2. In contrast, the rates of reduction of MV2+by e,b-(Ti02) in related systems are in the range 102-105 s-l, depending on the nature and concentration of surface complexing agent employed.29 The decay rate of photoproduced Ru(bpy)2L3+is evidently slowed by the inclusion of C16V2+into the supported bilayer, when roughly one-third of the total Ru(bpy)2L3+ population decays with a TK value (eq 3) that is an order of magnitude larger than the value observed without C16V2+. Trapping of e,b-(Sn&) by c,6v2+ competes with the recombination between Ru(bpy)2L3+and e,b-(SnOz) (eq 2). The subsequent reduction of the long-lived population of Ru(bpy)2L3+ back to Ru(bpy)2L2+could occur either by the direct (bimolecular)reaction between C16V'+ and Ru(bpy)2L3+ or else via a thermally
activated process involving electron transfer from C16V'+ back to the conduction band of SnO2; the reduction of Ru(bpy)2L3+ by OH- is expected to be too slow to account for the observed decay.30 Evidence that bimolecular reactions between the viologen and Ru-polypyridine species can occur is provided by the oxidative decomposition of c16v2+. Hexadecylviologen is apparently irreversibly oxidized to a 3-one product similar to the one obtained by the 'OH-induced two-electron oxidation of methylviologen in aqueous solution.22 While the generation of free hydroxyl radical is unlikely in our system, Ru(bpy)2L3+ may be a powerful enough oxidant to effect the oxidation of c16v2+according to Scheme 1, which is based on the mechanism proposed by Bahnemann et a1.22for methylviologen. The one-electron oxidation of c16v2+by Ru(bpy)2L3+together with the addition of OH- (or H20) has the net effect of the addition of *OH to the 3-position of the pyridylium ring to result in the formation of a carbon-centered radical. The subsequent oxidation of this radical by 0 2 leads to the 3-one cation, 3,4-dihydro- 1,l '-dihexadecyl-3-oxo-4,4'bipyridinium ion (Scheme 1).
Conclusion The experiments described above show that asymmetric supported surfactant bilayer colloids are readily prepared from combinations of colloidal SnOz, DDMABr, and dic8Pc. These sols are opalescent and amenable to characterization by standard spectrophotometric techniques. Ru(bpy)2L2+ is incorporated into these sols so that almost all of the molecules are located in the surfactant layer adjacent to the surface of the SnO2 particle. These molecules photosensitize electron injection into the conduction band of SnO2 with a quantum efficiency close to unity, and the conduction band electron can subsequently reduce hexadecylviologen which is also incorporated into the supported bilayer. The capability to photogenerate relatively long-lived oxidizing and reducing species within the inner monolayer of a surfactant bilayer assembly makes this system attractive for investigation of electron transfer reactions across the bilayer. The use of secondary electron acceptors that can be reversibly oxidized and reduced should alleviate the problem of oxidative decomposition in these systems. Experiments with Mn(II1)porphyrins instead of c16v2+are currently in progress.
Acknowledgment. Support for this project was provided by NSF Grant CHE-9208551 and by the Center for Photochemical Sciences at Bowling Green State University. References and Notes (1) Ford, W. E.; Rodgers, M. A. J. J . Phys. Chem. 1994, 98, 7415. (2) Schanze, K. S.; Sauer, K. J . Am. Chem. SOC. 1988, 110, 1180. (3) (a) Momson, S. R. Electrochemistry at Semiconductor and Oxidized Metal Elecrrodes; Plenum: New York, 1980. (b) Schmickler, W.; Schultze, J. W. In Modern Aspects of Electrochemistry; Bockris, J. O'M., Conway, B. E.,White, R. E., Eds.; Plenum: New York, 1986; Vol. 17, pp 357410. (c) Kelsall, G . H.; Gudyanga, F. P. J. Electroanal. Chem. 1990,280,
267. (4) (a) Meguro, K.; Adachi, T.; Fukunishi, R.; Esumi, K. Langmuir 1988, 4 , 1160. (b) Esumi, K.; Sakamoto, Y.; Meguro, K. Colloid Polym. Sci. 1989, 267, 525. (c) Esumi, K.; Sakamoto, Y.; Meguro, K. J . Colloid Inte8ace Sci. 1990, 134, 283.
Photosensitization of Colloidal SnOp (5) Capovilla, L.; Labbe, P.; Reverdy, G. Langmuir 1991, 7, 2000. (6) (a) Calvin, M. J. Membr. Sci. 1987, 33, 137. (b) Hurst, J. K. In Kinetics and Catalysis in Microheterogeneous Systems; Griitzel, M., Kalyanasundaram, K., Eds.; Marcel Dekker: New York, 1991; pp 183226. (c) Lymar, S. V.; P m o n , 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.; ColeHamilton, D. J. Chem. SOC. Rev. 1991, 20, 49. (7) (a) Kuhn, H. J. Photochem. 1979, 10, 111. (b) Arden, W.; Fromherz, P. J . Electrochem. SOC. 1980, 127, 370. (c) Mobius, D. In Kinetics of Nonhomogeneous Processes; Freeman, G. R., Ed.; John Wiley & Sons: New York, 1987; pp 533-573. (d) Ulman, A. An Zntroduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (8) Information about the micellar properties of short-chain phosphatidylcholines can be found in the following references and references cited therein: (a) Bian, J.; Roberts, M. F. J. Colloid Interfnce Sci. 1992, 153, 420. (b) Greathouse, D. V.; Hinton, J. F.; Kim, K. S.; Koeppe, R. E., 11. Biochemistry 1994, 33, 4291. (9) Ford, W. E.; Rodgers, M. A. J. J . Phys. Chem. 1994, 98, 3822. (10) Ford, W. E.; Rodgers, M. A. J. J . Phys. Chem. 1992, 96, 2917. (1 1) Okuno, Y. Laboratory of Chemical Biodynamics Quarterly Report No. 53; Lawrence Berkeley Laboratory, University of Califomia: Berkeley, 1976; pp 142-151. (12) Yoshimura, A,; Hoffman, M. Z.; Sun, H. J . Photochem. Photobiol. A: Chem. 1993, 70, 29. (13) (a) Okahata, Y.; Shimizu, A. Langmuir 1989, 5, 954. (b) Hu, N.; Rusling, J. F. Anal. Chem. 1991, 63, 2163. (14) Kerker, M. The Scattering of Light and Other Electromagnetic Radiation; Academic Press: New York, 1969; pp 325-328. (15) (a) Mulvaney, P.; Grieser, F.; Meisel, D. Langmuir 1990, 6, 567. (b) Bedja, I.; Hotchandani, S.; Kamat, P. V. J . Phys. Chem. 1994, 98, 4133. (16) (a) Plonka, A. Time-DependentReactivity of Species in Condensed Media; Springer-Verlag: Berlin, 1986. (b) Drake, J. M.; Klafter, J.; Levitz, P. Science 1991,251, 1574. (c) Scher, H.; Slesinger, M. F.; Bendler, J. T. Phys. Today 1991, 44, 26. (17) Nonlinear regression using the Marquardt-Levenberg algorithm was performed with SigmaPlot (Jandel Corp.). (18) Watanabe, T.; Honda, K. J . Phys. Chem. 1982, 86, 2617. (19) As a first approximation, the absorption spectrum of C I ~ V ' +can be assumed to be the same as that of the methylviologen radical in water,18 whose values of E at 395,452, and 602 nm are 4.2 x lo4, 800, and 1.3 x 104 M-] cm-I, respectively. The oxidation of Ru(bpy)2L2+to Ru(bpy)2L3+
J. Phys. Chem., Vol. 99, No. 14, 1995 5145 results in AE'S of -0.3 x 104 and -1.4 x lo4 M-' cm-I at 399 and 452 nm, respectively; the AE value at 602 nm is negligibly small.' (20) Electron injection into Sn02 particles can also result in absorbance increases at these wavelength^,'^^ but the difference absorption spectrum that we observe upon electron injection from *Ru(bpy)2L2+ (eq 1) is dominated by changes due to the oxidation of Ru(bpy)ZL*+. This spectrum is practically identical to the one published previously for a silylated analogue of Ru(bpy)2L2+ (see Figure 8 in ref 9) and shows a net bleaching at 399 nm. (21) 0 2 removal by AI bubbling of a SnO2 sol containing Ru(bpy)2L2+ (photosensitizer) and 1-dodecyl- 1'-methyl-4,4'-bipyridinium2+ (viologen) has an imperceptible effect on the yield and decay of the viologen radical that is produced. Ar bubbling of the sols containing surfactant is problematic because of foaming. Although altemative methods of 0 2 removal are possible, such attempts did not seem to be necessary in view of the lack of reactivity of e,b-(Sn02)9 and the viologen radical in the above experiment toward dissolved oxygen. (22) Bahnemann, D. E.; Fischer, C.-H.; Janata, E.; Heinglein, A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2559. (23) (a) Huo, Q.;Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schuth, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (b) Wangnerud, P.; Jonsson, B.; Langmuir 1994, 10, 3268. (24) The critical micelle concentrations of DDMABrlS and diC8PCSa are 0.16 and 0.2 mM, respectively, so that the concentrations of surfactant molecules dissolved in the bulk aqueous phase may be appreciable. These concentrations are relatively small compared to the overall values of 1.51.6 mM for DDMABr and 3.6-4.4 mM for diCsPC, however. (25) Soderlind, E.; Bjorling, M.; Stilbs, P. Langmuir 1994, 10, 890. (26) The model described in ref 23b suggests that a water layer having a thickness of ca. 3 8, may exist between the Sn02 surface and the adsorbed bilayer. (27) Mulvaney et al.I5" concluded on the basis of radiolysis experiments that trapping of electrons as Sn3+ centers within the lattice of colloidal SnO2 particles is minimal, whereas Bedja et al.'5bfound evidence for such traps but concluded that they are shallow. (28) Frei, H.; Fitzmaurice, D. J.; Gratzel, M. Langmuir 1990, 6, 198. (29) Moser, J.; Punchihewa, S.; Infelta, P. P.; Gratzel, M. Langmuir 1991, 7, 3012. (30) Ghosh, P. K.; Brunschwig, B. S.; Chou, M.; Creutz, C.; Sutin, N. J . Am. Chem. SOC.1984, 106, 4772. JP9427 138
