J. Phys. Chem. 1994, 98, 13058-13063
13058
An Examination of the Role of BzO3 Lewis Acid Sites in Electron Transport on Porous Vycor Glass Jianwei Fan Department of Chemistry and Biochemistry, Manhattan College, Riverdale, New York 10471
Harry D. Gafney" Department of Chemistry and Biochemistry, City University of New York, Queens College, Flushing, New York I1367 Received: January 21, 1994; In Final Form: August 17, 1994@
Photoinduced disproportionation of Ru(bpy)?+ on porous Vycor glass occurs within a fixed array of reactants by means of a mobile, photodetached electron. The absence of emission quenching when Ru(bpy)? cation exchanges onto the glass and the band gap of SiOz, the principal constituent of the glass, preclude direct electron injection into a conduction band. Instead, conduction of the photodetached electron is thought to occur via surface conduction, where the photodetached electron populates intermediate surface acceptor sites. These are shallow energy wells from which the electron can be thermally activated, but the specific identity of the site is not presently known. One possibility is the BzO3 Lewis acid sites present in the glass, and data presented here show that selectively neutralizing these BzO3 sites with chemisorbed N H 3 changes the quantum efficiency of disproportionation. The same chemistry is shown to occur in the presence or absence of chemisorbed NH3, but a surprising increase in the quantum efficiency of R ~ ( b p y ) 3 ~disproportionation + in the presence of chemisorbed NH3 indicates that the B2O3 Lewis acid sites are not involved in electron transport between the immobilized ions.
Introduction In the photoredox chemistry of Ru(II) diimines, heterogeneous media are being examined as a means of curtailing thermal reversibility and converting the one-photon-one-electron events into multielectron-transfer Semiconductive metal oxides are thought to improve charge separation by electron injection into the conduction band of the semiconductor and band bending at its Supports that are not usually thought of as electron conductors also improve charge seperation. Mallouk and co-workers have shown that zeolite L particles spontaneously organize a molecular triad, resulting in a significant improvement in the lifetime of the charge-separated Thomas and co-workers report that incorporating Ru( b p ~ ) 3 ~in+ silica colloids enhances electron transfer between the complex and methylviologen (MV2+), while photolysis of Ru(bpy)3*+ adsorbed onto dry cellulose leads to disproportiona t i ~ n . ~A . ' ~spontaneous partitioning of R~(bpy)3~+ and MV2+ in porous Vycor glass (PVG) results in a pronounced increase in the lifetime of the photoproducts in the boundary region between the partitioned adsorbate^.^,'^ In the photoinduced disproportionation of R~(bpy)3~+ in PVG, charge separation occurs even though the driving force for the thermal backreaction,
is the largest that will be encountered in a Ru(bp~)3~+ photoinduced electron t r a n ~ f e r . ~ . ' ~ In the latter case, emission polarization and macroscopic mobility measurements indicate that photoinduced disproportionation occurs within a fixed m a y of reactants immobilized on the glass surface!J6 Flash photolysis reveals a photodetached electron intermediate, and the quantum yield of [Ru(bpy)z(bpy-)]+ formation, (PR,,~),initially increases with
* Abstract published in Advance ACS Abstracts, November
15, 1994.
Ru(bpy)32+ loading, reaches a miximum, and then declines! @R,,(I) maximizes at a loading corresponding to a mean separation between the Ru(bp~)3~+ reactions of 50 f 10 A and approaches zero at loadings corresponding to mean adsorbant spacings of 2200 and 1 1 3 A, respectively. The separation corresponding to the maximum yield agrees with electron migration distances on PVG,17,18while the adsorbate spacing at higher loading, 5 13 A, correspondings to a contact interaction, Le., monolayer coverage? These data suggest that charge separation in this glass is a consequence of the immobility of the reactants and their distribution on the glass surface. Electron transfer occurs when the redox partner is within the electron migration, ca. 50 A, and the redox products are stable when the distance between them exceeds that for the thermal backreaction, I 1 3 A.4J5J6 The absence of emission quenching when Ru(bp~)3~+ cation exchanges onto the glass and the band gap of SiOz, the principal constituent of the glass, preclude direction electron injection into a conduction band?,16 Instead, two-photon excitation leads to ionization, and conduction of the photodetached electron on this glass is thought to occur via surface conduction where the photodetached electron populates intermediate surface acceptor sites!J6 The temperature dependence of @p,,,(~) suggests that these acceptor sites are shallow energy wells, 16.78 f 0.11 kcaymol, from which the electron can be thermally activated, but nevertheless present an energy barrier to immediate recombination.16 One possibility is the BzO3 Lewis acid sites in the glass. These sites, which exist throughout the pore structure of the glass, are a consequence of the method of manufacture of the glass, Le., acid leaching of a cooled, phase-separated 96% SiOz, 3% B203, and 1% NazO and A1203 melt.15 Although the amount of boron remaining in the leached glass is relatively small, 2.6 f 0.1% in the first 50 A, the majority exists as B203 sites on the pore surface, where R~(bpy)3~+ is adsorbed and electron transfer occurs. Being a Lewis acid, B2O3 could act as an electron acceptor but, in forming a radical species, would remain sufficiently reactive to liberate the electron on thermal
0022-365419412098-13058$04.50/0 0 1994 American Chemical Society
Role of B2O3 Lewis Acid Sites in Electron Transport activation. At least in theory, these sites could act as a surface acceptor site and promote electron conduction on the glass surface. These experiments were initiated to explore the role of these sites in the photoinduced disproportionation of R~(bpy)3~+(ads) (ads designates the adsorbed complex). This reaction proceeds via a mobile, photodetached electron? and as such, its quantum efficiency reflects the mechanism of electron transport on the glass surface. Rather than vary the amount of boron in the glass, which is accompanied by changes in glass morphology, surface area, and adsorbate spacing, we have taken advantage of the difference in the adsorption of NH3 to selectively neutralize the B203 sites.19 Data presented here show that the mechanism by which the disproportionation of R~(bpy)3~+(ads) occurs is unaffected by the presence or absence of coadsorbed NH3. However, neutralizing the electron-accepting properties of the B2O3 sites leads to a significant improvement in charge separation in this glass.
Experimental Section Materials. [Ru(bpy)s]C12 was prepared by the method of Palmer and Pipesoand twice recrystallized from distilled water. FTIR spectra of the crystalline complex and absorption, emission, and resonance Raman spectra of aqueous solutions of the complex agreed with published spectra.21-26 Tetramethyl orthosilicate (TMOS)(95%, Aldrich), methanol (spectroscopic grade, Seelze-Hanover), ammonium hydroxide (reagent grade, J. T. Baker), and gaseous NH3 (199%, Linde) were used as received. Code 7930 porous Vycor glass (Coming) in the form of 25 x 25 x 2 mm3 polished plates was extracted and calcined according to previously described procedures!.27 Pieces of PVG were crushed, and the 250-1000 p m diameter particles used in these experiments were isolated with standard sieves. The glass particles were calcined at 650 "C for at least 72 h prior to use. All PVG samples were stored at 550 "C until needed, at which point the sample was cooled to room temperature under vacuum and impregnated. Based-catalyzed TMOS/CH30H/H20 xerogels were prepared by previously described procedures28and cast on Hg to obtain an optically flat surface. After drying in air at room temperature, the xerogels were calcined at 550 "C for 24 h and stored under vacuum until needed. Sample Preparations. The PVG plates and the TMOS/CH3OHM20 xerogels were impregnated by previous described procedures where the moles adsorbed was calculated from the change in absorbance of the impregnating solution!*29 Samples containing from to mol of R~(bpy)3~+ adsorbedg of glass were obtained by a single exposure to the impregnating solution. Loadings of z mol/g required multiple exposures, in which case the moles of R~(bpy)3~+ adsorbed was determined spectrally after each exposure to the impregnating solution. The sample was then dried at room temperature under vacuum, and the impregnation process was repeated until the desired loading was achieved. Powdered PVG was impregnated by adding a weighed amount of glass to a minimum amount of a Ru(bp~)3~+ solution. The mixture was kept in a covered flask until the solution turned colorless. At this point, the solvent was removed under vacuum, and the loading was calculated from the initial concentration of Ru(bpy)3*+ solution. All experiments were performed on dry samples where 199.99% of the water incorporated during impregnation was removed under vacuum (v I Torr) at room temperature?
J. Phys. Chem., Vol. 98, No. 49, 1994 13059
Photochemical Procedures. PVG plates impregnated with R~(bpy)3~+ were mounted in previous described quartz or Pyrex cells."J6 All samples were evacuated (v I Torr) and irradiated either under vacuum or in the presence of coadsorbed NH3. In the latter case, the impregnated powdered or plate samples were equilibrated with NH3 for 30 min and then maintained under a dynamic vacuum (p 1 Torr) at room temperature for at least 4 h to remove the physisorbed NH3 (see below). The samples were irradiated with 457.9 nm light from a Spectra-Physics Model 164-08 Ar+ laser, and the intensity incident on the sample, 0.1 W, was measured with a Coherent Model 210 power meter. The absorbed intensity was calculated from the incident intensity and the average Ru( b p ~ ) 3 ~absorbance. + The rate of photoinduced disproportionation was determined spectroscopically by monitoring the rate of [Ru(bpy)2(bpy-)]+ formation at 510 nm. The moles of product formed was calculated by previously described procedures, and the reported quantum yields, @R,,o), were calculated from the initial rates of [Ru(bpy)2(bpy+)]+ f ~ r m a t i o n . ~ ,Relative '~ values of @R,,(I), measured in the presence and absence of chemisorbed NH3 as a function of initial Ru(bpy)32+(ads)loadings, were calculated according to the equation
where the subscripts 1 and 2 designate the reference and the specific sample, respectively. d A d d t represents the initial rate of [Ru(bpy)2(bpy-)]+ formation, measured at 510 nm, while 1 represents the fraction of the incident intensity absorbed by R~(bpy)3~+(ads) in the specific sample. All photolyses were limited to 120%conversion, and the absorbance of R~(bpy)3~+ (ads) at the excitation wavelength, A, was taken as the average of the initial and final absorbance. The reported quantum yields for different loadings are the average of at least three independent measurements under the specified conditions. Physical Measurements. UV-visible spectra were recorded on a Aviv Model 14DS spectrophotometer relative to the untreated glass. Emission spectra were recorded on a previously described Perkin-Elmer Hitachi MPF-2A emission spectrophot ~ m e t e r . ~ .Diffuse '~ reflectance FTIR (DRIFT) spectra were recorded on a Nicolet 5/20 DX FTlR equipped with a Harrick Scientific diffuse reflectance accessory. Blank or impregnant powdered PVG samples (particle diameter 1 2 5 0 mm) diluted 1:30 with KBr were placed in the sample chamber of the DRIFT accessory. The sample chamber was connected to an external vacuum line by two lines. One was used to evacuate the sample while the other was used to transfer NH3 to the sample. Unless otherwise specified, all measurements were carried out with samples at room temperature. The general procedure was to evacuate the sample to 5 Torr, record an initial spectrum, and then expose the sample to 1 atm of NH3. After reaching equilibrium, which occurs in a few minutes at room temperature, the sample was maintained under a dynamic vacuum (p I Torr),and spectra were recorded as a function of time to monitor NH3 desorption. EPR spectra of the photoproducts were recorded on a IBMBruker 2oOE-SRC spectrometer. Powdered PVG samples impregnated with R~(bpy)3~+ were placed in previously described EPR cells and evacuated! As in the optical experiments, the samples were evacuated and then either irradiated in vacuo or equilibrated with a given pressure of NH3, evacuated to remove the gaseous and physisorbed NH3, and irradiated in the presence of chemisorbed NH3. All samples were photolyzed
13060 J. Phys. Chem., Vol. 98, No. 49, 1994
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Fan and Gafney
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3333
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3722
3366
3544
3189
24
s o 0
3722
3900
3544
3366
3189
Wmanmbex(cm- 1)
Figure 1. (a) FTIR spectra of a calcined PVG (i) in vacuo and (ii) exposure to 1 atm of N H 3 . (b) FTIR spectra recorded during the desorption of NH3 from the PVG: (i) 10 min, (ii)35 min, (iii) 1.5 h, (iv) 2 h, (v) 3 h, and (vi) 16 h of evacuation of a PVG sample which has been exposed to NH3 for 30 min.
in the cavity of the EPR with a 350 W high-pressure Hg arc lamp (Illumination Industries). The incident light (2300 nm) was passed through a water-filled 12 cm quartz cell to remove IR radiation, and spectra were recorded as a function of photolysis time. EPR experiments were carried out at room temperature, and spectra of samples irradiated in the presence of coadsorbed NH3 were referenced to spectra obtained from identical experiments carried out in the absence of NH3.4 Emission lifetimes in the presence and absence of NH3 were determined by previously described procedures. l6 All samples were excited with 355 nm harmonic (7 ns fwhm, 0.01 mJ/ pulse), and the emission intensity from the front surface of the sample was monitored at 600 nm.
Results N H 3 Adsorption. Exposing PVG to NH3 (1 atm) results in a loss of the free and associated SiO-H bands at 3744 and 3656 cm-' accompanied by appearance of a series of bands in the 3400-3200 cm-' N-H region (Figure la). Evacuating the sample at room temperature causes an immediate loss of the 3333 cm-' band, due to gas phase NH3 in the sample chamber, followed by a slow decline in the intensities of the 3320 and 3400 cm-' bands (Figure lb). However, no detectable change occurs in the intensities of the 3365 and 3280 cm-' bands. The decline in the 3320 and 3400 cm-' bands occurs with the reappearance of the 3744 cm-' silanol band. After 16 h of pumping (p 5 Torr) at room temperature, the bands at 3320 and 3400 cm-' have disappeared, and the spectrum (Figure lb) consists of an intense silanol band at 3744 cm-' and bands at 3365 and 3280 cm-'. Heating the sample under vacuum to 150 "C does not change the spectrum, but at 200 O C , the 3365 and 3280 cm-I bands disappear. Unlike the decline in the 3320 and 3400 cm-' bands, however, loss of the 19,28v30
L
3122
3544
3366
3189
Wawa.r).r(o 1)
Figure 2. (a) FTIR spectra of a calcined xerogel (i) in vacuo and (ii) exposure to 1 atm of NH3. (b) FTIR spectra recorded during the desorption of NH3 from the xerogel: (i) 30 min, (ii) 1 h, (iii) 3 h, and (iv) 16 h, of evacuation of a xerogel sample which has been exposed to NH3 for 30 min.
3365 and 3280 cm-' bands does not occur with a concurrent increase in the intensity of the 3744 cm-' silanol band. Identical experiments were carried out with base-catalyzed TMOS/CH30WH20 xerogels to further probe the mechanism of NH3 adsorption. Like PVG, calcined, base-catalyzed TMOS/ CH3O/H2O xerogels are nodular, porous glasses that possess surface SiOH Bronsted acid sites. Unlike PVG, however, xerogels do not process B2O3 Lewis acid sites. Consequently, the xerogels offers a means of further distinguishing the IR bands indicative of NH3 adsorption onto B2O3 sites from adsorption onto the surface silanol groups. As with PVG, exposing the calcined xerogel to NH3 (1 atm) results in an immediate loss of the 3740 cm-l SiO-H band and the appearance of bands at 3400 and 3333 cm-' (Figure 2a). Removing gas phase NH3 under vacuum eliminates the 3333 cm-' band and reveals bands at 3400 and 3320 cm-' (Figure 2b). The latter are identical to those found with PVG, but the 3365 and 3280 cm-' bands found in the NH3-PVG spectra (Figure 1) are not present in the NH3-xerogel spectra (Figure 2). Pumping at room temperature 0, I Torr) causes the 3400 and 3320 cm-' bands to disappear with a concurrent increase in the intensity of the 3740 cm-' SiO-H band (Figure 2b). The data fully agree with the original assignment of Cant and Little, who assigned the 3400 and 3320 cm-' bands to NH3 adsorbed onto the silanol groups and the 3365 and 3280 cm-' bands to NH3 adsorbed onto the Bz03 Lewis acid sites.lg. On both glasses, the adsorption and desorption of NH3 are unaffected by the presence of R~(bpy)3~+(ads). Exposing PVG samples containing from 1.2 x to 3.8 x mol of Ru(bp~)3~+(ads)/g to 1 atm of NH3 leads to spectral changes identical to those found in the absence of the complex. N-H bands at 3400,3365,3320, and 3280 cm-' appear accompanied by a decline in intensity in the SiO-H region. Although the
J. Phys. Chem., Vol. 98, No. 49, 1994 13061
Role of BzO3 Lewis Acid Sites in Electron Transport I
300
I
350
I
I
I
400
450
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550
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600
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650
1
700
N.nometar(nm)
Figure 3. W-visible spectra recorded (a) before and (b) after the sample in the presence of chemisorbed photolysis of a R~(bpy)3~+(ads) = 457.9 nm, P = 0.1 W. NH3.
loadings examined in these experiments correspond to Ru( b p ~ ) 3 ~surface + coverages of z(bpy-)]+(ads)formation. EPR spectra recorded during a 2300 nm photolysis of powdered PVG containing 3 x mol of R~(bpy)3~+(ads)/g in vacuo show the growth of an EPR resonance with g = 2.01 and a peak-to-peak line width of 25 f 2 g. The spectrum is identical to that previously found4 and, consistent with the increase in absorbance at 510 nm, is assigned to the formation of [Ru(bpy)~(bpy-)]+(ads). Equivalent results occur in the presence of coadsorbed NH3. In these experiments, samples containing Ru(bpy)?+(ads) were exposed to 1 atm of NH3 for 30 min and then evacuated (p 5 Torr) at room temperature for ca. 4 h to remove gas phase and physisorbed NH3 prior to photolysis. A 457.9 nm photolysis of R~(bpy)3~+(ads) in the presence of chemisorbed N& leads to spectral changes equivalent to those in the absence of NH3 (Figure 3). The decline in absorbance at 452 nm, indicative of R~(bpy)3~+(ads) consumption, is accompanied by an increase in absorbance at 5 10 nm, indicative of [Ru(bpy)~(bpy-)]+(ads) formation. EPR spectra, which show the growth of a resonance with g = 2.01 f 0.01 and a peak-to-peak line width of 25 f 2 G (Figure 4), are identical to those recorded during photolysis in the absence of coadsorbed NH3. Using the molar extinction coefficients for R~(bpy)3~+(ads), E = 1.49 x lo4 M-' cm-' at 450 nm, and for [Ru(bpy)2(bpy-)]+, E = 1.2 x 10" M-' cm-I at 510 nm, the UV-visible changes yield a reaction stoichi-
3300
J4w
3500
3600
Magnetic field(G)
Figure 4. EPR signal generated during the photolysis of a Ru(bpy)3*+(ads) sample in the presence of chemisorbed NH3 with a 350 W Hg lamp. Microwave power = 2 mW, microwave frequency = 9.70 GHz, and gain = 8 x lo5.
ometry of 2.2 f 0.8 mol of R~(bpy3~+(ads) consumed/mol of [Ru(bpy>z(bpy-)]+(ads)formed in the presence of chemisorbed NH3 (Figure 3) and 2.1 & 0.6 in the absence of NH3, i.e., in vacuo. The latter values are in excellent agreement between themselves and in excellent agreement with the previously determined value of 2.1 f 0.2 in V ~ C U O . Although ~ the same chemistry occurs in the presence and absence of NH3, the quantum efficiencies of the reaction are quite different. Values of &(I) as a function of initial Ru(bpyh*+(ads) loading and in the presence and absence of NH3 are presented in Figure 5. All photolyses were carried out with 457.9 nm light at an incident power of 0.1 W, and relative values of @ R ~ ( I ) were calculated from the initial rates of [Ru(bpy)z(bpy-)l+ formation according to eq 2. In vacuo, as Ru(bpyhZ+(ads) loading increases from 9.4 x lo-* moVg, @R,,(I) increases and moVg and then reaches a maximum value at 2.09 x declines as initial loading increases to 1.13 x moVg. The normalized values of @ R ~ ( I )determined in these experiments (Figure 5) are in excellent agreement with previous measurements? In both series of experiments, the maximum value occurs at a loading of (2.0 f 0.1) x moVg, and @RU(I) declines with higher and lower loadings. Carrying out the experiments in the presence of coadsorbed NH3 does not change the dependence on initial loading. @R~(I). initially increases with increasing loading, reaches a maximum value at (1J x 0.1) x mol of Ru(bp~)3~+(ads)/g, and then declines at higher loadings (Figure 5). However, the relative values of @ R U ~ )measured in the presence of coadsorbed NH3 are ca. 2.5 times larger than those measured in the absence of NH3.
Discussion PVG is obtained by cooling a borosilicate melt below its phase transition temperature. l5 The boron oxide-alkali oxide
Fan and Gafney
13062 J. Phys. Chem., Vol. 98, No. 49, 1994
Ij 0.8
Log(mo1eslgm)
Figure 5. Dependence of @R"(Q on the moles of Ru(bpy)&ads)/g: (0)in vacuo, (A)in the presence of chemisorbed NH3, and (0)in vacuo. from ref 4.
phase separates and, on acid leaching, yields a transparent, microporous glass. SEM and "EM reveal a nodular material, and BET measurements yield a surface area of 183 f 15 m2/g and a pore diameter of 100 & 13 A.28 Although acid leaching removes the majority of the boron to create this random array of interconnected pores, XPS shows that 2.6 f 0.1% boron remains in the first 50 8, of the glass.28 Assuming the same distribution of B throughout the glass, and a structure equivalent to that of B2O3 with a B-0 bond length of 1.3 8,, the calculated surface coverages indicate that boron oxides make up 20-35% of the surface. This is clearly an approximation, but it illustrates the surprisingly high surface coverage achieved with what, a priori, appears to be a relatively small amount of boron. The calculated availability of these sites and their Lewis acid properties led to these experiments examining the role of the B2O3 sites in the photoinduced disproportionation of R~(bpy)3~+. On the basis of IR data and differences in the desorption temperatures, Cant and Little proposed that NH3 physisorbs onto PVG by hydrogen bonding to the silanol groups and chemisorbs by reacting with the B2O3 Lewis acid sites to from the B2O3NH3 donor-acceptor complex.l9 Physisorbed NH3 desorbs under vacuum at room temperature, whereas chemisorbed NH3 requires temperatures of 200 O C . DRIFT spectra recorded in our experiments agree exactly with these assignments. Exposing PVG to 1 atm of NH3 (Figure la) results in an immediate loss of the 3744 cm-' SO-H band and the concurrent appearance a series of bands in the 3400-3200 cm-' region. After removal of the gas phase NH3, further pumping at room temperature reduces the intensities of the 3320 and 3400 cm-' bands with the concurrent reappearance of the 3744 cm-' SiO-N. Consistent with Cant and Little's original assignment^,'^ the 3320 an 3400 cm-' bands are assigned to physisorbed NH3, since these bands decline under vacuum at room temperature, and the decline in their intensity is accompanied by a corresponding increase in the intensity of the 3744 cm-' SiO-H band. Physisorption occurs via hydrogen bonding to the silanol groups,
and the changes in these N-H bands occur with concurrent, but opposite, changes in the intensity of the 3744 cm-' SiO-H band. The bands at 3365 and 3280 cm-' are assigned to chemisorbed NH3 where NH3 adsorbs via the formation of a NH3B2O3 donor-acceptor complex. The intensities of these bands are unaffected by pumping at room temperature. Instead, desorption requires higher temperatures, and the loss of the 3365 and 3280 cm-' bands is not accompanied by a concurrent increase in the SiO-H bands (Figure lb). Additional support for these assignments occurs in the experiments with TMOS/ CH3OH/J&O xerogels. Although more random with respect to the range of pore sizes, morphologically, base catalyzed, calcined TMOS/CH30H/H20 xerogels are similar to PVG28and, like PVG, exhibit an intense SiO-H band at 3740 cm-' (Figure 2a) indicative of surface silanol groups. Exposure to NH3 leads to the appearance of N-H bands at 3400 and 3320 cm-' accompanied by a decline in the intensity of the 3740 cm-l SiO-H band (Figure 2a). The spectral change, which is identical to that obtained with PVG, is reversible under vacuum at room temperature and assigned to physisorption where NH3 hydrogen bonds to the surface silanol groups. Consistent with the absence of boron in the xerogels, the 3365 and 3280 cm-l bands assigned to chemisorbed NH3 in PVG are not present in the xerogel spectra (Figure 2b). R~(bpy)3~+ cation exchanges onto these glasses by displacement of the slightly acidic silanol protons and, with the exception of a decline in the relative intensity of the bipyridine-localized, 286 nm x-x* transition, retains the spectral properties fround in aqueous solution. NH3 adsorption and desorption occur without a change in complex's spectrum even at the higher temperature, 80 "C, required for desorption of chemisorbed NH3. Consequently, the differences in the desorption of physi- and chemisorbed NH3 can be exploited to selectively neutralize the B2O3 sites in the presence of the adsorbed complex and, in turn, examine the role of these sites in electron transfer. A 457.9 nm photolysis of R~(bpy)3~+(ads) leads to disproportionation,4
-
2R~(bpy),~+(ads)
Identical chemistry occurs in the presence and absence of coadsorbed NH3. Photolysis of R~(bpy)3~+(ads) in vacuo leads to the appearance of an EPR resonance, g = 2.01, and a peakto-peak line width of 25 f 2 G that increases with increasing irradiation time. The latter is equivalent to that previously found4 and is assigned to the formation of [Ru(bpy)2(bpy-)l+(ads). The decline in the 452 nm absorption and corresponding increase at 510 nm yield a stoichiometry of 2.1 f0.6 mol of R~(bpy)3~+(ads) consumedmol of [Ru(bpy)z(bpy-)]+(ads)formed, which is in excellent agreement with the previously determined value of 2.1 f 0.2.4 Identical changes occur in the presence of chemisorbed NH3. EPR and W-visible c o n f i i the formation of [Ru(bpy)2(bpy-)]+(ads), and stoichiometry measurements yield 2.2 f 0.8 mol of R~(bpy)3~+(ads) consumed/mol of [Ru(bpy)2(bpy-)]+(ads) formed in the presence of coadsorbed N H 3 . Chemisorbed NH3 does not change the chemistry or the lifetime of the photoredox properties, but it does change the efficiency of the reaction. Current data indicate that disproportionation in PVG occurs within a fixed array of immobilized adsorbates by means of a photodetached The dependence of @ R ~ ( I ) on the moles of R~(bpy)3~+(ads)/g obtained in this series of experi-
Role of B2O3 Lewis Acid Sites in Electron Transport ments is in excellent agreement with past measurements (Figure 5 ) and establishes the consistency of the data in different samples of PVG. In vacuo, both sets of data show that @R,,(o maximizes at a loading of (2.0 f 0.1) x moVg and declines with lower and higher loadings. In the presence of chemisorbed NH3, the maximum occurs at 1.8 x mol of Ru(bpy)3*+(ads)/g and also declines with lower and higher loadings. Although the same dependences is obtained in the presence of coadsorbed NH3 (Figure 5 ) , the individual values of @R,,(I) in the presence of coadsorbed NH3 are ca. 2.5 times larger than those in the absence of NH3. If the B2O3 Lewis acid sites are involved in the migration of the photodetached electron, then neutralizing their electron-accepting properties is expected to either reduce @~,,(l)or at least bias the observed dependence on the initial R~(bpy)3~+(ads) loading. Neither occurs. Chemisorption of NH3 and formation of a NH3-B203 donor-acceptor complex increase the quantum efficiency of disproportionation uniformly at all loadings. This implies that the B203 Lewis acid sites are not involved in charge separation. Charge separation on this glass occurs when the mean separation between the redox partners is within the electron migration distance but exceeds that required for the thermal back-reaction. Within this model, at low loadings, 1 2 x moVg, @R,,(I) is principally determined by electron migration distance, whereas at high loadings, 2 2 x moVg, @R,,(I) is principally determined by the thermally activated back-reaction. Assuming that the quantum efficiency of photoionization of R~(bpy)3~+(ads) is unaffected by coadsorbed NH3, since its absorption and emission spectra are unaffected, the absence of a bias in Figure 5 that is attributable to chemisorbed NH3 implies that the B2O3 sites are not involved in either the conduction of the photodetached electron or the thermal back-reaction.
Conclusion Although estimates of surface coverage based on the amount of boron in PVG suggest that between 20% and 35% of the surface could be BzO3 Lewis acid sites, selective neutralization of these sites increases the quantum efficiency of disproportionation in PVG. Consistent with the observation that photoinduced disproportionation of Ru(bp~)3~+(ads) occurs in xerogels and other Si-based supports that do not possess B2O3 Lewis acid sites, these sites are not involved in charge separation in PVG. ‘Although modeling charge separation in PVG indicates a need for a surface acceptor sites, the data gathered here indicate that the BzO3 Lewis acid sites are, in fact, detrimental to charge separation.
J. Phys. Chem., Vol. 98, No. 49, 1994 13063 Acknowledgment. Support of this research by the Research Foundation of the City University of New York and the National Science Foundation (CHE-8913496) is gratefully acknowledged. H.D.G. also thanks Corning Inc. for samples of porous Vycor glass. References and Notes (1) Willner, I.; Degani, Y. J . Am. Chem. SOC. 1983, 105, 6228. (2) Willner, I.; Otvos, J. W.; Calvin, M. J. J. Am. Chem. Soc. 1981, 103, 3203. (3) Milosavijevic, B. H.; Thomas,J. K. J . Phys. Chem. 1983,87,616. (4) Kennelly, T.; Gafney, H. D.; Braun, M. J. Am. Chem. SOC. 1985, 107, 4431. (5) Shi,W.; Gafney, H. D. J. Am. Chem. SOC. 1987, 109, 1582. (6) Vermeulen, L. A.; Thompsom, M. E. Nature 1992, 358, 656. (7) Slama-Schwok, A.; Ottolenghi, M.; Avnir, D. Nature 1992, 355, 656. (8) Gratzel, M. Acc. Chem. Res. 1981, 14, 376. (9) Vlachopoulos, N.; Liska, P.; Augustynski, J.; Gratzel, M. J . A. Chem. SOC. 1988, 110, 1216. (IO) Lehn,J. M.; Sauvage, J. P.; Ziessel, R. Nouv. J. Chim. 1980, 4 , 623. (11) Novak, A. J. Photochemical Conversion and Storage of Solar Energy; Connolly, J. S . , Ed.; Academic Press: New York, 1981; p 271. Appl. Phys. Lett. 1977, 30, 567. (12) Krueger, J. S.; Mayer, J. E.; Mallouk, T. E. J. Am. Chem. SOC. 1988, 110, 8232. (13) Li, Z.; Lai, C.; Mallouk, T. E. lnorg. Chem. 1989, 28, 178. (14) Wheeler, J.; Thomas, J. K. J . Phys. Chem. 1982, 86, 4540. (15) Gafney, H. D. Coor. Chem. Rev. 1990, 104, 113. (16) Fan, J.; Shi, W.; Tysoe, S.; Strekas, T. C. Gafney, H. D. J. Phys. Chem. 1989, 93, 373. (17) Wong, P. K. Photochem. Photobiol. 1974, 19, 391. (18) Wong, P. K.; Allen, A. D. J. Phys. Chem. 1970, 74, 774. (19) Cant, N. W.; Little, L. H. Can. J. Chem. 1964,42, 802; 1965, 43, 1252. (20) Palmer, R. A.; Piper, T. S. lnorg. Chem. 1966, 5, 864. (21) Mallick, P. K.; Danzer, G. D.; Strommen, D. P.; Kincaid, J. R. J . Phys. Chem. 1988, 92, 5628. (22) Konig, E.; Lindner, E. Spectrochim. Acta 1972, 28A, 1393. (23) Sutin, N.; Creutz, C. Adv. Chem. Ser. 1978, 168, 1. (24) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159. (25) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85. (26) Bradley, P. G.; Kress, N.; Homberger, B. A.; Dallinger, R. F.; Woodruff, W. H. J . Am. Chem. Soc. 1981, 103, 7441. (27) Darsillo, M. S.; Gafney, H. D.; Paquette, M. S. J. Am. Chem. SOC. 1987, 109, 3275. (28) Mendoza, E.; Wolkow, E.; Sunil, D.; Wong, P.; Sokolov, J.; Rafailovich, M. H.; den Boer, M.; Gafney, H. D. Langmuir 1991, 7, 3046. (29) Shi, W.; Wolfgang, S.; Streaks, T. C.; Gafney, H. D. J. Phys. Chem. 1985, 89, 974. (30) (a) Iler, R. K. The Chemistry of Silica; Wiley-Interscience: New York, 1979; p 639. (b) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Dekker: New York, 1067; p 79.
