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J. Phys. Chem. 1994,98, 5 120-5 124
Photoreduction of Methylviologen in Silica Gels of Different Pore Sizes Bosong Xiang and Larry Kevan’ Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: January 6, 1994: In Final Form: March 12, 1994”
Dimethylviologen dichloride (MV2+(C1-)2) was introduced into silica gel pores by impregnation or addition to the sol during sol-gel synthesis. The evacuated silica gel samples containing MV2+ were reduced by 320-nm irradiation to form a stable dimethylviologen cation radical MV+, which was detected by electron spin resonance and diffuse reflectance spectroscopies. The counteranion C1- is shown to be the electron donor. The photoyield and stability of the photoproduced MV+ radical depend on the pore size of the silica gel which ranged from 1.7 to 14 nm. In a large pore silica gel the MV+ photoyield is larger and the MV+ cation radical is more stable than in a small pore silica gel. Replacing the two methyl groups of dimethylviologen with buty and heptyl alkyl chains did not affect the photoyield or stability of the photoproduced viologen cation radical in the silica gels. It is suggested that stability of the photoproduced MV+ is controlled by its distance from C1 trapped within the pores. The trend of increasing MV+ stability and photoyield with increasing pore size is due to a larger separation distance between the MV+ and the C1 trapping site in the larger pores. It is found that the photoyields are about 5 times greater in impregnated vs sol-gel synthesized samples. Thus, impregnation seems to be the optimum method for preparation of the most efficient photoactive assemblies.
without a molecular electron acceptor.30 For room temperature photoionization it was found that larger photoyieldswere obtained in smaller pores, consistent with greater mobility and reactivity of TMB+ in larger pores. Here, we study the photoreduction of MV2+without a molecular electrondonor in silica gels of different pore sizes. It is demonstrated that the chloride counterion is the electron donor and that larger photoyields are obtained for larger pore silica gels which is just the opposite pore size effect as found for the photooxidation of tetramethylbenzidine. Comparison is also made between MV2+incorporated by impregnation and by addition to the sol during sol-gel synthesis of silica gels.
Introduction
Artificial photoredox systems are attractive for light energy utilization.1-3 Current research is oriented toward the design of efficient photoredox systems which can inhibit back electron transfer and lead to relatively stable charge separation between the electron donor and the electron ac~eptor.’~ It has become clear that heterogeneous systems, such as micelles, vesicles, molecular sieves, and silica gels, are better host systems than homogeneous systems. Dimethylviologen (MV2+), commonly known as methylviologen, is an efficient electron acceptor for artificial photoredox system^.^-^ MV2+can be reduced to MV+radical by ~ h e m i c a l , ’ ~ ~ ~Experimental Section electrochemical,l5 and photochemical1G28means. Most of these Sol-Gel Synthesis of Silica Gel. Tetraethylorthosilicate [Sistudies include an added molecular electron donor, such as alcohols ( O C Z H ~ )was ~ ] purchased from Aldrich Chemical Co. Formand bipyridium salts, to the system. The photoproduced MV+ amide (HCONH2), which was used to control the pore size of radicals are usually unstable at room temperature due to back the synthesized silica gel, was purchased from Fisher Scientific electron transfer or other reactions. However, stable charge Co. Si(OCzH5)4, H20, CHjCHzOH, and HCONH2 were in a separation is typically obtained at 77 K.18-23Also, stable charge volume ratio of 25 mL:20 mL:20 mL:V(HCONH2), and several separation has been found at room temperature in some solid drops of concentrated HCl were added to obtain pH 1. After media. Walszczak and Stradowski studied the photoreduction stirring for several minutes, a clear homogeneous sol solution was of methylviologen adsorbed on paper, cotton, cellophane, and obtained. The sol was put into a closed plastic jar in an oven at wool without an added electron donor molecule and observed 65 O C for 2 days for gelation of the sol. Then the jar was opened, stable photoproduced MV+ radicals at room t e m p e r a t ~ r eDutta .~~ and Turbeville studied methylviologen photoreduction in zeolite and the residual liquid was removed. The shrunken gel in the open jar was dried at 65 OC for 4 days. The dried pieces were with Ru(bpy)32+as the electron donor and found that part of the ground into a powder with a mortar and pestle. Three pore sizes photoproduced MV+ radicals had a long life.27 were prepared by using 0-, 5-, and 20-mL volumes of HCONHz Porous silica gel has been recently studied as a host system for to form average pore diameters of 1.7, 2.6, and 3.6 nm.30 photoredox reactions.”7 Using the sol-gel technique, silica gel Preparation of Electron Acceptors. 1 ,I ‘-Dimethyl-4,4’-bipycan be synthesized at room t e m p e r a t ~ r e . Organic ~~ molecules can then be added to the sol solution during the room temperature ridinium dichloride (methylviologen dichloride hydrate) was purchased from Aldrich Chemical Co. The MVZ+(C1-)2content synthesis process and are “trapped” in the synthesized silica gel.4-7330 However, recent work has suggested that organic was 85 wt 5% based on an extinction coefficient of 2.07 X lo4 M-1 cm-l.15 molecules can be equivalently “trapped” by impregnation after synthesis.30 Part of the photoproduced MV+ radicals in sol-gel 1 , I ‘-Dimethyl-4,4‘-bipyridinium dibromide (methylviologen synthesized silica gel with pyrene as an electron donor last for dibromide) was prepared from 2.50 g (8.26 X 10-3 mol) of several hours at room temperature at quite low molecular MV2+(C1-)2dissolved in 10 mL of water added to 9.53 X 10-3 concentration^.^ mol of Ag2S04 (Aldrich) dissolved in 1000 mL of water. The precipitate (AgCl) was removed by filtration. Then 9.53 X 10-3 We have studied the photoionization of tetramethylbenzidine mol of BaBr2 (Aldrich) dissolved in 20 mL of H 2 0 was added (TMB), an electron donor, in silica gels of different pore sizes and confirmed the long-term photooxidation of TMB to TMB+ to the clear filtrate solution. After the precipitate (BaS04) was removed by filtration, part of the water was removed by evaporation to obtain a 85 mM MV2+(Br)2/H20 solution. 0 Abstract published in Aduance ACS Abstracts, April 15, 1994. 0022-365419412098-5 120$04.50/0
0 1994 American Chemical Society
Photoreduction of Methylviologen in Silica Gels I ,1 '-Dimethyl-4,4'-bipyridiniumdiperchlorate (methylviologen diperchlorate) was prepared from 2.50 g (8.26 X 10-3 mol) MV2+(C1-)2dissolved in 20 mL of water added to 19.06 mol of AgC104dissolved in 50 mL of water. After the precipitate (AgC1) was removed by filtration, the volume was adjusted to obtain a 85 mM MV2+(C104-)2/H20 solution. 1 ,I '-Dibutyl-4,4'-bipyridiniumdichloride (butylviologen ( B P ) dichloride) was prepared from 4,4'-dipyridyl (Aldrich) dissolved in 75 mL of dimethylformamide added to 25 mL of CH3(CH2)3Br. After 3-h reflux this solution was filtered, and a yellow precipitate was collected. The precipitate was dissolved in 100 mL of H20, and the solution was mixed with 22.8 g of Ag2S04 solution in 2000 mL of H20. The resulting solution was filtered to remove the AgBr precipitate. The filtrate was titrated with a BaCl2 solution (1 1.0 g of BaC12 in 200 mL of H2O) until no more AgBr precipitate was formed. The precipitate was removed by filtration, and the final solid product was collected after water was removed by evaporation. The IH N M R spectrum in D20 of the product showed peaks at 0.83 (CH3), 1.25 (CH2), 1.91 (CH2), 4.55 (CH2-N), 8.35 (aromatic, 0 to N), and 8.95 ppm (aromatic, a: to N). 1,1 '-Diheptyl-4,4'-bipyridinium dichloride (heptylviologen (HV+) dichloride) was prepared from 2.00 g (3.89 X 10-3 mol) of 1,l '-diheptyl-4,4'-bipyridinium dibromide (Aldrich) dissolved in 100 mL of H20 and mixed with a Ag2S04 solution (1.45 g of Ag2S04 in 300 mL of H2O). The resulting solution was filtered to remove the AgBr precipitate. Then 1.07 g (5.13 X 10-3 mol) of BaC12 was dissolved in 30 mL of HzO, and the solution was added to the above filtrate. The precipitate (BaS04) was removed by filtration, and then water was removed by evaporation. The solid product was dissolved in 100 mL of CHCl3/CH30H (80/ 20 v/v), and the undissolved solid was removed by filtration. After removing the solvent by evaporation, the final solid product was collected. The 1H N M R spectrum in D20 of the product showed peaks at 0.70 (CH3), 1.0-2.0 (CHz), 4.60 (CH2-N), 8.40 (aromatic, 0 to N), and 8.95 ppm (aromatic, a to N). Sample Preparation and Measurement. The electron acceptors were introduced into silica gel by two methods: impregnation and addition to the sol solution during sol-gel synthesis. For the impregnation method, 3.0 mL of silica gel powder was immersed in 10 mL of 8.5 mM MV2+(Cl-)2 water solution for 1 day or longer. The amount of the MV2+ adsorbed in the silica gel was measured from the decrease of MV2+ in the liquid measured optically at ,A = 257 nm with an extinction coefficient of 2.07 X 104 M-1 cm-1. The MV2+(C1-)2 impregnated powder samples were designated IPMX where X is the average pore diameter of the silica gel in nanometers. MV2+(Br)2and MV2+(C104-)2 impregnated samples were prepared similarly. In addition to the sol-gel synthesized silica gels with 1.7-, 2.6-, and 3.6-nm pore diameters, three commercial silica gel powders, Sigma Chemical No. S-41333, Aldrich Chemical No. 28,860-8, and Aldrich Chemical No. 24,398-1 with2.5-,6.0-,and 14-nmporediameters, respectively, were used for impregnation samples. In thesecond method, MV2+(C1-)2,BV2+(C1-)2,0rHV2+(C1-)2 was added into the sol solution to obtain 8.5 mM concentration during the sol-gel synthesis. The amounts of the molecules trapped in the synthesized silica gel were determined by optical absorption from the difference in the amount of molecules in the sol and the amount of molecules in the residue solution before drying. The synthesized silica gel powders containing MV2+, BV+, and H V + aredesignated SPMX, SPBX, and SPHX, where X is the average pore diameter of the silica gel in nanometers. The sample powders were filled into 2-mm-i.d. by 3-mm-0.d. Suprasil quartz tubes to about 18 mm in height for ESR measurements. The tubes were connected to a vacuum line, pumped to about 3 X 10-3 Torr, and sealed. For diffuse reflectance measurements the sample powder was put into a cylindrical quartz sample cell (19-mm diameter by
The Journal of Physical Chemistry, VoL 98, No. 19, 1994 5121
A V
-4F20 G I
,*
Figure 1. ESR spectra at room temperature of SPM1.7, SPBl.7, and SPH1.7 sample types after 20-min room temperature irradiation at 320
nm. 1-mm path length), which was pumped to about 3 X 10-3 Torr and sealed. A Cermax 150 xenon lamp (ILC-LX15OF) was used for irradiation. The light was passed through a 10-cm water filter and a Corning 7-54 glass filter to give 320-nm irradiation at 1.3 X 106 erg cm-2 s-1 in the range from 240 to 400 nm. ESR spectra were recorded on a Bruker 300 ESR spectrometer with 100-kHz magnetic field modulation of 5 G and a microwave power of 2 mW. Doubly integrated intensities of the ESR spectra were determined with the Bruker software. The photoreduction yield is defined as the doubly integrated ESR intensity divided by the moles of viologen (MV2+, B V + , or H V + ) in the silica gel powder using the same arbitrary unit for all samples. A Perkin-Elmer Model 300 spectrometer with integrating sphere accessory was used to record the diffuse reflectance spectra. The silica gel without electron acceptors was used as the reference. Results
All the samples with MV2+ introduced by impregnation or addition to the sol were colorless and ESR silent before photoirradiation. After being irradiated by 320-nm light at room temperature for 20 min, they turned blue and showed a symmetric single ESR signal (Figure 1) with g = 2.004 and a peak-to-peak line width of 15 G. This ESR signal and the blue color are typical ofthe MV+ r a d i ~ a l . ~ Thediffuse ~ J ~ , ~ ~ reflectancespectra (Figures 2 and 3) of these MV2+-containing samples recorded after 20min-room temperature irradiation are the same as the absorption spectra of the MV+ radical in solution and in X zeolite in the region from 350 to 800 nm.12J3,31The characteristic absorption bands at 390 and 605 nm of the MV+ radical are clearly seen. This shows the existence of MV+ radical in the photoirradiated silica gel samples and confirms the photoreduction of MV2+into MV+ in thesilica gel. So the ESR spectra arereasonably assigned to the photoproduced MV+ radical. The ESR signal and the blue color disappeared immediately when the sample tubes were exposed to air or oxygen. The photoproduced MV+ radicals in silica gel are thus unstable to oxygen. The doubly-integrated ESR signal intensities of MV+ are roughly linear with the MV2+ solution concentration up to 10-1 M MV2+in impregnated (IPM) samples and up to M MV2+ in sol-gel synthesized (SPM) samples with 1.7-nm pore sizes. For the same solution concentration of MV2+ the MV+ yield is approximately 5-6 times greater in SPM vs IPM samples. This difference reflects the greater amount of MV2+adsorbed per unit
Xiang and Kevan
5122 The Journal of Physical Chemistry, Vol. 98, No. 19, 1994 400 i
IPM 1.7
0 min
0.5
i/\
0 220
Anion
300 min
after irrad.
1000
800
600
400
Figure 4. Relative photoyields (ESR intensity per mole viologen) at room temperature of MVt in an IPM14 sample for CI-, Br, and Clodcounterions after 20-min room temperature irradiation at 320 nm. -
0
-b
Wavelength, nm
Figure 2. Diffuse reflectancespectra at room temperature of the IPMl.7 sample type before irradiation (0 min), after 20-min room temperature irradiation at 320 nm, and 300 min after the 20-min irradiation.
-f-
f -
IPM14
IPMP.5
SPM 1.7
0
100 200 Irradiation Time, min
300
Figure 5. Increaseof ESR intensitiesat room temperatureof IPM14 and IPM2.5 sample types vs 320-nm irradiation time.
I _
SPB
\
--
-,-
1.7
- --- -- - - SPH 1.7
0 220
‘--------_ 400
600 Wavelength, n m
800
Figure 3. Diffuse reflectance spectra at room temperature of SPM1.7, SPB1.7, and SPH1.7 sample types before irradiation (- - -) and after 20-min room temperature irradiation (-) at 320 nm.
concentration in SPM vs IPM samples. Thus, MV2+ is incorporated more efficiently into silica gel by sol-gel synthesis in comparison with impregnation after synthesis. The synthesized samples containing BVZ+ (SPB1.7) and HV2+ (SPH1.7) also showed symmetric single ESR signals (Figure 1) after 20-min irradiation with g = 2.004 and a peak-to-peak line width of 11.4 G. Their diffuse reflectance spectra (Figure 3) were similar to that of MV+ in the SPM1.7 sample in Figure 3 and showed absorption bands at 390 and 605 nm. We conclude that BV+ and HV+ radicals are also produced by 320-nm photoreduction of BVZ+ and HV2+ in silica gel. The photoreduction of methylviologen chloride has been shown to involve electron transfer from the chloride ion.17923 The monophotonic photoreduction efficiency is greatly reduced with bromide as the counterion, presumably due to enhanced intersystem crossing by stronger spin-orbit coupling known as the
heavy atom effect.” In Figure 4 we show MV+ photoyields from MV2t(Cl-)2, MV2+(Br)z,and MV2+(C104-)2 impregnated into silica gel with 14-nm pores. The photoyield of MV+ decreases in the order C1- > B r >> C104-. This supports C1- as the electron donor under photoexcitation of theviologen. The yield with C104is very low because it is a very poor electron donor. Note that relative photoyields can be measured reliably. However, absolute yields are not known because the light intensity absorbed in a highly scattering solid system is not easily measured. Figure 5 shows the increase in ESR intensity of MV+ with irradiation time in large pore (IMP14) and small pore (IPM2.5) impregnated samples. These show somewhat different time behaviors; note the two different vertical scales. The initial slopes are not comparable because they must be corrected for the differing amounts of viologen adsorbed by these two sample types, which is about 5 times more per unit weight of silica gel for the 14-nm pore sample. Thus, the amount of MV2+ adsorbed per unit weight of silica gel increases a little less than linearly with pore size for impregnated samples. In contrast, for sol-gel synthesized samples (see below) the amount of MV2+ adsorbed is little affected by pore size. An irradiation time of 20 min was selected for comparative yield and decay studies. Figure 6 shows the MV+ photoyields (ESR intensity per mole viologen) of the MV2+ impregnated silica gel samples with the different pore sizes. It can be seen that the photoyields of the large pore samples, IPM14 and IP6.0 are much larger than those of the small pore samples, IPM2.5 and IPM 1.7. This is supported by deeper blue colors for IPM 14 and IMP6.0 compared to lighter blue colors for the IPM2.5 and IPM1.7 samples. It is clear that the photoreduction of MV2+ to MV+ depends strongly on the silica gel pore size for impregnated samples. The stability of the photoproduced MV+ radicals after irradiation in impregnated silica gels is shown in Figure 7. The MV+ radicals in the large pore samples, IPM14 and IPM6.0, are very stable. The sample color and ESR intensity for these two large pore samples remain about the same for several months when kept in the dark at room temperature after photoirradiation. In contrast, the MV+ radicals in the small pore size samples,
The Journal of Physical Chemistry, Vol. 98, No. 19, 1994 5123
Photoreduction of Methylviologen in Silica Gels 400 1
x
c
1.21
SPM3.6 SPM2.6
n.4
a 0
L
0
5 10 Pore Size, nm
15
Figure 6. Relative photoyields (ESR intensity per mole viologen) at roomtemperatureofMV+inIPMlI,IPM6.O,IPM2.5and IPM1.7sample at 320nm. Theestimated typesafter 20-minroomtemperatureirradiation errors are 20%. (0
g
1.0
.
c
C
;0.81\
\ 0 -0 .
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100 200 Decay Time, min
IPM6.0 IPM2.5
IPM1.7
300
44 1
D
I
2 3 Pore Size, nm
100 200 Decay Time, min
300
Figure 9. Decay of relative ESR intensities at room temperature of SPM3.6, SPM2.6, and SPMl.7 sample types with time after 20-min room temperature irradiation at 320 nm.
The photoyields and stabilities of the BV+ and HV+ radicals in SPBl.7 and SPHl.7 samples under the same conditions as for SPM1.7 were measured by ESR. Both the photoyields and the stabilities of BV+ and HV+ were the same as for MV+. So replacing both methyls with longer chain butyl and heptyl groups has no effect on the photoyield and stability of the photoproduced viologen radicals. Since C1- is the electron donor, C1atoms are presumably formed. However, even if trapped, these are not detected by ESR as has been found previously for halogen atoms in solid matrices.32
Discussion
Figure7. Decay OfrelativeESRintensitiesatroom temperatureofIPM14, IPM6.0, IPM2.5, and IPM 1.7 sample types with time after 20-min room temperature irradiation at 320 nm. 161
0.04 0
4
Figure 8. Relative photoyields (ESR intensity per mole viologen) at room temperatureof MV+vs pore size in SPM3.6,SPM2.6,and SPM 1.7 sample types after 20-min room temperature irradiation at 320 nm. The estimated errors are 20%.
IPM2.5 and IPM1.7, decayed substantially over 100 min after irradiation. The decay is not a single-exponential function as is typically found in solids. This is consistent with a distribution of distances between MV+ and C1 which control the decay (see Discussion), MV+ in the smaller pore IPM 1.7 sample decayed much faster than that in the IPM2.5 sample. Figure 2 also shows that the MV+ absorption bands a t 390 and 605 nm totally disappear in the IPM1.7 sample after 300 min. Also, no ESR signal could be detected 1 day after photoirradiation of the IPMl.7 and IPM2.5 samples. If the photoionization is done at 77 K, the viologen radical ESR spectrum is stable and no decay is seen. Figures 8 and 9 show similar photoyield and decay data for MV+ vs pore size in the sol-gel synthesized samples. These samples support the trends found for impregnated samples. However, there are significant differences. The photoyields are about 5 times larger for impregnated samples compared to solgel synthesized samples of the same pore size. Also, the decay is significantly faster in the sol-gel synthesized samples as can be seen by comparing the decay curves in Figures 7 and 9. After 24 h the MV+ signal has totally decayed in the synthesized sample; no ESR signal and no blue color can be detected.
The ESR and optical data in Figures 1-3 clearly show that the methylviologen radical cation, MV+, is formed by the photoirradiation of both sol-gel synthesized and impregnated silica gels with incorporated methylviologen. Figure 4 also shows that the electron donor for the photoreduction is the chloride ion as the counterion of the methylviologen. The trend of photoyield with pore size shown in Figures 6 and 8 clearly indicates that the photoreduction yield increases with an increase in pore size a t least up to 6-nm pore size. With the impregnated samples in Figure 6, it appears that the trend with pore size reaches a maximum and plateaus for pore sizes greater than 6 nm. Figures 7 and 9 show that the decay of the photogenerated MV+ as a function of pore size for both impregnated and sol-gel synthesized samples is faster for smaller pore sizes. Thus, smaller photoyields correlate with faster decay, which suggests that thenet photoyieldiscontrolled by thestability of the photoproduced methylviologen radical cation. This overall picture is further supported by comparison between the sol-gel synthesized and the impregnated samples. The photoyields are about 5 times less, and thedecay is faster for the sol-gel synthesized samples. So again samller photoyields correlate with faster decay. The above trend with pore size is exactly opposite to the trend found previously for the photooxidation of tetramethylbenzidine (TMB) in these same two types of silica In this system the silica gel framework is the electron acceptor. There, the photoyield decreased with increasing pore size, and the decay of the tetramethylbenzidine cation was faster for larger pores. The major factor controlling the stability of the photoproduced TMB+ was suggested to be its mobility within the pores. It was concluded that there was greater mobility for the photoproduced TMB+ in the larger pores which led to faster decay and lower photoyields. In contrast, for the photoreduction of methylviologen the electron donor is concluded to be a chloride ion. After donating an electron the chloride ion becomes a chlorine atom, which should be more mobile than a chloride ion in the silica gel pore and which is transiently trapped at an oxygen site such as Si-0- on the pore surface. Thus, in larger pores the average MV+ to C1 separation distance is greater due to the rapid migration of C1. The effect of pore size on this average separation distance appears to be the controlling factor which determines the net MV+ photoyield and decay rate. Thus, in larger pores the greater
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The Journal of Physical Chemistry, Vol. 98, No. 19, 1994
MV+ to C1 or Si-OCl- separation distance controls the decay and leads to larger photoyields and slower decay in larger silica gel pores. Significant migration of the alkylviologen radical cation in the decay process seems improbable since the alkyl chain length does not affect the decay rate. The sol-gel synthesized silica gel has a pore size distribution around its average pore diameter. The methylviologen molecules introduced into the silica gel pores during the sol-gel synthesis are approximately equally distributed among the pores of various sizes. However, introduction of methylviologen by impregnation should favor more efficient incorporation into the bigger pores of the silica gel. Thus, for silica gels of same average pore diameter, impregnation populates a larger average pore size than does sol-gel synthesis for an added photoactive molecule. This is a possible explanation for the greater MV+ photoyield and stability in impregnated vs sol-gel synthesized samples. No effect of alkyl chain length for dialkylviologens was found on the photoyield or the stability of the alkylviologen radical cation in the silica gel systems for the same pore size. This is consistent with the dominant factor in determining the viologen cation radical photoyield and stability being the initial separation distance between the cation radical and the C1 species which is independent of the alkyl chain length. It also suggests that migration of the alkylviologen cation is likely unimportant in the decay process. Simple electron tunneling also does not dominate the decay since the decay is inhibited at 77 K. Conclusions
Methylviologen (MV*+) with C1- as its counteranion in silica gel can be reduced to form stable methylviologen cation radical (MV+) by ultraviolet irradiation. The chloride ion is shown to be the electron donor. The photoreduction of methylviologen chloride shows a distinct trend of increasing photoyield with increasing pore size which correlates with faster decays of the photoproduced radical cation in the smaller pores. It is of particular interest that both of these trends are exactly opposite from the photooxidation of tetramethylbenzidine in similar silica gels. It is suggested that the stability of the photoproduced MV+ is controlled by the initial separation distance from C1. The trend of increasing MV+ stability and photoyield with increasing pore size is due to a larger separation distance between the MV+ and the C1 species in the larger pores. It is also found that increasing the alkyl chain of the viologen up to heptyl does not affect the photoyield or stability of the photoproduced viologen cation in pores of the same size so that migration of the viologen radical cation is not a dominant factor. Of perhaps greatest interest is
the fact that the photoyields are about 5 times greater in impregnated vs sol-gel synthesized samples, which suggests that impregnation is the optimal method to incorporate photoactive molecules into heterogeneous systems such as silica gel. Acknowledgment. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, US.Department of Energy. References and Notes (1) Kirch, M.; Lehn, J. M.; Sauvage, J. P. Helu. Chim. Acta 1979, 62, 1345. (2) Borgarello, E.; Kiwi, J.; Pelizzetti, E.; Visca, M.; Gratzel, M. Nature 1981, 289, 158. (3) Connolly, J. S. Photochemical Conversion and Storage of Solar Energy; Academic: New York, 1981. (4) Slama-Schwok, A.; Avnir, D.; Ottolenghi, M. J . Phys. Chem. 1989, 93, 7544. (5) Slama-Schwok, A,; Avnir, D.; Ottolenghi, M. J . Am. Chem. SOC. 1991, 113, 3984. (6) Slama-Schwok, A.; Avnir, D.; Ottolenghi, M. Photochem. Photobiol. 1991, 54, 525. (7) Slama-Schwok, A,; Avnir, D.; Ottolenghi, M. Nature 1992, 355, 240. ( 8 ) Krueger, J. S.; Mayer, J. E.; Mallouk, T. E. J . Am. Chem. SOC.1988, 110,8232. (9) Vermeulen, L. A,; Thompson, M. E. Nature 1992, 358, 656. (10) Homer, R. F.; Tomlinson, T. E. Nature 1959, 184, 2012. (11) Hyde, P.; Ledwith, A. J. Chem. SOC.,Perkin Trans. 2 1974, 1768. (12) Wolszczak, M.; Stradowski, Cz. Radiat. Phys. Chem. 1989,33,355. (13) Bockman, T. M.; Kochi, J. K. J. Org. Chem. 1990,55, 4127. (14) Kosower, E. M.; Cotter, J. L. J . Am. Chem. SOC.1964, 86, 5524. (15) Watanabe, T.; Honda, K. J . Phys. Chem. 1982,86, 2617. (16) Hopkins, A. S.; Ledwith, A.; Stam, M. F. Chem. Commun. 1970, 494. (17) Ebbesen, T. W.; Ferraudi, G. J . Phys. Chem. 1983, 87, 3717. (18) Colaneri, M. J.; Kevan,L.;Thompson, D. H. P.; Hurst, J. K.J. Phys. Chem. 1987, 91, 4072. (19) Sakaguchi, M.; Kevan, L. J . Phys. Chem. 1989,93, 6039. (20) Colaneri, M. J.; Kevan, L.; Schmehl, R. J . Phys. Chem. 1989, 93, 397. (21) Sakaguchi, M.; Kevan, L. J . Phys. Chem. 1991, 95, 5996. (22) McManus, H. J. D.; Kevan, L. J . Phys. Chem. 1991, 95, 10172. (23) McManus, H. J. D.; Kang, Y. S.; Kevan, L. J . Phys. Chem. 1992, 96, 2274. (24) Kaneko, M.; Motoyoshi, J.; Yamada, A. Nature 1980, 285, 468. (25) Walszczak, M.; Stradowski, Cz. Radiat. Phys. Chem. 1985,26,625. (26) Morishima, Y.; Furui, T.; Nozakura, S. J. Phys. Chem. 1989, 93, 1643. (27) Dutta, P. K.; Turbeville, W. J. Phys. Chem. 1992, 96, 9410. (28) Wheeler, J.; Thomas, J. K. J . Phys. Chem. 1982, 86, 4540. (29) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic: New York, 1990. (30) Xiang, B.; Kevan, L. Colloids Surf. A 1993, 72, 11. (31) Yoon, K. B.; Kochi, J. K. J . Am. Chem. SOC.1988, 110, 6586. (32) Jen, C. K.; Foner, S. N.; Cochran, E. L.; Bowers, V. A. Phys. Reu. 1958, 112, 1169.
