J. Phys. Chem. 1903, 87,4675-4681
4675
Heterogeneous Preparation of Singlet Oxygen Using an Ion-Exchange-Resin-Bound Tris( 2,2'-bipyridine)ruthenium( I I ) Photosensitizer Scott L. Buell and J. N. Demas' Department of Chemistry, Universlty of Virginia, Charlottesville, Vlrglnia 2290 1 (Recelved: *rch 24, 1983)
A heterogeneous photocatalyst utilizing [Ru(bpy),12+(bpy = 2,2'-bipyridine) bound to a Dowex 5OW-X1 cation-exchangeresin, [D]-Ru(bpy),has been prepared. The sensitizer is easily prepared and exhibits a limiting quantum yield of 'OZproduction in methanol of 0.90. Usable 'OZformation efficiencies are -80%. As a 'Oz generator in methanol, [D]-Ru(bpy)is comparable to homogeneous rose bengal and more efficient than other heterogeneous sensitizers. Hydration of the sensitizer decreases the photooxidationyield, but even water-saturated sensitizers exhibit usable 'OZyields of >20%. Hydration of the resin with DzO instead of H 2 0 also decreases the photooxidation yields but to a much lesser extent (yield > 40%). The analogous [D]-Ru(phen) (Ru(phen) = tris(1,lO-phenanthroline)ruthenium(II))behaves similarly. Spectral characteristicsof [D]-Ru(bpy)are virtually identical with those of homogeneous [Ru(bpy),I2+.In order to explain the photosensitization and luminescence quenching properties of this system, a two-phase microheterogeneous solvent domain model was developed. This model requires separation of the solvent into a local water environment around the sensitizer. Using a simplified planar diffusion model, we obtained reasonable estimates of the water layer thickness. A microscopic picture of the structure of the photosensitizer is developed.
m72. -.
Experimental Section Materials. Tetramethylethylene (TME) (99+ % Gold Label, Aldrich) in anhydrous methanol, MeOH (Mallinckrodt) or MeOD (99.5+% Gold Label, Aldrich), yielded clear solutions which were used in the photooxidation studies. TME concentrations were -0.13 M. Water was redistilled from alkaline permanganate. DzO (99.7 % , Aldrich) was used as received. [Ru(bpy),]Cl, (G. F. Smith Chemical Co.) was recrystallized from water. Dowex 5OW-X1 (50-100 mesh) cation-exchange resin (BioRad) was washed with concentrated HC1 and then distilled water. [D]-Ru(bpy) was prepared by mixing aqueous [ R u ( b p ~ ) , ] ~with + the resin; after 2 weeks the bright orange beads were filtered and washed well with water and then MeOH. The [ R u ( b p ~ ) ~was ]~+ completely (>99.8%) bound to the resin (-4% charge coverage). [D]-Ru(phen) was prepared similarly from [Ru(phen),] (C104)3(phen = 1,lO-phenanthroline). There was no detectable leaching of the complexes into water or MeOH even on extended standing. Rose bengal (Fisher) was recrystallized from MeOH. Commercial polymer-bound rose bengal sensiti~er,'~ [PI-rose bengal, was used as received. All other chemicals were reagent grade and used as received. The Dowex resin is highly hydrated (-80-85 wt % water); the Dowex-bound photosensitizers have similar water affinities. The Dowex-sensitizers were initially used in a relatively wet state. Immediately prior to use, they were extensively washed with MeOH, filtered, and aspirated to remove free solvent. Photooxidation yields were irreproducible, suggesting an erratic water content. Subsequent findings of a large variation of photooxidation yield with resin water content supported this. All further work was carried out by using washed and exhaustively vacuum-dried sensitizers. The photosensitizers were reused after an extensive methanol wash and drying. For studies with water-impregnated sensitizers, all photolysis experiments, except one, utilized a weighed amount of water or DzO added directly to the dried
(15) Bloasey, E. C.; Neckers, D. C.; Thayer, A. L.; Schaap, A. P. J.Am. Chem. SOC.1973, 95, 5820. (16) Williams, J. R.; Ortor, G.; Unger, L. R. Tetrahedron Lett. 1973, 46, 4603.
(17) Hydron Laboratories, Inc., 783 Jersey Avenue, New Brunswick, NJ 08902.
Singlet oxygen has received increasing attention since the chemiluminescence of the NaOCI-H2O2reaction was shown to arise from loz.' 'Oz is important in photos y n t h e ~ i s , and '~~ a~tinometry.~ Efficient IOz generation is an important area of study since existing methods have shortcomings. For example, radiofrequency discharge tubeslO or direct laser excitation" requires expensive equipment and chemical method~'J~-'~ may have side reactions. Problems with the common dye sensitization method include sensitizer-product separation and solvent incompatibilities of dye and IO2 acceptor. Heterogeneous photosensitizers overcome some of the problems of homogeneous sensitizers. Rose bengal,15J6eosin, and methylene blue16 have been bound to polymers, but the IO2 formation efficiencies are lower than for the corresponding homogeneous systems. We report a new heterogeneous polymer-bound photosensitizer, [D]-Ru(bpy),which is as efficient a 'Oz generator as is [ R u ( b p ~ ) ~in] homogeneous ~+ solution^.^ [D]-Ru(bpy) is based on [ R u ( b ~ y ) ~(bpy ] ~ += 2,2'-bipyridine) bound to a Dowex cation-exchange resin. Our systems provide an intimate probe of the detailed sensitizer-oxygen-substrate interactions in heterogeneous photosensitizers and may offer advantages over existing polymer sensitizers. (1) Khan, A. U.; Kasha, M. J . Chem. Phys. 1963, 39, 2105. (2) Foote, C. S. Pure Appl. Chem. 1971,27, 635. (3) Kearns, D. R. Chem. Reu. 1971, 71, 395. (4) Zweig, A.; Henderson, W. A., Jr. J.Polym. Sci.,Part A-I 1975,13, 993. (5) Ranbv. B.: Rabek. J. F. 'Photodeeradation. Photo-oxidation and Photbstabilization of Polymers"; Wileyl New York, 1975. (6) Bland, J. J. Chem. Educ. 1976, 53, 274. (7) Huber, J. E. Tetrahedron Lett. 1968.29, 3271. (8) Mazur, S.; Foote,C. S. J.Am. Chem. SOC.1970, 92, 3225. (9) Demas, J. M.; McBride, R. P.; Harris, E. W. J.Phys. Chem. 1976, 80, 2248. (10) Noxon, J. F. Can. J. Phys. 1961, 39, 1110. (11) Matheson, I. B. C.; Lee, J. Chem. Phys. Lett. 1970, 7, 475. (12) Khan, A. U.; Kasha, M. J . Chem. Phys. 1964,40,605. (13) Foote,C. S.; Wexler, S. J. Am. Chem. SOC.1964,86, 3879. 1967,89, (14) Wasserman, H. H.; Schaeffer, J. R. J. Am. Chem. SOC.
0022-365418312087-4675$0 1.5010
0 1983 American Chemical Society
4676
The Journal of Physical Chemistty, Vol. 87,No. 23, 1983
A
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LENS
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Buell and Demas
CELL
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Flgure 1. Photolysis setup. A MgOcoated slide replaces the celi for the reflectance measurement.
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0
10
20 t
[D]-Ru(bpy);the resin was equilibrated for >1 h. Water contents are expressed as weight percent (wt %). The deuterium studies used an equal molar amount of D20 but are expressed in terms of an equivalent weight percent of H20. In the deuterium studies MeOD replaced MeOH. Luminescence and Spectral Measurements. Lifetime studies were carried out on a nanosecond nitrogen laser decay time apparatus.ls The relative luminescence quantum yield as a function of excitation wavelength was determined with a luminescence quantum counter comparator.lg The 1cm thick quantum counter cell was filled with an aqueous slurry of [D]-Ru(bpy)which was optically dense over the 360-500-nm excitation range. The rearviewed luminescence was compared to a Rhodamine B (5 g L-' in MeOH) reference counter; spectra were corrected for the Rhodamine B response.20 Absorption spectra were measured on a Cary 14 spectrophotometer. Corrected luminescence spectra were measured on an SLM Model 8000DS spectrofluorimeter.21p22 The emission of [D]-Ru(bpy) was obtained in a 1-mm cell. Backside viewing at 45" minimized stray light. Ancillary Measurements. One pair of photolysis experiments was performed to test for aging effects. Two photolyses were carried out with a methanol/water/TME mixture added to the dried sensitizer. Conditions were identical except that in one case the solution was photolyzed immediately while in the other the photolysis was delayed 7 h. Since the luminescence lifetime of [D]-Ru(bpy) depended on the degree of sensitizer hydration, solvent equilibration of the resin was followed by the decay time. In one study a sample of dried sensitizer was suspended in deoxygenated MeOH. Deoxygenated water was added; this water was sufficient to hydrate the resin to 93 w t 70. The sample's luminescence lifetime was then monitored vs. time. The solution warmed on adding water. To determine the importance of temperature effects, the temperature of a sample of dried resin, suspended in MeOH, was monitored with a thermistor following the addition of water. Photolysis Measurements. Photolysis experiments were carried out with the setup of Figure 1. Samples (typically 0.25 g) in 12 mL, 2.5 cm diameter cells were bubbled with methanol-saturated O2 for -3 min before and during photolysis. An optically dense suspension was obtained by mixing with a magnetic stirrer. Excitation was at the 488-nm line from a Coherent CR-5 ionized Ar laser. The beam intensity (35-60 mW in a -1.5 cm diameter beam), measured before and after each 100-9 run with a Scientech 391 power meter, was stable to 20%. Even when water is introduced indirectly via a MeOH/TME/ water mixture, 4omis similarly reduced. Thus, the resin rapidly scavenges solvent water. In contrast, for homogeneous methanol rose bengal sensitizations, a high water concentration (32 w t % water) did not affect +obsd. This water concentration was the highest one that yielded clear solutions. Since the solvents in all heterogeneous photosensitizations were clear, the maximum bulk water concentration in these experiments never exceeded 32 w t % water. (29) (a) Demas, J. N.; Diemente, D.; Harris, E. W. J. Am. Chem. SOC. 1973,95, 6864. (b)Demas, J. N.; Harris, E. W.; McBride, R. P. J. Am. Chem. SOC.1977, 99, 3547.
4678
The Journal of Physical Chemistry, Vol. 87,
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Eueil and Demas
No. 23, 1983 2 5
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Flgure 6. Luminescence lifetime Stem-Volmer oxygen quenching plot (-22 “C)of dried [D]-Ru(bpy): (---, ‘) [D]-Ru(bpy) in MeOD (y offset 1.5); (-, X) [D]-Ru(bpy) in MeOH (y offset 1.0); (--, 0) [D]Ru(phen)in MeOH (yoffset 0.5); (-.-, +) [D]Ru(bpy) in H,O. All fits are for three data points (N2, air, and O2 saturated).
Typical luminescence decay curves are shown in Figure 5. The deoxygenated and oxygen-saturated sensitizers gave slightly nonexponential decays. Mean luminescence lifetimes, 7,were determined from linear least-squares fits to the semilogarithmic plots. The decay times in deoxygenated solvents, 70)s, are given in Table I. Figure 6 shows typical Stern-Volmer quenching plots (70/7- 1 vs. [O,]); the slopes of these curves equal the Stern-Volmer quenching constants, Ksv’s. Ksv’s and the bimolecular oxygen quenching constants, kq)s (=Ksv/TO), are given in Table I. Since the oxygen concentration in MeOH and water differ by a factor of 10,KSV’Sand k,’s for the mixed solvent systems depend upon whether one assumes a local MeOH or water environment. We report Ksv’s and kqls in Table I assuming both a pure MeOH and a pure water environment around the sensitizer (purewater values are in parentheses). Quenching data for [D]-Ru(bpy) in pure water and in pure MeOH are included; [02]’s were taken as that for the pure solvent. Figure 7a shows that the addition of water to a MeOH suspension of dry [D]-Ru(bpy) produced a sharp decrease in 7 , followed by rapid equilibration. This turned out to be a trivial thermal effect caused by the heating due to the addition of the methanol. Figure 7b shows that the rise time of the lifetime data parallels the sample cooling. Hydrational equilibration of the resin is at least as rapid as thermal equilibration.
-
Discussion We begin by excluding the possibility that our heterogeneous sensitizations arose from a steady-state concentration of sensitizer dissolved in the bulk solvent. The binding of the sensitizers is so tight that there is no detectable sensitizer concentrations in MeOH or MeOH/HzO solvent measured by absorption spectroscopy (C1 pM). The suspended sensitizer absorbs virtually all of the light. Traces of free sensitizer could not account for the high bobsd’s
The kinetics of lo2reactions in homogeneous solutions are summarized by
D
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20
30
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Figwe 7. Equilibration of [D]Ru(bpy)on adding water to the dry resin suspended in methanol. (a) 7 vs. time; H20 added at time zero. The initial point at 805 ns is the lumlnescence lifetime before the addition of water. (b)Temperature vs. time; (pl) resin added to methanol, (p2) water added. Note the diffwent thne scales in a and b. The conditions are not exactly the same between a and b (see text).
state. T is the TME IO2 trap and TO2is its hydroperoxide. $’is the efficiency of population of *D on excitation. This scheme is inadequate for our heterogeneous photosensitizers. In the scheme where the ‘02is formed directly in the bulk MeOH, the large changes in (bobsd on addition of water or with solvent deuteration are not explained. We first rule out several possible explanations of these discrepancies. are not due to changes in 4’ (eq 4) The changes in with the sensitizer’s environment. 4’ will be independent of whether the complex is in an H 2 0 or D20 environment and this is inconsistent with the large deuterium effect on Further, $’for [ R u ( b ~ y ) ~is] ~unity ’ in pure MeOH or waterB so it is unlikely that binding to the resin would greatly affect 4’. This last point is further supported by the invariance of the luminescence efficiency with excitation wavelength (Figure 4) which is the same as observed for Ru(I1) complexes in homogeneous solutions. The low dobsd’sof the hydrated resin are not due to inefficient quenching of *D. $obd varies strongly with the resin’s H 2 0 or DzO content even after correcting for different sample quenching efficiencies (vide infra). The changes in $obd with increasing water content are not caused by decreased ‘02 scavenging efficiency for TME (eq 7 and 8) arising from water leaching into the bulk solvent. The homogeneous photosensitizations with rose bengal show that this scavenging efficiency is unaffected by water in the methanol over the range encountered in our experiments. Two-Phase Photosensitization Model. We now develop a two-solvent phase model which explains the behavior of [D]-Ru(bpy). Dowex 5OW-X1 is a porous, sulfonic acid cation exchanger. [ R ~ ( b p y ) ~tightly ] ~ + associates with the acid groups yielding a dispersed sensitizer in a permeable matrix. In pure MeOH, the porous resin solvates with methanol which allows O2 and TME to diffuse throughout it. ‘02 formed on sensitizer quenching is generated directly in the TME-containing MeOH. The distance before encountering a TME trap is the same as if the ‘02had been generated homogeneously. Thus, $omfor dry [D]-Ru(bpy) and for [ R ~ ( b p y ) ~in] ~MeOH + are comparable as expected. When water is introduced, however, the strongly hydrophilic resin hydrates. We envision this resulting in a
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983 4679
Heterogeneous Preparation of Singlet Oxygen RES IN
TABLE 11: Photosensitization Properties of Dowex Sensitizers at 22 "C
i
[ DI-Ru(bpy)/MeOH 0% H,O 92%
H,O
94%
n1o H,O n1o
\ I
\
Flgure 8. Twodomain microheterogeneous model of [D]-Ru(bpy). - - - represents methanctwater boundaries fa Merent water bdlngs on the resin. D represents the bound photosensitizer.
nearly two-phase microheterogeneous environment. There is a hydrated, hydrophilic region around the photosensitizer, and an essentially pure MeOH-TME bulk solvent region well removed from the resin's polar regions. A single resin pore is shown schematically in Figure 8. In this model, the water layer over the sensitizer thickens with increased water loading. Since TME is water insoluble, '02formed in the water layer must diffuse across this layer to be trapped by the TME. Thus, d o h d depends on the water layer thickness and the lo2lifetime in this layer. We quantitate this two-phase model by expanding on the homogeneous model (eq 4-8).
0.88 0.78
50 25
0.82 0.76 0.78
40
[ D 1-Ru(phen)/MeOH 0%H,U
RES IN
77 46
0.90 0.90 0.90 0.90
1.0 0.67 0.68
0.90
0.57
0.38
0.68 1.0 Moles of hydroperoxide formed per einstein adsorbed. Calculated from eq 9 using @' = 1.0 and PR = 0.98 for H,O, or 1.00 for D,O. 0.85
57
equal-thickness layers, PE should be greater for D20than for H 2 0 as is observed (Table I). This result further substantiates that '02is the reactive species. PR is related to the '02scavenging ability of TME.
PR = [TMEl/([TMEl + P) = 7(%2)
kd/krx
=
l/kd
(W (Ilb) (114
6 is the [TME] where half of the '02is trapped. 7('02) is the lifetime in the absence of the scavenger. For MeOH,
@ = 0.0027 M.32 For the MeOD P C 0.0005 M.33 At our TME concentration of 0.13 M, PR = 0.98 for MeOH and
1.00 for MeOD.34 Table I1 shows 4obd, P , and PF for [D]-Ru(bpy) and 4obsd = 4 /pBpFpEpR (9) [D]-Ru(phen)along with &e PE'Sderived from eq 9. For the anhydrous resin in MeOH, PE = 1,which yields PF of 4'is defined above. Pe is the probability of quenching *D 0.90; this agrees within experimental error with the value by 02.PFis the probability that a quenching encounter of 0.855 obtained for homogeneous [ R u ( b p ~ ) ~photo]~+ forms lo2.PE is the probability that the '02successfully oxidation~.~ Henceforth, we assume that PF = 0.90 for crosses the water barrier and escapes into the bulk solvent. [D]-Ru(bpy). PF for [D]-Ru(phen) also agrees well with PR is the probability of reaction of lo2with the TME in the homogeneous value of 0.75.29 These results again the bulk solvent. demonstrate the minor perturbation of the sensitization On the basis of the observed spectral similarities between the heterogeneous and homogeneous [ R u ( b p ~ ) ~ ] ~ +properties of the complexes on binding them to the Dowex. Our two-domain heterogeneous sensitizer model is supsensitizers, we assume that 4' equals the homogeneous ported by the D20 studies. Replacing H 2 0 with D,O solution value of 1.00.2s PQ is given by should increase ~('0~) in the aqueous layer and, thus, inPQ = 1 - ( 7 ( 0 2 ) / 7 0 ) (10) crease PE as is observed. Little of the increase in C#Jobsd can be due to an increased ~('0~) in the bulk solvent; PR is 0.98 ~(0 is the ~ )sensitizer's luminescence lifetime under the in the MeOH and 1.00 in the MeOD. Further, 7(102) is oxygen-saturated photolysis conditions. in the bulk phase were twice as long as in MeOH. If ~('0~) On the basis of the spectral similarities of [D]-Ru(bpy) the dominant factor in determining 4 0 ~ , for the pure and homogeneous [ R u ( b p ~ ) ~sensitizers, ]~+ we assume that methanol system should be lower than for the D20-conPF is solvent independent. Extensive studies using diftaining systems. The opposite is observed to be the case ferent scavengers have shown that '02is the reactive and the two-phase model accounts for this observation. species in homogeneous sensitizations using [ R u ( b p ~ ) ~ ] ~ + Several conclusions can be drawn from the effect of and related c o m p l e ~ e s .The ~ ~ deuterium isotope effects adding a ternary MeOH/TME/water mixture to the dry are also consistent with a '02reactant (vide infra). We sensitizer (Table I). The resin quickly scavenges the bulk assume that lo2is the reactive species in our heterogeneous water, and dObsd is reduced by about the same degree that sensitizations. We make no assumptions about the lo2 it would be if the water were added directly to the resin. production mechanism; we favor energy transfer,29but an The invariance of C#JoM with incubation period shows that electr~n-transfer~~ pathway has been proposed. PE accounts for the two phases and the insolubility of (31) (a) Merkel, P. B.; Kearns, D. R. J.Am. Chem. SOC. 1972,94,7244. TME in water. In anhydrous resin, IO2 is generated in the (b) Gollnick, K. In 'Singlet Oxygen Reactions with Organic Compounds TME/MeOH solvent, and PE is unity. In the hydrated and Polymers"; Ranby, B., Rabek, J. F., Eds.; Wiley-Interscience: New resin, lo2must diffuse across the water barrier to react York, 1978;pp 111-34. (32)Foote,C. S. Acc. Chem. Res. 1968,I, 104. with the TME. PE, therefore, depends upon the layer (33)This value was derived from eq llb and ~ ( ~in 0, MeOD; ) k, was thickness and the '02lifetime, #02),in the water layer. assumed to be the same for MeOH and MeOD. We assume a ~('0~) in ~ ( ~ is -2 0 ~ ps ) in H 2 0 and 20 ps in D20.31 Thus, for MeOD of >35 ps which is the i(lOz) in a 1:l MeOD/D,O mixture.31 The (30)(a) Lin, C.-T.; Sutin, N. J. Phys. Chem. 1976,80, 97. (b) Lin, C.-T.; Bottcher, W.; Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem. SOC. 1976,98, 6536.
same lifetime was used for both anhydrous and aqueous conditions. (34)Because @
