Langmuir 1991, 7, 2991-2998
2991
Photophysics and Oxygen Quenching of Transition-Metal Complexes on Fumed Silica E. R. Carrawayt and J. N. Demas* Chemistry Department, University of Virginia, Charlottesuille, Virginia 22901
B. A. DeGraff Chemistry Department, James Madison University, Harrisonburg, Virginia 22807 Received March 6,1991.I n Final Form: May 6, 1991 Pressed fumed silica has unique mechanical, chemical, and optical properties. It forms dense (1g/mL), optically transparent disks, yet has a surface area of 200 m2/g,which is the same as the unpressed material. The disks are highly porous and respond quickly to changes in gas composition. Before pressing, the silica can be impregnated by adsorption with a variety of materials. As a means of probing this surface and determining its potential utility, we have adsorbed luminescent R u L ~ ~(L+= a-diimine) complexes and examined their luminescence and oxygen quenching. The adsorption sites are highly heterogeneous as shown by nonexponential decays in the presence or absence of an 0 2 quencher. By use of a newly developed method, quenching is shown to be predominantly or exclusively by dynamic rather than static processes. Modeling of the quenching indicates that the results are best fit by surface quenching by adsorbed O2that obeys a Freundlich adsorption isotherm, although a two-site model with different quenching constants yields reasonable fits. The high density, transparency, porosity, and ease of preparation of homogeneously dispersed high levels of adsorbed species suggest that pressed fumed silica has potential as a support for luminescence sensors and photocatalysts.
Introduction Luminescence of species on solid supports is becoming increasingly important, particularly in the area of fiber optic sensors,1v2which are frequently supported in polymers, in gels, or on surfaces. In contrast to more conventional homogeneous luminescence materials, these supports are frequently heterogeneous on a microscopic scale and give rise to complex and poorly characterized decay kinetics. This complexity results in obscure sensor response. There has been considerable work done on the photochemistry and photophysics of adsorbed organic species. A significant amount of work has been presented on adsorption of luminescent metal complexes on silicas and a1umina,314on porous Vycor glass (PVG),5in gels: and in clays7 and silicone^.^^^ However, satisfactory models of
* To whom correspondence should be addressed. + Current address: Environmental Engineering Science, California
Institute of Technology, Pasadena, CA 91125. (1) Kalyansundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: New York, 1987. (2) Chemical, Biochemical, and Enuironmental Fiber Sensors; Lieberman, R. A., Wlodarczyk, M. T., Eds.; Proceedings of SPIE; SPIE: Bellingham, WA, 1989; Vol. 1172. (3) (a) Leermakers, P. A,; Thomas, H. T.; Weis, L. D.; James, F. C. J . Am. Chem. SOC.1966,88,5075. (b) Willner, I.; Degani, Y. J.Chem. SOC., Chem. Commun. 1982,761. (c) Lochmuller, C. H.; Colborn, A. S.; Hunnicutt, M. L.; Harris, J. M. Anal. Chem. 1983,55,1344. (d) Turro, N. J.; Gould, I. R.; Zimmt, M. B.; Cheng, C. Chem. Phys. Lett. 1985,119,484. (e) Drake, J. M.; Levitz, P.; Turro, N. J.; Nitsche, K. S.; Cassidy, K. F. J. Phys. Chem. 1988,92,4680. (f) James, D. R.; Liu, Y.-S.; DeMayo, P.; Ware, W. R. Chem.Phys. Lett. 1985,120,460. (9) Simiarczuk, A.; Ware, W. R. J. Phys. Chem. 1989, 93, 7609. (4) (a) Kajiwara, T.; Hashimoto, K.; Kwai, T.; Sakata, T. J. Phys. Chem. 1982,86,4516. (b) Takemura, H.; Saji, T.; Fujihira, M.; Aoyagui, S.; Hashimoto, K.; Sakata, T. Chem. Phys. Lett. 1986,122,496. (c) Hashimoto, K.; Hiramoto, M.; Sakata, T.; Muraki, H.; Takemura, H.; Fujihira, M. J. Phys. Chem. 1987, 91, 6198. (d) Hashimoto, K.; Hiramoto, M.; Lever, A. B. P.; Sakata, T. J.Phys. Chem. 1988,92,1016. (e) Hashimoto, K.; Hiramoto, M.; Sakata, T. J.Phys. Chem. 1988,92,4636. (fj Wheeler, J.; Thomas, J. K. J.Phys. Chem. 1984,88,750. (g) Wheeler, J.; Thomas, J. K. J.Phys. Chem. 1982,86,4540. (h) Willner, I.; Otvos, J. W.; Calvin, M. J. Am. Chem. SOC. 1981, 103, 3203. (i) Wellner, E.; Rojanski, D.; Ottolenghi, M.; Huppert, D.; Avnir, D. J.Am. Chem. SOC. 1987,109,575.
surface quenching are still lacking particularly in the area of luminescence-quenching processes and their relationship to sensor design. We report a new and interesting solid silica support based on fumed silica. This material has the remarkable property of forming dense, very high surface area, optically transparent disks that can be used for a variety of spectroscopic, photophysical, and photochemical studies. To understand both the nature of this surface and its potential as a support for sensors and sensitizers, we examined the photophysics of Ru(L)s2+where L is an a-diimine (Phzphen = 4,7-diphenyl-l,lO-phenanthroline, phen = 1,lO-phenanthroline, and bpy = 2,2’-bipyridine) adsorbed on this surface.
Experimental Section Materials and Sample Preparation. Syntheses of the
complexes have been given elsewhere.10 (5) (a) Wolfgang, S.; Gafney, H. D. J.Phys. Chem. 1983,87,5395. (b) Wei,S.;Gafney,H. D.;Clark, J. B.;Perettie,D. J. Chem.Phys.Lett. 1983, 99, 253. (c) Kennelly, T.; Gafney, H. D.; Braun, M. J. Am. Chem. SOC. 1986,107,4431. (d) Shi, W.; Wolfgang, S.; Strekas, T. C.; Gafney, H. D. J. Phys. Chem. 1986,89,974. (6) McKiernan, J.; Pouxviel, J.-C.; Dunn, B.; Zink, J. I. J. Phys. Chem. 1989,93, 2129. (7) (a) Krenske, D.; Abdo, S.; Van Damme, H.; Cruz, M.; Fripiat, J. 3. J. Phys. Chem. 1983, 84, 2447. (b) Abdo, S.; Canesson, P.; Cruz, M.; Fripiat, J. J.; Van Damme, H. J.Phys. Chem. 1981, 85, 797. (c) DellaGuardia, R. A.; Thomas, J. K. J. Phys. Chem. 1983,87,990. (d) Ghosh, P. K.; Bard, A. J. J.Phys. Chem. 1984,88,5519. (e) Schoonheydt, R. A.; DePauw, P.; Vliers, D.; DeSchryver, F. C. J.Phys. Chem. 1984,88,5113. (0 Bergaya, F.; Van Damme, H. J . Chem. SOC.,Faraday Trans. 2 1983, 79, 505. (8) Wheeler, J.; Thomas, J. K. In Inorganic Reactions In
Organized Media; Holt, S. L.; Ed.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981; No. 177, p 97. (8) (a) Wolfbeis, 0. S. Chem. Anal. 1988, 77,129. (b) Wolfbeis, 0. S.; Posch, H. E.; Kroneis, H. W. Anal. Chem. 1986,57,2556. (c) Wolfbeis, 0. S.; Weis, L. J.; Leiner, M. J. P.; Ziegler, W. E. Anal. Chem. 1988,60, 2028.
(9) (a) Bacon, J. R.; Demas, J. N. Anal. Chem. 1987, 59, 2780. (b) Carraway, E. R.; Demas, J. N. Anal. Chem. 1991,63,332. (c) Carraway, E. R.; Demas, J. N. Anal. Chem. 1991,63, 337.
0 1991 American Chemical Society
Carraway et al.
2992 Langmuir, Vol. 7, No. 12, 1991
Cab-0-Sil, a product of the Cabot Corporation (Tuscola, IL), is fumed silicon di0xide.I' Adsorption and desorption processes are rapid since small pores do not entrap the molecules. In this study, the Cab-0-Si1silica is pressed into thin glasslike disks by using an ordinary KBr IR disk press.12 The disks have many excellentfeatures for optical measurements. They adsorb a variety of materials and the high surface area permits high adsorbate concentrations per unit volume, especially with compressed disks. Scattering is somewhat of a problem, but much less so than for powders. The disks are quite sturdy toward evenly applied facial pressure, but are very fragile with respect to uneven or lateral stress. Luminescencemeasurements were made on three ruthenium(11) complexes on pressed disks of M5 grade Cab-0-Sil. The complexes used were R~(bpy)~Z+, Ru(phen)32+,and Ru(Ph2hen)^^+. The M5 grade is formed from primary particles approximately 14 nm in diameter and has a surface area of 200 f 25 m2/g.I1 The disks were pressed by using a standard IR press. About 25 mg of the material is placed in the die and pressed (4400-5000 psi hydraulic pressure) for 15-20 min under vacuum pumping. This mass corresponds to about 0.25 mL of untreated material or 0.15 mL of adsorbed or solvent-treated material. Pressing for 15-20 min greatly improves the quality of the resulting disk as compared to a 10-min period. Pumping does not have a large effect. The vacuum must be slowly released to prevent fragmentation. A typical disk is 1 / 2 in. diameter and 0.13 mm thick. For samples containing complexes, the Ru(I1) complex was added before pressing. A small portion of the silica (about 0.1 g) was stirred with a water or methanol solution of the complex. Adsorption appeared to be immediate, but the two were stirred for about 30 min. The silica was collected on a fine sinteredglass frit and washed with copious amounts of the solvent. From the initial complex concentration and the concentration of the wash solution (determinedspectrophotometrically), the number of moles adsorbed per gram of silica was calculated. The silica was dried overnight in a vacuum oven (40-60 "C). Disks were stored in a desiccator or vacuum desiccator. Surface Area Determination. The surfaceareas of the loose material and of pressed disk pieces were measured with a Micrometrics rapid surface area analyzer, Model 2205, with Ar as the probe gas. Oxygen Quenching Measurement. For 0 2 quenching studies an evacuable sample holder was constructed. Disk samples were held directly on the quartz window with very small pieces of RTV-118 film or cellophane tape.12 In a quenching measurement, the cell was evacuated, small amounts of oxygen were admitted, and (after 1 min for equilibration) the emission intensity or decay time was determined. The intensity measurements were made quickly, at a single wavelength near the emission maximum, but single-photon counting (SPC) decay time measurements required 30 min. Therefore, for 7 determinations the average pressure during the measurement was used. SPC excitation profiles and backgrounds were recorded before or after each quenching series. Measurements of emission intensity of silicasamples as oxygen was adsorbed and desorbed showed very little hysteresis. The increase in Zo/Z on desorption was generally 0-5%. Intensity experiments performed immediately following one another gave indistinguishable Stern-Volmer plots, however, and therefore this loss in intensity does not affect quenching results. Gas Adsorption Isotherm Apparatus. Gas-phase adsorption isotherms were measured on Cab-0-Si1 with an adsorption apparatus described by Ross and Olivier.l2J3 Solution Adsorption Isotherm Measurements. A few measurements were made to determine adsorption isotherms for the complexes on Cab-0-Sil. These adsorptions were performed (10)(a)Lin,C.-T.;Bottcher,W.;Chou,M.;Creutz,C.;Sutin,N. J.Am. Chem. SOC.1976,98, 6536. (b) Hauenstein, B. L., Jr.; Dressick, W. J.; Gilbert, T. B.;Demas, J. N.; DeGraff, B. A. J.Phys. Chem. 1984,88,1902. (11)Cab-0-Sil Fumed Silica Properties and Functions; Cabot Corporation: 1987. (12)Carraway, E. R. Excited State Quenching of Immobilized Ruthenium(I1) Complexes. Ph.D. Thesis, University of Virginia, 1989. (13)Ross,S.;Olivier, J. P. OnPhysical Adsorption; Wiley-Interscience: New York, 1964;Chapter 2.
similarly to the sample preparations described earlier. The aliquots were stirred together for 30 min at constant temperature. To reduce scatter background from suspended silica particles, solutions were centrifuged at 10 000 rpm for 20-30 min. The supernatant was analyzed by adsorption or emission. Solution adsorption data were fit to the Langmuir equationI4
where CZis the solution concentration, ni is the moles of solute adsorbed per gram of adsorbent, ns is the limiting value of ni or a measure of the capacity of the adsorbent, b is a constant related to adsorption intensity and equals K/al where K is the equilibrium constant for adsorption, and a1 is the activity of the solvent. Alternatively, b equals b'eQIRTwhere b' is a new constant and Q is the net adsorption enthalpy. Absorption and Luminescence Spectroscopy. Absorption spectra were run on a Hewlett-Packard 8452 diode array spectrophotometer. The blank scans for the single-beaminstrument are normally scans of air, using the empty disk holder as a mask. An undoped disk was measured to determine the baseline. Because of variations in disk thickness and placement and the rising UV background, the subtraction of blank disk baselines was not straightforward, and no completely satisfactory way of obtaining reliable backgrounds over the entire visible-UV region was obtained. Emission and excitation spectra were recorded on a Spex Fluorolog 2 spectrofluorometer by using front face viewing. Emission spectra were corrected for support background and instrument response. For steady state studies, the background from the Cab-0-Si1 was relatively small. Because of sample placement irreproducibilities,the background was removed by extrapolating it from a region where the complex did not emit. Decay Time Measurements. Luminescence decays were acquired with a time-correlated single-photon counting system (SPC).lS The excitation source is a Photochemical Research Associates (PRA) Nz-filled flash lamp. Filters isolated the 337nm line. The 4-pstime base was chosen for almost all acquisitions. Emission monochromator band widths were commonly 16 or 32 nm. Since the excitation pulse from the flash lamp exhibits an afterglow or tail, all fits were by a nonlinear least-squares deconvolution Marquardt program.16J6 The fits could include up to four exponentials. The quality of the fit was assessed by several statistical values (mainly the reduced x2) and by the appearance of the residuals plot. The emission decays of the silica samples were very nonexponential. There was a dominant very short decay-time (5 ns) contribution from the support itself along with minor longer components (20-100 ns). The decays could all be accurately reproduced with a sum of three or four exponential decays
Cai
~ ( t ) exp(-t/ri) (2) where a's are weighting factors and the 7's are decay times. The number of components was varied until a statistically valid reduced x2 and random residuals plot were obtained. A fourcomponent fit was usually necessary. Lifetime fits of different samples,using similar fitting regions, yielded essentiallythe same decay-time components, but showed small variations in the a's. Attempts to simply subtract out the observed sample blank were unsuccessful and indicate that the shortest deconvoluted decay times describe both background and sample decay. This is consistent with distributions of background and sample decay times, which overlap in this short decay-time region. Analysis of background decays showed that most of the intensity is due (14)Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.; John Wiley & Sons: New York, 1982. (15)(a) OConnor, D. V.; Phillips, D. Time-Correlated Single Photon Counting; Academic Press: New York, 1984. (b) Snyder, S.W.; Demas, J. N. Anal. Chem. 1989,61, 2704. (16)(a) Demas, J. N.ExcitedStateLifetimeMeasurements;Academic Press: New York, 1983. (b) Bevington, P. R.Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill: New York, 1969. (c) Demas, J. N.; Demas, S. E. Scientific Computing and Interfacing on Personal Computers; Allyn & Bacon: New York, 1990.
Langmuir, Vol. 7,No. 12,1991 2993
Transition-Metal Complexes on Fumed Silica 2.00
"I
Methanol Solution
lI - - - - C a b - 0 - 3
Disc
1111
Q)
0 1.25 C
W
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Figure 1. Two undoped pressed Cab-0-Si1 disks (lower center)
and one disk doped with R~(phen)3~+. There is a very slight cloudiness to the disks. Even to the eye, the disks are all but invisible on the paper unless the reflections are correct. The photograph is overexposed to enhance visibility. The pile of Cab-0-Si1weighs about 60 mg or the same as the total weight of all three disks. to a very short decay of about 5 ns with small contributions from longer components of about 20 and 200 ns. We reduced the complex decays to a single quantity, the preexponential weighted-mean lifetimegb (3)
This was useful for comparison with the intensity quenching data for assessing the extent of static quenching (vide infra).
Results Figure 1 is a photograph of several disks and a comparable amount of the unpressed Cab-0-Sil. The exceptional clarity and approximately 30: 1 reduction in volume is clearly visible. The disks show a slight cloudiness that is not visible in the photograph. The disk density is =lg / ~ m as - ~opposed to 0.032 g/mL of the bulk material. Since the density of quartz is 2.6, and of lechaterlierite and opal 2.2, the disks are approximately one-half dead volume or space. The reported surface area of the unpressed Cab-0-Si1 powder is 200 f 25 m2/g.11 We measured 191m2/g for the Cab-0-Si1powder and 196 f 8.5 m2/gfor disk pieces. Thus, compaction has no detectible effect on the surface area. Virtually instantaneous stabilization of intensity readings in the quenching experiments establishes a very open porous structure. The very high degree of quenching a t higher oxygen pressures shows that the entire disk volume is accessible to gas. We compare our adsorption studies for M5 fumed silica with other silicas. Cab-0-Si1 has a significantly higher adsorption capacity (=lo0 pmol/g) than the silica reported by Markham and Benton (68-11 pmol/g a t 0 "C and 100 "C, re~pectively).'~In contrast, porous Vycor glass5a a t room temperature appears to have a somewhat higher 0 2 adsorption capacity than Cab-0-Sil. Figure 2 shows a typical absorption spectrum of Ru(Ph2~hen)3~+ in a pressed disk and in solution. Other than perhaps some broadening and loss of structure, there are no significant changes in band energies or relative intensities. Similar results were obtained for the other complexes. Figure 3 shows emission spectra for the complexes in different solvents and on Cab-0-Sil. We include the (17) Markham,
E.C.; Benton, A. F.J.Am. Chem. SOC.1931,53,497.
\
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Wavelength (nm) Figure 2. Absorption spectra of R~(Ph2phen)3~+ in methanol and on a Cab-0-Si1 disk with background subtraction. polymeric support GE silicone rubber RTV-118 for There is a significant solvatochromismfor Ru(bpy)s2+,a smaller but still discernible solvent effect for R ~ ( p h e n ) 3 ~and + , virtually no solvent sensitivity for Ru(Ph2~hen)3~+. Figure 4 shows an 0 2 adsorption isotherm for Cab-0-Si1 powder. Included are fits using both Langmuir and Freundlich isotherms. Both isotherms are equivalent within our experimental error. Figure 5 shows solution adsorption isotherms for Ru( ~ h e n ) 3 ~and + R~(Ph2phen)3~+.Table I includes the adsorption parameters. There were variations in the luminescence quenching behavior as a function of the pumping time on the disks. Figure 6 shows 0 2 quenching curves as a function of pumping time after preparation. The large variations in behavior cease after about 10 days. The effect is completely reversed by exposure to solvent. If the disk is rewet with methanol, dried overnight in the vacuum oven, and pumped overnight, the intensity quenching by oxygen returns to the initial one-day result. Figure 7 shows a Stern-Volmer intensity-quenchingplot along with the corresponding 7M plot for R u ( b p y ) ~ ~Two +. forms are presented for the 7M data. TM was directly determined from all the decay-time component fits; this makes no attempt to correct for the short-lived support component. Alternatively, the 5-ns decay-time component was deleted from the computation of 7M;since there is a 5-ns support component, this partially compensates for the blank. Figure 8 shows the intensity 0 2 quenching curve for the three complexes in Cab-0-Si1 disks. The solid lines are the best fits to a Freundlich adsorption model (vide infra).
Discussion There is little apparent variation in absorption spectra of the complexes on adsorption on the Cab-0-Sil. On the emission there are small solvent effects for Ru(bpy)32+, lesser ones for R~(phen)3~+, and negligible ones for Ru(Ph2~ h e n ) 3 ~ +There . may be analogous shifts in the absorption spectra, but we cannot detect such small changes because of their greater breadth. We attribute the variation in solvent effect to a shielding of the excited state of the molecule from the environment. Excitation is localized in the a-diimine portion of the
Carraway et al.
2994 Langmuir, Vol. 7,No. 12, 1991
0.80
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Equilibrium Oxygen Pressure (cm Hg) Figure 4. Adsorption of oxygen on unpressed Cab-0-Si1Silica with fits to the Langmuir and Freundlich adsorption isotherm models.
600
.
700
,
800
Wavelength (nm)
0.80 ;
c2 ( R U ( L ) d C W 2 PM) Figure 5. Langmuir plots for adsorption of Ru(phen)s2+(+) and of Ru(Ph~phen)3~+ (*) on loose Cab-0-Si1 from methanol.
Table I. Langmuir Adsorption Isotherm Parameters for Adsorption of Ru(I1) Complexes on M5 Cab-0-Si1 complex ns,pmolla b Ru(phenMClO4h 2.12 0.214 Ru(Phzphen3(ClO& 0.685 0.397 0.20
Figure 3. Corrected emission spectra of Ru(bpy)gP+(A) in water and methanol, on a Cab-0-Si1 disk, and in RTV-118 film, of Ru(phn)z2+(B)in methanol, on a Cab-0-Si1 disk, and in RTV118film, and of R~(Ph~phen)~z+ (C) in methanol, on a Cab-0-Si1 disk, and in RTV-118 film. complex.'8 For Ru(bpy)~Z+the excited portion is very the shielding is greater. For exposed, while for Ru(phen)~ Ru(Phzphen)~~+ the phenyl groups have a very large shielding effect on the luminescence center and make the complex largely insensitive to solvent variations. QuenchingModels. Even in the absence of quencher, the nonexponential decays are clearly a sign of heterogeneity. With such a large number of exponentials, it is (18)Krug, W. P.; Demas, J. N. J. Am. Chem. SOC.1979, 101, 4394.
unlikely that the decay times represent four true decaytime components, especially since no consistent pattern is seen in the changes in the fitting 7's as the oxygen pressure varies. Thus, the components should be considered as fitting parameters with no direct physical significance. The nonlinear intensity Stern-Volmer plots also establish a heterogeneous quenching model. This is not surprising since the decay curves are complex. We turn now to the question of static versus dynamic quenching. In homogeneous media with only a singlecomponent exponential decay, the intensity and lifetime forms of the Stern-Volmer equations are 70/7
1+ K,,[Ql
(4)
where [Q]is the quencher concentration, 7's are lifetimes, I's are intensities, K,, and kz are the Stern-Volmer and bimolecular quenching constants, respectively, and Keqis
Transition-Metal Complexes on Fumed Silica
Langmuir, Vol. 7,No. 12,1991 2995 6.00 3 a
0
- 5.00
A
4.00
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,
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Oxygen Pressure (cm Hg) Figure 6. Changes in intensity-quenching plots for Ru(bpy)32+ on Cab-0-Si1 with pumping. The number of days of pumping is indicated.
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Oxygen Pressure (cm Hg) Figure 7. Intensity (*) and lifetime quenching of Ru(bpy)32+on Cab-0-Sil. Triangles indicate T M calculated from a fourcomponent fit from the data peak on decays without baseline subtraction. Circles indicate T M calculated from the above but with the shortest -541s component omitted. the association constant for quencher binding to the luminescent species. The subscript 0 denotes the value in the absence of quencher. If plots of T O / T or l o / l versus quencher concentration are linear and match, quenching is purely dynamic (i.e., Keq = 0). If l o / l is above 7 0 / 7 , static quenching is present. Fitting both intensity and lifetime curves allows determining the static- and dynamicquenching contributions. This technique fails for our heterogeneous systems that cannot be characterized by a single decay time. For ascertaining the relative contributions of static and dynamic quenching, we used the method described earlier.gb The decays were deconvoluted by using a sumof-exponentials impulse response and the preexponential weighted-mean lifetime, TM, was computed from eq 3. Even though no physical significance is ascribed to the a's and 7'8, the calculated T M is still an accurate representation of this quantity.gb Further, the following is true in the absence of static quenching: = 7MO/TM (7) where TMO is TM in the absence of quencher. Thus, if we IO/'
25.00
-,
0.00 0.00
20.00
40.00
60.00
80.00
Oxygen Pressure (cm Hg) Figure 8. Stern-Volmer intensity quenching of Ru(bpy)s2+(A), Ru(phen)S2+(B),and Ru(Phzphen)a2+(C)on silica disks. The solid lines are the best fits using a two-parameter Freundlich adsorption model. use TM'S rather than single-exponential decay times in our Stern-Volmer equation, we can compare intensity and T data in microheterogeneous systems to assess the extent of static quenching. Given the difficulty of the TMO/TMmeasurements, Figure 7 shows reasonable agreement between the T M O / T M and
Carraway et al.
2996 Langmuir, Vol. 7, No. 12, 1991
Io/I data. The two agree very well up to about 20 cmHg of oxygen, but there is a suggestion of some static quenching a t higher pressures. However, even if the static component is real, there is really only a small static-quenching component since Stern-Volmer plots amplify the differences. From the intensity data, the sample is 88% quenched a t the highest oxygen concentration where the discrepancy is largest. However, the T M ~ / T M data indicate 8 1 4 4 % of dynamic quenching. Thus, the quenching is dynamic up to about 1 atm of air, and perhaps there may be a small 4-7% static-quenching component a t higher oxygen levels. Earlier we reported an analogous set of data for Ru(phen)32+where the data are better because the complex is more l u m i n e ~ c e n t .The ~ ~ agreement between the two Stern-Volmer plots indicated no evidence (((6% out of 80 % quenching) for static quenching. We conclude that all quenching is predominantly, if not exclusively, dynamic with a t most a minor static-quenching component. We use a dynamic-quenching-only model in all further analyses. For such a complex system we can envision several possible quenching mechanisms. The complex decay curves clearly indicate a heterogeneous distribution of lifetime sites. Site quenching could be by gaseous 0 2 and/ or adsorbed 02.For adsorbed 0 2 the Langmuir and Freundlich adsorption models are two possible models for 0 2 surface concentrations. The Langmuir isotherm model is one of the simplest, most used formulations describing adsorption phenomena up to monolayer ~ 0 v e r a g e . l ~
6 = [QIads/[QIads~= bP/(1 + bP)
(8)
where 6 is the fractional surface coverage of the adsorbate, [&lads is the surface concentration of the adsorbate (quencher) and [Q]ads~is the maximum surface concentration of the quencher (presumably a monolayer), P is the adsorbate gas-phase pressure, and b is related to the adsorbate adsorption energy. The surface quencher concentration is
The Freundlich isotherm is an extension of the Langmuir isotherm, which provides an empirical parameter for variation in the interaction energy between adsorbate and support.14 The Freundlich adsorption isotherm for gas-solid adsorption is
6 ap'l" (10) where 6 is the fractional surface coverage of the adsorbate, a is a collection of constants, P is the equilibrium gasphase pressure, and n is an empirical parameter related to the intensity of the adsorption. The Langmuir model assumes that all sites have the same binding energy, while .the Freundlich model acknowledges that there will be a distribution of site energies with preferential binding to the most energetic sites. For multiple emitting sites with different quenching constants, the Stern-Volmer equation is
where f o i is the steady-state or integrated fraction of light emitted from the ith component and Ksvi is its SternVolmer quenching constant.
Based on the above considerations, two primary questions need addressing: the nature and distribution of sensitizer sites on the adsorbate and the mode of quencher delivery to the sensitizer (i.e., surface adsorption followed by diffusional quenching of surface-adsorbed species or direct gas-phase quenching). In the surface-quenching case, [Q] in the equations is the surface concentration. We address these considerations with several concrete models that we explore below: (A) Two independent binding sites with one site quenched and the other unquenched. (B) Two independent binding sites with both sites quenched by gaseous diffusion with different quenching constants. The appropriate equation is
Io/I =
(
fol
1 + KsvI*[QI
+ 1 + Ksvz*[Q]) fo2
)-l
(12)
Model A is a special case of this equation with Ksv2 = 0. (C)Model B along with a static-quenching component. (D)Gaussian distribution of lifetime sites with each site quenched with the same bimolecular rate constant. That is Ksvi = kzq, with 122 independent of site. T ~ ' Sless than 0 were excluded from the computation. This is eq 11 expanded to an integral with foi equal to a Gaussian. (E) Diffusional surface quenching by adsorbed oxygen. Oxygen adsorption follows a Langmuir isotherm. Substitution of this concentration expression into the SternVolmer equation gives a two-parameter Stern-Volmer equation
& / I = 1 + KSV
[QI adsMbP 1
+ bP
(F) Langmuir surface diffusional quenching of model E coupled with gaseous diffusional quenching with each site being quenched with the same bimolecular rate constant. ( G )Langmuir surface diffusional quenching of model E where we assume a Gaussian distribution of lifetime sites, with each site being quenched with the same bimolecular rate constant. (Le., Ksvi = k 2 ~ i ,with k2 independent of site). (H) Diffusional quenching by adsorbed oxygen only with oxygen adsorption following a Freundlich isotherm. Substitutions similar to those made in the Langmuir isotherm lead to the followingtwo-parameter Stern-Volmer quenching expression
Io/I = 1 + K ~ ~ [ Q l a d s ~ a P l ' " (14) Of course, since the decays are highly nonexponential, the parameters of several models represent lumped parameters that are some average over the available sites, lifetimes, and quenching constants. However, within the framework of lumped-parameter models, we can assess which models accurately reproduce the observed data. Model A gives extremely poor fits. The somewhat more complex version B produces significantly better fits, but there are clear small systematic deviations. Model C adds a static-quenching component to model B and gives reasonable fits; these fits are not as good as the simpler Freundlich adsorption model. Further, the best-fit static binding constants are negative, which is physically impossible. For this reason, and because the TM quenching data suggest the absence of static quenching, we discount this model. Model D fits worse than any other model and the peak of the distribution approaches zero decay time. This model cannot duplicate the sharp hook of the Stern-Volmer quenching plots. Since a two-site quenched system gives
Langmuir, Vol. 7, No. 12, 1991 2997
Transition-Metal Complexes on Fumed Silica Table 11. Summary of Freundlich Fits to 0 2 Quenching Data .
__.-
run 1 run 2 run 3 run 4
disk 2
Ru(phen)32t run 1 run 2 disk 2
Ru(Ph*phen)$+
0.33 0.38 0.13 0.13 0.16
0.70 0.55 0.57
0.98
0.42
0.45 1.10 1.87
0.44 0.44 0.57
0.57 0.61
a reasonable fit, presumably this dual Gaussian distribution with different quenching constants could fit the data even more reliably, but the large number of parameters (two center decay times, a ratio of peak concentrations, two widths, and two quenching constants) makes this an undesirable model. Model E's surface quenching with a Langmuir adsorption isotherm yields very poor overall fits, which are still about an order of magnitude better than the single Gaussian distribution (D), but worse than the two-independentsite models (A and B). Further, even adding a dynamic gas-phase quenching component (model F) improves the fit noticeably, but significant systematic errors are still present. Surface quenching with a Langmuir isotherm and a Gaussian distribution of sites with the same bimolecular quenching constant (model G) only minimally improves on the simple Langmuir model (E). The distribution of sites peaks near a zero decay time and is extremely wide. Figure 8 shows the best Freundlich quenching model (model H) plots. The fits are all within experimental error even though only two variable parameters are used. Of course, adding a distribution to the lifetime could further improve the fits, but since they are already within our experimental error, such a refinement is not justified. The fitting parameters are given in Table 11. In spite of the large differences in quenching between complexes, the similarity of the l / n values for the different complexes is consistent with the l / n term reflecting common surface adsorption phenomena. Further, the l/n's are compatible with the 0 2 adsorption isotherm measured directly. For comparison, the 0 2 quenching for these complexes in RTV-118 silicone rubber are much less hooked than for Cab-0-Sil. Further, the silicone data are best fit by two independent sites each with a different quenching constant.gc In contrast to the silica, the power law Freundlich quenching model is markedly inferior to the twosite model for the silicone system. Surface Adsorptionand Changes. From the solution adsorption data for the metal complexes, we see that the main difference between the complexes is the limiting surface concentration. In both cases surface coverage is low. Surface coverage as calculated from the reported surface area of M5 Cab-0-Sill1 (200 f 25 m2/g) and the estimated radius of the complexes, about 6.3-10.2 A. Using 6.3 A as the radius for Ru(phen)3(C10& and the above limiting concentration yields a surface coverage of about 1%.With its larger radius, Ru(Ph2phen3)(ClO& has very nearly the same surface coverage. The slightly higher value of b for Ru(Ph~phens)(C104)2 is a reflection of the steeper slope or quicker attainment of full coverage observed in the plots of the raw data, surface concentration versus solution concentration. This may be due to enhanced interactions withless polar regions
of the silica surface with this complex relative to Ru( ~ h e n ) 3 ~and, + , thus, a greater driving force for adsorption from the polar solvent. The variation in quenching behavior with pumping time leaves no doubt that the character of the silica surface changes with pumping time and that the interaction between the adsorbed complex and oxygen is sensitive to this change. However, in the absence of quencher, the emission intensity (10)and decay-time decay parameters change very little over the same period during which the quenching behavior has undergone a large change. Relevant studies of the chemistry of silica surfaces are most often done as a function of temperature and not reduced pressure. The silica surface itself consists of silanol and siloxane groups. Adjacent silanol groups exhibit hydrogen bonding. Adsorbed molecular water may also be present. For Cab-0-Sil, surface moisture can be removed by heating to 110 OC or evacuation a t room temmm of mercury.'l Heating above 110 OC perature a t reversibly dehydrates adjacent hydroxyl groups resulting in formation of a new siloxane group. This process may be reversed by exposure to air a t 100% relative humidity or immersion in water, depending on the temperature and degree of dehydration. Zhuravlev and co-workerslg have studied the concentration of surface hydroxyl groups on various silicas. They determined that treatment of amorphous silica under vacuum a t 180-200 OC results in removal of water and retention of hydroxyls. The average silanol number, (YOH (the number of OH groups/nm2),is determined to be 4.554.9 for a number of types of silica of various surface areas and preparation methods. However, none of these silicas were prepared by a flame process, as is Cab-0-Sil. The Cab-0-Si1 silanol number is 3.5-4.5." This discrepancy is probably explained by the flame synthesis of Cab-0-Sil; higher temperatures may lead to a higher concentration of unhydrolyzable surface siloxane groups. While the two studies discussed above show obvious differences, the observed changes in quenching behavior are consistent with dehydroxylation and reversible rehydroxylation under the conditions described in the Cab0-Si1 technical data. Thus, the oxygen quenching data on pumping reflect changing relative amounts of surface hydroxyl and siloxane groups with a concomitant change in 02 surface diffusion properties or coverage.
Conclusions The high degree of surface heterogeneity as judged by the large number of exponential terms in the decay-time measurements makes it difficult to conclusively model the surface quenching. Multiple sites or distributions of sites must be present. However, static quenching is certainly not a significant component. We turn now to a consideration of the molecular quenching mechanisms. In terms of models, the dynamic quenching can best be described by a Freundlich surface quenching picture. Further, a t 1 atm of 02, the average distance to an adsorbed oxygen molecule is about 15 A (based on a uniform distribution of molecules), which is certainly close enough to give diffusional surface quenching. Independent corroborative evidence for the importance of surface quenching comes from the variation in quenching behavior with pumping time. It is difficult to envision how changes in the siloxane density on the surface would so dramatically affect a gaseous diffusional quenching. (19) Zhuravlev, L. T. Langmuir 1987, 3, 316.
2998 Langmuir, Vol. 7, No. 12, 1991
However, changes in the surface properties could have a profound effect on surface adsorption and diffusion. We cannot discount diffusional gaseous quenching. Kinetic gas theory indicates that each complex sustains 5-10 collisions with an 02 molecule per nanosecond a t 1 atm of 0 2 . The experimental quenching demonstrates that the gaseous quenching, if present, must be rather ineffective on a per collision basis. When compared with typical Ru(I1) complexes in solution, our maximum observed rate constants are about 2 X lo8M-' s-l assuming only gaseous diffusional quenching compared with (2-6) X 10-9 M-1 s-l in fluid solutions. The need to consider a contribution from gaseous diffusion is shown by the fact that a bimodal Gaussian distribution of sites with gasphase or surface quenching also provides a reasonable fit to the data. At this time we favor a quenching model that has a significant contribution of surface quenching with an unknown amount of gaseous diffusion. In spite of having only two parameters, the Freundlich model provides the best experimental fits. Further, even if not correct in all details, it provides a superb empirical model for generating calibration curves and predicting the response of sensor
Carraway et al.
systems based on inorganic complexes adsorbed on complex surfaces. Improvements in resolution and modeling of these complex systems are going to require finer resolution of the decay-time data. We do not feel that with the quality of our current data any attempt to resolve the decays into distributions would be acceptable. Our long flash tail contributed a serious and bothersome long-time contribution to our data and exacerbated interference from the silica blanks. Use of a crisper excitation source would make for cleaner resolution of the short-lived components and minimize deconvolution uncertainties.
Acknowledgment. We gratefully acknowledge support by the National Science Foundation (Grants CHE 8600012 and 88-17809). We thank Coleman Jones and Professor F. E. Wawner, Jr., for assistance with the surface area measurements and Robert Paine for pointing out the unique ability of Cab-O-Si1to glass on pressing. We thank Hewlett-Packard for the gift of the 8452A spectrophotometer and Henry Wilson for his kind assistance. 15158-62-0; Ru(phen)a2+,22873Registry No. R~(bpy)3~+, 66-1;R~(Ph*phen)3~+, 7782-44-7; 02,7782-44-7;silica, 7631-86-9.
