J. Phys. Chem. 1995, 99, 9886-9892
9886
Quenching of Tris(2,2'-bipyridine)ruthenium(lI) Luminescence by Cobalt(II1) Polypyridyl Complexes in Different Sites in and on Clays Amir Awaluddin,' Roberto N. DeGuzman,' Challa V. Kumar,*T' Steven L. Suib,'y'>* Sandra L. Burkett,a and Mark E. Davis*** Department of Chemistry U-60, University of Connecticut, Storrs, Connecticut 06269-3060; Institute of Materials Science and Department of Chemical Engineering, University of Connecticut, Storrs, Connecticut 06269-3060; and Division of Chemistry and Chemical Engineering, Califomia Institute of Technology, Pasadena, Califomia 91125 Received: September 30, 1994; In Final Form: March 22, 1995@
The effect of quenching of tris(2,2'-bipyridine)ruthenium(II) [R~(bpy)3~+] luminescence by C0(phen)3~+in various locations in and on bentolite-H clay has been investigated. Ru(bpy)32+incorporated into the interlayer region of bentolite-H by ion exchange exhibited spectral properties that were distinct from the R~(bpy)3~' incorporated by incipient wetness methods. For example, the samples showed different diffraction, luminescence emission, and lifetime decays. The method of preparation, the nature of counterion (chloride or cyanide), and the presence of a quencher [ C ~ ( p h e n ) ~ influence ~+] the emission properties. For example, incipient wetness methods allow deposition on outer surfaces (OUT) of the clay whereas ion-exchange leads to deposition primarily in the interlayer spacing (IN) based on spectroscopic and diffraction data. The luminescence intensities and lifetimes decrease in the order Ru(0UT) > Ru(IN) > Ru(OUT)/Co(IN) > Ru(IN)/ Co(0UT) > Ru(OUT)/Co(OUT) Ru(IN)/Co(IN). Different methods of preparation and treatment result in location of R ~ ( b p y ) 3 ~at+different internal or external sites in the clay which give different luminescence and lifetime properties. By controlling the locations of Ru(I1) and Co(II1) (inside or outside), the quenching rates could be modulated. The location of the Ru(I1) complex seems to control both the photophysical properties and the efficiency and rate of quenching by Co(II1). The Ru(1N) and Co(1N) sample is very similar to the Ru(0UT) and Co(0UT) sample. These two samples are considerably different from the Ru(IN)/Co(OUT) sample and the Ru(OUT)/Co(IN) sample.
SCHEME 1: Clay Systems under Investigation
Introduction Ru(bp~)3~+ has unique excited-state properties'-'s and has therefore been used as a probe to study the physicochemical properties of clay minerals. I 6 - l 7 R~(bpy)3~+ luminescence can be quenched by a wide variety of inorganic and organic substrates and this has been the focus of much i n t e r e ~ t . ' ~ . ' ~ Depending on the nature of the quencher, quenching reactions may proceed by energy transfer or electron transfer mechanisms (or both).9-'5i20-22 In this paper we report the results of a study of the quenching of R ~ ( b p y ) 3 ~luminescence + by C0(phen)3~+at various sites. Both Ru and Co complexes have been deposited on the surface (designated as OUT)by the incipient wetness method or in the interlayer region of the clay (called IN) by ion exchange. X-ray diffraction was used to monitor the incorporation of these complexes into the clay. Steady-state luminescence and luminescence lifetime measurements were used to probe the emission and quenching characteristics of these systems. Locations of the Ru and Co complexes in these materials are shown in Scheme 1. Synthetic details used to prepare the materials depicted in Scheme 1 are given in the Experimental Section. We have varied the location of the Ru and Co species in order to systematically separate these species. Clays are convenient spacer layers because the (001) reflection provides a direct measure of expansion of the clay layers during +Department of Chemistry, UC. Institute of Materials Science and Department of Chemical Engineering,
uc.
5 Califomia Institute of Technology.
* To whom correspondence should be addressed.
@
Abstract published in Advance ACS Abstracts, May 15, 1995.
Ruoun
0
inter~alation.~~' Antenna molecules such as R ~ ( b p y ) 3 ~can + be used to absorb light and potentially transfer this energy to other parts of the system.1,2,4 Electron-transfer,energy-transfer, and chemical reactions may all occur. C0(phen)3~+was chosen as a quencher molecule because it is believed that a contact or short-rangeelectron-transfer mechanism occurs with Ru(bpy)s2+ rather than a, long-range or Forster (dipole coupling) mechan i ~ m . ~ The ' proximity of the Ru and Co complexes will therefore be an important factor in electron transfer mechanisms in such clay systems.
Experimental Section
A. Adsorption of Ru(bpy)3*+in the Interlayer Region and on the Surface of Clays. I . Ru(bpy)jZ+inside Clays. A solution of Ru(bpy)3(Cl)2 (100 mL of M) (Aldrich) was added to 1 g of bentolite-H (E.C.C. America, Inc.) and was stirred for 24 h. The ion-exchanged clay particles were filtered
0022-3654/95/2099-9886$09.00/0 0 1995 American Chemical Society
Quenching of Tris(2,2’-bipyridine)ruthenium(II) Luminescence and washed with distilled deionized water until a negative test for chloride ions with silver nitrate solution was obtained. Samples were freeze-dried. 2. Ru(bpy)j2+outside Clays (Incipient Wetness Method). A solution of Ru(bpy)s(C1)2 (2 mL of M) was added to 1 g of bentolite-H. The volume of liquid used was just enough to cover the clay particles. The samples were immediately freezedried. 3. Both Ru(bpy)j2+and Co(phen)3”+inside Clays. Solutions of R~(bpy)3~+ (50 mL of M) and C0(phen)3~+(50 mL of lop6M) were added to 1 g of bentolite-H. The mixture was stirred for 24 h, filtered, washed with distilled deionized water, and freeze-dried. 4. Ru(bpy)j2+inside Bentolite-H, with Co(phen)j3+outside Bentolite-H. A solution of R~(bpy)3~+ (100 mL of M) was added to 1 g of bentolite-H and stirred for 24 h. The yellow colloidal solution was filtered, washed with distilled deionized water, and freeze-dried. The pale yellow solid obtained was placed in a watch glass, and 2 mL of M of C0(phen)3~+ was added by incipient wetness procedures. The mixture was freeze-dried. 5. R ~ ( b p y ) 3 ~outside + Bentolite-H, and Co(phen)j3+inside Bentolite-H. The procedure was the same as for 4 except Ru( b ~ y ) 3 ~and + C0(phen)3~+were interchanged. 6. Both Ru(bpy)j2+ and Co(phen)d+ outside Clays. Solutions of Ru(bpy)j2+ (1 mL of M) and C0(phen)3~+(1 mL of M) were added to 1 g of bentolite-H. These solutions were just enough to cover the clay particles. The sample was freeze-dried. 7. The same preparations from 1 to 6 were done using Ru(bpy)s(CN)z to study the counterion effect. The clay material used for these studies, bentolite-H is montmorillonite based. It has a cation-exchange capacity of 100 mequiv/100 g. Its composition is 68.1% Si02, 13.5% Al2O3, 2.9% MgO, 0.7% Fe2O3, 0.9% CaO, 3.5% Na20, 0.1% K20, and 0.2% TiO;?. B. Luminescence. Luminescence excitation and emission spectra were recorded with a Spex Model 1680 B doublemonochromator luminescence spectrometer. Samples (approximately 0.25 g) for luminescence and lifetime measurements were loaded in quartz tubing (3 mm inside diameter) and degassed in a vacuum line at about Torr for about 3 h and sealed. Lifetime experiments were done by using a timecorrelated single photon counting method. Decays were deconvoluted by a least-squares method to obtain the best fits to the data. The data were fit between channels 20 and 256 (inclusive). Attempts to fit data to an energy transfer mechanism were not successful. C. X-ray Powder Diffraction. X-ray powder diffraction (XRD) experiments were done on a Scintag Model PDS 2000 diffractometer. Samples were mounted on glass slides by sprinkling powder onto the slides in order to avoid preferential ordering. D. Infrared and Raman. IR spectroscopy was performed on a Nicolet System 800 FTIR Instrument. IR samples were prepared as KBr pellets. Raman spectroscopy was performed using the FT Raman accessory for the Nicolet System 800 (Nd: YAG laser, wavelength = 1064 nm, operating at 600 mW power).
Results A. X-ray Diffraction. The intercalation of R~(bpy)3~+ into the interlayer region of bentolite-H was monitored with X-ray powder diffraction. Table 1 summarizes the observed d spacings of the intercalated clays. The d spacings for the ion-exchanged
J. Phys. Chem., Vol. 99, No. 24, 1995 9887 TABLE 1: Observed d Spacings (hi) of the 001 Line for Different Bentolite-H Samples after the Incorporation of Ru(I1) Complexes sample location Ru(bpy)3(Cl)z Ru(bpy)3(CN)3 1. R ~ ( b p y ) 3 ~ + IN 14.416 14.384 2. R ~ ( b p y ) 3 ~ + OUT 10.863 9.400 3. R ~ ( b p y ) 3 ~ + IN 14.178 15.220 C0(phen)3~+ IN 4. Ru(bpy)3’+ IN 13.51 14.638 C0(phen)3~+ 5. R~(bpy)3~+
OUT OUT
14.142
15.092
C0(phen)3~+ 6. Ru(bpy)3’+
IN OUT
12.072
9.37
C0(phen)3~+
OUT
samples were larger than samples made by incipient wetness method. These results indicate that R~(bpy)3~+ is intercalated into the interlayer region of the clay for the ion-exchange preparations. B. Luminescence. Emission spectra of Ru(bpy)3*+ incorporated into bentolite-H under various conditions are shown in Figure 1. Figure l a is the luminescence emission spectrum of R ~ ( b p y ) 3 ~incorporated + by ion exchange. The emission spectrum of R~(bpy)3~+ adsorbed outside of the clay is shown in Figure 1b. The spectrum has a maximum at 623 nm. Spectra of R~(bpy)3~+ in the presence of C0(phen)3~+are shown in Figure lc-f. It is noteworthy that emission maxima for Ru( b ~ y ) 3 ~and + C0(phen)3~+ intercalated into the clay were significantly blue shifted to 614 nm. Results of the steady-state emission studies are summarized in Table 2. For the Ru(bpy)3(C1)2samples, the emission maxima vary from 610 to 623 nm for different preparation methods. Ion-exchanged R~(bpy)3~+ in bentolite-H has an intensity about 20% lower than R~(bpy)3~+ introduced by incipient wetness. Similarly,the Ru(bpy)3(CN)2-exchanged sample has an intensity of about 9% less than the sample prepared by incipient wetness. The addition of Co(phen)s3+ results in a decrease in the luminescence intensity of the Ru(I1) complexes in all the samples. C0(phen)3~+is not luminescent and is a known quencher of R~(bpy)3~+ luminescence. Quenching by Co( ~ h e n ) 3 ~is+seen for samples containing Ru(bpy)s(C1)2 as well as Ru(bpyMCN)2. Luminescence lifetime data for R ~ ( b p y ) 3 ~with + or without C0(phen)3~+adsorbed at different sites in bentolite-H are summarized in Table 3. All of the lifetime data show doubleexponential decays. The sum of the squares of residuals appears to be random over the regions used to determine lifetimes since the deviations are evenly distributed around zero. An example of a fit to a double-exponential decay is shown in Figure 2. The solid line is the calculated curve and the dots are individual data of the lifetime decay. The dotted line is the trace of the lamp profile which shows a small noise level past 50 ns. C. Infrared and Raman. Raman spectra for Ru(IN) where Ru = Ru(bpy)3(CN)2 are shown in Figure 3a. Peaks are observed at 2058, 1575, 1453, 1107, 1085,914,840,787,706, and 423 cm-’ which are consistent with R~(bpy)3~+ on bentoliteH. Similar Raman data for Ru(0UT) are shown in Figure 3b. Note that the band at 1085 cm-I is much more pronounced for Ru(0UT) than Ru(1N). The mixed Ru(IN)/Co(OUT) system in Figure 3c shows a Raman spectrum very similar to that of Ru(IN). Raman data for Ru(OUT)/Co(OUT) in Figure 3d show the absence of a peak at 1084 cm-I which was pronounced for Ru(OUT). Infrared data for Ru(1N) and Ru(0UT) are shown in Figure 4a,b, respectively. Bands at 3626, 3443, 1640, 1506, 1091, 1041,914,844,795,696,676,520, and 471 cm-’ are observed in both spectra although the band at 1506 cm-I is more
9888 J. Phys. Chem., Vol. 99, No. 24, 1995
Awaluddin et al.
d
a
150,000 100,000
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50,000 -
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I
500
550
650
600
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800,000
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Wavelength (nm)
Wavelength (nm)
Figure 1. Luminescence emission of Ru(bpy)3Clz-modifiedbentolite-H: (a) Ru (IN); (b) Ru (OUT); (c) Both Ru (OUT) and Co (OUT); (d) Both Ru (IN) and Co (IN); (e) Ru (IN) and Co (OUT); (f) Ru (OUT) and Co (IN). TABLE 2: Luminescence Peak Wavelengths of the Different Intercalated Clay Samples
1. R~(bpy)3~+ IN 619 1.21 x lo6 637 3.07 x lo6 2. Ru(bpy)3” OUT 623 1.50 x lo6 636 3.37 x lo6 3. R~(bpy)3~+ IN 614 2.94 x lo5 636 1.18 x lo5 C0(phen)3~+ IN 4. Ru(bpy)3’+ IN 617 6.10 x lo5 616 2.98 x lo5 C0(phen)3~+ OUT 5. Ru(bpy)3’+ OUT 616 1.04 x lo6 630 4.39 x lo5 C0(phen)3~+ IN 6. Ru(bp~)3~+ OUT 610 3.23 x lo5 630 1.68 x lo5 C0(phen)3~+ OUT a Wavelength (nm). Intensity (countsh). pronounced for Ru(0UT) than Ru(1N). Assignments3’ of IR and Raman data are given in Table 4. Note that the descriptions IN and OUT refer to the majority of ions in these samples.
Discussion The methods of ion-exchange and incipient wetness were chosen to obtain different intercalated clay samples as depicted
TABLE 3: Lifetimes Determined by Time-CorrelatedSingle Photon Counting for Ru(bpy)3(CI)z sample location TI0 t2a 1. R~(bpy)3~+ IN 201 & 10 331 z t 7 2. R~(bpy)3~+ 383 f 5 OUT 278 17 3. R~(bpy)3~+ IN 128 & 11 228 f 57 C0(phen)3~+ IN 4. R~(bpy)3~+ 353 f 11 IN 178 3 OUT C0(phen)3~+ 5. Ru(bpy)3’+ OUT IN C0(phen)3~+ 6. Ru(bpy)3’+ 255 f 38 OUT 134 f 2 C0(phen)3~+ OUT a Lifetimes are in nanoseconds, and errors are given as standard deviations.
* *
in Scheme 1. Introduction of Ru(bpy)s2+ by ion-exchange results in intercalation of Ru(bpy)s2+ into the interlayer of the clay, as demonstrated by the X-ray diffraction results of Table 1. The nonintercalated clay has a dml spacing of 13.7 A. An increase in the d spacing of the 001 plane is related to the expansion of the interlayer distance. The interlayer spacing of clay depends on many factors such as the size and orientation of intercalated guest molecules, the extent of hydration and
J. Phys. Chem., Vol. 99, No. 24, 1995 9889
Quenching of Tris(2,2'-bipyridine)ruthenium(II) Luminescence y
3.11,
I
,
1
1
1
4000
3600
3200
1
1
2800 2400
1
1
2000 1600
l
1200
I
800
I
400
WAVENUMBER
Figure 4. Infrared spectra of Ru(bpy),(CN)2 systems: (a) Ru(1N); (b) Ru(0UT).
TABLE 4: Assignments37of IR and Raman Bands for Ru Complexes band (cm-I)
1
I
~00.0
I
aoo.0
=o.
I
!
0
400.0
I
I
100.0
band (cm-')
assignment
3626
OH stretch, clay
1085
3443
NH stretch
1041
2058 1640
CN stretch bpy symmetric mode, NH deform. bpy symmetric mode, NH deform. bpy symmetric mode, NH deform. bpy symmetric mode, NH deform. bpy symmetric mode, NH deform. bpy symmetric mode, NH deform.
795 787
bpy symmetric mode, NH deform. bpy symmetric mode, NH deform. C H stretches, bpy C H stretches, bpy
706
CH stretches, bpy
696
CH stretches, bpy
676
CH stretches, bpy
520
CH stretches, bpy
47 1
MLCT
1575
000.0
1506
TIME I NANOSECONDS
Figure 2. Luminescence lifetime decay of Ru(0UTj.
1453 1107 1091
I
I
I
3200
2800
2400
I
I
I
2000 1600 1200 RAMAN S H I R (cm-')
I
I
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400
Figure 3. Raman spectra of Ru(bpy)3(CN)z systems: (a) Ru(1N); (b) Ru(0UT); (c) Ru(1Nj and Co(0UT); (d) both Ru(IN) and Co(1N). dehydration conditions, and the nature of c o ~ n t e r i o n s .Ideally, ~~ the d spacing should be the same as the diameter of the largest cations or molecules present in the interlayer space. However, d spacings can sometimes be smaller than the diameter of the cations present due to strong interactions between the cations and the negatively charged layers. The interlayer space can also be changed by deforming the tetrahedral-octahedraltetrahedral sheet structure of the clay itself (which is typical for a smectite clay such as bentolite-H), which can cause the (001) peak in the X-ray diffraction pattem to broaden.23 A reviewer has suggested that the X-ray data of Table 1 are not likely due to deformation of the clay sheets but rather to interstratifaction especially due to the low loadings used in this work. Incipient wetness methods result in the association of Ru( b ~ y ) 3 ~onto + the surface of the clay, and as a result, the
assignment
interlayer distance of the clays should not be affected. On the basis of these results, we have labeled the different intercalated clay samples as either IN (prepared by ion exchange) or OUT (prepared by incipient wetness) as a shortened nomenclature. The Raman and infrared data in Figure 3 and 4 and Table 4 clearly show that Ru(1N) and Ru(0UT) are similar except for the intensities of the Raman bands at 1084 cm-' and infrared bands at 1506 cm-I. These differences may be due to differences in CN-/Cl- content for the IN and OUT systems since Ru(0UT) samples prepared by incipient wetness methods would be expected to contain more residual CN- than Ru(1N) samples prepared via ion-exchange methods. This may lead to differences in site locations in the IN and OUT systems that cause differences in the bpy IR and Raman modes. We do not understand why the CN- band is not observed for the CN- precursor materials. These FTIR data may suggest that significant loss of CN- occurs during synthesis, perhaps due to complexation effects. It is also not clear why the FTIR and Raman spectra are dominated by the Ru(bpy)3*+ complex, when it is clear that Co is present in the Co(1N) and Co(0UT) systems from surface (XPS) and bulk analysis. In any event, the infrared and Raman data suggest that the noncentrosymmetric structure of the Ru(bpy)3*+complex is still intact and there is no loss of the bpy ligand in such systems. Free bpy was also not observed by luminescence emission (2 max for free bpy = 420 nm).24a The infrared and Raman data also support the conclusions from the luminescence data that R~(bpy)3~+ has been deposited and exchanged intact. Such data suggest that it should be reasonable then to compare Ru(1N) and Ru(0UT) luminescence emission and quenching data since
Awaluddin et al.
9890 J. Phys. Chem., Vol. 99, No. 24, 1995 the resultant clay materials appear to have similar Ru complexes in similar geometrical environments both on the surface and in the interior of the clay particles. Ru(1N) gives a lower luminescence intensity compared to Ru(OUT) for both the chloride and the cyanide forms. The more significant reason may be that the clay layers quench the luminescence intensity of R~(bpy)3~+.There may be less scattered light for interior versus scattering off of exterior sites. Another significant factor appears to be the amount of iron present in the lattice of the aluminosilicate materiakg Ferric and ferrous ions are known to quench R ~ ( b p y ) 3 ~ via + electrontransfer mechanisms.22 The iron usually present in the most abundant amount in the clay mineral is typically in the ferric state. Addition of the luminescence quencher C0(phen)3~+generally results in a decrease of luminescence intensity. For Ru(IN), addition of Co(1N) gives an even smaller intensity compared to Co(0UT). In the Ru(0UT) system, Co(0UT) gives less intensity compared to Co(1N). These observations indicate that when Ru(bpy)3*+ and Co( ~ h e n ) 3 ~are + introduced into/onto the clay using the same method (such as in samples 3 and 6), the luminescence intensities are lower than when Ru(bpy)3*+ and C0(phen)3~+ are introduced using different methods (as in samples 4 and 5). It appears that the use of similar preparation conditions result in the incorporation of both the luminescent probe and the quencher to similar sites; there are more probe-quencher interactions resulting in a diminished luminescence emission. That is, quenching is most efficient in IN-IN and OUT-OUT materials. The emission band near 615 nm is due to a metal to ligand charge transfer (MLCT) transition. Red shifts in emission maxima are indicative of hydrophilic interactions whereas blue shifts are due to hydrophobic interaction^.',^,^^ Such shifts have been observed for Ru(bpy)32+ in clay'*6and zeolite24systems. Ion pair formation can also stabilize transient dipole interactions in the excited state and this will tend to decrease the energy of the MLCT transition. Emission maxima (wavelength) for Ru(bpy)3(Cl)z on all six samples (Table 2 ) show the following trend: Ru(0UT) > Ru(1N) > Ru(IN)/Co(OUT) Ru(OUT)/Co(IN) > Ru(IN)/Co(IN) > Ru(OUT)/Co(OUT) (1) These data suggest that the Ru(OUT)/Co(OUT) and Ru(IN)/ Co(1N) samples have metal complexes in more hydrophobic sites than the other samples. The existence of such hydrophobic sites and an analogy to oases (hydrocarbordorganometallic) in deserts (inorganic) was first proposed for Ru(bpy)3(Cl)z silica gel by de Mayo et a1.26 For example, organic luminescent materials such as pyrene were found to aggregate in certain regions of the surface of silica gel as evidenced by the observed characteristic excitation spectra of ground-state associated dimeric species. Similar light-induced electron transfer processes and oaddesert scenarios are also important in biological systems such as enzymes that have hydrophobic and hydrophilic pockets. The shape selective properties of zeolites36 can be related to the lock and key models used to describe biological activity. Such comparisons are markedly bome out in studies of zinc zeolites that mimic catalytic activity of zinc-containing enzymes.38 Similar emission data for Ru(bpy)3(CN)2 samples (Table 2 )
show a different trend as summarized below: Ru(1N)
-
- -
Ru(0UT) Ru(IN)/Co(IN) > Ru(OUT)/Co(IN) Ru(OUT)/Co(OUT) > Ru(IN)/Co(OUT) (11)
The ordering of the emission maxima for the CN- anion derivatives of eq I1 is markedly different from that in eq I. Such data suggest that the anion used during ion-exchange and incipient wetness synthesis procedures influences the resulting microenvironment of the metal complexes on and in the bentolite H clay. In general, the data suggest that CN- anions force transition metal complexes into hydrophilic sites [emission 1 = 637-630 nm] as compared to C1- anions due to the degree of blue ~ h i f t . ' ~ ,This * ~ generalization has exceptions such as the CN- precursor for Ru(IN)/Co(OUT) [616 nm]. This may be due to differences in loading of the different metal complexes or differences in the anion concentrations and locations. There is no emission at 420 nm in any of these systems suggesting that free bpy is not present as was found for some zeolite systems.24a Differences in anion precursor may not explain all the differences in emission of these systems. For example, Ru(IN) and Ru(IN)/Co(IN) show different luminescence. We do not know why this is the case. The trends in eqs I and I1 also suggest there are specific differences in site preference when different anion precursor complexes are used for a 1:l comparison of the same location of each of the six pairs of samples. Regarding electron transfer, note that the most efficient quenching for either the C1- or CNprecursor complexes occurs for Ru(IN)/Co(IN). The efficiency of quenching appears to be most related to the specific method of preparation (which determines the proximity of the Ru and Co complexes) rather than to environmental (hydrophobic, hydrophilic) conditions. The differences in these sites may be related to specific steric interactions on the surface or inside or variations in charge densities which lead to changes in polarizabilities and concomitant polarities. It is clear that Ru(bpy)3(CN)2 and Ru(bpy)3(Cl)z lead to occupation of sites with different polarities as evidenced by trends of eqs I and 11, and the luminescence data of Figures 1 and 2 and Tables 2 and 3. Luminescence lifetime experiments were performed to determine the number of emitting excited-state species present. Data for luminescence lifetime results show double-exponential decays, indicating the presence of at least two excited state species; this can be attributed to two different locations of the R ~ ( b p y ) 3 ~ion. + An altemative explanation may be that two different Ru coordination environment, may be present due to partial loss of ligands. Table 3 shows that the lifetime decay for R~(bpy)3~+ inside of the clay is shorter than that for outside of the clay. Again, this may be due to the presence of iron impurities which quench Ru(bpy)3'+ by electron transfer. The addition of quencher [C0(phen)3~+]yields luminescence lifetimes that are shorter than for R~(bpy)3~+ alone. The quenching reactions of R ~ ( b p y ) 3 ~by+ C0(phen)3~+proceed primarily by an electron-transfer me~hanism.~'Table 3 also shows that both R ~ ( b p y ) 3 ~and + C0(phen)3~+inside have the shortest lifetime decay, probably due to the closer proximity of R ~ ( b p y ) 3 ~and + C0(phen)3~+so that electron transfer can proceed efficiently. The presence of double exponential behavior for emission of R~(bpy)3~+ species in bentolite H is not unusual. Albery40 has proposed a generalized model for such nonexponential systems. We do not suggest that anything is unusual about the present system, however, single exponential behavior for Ru( b ~ y ) 3 in ~ +zeolites24aand clays44has been reported previously.
Quenching of Tris(2,2’-bipyridine)ruthenium(II) Luminescence
J. Phys. Chem., VoE. 99, No. 24, 1995 9891
These data suggest that when both Ru(bpy)s2+ and Coreported here plays a dominant role in the emission properties (phen)j3+ are present on the outside of the clay, they occupy of these specific systems. different sites and are significantly separated from each other. It is also well known in such clay systems that segregation This may be due to the different p o l a r i t i e ~ ~of~ -the ~ ~two and aggregation of R~(bpy)3~+ and infrastratification effects can species. Similar results for organic moieties on clay, alumina, O C C U ~ . ~ ~ -We ~ ’ envision in such cases that the localized square silica, and zeolites surfaces have been obtained p r e v i o ~ s l y . ~ ~ - ~lattice ~ model of Habti et al.16amay be useful in understanding such quenching effects. The idea that antenna molecules that are present on the surface of the clay are more efficient than absorbers in the bulk of the Finally, a reviewer has suggested that the IR and Raman data clay for light absorption may not be the case here35. It is clear do not clearly establish the location of the adsorbed species but from Figure 1 and Tables 2 and 3 that the IN-IN and OUTonly show that the complex is intact after adsorption. In this OUT materials here are more efficient as regards electron regard, the XRD and XPS data are of most importance in transfer than the other materials we have studied. On the other locating the majority of the species in the different samples. hand, it may be possible that the quenching efficiency of surface species is greater than internal sites, Le., Fe3+,6c,39 although we Conclusions have observed no evidence ([Fe3+]gradients) that would support Location of R~(bpy)3~+ and Co(phen)j3+either on the surface this assertion. or in the interlayer space of bentolite-H results in markedly We note that the concentrations of Ru(bpy)3*+ introduced different spectroscopic and diffraction properties. The lumiduring ion exchange and during incipient wetness are not exactly nescence emission spectra and lifetime decays indicate that the same. The relative amounts of Ru(bpy)3*+ on the surface adsorption on the surface or in the interlayers provide different of the clay samples, however, have similar concentrations on environments for electron transfer to take place. The excited the basis of X-ray photoelectron spectroscopy studies. In states of R ~ ( b p y ) 3 ~in+the presence or absence of C0(phen)3~+ general, it has been found from microanalysis that about oneexhibit double-exponential decays, suggesting the presence of tenth of the total sites introduced via ion exchange are at least two types of R~(bpy)3~+ species or environments in these incorporated on the external surface. There is therefore about systems. The implications of the presence of these two species a 10-fold increase of surface Ru(bpy)3*+ in the neat clay or environments are that further improvements in preparation materials and about a 5-fold higher concentration in the mixed of such materials may be necessary to enhance the efficiency Ru(IN)/Co systems as compared to Ru(0UT). of electron transfer and that quencher molecules interact We have chosen very low concentrations of R ~ ( b p y ) 3 ~and + differently with such species. Within experimental error, the C0(phen)3~+for several reasons. First of all, at high concentraIN-IN and OUT-OUT materials are considerably more tions, concentration quenching or self-absorption effects can be efficiently quenched than the IN-OUT and OUT-IN samples. severe. The low concentrationsof Ru(bpy)3*+,generally 1- 100 An important criterion that must be considered for efficient nmol), are in the linear concentration range of the luminescence electron transfer in these systems appears to be the proximity instrument. We do not imply that the absolute intensities are of the donor and acceptor. unambiguous. Luminescence of solid particles involves scattering events and is dependent on how particles pack. Ratios Acknowledgment. The support of the National Science of intensities are most meaningful in such circumstances. Foundation under Grant CBT 8814974 is gratefully acknowlStereochemical and site occupancy factors (vide infra) can also edged. We thank Professor Harry B. Gray for helpful discusbe important. sions. 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L; Kim, Y, I.; Atherton, S. J.; Schmehl, R. H.; Mallouk, T. E. J. Am. Chem. a concentration averaged quenching effect. For example, CoSOC. 1992, 114, 8081-8087. (g) Wuttke, D. S.; Gray, H. B.; Fisher, S. L.; (OUT) has about 1 nmol of C~(phen)3~+ and has about the same Imperial, B. J. Am. Chem. SOC.1993, 115, 8455-56. effect on luminescence intensity of Ru(0UT) as Co(1N) which (2) (a) Gust, D.; Moore, T. A. In Advances in Photochemistry; Volman, has about 50 nmol of C0(phen)3~+had on Ru(IN). This type D., Hammond, G., Neckers, D. C., Eds.; Wiley: New York, 1991, Vol. 16. (b) Baral, S.; Fendler, J. H. in Photoinduced Electron Transfer; Fox, of analysis suggests that it is important to consider not only M. A,, Chanon, M, Eds.; Elsevier: Amsterdam, 1988; Part B, p 541. relative ratios of intensities of luminescence but also the relative (3) (a) Ollis, D. F.; Pelizetti, E.; Serpone, N. Environ. Sci. Technol. concentrations of all species and their specific locations. 1991,25, 1522-1529. 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