17632
J. Phys. Chem. 1995,99, 17632-17639
Supramolecular Assemblies of Tris(2,2'-bipyridine)ruthenium(II) Bound to Hydrophobically Modified a-Zirconium Phosphate: Photophysical Studies Challa V. Kumar* and Zeena J. Williams Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060 Received: May 18, 1995; In Final Form: July 11, 1995@
Interaction of tris(2,2'-bipyridine)ruthenium(II) dichloride (Ru(b~y)3~+) with n-butylammonium salt of a-Zirconium phosphate (BAZrP) causes extensive red shifts of the metal-to-ligand charge-transfer (MLCT) absorption band of the metal complex (from 452 to 480 nm). In contrast, the luminescence maximum is blue shifted, from 610 nm in aqueous media, to 580 nm when the metal complex was bound to BAZrP. Such a blue shift in the emission was observed when R~(bpy)3~+ was present in rigid matrices such as ice, or ethanolmethanol glasses at low temperature. Therefore, the microenvironment surrounding the metal complex in BAZrP is rigid and does not permit the relaxation of the initially produced metal-to-ligand charge-transfer (MLCT) excited state even at room temperature. Upon binding to the zirconium phosphate, the emission yield of R ~ ( b p y ) 3 ~is+increased nearly 5-fold when compared to aqueous solutions. Such large increases in the emission were not observed with Ru(bpy)32+in other heterogeneous media. The absorption and emission spectral maxima are independent of loading of the metal complex, strongly suggesting the formation of islands or pools of these metal complexes at the interlayer regions of BAZrP. The luminescence intensities decrease with loading of the metal complex indicating the strong self quenching of the luminescence within these pools. Stern-Volmer plots were constructed from the steady-state data and the estimated KSVvalue was 1.1 x lo6 m2/mol. Powder X-ray diffraction patterns of the samples indicate that intercalation of Ru(bpy)32+ into BAZrP increases the interlayer separation from 18.6 to 19.5 A. Time-resolved emission studies indicate the presence of two distinct species, a major long-lived component of -1500 ns ('90%) and a minor shortlived component of -400 ns (90% of the emission from the donors. Remarkably, the rate of energy transfer from the donor to the acceptor was found to increase with the donor concentration, implying rapid donor to donor energy migration (antenna effect) within these assemblies. In the absence of BAZrP, no such rapid transfer was ~bserved.'~ The ease with which the above mentioned supramolecular assemblies can be prepared and characterized provided inspiration for the current work. The study of energy transport in such assemblies, prepared from chromophores that absorb in the visible region, will be of practical value. The energy trapped by such assemblies can be converted into chemical potential via photoinduced electron transfer reactions, mimicking the photosynthetic apparatus. In this context, we report the high affinity binding of a large hydrophobic metal complex, Ru(b~y)3~+, to BAZrP.I7 Binding of R~(bpy)3~+ to BAZrP leads to spectacular changes in its photophysical properties. These observations illustrate the unique environment offered by BAZrP. Binding of R~(bpy)3~+ was found to be very sensitive to the presence of co-ions, and hydrophobic ligands. Luminescence from R~(bpy)3~+ present in the interlayer regions can be quenched at high rates by electron deficient metal complexes such as C0(bpy)3~+.'~ R~(bpy)3~+ was chosen for the current studies for several reasons. It absorbs in the visible region, and hence the window
of light absorption can be shifted to longer wavelengths when compared to the hydrophobic antenna systems described above.14-16 R~(bpy)3~+ is quite stable to heat and light, making it a convenient chromophore to construct artificial lightharvesting complexes. The excited state of R~(bpy)3~+ is both an excellent oxidant and a reductant, and therefore a multitude of energy- and electron-transfer reactions can be carried out using this chromophore.18 The photophysical and photochemical properties of R~(bpy)3~+ is well do~umented,'~ and it has been used as a reporter molecule to investigate a number of organized media.20 Current studies complement these investigations and provide a sharp contrast. The binding of the metal complex to BAZrP results in a large blue shift of the luminescence and lengthening of the luminescence lifetime.
Experimental Section The n-butylammonium salt of a-zirconium phosphate ((CagNH3)2Zr(P04)2*H20, denoted as was a generous gift from Professor G. Rosenthal, Department of Chemistry, University of Vermont. Its synthesis and characterization were published previously.21 Concentration of BAZrP is expressed either as the weight percent or as mmol dm-3 of exchangeable cationic sites, using 5.4 m o V g as the conversion factor, obtained from ion-exchange studies.22 The phosphate suspensions were stirred periodically to prevent sedimentation, and BAZrP concentrations were kept low (0.008%) to minimize scattered light from interfering with the spectral measurements. Tris(bipyridine)ruthenium(II) dichloride was obtained from Aldrich Chemical Co., and has been used as received. The Ru( b ~ y ) 3 ~concentration + was varied from 5 to 40 pmol dm-3 to study the effect of surface coverage on the luminescence properties of the metal complex. In a typical binding experiment, an aqueous solution of R~(bpy)3~+ was added to a dilute aqueous suspension of the phosphate such that the final concentration of BAZrP has been adjusted to 0.008% (432 pmol dm-3 exchangeable cationic sites). After equilibration for 24 h, no further changes in the luminescence properties were noticeable. Powder diffraction patterns of the BAZrP with and without the metal complex were recorded on a Scintag XDS 2000 X-ray diffractometer using Cu K a radiation'(A = 1.5406 A). Samples were spread on a glass slide and were exposed to the X-radiation. The powder diffraction patterns of BAZrP matched with the published results.21 The absorption spectra were recorded on a Perkin-Elmer Lambda-3B absorption spectrophotometer and the luminescence spectra were recorded on a Perkin-Elmer LS-5 fluorescence spectrometer. Both spectrometers were interfaced with an Apple Macintosh computer, and all the necessary software for the operation of these instruments has been developed in our laboratory. For the luminescence titrations, the BAZrP suspensions and R~(bpy)3~+ were mixed in different proportions. This resulted in a series of suspensions with a constant concentration of BAZrP but with varying concentrations of R~(bpy)3~+. Airsaturated aqueous solutions were used without degassing. For the luminescence measurements, the excitation and emission slits were adjusted to 3 and 5 nm, respectively. All quenching experiments were carried out by adding small volumes of a concentrated solution of the quencher. Samples were excited at 452 nm such that light absorption by the quencher was negligible even at the highest concentration. The absorption spectra of a suspension of BAZrP containing both R~(bpy)3~+ and the quencher were similar to the absorption spectra of individual components bound to BAZrP. Luminescence quenching data were analyzed using the Stern-Volmer equation,23(eq l), where Io and I are the emission intensities in the absence, and in the presence of the quencher [Q], respectively.
17634 J. Phys. Chem., Vol. 99, No. 49, 1995
Kumar and Williams 0.35
r
I
0.30
The Stern-Volmer quenching constant ( K ~ v was ) obtained from linear plots, using eq 1. The bulk concentrations of the Ru(I1) can be converted to the surface concentrations using available surface area as 100 m2/g of BAZrP. Thus, when 0.008% BAZrP suspensions are used, 1 m o m of the metal complex corresponds to l/8 mol/m2. The estimated error in the quenching constants did not exceed f 5 % , unless specified otherwise. Luminescence lifetimes were measured on a time-correlated single-photon-counting spectrometer, built in our laboratory. A nanosecond flash lamp (Edinburgh Instruments, Model F-199), filled with nitrogen, was used to generate light pulses with full widths at half-maximum in the range 2-3 ns. The luminescence decay traces were deconvoluted using software from PRA, Inc., The goodness of the fits to the experimental data were judged using a number of criteria such as the Durbin-Watson parameter, x2, residuals and the correlation function.24 Each luminescence lifetime measurement was repeated three times, and each data set has been deconvoluted using different starting guesses to ensure that the fits were unique and reproducible. Samples were equilibrated for 24 h, and no further changes in the luminescence properties were detected after this time period.
Results
Absorption Spectra. The electronic absorption spectrum of R~(bpy)3~+ changes dramatically, upon binding to BAZrP. The absorption spectra of the metal complex (5, 10, 15, 30 and 40 pM) in the presence of BAZrP (0.008 wt %, curves 2-6) and in the absence of BAZrP (curve 1) are shown in Figure 1. The spectra were offset for the sake of clarity, and highlight the differences and similarities among them. Upon binding to BAZrP, the intense, broad metal-to-ligand charge-transfer (MLCT) band of Ru(bpy)32+is shifted from 452 nm (in aqueous solution) to 480 nm, with a weak shoulder appearing at 440 nm. The relative intensities of the two bands also change upon binding to BAZrP. For example, in the presence of BAZrP the peak at 480 nm is of much greater intensity than the peak at 440 nm. Although curve 6 (40 pM metal complex, 0.008% BAZrP) looks different from the rest, it was found to be the sum of the absorption spectra of the bound and free chromophores. Thus, subtraction of curve 5 from 6 resulted in a spectrum superimposable on curve 1 (supporting information Figure 1). Thus, binding to BAZrP induces a large red shift of the MLCT band. Most notably, the absorption spectra of the bound complex are independent of loading of the metal complex. Powder Diffraction Patterns. The powder diffraction pattern of BAZrP with and without R~(bpy)3~+ indicate that intercalation of the metal complex changes the interlayer separation from 18.6 to 19.5 A. These changes are comparable to the reported values for BAZrP and ZrPS intercalated with R ~ ( b p y ) 3 ~ + The . ~ ~ interlayer ,~~ separation was found to be independent of loading of the metal complex. Emission Spectra. The luminescence spectra of Ru(bp~)3~+ recorded in the absence of BAZrP (curve 1) and in the presence of 0.008% BAZrP (5, 10, 15, and 20 pM of the metal complex, curves 2-5, respectively) are shown in Figure 2. At a constant concentration of Ru(bpy)32f (20 pM), the addition of BAZrP increases the luminescence intensity by a factor of 4.8 (curves 1 and 5), and blue shifts the emission maximum from 610 to 580 nm. The intensity increases observed here are much greater than what was reported with other layered materials.20%26 Further increase of Ru(bp~)3~+ concentration (0.008% BAZrP) results in a progressive decrease in the overall luminescence intensity
8
0.25
5
0.20
3
0.15
3
0.10 0.05
0.00 400
440
480 520 560 WAVELENGTH (nm)
600
Figure 1. Absorption spectra of Ru(bpy),*+ with BAZrP (0.008%, curves 2-6) and without BAZrP (curve 1). Concentrations of the metal complex for curves 1-6 were 10,5, 10, 15.30, and 40pM, respectively.
Figure 2. Luminescence spectra of Ru(bpy)32i with increasing concentrations of the metal complex in the presence of BAZrP (0.008%, 452 nm excitation). Spectra 2-5 contain 5 , 10, 15, and 20 p M of the metal complex, respectively. Spectrum 1 was obtained with 20 p M Ru(bpy)32+ and without BAZrP.
(Figure 3A). The emission intensity reaches a maximum at a concentration of 25 pM ruthenium and decreases at higher concentrations. A plot of the emission maxima in nanometers, as a function of [R~(bpy)3~+] shows a small but gradual red shift of the emission maximum with increasing loading (Figure 3B). Even at 40 pM Ru(bpy)32+ the maximum is still at 584 nm, not far from that of the bound metal complex (580 nm). Again, at high loadings the emission spectra were found to be composed of spectra arising from the bound and free metal complexes. A large fraction of the emission was still observed from the bound chromophore, and the emission spectra of the bound probe were essentially independent of loading. The blue shift observed upon binding to BAZrP is in contrast to what was observed when the metal complex binds to micelles, zeolites, lipid bilayers, polyelectrolytes, and DNA.27 Time-Resolved Studies. Time-resolved luminescence studies can be helpful in probing the nature of the emission and its time dependence. In aqueous solutions, the luminescent state of R~(bpy)3~+ was found to follow single exponential kinetics. However, when bound to BAZrP, it is much longer lived a d exhibits a biexponential decay. A typical decay trace of the luminescence (20 pM R~(bpy)3~+, 0.008% BAZrP) is shown in Figure 4. The decay was fitted to a major long-lived component (1340 f 90 ns, 93 f 6%) and a minor short-lived component (430 f 30 ns, 7 f 5%). The biexponential fit to the data was reproducible, with a x2 of -0.99. Goodness of the fit was also confirmed after examining the residuals, correlation function, and the Durbin-Watson parameter. When the concentration of R~(bpy)3~+ was varied from 5 to 40 pM (0.008% BAZrP), the data were successfully fitted to a sum of two exponentials at all concentrations of the metal complex (Table 1). Lifetime of the short-lived component was comparable to that of the free chromophore in aerated aqueous solutions (383 f 6 ns). The major component was nearly 5 times longer lived than that of the free chromophore. Time-
Supramolecular Assemblies
J. Phys. Chem., Vol. 99,No. 49, 1995 17635
9000 0.008% BAZrP
=o 4
a
I
. mo , . m,o .1 ao , 'i 10 , .1 , .i m, a .i m,o .I O,O .110,
40 .
0
Y
4"
140
40
I
1 0
I
0
P
585
-
0.008% BAZrP
503
-.r;7.,00
I
10
20
30
40
50
------_.-_--__-__________ I
:--*------I
I
1.0
[ R O P Y ) ~ ~PM +I Figure 3. (A, top) Plot of luminescence intensity at the peak vs the concentration of Ru(bpy)s2+ in the presence of BAZrP (0.008%). The intensity for Ru(bpy)32+ (20 p M , 0.008% BAZrP) was found to be nearly 4.8 times greater than in the absence of BAZrP. Thus, binding to the layered phosphate dramatically increases the emission yield. (B, bottom) Plot of peak position vs the concentration of R ~ ( b p y ) 3 ~in+ the presence of BAZrP (0.008%). In the absence of BAZrP, the peak position was 610 nm, whereas in the presence of BAZrP (10 p M of Ru(bpy)3*+), the peak shifts to 580 nm. Thus, binding to the layered phosphate results in a large blue shift of the emission.
resolved emission spectra constructed from these decay traces, at various delay times, had maxima at 580 nm independent of the delay time. This is due to the dominance of the longer lived, major component to the overall emission intensity throughout the decay. The maxima observed in these spectra correlate well with the steady-state data presented above. Lifetimes observed for the long-lived component are much longer than what was observed for this complex with hectorite,20a,26 montmorillonite,28 and a-zirconium sulfophenylphosphonate (ZrPS).29 Discussion Binding of Ru(bpy)3*+ to BAZrP results in a large red shift of the absorption bands from 420 and 452 nm for the free probe to 440 and 480 nm for the bound probe. The bound probe spectral maxima are independent of loading (Figure 1). Thus, curves 2-5 are superimposable with one another, and curve 6 is the sum of the spectra from the bound and free chromophores. For example, subtraction of curve 5 (30 pM, R~(bpy)3~+) from 6 (40 p M , R~(bpy)3~+) resulted in a spectrum that is superimposable with that of the free metal complex (curve 1) (supporting information Figure 1). Thus, at concentrations greater than -25 pM of R~(bpy)3~+ the absorption maximum begins to blue shift due to the accumulation of free chromophores. Therefore, binding of the metal complex to BAZrP (0.008%) is nearly complete at -25 pM R~(bpy)3~+. Using these stoichiometric concentrations, and the available surface area (100 m2/g), we
I
8. 0 TlYL
/
I
B. 0
a,
4. 0
MICnOmLeowm
Figure 4. Time-dependent decay of Ru(bpy),*+ luminescence (20 p M ) in the presence of BAZrP (0.008%).In the absence of BAZrP, the decay follows a single exponential with a time constant of 380 ns, whereas in the presence of BAZrP, the decay trace could be fitted to a biexponential with lifetimes 430 (7%) and 1340 ns (93%).
TABLE 1: Luminescence Lifetimes and Intensities of Ru(bpy)$+ in the Presence of BAZrP (0.008%) t 2
t l
[R~(bpy)3~+]
luM) 5 10 15 20 30 40 10 (no BAZrP)
lifetime (ns) 368f171 495 f 6 6 464f23 430f33 383f73 372f40 383 f 6
intensity
(%I 312 8f 6 1Oi5 7 f 5 1115 14f3 100
lifetime (ns) 1556f58 1472%90 1300f76 1341 i 9 9 1209f178 1276f20
intensity (%)
97f3 92% 6 90f5 93f6 89%5 86f3
estimate the area per bound Ru(II) ion as 133 A2. This footprint corresponds to a spherical ion of radius 13 A in agreement with the diameter of Ru(bpyh2+ (14 A).30 A similar conclusion was also arrived at, from the emission titrations (Figure 3A,B). This stoichiometry corresponds to the binding of the metal complex to roughly 12-14% of the available cation exchangeable sites and indicates nearly complete coverage of all the available surface area. The MLCT band of R~(bpy)3~+ was shown to undergo red shifts with a decrease in solvent p ~ l a r i t y . ~ ~ ~ ~ ~ ' Therefore, current results may suggest a hydrophobic environment surrounding the metal complex or that it is due to the frozen solvent matrix present in the galleries of BAZrP. Since the absorption spectrum of the bound complex is independent of loading, the metal complexes are bound in segregated islands/ pools, surrounded by a hydrophobic/rigid environment (Scheme 2). If the metal complexes are distributed uniformly, then the spectra are expected to evolve with increased loading due to reduced Ru(I1)-Ru(I1) distances. For example, red shifts of the MLCT bands were observed when Ru(bpyhzf binds to ZrPS.20bIn these studies, strong concentration dependence of
17636 J. Phys. Chem., Vol. 99, No. 49, 1995 SCHEME 2: Descriptions of Uniform Dispersion and the Clustering of the Metal Complexes in the Galleries of BAZrP
Kumar and Williams
large blue shift of the emission. Similar blue shifts were observed with Ru(bpy)32+ bound to montmorillonite, at low temperature. The unrelaxed excited state, as in the present study, can provide a larger driving force for photoreactions. The rigid matrix surrounding the metal complex is useful for excitation migration within the pools. For example, if the excited state is relaxed by the surrounding solvent, then the 0 0 0 0 excitation will be trapped locally, and rapid migration of 0 0 0 0 excitation is less likelyG4 The emission maxima of the bound probe were also found 100001 to be independent of loading (Figure 3B), consistent with the absorption data. At Ru(bpy)s2+concentrations greater than -25 pM, the emission spectra are combinations of the spectra arising Clustering Uniform Dispersion from the bound and free probes. Thus, the emission and absorption spectra strongly suggest the formation of islands or the spectra on the loading was observed suggesting increased pools of these chromophores at the interlayer regions of BAZrP interaction between the metal complexes at high loadings. which grow in size with an increase in probe concentration until Interactions between the metal complexes within these pools/ binding is saturated at -25 pM of R~(bpy)3~+. assemblies can be investigated by observing the ligand x-n* The relative luminescence yields and their dependence on bands. The x-x* absorption band of Ru(bp~)3~+ (280 nq, for loading also give details about the packing of the metal complex aqueous solutions) is also broadened and split in the presence at the interlayer regions. For example, a plot of intensity vs of BAZrP (from excitation spectra, not shown). Such splitting concentration of ruthenium gave a bell-shaped curve with a was also observed with Ru(bp~)3~+ in ice, colloidal Si02,20a,28b maximum centered around 25 pM of the metal complex (Figure ZrPS,20band m~ntmorillonite.~ The splitting was suggested to 3A). A plot of normalized intensity (intensity divided by the be due to excitonic interactions among the bound metal ruthenium concentration) as a function of [R~(bpy)3~+] illustrates complexes and was taken as evidence for the binding of the a monotonous decrease of the normalized emission intensity metal complex in the interlayer galleries, as opposed to the outer (Figure 5A). Useful information can be obtained from this data ~urface.~In the case of BAZrP, splitting of the n-x* band by plotting the inverse of normalized intensity as a function of may be taken as an indication that the metal complexes bind in probe concentration. This resulted in a linear plot with a the interlayer galleries and interact with each other within the y-intercept equal to 1/1387 (Figure 5B) which corresponds to ruthenium pools. However, these aggregates differ from that of the aggregates described a b o ~ e . ~ ,For ~ ~example, ~ , ~ , ~ ~ ~the emission intensity, at infinite dilution of the chromophore. Using this value as I,, a Stem-Volmer plot (eq 1) was aggregation in ZrPS, a-ZrP, and Si02 was suggested to result constructed for the self-quenching of Ru(II) emission (supporting in an extensive red shift of the emission as well as efficient information, Figure 2). The plot was linear with a slope equal self-quenching of the excited state. In the present case, the to 1.4 x 105/M, a value much larger than what has been emission is blue shifted and the excited-state lifetimes are observed with Ru(bp~)3~+ bound to ZrPS or HAZrP. There lengthened enormously. Perhaps, the n-butylammonium ions are, at least, two factors that influence the observed Stemseparate and solubilize the metal complex within these pools. Volmer slope. The bulk concentrations used in the plots are Then, the environment sensed by Ru(bpy)3*+ will be considermuch smaller than the presumably high local surface concentraably hydrophobic, consistent with the above spectral observations of Ru(I1). Therefore, using local concentrations (in mol/ tions. m2) will convert the above value to 1.1 x lo6 m2/mol. Second, The large blue shift in the emission maximum (30 nm) diffusion in the interlayer regions was suggested to be much observed in the presence of BAZrP (Figure 2) is unexpected. smaller than in the bulk phase and this should result in lower For example, the emission maxima recorded for R~(bpy)3~+ with KSV values. In the present case, the estimated KSV value ZrPS, a-ZrP, n-hexylammonium salt of a-ZrP (HAZrP), and corresponds to a large bimolecular quenching rate constant. For smectite clays were at wavelengths >604 nm.19b,24,31 Our results example, using the lifetime of the dominant component as 1500 indicate that the microenvironment surrounding the metal ns (Table l), a self-quenching rate constant of 8 x 10" m2/mol complex in BAZrP is quite distinct from these media. On the s is estimated. This value is to be compared with the diffusion other hand, the emission maximum was shifted to 580 nm in coefficient of ~ m ~ /for s ,the ~ interlayer ~ regions, ice, ethanol-methanol glasses at low temperature, and when which is several orders of magnitude slower than diffusion in bound to colloidal Si02.5*20a3b328b The blue shift was suggested aqueous solutions. This large value of k, is simply not due to to be due to a rigid environment surrounding the metal complex, static quenching because time-resolved studies indicate that the and the initially produced luminescent state is not relaxed due excited-state lifetimes are reduced with increased loading. The to the frozen solvent surrounding it. Thus, the current observarapid quenching observed here is perhaps, at least in part, due tions strongly suggest a rigid, nonpolar environment surrounding to transfer of excitation from chromophore to chromophore the metal complex. This is consistent with the binding of the within a pool of bound chromophores until it is quenched by a probe in the interlayer regions, lined with butylamine chains. trap.34 Experiments are in progress to investigate such migraIf the probe were to be externally located, the aqueous tion. Excitation migration was recently observed with aromatic environment surrounding it would relax the initially produced chromophores bound to BAZrP.15 As discussed above, the excited state and thus result in a red shift of the emission. The relative emission yields and not the spectral maxima depend hydrophobic chains of butylamine are expected to eliminate free on loading. water from the interlayer regions. Water, if present, will be strongly hydrogen bonded to the ZrP frame work such that it Time-resolved studies clearly indicate a dominant, long lived, cannot reorient when R~(bpy)3~+ excited states are produced. blue shifted, luminescence component when Ru(bpy)32+binds The hydrocarbon tails of the amine and the rigid ZrP frame to BAZrP (Table 1). These observations are consistent with work fail to relax the highly polar MLCT state, resulting in the the large blue shift observed in the steady-state measurements
ogo
J. Phys. Chem., Vol. 99, No. 49, 1995 17637
Supramolecular Assemblies
r
loOO lo00
i
800-
1 .o
0.008% BAZrP
0
-
400
-
kq = 7.2~10E9IMs
0.9
L
600
0.008% BAZrP '
0 0.8
0
0 0
0.7 0
200t
'
0.6 0
I 10
20
30
40
[ R u ( ~ P Y ) ~PM ~+I Figure 6. Plot of the observed luminescence decay rate (intercalated component) as a function of Ru(bpy)3*+concentration (0.008% BAZrP). The slope of the linear plot corresponds to the self quenching rate constant. This value is several orders of magnitude larger than anticipated, due to the very slow diffusion at these interlayer regions.
"
.
0
0
10
20
30
40
Figure 5. (A, top) Plot of the normalized intensity of Ru(bpy),*+ luminescence as a function of ruthenium concentration (0.008% BAZrP). The intensities were normalized by dividing with the corresponding concentration of the metal complex. Thus, the normalized intensity monotonously decreases with probe concentration. (B, bottom) plot of lhormalized intensity of Ru(bpy),2+ luminescence as a function of ruthenium concentration (0.008% BAZrP) with a y-intercept equal to 111387. It corresponds to the normalized intensity of the probe at infinite dilution. This value was then used to construct a Stem-Volmer quenching plot which resulted in a quenching slope of 0.14/pM.
(Figure 2). The long-lived major component can be assigned to the excited state associated with the metal complex bound in the interlayer regions of BAZrP (intercalated component). A small fraction of the probe will also be bound to the outer regions, and at the edges of BAZrP platelets, and this fraction (-10%) will be exposed to water. Since, water was known to quench the R~(bpy)3~+ excited state,35emission from this species will be short lived. These assignments are also consistent with the spectral maxima and relative quantum yields. The long lifetime of the intercalated component and the large blue shift of the emission can be interpreted using the energy gap law.36 The increased gap between the ground state and the luminescent state, observed in the current studies, i s expected to reduce the nonradiative rate constant. Excited-state lifetimes meassured at 77 K indicate only a weak temperature d e p e n d e n ~ e .These ~~ results suggest that triplet ligand field state does not play a major role in the luminescent state decay. In contrast to the current results, the luminescence lifetimes of the bound Ru(bpy)3*+were found to be drastically shorter with montmorillonite, laponite, and ZrPS.5,20a,b,2sb,32*33 Thus, BAZrP protects the excited state and increases the luminescence yield.
Time-resolved emission spectra recorded at various time intervals (after excitation with nanosecond light pulses) show only the blue-shifted emission with a peak at 580 nm. The emission maxima do not change with the delay time, indicating the dominance of the long-lived, blue-shifted emission to the overall luminescence spectrum. The microenvironment surrounding the probe is perhaps nonpolar and does not permit the relaxation of the initially produced luminescent state, even on long time scales. Similar results were also observed when the probe was bound to laponite with emission maxima around 580 nm but with significantly shorter luminescence lifetimes.lga Thus, BAZrP provides an interesting environment for enhanced, blue shifted emission from R~(bpy)3~+. The lifetime of the intercalated component was found to depend on loading (Table 1). For example, a plot of l/z as a function of ruthenium concentration gave a linear plot with a slope equal to 7.2 x 109/M s (or 5.8 x 1O'O m2/mol s, Figure 6). This value is an order of magnitude smaller than the rate constant estimated from steady-state data (8 x 10" m2/mol s). The later value was estimated from the extrapolation of the intensities and assuming a liftime of 1500 ns based on the major, long-lived component. It is also possible that there is some contribution from static quenching to the steady-state values. In either case, these rate constants are larger than the diffusional rate constant suggested for the interlayer regions of layered materials.29 Part of this enhanced quenching rate can be attributed to increased local concentrations at the interlayer regions, but it alone may not account for the enhanced rates. For example, such enhanced self quenching rates were not observed in ZrPS, HAZrP, or Si02 suspensions, where the local concentrations are also presumably very high.29,32,28b In fact, the self-quenching rate constant observed for Ru(bpy)s2+bound to ZrPS was found to be much slower (kq = 7 x 105M s, calculated using bulk concentrations) than in water. If the probes form pools of chromophores and excitation can migrate within these pools, it can result in enhanced self-quenching. Such rapid excitation migration and self quenching is wellknown in the photosynthetic antenna systems.38 This suggestion is consistent with the observed splitting of the ligand centered n-n* absorption band, described earlier. However, there is no direct evidence for such migration in the present case. The large blue shift of the emission maximum with little or no relaxation of the initially produced luminescent state makes the excitation migration thermodynamically plausible. Thus, BA-
Kumar and Williams
17638 J. Phys. Chem., Vol. 99, No. 49, 1995 ZrP provides a unique environment different from ZrP, ZrPS, montmorillonite, and smectite clays. A variety of quenching and displacement experiments also support the binding of the probe in the interlayer regions and the unique environment experienced by R~(bpy)3~+. When the n-hexylammonium salt of a-ZrP (HAZrP) was used for the binding studies, instead of BAZrP, the above-described changes in the luminescence and absorption properties of R~(bpy)3~+ were not observed. For example, the emission maxima were around 610 nm and were superimposable with spectra obtained with that of the aqueous solutions of R~(bpy)3~+. Our results are in complete agreement with previous reports of R~(bpy)3~+ binding to HA ZIP.^^ The longer, hexyl chains make the interlayer regions much more hydrophobic than in case of BAZrP, discouraging the binding of the hydrophilic ruthenium dication. The interlayer spacing in HAZrP is also much larger than the diameter of R ~ ( b p y ) 3 ~ +Binding .~~ of the metal complex to the outer layers was observed with a-ZIP where the interlayer spacing is too small to accommodate Ru(bpy)32+.9,10 These studies clearly point out the unique environment of BAZrP in contrast to ZrPS, a-ZrP, and HAZrP. Iodide anion was shown to quench the luminescence of Ru( b ~ y ) 3 ~at+diffusion controlled rates and luminescence quenching experiments have been used to probe the binding of Ru(1I) dications to various anionic organized media.l4*I5 Due to its negative charge, iodide will not be able to penetrate the interlayer regions of BAZrP and hence the metal complexes bound in the interlayer regions should be protected from the quencher. Addition of sodium iodide to BAZrP suspensions containing Ru(bpy)g2+,resulted in only poor quenching of the luminescence. Addition of potassium chloride/iodide released the metal complex into the aqueous phase, causing a large decrease in the luminescence intensity. These observations are consistent with the binding of the metal complexes away from the aqueous phase, in the galleries of BAZrP. In the absence of BAZrP, rapid quenching of the emission by sodium iodide was observed, consistent with the literature reports.39
Conclusions While the microenvironments provided by a-ZrP, ZrPS, HAZrP, and BAZrP are understandably different, BAZrP provides an environment that dramatically enhances the emission quantum yield and emission lifetimes of R~(bpy)3~+. From the stoichiometric concentration of the probe at the onset of the binding saturation, the footprint of the metal complex was estimated. The radius calculated from this area is in good agreement with the reported value for R~(bpy)3~+.*~ X-ray powder diffraction data indicate intercalation of the metal complexes in the BAZrP galleries. The spectroscopic properties of the metal complex provide useful information about the probe binding. The luminescence properties of our probe suggest that the environment contains little or no free water molecules in the interlayer regions. Water, if present, is frozen or immobile and cannot effectively solvate the metal complex. Thus, BAZrP provides an environment, simi€ar to rigid media, discouraging the relaxation of the excited state. The presence of short chain surfactant molecules in the galleries enhances the formation of pools of chromophores which are coupled electronically and perhaps participate in long-range energy transfer; additional studies are necessrry to evaluate this possibility. But the interaction does not lead to reduced excited-state lifetimes, as found with other layered materials. The interlayer regions of BAZrP are fairly protected from outside, as evidenced in the luminescence quenching experiments with NaI. These properties of BAZrP make it an interesting organized medium for a
variety of photochemical applications. We are currently investigating the utility of these assemblies to drive light-induced electron transfer reactions at the interlayer regions.'*
Acknowledgment. We are grateful to Professor G. Rosenthal, Department of Chemistry, University of Vermont, for the generous gift of the BAZrP sample. Financial support of this work by the Perkin-Elmer Corp., and the University of Connecticut Research Foundation, are gratefully acknowledged. Supporting Information Available: Absorption spectrum and Stem-Volmer plot (2 pages). Ordering information is given on any current masthead page. References and Notes (1) Photochemistry in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH Publishers: New York, 1991 and references therein. (2) (a) Kalyanasundaram, K. Coord. Chem. Rev., 1982, 46, 159. (b) Baral, S., Fendler, J. H. In Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.: Elsevier: Amsterdam, 1988; Part B, p 541. (c) Gust, D., Moore, T. A. In Advances in Photochemistry; Volman, D., Hammond, G., Neckers, D., Eds., Wiley: New York, 1991, Vol. 16. (3) A large number of examples are discussed in ref 1. Also see: Turro, N. J. In Molecular Dynamics in Restricted Places; Klafter, J., Drake, J. M., Eds.; John Wiley: New York, 1989; p 387. Balzani, V.; Scandola, F. SupramolecularPhotochemistry,Ellis Honvood: New York, 1991. Awaluddin, A,; DeGuzman, R. N.; Kumar, C. V.; Suib, S. L.;Burkette, S. L.; Davis, M. E. J. Phys. Chem. 1995, 99, 9886. (4) Fox, M. A.; Jones, W. E.; Watkins, D. M. Chem. Eng. News 1993, March 15, 38. Fox, M. A. Adv. Photochem. 1986, 13, 237. Webber, S. E. Chem. Rev. 1990, 90, 1469. Baxter, S. M.; Jones, W. E.; Danielson, E.; Worl, L.;Strouse, G.; Younathan, J.; Meyer, T. J. Coord. Chem. Rev. 1991, I l l , 47. ( 5 ) Thomas, J. K. Acc. Chem. Res. 1988, 21, 275. (6) Seiler, M.; Dum, H.; Willner, I.; Joselevich, E.; Doron, A; Stoddart, J. F. J. Am.Chem. SOC.1994, 1 1 6 , 3399. (7) Inorganic Ion Exchange Materials, Clearfield, A., Ed.; CRC: Boca Raton, FL, 1982. (8) Ungashe, S. B.; Wilson, W. L.;Katz, H. E.; Scheller, G. R; Putvinski, T. M. J. Am. Chem. SOC. 1992, 114, 8717; Vermeulen, L. A,; Thompson, M. E. Nature (London) 1992, 358, 656. (9) Clearfield, A. J. Mol. Catal. 1984, 27, 251. (10) Alberti, G.; Casciola, and Constantino, U. J. Colloid Znterface Sci. 1985, 107, 256. (1 1) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.;Wilson, W. L;Chidsey, C. E. D. Science (Washington D.C.) 1991, 254, 1485. (12) Vermeulen, L. A.; Snover, J. L.Sapochak, L. S.; and Thompson, M. E. J. Am. Chem. SOC.1993, 115. 11767. (13) Lehn, J.-M. In Frontiers in Supramolecular Organic Chemistry and Photochemistry; Schneider, H.-J., Dum, H., Eds.; VCH Publishers: New York, 1991, p 1. Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A,; Pewitt, E. B. J. Am. Chem. SOC.1985,107.5562. Wasielewski, M. R. Chem. Rev. 1992,92,435. Dum, H.; Schwarz, R.; Andreis, C; Willner, I. J. Am. Chem. SOC. 1994, 115, 12362. (14) Kumar, C. V.; Asuncion, E. H; Rosenthal, G. L. Microporous Mater. 1993, I, 123. (15) Kumar, C. V.; Asuncion, E. H; Rosenthal, G. L. Microporous Mater. 1993, 1, 299. (16) Kumar, C. V; Chaudhari, A. J. Am. Chem. SOC.1994, 116, 403. (17) Kumar, C. V. Abstracts of the North East Regional Meeting of the American Chemical Society, Burlington VT, 1994; Abstracts of the 208th National Meeting of the American Chemical Society, Washington D.C., 1994. (18) We recently reported the rapid quenching of ruthenium luminescence by Co(II1) complexes, submitted to Science. For useful reviews, see: Kalyanasundaram, K. Coord. Chem. Rev. 1982.46, 159; Juris, A,; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P; von Zelewsky, A. Coord. Chem. Rev. 1988,84, 85. (19) Lin,C.-T.; Sutin, N. J. Phys. Chem. 1976,80,97. Creutz, C; Sutin, N. Inorg. Chem. 1976, 15,499. Purugganan, M. D.; Kumar, C. V.; Turro, N. J; Barton, J. K. Science (Washington D.C.) 1988, 241, 1645. (20) (a) Kuykendall, V. G; Thomas, J. K. J. Phys. Chem. 1990.94.4224. (b) Colon, J. L.; Yang, C.-Y.; Clearfield, A; Martin, C. R. J. Phys. Chem. 1988, 92, 5777. (c) Turro, N. J.; Kumar, C. V.; Grauer, Z; Barton, J. K. Lungmuir 1987,3, 1056. (d) Vliers, D. P.; Schoonheydt, R. A; De Schryver, F. C. J. Chem. SOC.,Faraday Trans. 1985, 81, 2009. (21) Rosenthal, G. L; Caruso, J. J. Solid State Chem. 1991, 93, 128. (22) The estimated surface area of BAZrP is 100 m2/g, and it is capable of exchanging 54 mM of cationic sites per gram (ecg); see: Wan, B.-Z.;
J. Phys. Chem., Vol. 99, No. 49, 1995 17639
Supramolecular Assemblies Anthony, R. G.; Peng, B. Z; Clearfield, A. J. Catal. 1986, 101, 19. The concentrations are expressed as wt % or as mM of ecg. (23) Turro, N. J. Modern Molecular Photochemistry; BenjamidCummings: Menlo Park, CA, 1978. (24) Demas, J. N. Excited State Lifetime Measurements; Academic Press: New York, 1983. (25) Rillema, D. P.; Jones, D. S.; Levy, H. A. J. Chem. Soc., Chem. Commun. 1979, 849. (26) Ghosh, P. K; Bard, A. J. J. Phys. Chem. 1984,88, 5519. Also see ref 19. (27) Binding of Ru(I1) complex to these media, in general, stabilizes the MLCT state and hence a red shift of the emission was observed; see refs 19 and 24. (a) Kumar, C. V.; Turro, N. J; Barton, J. K. J. Am. Chem. SOC. 1985, 107, 5518. (b) Duveneck, G. L.; Kumar, C. V.; Turro, N. J; Barton, J. K. J. Phys. Chem. 1988, 92, 2028. (c) Wheeler, J; Thomas, J. K. J. Phys. Chem. 1982, 86, 4540. (28) (a) Nakamura, T; Thomas, J. K. Langmuir 1985, 1, 568. (b) DellaGuardia, R; Thomas, J. K. J. Phys. Chem. 1983, 87, 990. Iverson, B. L.; Iverson, S. A,; Roberts, V. A. Science (Washington D.C.)1990, 249, number 4969, 659. (29) Colon, J. L.; Yang, C.-Y.; Clearfield, A; Martin, C. R. J. Phys. Chem. 1990, 94, 874. (30) Assuming that the metal complexes completely occupy all the available surface area (100 m2/g of BAZrP) and that one layer of metal complexes separate the ZrP platelets, the area per molecule was estimated. The diameter of the metal complex and the average metal-metal distance at the saturation point were calculated to be 13 A. This value is consistent
with the interlayer spacings calculated from the 001 reflections of the powder diffraction patterns as well as the known diameter of the metal complex. (31) Meisel, D.; Matheson, M. S ; Rabani, J. J. Am. Chem. SOC. 1987, 100, 117. (32) Vliers, D. P.; Collin, D.; Schoonheydt, R. A; De Schryver, F. C. Langmuir 1986, 2, 165. Also see refs 19b,c and 24. (33) Habti, A.; Keravis, D.; Levitz, P; van Damme, H. J. Chem. SOC. Faraday Trans. 2 1984, 80, 67. (34) van Grondelle, R. Biochim. Biophys. Acta, 1985, 811, 147. (35) Caspar, J. V.; Sullivan, B. P.; Kober, E. M.; and Meyer, T. J. Chem. Phys. Lett. 1982, 91, 91. (36) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193. (37) In frozen BAZrP suspensions (1:2, ethylene glycol to water) at 77 K more than 99% of the emission decays with a lifetime of 3950 f 20 ns. A weak second component was also observed, but its intensity is too low to measure accurately. At room temperature, the corresponding long- and short-lived components are 1602 f 20 (94%) and 424 f 35 ns (6%). The shallow temperature dependence is indicative of the diminished role of the triplet ligand field state in the luminescent state dynamics. (38) Kaufman, K. J.; Dutton, P. L.; Netzel, T. L.; Leigh, S. J.; Rentzepis, P. M. Science (Washington D.C.)1975, 188, 1301. Van Grondelle, R.; Gamer, J. J.; Rijgersberg, C. P. Biochim. Biophys. Acta 1982, 682, 208. Goedheer, J. C. Biochim. Biophys. Acta 1959, 35, 1. (39) Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem. SOC. 1977, 99,5615. JF'951374V
