IO009
J. Phys. Chem. 1991, 95, 10009-10018 at 308 nm, the increased fragmentation of the products of ArF laser ablation is understandable. Whether the distribution of products in ArF laser ablation near threshold is due to such secondary photolysis or to a significant photolytic contribution to the ablation mechanism as discussed above is an unresolved question. In general a number of competing processes may occur during the excimer laser ablation of any polymer. These include direct photolysis, multiple photon processes? and thermolysis following the relaxation of electronic energy to heat. It should be observed that the conversion of electronic to vibrational energy may occur partly by a process involving cage recombination of radicals initially generated by photolysis. The ablation of PET by XeCl laser irradiation occurs by rapid relaxation of the initial electronic excitation to heat, resulting in thermal decomposition. This
conclusion is reached on the basis of the observed ablation products, the lowering of the ablation threshold by preheating, the evidence of photoacoustic measurements, and the results of model calculations involving thermolysis rates. The available evidence also supports thermolysis as an important factor in the ablation of PET by ArF laser irradiation, although in this case the possibility of a contribution from photolysis cannot be excluded.
Acknowledgment. We are grateful for the skilful assistance of B. L. Tait. We thank Laser Applications Ltd. and the S.E.R.C. for the award of a C.A.S.E. studentship to J.S.and the S.E.R.C. for support by research Grants GR/D/28065 and GR/D/97740. Registry No. PET, 25038-59-9;XeCI, 55130-03-5; ArF, 56617-31-3; COZ, 124-38-9;CO,630-08-0; CH+74-82-8;CZHZ,74-86-2; CZHI,7485-1;C6H6,71-43-2.
Spectral Differences between Enantiomeric and Racemic Ru(bpy):’ Probable Causes
on Layered Clays:
Prashant V. Kamat; K. R. Copidas; Tulsi Mukherjee,*Vishwas Joshi,j Diiip Kotkar,s Vinit S. Pathak,g and Pushpito K. Chosh*.* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India, and Alchemie Research Centre, Thane- Belapur Road, Thane 400 601, India (Received: April 15, 1991; In Final Form: July I S , 1991)
The preferential self-annihilation (static and dynamic) of A,A-Ru(bpy)?+* over A- or A-Ru(bpy)?+* is reported for aqueous dispersions of sodium hectorite lightly loaded with the Ru(I1) chelate and subjected to pulsed laser excitation. By varying the loading level over a factor of ca.60, it is also shown that racemate emission falls off sharply with increased loading whereas emission from the enantiomeric adsorbate remains more nearly constant. The decrease in luminescence yield of racemate with increased loading is mainly associated with an attenuation in the peak emission intensity, I(O), as found from time-resolved measurements. It is proposed, based on these studies, that clays offer both quenching and nonquenching sites for sorption and that A,A-Ru(bpy),” prefers the latter at low loadings, the ions being clustered within such regions. Enantiomeric Ru(bpy),”, on the other hand, is more randomly distributed over the sites. The above model also permits rationalization of (i) observed changes in emission intensity with time, (ii) anomalies in the relative emission yields of Ru(bpy)?+* and Ru(phen):+*, and (iii) the effect of Zn(phen)?+ on emission. Finally, differences in binding modes of enantiomeric and racemic chelate forms also induce differences in the flocculation trends of dispersed clays, the effects being most prominent for freshly prepared ruthenium(l1) montmorillonite.
We have recently reported results of our studies on the absorption and emission spectral behavior of optically active Ru(bpy),2+ (bpy = 2,2’-bipyridine) and Ru(phen);+ (phen = 1,IO-phenanthroline) chelates adsorbed on naturally occurring layered c1ays.l These studies-which followed Yamagishi and Soma’s original observations concerning the degree of exchange of such chelate types on sodium montmorillonite2-indicated that the binding states of enantiomeric and racemic complexes of Ru(l1) are different on clay, in marked contrast to their behavior on other supports. Although some form of spontaneous chiral interaction is implicit in the studies on lightly loaded clays, no conclusive evidence could be obtained so far to suggest that the spectral differences reflect genuine interactions between optical antipode~.’~J Spectral variations could also arise from differences in distribution patterns of the chelates over environmentally distinguishable exchange sites, influenced as such patterns may be by interactions during the sorption process. Firm evidence on the origin of the observed effects would clearly be necessary to develop models of chiral interactions on clay^.^-^ To this end, we have compared the effect of excitation intensity on the time-resolved luminescence behavior of enantiomeric and racemic Ru(bpy)t+* lightly loaded on sodium ‘University of Notre Dame. *Bhabha Atomic Research Centre. Alchemie Research Centre.
hectorite.l*b* By varying the loading level over a factor of ca. 60, the site selectivity of the chelate forms could also be probed. (1) (a) Joshi, V.; Kotkar, D.; Ghosh, P. K. J . Am. Chem. Soc. 1986.108, 4650. (b) Kotkar, D.; Joshi, V.; Ghosh, P. K. Proc. I d . Narl. Sci. Acad. A 1986,52,736.(c) Joshi, V.; Ghosh, P. K.J . Chem. Soc., Chem. Commun. 1987,789. (d) Joshi, V.; Kotkar, D.; Ghosh, P. K. Curr. Sci. 1988,57,567. (e) Joshi, V.; Ghosh, P. K. J . Am. Chem. SOC.1989, 111, 5604. (f) Joshi, V.; Kotkar, D.; Ghosh, P. K. Proc. Ind. Acad. Sci. (Chem. Sci.) 1990, 102,
203. (2)(a) Yamagishi. A.; Soma,M. J . Am. Chem. Soc. 1981,103,4640. (b) Yamagishi, A. J. Phys. Chem. 1982,86,2472. (c) Yamagishi, A.; Fujita, N. J. Colloid Interface Sci. 1984,100, 1778. (d) Yamagishi, A. J. Coord. Chem. 1987,6,131 and references therein. (3) Villemure, G.; Bard, A. J. J . Electroanal. Chem. 1990, 283, 403. (4)(a) Pirkle, W. H.; Finn, J. M.; Hamper, B. C.; Schreiner, J.; Pribish, J. R. In Asymmetric Reactions and Processes in Chemistry; E M , E., Otsuka, S., Eds.; ACS Symposium Series No. 185; American Chemical Society: Washington, DC, 1982;p 256. (b) Pirkle, W.H.; Hyun, M. H.;Banks, B. J . Chromarogr. 1984, 316, 585. (c) Salem, L.; Chapuisat, X.;Segal, G.; Hilberty, P. C.; Minot. C.; Leforestier, C.; Sautet, P.J . Am. Chem. Soc. 1987, 109, 2887. (d) Lipkowitz, K. B.; Demeter, D. A.; Zegarra, R.; Larter, R.; Darden, T. J . Am. Chem. Soc. 1988, 110, 3446. (e) Metcalf, D. H.; Snyder, S.W.; Demas, J. N.; Richardson, F. S . J . Am. Chem. SOC.1990,112,5681 and references therein. ( 5 ) (a) Arnett, E. M.; Harvey, N. G.; Rose, P. L. Ace. Chem. Res. 1989, 22,13 1 and references therein. (b) Andelman, D. J . Am. Chem. SOC.1989,
111,6536. ( 6 ) Turro, N. J.; Kumar, C. V.; Grauer,
2.;Barton, J. K. Lungmuir 1987,
3, 1056.
(7)Kuykendall, V. G.; Thomas, J. K. J. Phys. Chem. 1990, 94, 4224.
0022-3654191 12095-10009%02.50/0 0 1991 American Chemical Society
10010 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
Kamat et al.
TABLE I: Values of Fitting Parameters Obtained from Double-Exponential Simulation of the Single-Photon-Counting Luminescence Decay Profiles of 20 uM A-(+),,- and A,A-Ru(bpy)a2+'at L a w and High Lording Levels on Sodium Hectorit@> Ru(l1) Wd 7 1 ,LCS 7% loadingC a I-a 72. PS X2 0.319 0.3 I 0.681 1.05 I .05 2 4389 A-(+)D 0.203 0.39 2 7877 0.797 1.02 1.02 A,.i 0.391 1.07 1.14 0.609 0.38 3566 32 A- ( +) D 0.306 0.38 0.694 0.84 0.93 32 4755 A J
+
'337-nm N 2 lamp excitation and 590-nm detection. b l ( t ) = I(O)[aexp(-t/r,) ( 1 - a) exp(-t/z,)]. Ccec of clay - I mequiv/p. dlnstrumentation parameters were maintained constant, and signal accumulation time was 45 min in all cases. The fitting was from 0 to 3 ps.
min. The solution was filtered and heated to 70 OC. A saturated NaCl solution (5 mL) was then added and the flask left standing under ambient conditions. The crystals obtained were collected on a frit and washed with ice-cold water. Chemical purities of all salts were checked with atomic absorption and elemental Experimental Section analysis. Doubly distilled water was used in all experiments. Sample Preparation Method. Clay/chelate samples were Materials. Hectorite from San Bernadino County, CA, prepared by one-shot addition of an appropriate volume of an (SHCa-I, Source Clay Minerals Repository, University of Misaqueous solution containing the metal complex(es) into a known souri), low-iron montmorillonite from Wyoming bentonite (GK volume of a clay dispersion. Racemic Ru(II) was prepared by 129, Georgia Kaolin Co.), illite from Silver Springs, MT (IMT- I , premixing aqueous solutions of the A and A enantiomers prior Source Clay Minerals Repository, University of Missouri), and to sorption on clay. kaolinite from Georgia (Hydrite PX, Georgia Kaolin Co.) were Instrumentation. UV-vis absorption spectra were recorded on employed in the present work. The clays were converted into their a Shimadzu Model UV- 160 spectrometer while steady-state luNa+-exchanged forms by mechanical stirring in 2 M NaCl (40 minescence studies were carried out on a Perkin-Elmer Model mL/g of clay) at room temperature for 4-5 days. This was LS-5, Hitachi F-4010, or SLM 8000 spectrofluorimeter. Clay followed by repetitive high-speed (ca. 14000 rpm) centrifugation aggregation was monitored on a Horiba Model LA-500 laser on a REM1 Model R-24 centrifuge (fitted with a 4 X 100 mL (He-Ne, h = 632 nm) diffraction particle size distribution anaR-24 1 rotor with a specified centrifugal acceleration of 2.18 X lyzer. Optical rotations were measured on a digital polarimeter 104gat 13 500 rpm) and redispersion of residue in water. Finally, (Jasco DIP-140) employing the sodium D line, and an elemental the clay dispersions were centrifuged for 30 min (hectorite & analyzer (Carlo Erba, Model 1106) was employed for carbon, montmorillonite, 5000-6000 rpm; illite and kaolinite, -4OOO rpm) hydrogen, and nitrogen analyses. Atomic absorption measureto remove heavier particles. Other than centrifugation, no dements were performed on a Perkin-Elmer Model 2380 spectromliberate attempt was made to remove clay impurities such as eter without background correction. carbonates in hectorite. For a few experiments, sodium hectorite Time-resolved emission studies were performed both a t highdispersions were fractionated as follows: The clay suspension was and low-excitation intensities. The former were carried out in centrifuged at 14000 rpm and the supernatant collected. The a I-cm quartz cuvette employing the 355-nm third harmonic (pulse residue was redispersed in water and centrifuged at 10000 rpm. width -6 ns) from a Quanta Ray DCR-1 YAG laser as excitation This process was repeated to yield dispersions at 6000 and 2000 source. The output laser power was constant (in the range 2-5 rpm. The strengths of the clay dispersions were determined by mJ/pulse) for a given set of experiments, the intensity at the cell drying a known volume of a dispersion on a glass slide and being varied by introducing 1075, 30%, and 65% transmittance measuring the weight of the residue. Cation-exchange capacities filters. Dose measurements were made by splitting off a fraction (cec) of the clay dispersions were obtained by measuring the of the beam with a quartz plate and directing this light to a uptake of A-(+)D-Ru(bpy)2+ and A,A-Ru(bpy)?+ from solutions pyroelectric monitor. The emission from the sample was monitored containing ca. 3-fold excess of the Ru(I1) ion. There were no at right angle with a monochromator, photomultiplier, and Teksignificant differences between the values obtained with the entronix R-7912 transient digitizer. A typical experiment consisted antiomeric and racemic forms, the cec values being 1.02, 0.94, of 5-10 replicate shots per measurement, and the average signal 0.26, and 0.19 mequiv/g, for hectorite, montmorillonite, illite, and was processed with an LSI-I1 microprocessor interfaced to a VAX kaolinite, respectively. Chloride salts of Ru(bpy)32+and Ru(phen)32+were synthesized according to literature p r ~ e d u r e s . ~ 1 1/70 computer. Time-resolved fluorescence measurements at low-excitation flux were performed with a Model 199 time domain The chelates were resolved with potassium antimonyl tartrate fluorescence spectrometer (Edinburgh Instruments) using either (Aldrich) and converted into the perchlorate salts.'"'O Chemical a coaxial metal gated nitrogen discharge lamp (A,,, 337 nm; and optical purities of these salts were found to be satisfactory. repetition rate, 25 kHz; fwhm, -1.5 ns) or a similarly fitted Chloride salts of tris( bipyridy1)- and tris(phenanthroline)zinc(II), hydrogen discharge lamp (&,, 310 nm; repetition rate, 30 kHz; -nickel(II), -cobalt(II), and -iron(II) were prepared as follows: fwhm, 1 ns) as excitation source and an EG&G Ortec single2 mmol of an appropriate metal salt (nickel(I1) chloride, amphoton counting (SPC) data acquisition system, interfaced with monium ferrous sulfate, cobaltous acetate, zinc(I1) chloride) and an LSI 11/23 (Plessey) computer. All instrumentation parameters 6.4 mmol of 2,2'-bipyridine/ 1,lO-phenanthroline were added to were maintained constant for a given set of experiments so that 25 mL of H20in a conical flask and the contents stirred for 30 the peak luminescence intensities could be compared. The decay profile from the peak count onward was analyzed for the lu(8) (a) Krenske, D.; Abdo, S.;Van Damme, H.; Cruz, M.;Fripiat, J. J. minescence lifetime by using a nonlinear iterative least-squares J . Phys. Chem. 1980,842447. (b) Abdo. S.;Caneswn, P.; Cruz, M.;Fripiat, fit method: I ( f ) = x B i exp(-f/+,), where Bi is the preexponential J. J.; Van Damme, H. J . Phys. Chem. 1981,85, 797. (c) DellaGuardia, R. factor and T~ the fluorescence lifetime of the ith component. A.; Thomas, J. K. J . Phys. Chem. 1983,87,990. (d) Ghosh, P. K.; Bard, A. J. J. Phys. Chem. 1984,88, 5519. (e) Habti, A.; Keravis, D.; Levitz, P.; Van Computer fits were evaluated based on minimum reduced x2 Damme, H. J. Chem. Soc., Faraday Trans. 2 1984,80.67. (f) Schoonheydt, values and the distribution of the weighted residues along the data R. A.; De Pauw. P.; Vliers, D.; De Schriver, F. C. J . Phys. Chem. 1984,88, channels. 5 1 13. (8) Van Damme, H.;Nijs, H.;Fripiat, J. J. J . Mol. Cafal.1984, 27, 123. (h) Nakamura, T.; Thomas, J. K. Langmuir 1985, I, 568. (i) Thomas, Results J. K. Arc. Chem. Res. 1988, 21, 275. (9) Braddock, J. N.; Meyer, T. J. J. Am. Chem. Soc. 1973, 95, 3158. Spectral Studies of Ru(II)/Clay. Previous studies have reported (IO) (a) Dwyer, F. P.;Gyarfas, E. C. J. Proc. R. Soc., N.S.W. 1949, 83, that smectite clay dispersions become unstable beyond a 10% 170; 1949, 83. 174. (b) Burstall, F. H. J. Chem. Soc. 1936, 173. (c) For loading of Ru(II).j Most such studies were conducted on clay assignment of absolute configurations, see: Mason, S.F.; Peart, B.J. J . Chem. SOC.,Dalton Trans. 1973, 949. fractions centrifuged at 5000-7000 rpm, with reported particle These and other studies enable us to comment more specifically on the binding states of the enantiomeric and racemic forms. Differences in binding modes are also manifest in the novel clay particle size data reported herein.
-
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 10011
Ru(bpy)32+on Layered Clays
-
....
A
[r
c
501
F
0
10
20 30 Yo Looding
40
50
60
Figure 1. Plots of steady-state emission intensity (&, 460 nm; &,, 590 nm) vs loading of 20 pM enantiomeric (A) and racemic (0)Ru(bpy)?+ exchanged on sodium hectorite (cec = 1 mequiv/g) fractionated at 14000 rpm.
5
0
10
15
TIME
25
20
30
35
45
40
/1i7sec
*i n
v
nm
Figure 2. UV-vis absorption spectra of 20 pM A-(+)D- (a), A-!-)D- (b), and A,A-Ru(bpy),2+ (c) in 2 g/L (A) and 0.125 g/L (B) sodium hectorite fractionated at 14000 rpm. All spectra were recorded against appropriate references.
0
sizes of 0.2 pm or less.3 Higher loading levels are afforded when finer dispersions are employed; thus, even when the loading was as high as 60% (20 pM Ru(I1) in 67 mg/L clay), the hectorite dispersion fractionated at 14000 rpm remained stable for at least a few hours," while a similar loading on coarser clay fractions led to instantaneous precipitation. The effect of loading on luminescence intensity is shown in Figure 1. There is a sharp decrease in the emission yield of the racemate with increased loading while the attenuation is marginal for adsorbed enantiomer. Table I provides data on the time-resolved emission profiles of 20 pM Ru(1I) in 2 and 0.125 g/L sodium hectorite obtained employing N, lamp excitation (A,, = 337 nm) and 590-nm ob( I 1 ) Curiously, the dispersions were less stable when dialyzed clay was employed.
0.4
08
1.2
1.6
2
2.4
PS Figure 3. Time-resolved luminescence profiles of 10 pM enantiomeric and racemic Ru(bpy)32+*in the absence [A-(+)D- (a), A,& (d)] and presence [A-(+)D- (b), A& (c)] of 20 pM A,A-Zn(phen),2+coadsorbed on I g/L aqueous sodium hectorite (see Table 111). All four profiles were recorded under identical conditions employing (A) low (337-nm N2lamp; I,, = 0 mJ/cm2) and (B) high (355-nm YAG laser; I,, = 1.5 mJ/cm2) excitation intensities. Steady-state spectra of the samples subjected to low and high excitation intensity are shown in the insets. The plots of P [ ( I d - 11)/(11, 11) vs X of (a) and (d) are also given in A.
+
servation wavelength. The most significant feature in the table is the pronounced decrease in the peak emission intensity, Z(O), of the racemate when its loading on clay is increased from 2 to 32%. Absorption spectra of these samples are shown in Figure 2.
Kamat et al.
10012 The Journal of Physical Chemistry, Vol. 95. No. 24, 1991
TABLE II: Effect of Excitation Intensity on the Values of Fitting Parameters Obtained from Double-Exponential Simulation of Luminescence Decry Profiles of 10 pM A-(+)D-, A-(-)D-, and A,A-Ru(bpy)?+' on 1 g/L Sodium Hectorite, in the Absence and Presence of Cordsorbed A,A-211(phen)~~* " Ru(l1) [Zn(Wl, g M Wb a Tlr PS I-a 72. PS X2
Low-Intensity Excitationc 0 20 0 20 0 20
A-(+)D A- (+) D A-(-)D
P-(-)D P,A P,A
3348 5393 3412 5404 7423 6109
0.236 0.058 0.359 0.095 0.047 0.162
0.225 0.197 0.264 0.245 0.208 0.337
0.764 0.942 0.641 0.905 0.953 0.838
0.584 0.782 0.702 0.880 0.880 1.034
1.14 1.14 1.40 1.30 1.36 1.16
0.456 0.636 0.487 0.556 0.516 0.562
0.585 0.840 0.526 0.878 0.758 0.901
1.60 1.04 1.75 1.61 1.47 2.48
High-Intensity Excitationd 0 20 0 20 0 20
A-(+)D *i-(+1D W-)D
A-(-)D &A &A
1.78 12.29 7.19 13.90 8.83 14.10
0.544 0.364 0.513 0.444 0.484 0.438
0.133 0.134 0.1I8 0.149 0.125 0.169
"The curves were fitted from 0 to 2.5 ps employing the equation / ( r ) = I(O)[aexp(-t/zl) + (1 - a) exp(-t/r2)], where I ( t ) and I(0) indicate the emission intensity at time f and time zero, respectively. *All instrumentation and other experimental parameters were kept constant for experiments at a given excitation intensity. C N 2lamp, A,, = 337 nm (I,,, ;z 0 mJ), A,, = 600 nm; dYAG laser, A,, = 355 nm (I,,, = 1.5 mJ), A,, = 600 nm. TABLE 111: Values of Parameters Obtained from Double-Exponential Simulation of the Time-Resolved Luminescence Profiles of A-(-)D- and A,A-R~(bpy)~*+'/HectoriteObtained at Three Excitation Intensities"$* Iexv
mJ/cm2 0 0.2 1.3 0 0.2 1.3
Ru(l1)
P A
P A,A P,A A,A
ff
0.359 0.507 0.445 0.047 0.509 0.500
TI,
us
0.264 0.183 0.130 0.208 0.222 0.150
1% us
0.702 0.560 0.554 0.880 1.050 0.811
x* 1.40 0.94 1.47 1.36 1.59 1.35
"Samples contained IO pM Ru(l1) in 1 g/L sodium hectorite. *Time-resolvedprofiles were simulated employing the equation / ( t ) = a exp(-t/r,) + ( 1 - a) exp(-t/i2).
0
I-
a
a w V V W v)
w
E
3 -1
" 4 z
I-
The effect of excitation intensity on the emission decay kinetics of enantiomeric and racemic Ru(Il)/hectorite (2% loading) was also studied, with and without coadsorbed A,A-Zn(phen)t+. The decay traces in Figure 3A were obtained employing 337-nm N, lamp excitation, the integrated area under a decay curve corresponding to the total luminescence output a t the observation wavelength of 600 nm. There is qualitative agreement between the ratios of these areas and the corresponding steady-state 1 .O/ emission intensity ratio (see inset) at 600 nm: a/b/c/d 1.812.1/2.2 for steady state vs 1.0/2.1/2.5/3.0 for time resolved. A similar set of experiments was performed with 355-nm YAG laser ( I .5 mJ/cm2) excitation (Figure 3B). The curves in Figure 3 have been simulated, and values of all parameters are given in Table 11. Comparison of the steady-state spectra (insets) in parts A and B of Figure 3 indicates that although samples subjected to low and high excitation intensity were similar in all respects, the decay profiles exhibited some differences at the two intensity levels. Most importantly, the racemate luminescence at t = 0 was sharply reduced when the excitation intensity was high. As a result, the trend I ( O ) , > I(O), > I ( O ) , > I(O), observed at lowexcitation flux changed to I(O)< > I(O),# > I(O), > f(O),. f(0)r, I(O),, I(O)495-nm
0
1 0 H ) 3 0 4 0 5 0 6 0 7 0 8 0 W
Time/lO-' sec Figure 10. Steady-state (310-nm excitation) and time-resolved (310-nm excitation, >490-nm detection) luminescence profiles of 24 pM Ru(phen)?+* in the absence (A-(-)D (b), A-(+)D (a), A,A (c)) and presence (A-(-)D (d), A-(+)D (e), A,& (f)) of 47 pM A,A-Zn(phen),*+ in 2 g/L sodium hectorite. All samples were sonicated for 5 min, and their emission profiles were recorded under identical conditions.
citation flux when Zn(I1) is not added. That there is an increase even of I(0)d/I(O)cwith excitation intensity indicates that, in the absence of Zn(II), adsorbed enantiomeric Ru(II)* also undergoes nondiffusional self-quenching, albeit to a lesser extent than the racemate. As shown in Figure 5, there is a pronounced effect of excitation flux on the decay profiles as well, the traces of I ( t ) vs r curves becoming steeper at higher excitation intensities although to different degrees for enantiomer and racemate. The static and dynamic self-quenching phenomena observed as a function of excitation flux may be ascribed to triplet-triplet annihilation (eqs 3 and 4).6*8.21922The third term in eq 3 would be significant only 3 R ~ ( b p y ) F *+ 3Ru(bpy)F*
-D
'Ru(bpy)F*
+
Ru(bpy)F
(3)
self-quenching of racemate over enantiomer with increase in ex(1 8) Small time-dependent changes in absorption and emission spectra of c l a y / R ~ ( b p y ) ~have ~ + been noted by others as well.'f' (19) The relative degree of steady-state luminescence enhancement in presence of Zn(1l) was similar with 310-nm and 460-nm excitations, Zn(I1) being completely nonabsorbing at the latter wavelength. (20) The effect of Zn(phen),'+ is perhaps most dramatic on kaolinite. ~ * + A,A-Ru* Discrepancies in the emission trends of A . A - R u ( ~ ~ ~ ) and (phen)32+*are eliminated altogether when Zn(l1) is coadsorbed with Ru(I1) ([Zn(Il)]/[Ru(lI)] = 3) on kaolinite.
(21) (a) Giannelis, E. P.; Nocera, D. G.; Pinnavaia, T. J. Inorg. Chem. 1987,26,203. (b) Lachish, U.; Ottolenghi, M.;Rabani, J. J. Am. Chem. Soc. 1977, 99, 8062. (c) Baxendale, J. H.; Rodgers, M . A. J. Chem. Phys. Leu. 1980.72,424. (d) Baxendale, J. A.; Rodgers, M.A. J. J . Phys. Chem. 1982, 86, 4906. (e) Komada, Y.; Yamuchi, S.; Hirota, N. J. Phys. Chem. 1988, 92, 6511.
10016 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
if the Ru(II)* concentration increases sufficiently so that two Ru(II)* ions can approach contact radius within the lifetime of the excited state. If self-annihilation is to influence the value of I(O), the process must occur within the duration of the laser pulse. For laser pulse width -6 ns and D = 1odcm2/s, static quenching would not be obsereved unless two or more Ru(I1) are within IO A ( I = ( 2 D ~ ) l /of~ each ) other prior to excitation. The high value of luminescence polarization (Figure 3A) of both forms of Ru(bpy)32+*in sodium hectorite ( P = 0.1 1 in clay dispersion vs 0.14 in a rigid matrix23) suggests that chelate mobility is severely restricted on clay in comparison to that in solution; Le., D
