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Anal. Chem. 1991, 63,337-342
Photophysics and Photochemistry of Oxygen Sensors Based on Luminescent Transition-Metal Complexes E. R. Carraway' and J. N. Demas* Chemistry Department, University of Virginia, Charlottesuille, Virginia 22901
B. A. DeGraff* Chemistry Department, James Madison University, Harrisonburg, Virginia 22807
J. R. Bacon* Chemistry Department, Western Carolina University, Cullowhee, North Carolina 28723
A detailed study of the photophysics and photochemistry of polymer-immobilized luminescent transition-metal complex oxygen sensors is presented. Emphasis is on understanding the underlying origin of the nonlinear Stern-Volmer quenching response. Microheterogeneity is important in both photophysical and photochemical behavior, and the nonlinear quenching responses in RTV 118 silicone rubber can be adequately described by a two-site model, although detailed lifetime measurements suggest a more complex Underlying system. Counterion studies with quenching counterions are shown to be useful probes of the structure of the complex in the polymer. While oxygen enhances photochemical instability, singlet oxygen is not directly implicated in sensor decomposition. I n the photochemistry there is at least one reactive and one much less reactive site, although the photochemistry and quenching measurements probably sample different populations of sites. The existence of reactive sites suggests that stability can be enhanced by a preliminary photolysis to eliminate the more reactive sites.
INTRODUCTION Luminescence-based optical sensors are becoming increasingly important, particularly in the area of fiber optic sensors (1). These sensors are frequently supported in polymers or gels or on surfaces. In contrast to more conventional homogeneous luminescences these supports are frequently heterogeneous on a microscopic scale and give rise to complex decay kinetics that can be characterized as sums of several exponentials or as distribution functions of exponentials (2-6). This complexity can frequently result in poorly understood sensors. An important class of luminescence sensors is 02-quenching sensors, which are based on the decrease of luminescent intensity and lifetime of the sensor material as a function of O2 tension (7-12). In homogeneous media with only a singlecomponent exponential decay, the intensity and lifetime forms of the Stern-Volmer equations with both static and dynamic quenching are ~ O / T=
1 + Ksv[Q1
(la)
Ksv = b o where [Q] is the quencher concentration, T'S are lifetimes, I's
* Current address: Environmental Engineering Science,California Institute of Technology, Pasadena, CA 91125.
are intensities, Ksv and k 2 are the Stern-Volmer and bimolecular quenching constants, respectively, and Keg is the association constant for binding of the quencher to the luminescent species. The subscript 0 denotes the value in the absence of quencher. If plots of T o / f or Zo/Z versus quencher concentration are linear and match, quenching is purely dynamic (i.e., K,, = 0 ) . If Zo/Z is above T ~ / T static , quenching is present. However, in many microheterogeneous systems, the multiexponentiality of the decay curves and the uncertainty of the fitting model preclude evaluating a single-exponential T for use in eq 1. Even the question of the relative contributions of static and dynamic quenching is difficult to address. Further, the Zo/Z plots are downward curved, which makes more accurate calibration difficult. We reported an O2 sensor based on quenching of a transition-metal complex R ~ ( P h ~ p h e n ) ~ (Ph,phen *+ = 4,5-diphenyl-1,lO-phenanthroline) in a silicone rubber (12). As with most quenching-based sensors, the response exhibits downward curvature. In view of the very long unquenched luminescence lifetimes of many transition-metal complex sensors (13, 14),these complexes seemed ideal to explore the mechanisms of the nonlinear Stern-Volmer plots and to develop models for predicting sensor response. Our goals were to examine in detail the photophysics and photochemistry of the quenching-based O2 sensor by using RUL,~+where L is an cu-diimine (2,2'-bipyridine, 1,lOphenanthroline, and their substituted analogues) in a silicone rubber. As we will show, quenching in these systems is purely dynamic, with the downward curvature resulting from site heterogeneity. Further, a very simple two-state model gives excellent fits to our experimental intensity-quenching results. The photochemistry also exhibits heterogeneity, which may prove useful for stabilizing sensors.
EXPERIMENTAL SECTION Chemicals. Tris complexes, RuLB2+,of the following ligands were prepared by literature methods (25). The abbreviations for the ligands are bpy = 2,2'-bipyridine, phen = 1,lO-phenanthroline, 5,6-Me2phen = Ph2phen = 4,7-diphenyl-l,lO-phenanthroline, 5,6-dimethyl-l,lO-phenanthroline, and 4,7-Me2phen = 4,7-dimethyl-1,lO-phenanthroline. The BPh4- salt of Ru(4,7Me,phen),?+ was prepared by metathesis from the perchlorate salt (16). In addition, the cis-Ru(phen),(CN), was prepared by literature methods (17). The one-part RTV 118 and the two-part RTV 615 were from General Electric Corp. The one-part air-cured Dow Corning silicone was a Silastic medical adhesive tape, Catalog No. 891. Samples were prepared as described earlier (12) by using =I mM solutions of the complex in CH2C12.Film thicknesses were typically 0.005-0.015 in. Earlier results showed behavior was independent of film thickness. The recommended slow evaporation procedure was used to improve film quality (12).
0003-2700/91/0363-0337$02.50/0@ 199 1 American Chemical Society
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Absorption spectra for the R u L P species in RTV 118 showed no new bands or significant spectral shift. This, coupled with the similarity of the emission spectra in dilute solutions and in the films, indicates that aggregation is not significant. Photophysical Measurements. Static dc luminescence measurements were made on a SPEX Fluorolog system (18). Luminescence lifetime measurements were made by using a homemade time-correlated single-photon-counting system (19, 20). Solution spectra were measured a t =lo0 bM. To survey different polymer-complex systems one-point inwere taken for several comtensity quenching data (Initrogen/Isir) plexes. Different polymers and methanol were used as the solvents. All measurements were made at room temperature (22 f 2 "C). Air measurements used room air. Previous measurements have shown that the systems are not affected by variations in humidity that include putting the sample in water (12). For the pressure dependence studies, the gas was pure oxygen and the pressure was varied with a pump. Photochemistry. Photochemical studies were performed by irradiating low optical density films in air in the Spex spectrofluorometer. Decomposition was monitored by following the emission intensity. Excitation bandwidth was set to the maximum of 16 nm to minimize photolysis times. Excitation was near the peak of the visible metal-to-ligand charge-transfer transition, and the emission was monitored at the wavelength of peak emission. Since emission intensity is directly proportional to concentration under our conditions and the only likely emitting species is the starting material, the decay of the emission intensity is directly proportional to the surviving concentration of the sensor material. Photochemical decomposition curves (luminescent intensities vs time) were fit to a simple first-order decay E ( t ) = K exp(-kObt) (2) where E ( t ) is the emission intensity as a function of time, K is a proportionality constant, and kobs is the photochemical rate constant. I n addition, since eq 2 did not give very good fits, we used a decaying exponential on a constant baseline: E ( t ) = K[(1 - F) exP(-kobst) -k F] (3) where F corresponds to the fraction of the total emission that is not lost from extended photolysis. We were also concerned that materials left in the polymer might be responsible for the degradation. To test this hypothesis, we exhaustively extracted a film in a Soxhlet extractor overnight with CH,CI,. Ru(phen)32+sensors were constructed by using the virgin and extracted films and photolyzed. Since our samples are optically dilute, the relative quantum yields were computed from $photo = kobs/e (4) where t is the molar extinction coefficient a t the excitation wavelength. Data Treatment. For ascertaining the relative contributions of static and dynamic quenching, we used the method described in the previous paper (22). The preexponential weighted mean lifetime, r M TM ~ ~ i 7 i / ~ ( Y 8 (5) was computed. The necessary a's and 7's were extracted from the decays by deconvolution assuming an impulse response that is a sum of exponentials; a Marquardt nonlinear least-squares algorithm was used (19,20,22).The success of the fit was judged by the magnitude of the weighted x:, the appearance of the weighted residuals plot, and various goodness of fit indicators such as the Durbin-Watson parameter. The number of components was varied until a statistically valid reduced x 2 was obtained. The computed best fit cy's and 7's were used to calculate rM from eq 5. Even if no physical significance can be ascribed to the T ' S and cy's, the following is true if there is no static quenching: I Q / I = 7MQ/7M (6) Thus, if we use rM's rather than single-exponential lifetimes in our Stern-Volmer equation, we can compare intensity and lifetime data in microheterogeneous systems to assess the presence of static quenching.
Table I. Sensitivity to Oxygen Quenching of Metal Complex-Polymer Systems Initrogenl lair
solvent/ support
Ru(4,7Ru(4,7RUMezphen),- Mezphen)- Ru(phed2- (Ph,phen)3(C104)z (Ph4Bh (CNh (C104h
methanol RTV 118 Dow 891 RTV 615
6.7 5.6
3.6 2.4
6.7 4.3 1.3 1.0
14.7 4.0 2.1
2.6
19.6 7.4 5.6 1.26
Table 11. Photochemical Decomposition Rates
complex
abs
0.15 Ru(b~~),(C104)," Ru(~hen)~(ClO~)~" 0.036 R~(5,6-Me~phen)~(C10,)~" 0.06 Ru(~hen),(Cl0~)2~ 0.1 Ru(phen)a(C104)zb*c 0.1
exponential 105k,b,, s-l 1.65 4.0 7.0 3.5 2.3
exponential plus baseline lo%&,,s-l F 9 (1.00) 13 (1.06) 22 (1.45) 14 (1.00) 11 (0.78)
0.73 0.55 0.43 0.61 0.55
a 450-nm excitation. * 420-nm excitation. Exhaustive 10-h Soxhlet extraction with CH,Cl, before formine sensor.
RESULTS Figure 1 shows the emission spectra of the three complexes in different media. The emission spectra of Ru(bpy),,+ is strikingly sensitive to media, Ru(phen),*+ much less so, and R u ( P h , ~ h e n ) , ~is + virtually media independent. Figure 2 shows O2 intensity-quenching data for Ru( P h , ~ h e n ) , ~along + with the earlier reported intensity data. The current data extend t o a full atmosphere of pure oxygen. Figure 3 shows intensity- and lifetime (rM)-quenchingdata for both Ru(bpy),,+ and R ~ ( p h e n ) , ~ + . Table I gives t h e one-point intensity-quenching data (Initrogen/Zair). For homogeneous media such as methanol, the Stern-Volmer plots for these complexes are known t o be linear, but no detailed studies were performed in the silicones. Table I1 shows the results of photochemical decomposition studies using both a single-exponential model and a singleexponential plus a baseline model. was normalized to 1.00 for R ~ ( b p y ) , ~for + the 450-nm studies. For the extraction study of Ru(phen),,+ at 420 nm, $photo was set to 1.00 for the sample that was not exhaustively washed. The single exponential with a zero baseline gave very poor fits using lo4+ photolyses. However, the deviations were much less obvious for our original 2000-s irradiations, and one might have been fooled into accepting the misleading results of the single-exponential fit (Table 11). Acceptable fits with no obvious systematic errors were obtained with the exponential plus a baseline model for the 2000- and 104-sirradiations. Figure 4 shows the intensity-quenching plots for Ru(bpy)?+, Ru(phen)32+,and Ru(Ph,phe&*+. The solid lines are best fits using model 2 described later. The estimated uncertainties on the Zo/I's are 1-2% based on the precision of the individual intensity measurements. These uncertainties also match the residuals in our modeling.
DISCUSSION The luminescence of all the complexes arise from ligand to metal charge-transfer (MLCT) excited states where the emitting state is derived from a configuration involving promoting a metal d electron t o a ligand A* antibonding orbital. The long microsecond lifetimes arise from the emitting states containing a high degree of triplet character; however, t o denote them as triplets and the emissions as phosphorescences is probebly a misnomer. Due to the high atomic number of Ru, the emitting states are best described as spin-orbit states
ANALYTICAL CHEMISTRY, VOL. 63,NO. 4, FEBRUARY 15, 1991
339
*
1
1
0.80
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0.60 0)
4
-C
20.00
*
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*
15.00 7
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U
0'40
[r
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5.00
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~
1
0.00 500
600
700
BOO
0.00 0. 0
Wavelength (nm)
o
.
o
o
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xxxx
Demas-Bacon
v 1 m 1 I *
40.00
I I
I
I I I
I
lo/l
I I I 4 8 I I I
60.b0
Oxygen Pressure (cm Hg)
8O.bO
Figure 2. Current O2intensity-quenchingdata for Ru(Ph,phen);+ in RTV 118 (asterisks)along with the earlier reported intensity ( X ) data
j
0.80
( 72).
0.60 0)
.w
C Q)
0.40
K 0.20 J
0.00
500
600
700
800
Wavelength (nm) Figure 1. Emission spectra (450 nm) of Ru(bpy);+ (B), and Ru(Ph,phen),2+ (C) in different media.
(A), Ru(phen);+
rather than as either singlets or triplets (23). The emissions can frequently show significant alterations with changes in solvent polarity or donor/acceptor properties due to the fact that the emitting states involve large radial changes in charge distribution. This is seen clearly for Ru( b ~ y ) (Figure ~ ~ + 1A). The solvent perturbations are strongly attenuated for R ~ ( p h e n ) and ~ ~ +all but eliminated for Ru(Ph2phen)32+.This decreasing sensitivity to solvent pertur-
bation is a consequence of the excitation being localized in the metal a-diimine portion of the complex (-N=C-C=N-) (24). The more extended the complex, the greater the shielding of the excited portion and the smaller the solvent perturbations of the emission spectrum. In particular, the bulky phenyl groups are extremely effective a t shielding the excited state from solvent perturbations. Similarly, micelles, surfactants, cyclodextrins, and intramolecular hydrocarbon tails can shield excited states of Ru(II), Os(II), and Re(1) complexes from solvent perturbations (25-27). It is difficult to put precise errors on the TOM/TM data, since they are so indirectly derived. The simulation of the previous paper suggests that good precision is possible. However, experimentally, with our data and instrumentation for a variety of systems, we find that experimental uncertainties, especially a t high degrees of quenching exceed those of the simulations. On the basis of our observed noise levels for different systems, it appears that for the R ~ ( b p y ) ~ and ~+ R~(phen)~ data ~ + of Figure 3, TOM/TM versus Io/I can be considered within experimental error of each other, and O2 quenching is essentially purely dynamic. The slight fall of the TM data for R ~ ( p h e n ) is ~ ~probably + within experimental error. However, even if the fall-off is real, the total quenching is 79% at high O2 concentrations while dynamic quenching is 75%. Static quenching, if present at all, could account for no more than a few percent of the total quenching. The downward curvature of the Stern-Volmer plots necessitates a model more complex than a single species quenched bimolecularly. Microheterogeneity is required to explain these results. We evaluated two mechanistic models: (1) The complex exists in two distinctly different environments, with one being quenchable and the other being unquenched. (2) The complex exists in two distinctly different environments, with both being quenchable but with different rate constants. Model 1 is the minimal dynamic model giving downwardcurved Stern-Volmer quenching. It assumes two independent sites with only one having significant sensitivity to oxygen quenching. Model 2 is a more realistic two-site model that includes quenching of both sites. The basic equation for model 2 is
where the fO:s are the fraction of the total emission from each
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 4, FEBRUARY 15, 1991
25'00
* * *
1
0
* 0
I
b
2.00
d
*
5
10.00
I
0
5 -
0
1.50
-
*O 5.00
O$*
*
***** 00000
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0.00
* 3
4.00
e a
*
-
*
60.00
80.00
5*00 4.00 0
1 J
*
a
0
73.00 0
-
0
L
*
0
5
40.00
20.00
Oxygen P r e s s u r e (cm Hg)
j
2.00
*
-
lo
if-
1.00 f 0.00
!
-
20.00
?1 I
I -
1
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I I I b
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r
I I I
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80.00
60.00
Oxygen P r e s s u r e (cm Hg) 1.00 f r , , , , , , l , t , , " " " ' t ~ ~ , ~ ~ ~ ~ ~ ~ , ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 0.00
20.00
40.00
60.00
3
80.00
Oxygen Pressure ( c m Hg)
i
Figure 3. Intensity- (asterisks)and rMlifetime- (0)quenching data for both Ru(bpy),*+ (A) and R~(phen),~+(B).
Table 111. Model 2 Oxygen-Quenching Fitting Parameters
for R U L P sample
Ru( b p y P film 1 0.57 film 2 0.61 Ru(phen)32+ 0.65 Ru(4.7-Ph,~hen)~*+0.97
x2
f02
fill
0.070 0.056
0.197 0.4
0.43 0.39 0.35
0.03
0.0069
0.0074
0.0053 0.015 0.021
0.0046
72.00 0 -
4
0.025 0.19
component under unquenched conditions and the Ksvi's are the associated Stern-Volmer quenching constants for each component. Model 1 is a special case of model 2 with KsVz = 0. The equation of model 1 has been used to generate calibration curves for luminescence sensors (8). In addition, we also tested the widely used, but purely empirical, power law quenching model that accounts for the downward curvature of the Stern-Volmer plots.
lo/l= 1 + K [ Q J m (8) where K and m are fitting parameters with no physical significance. Model 1was unacceptable compared to model 2. Model 1 gave distinctly inferior fits compared to model 2 with x's
1 .oo
0.00
'rcmrr-r~~n-rnn-q
20.00
40.00
60.00
80.00
Oxygen P r e s s u r e (cm Hg) Flgure 4. Intensity 0,-quenching data for Ru(Ph,phen);+ (A), Ru(phen),'+ (B), and R ~ ( b p y ) , ~(C) + in RTV 118. The solid lines are the best fits using model 2.
that were 50-100% larger than for model 2. This result is physically reasonable, as it seems unlikely to have two species in a polymer with one being heavily quenched and the other virtually unquenched. Table I11 summarizes the best fit parameters for model 2 with the three complexes. Figure 4 shows the best fits to the experimental data. Clearly, the d a t a are fit within experi-
ANALYTICAL CHEMISTRY, VOL. 63, NO. 4, FEBRUARY 15, 1991
mental error. Similar fits were obtained with the lifetime data; due to much larger uncertainties in the rM’sthe results are less accurate (see Figure 3) and are not reported here (5). Model 1is inferior to the power law. Model 1 and the power law gave indistinguishable results for Ru(bpy),2+;however, for the other two complexes, the x2 were 2-5 times larger than for the power law. Model 2 was superior to the power law in all cases; the x 2 for Ru(phen)32+was 4 times smaller. For Ru(bpyIs2+,x 2 was 30% smaller. For the R ~ ( P h z p h e n ) ~x2 ~ +was , only 10% lower. However, this last case is not surprising as there is little curvature and the nonlinear corrections are minimal. Thus, model 2 provides superior fitting compared to the empirical power law or model 1. In view of the ability of data described by complex decay kinetics to be fit by relatively simple decay schemes, it is quite possible that a two-component model can fit the data but not be correct. We address this question. If a two-component model is truly correct, the lifetime data should be fit by two exponentials and the lifetime and contributions should correlate with the parameters derived from the intensityquenching data. For R ~ ( b p y ) , ~the + decays for the entire oxygen concentration range were dominated by two components, which account for 96-99% of the total emission intensity. Further, both components are quenched in a chemically reasonable fashion; the degree of quenching for each component changes monotonically with oxygen pressure, and the rate constants are reasonable. However, the relative contributions of the short- and long-lived emissions to the total emission intensity are 16-31% and 65-80’70 over the oxygen pressure 0-1 atm, respectively. These deviate noticeably from the 40/60 ratio derived from the intensity data (Table 111). For Ru(phen),2+three lifetime components are required to account for about 90% of the emission; the components are not quenched in a monotonicaUy decreasing fashion, and their contributions to the total emission intensity are inconsistent with a physically plausible model. Thus, while a two-site model may be a chemically reasonable model for Ru(bpy),2+, it is certainly incorrect for Ru(phen),*+. In view of the complexity of polymeric systems, it seems likely that the two-state model is also incorrect for R ~ ( b p y ) , ~ + . Even though the two-site model may be chemically incorrect, it is excellent for fitting intensity-quenching curves. The agreement is not surprising given the well-known ability of two exponentials to give excellent fits to complex decay curves made up of distribution functions of exponential decays especially a t the count levels used on most single-photoncounting instruments. Thus, while the two-site model is not the full chemically correct model, it has excellent predictive and calibration properties, has a chemically sound basis, and (at least for inorganic complex sensors) is preferable to the less accurate power law calibration equation (8). Our results demonstrate clearly that the lifetime data are more sensitive to subtleties of the micromechanistic photophysics. In this case we are able to establish inadequacies of model 2 that were not detected by intensity-quenching measurements only. It is also clear that resolution of the detailed mechanism in these complex polymer systems will require even better lifetime data than we are able to obtain with a conventional flashlamp-based time-correlated photon-counting system. Probably our largest source of error was the long tail on our 5-ns (fwhm) excitation source, which gave a contribution of fluorescence at long times and required deconvolution for all (including microsecond range) lifetimes and the relatively low peak counts (=lo4) (19). Photochemistry. We have observed a low-level decomcomplex under intense, proposition of the R~(Ph,phen),~+ tracted irradiation. The effect is enhanced in the presence
341
of oxygen. Oxygen quenching of the complexes is an efficient source of singlet oxygen (loz)in homogeneous (28) and heterogeneous media (29). Since lozis a well-known reactive species, it may attack and destroy the complexes. T o test this possibility, we exploited the anticipated differences in reactivity of different complexes to loz attack. In 1 , l O phenanthroline the bridging 5,6 double bond is much less aromatic than the fully aromatized pyridine rings, has much more double bond character and is, thus, a likely target of singlet oxygen attack. To test this hypothesis, we photolyzed R ~ ( b p y ) (completely ~~+ lacks the reactive double bond), Ru(phen)32+(has the bond without activating groups), and Ru(5,6-Mezphen)32+, which has a much more reactive double bond. For example, tetramethylethylene is an extremely reactive single-oxygen scavenger (30). The kobs’sof Table I1 for a decay without a baseline suggest that there is an increasing reactivity with activation of the double bond. However, this apparently rapid decay of the phen complexes relative to the bpy one is spurious; this is a consequence of the larger limiting photolysis levels for the bpy complex and not the higher reactivity of the other complexes. This is seen by examining the kok’s for the exponential plus baseline fits, which reveal that the kokk differ by only a factor of 2.4. However, when corrected for the extinction coefficients of each complex, the dphob are 1.00:1.06:1.45,which shows no significant activation of the complexes on going from the bpy complex (no reactive double bond) to the Ru(5,6-Me2phen);+ complex, which should be quite reactive towards singlet oxygen. We conclude that singlet oxygen is not the primary cause of sensor deactivation. In the test of the unextracted and Soxhlet-extracted sensor films, the Ru(phen),2+ photochemistry was very similar in both sensors with, perhaps, an =20% lower reactivity of the extracted film. Thus, if polymer components are responsible for the reactivity, they are not readily extractable. The photochemical results do shed light on heterogeneity. If all sites were equally reactive, the simple exponential fit would apply, but this model does not give acceptable fits. An exponential plus a baseline gives satisfactory fits. These results demonstrate unequivocally that there is a t least one reactive site and one much less reactive site. Interestingly, one of the biggest factors stabilizing the complexes is not kok, but F , the fraction of the unreactive form. Thus, the reactive fractions are 27%, 45%, and 57% for Ru(bpy):+, Ru(phen)?, and R ~ ( 5 , 6 - M e ~ p h e n )respectively. ~~+, The Fs do not appear to correlate with the fractions of the two forms in the quenching measurements. While fol and F for R ~ ( p h e n ) , ~are + similar, fol for R ~ ( b p y ) , ~is+slightly lower than for R ~ ( p h e n ) , ~ but + , F is much higher. Thus, the evidence suggests that the two components in the quenching and the photochemical experiments do not represent the same population. A corollary to the photochemical results is that most of the photochemistry occurs early. Therefore, to stabilize sensors, a preuse photolysis may destroy the more reactive component and leave only the more stable form. Comparison of Complexes a n d Supports. Table I shows a number of interesting features. There is a wide range of behavior depending on complex and support. Note, in particular, the different behavior with different counterions for R ~ ( 4 , 7 - M e , p h e n ) ~ ~T+h. e BPh4- systems have oxygenquenching sensitivities much lower than those of the analogous ClO,- systems, and the differences are highly support dependent. In methanol there is no difference between the two species, while in RTV 615, the BPh,- salt is completely unquenched; this is in marked contrast to the perchlorate. Tetraphenylborate is a quencher of the excited states of our metal complexes. Earlier, we exploited this fact to de-
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 4, FEBRUARY 15, 1991
termine whether the counterion was closely associated with the metal complex when the complex was bound to micelles. The closer the association, the more effective the counterion a t quenching the complex (16). A similar argument would apply here. The less polar the environment, the more strongly the complex would ion pair with the quenching BPh4-, the shorter the lifetime, and the less the degree of oxygen quenching. Our results allow us to rank the polarity of the different polymers by their tendency to promote ion pairing. In methanol, the solution is too dilute for the BPh,- to quench, and as expected, we see no differences between the perchlorate and BPh4- salts. In all the polymers, however, there is evidence for ion pairing; the Znitrogen/Zair values are always smaller for the BPh,- salts. The less polar the polymer the stronger the ion pairing and the poorer the O2 quenching. Thus, experimentally, the polymer polarity decreases on going from RTV 118 to Dow 891 to RTV 615. Even without BPh4-, there are large differences in polymer performance. RTV 615 has the poorest performance. This is a two-part polymer that is initiated with a free radical catalyst. We suspect that some of the catalyst persists in the polymer and quenches the sensor, thus degrading its performance. Even extended extraction with CH,Cl, did not seem to reduce the quenching. Also, one cannot use oxygen quenching in a homogeneous solvent such as methanol as a predictor of sensor performance in polymers. The methanol data would predict that Ru(phen),(CN), would be markedly superior to Ru(4,7Me2phen),:+, yet in all the polymer systems, the Ru(4,7Me?phen)?+ is superior. Similarly, the methanol data predicts that Ru(Ph,phen)$+ will be the best sensor. While this is true in RTV 118 and Dow 891, Ru(Ph2phen),*+is poorer than the other complexes in RTV 615.
ACKNOWLEDGMENT We thank Seth Snyder for his assistance with the timecorrelated single-photon lifetime system and with the deconvolution software.
LITERATURE CITED (1) Chemical, Biochemical, and Environmental Fiber Sensors. froceedings of SfIE-the International Society for Optical Engineering; Lie-
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RECEIVED for review August 3,1990. Accepted November 1, 1990. We gratefully acknowledge the support of the National Science Foundation (Grants CHE 86-00012 and 88-17809).
