14922
J. Phys. Chem. 1996, 100, 14922-14927
Kinetics of Quenching of Ketone Phosphorescence by Oxygen in a Glassy Matrix John M. Charlesworth* and Tiang Hong Gan Aeronautical and Maritime Research Laboratory-DSTO, PO Box 4331, Melbourne, Victoria 3001, Australia ReceiVed: January 31, 1996; In Final Form: May 14, 1996X
Measurements of oxygen diffusion in polystyrene, poly(methyl methacrylate), and poly(vinyl chloride) films varying in thickness from 7 to 57 µm were obtained at 22 °C by following the kinetics of quenching of phosphorescence of camphorquinone dissolved in each polymer. Values of diffusion coefficients D were (2.2 ( 0.5) × 10-7, (1.4 ( 0.3) × 10-8, and (1.3 ( 0.1) × 10-8 cm2 s-1, respectively, in reasonable agreement with literature data. The decreasing trend in D in this series is consistent with increasing polymer properties including cohesive energy density and diffusion activation energy. Comparison of the diffusion-limited rate constant kd, calculated from D, and the bimolecular rate coefficient kq, determined from lifetime and steady state intensity measurements, indicates that triplet CQ immobilized in glassy polymers is quenched with an efficiency kq/kd J 4/9. This result is subject to some uncertainty due to the errors in kq and kd; however, a value of kq/kd > 4/9 must be considered possible in view of recent reports implying that intersystem crossing between excited complexes of all multiplicity is important in systems where the phosphor is a ketone immobilized in a high-viscosity matrix.
Introduction Optical sensing of oxygen at room temperature by measurement of the quenching of the triplet state of excited camphorquinone (CQ) immobilized in a number of polymeric and silica matrices has recently been reported.1 Phosphorescence methods are generally considered to be more sensitive than fluorescence techniques for oxygen determinations because of the much longer lifetime of the excited triplet state (10-6-10 s) compared to the excited singlet state (10-10-10-3 s), which greatly enhances the chance of encounter between the reactive species. It has been found that oxygen quenching of CQ immobilized in polymers is essentially a dynamic process, resulting in linear intensity and lifetime Stern-Volmer (SV) plots for polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(vinyl chloride) (PVC) matrices.1 The Stern-Volmer equation2 describing the phosphorescence emission intensity in the absence (I0) and presence (I) of oxygen at concentration [O2], and corresponding triplet lifetimes (τ0 and τ) is expressed as
I0/I ) τ0/τ ) 1 + kc[O2]
(1)
The resultant dynamic SV constant (kc) provides an adequate measure of sensor performance at equilibrium and forms a basis for making limited generalizations regarding phosphor and/or support characteristics. However, in order to make predictions about the behavior under a variety of conditions an understanding of the quenching kinetics and reaction mechanisms is required. The key parameters involved in defining the quenching process are the bimolecular quenching constant (kq) and the diffusion-controlled bimolecular reaction rate constant (kd). Derivation of kq for most systems, including very viscous polymer solutions, involves an expansion of the SV dynamic quenching constant,2 i.e.:
kq ) kc/Sτ0
(2)
This expression takes into account the triplet lifetime and the solubility, S, of oxygen in the polymer. Provided quenching X
Abstract published in AdVance ACS Abstracts, August 1, 1996.
S0022-3654(96)00325-5 CCC: $12.00
occurs at every encounter, then kq equals the collision rate kd. The parameter kd is related to the diffusion constant by the Smoluchowski equation,2 i.e.:
kd ) 4πNRD/1000
(3)
where N is Avogadro’s number, R is the sum of the interaction radii of the quencher and phosphor, and D is the sum of the diffusion constants of the interacting species. For a relatively large polar molecule dispersed in a glassy matrix D effectively becomes the diffusion constant of the penetrant. In practice the probability of triplet quenching on every collision with oxygen will be less than unity since kq is controlled by a number of factors, which we discuss below. The quenching efficiency, γ, is therefore defined as the ratio of the bimolecular quenching constant to the bimolecular diffusion rate constant,2 i.e.:
γ ) kq/kd
(4)
Previous studies which examined phosphorescence quenching by oxygen in glassy polymers have focused on the poly(styreneco-1-naphthyl methacrylate) system3 and naphthalene/PMMA mixtures.4 In one of these studies3 it was reported that the quenching efficiency was low, with γ equal to approximately 1/9. It is known from prior experiments in solution that γ depends on the properties of the solvent and the electronic energy levels of the phosphor. In particular, it has been shown that there are significant differences in the behavior of aromatic molecules compared to phosphors containing the ketone group, and differences between solvents such as hydrocarbons and more polar or viscous liquids.5 We therefore extend previous work in this area by measuring both the diffusion and quenching constants for oxygen in several polymers containing a ketone phosphor. Experimental Section AR grade camphorquinone (Figure 1) was obtained from Aldrich Co. and consisted of a mixture of the two optical isomers. This material had a strong visible absorption maximum at 470 nm and a visible phosphorescence emission peak at 560 © 1996 American Chemical Society
Quenching of Ketone Phosphorescence by Oxygen
J. Phys. Chem., Vol. 100, No. 36, 1996 14923 emission intensity as a function of time after the inert atmosphere surrounding the films is rapidly replaced by air.3,6 In this situation the rate of diffusion of the quenching gas into the film is anticipated to follow Fick’s second law:7
∂c ∂2c )D 2 ∂t ∂x
(5)
Here c(x,t) is the concentration of the dissolved gas at time t and distance x within the film. We assume throughout this work that there is a Henry’s law type dependency between the gas concentration above the film and the concentration in the upper layer and that the oxygen concentration in the gas phase is constant. Solution of this equation leads to the following expression relating the amount of diffusant absorbed by the film to the exposure time.7
Mt Figure 1. Energy-minimized structure of camphorquinone showing maximum and minimum collisionally active molecular dimensions O2a-Ha and O2a-O2, respectively.
nm. The PMMA and PS were spectroscopic grade (Aldrich), and the PVC was a commercial unplasticized product (ICI Aust.). All polymers showed no fluorescence or phosphorescence at any of the wavelengths investigated, and all materials were used without further purification. Details regarding the preparation of PS, PMMA, and PVC films incorporating CQ as the phosphor have been described elsewhere.1 The thickness of the films was measured using a Mitutoyo digital micrometer at several positions reading to an accuracy of approximately (1 µm. For most measurements the solvent-free films were 7-57 µm thick and contained 10% (wt/wt) of CQ. Varying the concentration of CQ in the range 5-20% did not alter the results.1 After the glass-cast films were dried overnight in a vacuum oven at 100 °C, these were peeled, cut to an appropriate size, and mounted in a holder fitting diagonally inside a standard 1 cm square cuvette located in the sample compartment of the spectrofluorimeter. The cuvette was maintained at 22 ( 1 °C and was fitted with an air-tight stopper with inlet and outlet lines for gas flow. High-purity nitrogen, air, oxygen, or mixtures were passed via a switching valve into the cuvette in which both sides of the free-standing films were exposed. Mixtures of oxygen and nitrogen were produced using calibrated flow meters. All spectroscopic measurements were accomplished using a Perkin-Elmer LS-50 scanning spectrofluorimeter in the shorttime phosphorescence mode with Fluorescence Data Manager software. The light source was a xenon discharge lamp, equivalent to 20 kW for 8 µs duration, with a pulse width at half-peak height 0. A plot of ln(1 - Mt /M∞) against t should therefore be linear with a slope of Dπ2/ L 2. In the system under study the change in mass is due solely to the reversible uptake of oxygen and this can easily be related to the emission intensity using the Stern-Volmer equation. This leads to the following expression:
[O2]t [O2]∞
)
( ) /( ) I0 -1 It
I0 -1 I∞
(7)
where [O2]t and [O2]∞ are the time-dependent and equilibrium concentrations of oxygen in the film respectively, and It and I∞ are the emission intensity at time t and at equilibrium, respectively. Combining eqs 6 and 7 gives the following expression relating phosphorescence intensity to time immediately after exposure of the free standing film to oxygen:
ln[I0(It - I∞)/It(I0 - I∞)] = A -
Dπ2t L2
(8)
where the preexponential constants in eq 6 have been combined as A. Fitting the experimental data to eq 8 enables a value of D to be determined for each polymer/phosphor system and the collisional rate constant, kd, can then be calculated using eq 3. Typical data illustrating the normalised emission intensities during the cycling of 10% oxygen and pure nitrogen for PMMA films containing 10% camphorquinone, and ranging in thickness from 8 to 29 µm, are shown in Figure 2. These data confirm that at equilibrium the reduction in emission intensity is independent of film thickness, that the process is essentially entirely reversible, and that increasing the film thickness increases the equilibration time. Data for all three polymerphosphor systems with different film thicknesses are shown plotted according to eq 8 in Figures 3-5. Excellent agreement
14924 J. Phys. Chem., Vol. 100, No. 36, 1996
Figure 2. Plots showing the normalized change in emission intensity for PMMA films of varying thickness containing 10% camphorquinone exposed to cycling of 10% oxygen and pure nitrogen.
Charlesworth and Gan
Figure 5. Data showing the oxygen quenching of phosphorescence for the three PVC/CQ films, plotted according to the theoretical model for Fickian diffusion combined with Stern-Volmer theory.
TABLE 1: Diffusion Coefficients and Polymer Propertiesa Tg (K)b Ed (kJ mol-1)b Ecoh (J cm-3)b PS PMMA PVC
373 378 360
29.7 31.8 54.4
303 345 369
D(cm2 s-1) (2.2 ( 0.5) × 10-7 (1.4 ( 0.3) × 10-8 (1.3 ( 0.1) × 10-8
a Glass transition temperature (T ), activation energy of diffusion g (Ed), and cohesive energy density (Ecoh) for polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(vinyl chloride) (PVC), and measured diffusion constants (D) of oxygen at 22 °C. b Data from Van Krevelen.8
Figure 3. Data showing the oxygen quenching of phosphorescence for the three PS/CQ films, plotted according to the theoretical model for Fickian diffusion combined with Stern-Volmer theory.
Figure 4. Data showing the oxygen quenching of phosphorescence for the three PMMA/CQ films, plotted according to the theoretical model for Fickian diffusion combined with Stern-Volmer theory.
with Fickian behavior is observed and the diffusion coefficients calculated from the curves are (1.3 ( 0.1) × 10-8, (1.4 ( 0.3) × 10-8, and (2.2 ( 0.5) × 10-7 cm2 s-1 for PVC, PMMA, and PS, respectively. The errors in these data represent 1 standard deviation on the average value for measurements on three films of different thickness for each polymer. For comparison, values available in the literature range from 1.2 × 10-8 to 3.6 × 10-8 cm2 s-1 for PVC,8-10 1.2 × 10-8 to 3.7 × 10-8 cm2 s-1 for PMMA,9,11-14 and 1 × 10-8 to 1.2 × 10-6 cm2 s-1 for PS.8,13-16 We can also compare our results with those derived for the quenching of singlet oxygen by several quenchers dissolved in
PS and PMMA, as reported by Ogilby et al.17 The systems studied by these workers are such that quenching occurs at the diffusion controlled limit, i.e., kq ) kd, particularly if nickel(II) bis[diisopropyl dithiophosphate] (N) is used. The values of kq presented by Ogilby et al.17 are 2 × 108 s-1 M-1 for deactivation by N in PS and 1.4 × 107 s-1 M-1 for deactivation by N in PMMA at 20 °C. In order to compare values of kd for different quenching species we can apply the Smoluchowski equation. The N/O2 systems studied by Ogilby et al.17 have a much larger interaction radius than the camphorquinone/O2 system that we use. We have performed an MM+ calculation and find that the van der Waals radius of N is around 9 Å. This is approximate because the molecule has several rotatable groups which may not truly minimise. Also the molecule is not spherical and may not be completely collisionally active; however, we do not think this value is in error by more than 1 or 2 Å. As described below, we take the van der Waals radius of O2 to be 2.1 Å. This gives a value of about 11 Å for R. Substitution in the Smoluchowski equation together with the experimentally measured values of kq, i.e., kd in this system, leads to D(O2/PMMA) ) 1.7 × 10-8 cm2 s-1 and D(O2/PS)) 2.4 × 10-7 cm2 s-1, in excellent agreement with our experimental data. We conclude that our data is in good agreement with the range of literature values measured using a variety of techniques. The small differences could be attributed to several factors including sample morphology, molecular weight, and thermal history, all of which are known to influence diffusion behavior.7 Within the series studied by us it appears that the trend in diffusion constants is consistent with some of the effects of the known properties of the polymers as expressed in literature data.7,8,18 These are listed in Table 1 and include glass transition temperature (Tg), activation energy of diffusion (Ed), and cohesive energy density (Ecoh). A decrease in the Tg of the polymer does not inevitably increase the diffusion constant because the fractional free volume in the polymer, through which small molecules may move, is approximately constant below
Quenching of Ketone Phosphorescence by Oxygen
J. Phys. Chem., Vol. 100, No. 36, 1996 14925
TABLE 2: Rate Constant Parametersa kc (atm-1) S (mol L-1 atm-1) τ0 (10-3 s) kq (L mol-1 s-1) kd (L mol-1 s-1) kq/kd average range
PS
PMMA
PVC
286 (7.8 ( 1.6) x 10-3 b 0.81 (4.5 ( 0.9) x 107 (9.7 ( 3.4) x 107
31 (8.7 ( 1.7) x 10-3 b 0.80 (4.5 ( 0.9) x 106 (6.1 ( 2.0) x 106
7.3 (2.6 ( 0.3) x 10-3 c 0.76 (3.7 ( 0.5) x 106 (5.9 ( 1.2) x 106
4.2/9 2.7/9 to 7.7/9
6.6/9 4.3/9 to 11.9/9
5.6/9 4.1/9 to 8.0/9
a Stern-Volmer constant (k ), oxygen solubility (S), triplet lifetime (τ ), bimolecular quenching constant (k ), diffusion rate constant (k ), and c 0 q d quenching efficiency (kq/kd) for oxygen in PS, PMMA, and PVC. b Peterson.14 c Brady et al.10
Tg.7 A correlation exists between the activation energy for diffusion and the diffusion constant and this is related to the energy needed to enable the penetrating molecule to jump into another hole. Similarly, the observed correlation of diffusion constant with the cohesive energy density, which is defined as the energy needed to break all the intermolecular contacts in a given volume, is consistent with the argument that the diffusion coefficient should decrease as it becomes harder to create a pathway for the diffusing species. The values of kd calculated from D using the Smoluchowski equation are listed in Table 2. In this calculation we assume that R ) 5.8 ( 0.7 Å, where the hard sphere collision radii of CQ and oxygen are 3.7 ( 0.7 and 2.1 Å, respectively. The value for oxygen is simply the van der Waal’s diameter of oxygen plus half the bond length. The value for CQ was determined from molecular mechanics calculations, using half the interatomic distance, plus van der Waal’s radii, in the energyminimized structure of CQ. The maximum possible collisional radius includes the distance between O2a and Ha (5.7 Å), as shown in Figure 1. In this case the entire CQ molecule is defined as collisionally active rather than only the carbonyl groups. Some evidence to support this hypothesis comes from quantum mechanical calculations which indicate that the electronic energy in the π* orbitals in ketones can, in effect, migrate into other bonds in the molecule.19 The minimum collisionally active radius is based on the size of the dicarbonyl section of the molecule. The torsional angle O2a-C2-C2O2 in Figure 1 is only -1.6° and therefore electronic energy is delocalized over the overlapping π orbitals of both carbonyl groups. The interatomic distance between O2a and O2 is 2.9 Å. b. Bimolecular Quenching Rate Constants. The dynamic SV constant, kc, was determined from measurements of the change in both intensity and lifetime as a function of oxygen concentration using equation 1. Several plots illustrating the results of these measurements are given in Figure 6. The triplet lifetimes were determined from the slope of plots of the logarithm of intensity vs time. Some examples for each of the three systems in the absence of oxygen are shown in Figure 7. It can be seen that the decay of triplet CQ in all three polymers is first order over several half-lives. Values of kq were determined using eq 2 together with literature values for the solubility of oxygen in each polymer10,14 and the measured values of τ0. These data are listed in Table 2. The accuracy of the solubility measurements obviously influences the calculated kq and therefore we have chosen values of S for PMMA and PS which were obtained by the one worker using the same apparatus and conditions for both polymers.14 These values represent an average of several measurements which appear to show a slight positive deviation from Henry’s law. Consequently, a range of possible values of kq will be present which we approximately estimate will have an uncertainty of at least (20%. The solubility data for oxygen in PVC were taken from
Figure 6. Phosphorescence intensity (I) and lifetime (τ) data for quenching by oxygen of CQ in each polymer, plotted according to Stern-Volmer theory.
an investigation in which multiple measurements were performed.10 A reproducibility of (13% in S is quoted in this study. c. Kinetic Scheme. The mechanism of triplet quenching by oxygen has been investigated for aromatic hydrocarbons in solution5,20,21 and solid polystyrene matrices,3,22 aromatic ketones, amines and quinones in solution,23-25 and carbonyl compounds in the vapor phase.26 The enhancement of spinforbidden excited state decay of excited triplet state molecules by quenchers which have unpaired electrons is considered to be due to interaction of the two species in a collision complex. A description of the kinetics involved in this intermolecular process is given by the series of reactions5 in Scheme 1.
SCHEME 1 gikd
ket
M* + 3O2 {\ } 1(3M‚‚‚3O2)* 98 1M + 1O2* k
3
-d
gikd
kisc
M* + 3O2 {\ } 3(3M‚‚‚3O2)* 98 1M + 3O2* k
3
-d
gikd
M* + 3O2 {\ } 5(3M‚‚‚3O2)* k
3
-d
(R1)
(R2)
(R3)
In this system, which is governed by spin statistics,5 the reaction pathways all lead to the collision complex i(3M‚‚‚3O2)* which has singlet (i ) 1), triplet (i ) 3), or quintet (i ) 5) properties. The term gi corresponds to a statistical probability weighting factor which may be 1/9, 3/9, or 5/9. In terms of the above rate constants, kq can be expressed as
kq )
[
(
)]
kd kisc ket +3 9 ket + k-d kisc + k-d
)
kd [P + P ] (9) 9 1 3 3
14926 J. Phys. Chem., Vol. 100, No. 36, 1996
Figure 7. First-order decay plots of the phosphorescence of CQ in each polymer in the absence of oxygen.
At room temperature in solution, aromatic hydrocarbons with ππ* triplet states are quenched at a rate kq ) kd/9, which is in agreement with theoretical calculations predicting that oxygenenhanced intersystem crossing is negligible in these systems.23 Another method which is commonly used to interpret the kinetics is in terms of the experimentally measurable efficiency of singlet oxygen production.23 This parameter is defined as S∆ and is equivalent to P1/(P1 + 3P3). In practice, S∆ values less than unity (i.e., P3 > 0) have been found for a variety of aromatic hydrocarbons and ketones.23 Particularly for carbonyl compounds such as CQ with nπ* triplets, S∆ varies between 0.25 and 0.35. The low S∆ values imply that enhanced intersystem crossing is an important deactivational channel, with the recurrent observation that kd/9 e kq e 4kd/9, although in the case of triplet benzophenone quenching, S∆ < 1 even when kq e kd/9.21 Participation of charge transfer (CT) interactions, as exhibited by the dependence of kq and S∆ on the oxidation potential of substituted naphthalenes,28 may explain the variation of observed S∆ and kq combinations. Even though the mechanism of triplet quenching by oxygen appears to be rather intricate, Scheme 1 has been successfully used to explain the behavior of a comprehensive range of aromatic hydrocarbons, ketones, and amines, including molecules with quinone, cyano and heavy atom substituents, with nπ* and ππ* triplets.23 It follows from Scheme 1 that if ket . k-d . kisc, P1 ) 1 and P3 ) 0, and energy transfer is the dominant mechanism. This is confirmed by the observation that P1 is 6-9 times greater than P3 for compounds with positive ∆Gel in solution,28 i.e., k-d . kisc. Because k-d is inversely proportional to viscosity,5 it seems reasonable to assume that for phosphors immobilized in a glassy polymer matrix, the increased viscosity relative to a mobile liquid solution causes a reduction in k-d, whereas ket is unlikely to be affected.5 Some evidence has been reported indicating that kisc approaches k-d at low triplet energies5 and therefore in a very viscous medium it may be possible for kisc to exceed k-d. Recent work by Smith29 suggests that CT states with a lower energy than the localized triplet state also enables kisc to exceed k-d. Based on the spin-statistical limitations of the system, for triplet CQ in a rigid polymer, it appears that even taking into account the above factors, the quenching ratio kq/kd can have an upper limit of no more than 4/9. The only direct experimental observation previously published which exceeds this value relates to N-methylindole in benzene at room temperature where kq slightly greater than 4kd/9 has been found.30 Our calculated values for kq/kd are listed in Table 2. The average values are 4.2/9 for PS, 6.6/9 for PMMA and 5.6/9 for PVC. These results are equal to or higher than the predictions of Scheme 1 as it applies to a high viscosity medium. The small dependency of the rate
Charlesworth and Gan constant ratio on the polymer matrix may or may not be significant since the calculated values depend on the accuracy of the oxygen solubility data available in the literature. In order to better explain the possibility that kq/kd may be greater than 4/9, it is necessary to invoke the prospect that the process is not limited by the spin statistics outlined in Scheme 1. This possibility has been recently raised by McLean and Rodgers25 for several ketones in toluene at low temperatures where viscosity is high. By studying the temperature dependence of the rate of oxygen quenching over the range 190-360 K these workers obtained evidence from activation energy plots that quenching proceeds via singlet, triplet and quintet states. This result further implied that ISC operates between excited complexes of all multiplicity and that the rate constant for ISC from quintet to triplet states must be comparable to, or exceed, k-d. These workers concluded that the limit of 4kd/9 for kq can be surpassed. This mechanism may afford an explanation for the current results but unfortunately with the precision which we can obtain from the data it is not possible to say with confidence whether our values are greater than or equal to 4/9. Conclusions We have studied the diffusion of oxygen in thin films of polystyrene, poly(methyl methacrylate) and poly(vinyl chloride) at 22 °C by following the kinetics of quenching of phosphorescence of camphorquinone dissolved in each polymer. Values of diffusion coefficients thereby obtained are in reasonable agreement with literature data. Comparison of the diffusion limited rate constant kd, calculated from D, and the bimolecular rate coefficient kq, determined from lifetime and steady state intensity measurements, indicates that triplet CQ immobilized in glassy polymers is quenched with an efficiency kq/kd J 4/9. This result suggests that the process may not be limited by spin statistics as has previously been suspected and that it is possible that quenching can proceed via singlet, triplet, and quintet states. This result is subject to some uncertainty due to the errors in kq and kd; however, a value of kq/kd > 4/9 must be considered possible in view of recent reports implying that intersystem crossing between excited complexes of all multiplicity is important in systems where the phosphor is a ketone immobilized in a high-viscosity matrix. References and Notes (1) Charlesworth, J. M. Sensors Actuators B 1994, 22, 1. (2) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, Plenum Press: New York, 1983. (3) Andrews, M. M.Sc. Thesis, University of Toronto, 1978. (4) Jones, P. F. J. Polym. Sci., Polym. Lett. 1968, 6, 487. (5) Gijzeman, O. L. J.; Kaufman F.; Porter, G. J. Chem. Soc., Faraday Trans. 2 1973, 69, 708. (6) Shaw, G. Trans. Faraday. Soc. 1967, 63, 2181. (7) Crank J., Park, G. S., Eds. Diffusion in Polymers; Academic Press: London, 1968. (8) Van Krevelen, D. W. Properties of Polymers, Elsevier, Amsterdam, 1990. (9) Sotelo Lerma, M.; Iwamoto, K.; Seno, M. J. Appl. Polym. Sci. 1987, 33, 625. (10) Brady,T. E.; Jabarin, S. A.; Miller,G. W. Polym. Sci. Technol. 1974, 6, 301. (11) Higashide, F.; Omata, K.; Nozawa, Y.; Yoshioka, H. J. Polym. Sci. 1977, 15, 2019. (12) Kaptan, Y.; Peckan, O.; Arca, E.; Guren, O. J. Appl. Polym. Sci. 1989, 37, 2577. (13) MacCallum, J. R.; Rudkin, A. L. Polymer 1978, 14, 655. (14) Peterson, C. M. J. Appl. Polym. Sci. 1968, 12, 2649. (15) Gao, Y.; Ogilby, P. R. Macromolecules 1992, 25, 4962. (16) Benson, R.; Geacintov, N. J. Chem. Phys. 1974, 60, 3251. (17) Ogilby, P. R.; Dillon, M. P.; Kristiansen, M.; Clough, R. L. J. Phys. Chem. 1992, 25, 3399.
Quenching of Ketone Phosphorescence by Oxygen (18) Brandrup, J., Immergut, E. H., Eds. Polymer Handbook; Wiley: New York, 1975. (19) Arnett, J. F.; Newkome, G.; Mattice, W. L.; McGlynn, S. P. J. Am. Chem. Soc. 1974, 96, 4385. (20) McLean, A. J.; Rodgers, M. A. J. J. Am. Chem. Soc. 1992, 114, 3145. (21) McLean, A. J.; Rodgers, M. A. J. J. Am. Chem. Soc. 1993, 115, 4786. (22) Benson, R.; Geacintov, N. J. Chem. Phys. 1972, 59, 4428. (23) Grewer, C.; Brauer, H. D. J. Phys. Chem. 1994, 98, 4230. (24) Redmond, R. W.; Braslavsky, S. E. Chem. Phys. Lett. 1988, 148, 523.
J. Phys. Chem., Vol. 100, No. 36, 1996 14927 (25) McLean, A. J.; Rodgers, M. A. J. J. Am. Chem. Soc. 1993, 115, 9874. (26) Cebul, F. A.; Kirk, K. A.; Lupo, D. W.; Pittenger, L. M.; Schuh, D. M.; Williams, H. R.; Winston, G. C. J. Am. Chem. Soc. 1980, 102, 5656. (27) Kawaoka, K.; Khan, A. U.; Kearns, D. R. J. Chem. Phys. 1967, 46, 1842. (28) McGarvey, D. J.; Szekeres, P. G.; F. Wilkinson, Chem. Phys. Lett. 1992, 199, 314. (29) Smith, G. J. J. Am. Chem. Soc. 1994, 116, 5005. (30) Garner, A.; Wilkinson, F. Chem. Phys. Lett. 1977, 45, 432.
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