396
J . Phys. Chem. 1988, 92, 396-402
Solid-State Endoperoxide Thermolysis. The Klnetlcs of Oxygen Release into the Gas Phase Allen J. Twarowski,*+Lisa Good, and George Bus& KMS Fusion, Inc., Ann Arbor, Michigan 48106 (Received: May 18, 1987; In Final Form: August 3, 1987)
The release of molecular oxygen from thin films of 1,4-dimethyl-2-poly(vinylnaphthalene1,4-endoperoxide) into the gas phase is reported. The rate of the thermolysis reaction is temperature dependent with an activation energy of 91 kJ/mol. The fraction of oxygen released into the gas phase in the electronically excited IA state depends on film thickness. A simple model of diffusional transport which also includes quenching of electronic excitation describes the variation in film thickness of the 0, ]Ag yield. For film thicknesses of less than 50 nm, more than 10%of the oxygen released into the gas phase from the solid polymer is electronically excited.
Introduction The transannular addition of singlet oxygen (l Ag) to aromatic Many of these hydrocarbons produces 1,4-endoperoxides.'" molecules decompose only very slowly at room temperature, and thus for all practical purposes they are considered stable. However, upon heating most undergo therm~lysis.~,' In many cases thermolysis gives back the parent hydrocarbon and singlet oxygen.6-14 Naphthalene and anthracene derivatives are the most extensively studied&14of the aromatic hydrocarbon endoperoxides which undergo reversible oxidation and thermolysis. The thermolysis of 1,4-dimethyInaphthalene in solution produces singlet oxygen The reaction proceeds with with yields approaching 100%.Lf13 an activation energy of about 100 k.J/m~l,"'~close to the quantity of electronic energy stored in O2 lAe. The activation energy for the thermolysis of endoperoxide to polyacene and singlet oxygen represents an upper limit for the reaction enthalpy. Thus, the fact that the activation energy for thermolysis of naphthalenederived endoperoxides is approximately the same as the electronic energy of product singlet oxygen molecules implies that thermal energy is efficiently converted to electronic excitation during the course of the thermolysis reaction. This high efficiency suggests that endoperoxides may be useful materials for electronic energy storage. Recent experimental results have been reported for endoperoxides of polymer-bound naphthalene moieties.l3-l4 These experiments were conducted in solution, using soluble polymers of low molecular weight. The single oxygen production efficiencies and thermolysis activation energies were similar to those for the endoperoxides of 1,4-dimethylnaphthalene. In this paper we report our results for the release of oxygen into the gas phase from solid films of a polymeric endoperoxide, 1,4-dimethyl-2-poly(vinylnaphthalene 1,4-endoperoxide), 2-PVNE. Both singlet oxygen yields and total oxygen release were determined. Our examination of the thermolysis kinetics of polymeric endoperoxides in the solid state is motivated by the need for high efficiency, clean sources of singlet oxygen for the oxygen-iodine transfer laser.I5 Chlorine/basic hydrogen peroxide (CBHP) generators presently are used for these lasers. In CBHP generators, chlorine gas is bubbled through a solution of hydrogen peroxide in a strong base such as potassium hydroxide. Chlorine is consumed from the bubbles, and singlet oxygen is produced in its place. Along with the singlet oxygen, quenching impurities are introduced into the gas stream. These include unreacted chlorine, water vapor, hydrogen peroxide, and even droplets of solution. Solid endoperoxide singlet oxygen generators should provide a significant advantage over CBHP generators since impurities should be virtually eliminated. Resent address: Rockwell International Science Center, Thousand Oaks, CA 91360. *Present address: Los Alamos National Laboratories, Los Alamos, NM 87545.
Singlet oxygen is released into the gas phase upon thermolysis of a solid endoperoxide film in a two-step process consisting of a unimolecular thermolysis reaction, which produces singlet oxygen, followed by diffusion of the oxygen to reach the solid/gas interface. The net reaction rate is a convolution of these two processes; however, if the rates for the individual processes differ substantially, one of the steps will be rate determining. For an estimate of the rate of thermolysis of solid 1,4-dimethyl-6-poly(vinylnaphthalene 1,4-endoperoxide) we use the kinetic parameters obtained for this same compound in solution. Saito et aL13 report an activation energy of 108 kJ/mol and a frequency factor of 5.8 X 1014s-I. These parameters lead to a value in the range of 3 s-l for the thermolysis rate constant at 120 OC.
Not all of the O2 'Ag produced by the primary thermolysis step will survive to be released into the gas phase in the excited state. While the singlet oxygen molecule diffuses through the polymer film it encounters C-H bonds which are effective quenchers of excited oxygen. The longer the distance to the surface to which the singlet oxygen must diffuse, the more likely it is that it will be deactivated. We can derive a crude estimate of the film thickness which will release singlet oxygen without extensive . L deactivation from the diffusion relation, L E ( O T ) ~ / *Here is the film thickness and T is the lifetime of the singlet oxygen molecule. The lifetime of O2 lAe varies dramatically with its environment. In fully halogenated hydrocarbons such as some freons the lifetime of O2 'Ag is on the order of milliseconds at room temperature;" however, in solvents with 0-H or C-H bonds the
(1) Moureu, C.; Dufraisse, C.; Dean, P. M.; Hebd. Seances Acad. Sci. 1926, 182, 1440. (2) Gollnick, K.; Schenck, G. 0. In 1,4 Cycloaddition Reactions; Hamer, J., Ed.; Academic: New York, 1967; p 255. (3) Rigaudy, J. Pure Appl. Chem. 1968, 16, 169. (4) Denny, R. W.; Nickon, A. Org. React. 1973, 20, 133. (5) Saito, I.; Matsuura, T. In Singlet Oxygen; Wasserman, H. H., Murray, R. W., Eds.; Academic: New York, 1979; p 511.
(6) B l d w o r t h , A. J.; Eggelte, H. J. In Singlet Oxygen; Frimer, A. A,, Ed.; CRC Press: Boca Raton, FL, 1985; Vol. 11, p 93. (7) Murray, R. W. In Singlet Oxygen; Wasserman, H. H., Murray, R. W., Eds.; Academic: New York, 1979; p 59. (8) Wasserman, H. H.; Scheffer, J. R. J. Am. Chem. SOC.1967, 89, 3073. (9) Wasserman, H. H.; Larsen, D. L. Chem. Commun. 1972, 253. (10) Larsen, D. L. Ph.D. Dissertation, Yale University, 1973. (1 1) Schaefer-Ridder, M.; Brocker, U.; Vogel, E. Angew. Chem. 1976,88, 267. (12) 218. (13) (14) 6329. (15)
Turro, N. J.; Chow, M. F.; Rigaudy, J. J . Am. Chem. SOC.1981,103, Saito, I.; Nagata, R.; Matsuura, T. Tetrahedron Lett. 1981,22,4231. Saito, I.; Nagata, R.; Matsurra, T. J. Am. Chem. SOC.1985, 107,
McDermott, W. E.; Pchelkin, N. R.; Benard, D. J.; Bousek, R. R. Appl. Phys. Lett. 1978, 32, 469. (16) Yasuda, H.; Stannett, V. In Polymer Handbook, 2nd ed.; Brandrup, J., Immergut, E. H., Eds.; Wiley: New York, 1975; pp 111-229.
0022-3654/88/2092-0396$01.50/00 1988 American Chemical Society
Solid-state Endoperoxide Thermolysis lifetime is much shorter. The 2-PVNE films which we are investigating have numerous C-H bonds and a reasonable estimate of the room temperature lifetime of O2 lA in these materials is 20 ps-the approximate lifetime of singlet oxygen in similar materials such as benzene or cy~lohexane.~’The lifetime is expected to decrease by only a factor of 2 in raising the temperature from 20 to 120 “C,based on measurements of the temperature dependence of ‘A lifetimes in chloroform. Using an estimate for the diffusivity, D, calculated from data given in ref 16 for polys, we (styrene), and a singlet oxygen lifetime the order of estimate a critical thickness of 100 nm from the diffusion equation. For films much thicker than this the fractional IA yield is expected to decrease.
Experimental Section A. Materials. The monomer, 1,4-dimethyl-2-vinylnaphthalene, was prepared at Wayne State University by Professor A. P. Schaap. Its synthesis and characterization are reported elsewhere.19 Polymerization of this material was accomplished by refluxing a solution of 2 g of the monomer with 0.2 g of a freeradical initiator, 2,2’-azobis(2-methylpropanenitrile)(AIBN), in 50 mL of degassed benzene for approximately 4 h. The dropwise addition of the benzene-polymer solution to 200 mL of methanol precipitated the polymer. The polymer was dried and then dissolved in 70-80 mL of methylene chloride. Enough methylene blue dye was added to the solution to give an optical density of 1-2 at 590 nm. Oxygen was bubbled through the mixture while it was irradiated for 4-8 h with a 250-W sodium lamp. Aliquots were taken periodically and the absorption at 290 nm was measured. Peroxidation of the naphthalene ring resulted in a significant decrease in the absorbance at 290 nm and this result was used to determine when to terminate the photoperoxidation reaction. Upon completion of the reaction, the solution was filtered through charcoal to remove most of the methylene blue dye, the solvent was evaporated, and copious quantities of methanol were used to precipitate the 2-PVNE. The solid 2-PVNE was dried and stored at reduced temperature in a freezer. The yield of solid material obtained was 90% of the weight of polymer originally dissolved. The solid was 75% 2-PVNE, the balance being the starting polymer. B. Film Deposition and Characterization. Thin films of 2PVNE were deposited onto Pyrex substrates which were overcoated with a thin conductive film (see below). The polymer film deposition technique consisted of flowing a solution of the endoperoxide dissolved in methylene chloride onto a spinning substrate. The substrate was mounted on a platform which was rotated a t 19 rps (measured with a General Radio Strobotac). The concentration of the dissolved polymeric endoperoxide determined the amount of polymer deposited on the substrate, and thus the film thickness. In the discussions which follow, the concentrations of polymer and the light absorption coefficients are given in units of mol/L and L/(mol cm), respectively. In each case, the quantity “mole” refers to monomeric units of the polymer. A series of experiments was performed to determine the correlation between the deposition solution concentration and the number of moles of polymer deposited on the substrate per unit area. In these experiments the unperoxidized 1,4-dimethy1-2poly(vinylnaphthalene), 2-PVN, was deposited on quartz substrates and the characteristic naphthalene absorbance at 290 nm was measured with an IBM 9430 spectrophotometer. The extinction coefficient for absorbance of 2-PVN at 290 nm was determined to be 5200 L/(mol cm) from solution measurements (methylene chloride was the solvent) and this value was assumed to be the same for the absorption peak of the solid film in the neighborhood of 290 nm. Assuming Beer’s law, the molar surface density in mol/m2 can be calculated from the measured absorbance of the (17) Kearns, D. R. Singlet Oxygen; Wasserman, H. H., Murray, R. W., 1979; p 115. (18) Long, C. A.; Kearns, D. R. J. Am. Chem. Sor. 1975, 97, 2018. (19) McCall, D. Ph.D. Dissertation, Wayne State University, 1985.
Eds.; Academic: New York,
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 397
”
0
5
10 15 20 SOLUTION CONC. ( x
25
30
35
moledliter)
Figure 1. Surface density of 2-PVN films (mol/m2)as a function of the concentration of the spin-coating solution (mol/L).
I 1 SOLUTION CONC. (moledliter)
Figure 2. Thickness of 2-PVN films as measured with a profilometer as a function of spin-coating solution concentration.
thin films of 2-PVN and the extinction coefficient. Figure 1 shows the correlation between molar surface density and solution concentration. A reasonably good fit to a linear function is obtained, the least-squares fit giving a slope of 0.0096 mol/m2 of deposited polymer per mole/liter of solution. As has been stated above, the absolute value of this correlation assumes that the extinction coefficient for absorbance of the pendant naphthalene unit does not change from solution phase to solid phase. This assumption was tested by depositing 2-PVN on a quartz substrate from a 0.028 M solution, measuring the absorbance of the deposited polymer film at 290 nm and then carefully washing the 2-PVN off the plate with methylene chloride and measuring the absorbance of the resulting solution. Because the area of the deposited film is known and the volume of the resulting solution is measured, the moles/meter* of the 2-PVN film can be independently calculated from the solution-phase absorption mol/m2 while results. The latter measurement gave 3.3 X the film measurements gave 2.7 X lo4 mol/m2. While this disagreement might appear to argue that the molar extinction coefficient of 2-PVN differs between the solid phase and solution, the reproducibility of the film absorption measurements proved much better than that of the washed film/solution measurements. The film measurements were used to correlate molar surface density of the deposited films with the concentration of the depositing solution, realizing that these values may be subject to systematic error of as much as 20%. An assumption implicit in the further use of this correlation is that the mass density of a 2-PVN film is essentially identical with that of a 2-PVNE film (which is 75% endoperoxide and 25% 2-PVN). This assumption was examined with the film thickness measurements described below. The mean thickness of the deposited 2-PVN films can be calculated from the molar surface density and the mass density. Assuming a mass density of 1 g/cm3, a value which is within a few percent of the density of most hydrocarbon-substituted naphthalene solids, the thicknesses of the 2-PVN films embraced the range 0-50 nm. The film thickness can be correlated with the concentration of the depositing solution yielding a value of 9.6 nm of film per 1 g of polymer/L of solution. An independent measure of film thickness was attempted using a Dektak Profilometer; however, the use of this instrument required the deposition
398 The Journal of Physical Chemistry, Vol. 92, No. 2, 1988
Twarowski et al.
Endoperoxide Substrate TOP VIEW To electrical feedthrough
-111:
Endoperoxide film
Pyrex substrate
Conductive metal oxide coating To electrical feedthrough
+Teflon
mask
Copper electrodes (clamps)
Figure 4. Measured thermal profile of substrate surface.
FRONT VIEW OF SUBSTRATE WITH TEFLON MASK
ExperimentalApparatus
Conductively coated substrate
Copper foil
-/ /
el-
I
/--To controller
Capacitance manometer
-
MPLE CHAMBER
Teflon mask/
Figure 3. Geometry of the 2-PVNE film on the substrate surface.
of thicker films. Three films of 2-PVNE with up to 1600 nm nominal thickness were deposited on Pyrex substrates. In Figure 2 the thickness as measured by the profilometer is plotted against solution concentration. Each value plotted is the mean of at least nine measurements made on four different substrates. The thickness measurements proved to be quite reproducible and the standard deviation from the mean value is smaller than the symbol size in each ca$e. A straight line is drawn through the data points representing the two thinner films and the origin. The data point representing the thickest film deviates substantially from the correlation line drawn using only the thinner film data. This deviation may be significant and may be due to an increase in viscosity at solution concentrations approaching 1 M. From the slope of the correlation line of Figure 2, a value of 8.4 nm of film thickness per 1 g of polymer/L of solution is calculated. This value deviates from that calculated by using spectroscopic data by only 13%. The kinetics experiments reported below were carried out with films of less than 500 nm and the correlation derived from Figure 2 was used for the calculation of film thickness from solution concentrations. C. Polymer Film Heating. The heating of the 2-PVNE films was accomplished by passing an electrical current through a thin conductive coating bonded to the surface of the Pyrex substrate. The conductive coating was deposited on 1-in. square Pyrex substrates by Metavac Inc., Flushing, NY, and consists of a proprietary metal oxide coating with a resistance in the range of 20-40 ohms/square. The endoperoxide was deposited directly on top of the conductive metal oxide coating so that the temperature of the thin endoperoxide film tracked the front surface temperature of the heated plate. Electrical contact was made to the substrate conductive coating by using the following procedure (refer to Figure 3). Copper foil with an adhesive backing was fixed to a '/*-in. strip at two opposite ends of the plate. The edge of the junction between copper foil and substrate was painted with a conductive (silver-filled) fluorocarbon resin (Acheson Electrodag 504) and mechanical contact was made to the copper foil with copper clamps which were attached to the rods of the electrical feedthroughs. The electrical current flowing through the conductive coating on the substrate was controlled in such a manner so as to quickly bring the temperature of the endoperoxide up to a target temperature and maintain thereafter a constant temperature for a period of several seconds. A voltage-controlled, regulated power supply (Kepco ATE 150-3.5M) provided the current for the heating of the substrate. The power supply was controlled by an
1 L -
To power supply 25 micrometer pinhole
Figure 5. Experimental apparatus.
'
IBM XT computer equipped with a Qua Tech WSB-10 waveform synthesizer board which could be programmed to provide any temporal profile desired. The temporal profiles of our current pulses have two regions. At early times the current is held constant at the maximum value permitted by impedance matching constraints of the substrate conductive film with the power supply. This brings the substrate temperature to its target value in the shortest possible time. After the target temperature is reached, the current is decreased at a rate which is proportional to the 1/4 power of time. This latter region of the heating pulse is designed to just maintain the endoperoxide film at the target temperature by matching the rate of heat conduction into the substrate. The temperature of the endoperoxide film was monitored with a Vanzetti TM2 Thermal Monitor custom modified to provide a response time on the order of 10 ms. The TM2 consists of a germanium detector, complete with appropriate wavelength filtering, radiation collection optics, and amplification electronics. This device provided a noncontact means of monitoring surface temperatures. The thermal monitor was used both to measure substrate thermal parameters needed in the computer modeling of the current pulse shape and to test and calibrate the temperature control system. Figure 4 shows oscilloscope traces comprising a set of thermal monitor signals which demonstrate that the surface of the substrate can be heated to over 80 O C in 0.1 s and maintained constant in temperature to better than 10% of the temperature increase for durations of a second or more. In fact, the temperature can be controlled for periods as long as 10 s (the longest times which we attempted to implement). The ability to heat an endoperoxide film quickly and then maintain the film at constant temperature has allowed the rate of thermolysis to be examined as a function of temperature. In practice with the present configuration of equipment the temperature of the endoperoxide can be raised from room temperature to 130 "C in less than 0.2 s. Above this temperature more than
-
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 399
Solid-state Endoperoxide Thermolysis 2.75 in
1.5 in Sapphire window
Germanium detector
I
TABLE I: Oxygen Release from 2-PVNE soln concn, obsd press. mol/L rise, Torr moles of 0, 7.7 x 10-7 0.29 0.084 3.9 x 10-7 0.15 0.040 0.0 19 1.9 x 10-7 0.073
calcd press. rise, Torr 0.080 0.040 0.020
Detection Geometry Figure 6. Optical geometry used in the O2'$ luminescence experiments.
half of the endoperoxide will have thermolyzed before the target temperature can be reached and so most of the kinetic measurements reported here were limited to lower target temperatures. D. Apparatus for Kinetics Experiments. The experimental equipment used to collect kinetic data is illustrated in Figure 5 . The sample chamber is a stainless steel cube, Teflon coated on the inside and equipped with ports on all six faces. The chamber is evacuated through one port with a 150 L/s diffusion pump, and a butterfly valve at the sample chamber port entrance is used to isolate the sample chamber when it reaches high vacuum. The substrate is clamped in place between two electrodes from an electrical feedthrough flange which occupies a port on the cube. The remaining ports of the cube are fitted with optical windows for viewing luminescence from O2 lA, with an FPI capacitance manometer for determining the quantity of gas released, and with a 25-pm pinhole connected to an Inficon Quadrex 200 quadrupole mass spectrometer. Primary emission from O2 lAg released in the thermolysis reaction of the endoperoxide was detected with a liquid nitrogen cooled Judson J16-D germanium diode equipped with a 1270-nm interference filter. The germanium diode signal was amplified with a Keithley 410A picoammeter, filtered with a 10-Hz low-pass cutoff filter, and displayed on an oscilloscope. To ensure that only gas-phase O2 'Ag was detected and to exclude emission from the solid, the geometry illustrated in Figure 6 was used. The germanium detector (1 cm in diameter), housed in a liquid nitrogen Dewar, was mounted 90" to the normal of the sample substrate surface (viewing the edge rather than the front of the substrate) and cooled to liquid nitrogen temperatures. The surface of the plate was masked with a flat 1/16-in.Teflon gasket which had 1/16 in. slots cut out every 1/8 in. This grid of Teflon strips was mounted so as to block the sample surface from direct view of the germanium detector. The edge of the sample substrate was also masked from view by the detector. These measures were taken to not only block possible l A emission originating in the polymer film from direct view of detector but more importantly to decrease the amount of blackbody radiation arriving at the detector from the heated substrate surface. Without these measures the blackbody radiation signal limits the detectivity to unacceptably large values. With the present arrangement of equipment, signals of less than 5 X lo-" A are easily measurable.
Results A . Kinetics of Total Oxygen Release. The heating of a 2PVNE film from room temperature to 130 "C for 2 s results in a pressure rise in the sample chamber. Subsequent reheating of the same sample does not significantly increase the sample chamber pressure. Comparison of the mass spectra of the gas before and after thermolysis reveals that gas released from the 2-PVNE film is almost entirely molecular oxygen (mass peak at 32). A small increase in the mass peak at 28 (less than 5% of the total increase in detectable mass) is attributed to nitrogen released from adsorption sites on the substrate or released from inclusion sites in the polymer film. No evidence of polymer pyrolysis resulting in volatile products is seen in the mass spectrum. The rise in pressure in the sample chamber upon thermolysis of the 2-PVNE film was measured with a capacitance manometer and is listed in Table I for three sample loadings, The results may
1
0
40
120 160 TIME, t - t,, (ms)
80
200
Figure 7. Natural logarithm of the capacitance manometer signal (see text) versus time after the target temperature is reached ( t - tk). ,S"' is the signal at long times; Skis the signal when the 2-PVNE film has reached target temperature.
be compared with pressure rises expected if all the endoperoxide is thermolyzed and releases oxygen into the gas phase. Using the correlation of deposition solution concentration and molar surface density along with the measured area of deposited 2-PVNE film, the number of moles of O2available for release into the gas phase was computed. (A factor of 0.75 needs to be included to allow for the fact that only 75% of the pendant naphthalene units are converted to endoperoxide.) Finally, from the ideal gas law and the measured sample chamber volume the maximum theoretical pressure rise was calculated (column 4 in Table I). The agreement UGLWGGII L U G UUbGlVGU QllU bQIbUIclLGU ylG33UIG 1130 13 yUlLG g W U ,
providing evidence that all the 2-PVNE has been thermolyzed and the product oxygen molecules released into the gas phase quantitatively. The rate of release of oxygen into the gas phase can be monitored with the capacitance manometer as a function of time. The response time of the capacitance manometer was measured by using a pulsed piezoelectric leak valve (Vacuum General Model 77-10M) which can be gated open for times as short as 2 ms. The pressure pulse which followed the introduction of a 2-ms burst of gas into the sample chamber was recorded and the time for the capacitance manometer signal to reach 90% of its final pressure reading was measured to be 0.09 s. Given this response time for the manometer, a rate constant for release of oxygen into the gas phase of 10 s-' or less can be accurately measured with the present apparatus. The release of oxygen from the 2-PVNE film is expected to be described by the equation
where we have assumed for the moment that kTb the elementary rate constant for thermolysis of endoperoxide, is the rate-limiting step in the release of oxygen from the polymer film. ( 0 2 ) 0 is the amount of available oxygen (initially in the form of endoperoxide). Equation 1 is integrated by using the following initial conditions. At time tk when the endoperoxide film has reached target temperature, the amount of oxygen which has been released is (02)k. Integrating eq 1 from time tk to t and rearranging terms gives
A plot of the natural logarithm function on the left as a function of delayed time, t - tk, should yield a straight line with a slope of kn. Figure 7 shows a typical plot of eq 2 from an experiment in which a 2-PVNE film is heated to 105 "C.The capacitance
400
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988
Y
Twarowski et al.
-
Y
0.8-
c 1
-
0.4 -
0-
- 0.41
0 I
I
.00257
.00262
I
I
.00267 .00272
dll
.00277 .
1TT
Figure 8. Arrhenius plot of the thermolysis rate constant, kn. Film 120 nm; 0, 240 nm; 0 , 4 9 0 nm. thickness:
*,
manometer signal, S, which should be linearly proportional to (02), is plotted in Figure 7 as a reminder of the experimental quantity actually being measured. A linear fit to the data gives the rate constant for release of oxygen at this temperature. The above experiment was repeated at several different temperatures and for several film thicknesses. Over the temperature range of 83-120 OC the rate constant changes from 1 to 10 s-'; however, it does not appear to depend on film thickness over the range of 100-400 nm. The absence of a thickness dependence to the oxygen release rate constant is evidence that it is the elementary thermolysis rate, and not the rate of diffusion out of the polymer, which limits the release of oxygen into the gas phase. The rate constant for thermolysis of naphthalene-type endoperoxides in solution exhibits a strong temperature dependence with an Arrhenius relation of the form
kTh= ko exp(-Ea/RT)
Figure 9. Germanium detector signal from a 240-nm 2-PVNE film heated a t 130 O C . A, first heating pulse; B, second heating pulse. '"I
(3)
Here ko is the preexponential factor, usually assumed to bear no temperature dependence, Ea is the activation energy, R is the gas constant, and T is the absolute temperature. Assuming the Arrhenius relation holds for the thermolysis of 2-PVNE films, a plot of the natural logarithm of kThagainst the reciprocal of temperature should give a straight line with a slope of -Ea/R and a y intercept of In (ko). Figure 8 shows this plot with the linear least-squares fit to our data. We have included data for all three film thicknesses since, as we stated above, the variation of kn with film thickness is not in evidence in our data. From the linear least-squares fit we obtain a value of 91 f 7 kJ/mol for the activation energy of thermolysis and (1.8 f 0.4) X 1013s-' for the preexponential term. The activation energy which we observe for the thermolysis of 2-PVNE in solid films is in reasonable agreement with the activation energies of similar compounds thermolyzed in solution. B. 0,'Ag Release. The signal from the germanium detector was amplified with a Keithley 410A picoammeter, filtered with a IO-Hz low-pass RC filter and displayed on an oscilloscope. Figure 9a shows an oscillograph of a typical signal obtained upon thermolysis of a 240-nm-thick 2-PVNE film heated to 130 O C . Figure 9b shows the background signal, attributed to blackbody radiation, obtained by reheating the same sample after allowing sufficient time for the substrate to cool from its original heating. This measurement was made with 10 Torr of nitrogen buffer gas present in the sample chamber. The buffer gas is necessary because we want to measure quantitatively the total yield of gas-phase O2 'A,. The peak signal recorded with the germanium detector will be directly proportional to the total amount of O2 lAg released into the gas phase from thermolysis of 2-PVNE, providing the time for diffusion through the buffer gas and out of the field of view of the germanium detector is longer than the response time of the detector and the release time of the thermolysis reaction, but shorter than the time for quenching of singlet oxygen by buffer gas. As a buffer gas we chose nitrogen because of its very low rate for O2 lAg deactivation.20 (20) Collins, R. J.; Husain, D.; Donovan, R. J. J. Chem. Soc., Faraday Trans. 2 1973.69, 145.
W-
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 TIME (s)
Figure 10. Effect of buffer gas pressure in the sample chamber on signal from the germanium detector. Buffer pressure: 0 , O Torr; 0 , 0 . 5 Torr; A, 10 Torr; A, 100 Torr; 0 , 760 Torr.
Figure 10 shows the detector signals recorded as a function of time (background has been subtracted) for thermolysis of 240-nm films of 2-PVNE at 130 O C . The buffer gas pressure was varied from 0 to 760 Torr. Without a buffer gas present a released O2 'Ag molecule rapidly collides with many of the surfaces present in the sample chamber. Even though most of the interior surface was Teflon coated to reduce the rate of quenching of singlet oxygen on the walls, nevertheless, the small amount of bare metal surface will rapidly deactivate O2 'Ag if it is unrestrained by a buffer gas from sampling the interior surfaces. With only 0.5 Torr of buffer gas, the germanium detector signal is dramatically increased. Apparently, this small pressure of buffer gas is sufficient to keep the singlet oxygen molecule from sampling much of the interior surface over the time period of our measurement. Upon further addition of buffer gas the released oxygen is not only kept from sampling many of the interior surfaces but at 10 Torr or more of buffer gas the released singlet oxygen is confined to the viewing region of the germanium detector for a few tenths of a second after its release into the gas phase. Figure 10 shows that release of singlet oxygen into 760 Torr of buffer gas results in a significant reduction in germanium detector signal. At this pressure, quenching of singlet oxygen by
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 401
Solid-state Endoperoxide Thermolysis
3=
x
1012
d = 5 x 10l2
I
' 40 ' 80 ' 120
0;
-
'4
It
2
'
160 ' 200 ' 240 FILM THICKNESS (nm) '
e ! 1
x
0,41/
p 0.2 0
0
1 '
" 20' " 40 60 FILM THICKNESS (nm)
40
60 POLYMER THICKNESS (nm)
80
lo1*
5! 0.6
(I)
20
Figure 12. Fractional yield of O2lA, released from 2-PVNE films upon heating. Model calculation used with 0 = 2.0 X 10l2cm-2.
1.2 + p =2
I
0
80
The inelastic limit was chosen as the appropriate limit to use to model the release of singlet oxygen into the gas phase. The singlet oxygen release model employs a fitting parameter, which is @,equal to the ratio of the quenching rate constant to the diffusivity of O2 lAg in the polymer. Figure 1 l b shows a comparison of the model calculations for detector signal with experimental data for the range of film thicknesses from 0 to 80 nm. The best fit between the model calculation and the experimental results is obtained for a value of @ of approximately 2 x 1OI2cm-2.
Figure 11. Germanium detector signal as a function of 2-PVNE film
Conclusions and Discussion
thickness: X, data; solid lines, model calculations (see text). Film thickness; A, 0-240 nm; B, 0-80 nm. 0 is in units of cm-2.
Oxygen is released quantitatively from solid films of 2-PVNE upon heating. At temperatures above 98 O C more than half of the available oxygen is released in less than 1 s. A mass spectrum of the gas-phase thermolysis products reveals no evidence of polymer decomposition other than oxygen release at temperatures as high as 112 O C . The rate constant for thermolysis of 2-PVNE shows an Arrhenius temperature dependence with an activation energy of 91 kJ/mol, a value somewhat lower than the 108 kJ/mol reported13J4 for similar polymer-bound alkylnaphthalene endoperoxides in solution. Because one of the thermolysis products, O2 IAg is 96 kJ/mol above its ground state, an activation energy of 91 kJ/mol implies that the stability of the poly(viny1naphthalene) product is comparable to that of the endoperoxide. The small difference of 5 kJ/mol in these values is probably outside the range of experimental error. A complication in our measurement of activation energy for thermolysis is the need for oxygen to diffuse out of the polymer film before it is detected. If the rate of diffusion of oxygen out of the polymer is slower than the molecular thermolysis reaction rate, the activation energy which we report could in principle be a measure of the barrier to diffusion in the polymer film rather than the thermolysis reaction step. However, if this were the case, then the apparent rate constant for appearance of oxygen would be expected to be inversely proportional to the film thickness. The lack of dependence of the rate constant we measure on film thickness lends support to our assignment of 91 kJ/mol as the activation energy for the thermolysis step. Of principal interest to us is the gas-phase singlet oxygen yield, or fraction of oxygen released in the excited state, from thermolysis of 2-PVNE. In particular, it is important to know how the yield depends upon thickness of the solid film, since in most potential applications it is desirable to use the thickest possible films which produce acceptably high yields. In the Results section and in the Appendix, a model is developed which is consistent with experimentally observed trends in the data, namely, a signal level which is proportional to film thickness for thin films, but is independent of thickness for thick films. The model predictions depend on two parameters: the ratio of the diffusion coefficient to the quenching rate constant for singlet oxygen in the film, and the probability of singlet oxygen quenching at the interface of the endoperoxide film with the solid conductive substrate. These parameters are evaluated from a best fit to the experimental data. The quality of fit (see Figure 11) is quite good, which suggests the model may contain the important features of the physical processes involved. The prediction of singlet oxygen
nitrogen is expected to become important on the time scale of the thermolysis reaction. Using a value of 1 X cm3/(mol S ) ~ O for the quenching rate constant of O2 *Agby nitrogen, we calculated an e-folding time of 0.5 s. Another matter of concern at higher pressures is the effect of confinement of released oxygen close to the polymer film surface for longer time. This surface is probably a fairly efficient quencher of singlet oxygen and frequent collisions with this surface may also contribute to the reduced germanium detector signal observed with 760 Torr of buffer gas. The percent of released oxygen which enters the gas phase in the l A state is determined by the relative rates of diffusion out of the endoperoxide polymer and of quenching of excitation by the polymer. It is expected, therefore, that the fraction of oxygen released electronically excited will be a function of the polymer film thickness (since diffusion out of the polymer is more rapid for thinner films). A cooled germanium detector was used to monitor the amount of O2 IA released into 10 Torr of nitrogen buffer gas from films of various thickness heated to 130 OC. Figure 1l a shows the results of these measurements. Note that the signal stays constant, as the film thickness decreases from 240 nm to about 40 nm, and then decreases with a further reduction in film thickness. This behavior can be easily understood if the quenching is thought of as limiting the time a O2IA molecule can spend diffusing through the polymer film. This in turn limits the number of oxygen molecules which reach the gas phase still in the excited state to those born within a fixed distance from the front surface. If the endoperoxide film is made thicker than this distance, the amount of excited oxygen reaching the gas phase will not change and hence the primary emission signal which is proportional to the amount of O2 'Ag in the gas phase will remain constant as the film thickness is increased beyond a critical value. A more quantitative but still simple calculation of the diffusion and quenching of O2 'A, in a solid material is discussed in the Appendix. This model of singlet oxygen release into the gas phase from a solid polymer film has two limiting cases differing only in how collisions of the O2I$ molecules with the conductive metal oxide surface are treated. In the elastic limit it is presumed that O2lAg is not deactivated by collisions with this surface while the inelastic limit presumes full deactivation. The conductive metal oxide surface is expected to efficiently quench O2 IAg analogous to the efficient quenching of singlet oxygen by metal surfaces.
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yields versus film thickness obtained from the model is shown in Figure 12. The accuracies of the determinations of singlet oxygen yields and projections for improvements depend heavily on the validity of the model used to describe the processes of release, diffusion, and quenching. The fact that the values for parameters obtained by fitting the model to experimental data are in good agreement with values obtained for quenching in solutions and for diffusion of oxygen in polymers increases the level of confidence in this model. Another factor which has significant impact on the reliability of the determinations of singlet oxygen yields is the accuracy of optical detector calibrations. Relevant aspects of some of the quantities that go into the calibration calculations are discussed in the Appendix. While it seems unlikely that calibration errors could be large, we are currently developing gas-phase chemical titration techniques as an independent test of optical calibrations. These results will be reported in a future publication. To qualify as a fuel source for a singlet oxygen-iodine transfer laser, the yield of O2 entering the laser gain region must exceed 15%. From the yield curve plotted in Figure 12, it appears that 2-PVNE films must have thicknesses less than 50 nm to be useful as a source of O2I$ for a chemically pumped oxygen-iodine laser. Free-standing films or microscopic polymer structures such as fibers or foam cells would be constrained to thicknesses of less than approximately 0.1 pm. Deuteriation or partial fluorination of the endoperoxide polymer may decrease the quenching rate of O2 IA, by a t least an order of magnitude, allowing the use of thicker polymer structures for a chemically pumped oxygen-iodine laser fuel source.
'$
Acknowledgment. This work was supported by the Air Force Weapons Laboratory, Kirtland Air Force Base, NM 87117, under contract F29601-85-C-0027. Appendix: Diffusion of Singlet Oxygen in 2-PVNE
The fraction of O2 lA, released from heated 2-PVNE films is calculated here with a simple diffusion model. The polymer film is treated as an infinite slab of homogeneous material characterized by a single, constant diffusivity, D, and a thickness, S. It is assumed that 0, lAg is generated, instantly at time t = 0, and that it is distributed uniformly throughout the film. Singlet oxygen is allowed to escape from only one of the two planar slab surfaces; the other surface is in contact with a conductive metal oxide surface. Collisions of O2 IA, with the conductive surface result in some degree of deactivation of the excited oxygen. We treat the limiting cases of complete deactivation (the inelastic limit) and complete reflection of the excited oxygen back into the polymer film without deactivation (the elastic limit). The problem of thermal diffusion in a slab with parallel A slab is boundaries is discussed by Carslaw and Jaeger.,' considered to occupy a space from -L to +L with the boundary conditions T = 0 at x = -L and +L. The initial temperature (To) is constant throughout the slab. The analogous problem in mass diffusion considers a slab with an initial concentration of diffusing (21) Carslaw, H. S.; Jaeger, J. C. Conduction of Heat in Solids, 2nd ed.; Oxford University Press; Oxford, U.K., 1959.
Twarowski et al. species, C,, uniformly dispersed at time t = 0 and with the boundary conditions C = 0 at x = -L and +L. Here C is the concentration of the diffusing species of interest. If the diffusing species is O2 lAg, the flux of this species crossing the boundary at x = +L into the gas phase is given by 2DCo m f l ~ x I , ==~ -E exp[-D(2n 1)2a2t/4L2] (4) L n=O Equation 4 presumes that the singlet oxygen molecules have an infinite lifetime. We need to multiply the flux in eq 4 by exd-k,t), the probability that the oxygen molecule is still excited at time t. Here k, is the pseudo-first-order quenching constant associated with deactivation of O2lis, by the polymer. Finally, this lifetime corrected flux is integrated over all time to give
+
In eq 5 we have included the area A of the boundary surface across which singlet oxygen diffuses before reaching the gas phase. The result is N , the total number of O2 'A, molecules which enter the gas phase. Equation 5 has one adjustable parameter, @ = k,/D, the ratio of the quenching constant to the molecular diffusivity of singlet delta oxygen in 2-PVNE. The sum in eq 5 was evaluated numerically for a range of values of L. The calculation was repeated for several values of the adjustable parameter, @, giving a family of curves of 0, IAS release as a function of film thickness. The manner in which L is related to the endoperoxide film thickness will determine whether the back surface is modeled as deactivating or nondeactivating. If the thickness, S, is chosen to be equal to L with the front surface at +L and the back surface at x = 0, the calculation prescribed by eq 5 will give the elastic limit. The inelastic limit results from the choice of S = 2L. Equation 5 was used to calculate the total number of moles of O2 lA, released into the gas phase as a function of film thickness. The experimental data is given in terms of amperes of current from a cooled germanium detector. It remains to convert one to the other for purposes of comparison between the theoretical calculations and the experimental results. We have chosen to convert the results of our theoretical calculations to units of signal current. The calculated signal current is given by signal = (0.5)(1.56 X 10-')(2.6 X 10-4)(0.57)(0.72)(1.7 X 10-3)N (6) The first term on the right is the responsivity of the germanium detector in amperes/watt at 77 K and 1300 nm estimated from manufacturers' data, the second term is the energy in joules of one photon at 1270 nm, the third term is the radiative rate constant in SKI,the fourth term corrects for the transmission of the interference filter, the fifth term allows for reflection losses at four surfaces (the sapphire window on the sample chamber and on the Dewar of the germanium detector), and finally the next to last term is the fraction of photons collected by a 1 cm diameter surface 6 cm from the source of photons. The factor assumes that all the IA is present at the center position of the sample chamber. This is not a severe approximation and any reasonable distribution of excited oxygen within the viewing region of the sample chamber will give approximately (within 10%) the same result. Registry No. 02, 1782-44-1.
