J . Phys. Chem. 1990, 94. 4926-4929
4926
( < I ) is the quantum yield for the formation of K. This calculated yield is much smaller than the proposed value of 3 X suggesting that such a double isomerization is not likely to be of importance in the light-induced transition from BR-548 to BR-570. From the data presented here, it is more likely that the K intermediate in the BR-548 photocycle is converted directly to BR-570 by the thermal isomerization of the C,,=N bond.
thermally equilibrated with BR-570 ( 1 3-trans, 15-anti) by the double isomerization of the CI3=Cl4 and CIS=N bonds over a 20-min period. The BR-548 photocycle involves C,3=Cl4 isomerization to form a bathochromic species, K ( 1 3-trans, I S-syn). Thermally, the reverse isomerization occurs from the K to reform BR-548 with a time constant of =40 ms. With respect to the light-induced conversion of BR-548 to BR-570, two mechanisms have been d i s c ~ s s e d . ~ ' . jIn~ -the ~~ the bathochromic (K) intermediate after absorbing light decays directly to BR-570. In the second,3s K converts into the K-610 intermediate of the trans photocycle ( 1 3-cis, 15-anti) after the absorption of light. Both processes, therefore, involve secondary photochemistry at the Schiff base. In the first mechanism, only a single isomerization at the C,,=N bond occurs, while in the second, a double isomerization a t both the C,,=C,, and the C,,=N bonds is required. If the double isomerization of K to K-610 occurs with the same rate as the slow (20 min) double isomerization between BR-548 and BR-570,j5 then the K K-610 quantum yield relative to that of dark-adapted, cis cycle can be estimated to be aK (40 ms/20 min) < 3 X where aK
Acknowledgment. We thank T. Brack, D. Blanchard, and S. Ruskin for their technical assistance in these experiments. This work was supported by a grant (GM 36628-03) from the National Institutes of Health and by the DuPont Corp. through its funding of a Japanese-American exchange program between the University of Tokyo and the University of Arizona. W.G. expresses his gratitude to the Deutsche Forschungsgemeinschaft which supported a visit to the University of Arizona as part of this collaboration. G.H.A. gratefully acknowledges the support of the Alexander von Humboldt-Stiftung and the hospitality of Prof. E. Schlag at the Technical University of Munich where part of this paper was prepared.
-
Time-Dependent Reaction Rates in the Thermolysis of a Poly(v1nylnaphthalene endoperoxide) M. R. Wixom K M S Fusion, Inc., P.O.Box 1567, Ann Arbor, Michigan 48106-1567 (Received: October 19, 1989)
Classical reaction kinetics with time-independent rate constants have failed to provide a satisfactory explanation of the rate of thermolysis leading to the release of excited ('Ag)molecular oxygen from polymer solutions and thin solid films of 1.4-dimethyl-2-poly(vinylnaphthalene 1,4-endoperoxide). First-order kinetics with time-dependent rates are shown to produce better agreement with the experimental data. The thermolysis rate constant is observed to have the time dependent k tu-', where a = 0.66 for polymer solutions and a = 0.38 for polymer thin films. The value of a indicates the distribution of lifetimes for excited oxygen. The lifetime distribution is broadened by local concentration effects in the polymer.
-
Introduction Polymer-immobilized naphthalenes have been developed as The naphthalenes chemical generators of singlet ('A8) and other polynuclear aromatics can undergo transannular addition reactions with singlet oxygen to form 1,4-endopero~ides.~-'~ Subsequent thermolysis of these endoperoxides produces the original aromatic system and singlet o ~ y g e n . ~ ! ~ *Thus, ~ @ laromatic ~ systems that form stable endoperoxides can provide a clean and rapid source of singlet oxygen which can be used in synthetic chemical reactions or chemical laser pumping schemes.
Several previous studies have reported kinetic data for thermolysis reactions of aromatic hydrocarbon endoperoxides. Naphthalene derivatives are among the most extensively studied. The naphthalene derivatives have included 1,4-dimethylnaphthalene ( D M N ) in s ~ l u t i o n ,poly( ~ 1,4-dimethyl-2-vinylnaphthalene) (2PVN) in ~ o l u t i o nand , ~ 2PVN in thin films.* The thermolysis reactions of the endoperoxides (DMNE and 2PVNE) in solution were reported to follow simple first-order kinetics consistent with the following reaction scheme:
( I ) Saito. 1.; Nagata, R.; Matsuura, T. Tetrahedron Lett. 1981, 22(42), 423 I . (2) Twarowski, A. J.; Good, L.; Busch, G . J . Phys. Chem. 1988,92, 396. (3) Saito, 1.: Nagata, R.; Matsuura, T.J . Am. Chem. SOC.1985, 107. 6329. (4) Twarowski, A.; Dao, P. J . Phys. Chem. 1988, 92, 5292. (5) McCall. D. B. Ph.D. Dissertation Part 2, Wayne State University. 1985. (6) Goilnick, K.; Schenck, G . 0. In 1,4 Cyclooddition Reactions; Hamer. J., Ed.; Academic: New York, 1967; p 255. (7) Rigaudy, J. Pure Appl. Chem. 1968, 16, 169. (8) Denny, R. W.: Nickon, A. Oig. React. 1973, 20, 133. (9) Saito, L.; Matsuura, T. In Singlet Oxygen; Wasserman, H. H.. Murray, R. W., Eds.; Academic: New York, 1971; p 5 1 I . ( I O ) Bloodworth, A. J.; Eggelte, H.J. In Singlet Oxygen; Frimer, A . A.. Ed.; C R C Press: Boca Raton, FL, 1985; Vol. 11, p 93. ( 1 I ) Murray. M. W. I n Singlet Oxygen; Wasserman, H. H , Murray, R. W., Eds.; Academic: New York, 1979; p 59. ( I 2) Wasserman, H . H.; Scheffer, J. R. J . Am. Chem. SOC.1967,89, 3073. (13) Wasserman. H. H.; Larsen, D. L. Chem. Commun. 1972, 2.53. (14) Larsen, D. L. Ph.D. Dissertation, Yale University, 1973. (15) Schaeffer-Ridder, M.; Brocker, U.; Vogel, E. Angew. Chem. 1976. 88, 267. (16) Turro, N . J.; Chow, M. F.; Rigaudy. J . J . Am. Chem. Soc. 1981. 103, 218.
02*
0022-3654/90/2094-4926$02.50/0
+sE
+ 02* k t h = 8 X 0, + s k, = 6 X N
s-'
cm3 s-'
(ref 17)
(I)
(ref 18) (2)
where E is the endoperoxide, N is the unoxidized naphthalene, and 02*is the excited singlet oxygen. The second reaction indicates a physical quenching mechanism where S is a solvent molecule. Since [SI is a constant = 1021 cm-; and k , [ S ] >> k t h , reaction 1 is the rate-limiting step. The thermolysis reaction rate can be conveniently determined by using UV absorption spectrometry to follow the rise in naphthalene concentration. For r c " temperature measurements, the naphthalene peak at 290 nm is recorded over a period of several hours or days. The integrated rate law for reaction 1 can be expressed as a function of the naphthalene concentration [N]: In
PI,
-
[Nlf -
P I
= -kt
(3)
P I 0
( 1 7) Twarowski, A . J . Phys. Chem. 1988, 92. 6580. ( I 8) Monroe, B. M. I n Singlet 02;Frimer, A. A., Ed ; C R C Press: Boca
Raton. FL. 1985: p 177
0 1990 American Chemical Society
Thermolysis of a Poly(vinylnaphtha1ene endoperoxide)
The Journal of Physical Chemistry, Vol. 94, No. 12, I990
4921
=
?
-2
0
30
60
90
\
1
120
150
time (10 s) Figure 1. Measured thermolysis rate for monomeric solutions of di-
methylnaphthalene endoperoxide (solid points) and vinylnaphthalene endoperoxide (open points) compared to the calculated first-order rate law given in eq 3 (solid lines).
time (1o3 s) Figure 2. Measured thermolysis rate for poly(2-vinylnaphthalene endo-
peroxide) in solution. h
0
where [N], is the concentration at t = 0 and [N], is the concentration at t = a. The thin film thermolysis rate was determined by measuring the oxygen release from heated fib" A similar integrated rate law was expressed as a function of the amount of oxygen released. In attempting to reproduce these measurements it became apparent that the rate of thermolysis for the polymer-bound naphthalenes was not in fact constant with time as assumed in the above reports. A systematic and reproducible variation of k with time was observed for both 2PVNE in solution and 2PVNE in thin films. The time dependence of k is attributed to the nonrandom distribution of polymer-bound reactants. This report describes the time-dependent reaction rates observed for the thermolysis of 2PVNE in solution and ZPVNE in solid films.
Experimental Section The naphthalene derivatives, 1,4-dimethyInaphthalene and 1,4-dimethyl-2-vinyInaphthalene,were prepared at Wayne State University by Professor A. P. Schaap. Their synthesis and characterization are reported e l ~ e w h e r e . Polymerization ~ and peroxidation of the vinyl monomer is also reported in detail el~ewhere.~ The polymerization reaction occurred by a free-radical mechanism initiated by azobisisobutyronitrile. The polymer molecular weights were determined by gel permeation chromatography. The weight determinations were made for 1 wt % solutions in chloroform. The number-average molecular weights for five different samples were all between 3000 and 5000 with dispersities close to 2. The endoperoxides were generally prepared within a few hours of starting the thermolysis rate measurement experiments and stored, if necessary, at -20 O C . The polymer was dissolved in methylene chloride. Thin films of 2-PVNE were deposited from solution onto quartz substrates by use of a spin coating technique. Film thickness was controlled by varying the concentration of the polymer solution. The thermolysis rates for the endoperoxides in both solution and thin film were measured by using UV absorption spectrometry to monitor the reformation of the naphthalene moiety. The naphthalene concentration was determined by recording the absorbance at 294 nm. To correct for background, the absorbance at 350 nm was subtracted from the 294-nm absorbance. Both the solutions and thin films were prepared to give final naphthalene absorbances close to 1 .O. The solution concentrations were 2 X lo4 M in chloroform. The thin-film thicknesses were nominally 0.35 pm. Absorption measurements were made on a IBM 9430 spectrometer. The absorption coefficient for 2PVN in solution was measured to be t = 5.52 X lo3 M-' cm-' at 294 nm. Results The monomer thermolysis rate was measured for both DMNE and VNE. The agreement with the first-order integrated rate law is shown in Figure 1. The observed rate constants are independent of time as indicated by the constant slope of the rate plot. The VNE thermolysis rate is somewhat lower than the D M N E rate as a result of the stabilizing effect of the vinyl substituent. The observed rates are in good agreement with the rates reported by
z 5.-- 0
:. I.(
.*
-0.4
5
0.
*.
**.-
-11
0
100
200
time (1 o 3 s) Figure 3. Measured thermolysis rate for poly(2-vinylnaphthalene endoperoxide) in a thin solid film.
Saito et aL3 for the similar compounds 1,2,4-trimethyl-6-vinylnaphthalene ( k = 2.0 X s-' at 30 "C) and 1,4-dimethyl-6s-I at 25 "C). vinylnaphthalene ( k = 4.7 X The thermolysis rate was also measured for polymer solutions (Figure 2) and polymer thin films (Figure 3). Both figures clearly show that the polymer thermolysis rates do not follow the linear first-order integrated rate law as do the monomer solutions. The nonlinear behavior shows that the reaction rate is continuously decreasing with time. The decreasing reaction rate can be attributed to an additional reaction channel available in the polymer-bound endoperoxides. The polymers contain both peroxidized and unoxidized naphthalene units. If the local concentration of unoxidized naphthalene is high, the possibility of the reverse reaction must be considered. 02*+ N
-
E
k, = 2
X
lo-' cm3 s-l
(ref 19)
(4)
In the monomer solution, the reverse reaction does not compete with quenching. In the polymer solutions some fraction of the thermolysis reactions will occur along regions of the polymer which are surrounded by a high local concentration of N. In polymer thin films a yet higher fraction of the thermolysis will occur in such regions of high N concentration. Since the local concentration of unoxidized naphthalene increases with time, the observed thermolysis rate appears to decrease as the reverse reaction competes more effectively with quenching. We have attempted to account for the reverse reaction by combining (1)-(4) to derive the following rate equations:
- - - kth[El d[N1 dt
[
-
[SI represents the concentration of the quenching species which is assumed to be any molecule with which a quenching collision (19) Stevens,
6846.
B.;Perez, S . R.;Ors, J . A. J .
Am. Chem. SOC.1974, 96,
4928
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990
Wixom 1.37
a = 0.96
y 0,
0 -
0.9-
. ..
0.6-
0.3t
cx = 0.66
m 0
cx = 0.34
a
a = 0.76
b
a=070 a=066
C
0.74 3.0
0 4 3.8
4.2
4.6
5
4.2
4.6
5
log t
log t Figure 4. A log-log plot of the instantaneous rate constant k versus time
for vinylnaphthalene endoperoxide monomer solution (solid circles), polymer solution (open circle), and polymer thin film (solid squares). The lines are least-squares fits to the data which give a slope of cy - 1
Figure 5. Time-dependent rate constant shown quite sensitive to naphthalene concentration [N]f at t = m. These three lines show the effect of varying [N], by f0.05 absorbance units.
according to eq 1Oc.
can occur, i.e. N , E, or, for solutions, solvent molecules. Therefore, [SI can be assumed to be constant -IO2’ and k,[S] be replaced by k,’ N 6 X lo3 s-I. Rewriting [E] = [N], - [N] in the above equation gives 0 0.3
Our attempts to fit the observed rate data to the above equation or to similar schemes have failed. The observed thermolysis rates always show systematic deviation from the rates calculated by varying the constants in the model equations. The deviations can be modeled more successfully by replacing the rate constants with a time-dependent rate coefficient. The time dependence is generally expressed by the function k ( t ) = Bra-’
0
50 “C for an hour or more at the end of the run to fully thermolyze any remaining endoperoxide. In practice small errors in concentration will occur as a result of solvent evaporation or optical changes in the sample. Figure 5 shows that errors in [NIf of f0.05 absorbance units can cause a to vary from 0.76 to 0.66. Nevertheless, despite the sensitivity to [N],, a values were generally close to 0.67 for polymer solutions and 0.40 for polymer thin films. The stronger time dependence for the thin film thermolysis rate constant is due to the higher local variations in naphthalene concentration relative to the variations that occur in the polymer solution. The vinylnaphthalene was polymerized by a free-radical mechanism giving relatively low molecular weight polymers with
0.9
1.2
1.5
log t Figure 6. A log-log plot of the instantaneous rate constant k versus time for the oxidation of poly(vinylnaphtha1ene) thin films from data provided in ref 17.
-
A 5000. The polymer chains consisted of a few hundred monomer units and probably contained a high degree of cross branching typical of free-radical polymerization. Initial attempts to control the molecular weight and cross branching have not succeeded. If the molecular weight and extent of cross branching could be increased, the polymer solution value for a should approach the value of a observed for thin solid polymer films. Time-dependent reaction rates can also be observed in the oxidation of PVN thin films. In ref 17 the porphyrin-sensitized photooxidation of PVN is reported as a function of illumination time. A reaction scheme is proposed that involves the same thermolysis, quenching, and reverse reactions as in the above scheme. The proposed reaction scheme also includes an additional reaction 02
log -
0.6
+E
--*
N
+ 202
(11)
kqE
which the author concludes is necessary to give a calculated rate that fits the experimental data. Even with the inclusion of this reaction, however, a systematic variation is still observed between the calculated rate and the experimental data. Figure 6 shows the data from Figure 7 of ref 17 replotted as a log-log plot. The least-squares fit to the data gives a value of a = 0.2. The scatter in the data at longer time is an artifact of trying to measure small changes in the instantaneous reaction rate after long times. It appears then that the inclusion of kqEin the reaction series is not necessarily implied by the experimental data. The possibility must be considered that a time-dependent rate coefficient could produce a better fit to the observed results. This explanation seems plausible in a system that is limited by the diffusion of excited oxygen from photosensitizer molecules to polymer-bound naphthalene molecules. Similar results have been observed in the reaction kinetics of species trapped in glassy matrices.2’ Conclusion
Time-dependent reaction rates appear to explain the observed thermolysis rates for polymer-bound endoperoxides. The timedependent rates arise from the nonrandom reactant distribution imposed by restricting the reactants to the polymer chains. The ~~
(20) Hamill, W . H.; Funabashi. K . Phys. Reo. B 1977, 12, 5523
(21 ) Plonka, A. Time-Dependent Reactivity of species
dia; Springer-Verlag: Berlin, 1986; pp 6-82
in
Condensed Me-
J . Phys. Chem. 1990, 94, 4929-4935 thermolysis rate for thin polymer films was more strongly time dependent (a = 0.34) than for polymer solutions (a = 0.66). The poorly stirred conditions for these reactions suggest that classical chemical kinetic models are not appropriate.
Acknowledgment. The helpful suggestions of an anonymous
4929
reviewer are gratefully acknowledged. L. A. Good and A. J. Twarowski assisted in collecting and modeling the data presented in this paper. Registry No. D M N E , 35461-84-8; V N E , 126950-45-6; V N E (hom126950-48-9; 0, 7782-44-7.
opolymer),
Solvation Dynamics in N-Methylamides Curtis F. Chapman, Richard S. Fee, and Mark Maroncelli* Department of Chemistry, 152 Davey Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: October 19, 1989; I n Final Form: January 9, 1990)
Solvation times in three homologous amides, N-methylformamide, N-methylacetamide, and N-methylpropionamide, have been detd. from measurements of the dynamic Stokes shift of the fluorescence spectra of two-probe solutes, prodan and coumarin 102. Single-particle reorientation times have also been measured in one of these solvents, N-methylformamide, by using NMR methods. The solvation dynamics are compared to two theoretical models, the simple continuum model and the dynamic MSA model. Although neither model predicts the time dependence of the response satisfactorily, the average solvation times observed are close to the solvent longitudinal relaxation time (TL) predicted by the simple continuum model. In contrast, the predictions of the dynamical MSA model are approximately 4 times slower than the observed solvation response. The failure of the latter model appears to result from an overestimation of the single-particle reorientation times of these solvents. Esimates of such single-particle times based on the NMR measurements are within a factor of 2 of TL. This similarity seems to account for the near equality of average solvation times to T L in the amides.
introduction In the past few years quite a number of experimenta11-8 and t h e o r e t i ~ a l ~studies -~l have considered the dynamical aspects of solvation in polar liquids with a view toward understanding the dynamical coupling between polar solvents and charge-transfer reactions.22 In such studies, the central focus has been on measuring how rapidly a solvent responds to changes in the charge distribution of a solute molecule and on understanding what solvent and/or solute attributes determine this response time. Work in this area has been summarized in several recent review article^:^-^^ so here we will only highlight some of the main results that have so far emerged. Theoretical treatments of the dynamics of polar solvation can be roughly divided into two approaches, which differ in the sophistication with which they model the solvent. The earliest treatments, provided by Bakshiev9 and Mazurenko,Io typify the first approach, which we will refer to as "simple continuum" model^.^-^^ Here the most elementary model of the solvent is adopted, that of a continuous, homogeneous fluid whose only relevant property is its bulk, frequency-dependent dielectric response. In such models the dynamics are simply related to solvent dielectric properties and are relatively insensitive to the solute attributes.12J6 All simple continuum models predict that solvation should proceed on a time scale that is approximately equal to the longitudinal relaxation time of the solvent, TL. This longitudinal relaxation time is a bulk dielectric property of the solvent that is much faster than the dielectric relaxation time T D (see Discussion). The T~ prediction of simple continuum models has served as an important benchmark against which to view experimental data. The second group of goes beyond a simple continuum representation by recognizing the molecular nature of the solvent in some way. The first such model was proposed by WolynesI5 and further developed by Rips et al.'8919and Nichols and Calef.20 This theory describes the solvent in terms of an equivalent dipolar hard-sphere system whose equilibrium structure can be calculated approximately within the mean spherical approximation (MSA). Although this "dynamical MSA" model of Author to whom correspondence should be addressed.
0022-3654/90/2094-4929$02.50/0
is only one of several molecular models for the dynamics,16*20,21 it is representative of them all, and at present it is the one most easily applied to the experimental data. The dynamical MSA model predicts much more complex dynamics than the simple continuum models. Rather than there being a , molecular model predicts single relaxation time such as T ~ this that a range of relaxation times, roughly spanning the range between T L and TD, should be present in the solvation response. ( I ) Castner, Jr., E. W.; Maroncelli, M.; Fleming, C . R. J . Chem. Phys. 1987, 86, 1090. Castner, Jr., E. W.; Bagchi, B.; Maroncelli, M.; Webb, S. P.; Ruggiero, A. J.; Fleming, G. R. Ber. BunsenGes. Phys. Chem. 1987, 92, 363. (2) Maroncelli, M.; Fleming, G. R. J . Chem. Phys. 1987, 86, 6221. (3) Su, S.-G.; Simon, J. D. J . Phys. Chem. 1987,91,2693. Simon, J. D.; Su,S.-G. J . Chem. Phys. 1987.87, 7016. (4) Su, S.-G.; Simon, J. D. J . Phys. Chem. 1989, 93, 753. (5) Nagarajan, V.; Brearley, A. M.; Kang, T.-J.; Barbara, P. F. J . Chem. Phys. 1987,86,3183. Kahlow, M. A.; Kang, T. J.; Barbara, P. F. [bid.1988, 88, 2372. Jarzeba, W.; Walker, G. C.; Johnson, A. E.; Kahlow, M. E.; Barbara., P. F. J . Phys. Chem. 1988, 92, 7039. (6) Kahlow, M. A.; Jarzeba, W.; Kang, T.-J.; Barbara, P. F. J . Chem. Phys. 1989, 90, 151. (7) Kinoshita, S.; Nishi, N.; Kushida, T. Chem. Phys. Lett. 1987, 134,605. ( 8 ) Declemy, A.; Rulliere, C. Chem. Phys. Lett. 1988, 146, I . (9) Bakhshiev, N . G. Opt. Spectrosc. (USSR) 1964, 16, 446. (10) Mazurenko, Yu. T. Opt. Spectrosc. ( U S S R ) 1974, 36, 283. (11) Bagchi, B.; Oxtoby, D. W.; Fleming, G . R. Chem. Phys. 1984, 86, 257. (12) Castner, Jr., E. W.; Fleming, G . R.; Bagchi, B. Chem. Phys. Lett. 1988, 143, 270; 1988, 148, 269. (13) van der Zwan, G.; Hynes, J. T. J . Phys. Chem. 1985, 89, 4181. (14) Loring, R. F.; Yan, Y. J.; Mukamel, S. J . Chem. Phys. 1987, 87, 5840. Loring, R. F.; Mukamel, S. Ibid. 1987, 87, 1272. (15) Wolynes, P . G . J . Chem. Phys. 1987, 86, 5133. (16) Freidrich, V.; Kivelson, D. J . Chem. Phys. 1987, 86, 6425. (17) Castner, Jr., E. W.; Fleming, G. R.; Bagchi, B.; Maroncelli, M. J . Chem. Phys. 1988.89. 3519. (18) Rips, 1.; Klafter, J.; Jortner, J. J . Chem. Phys. 1988, 88, 3246. (19) Rips, 1.; Klafter, J.; Jortner, J . J . Chem. Phys. 1988, 89, 4288. (20) Nichols 111, A. L.; Calef, D. F. J . Chem. Phys. 1988, 89, 3783. (21) Bagchi, B.; Chandra, A. J . Chem. Phys. 1989, 90, 7338, and refer-
ences therein. (22) See, for example: Hynes, J. T. J . Phys. Chem. 1986, 90, 3701. (23) Barbara, P. F.; Jarzeba, W. Acc. Chem. Res. 1988, 21, 195. (24) Simon, J. D. Acc. Chem. Res. 1988, 21, 128. (25) Maroncelli, M.; Maclnnis, J.; Fleming, G . R. Science 1989, 243, 1674. (26) Maroncelli, M.; Fleming, G . R. J . Chem. Phys. 1988, 89, 875.
0 1990 American Chemical Society
