OXIDATION AND CHEMILUMINESCENCE OF TETRAKIS(DIMETHYLAMINO) ETHYLENE
1507
Oxidation and Chemiluminescence of Tetrakis(dimethylamin0)ethylene. 111. Kinetics, Quantum Yield, and Mechanism of Luminescence
by Aaron N. Fletcher and Carl A. Heller Chemistry Division, Research Department, U.S. Naval Ordnance Test Station, China Lake, California 99656 (Received July 11, 1966)
The kinetics for the chemiluminescence from the autoxidation of tetrakis(dimethylamin0)ethylene (TMAE) has been measured. An equation giving absolute light output has been obtained. The chemiluminescent decomposition of an intermediate of the oxidation is also measured. Spectra, photoluminescent efficiency, and thermochemical data are given. A mechanism is proposed which covers both oxidation and chemiluminescent kinetics. Part of this mechanism is a reaction producing electronically excited TMAE.
Introduction Background. The and autoxidation1Pe5 of tetrakis(dimethy1amino)ethylene (TMAE) have been studied and mechanisms have been proposed for both. A problem is to show how the two phenomena can be explained by one mechanism. An energycoupling step which puts energy of the oxidation into an electronically excited species is needed. Paris2 wrote such a mechanism which, however, does not fit the autoxidation kinetic^.^^^ The other autoxidation mechanisms which have been proposed4s6 show no coupling to chemiluminescence. It will be the purpose of this paper to present additional data and to propose a plausible mechanism which couples the reaction energy to the light emission. The first' reported study of TMAE chemiluminescence showed that the spectrum matched the TMAE fluorescence spectrum while none of the products is fluorescent. Thus the energy from the oxidation of one or several TMAE molecules appears in an electronically excited TMAE molecule. Related to this is the fact that the autoxidation is first orderao5 in TMAE while the chemiluminescence is second order.2.a An energy-transfer mechanism from an excited product molecule to TMAE would explain these kinetics, but the quenching data indicate that only one excited species is presents6 Therefore, the coupling must occur in the oxidation chemistry. We have suggested a general equation as pertinent
to most steady-state chemiluminescent reactions.6 This is
I
= $e X 4 c X
(-4
(1)
where I is the light intensity, +e is the emission efficiency, 4o is the chemical efficiency, and - r is the rate of disappearance of one reactant. Winberg, Downing, and Coffman' made a similar suggestion of dividing the over-all efficiency into two factors-the chemical efficiency and the emission efficiency. For electronically excited TMAE in n-decane, the emission under chemiluminescent conditions is given by6 4e =
40
1
+ Ko2(02) + f(HA) 40
1
+ 48(HA)1 + 1040(HA)4 + 6400(02)
(11)
where r$o is the unquenched photoluminescence efficiency, Kol (M-l) the Stern-Volmer constant for chemi(1) H. E. Winberg, J. R. Downing, and D. D. Coffman, J. Am. Chem. SOC.,87, 2054 (1965). (2) J. P. Paris, Photochem. Photobiol., 4, 1059 (1965). (3) A. N. Fletcher and C. A. Heller, presented at the Symposium on Chemiluminescence held at Durham, N. C., March 31-April 2, 1965. (4) W. H. Urry and J. Sheeto, Photochem. Photobiol., 4, 1067 (1965). (5) A. N. Fletcher and C. A. Heller, J . Catalysis, 6 , 263 (1966). (6) A. N. Fletcher and C. A. Heller, Photochem. Photobiol., 4, 1051 (1965).
Volume 71,Number 6 AprQ 1067
1508
AARONN. FLETCHER AND CARLA. HELLER
luminescent quenching by 0 2 , and f(HA) the sum of quenching terms for alcohol monomer, (HA)l, and tetramer, (HA)4, concentrations. TMAE does not selfquench12,' so there is no term for it. The factor t$o has been given' as 0.03-0.05. The autoxidation rate5is given by
(-$)
=
F(HA) X (E) X (02) =
[(7.58 X
+ (3.48 X 10-2)(HA)1 +
(1.14 X 102)(HA)4]X (E) X (02) (111) where the molar reactant concentrations are TMAE (E), alcohol (HA), and oxygen (02). F(HA) is a function of all catalysts for the reaction. Catalysts' are any proton donors, and in our system are the glass wall, alcohol monomer, and alcohol tetramer. Alcohol is not consumed in the system to any measurable extent for autoxidation a t low concentrations of TMAEe5 The remaining factor, &, gives the number of excited molecules formed per reactant molecule oxidized. This does not have to be unity even for a simple elementary The energy coupling reactions will plainly be involved in The major products of reaction appear to result from two reaction paths and can be characterized by the reactions (CHa)zN
\
\
co
/
7(r800/0 (a)
/
(CHa)2N 0 0
E
+
/I
0 2
/I
+ (CHa)zN-C--C-N(CH&
+
2(CHa)zN* 3@20%
(b)
Urry'o states that recent work shows that the minor products all can come from the amino radicals. Although it seems likely that only one of these reaction paths is associated with light emission, there has been no evidence on this. In fact, the low quantum yields reported'J indicated that a minor reaction path might produce the light. Urry and Sheeto4showed that when TMAE is autoxidized over water, a rapid reaction produces dications, E*+,hydroperoxy anions, OzH-, and OH- in the water. These react further in the water to give a new set of products with the absence of light emission. Similar ions can be formed in acetonitrile or dimethyl sulfoxide by electrochemical means. Kuwata Tha Journal of Phyeical Chemktry
and Geske" produced E2+and the radical cation .E+ by this method. They show that an equilibrium exists
+E
E2+
2E+
Finally, oxygen electroreduc tion has been measured with and without proton donors present.12 Thus there are electrolytic cell potentials which permit calculation of the free energy of formation of various ion pairs of TMAE and 02. In addition to the foregoing material, there have been some results with which our measurements will disagree. These include quantum yield measurernents'J and the kinetic dependence on the second order of the alcohol concentration. These disagreements will be discussed after presentation of our results. Approach. We determined +o so as to know 4e quantitatively. Knowing 4e and dE/dt, we measured the absolute light emission to obtain 4cfrom eq I. For accurate measurement of the light, we needed to know the absolute emission spectra from dilute ndecane solutions. By removing oxygen and measuring the decay of light, we obtained kinetics for the later, light-producing portion of the over-all reaction. It was possible to get activation energies for the reactions involved. The over-all energy of the reaction was measured to show that the main sequence produces sufficient energy for the photons emitted. Calculations of energies of formation for various ions helped choose the most reasonable intermediates. On the basis of the new measurements, it was possible to write a mechanism which also fits the information presented in the Introduction. The energy-coupling step was made part of the oxidation mechanism since the chemiluminescent data made this seem even more necessary.
Experimental Section Material. TMAE and n-decane were purified as reported earlier.13 The 1-octanol was dried by molecular sieve 4A and distilled under 700 torr of helium. The oxygen was treated to remove water and carbon ~
(7) W. R. Ware, private communication. (8) C. J. Halstead and B. A. Thrush, Photochem. Photobwl., 4, 1007 (1965). (9) M. A. A. Clyne, B. A. Thrush, and R. D. Wayne, Trans. Faraday Soc., S O , 359 (1964). (10) W. H. Urry, private communication. (11) K. Kuwata and D. H. Geske, J. Am. Chem. SOC.,86, 2101 (1964). (12) D.L. Maricle and W. G. Hodson, Anal. Chem., 37, 562 (1965). (13) C. A. Heller and A. N. Fletcher, J. Phys. Chem., 69, 3313 (1965).
OXIDATION AND CHEMILUMINESCENCE OF TETRAKIS(DIMETHYLAMINO)ETHYLENE
dioxide. Quinine sulfate was used without purification. Reciprocating Flow System. The reciprocating flow system has been described in the second paper of this ~ e r i e s . ~It was used in the present paper for quantitative kinetic and light intensity measurements. A maximum flow rate of 500 cc/min was used. All kinetic measurements were made in n-decane solutions. The flow system was used only at 30". The reacting volume varied, being normally about 400-500 ml. The TMAE concentration was measured in a 1-cm spectrophotometric flow cell by a Beckman DK-2 spectrophotometer. Simultaneously, light emission was measured in a 128-ml light-emission flow cell in series with the spectrophotometric flow cell. The lightemission flow cell was made from a single slice of a 12cm diameter glass sphere. It was vacuum jacketed and silvered on the back of its spherical portion. Its flat glass portion was placed directly against the face of a 5-in. diameter Du Mont 6364 photomultiplier tube (S-11 response) with a thin film of glycerol between them. We assume 100% detection of the light produced within the light-emission flow cell. The additional component parts of the light-sensing apparatus were a Keithley Model 417 Picoammeter and a Hamner Nodel N-401 power supply. A General Electric Model 30 A/T24/7 tungsten lamp in an optical and electrical system recommended by the National Bureau of Standards14 was used to make periodic checks on the absolute light sensitivity of the phototube. Neutral density filters (Optics Technology, Inc.) were used to vary the light intensity of the tungsten lamp while Monopass filters (Optics Technology, Inc.) were used in the absolute spectral calibration of the phototube. Through the use of voltages between 400 and 1400 v applied to the phototube, light intensities over a range of lo8 in terms of anode current could be measured. Through electrical suppression of the dark current, measurements of anode current were made amp, although the usual between 10-lo and 3 X procedure restricted measurement to a maximum of amp in order to preserve the phototube calibration. These currents were related to an applied voltage of 836 v which was in the middle of the sensitivity range made available through varying the voltage applied to the phototube. Oxygen partial pressure in the gas phase was measured with a Beckman E-2 oxygen analyzer with a sensitivity of 0.1 torr of oxygen for oxygen pressure below 700 torr. Equilibrium between the gas and liquid phases was measured by time extrapolations of both TMAE absorbance and light intensity output when
1509
there was pumping of the gas phase through the liquid phase. If equilibrium was established, measurements following phase reequilibration would extrapolate back to those preceding equilibration. This method was valid during 3-min (and longer) runs, but was impossible to use during the fast 60-sec runs. Spectra. Photoluminescence and chemiluminescence spectra were obtained using a Bausch and Lomb 500mm monochromator and a Du R h t Type IC1927 (S-20 response) phototube which were calibrated by means of the tungsten lamp. A 6-ft length of 8-mm tubing was bent into a U-shape, silvered on the outside, vacuum jacketed, and connected to the flow system. The length of this U-tube was placed in line with the entrance slit of the monochromator. Excitation from the back side of the U-tube by 366-nm radiation was used for photoluminescence spectral measurements. A spectral slit width of 3.3 nm was used. Later, meassurements were also made using the Turner 210 absolute spectrofluorometer (G. K. Turner Associates, Palo Alto, Calif.). Static System. Small volumes (1-5 ml) of TMAE solutions were placed in 25-mm diameter cylindrical reaction flasks under Nf. The flasks were placed in a bath and were connected to a conventional system for handling gases. The reaction was started by removing N2and then adding 02. The solution was stirred a t a rate that was high enough so that it did not affect the reaction rate. Light was measured from above and 0 2 uptake was monitored by the pressure drop.13 The 0 2 could be removed rapidly and the bath temperatures could be changed fairly rapidly. This static system was used to measure 0 2 solubility,13 photoluminescent quenching,g and for temperature studies of dlldt. It was particularly useful for measuring light decay after removal of the oxygen. Photoluminescent Quantum Yield. The quantum yield of photoluminescence was measured on a Photovolt Corp. Model 540 fluorescence meter using the technique of Parker and Rees. l 5 Quinine bisulfate was used as the secondary standard. Two batches of quinine sulfate from hlerck were used in 0.1 N H804. A prepared solution from Harleco was used in two runs. Degassing of quinine solutions gave no effect. Excitation was at 366 nm. Heat of Reaction. The heat of reaction was measured in a rotating-bomb calorimeter. Two runs were made using 3 ml of TMAE with, respectively, 0.5 and 1 ml of octanol. The runs were finished within 30 min. (14) R. Stair, R. G. Johnston, and E. W. Halback, J . Res. Natl. Bur. Std., 64A,291 (1960). (15) C. A. Parker and W. T. Rees, Analyst, 87, 83 (1962).
Volume 71, Number 6 April 1967
AARONN. FLETCHER AND CARLA. HELLER
1510
I .o
PHOTOLUMINESCENCE, MAX 490 n m
0.9
0.8
2r 0.7 X
4
0 6
w
-
s W
-, -
0.5
The Photoluminescent Quantum Yield. These measurements involved comparing TMAE with the known quinine sulfate quantum yields. Table I shows the data for our runs. The Harleco solutions appear to contain a quenching impurity. The value = 0.35 =t 0.05 is considerably larger than that reported by Winberg, et aZ.,] who used a comparison with fluorescein. Their low values may be due to the presence of tetramethylurea as a quenching impurity. This would agree with our explanation for the wavelength shift of their spectra compared to our findings (see previous section).
E c
Tu
0.4
E?
0.3
w
Table I: Photoluminescent Quantum Yield of TMAE us. Quinine Bisulfate at 30'
0.2
0.1
0 WAVELENGTH, n m
Figure I. Emission spectra. Photoluminescence: TMAE = 1.4 X 10-8 M. Chemiluminescence: TMAE = 4.5 x 10-* A f . PO,= 300 torr; 1-octanol = 1.8 X lo-* M.
The products were analyzed by gas chromatography. The major products detected are those described by Urry and sheet^.^
Results Emission Spectra. In order to measure absolute intensity, we needed the spectral distribution of the emitted light. Figure 1 shows the corrected spectra of photoluminescence and chemiluminescence of TMAE in n-decane. A slight difference can be accounted for by self-absorption in the chemical system, which turns yellow or brown during reaction. l* We found a solvent effect for photoluminescencewith the peak shifting as much as 80 nm. The peaks for TMAE (true spectra using a Turner 210 spectrofluorometer) are: n-decane, 487 nm; neat TMAE, 505 nm; dioxane, 550 nm; 75% TMAE-25% tetramethylurea, 565 nm. The peak we find in neat TMAE differs from that previously reported' (515 nm). The difference is probably due to the presence of tetramethylurea in the earlier work. Tetramethylurea, of course, forms if oxygen has been admitted to the TMAE at any time. The Journal of Physical Chemktry
IEv
AE,
IQ?
%
in.2
%
AQP in.2
&E)
4.85 5.3 7.54 5.55 14.1 36.1 8.79 5.8 3.85 5.90 12.70
10 10 3.2 6.8 6.8 5.17 10.8 10.8 10.0 3.8 5 . 13c
9.81 9.9 11.92 11.72 10.7 7.3 18.12 11.7 13.17 3.87 1.87
0.32 0.39 0.35 0.32 0.32 (1.4)b 0.30 0.30 0.47 (0.89)* (0.68)
8.5 8.5 3.6 6.0 16.8 11.3 10.8 10.8 3.8 4.0 4.2c
Avd 0.35 f 0.05 Symbols: ZE and ZQ, per cent light absorbed in 1-cm cells; AE and AQ, areas under corrected spectral curves; qt, index of refraction of solvent i; &a(&)= 0.55, ref 15. * Quinine bisulfate solution from Harleco. The other reference solutions were made from quinine sulfate (Merck and Co.) dissolved in 0.1 N HzSO4. Calculated after 1:10 dilution. Others measured directly in a spectrophotometer or fluorometer. Values in parentheses are not included in the average.
Chemical Efiiency at Low Concentration-A bsolute Light Intensity Measurements. At TMAE concentrations below M, light intensity depends upon the square of TMAE concentration over a wide range of concentrations as shown in Table 11. Initial concentrations of TMAE of M were allowed to be autoxidized so as to decay to below M in these studies. Oxygen and 1-octanol were constant during each run. The very small buildup of products had no measurable effect on the light intensity as found by the reproducibility of the runs. The second-order effect on the light intensity over a wide range of conditions permitted us to write & =
OXIDATION AND CHEMILUMINESCENCE OF TETRAKIS(DIMETHYLAMINO)ETHYLENE
1511
100
I
Table 11: Order of Light Output in TMAE for TMAE up to M ; Light = K(TMAE)" (HAh M added I-octanol
Po2. torr
n
0.04 0.04 0.04 0.04 0.11 0.11 0.11 0.11 0.24 0.96
200 400 800 1200 200 400 800 1200 73 44
2.19 1.95 1.98 2.09 2.14 2.02 1.91 1.91 1.96 2.04
L
2 2
t
Y
I
1.0
0.0215M
\
5W
0.0165 M
a a
U 3
0
G X E. We now needed to find the dependence of the function G on the oxygen and 1-octanol concentrations. We could rewrite eq I as
Measurements of absolute light intensity permitted us to calculate &G. Table I11 shows that doG and therefore G are independent of both oxygen and 1octanol. Using $0 = 0.35 we get dc = 1.20E. Table 111 shows the error of our absolute light intensity measurements. T o get a reasonable accuracy
Table 111: Chemiluminescence Light Function Determinations for TMAE below 6 X lo-' M
(HA)o, M
Poi, torr
No. of determinations
8.49 13.9 25.0 25.0 7.3 21.0 35.8 35.8 50.0 50.0 207.8 207.8 575. 575. 961. 961.
700 700 7 12 1407 800 1400 708 1400 700 1400 58.1 162.0 23.2 118.7 20.7 44.1
4 3 14 17 5 6 9 12 11 9 11 3 5 6 10 5
1-Octanol
x
IO*,
+DOC,
einstein 1. mole-'
0.61 0.59 0.40 0.41 0.55 0.45 0.45 0.41 0.38 0.41 0.33 0.37 0.24 0.37 0.19 0.56 Av
0.42f0.03
s? '0 EO.Ol,U
n
0.004M I
,
,
,
500 1000 OXYGEN PRESSURE,TORR
Figure 2. Relative I / E a VI. oxygen pressure at four l-octanol concentrations. At these low TMAE concentrations, the value of the function Z/Ea does not depend on the TMAE concentration. Lines represent curves calculated from eq 11, 111, and IV before photometry system correction was made.
for $oG, we took 130 measurements. The fit of data to calculated curves using eq 11-IV is shown in Figure 2. To reduce the variables for each run, we plot the ratio I / E 2 which remains constant for each run. Note that the light intensity becomes essentially independent of oxygen pressure a t higher pressures. The variable I / E 2 now was only dependent upon the alcohol function F(HA)-the same function as given for autoxidation in eq 111. We can now write an equation for light intensity which is valid over a wide range of TMAE, 1-octanol, and oxygen concentrations. However, if we extrapolate to higher TMAE concentrations, a contradiction appears. That is, at E = 4.3 (neat TMAE), we get do = 5.2. Since 9. cannot exceed unity, we had to extend our kinetic measurements to concentrations M. above Chemical Eficiency at Higher Concentration-The Full Chemiluminescent Equation. At high TMAE concentration, several experimental difficulties must be overcome. The oxygen in the solution rapidly becomes depleted. Reaction products build up and both quench the excited species' and poison the 1-octanol catalyst by hydrogen bonding with it. We were thus limited to short runs using calculated values of dE/dt while relative light intensity measurements were taken. In this fashion, we obtained values of I/Ez up to 0.22 M TMAE. Volume 71, Number 6 April 1067
1512
AARONN. FLETCHER AND CARLA. HELLER
It was evident that the values fall off at higher concentrations of TMAE. To fit these data, we assumed the form, +o = +,'E/(E C ) . This form is suggested by the mechanism we will propose. We can substitute the above into eq I (expanded) and rearrange to get g(E C> = F(H-4) X (02) X E*/(light)(l+ CKiQt), where the g factor contains all geometrical and instrumental constants. Figure 3 shows the plot of the values vs. E and a least-squares-determined line which has a linear correlation coefficient16of 0.97. The slope is g and the intercept gC, which permits us to calculate C = 0.087. To get +o, we use the fact that a t low TMAE concentration (where C >> E ) , 4, = +,'E/C = 1.20E. Solving gives 4,' = 0.104. Using the above values, we can now write a complete kinetic equation for light intensity. This will be in the form of eq I.
+
+
.-
-
1
9 - 0.7 Y Y
-
0
0.0194 M
t
0.0188 M
A
0.0138M
1
X
c
-
0.6
I
2
2w 4
-
0.5
-
0.4
-
s
I-
X
X I
4
I
Z 0.3
-
0.2 0
1
1
1
1
t
0.I
I
I
I
I 0.2
TMAE. M
Figure 3. Light emission function a t high concentrations of TMAE for varying concentrations of 1-octanol monomer. Light is a relative value measured by the photomultiplier current.
E
+ 0.087 X
lO-')(HA)i
[(7.58 X lo+)
+ (1.14 X
+ (3.48 X
102)(HA)4] X E X (02) (V)
Concentrations are given in molarity with the same symbols as earlier. Oxygen pressure, measured in torr, was converted to molarity by the solubility conM-' torr-'. Alcohol concentrastantla 0.0132 X tions were calculated as in ref 5. Thus the constants in F(HA) are third-order rate constants since the alcohol association constants have been accounted will include a surface for. Of course the 7.58 X concentration of protons on the glass and be in M-' sec-'. The constants in eq V are given to three significant figures in order to ensure consistent results among the various calculations. We feel that the accuracy is within a *25Q/, relative error, over a range of lo6 in light intensity. Figure 4 shows calculated values of the chemiluminescent quantum yield (+e X 4,) and of light output over a range of TMAE concentrations. For this calculation, oxygen was held at 150 torr. The solid lines represent the region in which we have measured values of I and E. This figure is given to permit comparison of the TMAE system with other chemiluminescent systems. It illustrates the form of the mathematical solutions to eq V. An interesting minor point is that the wall enters The Journal of Physical Chemistry
the oxidation rate factor, but not the quenching factor. Experimental difficulty in measuring quenching constants is the reason for this difference. Temperature Eflect on the Over-All Reaction. In our static system, we measured only relative light inteaity vs. time. Figure 5 shows a typical run (ignoring the dashed line temporarily). When the reaction reaches a steady state, the decay of light with time should obey eq V. To check this, we can differentiate eq V to get (remembering only E and I change with time)
For our runs, initial E was between 0.1 and 0.01. Essentially, it was always smaller than C during the measurements so that
Measured values of kobed agree with the more accurate values of eq V a t 30". Of more interest was the temperature effect which essentially is that for F(HA). The over-all activation energy is 1 * 1 kcal mole-'. Thus F(HA) and the oxidation rate show little or no temperature dependence. (16) P. G. Hoel, "Introduction t o Mathematical Statistics," 2nd ed, John Wiley and Sons, Inc., New York, N. Y., 1954,p 117.
OXIDATIONAND CHEMILUMINESCENCE OF TETRAKIS(DIMETHYLAMINO)ETHYLENE
1513
MOLAR CONCENTRATION OF TETRAKISfDIMETHYLAMIN0)ETHYLENE
Figure 4. Calculated chemiluminescence quantum yield and light output as a function of the TMAE concentration for three concentrations of added 1-octanol. Solutions are at 30" and are in equilibrium with 150 torr of oxygen. Dashed lines represent values calculated (by eq V) outside the region were experimental values were obtained.
The justification for using eq VI1 and the static system for temperature effect measurements is the speed of operation. It also provides a check of the usefulness of eq V. Kinetic Measurements of the Intermediate Decay. When oxygen is removed rapidly from the static system, the light first increases end then decays (Figure 5, dashed line). We have measured this decay and have also carried out experiments to show that a true longlived intermediate is present. A t first glance, it appears possible that residual oxygen in the system accounts for the light. How-
ever, a slow removal of oxygen does not give an increase in the intensity of light. The sudden increase is explained best by the long-lived intermediate which continues to produce excited molecules in a solution with the strong quencher 02 removed. Other evidence for a real intermediate which decomposes to give light comes from low-temperature experiments. TMAE in hexane could be oxygenated at -30'. The oxygen was removed by flushing with helium for several minutes. Rapid warming gave a burst of light. Also, we could form the intermediate in n-decane a t Volume 71, Number 6 April 1987
1514
AARONN. FLETCHER AND CARLA. HELLER
TIME (MINUTES)
Figure 5 . Steady-state and intermediate decay curves; 2 ml of n-decane solution: TMAE = 0.01 M , 1-octanol = 0.01 M . Solid curve: P o n = 500 torr, rate constant (kobad) measured from A to 9.5 min. Dashed curve: a t point A oxygen was pumped off the stirred solution; rate constant (k') measured from 2 to 4.5 min Ends of both curves show where light level went below 10% of most sensitive photometer scale.
octanol concentration. At low T-IIAE concentrations, the energy of activation is 18 kcal mole-' and at high TMAE concentrations it is 12.5 kcal mole-'. This variation of the activation energy with the TMAE concentration must be explained by the mechanism. The considerable difference between the energy of activation for the light decay k' and that found by kobsd of the over-all reaction also shows that a different mechanism is involved in the two reactions. Energy of Reaction. The over-all reaction to form the major products was found to produce 140 kcal mole-'. This is considerably more than the 72 kcal needed to produce one quantum of light at 400 nm. Thus there will be no need of summing quanta of two reactions. Also, the main reaction sequence can provide the chemiluminescent step and we need assume no extra energetic side reactions.
Mechanism and Discussion
O", remove oxygen, and quickly cool to -30" where ndecane and TMAE are solids. A weak light could be seen for hours from this solid. Finally, in the n-decane solutions, we looked for any effect of continued stirring and pumping after initial oxygen removal. Quiescent or stirred and pumped solutions gave identical decay curves. This indicates an intermediate which does not decompose reversibly to give 0 2 , but only irreversibly to give light. The actual decay curves are first order in light so that -dI/dt = k ' l . The decay can be followed for two to three orders of magnitude of I. Table IV shows typical values of k' so measured. The values of k' are weakly affected by 1-octanol concentration and strongly affected by TMAE concentration and by temperature. Table IV : Decay Rates of Intermediates E,' '1.i
1-Octanol,"
0.01 0.1 0.1 0.43 0.43 0.43 4.25 4.25 4.25
0.2 0.2 0.2 0.02 0.04 0.1 0.1 0.1
M
0.1
Temp, OC
0.0 11.3 24 0.0
0.0 0.0 0.0 12.7 25
k', 8ec -1
0.0056 0.0228 0.052 0.0025 0.0031 0.0050 0.026 0.07 0.175
' Initial concentration.
From the data we have calculated activation energies. These vary with TMAE concentration, but not with 1The Journal of Phyeical Chemistry
Mechanism. We have picked the simplest mechanism which will fit all of the known facts. It is based upon the ions which were found by Urry and sheet^.^ In n-decane, these would be expected to form solvated ion pairs of triplets. We will write the mechanism in its basic form and discuss some details and complications later.
E
E.02
+
0 2
= Ea02
+ HA + [E2+02H-A-]soiv
+ E = [+E-E+OzH-A-]soiv [E2+02H-A-],01v +products + HA
[E2+OzH-A-]soiv
[+E-EfozH-A-]solv
(1) (2) (3)
(4)
f
E* or E
+ products + HA
(5)
We have not included the emission and dark reactions of E* since those will obviously yield as in eq 11. This mechanism is similar to the oxidation mechanism proposed in ref 5. We have added the coupling reaction 3 and the chemiluminescent reaction 5. These combine to provide the energy coupling necessary to explain chemiluminescence. Reactions 4 and 5 also explain the two major sets of oxidation product^.^ We will now examine this mechanism to show how it fits the available data. We will make two assumptions in order to obtain kinetic expressions from our mechanism. The first is that reaction 1 is rapidla enough to maintain a pseudoequilibrium with the charge-transfer complex, E * 0 2 . Thus E.02 = K1(E)(02). The second is the usual stationary-state assumption that d(intermediate)/dt = 0. This assumptionl7 appears reasonable at room temperature where k4 and ks are large relative to F(HA).
OXIDATION AND CHEMILUMINESCENCE OF TETRAKIS(DIMETHYLAMINO)ETHYLENE
Solving the steady-state equations
(-$)
= K1k2(HA)(E)(O2)
(VIII)
1515
about the reactions which simplify and permit mathematical solutions. Thus reaction 3 and its reverse may be considered either frozen, or to be in pseudoequilibrium, so that
Using this second assumption gives We have used the notation that k, and IC-, represent rate constants for forward and back reactions and K is an equilibrium constant. The 46 is the chemiluminescent quantum efficiency of the elementary reaction 5.
Comparison of the calculated eq VI11 with the measured kinetics of eq I11 shows agreement. It must be remembered that in practice HA is replaced by the polynomial F(HA) due to alcohol self-association. The emission factor, &,, will contain a different polynomial, f(HA). Equation VI11 is the same as that obtained for our oxidation mechai~isrn.~The rate depends upon only the production of ions once a steady state is reached. The extra TMAE molecule involved in reaction 3 is not oxidized and does not enter the oxidation stoichiometry or kinetics. This mechanism is consistent with the low activation energy found for the over-all reaction. There will be two temperature effects. The equilibrium quotient K1 has only a small AH1 temperature effect,13 but k2 presumably has a reasonable activation energy. The alcohol term, F(HA), actually contains three kz’s, one each for wall, monomer, and tetramer. Temperature definitely affects the alcohol self-association. At low temperature there will be more of the tetramer which is a better catalyst.5 Thus the two effects tend to compensate and give the observed low activation energy for the over-all reaction. The decay rate when oxygen is removed should involve only reactions 3-5 of our mechanism. The lack of effect of pumping on the stirred solutions show that reaction 2 is essentially nonreversible. This system of four reactions (since reaction 3 is reversible) cannot be simplified by a steady-state assumption. A complete solution’* of the differential equations will have the form
I
+ &eXzt
(-$)
+ k&3E1
Jcd
=
1
+ K3E
(X)
This equation indicates that the observed rate constant will approach k4 a t low TMAE concentration and ka a t high TMAE concentration. The physical meaning of this is that large concentrations of TMAE force the reaction toward the chemiluminescent path. It will be recalled that the activation energy for light decay did change with TMAE concentration. Both eq X and the physical picture predict this change if reactions 4 and 5 have different activation energies. The results give the activation energy for reaction 4 being greater than or equal to 18 kcal mole-’ and the activation energy for reaction 5 being less than or equal to 12.5 kcal mole-’. Not only does the change of activation energy with concentration fit the proposed mechanism, but also it conflicts with an energy-transfer mechanism. In such a mechanism, the form of c $ would ~ arise from the competition between the two reactions of the first electronically excited product, P*
+
P* +P heat P* E -+ P E*
+
+
(6) (7)
It seems very unlikely that these reactions would have large activation energies and certainly the value for reaction 6 would not be larger than that for reaction 7. Only one activation energy would be foundthat for the formation of P*. Thus these data reinforce the conclusions from the quenching experimentss to the effect that electronically excited TMAE is formed directly in the oxidation. It was noted (Table IV) that alcohol affects the decay rate constants. This is a small effect and appears likely to be a solvent effect. However, this might be expected to affectthe constant
= aleX1t
The parameters a and A cannot be found without knowing initial conditions which are not presently available. However, the observed first order means that we are operating where one term is large relative to the second term. Either of two opposite assumptions can be made
of eq IX. The fact that no such effect was found (17) S. W. Benson, “The Foundation of Chemical Kinetics,” McGraw-Hill Book Co., Inc., New York, N. Y., 1960, p 50. (18) See ref 17, pp 39-42.
Volume 71, Number 6 April I967
AARONN. FLETCHER AND CARLA. HELLER
1516
(Figure 4) may reflect the cancellation of effects in C or our admittedly poor accuracy in the high TMAE concentration measurements. This relatively simple mechanism fits all the kinetic data which we have measured. We now wish to show that the chosen intermediates are chemically and energetically reasonable. Intermediates. There is, as yet, no direct identification of the intermediates during oxidation in nonpolar solvents. Our quenching studies6 gave strong evidence that at least one intermediate is ionic. The known c h e m i ~ t r y ~ ~of' ~TMAE * ' ~ ~ ~also ~ makes ions seem likely. In nonpolar solvents, ions will occur as solvated ion pairs as shown in our mechanism. One species used in our mechanism requires special mention since it has not been identified as yet. Only our kinetic data give evidence for the dimer dication, +E-E+. Such dimers had been identified in anionradical reactions2' involving electron transfers. It seemed a reasonable intermediate in the equilibrium +E-E+ 1 '2.Ef E E'+ This also suggested that the cation radical, .E+,should be seen during autoxidation, as is indeed the case.22 The cation-radical esr signal intensity continued when oxygen was removed, suggesting that a stable species such as [.E+ (OR-) ]solv was formed in a side reaction. We used the electrochemical data to calculate energies of formation of various ions. These calculations can serve two purposes. First, they could show which species are most likely to form. Second, they could show whether the intermediates contain a reasonable proportion of the over-all reaction energy. The over-all energy of 140 kcal mole-' is a weighted average of reaction paths a and b. At least 72 kcal mole-' must remain to produce the electronic excitation and some energy will surely be "wasted" as vibrational energy. We can calculate the upper limit of the energy of formation of our intermediates using the electrolytic cell potentials for TMAE" and oxygen12in acetonitrile and dimethyl sulfoxide.
+
+ 02 + *E+O2*E + 02 +E2+0z2E
AF' (298'K)
(-0.9 kcal) (81.4 kcal)
+ C6HsOH + + 2(.E+) + H02- + CeHsO- (5.1 kcal) E + CaHbOH + E2++ H02- + CaH60- (-23 kcal)
2E
(8)
(9)
0 2
(10)
0 2 +
(11) These values of - AFO will be smaller in n-decane, but should maintain similar relative positions. Thus reaction 11, which is similar to our reaction 2, is the The J o u T of ~ Phyeicd ~ ~ Chemietry
only likely reaction. Even it will not be as favorable in n-decane. The apparent irreversibility of reaction 2 may be due to subsequent solvation of an initially formed ion pair. Stepwise solvation would lower the energy of the ion pair without necessarily affecting the kinetics. Part of the reason for the large reaction rate constant for the alcohol tetramer might be partly due to such ~ o l v a t i o n . ~ It can be seen that ionic intermediates are energetically reasonable and possible. It remains to be considered how the chemiluminescent reaction converts the chemical energy to electronic excitation. The Chemiluminescent Reaction. As written, reaction 5 is a unimolecular reaction producing an excited species. The first-order kinetics provided one reason for considering the reaction unimolecular. The fact that the reaction continued in frozen solutions also argues for a unimolecular reaction. The small effect of alcohol can be explained as a solvent effect. The quantum yield of our proposed unimolecular reaction was 0.104 = &' = &. Its activation energy was 12.5 kcal mole-'. The preexponential factor for reaction 5 will vary with alcohol concentration. A t 1-octanol added concentration of 0.1 M the rate constant is ks = 2 X lo*exp( - 12.500/RT) sec-'. It was possible to write a unimolecular reaction which appeared reasonable. Criteria for chemiluminescence are sufficient release of energy plus some reason for forming an electronically, rather than vibrationally, excited state. Chandross and SonntagZs have suggested that matching geometric configuration of the reactant and excited product molecule should increase the chemiluminescent probability. A principle of least motion might also be involved. Reaction 5' can be seen to fulfill such requirements. One of the two TMAE molecules in reaction 5' receives two electrons and must receive at least 72 kcal mole-' for it to be produced in an electronically excited state. Only electrons need to be moved in this reaction. The tetramethylurea which is formed will be vibrationally excited but with reasonable atomic spacing. The activation energy of reaction 5' can be due to the need to bring the OOH- closer to one of the TMAE molecules or else be due to the transfer of the proton to the alcohol before reaction (or both). Since there is a very large Stokes loss from the absorp(19) D.M.Lema1 and K. I. Kawano, J . Am. Chem. SOC.,84, 1761 (1962). (20) W.Carpenter, J. O T ~Chem., , 30, 3082 (1965). (21) J. Jagur-Grodrinski and M. Szwarc, PTOC. Roy. Soe. (London), A288, 224 (1965). (22) D.W.Moore, personal communication, 1966. (23) E. A. Chandross and F. I. Sonntag, J . Am. Chem. Soc., 8 8 , 1089 (1966).
OXIDATION AND CHEMILUMINESCENCE OF TETRAKIS(DIMETHYLAMINO) ETHYLENE
tion band at 270 nm13 to the photoluminescence emission band a t 485 nm, it is reasonable to expect that the geometry of the ground-state TMAE molecule is different from the geometry of the excited-state molecule. Since some physical distortion of even the cation radical, .E+, can be expected,” we believe that an important feature in the strong chemiluminescence of the TMAE autoxidation is due to the TMAE molecule, as it is formed in reaction 5’, being twisted and consequently being more closely related to the geometry of the electronically excited TMAE molecule than it is to the geometry of the centrosymmetric’ ground-state molecule.
N (CHd2
(CH3)ZX
+\G-c/
i\,
/Y
(CHS)SN
E*
+
N(CH3)2 4
(CHd2N
\ c=o
2
(5’)
/
+ c-c /Y
ROH \
(CH3)2N -0-0
\ N(CH3)2
\
H
R-OWe have written reaction 5‘ as producing tetramethylurea. Our reason for this is based on energetics. The other products consist of tetramethyloxamide and two amino radicals. This formation of radicals would seem to release less energy and be less favorable for chemiluminescence. However, this assignment of tetramethylurea to the luminescent path is very tentative. Comparison of Results. There have been two earlier studies1j2of the TMAE chemiluminescence with which we can compare our results; both gave integrated chemiluminescence quantum yields and one presented kinetic data. The quantum yields can be compared by integration of our eq V within the limits presented in the other works. Winberg, et al.,l reported an integrated quantum einstein mole-’ for pure TMAE over yield of 3 X water under air. Since the reaction products dissolve in water, the TMAE concentration remains almost constant with time. Quenching of the excited TMAE is almost entirely by O2 since the HSO solubility in TMAE is Iow. We calculate a yield of 2.1 X loa3for
1517
these conditions. The tenfold discrepancy can be accounted for by the results of Urry and sheet^.^ They found that the major initial products for the twophase TMAE-H20 system were TMAE2+ and hydroperoxide ion, while the major products from TMAE alone or dissolved in a hydrocarbon solvent was tetramet hylurea and tetramethyloxamide. Since light is emitted only from the TMAE phase of the TMAEHzO system, the differences in quantum yield can be explained by considering that the small portion of the TMAE which leads to tetramethylurea or tetramethyloxamide is related to the light-emitting step in the TMAE phase, whereas the formation of the separated TMAE2+ and hydroperoxide ions is a dark reaction as shown in our mechanism. Thus our higher calculated chemiluminescence quantum yield is due to the difference in the two systems under consideration. Paris2 has reported an integrated quantum yield of 3.7 X on complete consumption of TMAE with an initial concentration of 0.030 M in cyclohexane. His catalyst was 0.16 M methanol and the solution was under 760 torr of oxygen. We calculate a yield of 8.8 X 10-5 for his conditions where oxygen is the principal quencher. We believe the difference in the results to be due to his not correcting for the ( I trapping” of r a d i a t i ~ n ~ within ~ p ~ ~ the spherical vessel that he used in his work. Paris reported second-order kinetics for both TMAE and added methanol. We agree completely with the TMAE second order a t the lower concentrations but not with those for methanol. Paris corrected his light intensity measurements for the quenching due to the added methanol. However, he used photoluminescence quenching constants which have been shown to be lower than those for chemiluminescence in the case of TMAE.6 Furthermore, these corrections should have been added to the large oxygen-quenching term which he totally neglected. It would appear that by working over a limited portion of the complex curve for light intensity vs. added alcohol (combined with the above factors), Paris obtained a second-order relationship. Comparison with Earlier Mechanisms. Two earlier mechanisms of TMAE oxidation and chemiluminescence have been suggested. 2 , 4 The intermediates suggested by Urry and Sheeto4 differ from ours in being zwitterions rather than ion pairs or triplets. We favor the ions shown because no energy is lost forming covalent bonds until the elementary chemiluminescent reaction occurs. Also, zwitterions have not been (24) R. H. Gillette, Reg. Sci. Instr., 21, 294 (1950). (25) A. Shepp, J. Chem. Phys., 25, 579 (1956).
Volume 71, Number 6 April 1967
1518
NOTES
found analytically. However, the kinetics would be the same if we used zwitterions in the mechanism. Paris2 presented a completely different mechanism which fit his chemiluminescent kinetics. His work on TMAE2+in strong alkali suggested carbene radicals as intermediates. We consider the carbene mechanism unlikely on the following grounds. First, Urry and Sheeto's results in alkali media indicate that carbenes are not involved. Second, Carpenter2s was unable to find unsymmetric tetraaminoethylenes when a mixture of two different tetraaminoethylene dications reacted with alkali. Finally, Paris has not shown the existence of any of his reaction intermediates and must
use a large number of mechanistic assumptions in order to achieve a kinetic relationship which does not fit the present experimental data.
Acknowledgments. Many discussions of this work with Professor W. H. Urry and Dr. R. H. Knipe are gratefully acknowledged. Mrs. Edith Kirk performed much of the experimental work. Dr. C. H. Shomate performed the calorimetric work, and E. M. Bens performed the gas chromatographic analyses. This research was supported by the Bureau of Naval Weapons and the Army Munitions Command. (26) W. R. Carpenter, unpublished data.
NOTES
The rate expressions are
The Relative Importance of Homogeneous
R,'
and Heterogeneous Reactions'
Aerospace Corporation, El Segundo, California (Received October 7, 1966)
I n many problems of gas-phase kinetics, both gasphase and wall reactions play a role. I n order to understand the detailed chemistry, it is necessary to know the relative importance of the homogeneous and heterogeneous reactions. If both reactions are first order in some species C, then a crude approximation for the ratio of the gas-phase reaction R , to the wall reaction R , is N
k,Ro2/4D (for an infinite cylinder)
R,/R,
-
k,Ro2/9D (for a sphere)
(la) (lb)
where k, is the first-order homogeneous rate constant, D is the diffusion coefficient, Ro is the radius of the vessel, and Ro/2 and Ro/3 are, respectively, the ratio of volumeto-surface area for infinite cylindrical and spherical reaction vessels. Although eq 1 is useful for crude approximations, it may be considerably in error for many problems. Therefore, for more accurate results, it is necessary to derive and tabulate the correct expressions. The Journal of Phyacal Chemistry
k,C(r}
(2d
R,' = k,C{Ro) (2b) where the primes on R,' and R,' indicate local reaction rates, C ( r ] is the concentration of C a t some radial distance r, and C( Ro ] is the concentration of C at the
by J. L. Hudson and Julian Heicklen
R,/R,
=
wall. The wall reaction rate constant k , has the dimensions of velocity. The applicable diffusion expression is
0 = DV2C
+ R' - k,C
(3)
and is subject to the boundary conditions
-D dC/dr C
=
k,C
= finite
(for r = Ro)
(for r
=
0)
where R' is the uniform homogeneous rate of production of C throughout the vessel. In order to solve the diffusion equation, it is convenient to work with dimensionless quantities. Therefore, we define
K,
k,Ro2/D
(4%)
K, kwRo/D (4b) Direct evaluation of eq 3 leads to expressions for C ( T ): for an infinite cylinder _ _ _ ~ ~
(1) This work was supported by the U. S. Air Force under Contract No. AF 04(695)-669. The authors wish to thank Mrs. V. M. Armstrong for her assistance with the manuscript.
