1940
J. L.
Charlton and J. Saltiel
An Analysis of trans-Stilbene Fluorescence Quantum Yields and Lifetimesla James L. Charlton*'b and Jack Saltlel" Depaement of Chemistry, Florida State University, Tallahassee, Florida 32306 (Received February 1, 1977) Publication costs assisted by the National Science Foundation
Participation of equilibration of planar, It*, with twisted (phantom), lp*, conformations of the lowest excited singlet state of trans-stilbene was evaluated by analysis of the temperature dependence of fluorescence quantum yields. A computer assisted curve fitting technique was used to generate kinetic parameters consistent with measured quantum yields. The derived parameters indicate that the majority of the fluorescence occurs with very short lifetime at room temperature and above (e.g., 73%, 0.07 ns at 25 "C),rather than with the much longer fluorescence lifetimes which were measured recently. The magnitude, but not the trend, in the latter was accounted for with the derived parameters by choosing an appropriate value, kd, for the rate constant of 'p* decay, and assigning the measured lifetimes to the smaller delayed fluorescence component. The calculated and parameters are E,, = 4.07 kcal/mol, At, = 1.36 X 1013s-', EPt= 7.33 kcal/mol, kd/APt= 1.22 X = 6.4' X lo7 s-'. Use of k d = 6.05 X 10' 8-l is consistent with the longest reported high temperature lifetime.
Introduction Malkin and Fischer's2 and Dyck and McClure's3initial investigations of the temperature dependence of transstilbene fluorescence, Cpf, have been confirmed and refined by several more recent s t u d i e ~ . ~In- ~accounting for these and other observations concerning stilbene photoisome r i ~ a t i o ntwisting ,~ in the first excited singlet state of trans-stilbene was postulated as the activated process competing with fluorescence, eq 1-5, where 12, = 0 and the
shown that the time dependence of fluorescence intensity,
I&), is given by eq 10 where the X and C parameters are
I f ( t )= kf(Cle-'lt
+ C2e-'zt)
(10)
defined in eq 11-14. The function If(t)can be integrated hl.2
= 1/2{(x 4ktpkpt
+ kpt + kd) 7
[(kpt
+ kd - x ) 2 +
I' 'Y
(11)
la(f) I t
t hukf
It*--+
(1)
It*
ktp
It*+
+--P1
+ + ktp
x = kf
't t hu
ki,
from t = 0 to t = m to give the fluorescence quantum yield expression 15. Since the quantum yield is time inde-
*
kPt his
It*_,
(14)
3t*
[2 + 31
(4)
Gf
(5)
pendent it does not depend on I&) being a 6 pulse. Indeed a little algebraic manipulation reduces eq 15 to the more familiars steady-state-derived expression 16. The ad-
= kf
kd
P *-+ ru't t (1- @ ) I C
symbols have been defined previously.6 This mechanism predicts that the temperature dependence of Cpf is given by eq 6, an expression first applied to the data by Dyck q5f = kf/(kf + his Atpe-Etp lR T
+
1
(6) and McClureS3The rate constants for radiative decay, kf, and intersystem crossing, kis, are assumed to be temperature independent. A consequence of this treatment is the prediction of very short stilbene fluorescence lifetimes at moderate temperatures, eq 7,'O which appears to be in rf = l / ( k f + ki, + A t p e - E t p ' R T 1 conflict with the much larger fluorescence lifetimes reported recently.* To explain the discrepancy Birch and Birks suggested that the twisting process, eq 3, is reversible.* This modified mechanism gives rise to two coupled differential equations, eq 8 and 9. The solution to these
d['t*]/dt = I a ( t )- ( k f + ki, + k P t [' P*l
d['P*l/dt = k t p [ l t * I
- (h,t
ktp)['t*]
+ kd)['P*l
+ (8)
(9)
two equations for a 6 excitation pulse ([%*I = [%*loand ['p*] = 0 at time 0) has been given elsewhere8J1and it can be The Journal of Physical Chemistry, Voi. 8 1, No. 20, 1977
@f = Izf/lkf
+ kis + k t p k d / ( k p t + k d ) l
(16)
vantage of eq 15 is that each C,/X, term represents that amount of light emitted with time constant l/X,, whereas eq 16 does not readily yield information as to the time dependence of the fluorescence yield. While fast and slow decay components were present in the fluorescence of trans-stilbene above -40 "C, instrumental limitation allowed the detection of the former but not its determination.8 Since fitting I&) to eq 10 was not possible Birch and Birks chose to assume k,, kpt >> kf+ ki,, kd thus reducing eq 10 to a single exponentials whose time constant is given by eq 17, where Ktpis the equi-
librium constant k,/k,, for the twisting process and k, = kf ki,. By also applying the assumption of rapid equilibration between It* and Ip*,kPt >> kd, to eq 16 and combining it with eq 17, eq 18 was derived from which
+
@f/T
=
kf/(l + Ktp)
(18)
Analysis of trans-Stilbene Fluorescence
19141
TABLE I: Observed and Calculated trans-Stilbene Fluorescence Lifetimes
T.K
Obsd T . ns ~
333.2 323.2 313.2 303.2 298.2 273.2
1.65 1.62 1.60 1.57 1.54 1.37
TABLE 11: Parameters for Lifetime Fits t o Eq 19 Parameter
Calcd 7.b
ns
1.721 1.721 1.721 1,720 1.720 1.719
cal deg-' mol-' AHtp; kcal mol-'
AStp,
ns
T . ~
1.646 1.628 1.602 1.566 1.543 1.372
ka, s-
k,, s-'
From ref 8. From eq 17 using the parameters in ref 8. From eq 19 using AStp = -27.6 cal deg-' mol-', AHtp = -7.11 kcal mol-', k , = 5.89 X l o 8 s-' and k d = 1.04 x 109 ~ 1 . a
Birch and Birks calculated AH = 1.75 kcal molP and AS = 10.6 cal mol-' deg-l. Values of kf = 6.0 X IOs s-' and k d = 5.8 X lo8 s-l were obtained independently as limiting parameters at low and high temperatures.8 It was concluded that lp*is the dominant species in the temperature range from -40 to 60 "C (,f*' = 0.82-0.94).8 Several difficulties are immediately apparent with the Birch and Birks interpretation. Central to these is the assumption that the observed but not defined fast decay component does not contribute significantly to the fluorescence quantum yields, i.e., Cl/hl >> C2/Xz. This assumption is implicit in the use of eq 18 and requires that nearly identical fluorescence spectra and quantum yields be observed from cis- and trans-stilbene in the temperature region for which eq 18 holds, provided that IC* lp* twistingg represents the major decay process of the first excited singlet state of cis-stilbene. This requirement is in conflict with experimental observation as was first pointed out by Lewis, Magel, and Lipkin.12 Secondly,the observation that the fluorescence quantum yield of trans-stilbene in n-pentane at 25 "C is nearly unaffected by the presence of an atmosphere of oxygen over the solution (r#$/+f = 1.03 f 0.02)13is inconsistent with the large lifetime attributed to it. The possibility that Ip* is not quenched by oxygen does not seem viable particularly in view of the very efficient oxygen quenching of 3p*.14It was these difficulties which prompted the present analysis of the data.
-
Discussion Consideration of eq 17 readily shows that the kf and k d values derived by Birch and Birks are inconsistent with the observed high temperature lifetimes. If, as assumed, equilibrium between I t * and Ip* is complete then 7 must fall between l / k f (1.67 X lO-'s; ki, = 0 was assumed) and l/kd (1.724 X lo-' s), regardless of the magnitude of Ktp. Indeed, substitution of the Birch and Birks parameters into eq 17 does not generate the observed lifetimes as shown in Table I. It appears that the uniqueness in 7 and & is lost when they are combined in eq 18 as 4f/7. Also, incorrect values of kf and k d may have been generated in the extrapolations to high and low temperature limits, In order to avoid both eq 18 and the extrapolations for kf and kd a fit directly to eq 17 for the six highest temperature lifetimes was attempted. Equation 17 was rewritten as eq 19 with four parameters AS,,, AHtp, k,, and kd. Fitting of this equation to the observed lifetimes was 1+ eAStpIRe-AHtplRT 7 = k , + kdeAStplRe-AHtp/RT (19) carried out using DeTar's GENLSS iterative least-squares computer ~r0gram.l~ Since the operations of interchanging kd and k, and simultaneously changing the signs of AH,,
Fit 1 -25 to -32
Fit 2 + 2 5 to t 32
-5.1 to-7.1 5.80 X l o 8 to 5.89 X l o 8 1.04 X l o 9 to 1.4 X 10''
t 5 . 1 to +7.1 1.04 X l o 9 to 1.4 X 10" 5.80 X 1 0 8 t o 5.89 X l o 8
and AS,, leave eq 19 unchanged, eq 19 can a priori be satisfied by two families of solutions: (1) AHtp< 0, k, < kd and (2) A",, > 0, k, > kd. Satisfactory fits were obtained with several sets of the four parameters in eq 19, but the ranges of the parameters for the two families of solutions fall within the limits given in Table 11. Calculated lifetimes for a specific set are given in Table I to illustrate a typical case. Both families of solutions can be shown to be inconsistent with fluorescence quantum yields and photoisomerization observations. The first has k, values which are encouragingly close to kf = 5.9 X lo8 s-' calculatedL6 using the Birks-Dyson re1ati0nship.l~ However, when the predicted equilibrium constants are substituted into eq 18 the calculated fluorescence quantum yields are close to unity for the temperature range in Table I. The second family of solutions gives parameters which when substituted into eq 18predict fluorescence quantum yields which are in better agreement with observed values, however, the large k, values obtained require that ki, be much larger than expected from photochemical observationsg and low temperature fluorescence quantum The most obvious flaw of all these fits is the prediction of lASl 1 25 cal mol-l deg-l, a value which is clearly too large for a unimolecular equilibrium. It is, therefore, concluded that the assumption of ktP, k,, >> k,, k d leading to eq 17 is incorrect. If the observed If(t)represents single component exponential decay it should be associated with the slower component of eq 10, 7 = l / X l , since this is precisely the component of delayed fluorescence without the assumption that k,,, kPt >> k,, kd. However, since X1 contains six parameters and the lifetime data are limited one might expect extreme difficulty in fitting the temperature dependence of l / to~the X1 equation even with computer assistance. It seemed prudent to turn instead to an analysis of the fluorescence quantum yields since these are experimentally easier to obtain and data obtained by several groups2-' are in reasonable agreement and are much more sensitive to temperature changes than the reported fluorescence lifetimes.8 The fluorescence quantum yields of Sharafy and M u s ~ k a textrapolated ,~ to 298 K, combined with the appropriately scaled data of Birch and Birks8 (see Appendix A) were fit to eq 20 which is equivalent to eq 16
with k,, and kptexpressed with corresponding Arrhenius equations. In treatments of trans-stilbene fluorescence a constant value has generally been assumed for kf,2-6,8 However, recent experimental observations suggest strongly an index of refraction dependence for this parameter in agreement with theoretical prediction~.~' Accordingly, kf values were calculated assuming proporThe Journal of Physlcal Chemlstry, Vol. 8 1, No. 20, 1977
J. L. Charlton and J. Saltiel
1942
TABLE 111: Parameters for Eq 20
TABLE IV: Observed and Calculated Fluorescence Properties
Parameter
Value
Std dev
1% A tp Em, kcal/mol log kd/Apt E,,, kcal/mol
13., 4.0, -4.9, 7.3, 6.4,X 10’
0.2 0.2 0.8 1.3 1.2 x 107
his
I
90
I
130
I
I
I
I
I1
170
210
250
290
330
T,OK
T.K
+
tionality to n2 as described in Appendix A. Parameter values which give the best fit of eq 20 to the 4s; were found using GENLSS,“ Table IIL20 Using a fixed value of 6.0 X lo8s-l for kfalters the values of the parameters by less than 10% indicating that the index of refraction correction is relatively unimportant. A discussion of the standard deviations in Table I11 and their significance is given in Appendix B. Observed and calculated 4s; are given in Table IV and plotted in Figure 1. Significant deviations occur only at very low temperatures suggesting a medium effect close to a phase transition (possibly a decrease in hi, in this region). The parameters At, and Et, are insensitive to large changes in the high temperature & values in contrast to the parameters kd/A,t, E t, and kiB. Also shown in Figure 1are 4f values calculatec! by substituting the above parameters into eq 6. The best fit parameters and an assumed k d can be used to generate values for C1, X1, C2,and X2. These values and eq 15 can be used to compute the fraction of fluorescence, f 2 , associated with the shorter-lived “prompt” fluorescence lifetime (1/X2), eq 21. These calculations were carried out
(C2/b)/[(Cd~I+ ) (CZ/b)l
(21)
using k d = 6.05 x lo8 s-l since this value gives l / X 1 N 1.65 ns which is nearly identical with the observed fluorescence lifetime at the highest temperature. Calculated XI-’, XZ-l, and f z values are shown in Table IV. Changing k d does not significantly alter X2-l or fz, but results in a direct change in A1-l, i.e., k d N X1 at all temperatures. Conclusions The analysis of the temperature dependence of & indicates that at least 40% of the observed fluorescence is “prompt” even at 333 K. Thus, the assumption made by Birch and Birkss that the decay is predominantly “delayed” at high temperatures is not justified. Nonetheless, the magnitude of the high temperature fluorescence quantum yields and lifetimes do require reversibility in the ‘t* to lp* process as proposed by these authors. At The Journal of Physical Chemistry?Vol. 81, No. 20, 1977
A;’:
ns ns fqe,f 83.2 0.949 0.900 5.80 1.65 1.56 1.00 93.2 0.937 0.900 5.77 1.65 1.57 1.00 113.2 0.915 0.899 5.70 1.65 1.59 1.00 133.2 0.894 0.894 5.64 1.65 1.60 1.00 153.2 0.810 0.867 5.56 1.65 1.57 1.00 173.2 0.731 0.772 5.50 1.65 1.41 1.00 183.2 1.24 5.46 1.65 1.26 0.999 193.2 1.13 0.588 0.576 5.43 1.65 1.07 0.999 208.2 0.92 5.37 1.65 0.756 0.997 213.2 0.364 0.353 5.35 1.65 0.660 0.996 233.3 0.89 0.208 0.202 5.28 1.65 0.376 0.986 253.2 1.10 0.106 0.114 5.21 1.65 0.211 0.958 273.2 1.37 0.0720 0.0699 5.13 1.65 0.122 0.891 293.2 0.0510 0.0486 5.06 1.65 0.075 0.769 298.2 1.54 0.0442 0.0466 4.95 1.65 0.070 0.731 303.2 1.57 0.0415 0.0421 4.93 1.65 0.060 0.689 313.2 1.60 0.0382 0.0387 4.89 1.66 0.049 0.602 323.2 1.62 0.0370 0.0367 4.84 1.66 0.040 0.513 333.2 1.65 0.0359 0.0359 4.81 1.66 0.033 0.427 a From ref 8. From ref 5 and 8, see Appendix A. From eq 20, see text. MCH/MCP, T < 293.2 K ; MCH/IH, T > 298.2 K, see Appendix A. e For MCH/IH. From eq 21, using k d = 6.05 X lo’s-’, see text.
Figure 1. Observed 4 f (0from ref 5, A from ref 8 scaled to ref 5) and calculated 4 (solid line, eq 20, broken line, eq 6) for trans-stilbene. Break in lines is due to change in solvent, see text.
fz =
kf,d s-1 x A;:’
T,a
ns
d+tJ
cbfc
IO*
higher temperatures (>298 K) the observed fluorescence lifetimes should reflect the X1 component of eq 10 as the X2 component corresponds to lifetimes less than 0.070 ns. However, the calculated values of X i 1 are virtually constant (and equal to kd) whereas the recorded lifetimes decrease as temperature decreases. The simplest modification to the proposed mechanism which would give this temperature dependence is one in which k d increases as temperature decreases. If this temperature dependence takes the Arrhenius form k d = Ade+Ed/RT then substitution into eq 20 will change only the value of EPt(e.g., EPt= 7.3 Ed kcal/mol). There is insufficient data to assign Ed with confidence but its value certainly appears too small to alter the conclusiod that Et, < EPt. Although the negative temperature dependence of k d suggests equilibration with still another state, it does not seem prudent to speculate on a mechanism until more extensive measurements of both X1 and X2 are available. Approximate thermodynamic parameters for the %* and lp* interconversion can be calculated, but depend entirely on the accuracy of the high temperature data and the value assumed for kd. Thus, the present analysis gives mtP = Et, - Ept= -3.3 kcal mol-l and, for k d = 6.05 X lo8 s-l, Astp = R In (Atp/Apt)= -2 cal deg-I mol-I. These results do not address the interesting assignment of the lp* state to a doubly excited lA, statea8V2l While recent two-photon excitation spectroscopic observation provide strong experimental basis for the existence of such a state in trans-stilbene,22judgment should be reserved concerning its involvement in the isomerization process until more information is obtained concerning potential energy variation with torsional displacement in this state and in the initially produced lB, state. The present analysis does not suffer from the difficulties mentioned in the Introduction. The small oxygen quenching effect on Cbf is understandable since most of the fluorescence at 25 “Cis extremely short lived.23 This result also accounts for previous successful applications of eq 6 to the lower temperature fluorescence quantum yield^,"^ since Xz nearly equals (kf ki, + ktp) below 0 “C (cf. also dashed line in Figure 1). An interesting prediction of the analysis is that, as the temperature is increased, increasing
+
Analysis.of trans-StilbeneFluorescence
1943
TABLE V: Parameters Employed for Index of Refraction Calculations Solvent do a x l o 4 p x lo'
C
MCH/IH (3:2) 0.74570 9.51808 5.57578 0.7417 MCH/MCP (1:l) 0.77623 8.84883 4.26624 0.7310
Flgure 2.
Polar plots of variance vs. parameter values.
-
yields of It* fluorescence should be observable following excitation of cis-stilbene, provided that IC* 'p* is important as is generally t h ~ u g h t . ~ In fact, cis-stilbene fluorescence quantum yields may provide an accurate direct experimental measure of kfC1/X1.24
Acknowledgment. We thank Professor DeLos F. DeTar for valuable assistance in the use of GENLSS and the Florida State University Computing Center for funds to carry out these computations. Appendix A Choice of Fluorescence Quantum Yields. Birks and Birch based their analysis8 on the Malkin and Fischer fluorescence quantum yields2 measured in 3:2 methylcyclohexane/isohexane (MCH/IH) mixed solvent, which they extended to higher temperatures. While these quantum yields are generally in good agreement with more recent data of Sharafy and Muszkat5 in 1:l methylcyclohexane/methylcyclopentane (MCH/MCP), they differ significantly at 25 "C, the reference temperature in the Birch and Birks extension, 4f = 0.08 for ref 2 vs. 0.045 (extrapolated value) for ref 5. Other data, including our most recent obser~ations,~J~ suggest that the latter value should be preferred. For example, ref 4b gives & = 0.05 in 2:l MCH/IH, as does ref 6 in MCH,25both at 25 "C, ref 16 gives 4f = 0.035 in n-pentane at 30 "C and an unpublished value at 20 "C is 0.043.' While these values should reflect some variation due to the index of refraction dependence of ka18 the 0.08 value is clearly inconsistent with all the others. The analysis in this paper was therefore based on the Sharafy and Muszkat measurements. The data shown in Table IV were extracted from the paper by photocopying the figure on which they appeared and measuring the points with a ruler after enlargement using an opaque overhead projector. An extrapolated value of the data at 25 "C was obtained for use as reference in scaling the higher temperature quantum yields of Birch and Birks. For this purpose the change of solvent in the two studies was taken into account by as-
suming k f proportional to n2as discussed in the following section. T h e Index of Refraction Dependence of kf. A reference k f value could be obtained by using its theoretically calculated value of 5.9 X lo8 s-l at 30 "C in n-pentane16 or by choosing a value which gives T N 1 / X 2 in 3 2 MCH/IH at -90 "C, the lowest temperature for which an experimental fluorescence lifetime8 is available. The second approach was employed in the calculations after showing that use of the theoretical kfvalues as reference, or of a constant k f value for all temperatures, does not alter the conclusions in this paper. No index of refraction or density data could be found for the mixed solvents 3:2 MCH/IH or 1:l MCH/MCP. Densities for the pure liquids were obtained using the reported temperature dependence of the specific volume of MCH and assuming that the MCP and IH specific volume dependences follow those reported for MCH and isopentane, respectivelySz6Data from Egloff were used as reference.27 Where actual data for limited temperature ranges were available27they were found to agree well with the calculated values. The densities of the mixed solvents, d12,were then calculated assuming ideality and using the molar fractions, yn, of each component, i.e., eq 22, where dlZ(t) = Y l d l ( t ) + Y 2 d d t ) (22) t is the temperature in "C. These densities were fit to the usual eq 23, where dlzois the effective density of the two dlzt = d12 - a t - /3t2
(23)
component solvents at 0 "c, using GENLSS.15 They were then converted to indexes of refraction using the Eykman equation, eq 24.% Values of C for the mixed solvents were
estimated from those of the pure liquids27 using an equation analogous to eq 23. Relevant parameters are shown in Table V. Use of eq 23 and 24 over the entire temperature range may introduce some inaccuracy since it represents extrapolation into the region where the solvents become glasses. However, since the overall change in n is less than 10% in each solvent for the entire temperature range, -190 to +60 "C, no serious errors are anticipated. Values of k f , calculated from the relationship k f = (5.403)108n2/(1.4640)2,are shown in Table IV. Equations 23 and 24 and the relationship for kf were entered directly into GENLSS demonstrating the flexibility of this program. Appendix B Statistical Significance of Computer Fitting. Since the precision with which the parameters of eq 20 are defined by the quantum yield data is of importance in determining the significance of the analysis, further clarification is presented in this appendix. Using the search routine of GENLSS15 the parameters are individually stepped through a series of values about the convergence points and the variance in 4f calculated at each step. For instance the value of log A , is stepped and at each step the other parameters are adjusted for best fit of the calculated 4s; to the observed 4;s. The variance in 4f at each value of log At, is plotted in Figure 2.29 Similar plots are given for the other parameters. From these plots one can readily see the relative precision with which the parameters are defined by the data. The variance level of 1.82 X represents approximately the standard deviation of the best fit values. Convergence to the same solution occurs The Journal of Physical ch81??/Stty,Vol. 8 I, No. 20, 1977
P. Maruthamuthu and H. Taniguchi
1944
regardless of the choice of initial parameter values.
ReIferences and Notes (a) Supported by National Science Foundation Grant No. MPS 7602439. (b) Visiting Professor, 1976-1977; permanent address: Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 2N2. (a) S.Malkin and E. Fischer, J. Phys. Chem., 66, 2482 (1962); (b) ibid., 68, 1153 (1964). R. H. Dyck and D. S. McClure, J . Chem. Phys., 36, 2326 (1962). (a) K. A. Muszkat, D. Gegiou, and E. Fischer, J . Am. Chem. SOC., 89, 4814 (1967); (b) D. Gegiou, K. A. Muszkat, and E. Fischer, ibk!., 90, 12, (1966). S.Sharafyand K. A. Muszkat, J. Am. Chem. SOC.,93, 4119(1971). J. Saltiel and J. T. D'Agostino, J. Am. Chem. SOC.,94, 6445 (1972). J. Saltiel, A. Marinari, and D. W.-L. Chang, manuscript in preparation. D. J. S.Birch and J. B. Birks, Chem. Phys. Lett., 38, 432 (1976). For a review of stilbene photoisomerization see J. Saltiel, J. T. D'Agostino, E. D. Megarity, L. Metts, K. R. Neuberger, M. Wrlghton, and 0. C. Zafiriou, Org. Photochem., 3, 1 (1973). J. Saltiel, J. T. D'Agostino, 0. L. Chapman, and R. D. Lura, J . Am. Chem. SOC., 93, 2804 (1971). J. B. Birks, "Photophysis of Aromatic Molecules", Wley-Interscience, London and New York, 1970, pp 304-305. G. N. Lewis, T. T. Magei, and D.Lipkin, J . Am. Chem. Soc., 62, 2973 (1940). J. Saltiel and A. Marinari, unpublished observations. J. Saltiel and B. Thomas, Chem. Phys. Lett., 37, 147 (1976). D. F. DeTar, Comput. Programs Chem., 4, 71 (1972). A. Marinari and J. Saltiel, Mol. Photochem., 7, 225 (1976).
(17) J. B. Birks and D. J. Dyson, Proc. R . SOC. London, Ser. A, 275, 135 (1963). (18) J. Oimstead 111, Chem. Phys. Lett., 36, 287 (1976). (19) Reference 11, p 103. (20) For a discussion of uncertainties and significant figures in the parameters see ref 15. (21) G. Orlandi and W. Siebrand, Chem. Phvs. Lett., 30, 352 (1975). (22) T. M. Stachelek, T. A. Pazoha, and WM :. McClain, submitted for publication. We are grateful to these authors for a preprint of their paper. (23) Actually, if oxygen quenched 't' and 'p' with equal efficiency the expected 4 :/4,ratio according to the fluorescence distribution in Table IV woukl be 1.16 rather than 1.03, suggesting that the calculated parameters still overestimate the contribution of the slow component to the fluorescence yield. (24) A correction for dihydrophenanthrene formation would be req~ired.~ cis-Stilbene 4,measurements are planned as an extention of this work. (25) The observed value, 0.046, was rounded off in the publication. (26) R. Passerini and I. G. Ross, J . Sci. Instrum., 30, 274 (1953). (27) G. Egloff, "Physical Properties of Hydrocarbons", Vol. I, Reinhold, New York, N.Y., 1939; Vol. 11, 1940. (28) S.S. Kurtz, Jr., S. Amon, and A. Sankin, Ind. Eng. Chem., 42, 174 (1950). (29) The variance given in Figure 2 is the relative variance
v=
Yobsd - Y d c d ) / Y o b s d ] *
1/(n- P )
where Y is f, n Is the number of data points, and p the number of parameters being adjusted.
An in Situ Photolysis-Electron Spin Resonance Study of Some Reactions of Phosphate Radicals' P. Maruthamuthu and Hitoshi Taniguchl" Radiation Laboratory, University of Notre Dame, Nofre Dame, Indiana 46556 (Received June 6, 1977) Publication costs assisted by the U S . Energy Research and Development Administration
or its protonated forms, HP04Free radical intermediates formed in the reaction of phosphate radicals and HZPO,) with a number of fundamental organic and inorganic compounds have been studied by the in situ photolysis-steady state ESR method. The phosphate radicals were generated effectively by the photolysis of peroxodiphosphate (Pz02-)in aqueous solutions. Though the phosphate radical itself could not be detected directly, the ESR spectra of the phosphate radical adducts to fumaric and maleic acids and also the aci-anion of nitromethane were observed successfully. The pK, value for the proton dissociation in the phosphate group of the adduct to fumaric and maleic acids, -00CCH(OP03H-)CHC00-,has been determined to be 6.7. The reactions of phosphate radicals are rather similar to those of the related active species SO4-, e.g., they give rise to hydroxyalkyl radicals from aliphatic. alcohols, adduct radicals with unsaturated compounds, and inorganic However, there are notable differences in reactions radicals such as COS-from HC03- and PO:- from HPO:-. with aliphatic and aromatic carboxylic acids. From saturated aliphatic mono- and dicarboxylicacids, a-carbon radicals produced by hydrogen abstraction with phosphate radicals were predominantly detected in neutral aqueous solutions and phenyl-type radicals were not detected from.aromatic carboxylic acids. On the contrary the resultant radicals by decarboxylation were mainly observed in SO4-system. Clearly, direct oxidative electron transfer is not the predominant process in the reaction of the phosphate radical (HPOL) with carboxylic acids.
Introduction The close similarities between the peroxo anions of phosphorus and sulfur, namely, peroxodiphosphate (PzOg") and peroxodisulfate (SzO2-), suggest comparative studies of reactions of the radicals produced from these two oxidants. Peroxodiphosphate like peroxodisulfate reacts very efficiently with eaq- producing phosphate radicah2 In a recent paper,3 the results of the reactions of a number of organic compounds with the three acidbase forms4 of phosphate radicals H,PO,
-n+ pK, = 5.9
-H+
HP0,- Cp0,2pK,=
10.7
have been reported. The Journal of Physical Chemistry, Voi. 8 1, No. 20, 1977
Another possibility for generating phosphate and sulfate radicals is to subject the corresponding peroxo dianions to photoly~is.~,~ In fact a large amount of work has been done on photolytic reactions' as well as in situ photolysis-ESR investigations with peroxodi~ulfate.~,~ Edwards and c o - w o r k e r ~have ~ , ~ ~carried out the photochemical oxidations of ethanol and 2-propanol with peroxodiphosphate. They have postulated the formation of two phosphate radicals as the primary process P, 0 , 4 -
hv
2p0,z-
and compared the results with those of S202-. As yet, no other report has appeared on the in situ photolysis-ESR investigations of peroxodiphosphate. We have carried out
