370
N. MATAGA, H. OBASHI,AND T. OKADA
While the details of these arguments have yet to be fully worked out, they do suggest a novel manner of looking at the whole question of the relationship of transition state structure to those variables which affect reactivity including variation in polar substituent, solvent, ionic strength, and the like. Finally, we wish to note that catalysis of orthoester hydrolysis by anionic surfactants may be rather directly related to catalysis of glycoside hydrolysis by the enzyme, lysozyme. In the first place, it seems quite likely that the basic features of the mechanism of orthoester and glycoside hydrolysis are ~imilar.~’I n the second place, recent speculations on the mechanism of action of this enzyme based on detailed X-ray diffraction studies of the enzyme and ensyme-substrate complexes include facilitation of substrate protonation and electrostatic stabilization of a developing carbonium ion as important catalytic functions of the enzyme.28 These are, of course, precisely those functions
served by the anionic surfactants in the catalysis of orthoester hydrolysis.
Acknowledgment. We thank Dr. F. Puschel, Institute fur Fettchemie der Deutschen Academie der Wissenschaften, Berlin-Adlershof, DDR, for his gift of the isomeric hexadecyl sulfates, Dr. A. J. Stirton and Dr. J. K. Weil, Eastern Regional Research Laboratory, Agricultural Research Service, Philadelphia,, Pa., for their generous donation of numerous surfactants, and Dr. Ted Logan and Dr. Lawrie Benjamin, Miami Valley Laboratories of Procter & Gamble Coo, Cincinnati, Ohio, for their helpful discussions and the gifts of several surfactants. We also express our appreciation to Dr. R. W. Taft for his gift of methyl ortho-p-fluorobenxoate. (27) C. A. Vernon, Proc. Roy. SOC.,B167, 389 (1967). (28) C. C. F. Blake, L. N. Johnson, G. A. Mair, A. C. T . North, D. C. Phillips, and V. R. Sarma, {bid., B167, 378 (1967).
Electronic Excitation Transfer from Pyrene to Perylene
by a Very Weak Interaction Mechanism18 by N. Mataga,lb H. Obashi, and T. Okada Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, Japan (Received July 86, 1968)
The singlet-singlet electronic excitation transfer froin pyrene to perylene in high polymer matrices has been investigated by measuring the fluorescence decay curve of pyrene using the nanosecond-flash light for excitation. The results of measurements in polymethyl methacrylate and polystyrene clearly showed that the excitation transfer occurred by the dipole-dipole coupling of the very weak interaction mechanism, and Forster’s formula for the fluorescence decay curve in the case of fixed donors and acceptors was accurately satisfied. Although polystyrene has low-lying n-electronic energy levels compared to polymethyl methacrylate, the possibility of the indirect coupling through the r-electronic states of polystyrene was excluded.
Introduction A large number of investigations has been performed
methylanthracene to perylene in liquid solvents of various viscosities and also in polymethyl methacrylate
to elucidate the mechanism of long-range intermolec(1) (a) A preliminary report has been published: N. Mataga, H. Obashi, and T. Okada, Chem. Phys. Letters, 1 , 133 (1967). (b) T o ular electronic excitation transfer.2 The investigawhom correspondence should be addressed. tions by Bowen and B r o c k l e h u r ~ tas ~ ~Re11 as Bowen (2) Th. Forster: (a) “Fluoressens Organischer Verbindungen,” Vandenhoeck and Ruprecht, Gottingen, 1951 ; (b) Discussions Faraand L i ~ i n g s t o non ~ ~ the excitation transfer from 1day soc., 27, 7 (1959); (0) “Comparative Effect of Radiation,” chloroanthracene to perylene in liquid solvents and in M. Burton, J. J. Kirby-Smith, and J. L. Magee, Ed., John Wiley & Sons, New York, N. Y., 1960,p 300; (d) “Modern Quantum Chemisthe rigid glass at 77°K indicated the long-range excitatry,” 0. Sinanoglu, Ed., Part 111, Academic Press, New York, N. Y., 1965, 93, tion transfer which is approximately independent of the Solvent Viscosity, in accordance with Forster’s t h e ~ r y . ~ , (3) ~ (a) E. J. Bowen and B. Brocklehurst, Trans. Faraday SOC.,51, 774 (1956); (b) E.J. Bowen and R. Livingston, J. Amer. Chem. SOC., Their measurements, liowever, were neither very ac76, 6300 (1954). curate nor quantitative. (4)Th. Forster, Ann. Phys., 2, 65 (1948). Afelhuishb studied the excitation transfer from 9(5) w. H. Melhuish, J. Phya. Chem., 67, 1681 (1963). The JoUTnal of Physical Chemistry
ELECTRONIC EXCITATION TRANSFER FROM PYEENE TO PERYLENE (PMMA). He concluded that the rate constant for energy transfer was strongly dependent on solvent viscosity and the rate constant in PMNA was a little smaller than that calculated by Forster's formula for dipole-dipole coupling, whereas its value in liquid solvent extrapolated to zero fluidity agreed with the calculated value. Wc have made a detailed investigation on the excitation transfer from pyrene to perylene in fluid solvents by observing the fluorescence decay time of pyrene at various solvent viscosities and temperatures.6 The results have been analyzed by the method proposed by Yoltota and Tanimoto' to solve approximately the diffusion equation which includes the effect of the excitation transfer depending on the energy donor-acceptor distances. This analysis has shown that the observed rate of the energy transfer agrees exactly with that calculated by using Forster's formula. In the present report, we shall show further that the excitation transfer in the same system, pyrene-perylene, fixed in high polymer matrices occurs by the dipole-dipole resonance and Forster's mechanism is exactly satisfied. Although Bennett*has already made n similar measurement of the excitation transfer from pyrene to Sevron yellow L (a dye molecule) in cellulose acetate, we are concerned here with the case where both the energy donor and acceptor are aromatic hydrocarbon molecules.
Experimental Section ( a ) Apparafus and Measurements. Fluorescence spectra were measured by using an Aminco-Bowman spectrophotofluorometer which is calibrated to obtain correct quantum spectra. Absorption spectra were measured by a Cary 15 spectrophotometer. The fluorescence quantum yield of pyrene in high polymer matrix which is necessary to calculate the critical transfer distance (eq 1) was determined by using an ethanol solution of pyrene as a standard. The fluorescence decay curves were measured by an apparatus similar to the one described e l s e ~ h e r e . ~ The exciting light pulse and the emission pulse were passed through appropriate filters so as to excite pyrene at 320 mp, where the molar extinction coefficient ~ ( p y rene) = 30 X c(perylene), and to take out only the emission of pyrene. The measurements were made at room temperature as well as a t 77" K. (b) Materials and Preparation of Specimens. Pyrene was chromatographed on alumina and silica gel using n-hexane for elution, in a dark place. Chromatographed pyrene was sublimated under vacuum and zone-refined. Perylene mas chromatographed on alumina and silica gel in a dark place using a mixed solvent of n-hexane and benzene (6:4 in volume) for elution. After recrystallization from the mixed solvent, it was sublimated under vacuum.
371
Methyl methacrylate (AMA) was shaken with an aqueous solution of sodium hydroxide (5%) several times, washed with water, dried over sodium sulfate anhydride or magnesium sulfate, and immediately after it was passed through a column of activated silica gel, it was distilled in an atmosphere of nitrogen under reduced pressure. The method of purification of styrene was quite similar to that of XIMA. Solid solutions of pyrene and/or perylene in PAIMA were prepared by polymerizing the MMA solutions of pyrene and/or perylene as follows. The K U A solution in a glass tube was deaerated carefully by an ordinary evacuating system with an oil diffusion pump and a rotary pump. The deaerated solution was polymerized at 40" for 24 hr at first, Then the temperature of the system was raised gradually to BO". It took over 8 hr for this temperature change. After it was polymerized at BO" for 24 hr, the system was cooled gradually. It took over 24 hr until the solution was cooled to room temperature. It was confirmed by means of absorption spectrometry that no decomposition of the solutes occurred during the polymerization. If the polymerization is not complete, the solid solutions show the strong fluorescence of the pyrene excimer. The excimer fluorescence appears also when the solid solution is cracked in the course of polymerization. We have used clear glassy solutions polymerized completely of which no excimer fluorescence can be observed. The polystyrene (PST) solution was prepared by the procedure similar to the case of PhlMA solution. Namely, the deaerated styrene solution was polymerized a t 60" for 24 hr followed by a further 7 2 hr a t 90". We have prepared both solid solutions polymerized with the addition of azobisisobutylonitrile as an initiat,or and polymerized without it. Of course, the polymerization is followed by the volume decrease of the solution. We have determined the extent of the volume change and calibrated the concentrations of the solutes after the polymerization has been completed.
Results and Discussion In Figure 1, the fluorescence and absorption spectra of pyrene and perylene, respectively, in PMMA at room temperature are indicated. The spectra in PST are quite similar to those in PMMA although the former spectra are a little shifted to red compared to the latter spectra. From these spectra, the integral, RfQD(ij) r*(ij)d;/~4, where fQD(ij) represents the fluorescence (6) M. Tomura, E. Ishiguro, and N. Mataga, J . Phys. Soc. Japan,
22, 1117 (1967).
(7) M. Yokota and 0. Tanimoto, ibid., 22, 779 (1967). (8) R. G. Bennett, J . Chern. Phys., 41, 3037 (1964). (9) N. Mataga, M. Tomura, and H. Nishimura, Mol. Phys., 9, 367 (1966). Volume YS, Number B February 1969
372
N. MATAGIA, H. OBASHI,AND T. OKADA
-
-
- 4
Y.
-3
-
2
y .-I E 0
h '
B
- 2 .8+8
0
9.-
.-2 c
3
b - 1 2
/
Y
Q
.
. . a
*
+'
'
a0
quantum spectrum of pyrene normalized to unity and eA(fi) is the molar extinction coefficient of perylene, can be evaluated. The observed values for this integral were 3.4 X 10-14 in the case of PMMA solution and 2.7 X lo-" for the PST solution a t room temperature. According t o Forster, the critical transfer distance Ro in centimeters is given by
where n is the refractive index of the medium, N is Avogadro's number, K is the orientation factor for the transition moments of donor and acceptor, respectively, and Q F is~ the fluorescence quantum yield of the donor. ~ 0.70 in PMMA According to our measurements, Q F E and Q F E ~ 0.65 in PST a t room temperature. The refractive indices of the solid solvents are given as n(PMMA) S= 1.49 and n(PST) = 1.60. For the completely random distribution of acceptors in the ground state, the mean-square value of K will take the value 2/3. By using these values of the parameters, RO can be calcu1at:d as, Ro = 38 0 . d in PMMA and RO = 34 -I: 0.7 A in PST at room temperature. According to the measurement by i\lelhuish,l0 Q F in~ P?t!tMAis 0.61. If we use this value, Ro E 37 win PMMA. Some typical results of the fluorescence decay curve measurements are shown in Figures 2a, b and 3a, b. The fluorescence decay curves of pyrene in PMMA as well as those in PST are accurately exponential with TD (mean decay time of pyrene fluorescence) S= 380 nsec at room temperature when perylene is absent in the solution. I n the case of PMiLIA solution, we have made measurements also a t 77°K. I n this case, TD = 520
*
The Journal of Physical Chemistry
l b L
0
1
I
2
I
I
4
I
I
I
6
I
I
8
Figure 2. (a) Log p ( t ) vs. t relation in PMMA at room temperature; (b) log 6 ( t ) us. t'Izrelation in PMMA at room temperature. Concentrations of donor (CD)and acceptor (CA), respectively: (1)pyrene only; ( 2 ) 5.5 X l o V 3and 2.3 X M ; (4) 1.4 X M ; (3) 8.4 x lo-* and 3.5 X and 5.8 x 10-3 M . No initiator was added in the preparation of specimens.
nsec. This increase of the T D value at 77°K seems to indicate a little decrease of the radiationless transitions at 77°K even if the molecule is fixed in the high polymer matrix. (10)W. H. Melhuish, J . Opt. Soc. Am., 54, 183 (1964).
373
ELECTRONIC EXCITATION TRANSFER FROM PYRENE TO PERYLENE
0
2
4
t
6
8
(10"sec).
Co = T ~ / ~ N ~ where ' R ~ ~N A) ', is the number of acceptor molecules per millimole. In the present work, the ratio (CD/CA), where CD is the concentration of pyrene, was kept constant and the total concentration was varied. I n the case of PMMA solutions, two series of specimens with (CD/CA)= 2.38 and (CD/CA) = (1/5.88) were examined while (&/ CA) = 2.31 and 1.88 in the case of PST solutions. It has been confirmed that TD value in the absence of perylene is approximately independent of the CD value and the fluorescence decay curve of pysene in the presence of perylene depends only on CA for all specimens we have examined, in accordance with eq 2. The Ro values obtained from the fluorescence decay curves in PMhSA a t room temperature are given in Table I. Thus, the observed value of the critical transfer distance is given as, Ro = 36 =t2 8. Therefore, the Ro value calculated by using eq 1 agrees completely with the observed value within the experimental error. The Ro value observed at 77°K was a litjle larger than that at room temperature (Ro = 39 A). Although we have used the same CAvalue as in the case of the evaluation at room temperature, the CA value at 77°K may be a little larger than the value a t room temperature because of the volume contraction a t low temperature. Therefore, the true Ro value a t 77°K will be a little smaller than 39 A, thus, being rather close to the value obtained at room temperature.
Table I : Ro Values for PMMA Solid Solutions at Room Temperature CD
0
2
4
6
8
x
0.46 0.68 0.90 1.13 5.5 8.4 14
Figure 3. (a) Log p ( t ) us. 1 relation in PST at room temperature; (b) log 8 ( t ) us. t ' / z relation in PST a t room temperature. Concentrations of donor (CD)and acceptor (CA),respectively: (1) pyrene only; (2) 7.0 x 10-8 and 2.9 x 10-8 M ; (3) 1.04 X 10-2 and 4.4 x 10-3 M ; (4) 1.74 x 10-2 and 7.3 X 10-3 M. No initiator was added in the preparation of specimens.
When perylene is present in the solid solutions, the observed fluorescence decay curves of pyrene can be well fitted to eq 2 as is shown in Figures 2 and 3 p ( t ) = eXp[-t/TD
2(CA/cO)(t/~D)"']
exp[-2(Ca/Co) ( t / ~ D ) * ' ~ l (2) where CAis the concentration of the acceptors and COis the critical concentration related to ROby the equation, 6(t)
5
10-1 M
CA
x
lo-' M
2.7 4.0 5.3 6.7 2.3 3.5 5.8
Ro,
39 37 35 34 35 34 35
The observed ROvalues for PST solutions ase @veri in Table 11. The observed value, Ro = 37 4 1 A, appears to be somewhat larger than the value calculated by using eq 1. However, it does not seem t o be plausible that this small difference between the two Rovalues is very significant. Certainly, the difference may be within the limits of experimental accuracy in view of some uncertainties involved in the experiment. For example, there might be some inhomogeneities in the distribution of donor and acceptor molecules in the PST matrix which may realize preferential orientations of donor and acceptor on the polymer chains to some extent. Thus, the present results clearly show that the longrange excitation transfer from pyrene to perylene occurs Volume YS, h'umber d
Februavu 1960
JOHN M. HASCHKE AND HARRY A. EICK
374 Table I1 : Ro Values for PST Solid Solutions at Room Temperature CD
x
loes M
CA
x
M
RQ,
a. Without the Addition of the Initiator 7.0 2.9 39 10.4 4.4 38 17.4 7.3 36 7.1 9.4 11.8
b. W i t h Addition of t h e Initiator 3.8 5.0
6.2
38 37 37
by means of the very weak interaction mechanism and the interaction is due to the dipole-dipole term in the expansion of the interaction hamiltonian into the multipole-multipole interaction terms. Therefore, in the pyrene-perylene system, Forster's mechanism of dipoledipole coupling is verified accurately both in solid as well as in fluid solutions. The electronic interaction between the energy donor and acceptor molecules which is responsible for the excitation transfer is not limited to the direct interaction between the donor and the acceptor but also the indi-
rect interactions via the virtual excited states of the intervening solvent molecules might be effective for the transfer. Robinson and coworkerP suggested such a mechanism for the triplet-triplet excitation transfer in solid solution in view of very efficient triplet excitation transfer in the crystal of aromatic hydrocarbons. One may expect similar circumstance also for singlet-singlet excitation transfer in the pyrene-perylene-PST system since the energy difference between the fluorescence levels of PST and pyrene is not very large and there should be some interactions between the n-electron systems of PST and solute molecules in addition to the quite long fluorescence lifetime of pyrene. The possibility of the transfer by indirect interaction was excluded, however, by the experimental study on the triplet-triplet excitation transfer for several systems in the rigid glass at 77"K.12 Furthermore, as we have described already, the examination on the pyreneperylene system in PST does not show any indication of the indirect coupling effect. There is no essential difference between the experimental result in PST matrix and that in P M l I A matrix. (11) H. Sternlicht, G. C . Nieman, and G. W. Robinson, J . Chem. Phys., 38, 1326 (1963). (12) S. Siege1 and H. Judeikis, ibid., 41, 648 (1964).
The Vaporization Thermodynamics of Europium Monoxide by John M. Hawhke and Harry A. Eick Department of Chemistry, Michigan State Uniuersity, East Lansing, Michigan 48888
(Received J d y 89, 1968)
Europium monoxide prepared by the reduction of the oxide chloride with lithium hydride has been studied by the target collection Knudsen effusion technique and found t o vaporize according to the reaction 4EuO(s) + Eu3O4(s) Eu(g) (l),over the temperature range 1334-1758°K. At 1546"K, AH"* = 75.91 i 0.94 kcal/ gfw and A P T = 28.66 =k 0.61 eu. Heat capacity data for EuO(s) have been estimated, and second- and thirdlaw enthalpies for reaction 1 are presented. For EuO(s), AHr"zs.9 = - 145.2 4.1 kcal/gfw,A G = -139.0 ~ ~ &~ 4.1 kcal/gfw, and SDzSs = 15.0 f 3.0 eu.
+
*
Introduction The well-characterized lower oxides of europium (EuO and Eu304) are the only reported lanthanide lower oxides whose existence has not been questi0ned.l Because of its ferromagnetic behavior, europium monoxide has been examined extensively. However, as we have noted previously in a study of E U ~ Othe ~ , only ~ thermochemical values reported for europium oxide phases are the enthalpy of formation and high-temperature heat capacity of EuzOa and the enthalpy of formaThe Journal of Physical Chemistry
tion of EuO, all of which were obtained calorimetrically. Westruma has also estimated the absolute entropy of these phases. Along with samarium and ytterbium, divalent europium often behaves chemically like the alkaline earths (1) G. Brauer, H. Baernighausen, and N. Schults, 2. Anorg. Allg. Chem., 356, 46 (1967). (2) J. M. Haschke and H. A. Eick, J . Phys. Chem., 72, 4235 (1968). (3) E. F. Westrum, Jr., Advances in Chemistry Series, No. 71, American Chemical Society, Washington, D. C., 1967.
~
~
