OPTICALAND ESRSPECTRA OF TRAPPED ELECTRONS IN ORGANIC GLASSES
4599
Effects of Matrix Polarity on the Optical and Electron Spin Resonance Spectra of Trapped Electrons in Organic Glasses1 by Alfred Ekstrom and John E. Willard Department of Chemistry, University of Wisconsin, Madison, Wisconsin (Received June 26, 1968)
Trapped electrons produced by y irradiation of 13 organic glasses at 77°K have optical absorption maxima (eV) and esr line widths which increase smoothly with increasing polarity of the matrix molecules, from 3methylpentane to glycerol, consistent with a model in which the electrons are trapped in preexisting cavities in the matrix, the size of cavities which stabilize the electrons decreasing with increasing polarity. Trapped electrons in mixtures of alcohols in the glassy state show only one optical absorption maximum. This shifts from the energy characteristic of the first alcohol to that of the second as the mole fraction of the second is increased, indicating that the alcohol molecules are homogeneously mixed, and that each trapping site is composed of several molecules. Trapped electrons in mixtures of normal propanol and 3MP in the glassy state show two electron absorption peaks, one characteristic of each pure species, with preference for trapping in the alcohol phase, indicating aggregation of like molecules.
Introduction Considerable evidence is available on the properties of trapped electrons in both polar and nonpolar organic matrices2 as observed by visible-infrared absorption, e ~ r ~ ~electrical d , ~ ~ iconductivityJ6 ~ and luminescence measurements,6 but there is relatively limited evidence as to the physical nature of the trapping sites. It appears plausible that the same type of forces may be responsible for stable trapping in the solid state at 77°K as are responsible for the relatively unstable ‘‘solvation” of electrons in liquids at 300°K.8d~3e Jortner7 has proposed a hypothesis for the stabilization of electrons in liquids, in which the electron is regarded as creating a cavity in the liquid due to the short-range repulsion between the electron and the electrons of the medium. The electron is stabilized by the polarization of the molecules in the surrounding liquid. For polar molecules the trapping energy increases and the cavity size decreases with increasing polarity. I n qualitative agreement with this hypothesis, the absorption maxima of the solvated electron in a series of liquid alcohols a t 25” * and glassy alcohols at 77”K3eshow a red shift with decreasing dielectric constant of the alcohol. It might be expected that the line width of the esr signal of the electron would decrease with decreasing polarity of the medium, that is, with increasing minimum size of the trapping cavities and consequent decreasing interaction of the electron with the nuclei of the cavity walls. Such an effect seems to hold for several y-irradiated alcohols compared to ethers and for electrons prepared by the deposition of sodium in a few alcohols of different polarity.ad This paper extends the available data on the optical and esr spectra of trapped electrons in y-irradiated organic glasses covering a range in polarity from 3methylpentane (3MP) to glycerol. It examines the
correlation between the static dielectric constants, the visible-infrared absorption maxima, and the esr line widths, and between the absorption spectra of solvated electrons in the liquid state and trapped electrons in the glassy state. The properties of the electrons trapped in mixtures of components of differing polarity have also been studied.
Experimental Section All materials were of reagent grade quality, whose purity was checked by gas chromatography as being (1) This work has been supported in part by the U. .S. Atomic Energy Commission under Contract AT(l1-1)-1715 and by the W. F. Vilas Trust of the University of Wisconsin. (2) For recent reviews of this subject, see: (a) W. H. Hamill, “Ionic Processes in 7-Irradiated Organic Solids a t - 196OC,” a chapter in “Radical Ions,” L. Kevan, Ed., John Wiley and Sons, Inc., New York, N. Y., 1968; (b) J. E. Willard, “Radiation Chemistry of Organic Solids,” a chapter in “Fundamentals of Radiation Chemistry,” P. Ausloos, Ed., John Wiley and Sons, Inc., New York, N. Y . , 1968. (3) (a) D. W. Skelly and W. H. Hamill, J . Chem. Phys., 44, 2891 (1966); (b) J. Lin, K. Tsuji, and F. Williams, J . Amer. Chem. Soc., 90, 2766 (1968); (c) P. J. Dyne and 0. A. Miller, Can. J. Chem., 43, 2696 (1965); (d) J. Blandamer, L. Shields, and M. C. R. Symons, J. Chem. SOC.,1127 (1965); (e) B. G . Ershov, I. E. Makarov, and A. K. Pikaev, Khim. Vysokikh Energii, 1 , 472 (1967); High Energy Chem., 1, 414 (1967); (f) J. E. Bennett, B. Milne, and A. Thomas, J. Chem. SOC.,1393 (1967). (4) (a) K. Tsuji, H. Yoshida, and K. Hayashi, J. Chem. Phys., 46,810 (1967); (b) M. Shirom, R. F. C. Claridge, and J. E. Willard, ibid., 47, 286 (1967); (0) K. Tsuji and F. Williams, J . Amer. Chem. Soc., 89, 1526 (1967); (d) C. Chachaty and E. Hayon, J . Chim. Phys., 61, 1115 (1964); (e) D. R. Smith and J. J. Pieroni, Can. J . Chem., 43, 2141 (1965). (5) B. Wiseall and J. E. Willard, J. Chern. Phys., 46, 4387 (1967). (6) (a) K. Funabashi, P. J. Herley, and M. Burton, ibid., 43, 3939 (1965); (b) D. W. Skelly and W. A. Hamill, ibid., 43, 3497 (1965). (7) (a) J. Jortner, ibid., 30, 839 (1959); (b) J. Jortner and S. A. Rice, Advances in Chemistry Series, No. 50, American Chemical Society, Washington, D. C., 1965, p 7 ; (c) D. C. Walker, Quart. Rev., 21, 79 (1967); (d) J. Jortner, Radiation Res. SuppE., 4, 24 (1964); (e) X. Fueki, J . Chem. Phys., 44, 3140 (1966). (8) (a) M. C. Sauer, S. Arai, and L. M. Dorfman, ibid., 42, 708 (1965) ; (b) 8. Arai and M. C. Sauer, ibid., 44, 2297 (1966).
Volume 72, Number IS December 1968
ALFREDEKSTROM AND JOHN E. WILLARD
4600 greater than 99.9%. The alcohols were further distilled from 2,4-dinitrophenylhydrazine sulfuric acid solution to ensure the absence of ketonic impurities. Xlethyltetrahydrofuran (MTHF), methylcyclohexane (MCHx), and 3-methylpentane (3MP) were purified as previously described. Degassing of the purified samples was accomplished by repeated freeze-pumpthaw cycles and included direct pumping on the liquid to ensure removal of COZ. All experiments were made with glassy rather than crystalline samples unless otherwise specified. Isopropyl alcohol at a concentration of 1 mol % was present in all samples of CHIOH to prevent crystallization. Irradiations were made with a cobalt-60 source at a dose rate of 0.8-2.0 X 10l8eV g-I min-1, the total dose being, typically, 2.4 X 1019eV g-l. During the irradiation the samples were immersed in liquid nitrogen and positioned reproducibly adjacent to a cobalt-60 source. Samples for optical examination were contained in flat, high-purity quartz cells, 2 X 0.5 cm, with a normal path length of 1 mm. On completion of irradiation, they were transferred to an optical dewar for spectroscopic measurements, these operations being carried out in complete darkness to avoid bleaching of the trapped electrons. A Cary 14 spectrophotometer, modified to pass the infrared analyzing beam through the monochromator before passing through the reaction cell, was used. The esr measurements were made as previously describedg at a microwave power of ca. 0.01 mW. The esr line width measurements on the trapped electrons in alcohols were reproducible to f1 G.
Results The optical absorption spectra at 77°K of trapped electrons in ten 7-irradiated alcohol glasses observed in this work are all similar in shape to the spectra of solvated electrons in the liquid alcohols,8but are somewhat narrower and are shifted 100-200 nm toward shorter wavelengths. This is illustrated in Figure 1 by the spectra in ethanol. The absorption spectra of solvated electrons in liquid alcohols shift to shorter wavelength with decreasing temperature8 but do not show the decrease in the width of the absorption band observed when the electrons are trapped in a glass. The photon energy at the wavelength of maximum optical absorption by trapped electrons in yirradiated organic glasses at 77°K increases with increasing polarity of the matrix molecules, as shown in Table I and Figure 2 and reported earlier for a series of alcohols.ae This trend holds over a wide range from compounds which are essentially nonpolar to highly polar compounds. It is analogous to the trend8of the absorption maxima for the solvated electron in liquid alcohols (Table I). Paralleling the shift in the optical absorption maximum of the trapped electron with polarity of the matrix, there is a continuous variation in the line width of The Journal of Physical Chemietry
I
\
L I
400
I
600
800
nm
1000
1
Figure 1. Comparison of the absorption spectra of the solvated electron in liquid ethanol8 (- - - -) and the trapped electron in glassy ethanol at 77°K (-).
Table I: Absorption Maxima and Esr Line Widths for Electrons in Media of Different Polarity 7 3 0 0 ° K 7 777’KDBa
Glycerol Ethylene glycol Methyl alcohol Ethyl alcohol n-Propyl alcohol Isopropyl alcohol %-Butyl alcohol Isobutyl alcohol n-Pentyl alcohol &Butyl alcohol Methyltetrahydrofuran Diethyl ether 3-Methylpentane Methylcyclohexane
42.5 37.7 32.6 24.5 20.1 18.6 17.8 15.8 13 10.9 4.6 4.3 2.0 2.0
ErnaxSl’
Ernax,’
eV
eV
photon-1 photon-1
2.35 2.14 1.96 1.77 1.67 1.51 1.82
2.50 2.41 2.38 2.28 2.22 1.92 2.21 1.76 2.18 1.75f 0.99 0.89 0.75 0.73
AH,^ G
RQ,~ au
4.2
1.40 1.46 1.48 1.50 1.53 1.65 1.53 1.69 1.51 1.73 2.29 ’
3.6 3.0 3.0
2.42 2.64 2.67
15 14 12 12 10 8 9 7
a Static dielectric constant. Absorption maximum of “solvated” electron, ref 8. Absorption maximum of “trapped” electron. Esr line width. e Square potential well radius. Value taken Calculated from Ela * Ez, = 1.05n1/2Ro*,ref 3d. from ref 3f.
’
its esr singlet. The esr spectra are illustrated in Figure 3 and the relationship between the line widths and the energies of the optical absorption maxima is shown in Figure 4. When two alcohols such as ethylene glycol and isopropyl alcohol or ethyl alcohol and isobutyl alcohol are mixed in various proportions, frozen to the glassy state at 77”K, and yirradiated, the spectra of the trapped electrons produced show absorption maxima which vary continuously with change in composition, shifting toward shorter wavelength with increasing concentration of the more polar component (Figures 5 and 6). This observation is analogous to the shifts in absorption (9) M. Shirom and J, E. Willard, J. Phys. Chem., 72, 1702 (1968).
OPTICALAND ESRSPECTRA OF TRAPPED ELECTRONS IN ORGANIC GLASSES
4601
:;/: as 1.0
-,/
Figure 2. Variation of energy per photon at the absorption maximum of the trapped electron with the dielectric constant of the medium.
t I
700
600
500
0400
800
nm
Figure 5. Effect of matrix composition on the absorption spectrum of the trapped electron in ethylene glycol-isopropyl alcohol mixtures: -, pure ethylene glycol; - - -,pure isopropyl alcohol; * . * , 0.4 mf isopropyl alcohol in ethylene glycol; --, 0.7 mf isopropyl alcohol in ethylene glycol.
-
----
r
Figure 3. Comparison of ear spectra of the trapped electron in glassy MCHx (A) and ethanol (B).
I
0
I
I
02
OA mf
0.6
0.8
Figure 6. Effect of matrix composition on the energy of the photon at the absorption maximum of the trapped electron: x, ethylene glycol-isopropyl alcohol mixtures; 0 , ethyl alcohol-isobutyl alcohol mixtures.
Emox, eV photon-'
Figure 4. Esr line width ( A H ) vs. the optical absorption maximum (eV)for the trapped electron in a series of y-irradiated organic glasses at 77'K.
spectra observed in a series of liquid alcohol mixturess and in glassy ethylene glycol-water and glycerol-water mixtures.'O
I n contrast to the mixtures of two alcohols or alcohols and water, y-irradiated glassy mixtures of n-propyl alcohol with 3MP show two absorption peaks attributable to trapped electrons, one characteristic of npropyl alcohol and one characteristic of 3MP (Figure 7). Similarly, y-irradiated glassy mixtures of MTHF and ethyl alcohol have been reported" to yield two absorption peaks, one characteristic of each pure component. Figure 8 shows the change in concentration of trapped electrons in methylcyclohexane glass at 77°K as a function of y dose, as measured by infrared absorption a t 1600 nm. This confirms earlier results,12 obtained (10) B. G . Ershov, I. E . Makarov, and A. K. Pikaev, Khim. Vysokikh Energii, 1, 404 (1967); High Energy Chem., 1, 355 (1967). (11) L.Shields, J . Phys. Chem., 69,3186 (1965). (12) M. Shirom and J. E. Willard, J. Amer. Ch,em. SOC.,90, 2184 (1968).
Volume 78. Number 13 December 1968
ALFREDEKSTROM AND JOHN E. WILLARD
4602
77°K show no decrease in G(e-) up to at least 2 X 1020 eV g-l (curve C), indicating that, as in the case of methyltetrahydrofuran1l2 more electron trapping sites are available than in the hydrocarbons or that the entity which removes trapped electrons in the hydrocarbons is not available to do so in the alcohol. Incidental to these studies, it was noted that bleaching of the trapped electrons in alcohols, and particularly in methanol, resulted in a significant increase in the free-radical signal which underlies the electron singlet. This effect was absent in the case of nonpolar matrices, as previously reported.9
Discussion
Figure 7. Effect of matrix composition on the absorption spectrum of the trapped electron in n-propyl alcohol-3, pure n-propyl alcohol; methylpentane mixtures: , 0.3 mf n-propyl alcohol in 3-methylpentane; , 0.1 mf n-propyl alcohol in 3-methylpentane.
-.-
---L_
I
I
eV g-' x 10-20
Figure 8. The effect of y dose a t 77°K on the concentration of trapped electrons in (A) pure MCHx, (B) MCH 5.0% 2-methylpentene-1, and (C) ethanol. The absorption in MCHx was measured at 1600 nm (scale at left) and in ethanol at 600 nm (scale a t right).
+
by esr measurements and with less precise infrared techniques, which demonstrate that only a limited number of electron trapping sites are available in the glassy hydrocarbon matrix. Curve B of Figure 8 shows that the enhanced electron population known to be obtained in the presence of 2-methylpentene-15t1ais also subject to removal by products of the irradiation, and that the electron trapping sites are destroyed or preempted. Similar experiments on glassy ethanol at The Journal of Physical Chemiatry
The compounds used as matrices in the present investigation are all characterized by a negative electron affinity, and so have no vacant orbitals capable of accommodating an additional electron. Consequently, the stabilized electrons cannot be considered as localized on any particular molecule but must be stabilized by some other mechanism. This is further confirmed by the absence of hyperfine splitting of the electron esr signal. The similarity between the visible-infrared spectra of solvated electrons in liquid alcohols and of trapped electrons in glassy alcohols suggests that the environment of the electrons is similar in the two cases. However, the electron lifetimes in the glasses at 77°K are of the order of minutes to days as compared to microseconds in the liquids. This is, plausibly, the result of the fact that the stabilizing configuration of solvating molecules in the liquid is transitory, because it is subject to rapid disruption by thermal motions, whereas in the glass at 77°K trapping occurs in relatively stable voids frozen into the matrix. It is probable that reorientation of matrix dipoles under the influence of an electron is too slow a process to assist electron t r a ~ p i n g . l ~ *If' ~this is the case, the electrons must be trapped in the alcohols in preexisting cavities which have configurations of favorably oriented dipoles determined by the molecular dimensions of the alcohols and the nature of the polymeric aggregates resulting from hydrogen bonding. Compounds which have the highest static dielectric constant in the liquid phase apparently form cavities capable of the most stable trapping. There is evidence that even in the nonpolar hydrocarbon glasses, trapping occurs only in preexisting voids. The concentration of trapped electrons in hydrocarbon glasses previously investigated12 and further examined in the present work (Figure 8) passes through a maximum and then decreases with increasing dose, indicating that the number of trapping sites is (13) J. B. Gallivan and W. H. Hamill, J . Chem. Phys., 44, 2378 (1966). (14) L. Kevan, Progr. Solid State Chem., 2, 321 (1966). (15) R. Schiller, J . Chem. Phys., 47, 2281 (1967).
OPTICAL AND ESRSPECTRA OF TRAPPED ELECTRONS IN ORGANIC GLASSES limited, and that these become filled and subsequently removed or populated by nonparamagnetic non-ir-absorbing species. Consistent with the conclusion that preexisting voids are required is the fact that trapping of electrons and positive charge does not occur nearly as effectively in organic crystals as in the corresponding glasses which, because of their random structure, may be expected to have a higher concentration of voids.l8 This is true despite the fact that electrons and positive charge can be chemically trapped in the crystals by additives such as biphenyl” when the latter are present at concentrations as low as mf. The yield, G(biphenylide), is about 1, indicating that charge migrates extensively in the crystals without finding physical trapping sites. In the absence of scavengers it eventually encounters a positive ion and is neutralized. The Jortner theory, developed for liquid systems, assumes that the dipole orientation required for solvation occurs as a result of the presence of the electron. In the application of the theory to the glassy systems it must be assumed that the portion of the trapping interaction which is due to permanent dipoles is the result of configurations existing in the trapping matrix prior to the arrival of the electron. I n terms of the Jortner theory,’ the radii of the trapping voids may be estimated using the assumption that the energy per photon at the wavelength of maximum absorption is equal to the energy of the El, + E2, transition in a square potential well which is equal to 1 . 0 5 ~ ~ / 2 Rwhere 0 ~ , Rois the cavity radius. As noted earlier,ae the radii so obtained for the glasses listed in Table I increase progressively with decreasing polarity of the matrix. The increasing esr line width with increasing polarity is consistent with this theory, if broadening of the esr line is attributablel**lsto interaction of the electron with the nuclear magnetic moments of the hydrogen atoms in the cavity wall. That such interaction is important is demonstrated by the fact that the line width of the trapped electron signal is 6 G in C2H50D,19 as compared to 12 G in C2H,OH, as a result of the lower nuclear magnetic moment of D than of H. The fact that the A,, values and the esr line widths of the hydrocarbons fall in a consistent sequence, as a function of polarity, with those of the alcohols, is strong evidence that polarity rather than more specific features of the trapping species are dominating, and adds qualitative support to the applicability of the Jortner model to electron trapping in glasses. The curves of Figures 2 and 4,as drawn, extrapolate to the origin. This is consistent with the fact that the transition energy, El, + EZp = 1.05a2/2R02,approaches
4603
zero as Ro becomes very large. An electron in a very large or infinite cavity is essentially a free electron. Such a free electron would be expected in an environment of zero polarizability. Similarly, a free electron would be expected to exhibit a “zero” esr line width, consistent with the trend of the extrapolation of Figure
4. A further interesting aspect of Figure 2 is that the value of the transition energy appears to approach a limiting value with increasing dielectric constant of the medium. Such a behavior may be expected if the radius of the cavities suitable for trapping electrons approaches a limiting value. I n n-propyl, n-butyl, and n-pentyl alcohols the transition energies are somewhat higher than the general trend but the branched isomers of these compounds give consistent values, suggesting that the linear molecules orient in such a way as to provide smaller cavities with greater trapping ~tability.~‘ The observation that electrons in mixtures of alcohols (Figure 5) give only one absorption peak, the maximum of which shifts to the red as the mole fraction of the less polar component increases, suggests random intermolecular hydrogen bonding of the two alcohols to provide traps of polarity intermediate between those of either pure alcohol. The results confirm the concept that the electron is not trapped by an individual alcohol molecule but is contained by a group of molecules, the absorption energy being related to the composition of the whole group, as is also the case for solvated electrons in liquid alcohols.* The absorption spectra of the electron trapped in n-propyl alcohol-3-methylpentane mixtures (Figure 7) show that the electron is preferentially trapped in an environment of n-propyl alcohol molecules, even a t low n-propyl alcohol concentrations. As a result of their ability to form hydrogen bonds, alcohols are extensively aggregated in the form of dimers, trimers, and tetramers in a nonpolar solvent, even at very low alcohol concentrations, and this aggregation would be expected to be enhanced at low temperatures. These aggregates presumably form suitable traps for the electron, and are present in a sufficient concentration to compete favorably with those formed by 3MP molecules. (16) M. A. Bonin, J. Lin, K. Tsuji, and F. Williams, Advances in Chemistry Series, No. 82 American Chemical Society, Washington, D. C., 1968, Vol. 11, p 269. (17) A. Ekstrom and J. E. Willard, unpublished results. (18) P. N. Moorthy and J. J. Weiss, Advances in Chemistry Seriee, No. 50, American Chemical Society, Washington, D. C., 1965, p 180. (19) D. R. Smith and J. J. Pieroni, Can. J. Chem., 45, 2723 (1967).
Volume 72,Number 18 December 1068
