Thermoluminescence of aliphatic oligoesters irradiated by electron beams—II

Radiat. Phys. Chem. Vol. 16, pp. 307-313 Pergamon Press Ltd., 1980. Printed in Great Britain

THERMOLUMINESCENCE OF ALIPHATIC OLIGOESTERS IRRADIATED BY ELECTRON BEAMSmII EFFECT OF ELECTRON SCAVENGERS ON THE LUMINESCENCE IN G L A S S Y M A T R I X MINORU TSUMURA, NAOTO OMI and YOSHIMASA HAMA Science and EngineeringResearch Laboratory, Waseda University, 3-4-10kubo, Shinjuku, Tokyo 160, Japan (Received on 29 October 1979; in revised form 25 December 1979) AbstractmThermoluminescencemechanism of glassy aliphatic oligoester undoped and doped with some kinds of electron scavengers is discussed, comparing with the tbermoluminescence of polycrystalline oligoester. A remarkable decrease of the thermoluminescence intensity was observed in the glassy undoped matrix in comparison to the polycrystalline one. This is due to the trap depth difference between the glassy and polycrystalline matrices. In the glassy matrix doped with electron scavengers, an intense glow was observed at around 140 K for all matrices. The luminescence center of the doped matrix is the excited state of the electron scavenger and is different from that of the undoped matrix in which it is the ester cation of the oligoester. It is expected that energy transfer from the excited charge transfer complex to the electron scavenger occurs in the reaction of the ester cation with the scavenger anion. The appearance of the fluorescence component in the thermoluminescence spectrum is dependent on the kind of the matrix. INTRODUCTION IRRADIATION of organic glasses by ionizing radiation at low temperatures produces many species in the materials, such as trapped electrons, cations and anions, etc. These active species have been investigated by many authors by using several methods such as pulse radiolysis,cl) photo-absorption,(') electron spin resonance,(3) and luminescence measurements,c'' Thermoluminescence (TL) measurement also offered a useful method to study the active species produced by irradiation. In our recent work on T L of irradiated aliphatic oligoester, it has been reported that the pregiow and the lower temperature glow (a-glow) can be attributed to recombination of the trapped electron with the ester cation and that the higher temperature one (/3-glow) to the dimerization of acyl radicals. (s) In that work the experiment has been carried out in the polycrystalline state of the aliphatic oligoester. We have found later that the oligoester of ethylene glycol and adipic acid, which is denoted as (2,4) oligoester in the previous work, °) could be prepared in glassy state by rapid cooling from the melt. Moreover, in a preliminary experiment on TL of irradiated glassy (2, 4) oligoester, we have obtained results different from those in the poly-

crystalline state: the l o w e r temperature glow (aglow) and the higher temperature one (~8-giow) were both very weak compared with those in the case of polycrystalline state. In the present paper, the difference in T L between the polycrystalline state and the glassy state is first made evident and TL mechanism of the glassy (2,4) oligoester doped with some electron scavengers is investigated. EXPERIMENTAL Sample preparation. The sample used was the ester made of ethylene glycol and adipic acid. which has been denoted as (2, 4) oligoester in the previous paper. ~ This (2,4) oligoester could be made glassy by cooling it rapidly from the melt by using liquid nitrogen but it became polycrystaUine in slow cooling. Biphenyl, terphenyl, benzophenone and carbon tetra bromide of extra pure grade were used without further purification as the electron scavenger. The (2, 4) oligoester could be made glassy even when the electron scavenger was added in it. The sample which is melted after being sealed in the quartz cell in a vacuum of about 10-3 tort was cooled rapidly down to liquid nitrogen temperature. Irradiation~ In the TL measurement, the sample was irradiated at 83 K to the dose of i-3 Mrad by 1.2 MeV electron beams from a Van de Graaff accelerator. In the electronic absorption and the ESR measurements, the sample was irradiated at 77 K to the dose of 0.6 Mrad by y-rays from a 6°Co source. 307

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TL measurement. The details of the TL measurement were described in the previous paper,tTJ The sample irradiated at 83 K was warmed up at a constant rate of 2.5 K/min to room temperature. ESR measurement. ESR measurement was carried out by using an X-band spectrometer with 100KHz field modulation manufactured by JEOL Co. Ltd. The ESR data was analyzed by JEC-6 spectrum computer manufactured by the same company. Absorption measurement. Spectrophotometer MPS5000 manufactured by Shimazu Seisakusho Co. Ltd. was used for the absorption spectrum measurement. In the measurement of the temperature dependence of the absorption spectrum, the sample was warmed up at a constant rate of 2 K/rain.

of the undoped (2, 4) oligoester, are shown in Fig. 2. A broad glow curve was observed at around 140 K for all samples. This component is much more intense compared with that in the case of the undoped sample. In the sample doped with biphenyl or terphenyl, a rather weak glow, compared with the glow appearing at 140 K, was observed at 220 K. This component could not be observed in the sample with benzophenone. (ii) T L spectra Figure 3 shows the T L spectrum of the sample doped with biphenyl, benzophenone or terphenyl, together with that of the unripped sample and of the polyethylene glycol matrix. The measurement was carded out quickly in a short period while the glow is appearing. The same spectra were observed even if the measurement was carried out at different temperatures during the glow. Since the T L spectra did not coincide with the fluorescence or phosphorescence spectra of the (2,4) oligoester, °~ the spectra appearing at the wavelengths longer than 380 nm and that of 290-380 nm could be assigned to the phosphorescence spectra and to the fluorescence spectra from the dopants, respectively. The sample doped with benpzophenone, however, did not have the fluorescence component and the one doped with the biphenyl had a weak fluorescence component. On the other hand, if polyethylene glycol matrix was used with biphenyl as dopant, the fluorescence component larger than that from the oligoester matrix was obtained, as is shown by Fig. 3-D.

RESULTS (i) G l o w curves Glow curves of the (2,4) oligoester irradiated by electron beams at 83 K are shown in Fig. 1, together with temperature dependence of tan 8 measured by dielectric measurement. The solid line and the broken line show the glow curves observed in glassy and polycrystalline states, respectively. It can be seen that there is a remarkable difference between two curves: the glow intensity of the glassy sample is far smaller than that of the polycrystalline one. In particular, the decrease in the intensity of the /3-glow is considerable. Moreover, it was found that sudden gas evolution and crystallization occur at around 260 K in the glassy sample. Glow curves of the glassy (2,4) oligoesters doped with biphenyl, terphenyl and benzophenone, measured in the same condition as that

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TEMPERATURE(K) Fig. 2. Thermoluminescence glow curves of (2,4) oligoester doped with various electron scavengers: A, biphenyi; B, benzophenone; C, terphenyl.

(iii) A b s o r p t i o n spectra o f irradiated sample When the doped samples are irradiated at 77 K, they acquire color. Absorption spectra of the samples doped with 0.01 moll! of electron scavengers, biphenyl, terphenyl and benzophenone, are shown in Fig. 4. These spectra can be assigned to the anion of the dopant from the results given by Shida et al.
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temperature dependence of the spectrum for the sample doped with terphenyl and biphenyl, the spectral shape did not change, but only the spectral intensity decreased with temperature. In the sample doped with benzophenone, however, both spectral shape and intensity changed remarkably as shown in Fig. 4-C. The spectral peak appearing at 760 nm at 77 K, which is due to benzophenone anion, shifted gradually to a shorter wavelength and the intensity also decreased with elevation of temperature. At same time, a new peak appeared at around 560 nm which grew up gradually with elevation of temperature. This spectrum can be assigned to ketyl radical (d~-(~(OH)-d,) as reported by Shida et dl. ¢9) Figure 5 shows temperature dependence of the absorption intensities of biphenyl anion, benzophenone anion and ketyl radical observed at Amax= 650, 760 and 560 nm, respectively. It can be found that the decrease of benzophenone anion corresponds just to the increase of ketyl radical.

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(iv) E S R s p e c t r u m ESR spectra of 1 wt% biphenyl and 1 wt% benzophenone in the glassy (2,4) oligoester matrix are shown in Fig. 6. Irradiation and measurement were carried out at 77 K. When the sample doped with biphenyl is illuminated by visible light at 77 K after the irradiation, the spectrum shown by Fig. 6-A-b was observed. It can be found by subtracting Spectrum b from Spectrum a that decay of a singlet spectrum occurred by illumination of visible light. Since the absorption spectrum due to the biphenyl anion also disappeared by illumination of visible

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Fig. 4. Absorption spectra of -#-irradiated (2,4) oligoester doped with various electron scavengers: A, terphenyl, observed at 77 K; biphenyl, observed at 77 K, C, benzophenone, observed at 77 K (a), 120 K (b) and at 200 K (c); D-a undoped (2, 4) oligoester and D-b doped with CBr,, observed at 77 K.

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Fig. 5. Temperature dependence of absorption intensities of: O, biphenyl anion; , , benzophenone anion, and A, ketyl radical, observed at Am~x-- 650, 760 and 560 rim, respectively.

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FIG. 6. First derivative ESR spectra of y-irradiated (2, 4) oligoester with electron scavengers: A-a, biphenyl, observed at 77 K after ~-irradiation; A-b, observed after illumination of A-a by visible light at 77 K: B, benzophenone observed at 77 K. light, it is concluded that this singlet spectrum arises from the biphenyl anion. The residual spectrum is composed of a doublet spectrum and a slight singlet spectrum. The doublet spectrum ~:an be assigned to the ester radical anion ( - C H r - O - C O - C H r - ) produced in (2, 4) oligoester as was pointed out by our previous work. ~6>The other slight singlet spectrum may be attributed to the biphenyl anion which could not be bleached by illumination of visible light. The yield of the biphenyl anion increased with concentration of biphenyl and at 10% biphenyl in the (2, 4) oligoester the dominant production was biphenyl anion. DISCUSSION (i) TL mechanism of undoped glassy (2, 4) oligoester As shown in Fig. 1, two glows (a- and/]-glows) for undoped glassy oligoester are so weak in comparison with those of the polycrystailine one. It has been reported in our previous paper ~5~that the a and/]-glows are caused by recombination of trapped electrons with ester cations and dimerization of acyl radicals, respectively. It is evident that the difference in intensity between these glows

results from the difference in the physical state of the matrix. If the a-glow is due to recombination of trapped electrons with cations as was mentioned by Hama et al.,~> it should be considered that the yield of trapped electrons are far smaller in the glassy oligoester than in the polycrystailine one. The reasons for this large decrease of the yield may be as follows: (a) In the glassy oligoester, electrons produced by irradiation are trapped by ester groups to form ester anions. (b) Even if the electrons are trapped at some sites other than the ester group, they transfer easily to the ester group. This will occur when the depth of the traps in the glassy oligoester is shallower than that of the polycrystalline material. (c) Large difference in the intensity of the preglow observed immediately after irradiation at 77 K between two physical states could not be observed. This suggests that electrons trapped by shallow traps have already disappeared by recombination with cations or by transferring to ester groups during the stage of the preglow. Reasons of the decrease of the/]-glow will now be considered. It has been suggested that the appearance of the/]-glow is caused by dimerization

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of acyl radicals which result from decomposition of ester anion by the reaction My + CO-O-CHz(1)

+ CO-CH As is described above, a large amount of ester anion should be produced in the glassy matrix. Therefore, if reaction (1) occurs also in the glassy matrix, intense ~-glow is expected to occur. However, gas evolution has been observed in the vicinity of the temperature at which the ~-glow appeared. The mass spectroscopic analysis of the evolved gas indicated that CO2 (83%) and CO (10~) are the dominant products. Since the production of ester anions at low temperature in the glassy matrix has certainly been confirmed similarly as in the polycrystalline one by ESR measurement, it is surmised that gas evolution reaction but not the dimerization of acyl radical occurs predominantly in the region of the /3-glow temperature. This reaction may be written as (2)

---CHz-O-CO --~M3 + CO2

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--CH~4~O -> M4 + CO.

The new species, M3 and M4, in reactions (2) and (3) could not be identified in the present work yet. They should have decayed in quite a short time after production. The reaction concerned should be a non-radiative one, since almost no /~-glow could be observed in the glassy matrix. (ii) Electron scavenger e•ect on TL. (ii)-a. Luminescence center. In the glassy matrix doped with three types of electron scavengers, a greater glow peak was observed in the vicinity of 140 K, in comparison with the case of the undoped oligoester. The TL spectra of these doped matrices did not result from the ester cation but were due to emission from the electron scavengers. Therefore, the luminescence center of the doped matrix should be the excited state of the electron scavenger. The TL spectra of the oligoester doped with benzophenone did not have the fluorescence component and for the matrix with biphenyl some trace of the fluorescence was observed. This suggests that only the excited triplet state can be formed in the matrix with benzophenone and that both excited singlet and triplet states can be formed in other matrices. (ii)-b. Luminescence mechanism. As is described above, the anion of the electron scavenger

is formed in the glassy matrix by irradiation at low temperature. In the irradiated matrix, there should also be a large amount of cations comparable with that of anions. In the present work, the cation could not be detected directly in ESR and absorption measurement. It can be inferred, however, that there exists a large number of oligoester cations also in the doped matrix, since the luminescence center of the undoped matrix is the oligoester cation -CHz-O--CO--CHz- as is revealed in the TL experiment of the undoped matrix. On the other hand, the luminescence center of the doped matrix was found not to be the ester cation but the electron scavenger itself. It is considered in general that the luminescence center are cations produced by irradiation and that the emission is caused by recombination of cations with electrons. In the doped matrix, however, almost no trapped electrons exist, since a large amount of anions are produced from electron scavengers. Comparing the glow curve with the absorption decay curve of anions, shown in Figs. 2 and 5, it can be seen that the luminescence is related to the decay of the anions. This fact indicates that the excited state of the electron scavenger is produced by the reaction of the scavenger anion with some other molecule. The occurrence of the reaction of the scavenger anion with the ester cation existing near the anion may be most probable, since at low temperature at which the glow appears, a large scale molecular motion such as the microbrownian motion are not allowed. In fact, it is confirmed that the glass transition temperature of the (2, 4) oligoester is about 200K by dielectric loss measurement (Fig. 1). Recently, Meggitt and Charlesby have pointed out that in the TL study of irradiated polyethylene in which biphenyl is soaked, the luminescence occurs when the holes, trapped in the polymer matrix following irradiation, are released thermally and migrate towards the biphenyl anions. ~1°~In the present work, a similar migration of matrix holes towards the scavenger anions which is caused by a local motion of the ester groups may be possible, since there are a large number of ester groups in the vicinity of the matrix ester cations produced by irradiation. These suppositions lead to the following conclusion concerning the mechanism of the glow observed at lower temperature in the doped matrix: (1) When the matrix is irradiated at low temperature, the cations (M ÷) of (2, 4) oligoester and the anions (A-) of scavengers are produced in the matrix. On warming the matrix, these cations

Thermoluminescence of aliphatic oligoesters and anions react through the following processes (4)

M * + M ~ M + M ÷ (hole migration),

(5) M * + A - - - * ( M .... A)*-.* M + A * - , M + A + hz,, where ( M .... A)* means a state like the excited charge transfer complex. (2) In reaction (5), if the scavenger is benzophenone the emission results only from the excited triplet state. In the cases of other scavengers, it results from the excited triplet and singlet states, but for biphenyl the singlet component is weak. On the other hand, the weak glow which was observed at around 220 K for the matrix doped with biphenyl and terphenyl, but not with benzophenone, is related apparently to the glass transition of the matrix. This glow could be caused by the same reaction as that at lower temperatures, but the anion and the cation involved in this reaction should be separated rather far apart. The absence of the higher temperature glow in the matrix doped with benzophenone is due to the formation of ketyl radical from the benzophenone anion with elevation of temperature. (iii) Quenching effect o f the fluorescence c o m ponent in the T L spectrum As is described above, there is a difference in the appearance of the fluorescence component of the T L spectra for the different electron scavengers. It is supposed that this difference can be attributed to the energy transfer process in the reaction ( M ..... A)*--, M + A* in reaction (5). It is known that benzophenone has no fluorescence but only phosphorescence due to the strong intersystem crossing from the singlet state to the triplet state."" Therefore, the excited benzophenone,

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produced through reaction (5), also drops to the lowest triplet state by radiationless intersystem crossing. It is concluded that this process does not depend on the kind of the matrices used in the present experiment. On the other hand, in the case for scavengers such as terphenyi and biphenyl, fluorescence appears but its intensity depends on the kind of the matrix. This suggests that the quantum yield of intersystem crossing to the triplet state depends on the kind of the matrices in such case. Acknowledgement--We wish to thank Dr. K. Shinohara for helpful discussions.

REFERENCES I. (a) S. ARAI and M. C. SAUER, J. chem. Phys. 1966, 44, 2297. (b) S. ARAi,A. KmA and M. IMAMURA, J. phys. Chem. 1975, 80, 1968. 2. E. T. KAISERand L. KEVEN (editors), Radical Ions. p. 321, Interscience, New York, 1968; Ionic Process in y-Irradiated Organic solids at - 196 C, by W. H. HAMILL.

3. K. TsuJI, H. YOSHIDA and K. HAYASHI, J. chem. Phys. 1967, 46, 810. 4. (a) K. FUNABASrn,P. J. HERLEY and M. BURTON,J. chem. Phys. 1965, 43, 3939. (b) J. KROrl and J. MAYER. Int. J. Radiat. Phys. Chem. 1974, 6, 423. 5. M. TSUMURA,S. TAKAHASHI,N. OMI and Y. HAMA, Radiat. Phys. Chem. 1980, 16, 67. 6. T. Ooi, MIMURA, Y. HAMA and K. SmNONARA, J. Polymer Sc£ Polymer Chem. Ed. 1976, 14, 813. 7. Y. HAMA, K. N]SHI, K. WATANABEand K. SmNOHARA, J. Polymer Sci., Polymer Phys. Ed. 1974, 12, 1109. 8. T. SHXDAand W. H. HAMILL.J. chem. Phys. 1966, 44, 2375. 9. T. SrlIDAand W. H. HAMILL,J. Am. chem. Soc. 1966, Jill, 3683. 10. G. C. MEGGrFr and A. C~ARLESBY, Radiat. Phys. Chem. 1979, 13, 45. 11. A. LAMOLAand G. HAMMOND,J. chem. Phys. 1965, 43,/ 2129.