B. Brocklehurst, J. S. Robinson, and D. N. Tawn
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Emission from Aromatic Radicals in Ion Recombination Luminescence rockfehurst,* J. S. Robinson, and
D. N. Tawn
Department of Chemistry, The University, Sheffield, S3 7 H f , United Kingdom (Received March 31. 7972)
Thermoluminescence occurs when solutions of aromatic hydrocarbons are irradiated with y rays or ultraviolet light and then allowed to warm up, releasing trapped ions. The luminescence spectra consist of emissions from the solute molecules and from related radicals, e . g . , benzyl from toluene: loss or gain of a hydrogen atom may occur. Carbanions are likely precursors of the excited radicals since infrarec! stimulated emission comes from the molecules, not the radicals, in most cases. At high doses, ground state radicals are formed and then trap charge in a second radiation-dependent step. Surprisingly, radical emission is sometimes observed at low doses (down to 1.5 krads): possible mechanisms are discussed.
Introduction Ions can be trapped in organic glasses a t 77 K:1-4 they can be produced by high-energy radiation or ultraviolet light; the latter usually requires a long-lived triplet state which is ionized by a second photon. Recombination of the ions, especially in alkane glasses, often produces lumin e ~ c e i i c e which ~ , ~ can be used to study the mechanisms involved. Some recombination takes place at 77 K (isothermal luminescence) because of the slow diffusion of electrons; this can be speeded up by irradiating in the absorption bands of electrons or anions (infrared stimulated emission; prolonged exposure to visible or infrared light bleaches out the ions). Recombination also occurs on warming (thermoluminescence) when molecular ions as well as electrons become mobile. Burton and other^^-^ have studied the weak ion recombination luminescence of the pure alkanes, where the identity of the emitters is still uncertain:6 they may be radicals. When aromatic solutes are added, positive and negative charge is transferred to them from the solventlJ and the luminescence usually consists of fluorescence and phosphorescence of the In some cases, the emitter may be a radical derived from the solute: e . g . , toluene solutions give benzyl emission in thermoluminescence after both photolysis7 and radiolysis;8 durene similarly ~ ~ l ~ on the loses a hydrogen atom to give d ~ r y l . Styrene, other hand, gains a hydrogen atom: its thermoluminescence in the green has been shown to be due to the a rnethylbenzyl radical .I1 By analogy, similar emission from ~ ) ~ e n y ~ a c e ~solutions ~ l e n e is probably due to the a-styryl radicaLI2 Formation of ground state radicals by photolysis and radiolysis is well known, but the mechanisms by which excited radicals are produced in ion recombination are not yet clear. In some of the previous work, very high doses were k e d so that trapping of charge by previously formed radicals was likely.9J1 In this paper, results obtained mainly a t very low doses are presented. Experimental Section Some of the methods used have been described prev i 0 u s i y . ~ ~ ,In ~ 3brief, 25-cm3 samples were made up in high-purity silica tubes (internal diameter, 15 mm) and degassed by repeated freezing and pumping. A 2 : 1 mixture of methylcyclchexane and isopentane was used as thq solvent. Indene was shaken with 6 N HC1, refluxed with The Journal of Pliysi6al Chemistiy, Vol. 76, No. 25, 7972
40% sodium hydroxide for 2 hr, and passed twice down a silica gel column. After distillation under nitrogen at reduced pressure, it was used as quickly as possible to avoid polymerization. This procedure greatly reduced the initially observed phosphorescence, probably due to carbazole impurity. Phenylacetylene was purified by gas-liquid chromatography. Toluene (Analar reagent grade) and other solutes were used as received. Sulfur hexafluoride was distilled on the vacuum line. Samples were radiolyzed with a 6oCo source, dose rate approximately 750 rads/min; exposure to light was reduced as far as possible to avoid bleaching. Photoelectric intensity measurements were usually made with an Aminco spectrofluorimeter, with the excitation source off for thermoluminescence and isothermal luminescence, or set to 750 nm with a red filter inserted for infrared stimulated emission. Quoted wavelengths have not been corrected for instrument response. For thermoluminescence measurements, samples were allowed to warm up quite quickly (see Figure 1) in a precooled empty dewar: temperatures were measured with a thermocouple a t the center of the sample in a separate experiment. The method has disadvantages: there is a temperature gradient inside the sample which partly blurs out details of the glow curves and the sample thickness is such that significant reabsorption of fluorescence occurs. On the other (1) W. H. Hamill in "Radical Ions," E. T. Kaiser and L. Kevan, Ed., Interscience, NewYork, N. Y., 1968. (2) J. E. Willard, Mol. Cryst. Liquid Cryst., 9, 135 (1969) (3) F. Kieffer and M. Magat in "Actions Chimiques Bioiogiques des Radiations," Vol. 14, M. Haissinsky, Ed., Masson et Cie, Paris, 1970, p 135. (4) B. Brocklehurst, Radiat. Res. Rev., 2, 149 (1970). (5) M. Burton and K. Funabashi, Mol. Cryst Liquid Cryst. 9, 153 (1969). (6) B. Brocklehurst and d . S . Robinson, Chern. Phys. Lett., IO, 277 (1971). (7) W.A.'Gibbons, G. Porter, and M. i. Savadatti, Nature (London). 206, 1355 (1965). (8) B. Brocklehurst, R. D. Russell, and M, I. Savadatti, Trans. Faraday SOC., 62,1129 (1966). (9) C. Deniau, A. h r o u l e d e , F. Kieffer, and J. Rigaut, J. Lurnin., 3, 325 (1971). (10) A. C. Albrecht and F. P. Schwarz, Proceedings of the Tenth Czechoslovak Annual Meeting on Radiation Chemistrv, Marianske La z n k 1970, p 197. (11) A. Deroulede, F. Kieffer, E. Migirdicyan, and J . Sigaut, J. Chim. Phys., 67, 1931 (1970). (12) 8. Brocklehurst, J. S. Robinson, and D. N. Tawn, Chem. Phys. Lett.. 12. 610 (1972). (13) B. Brocklehurst and R D. Russell, Trans Faraday SOC.,65, 2159 (1969).
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Aromatic Radical Luminescence hand, rapid warm-up of thick samples produces high light intensities so that spectra can be recorded easily a t very low doses. High intensities of ultraviolet light were obtained with a Rayonet photochemical reactor, but most photolysis measurements were made with a Hanovia coiled low-pressure mercury arc" intensities were measured with a ferrioxalate actinometer. Thermoluminescence intensities were much lower than with rhdiolysis; the intensity soon saturates because of the efficient bleaching out of trapped electrons. Spectra could be recorded on the fluorimeter a t high doses. at low doses, a filter-photomultiplier combination was used to obtain greater sensitivity. Partial bleaching of irradiated samples was carried out by placing the sample in a silica dewar surrounded by a Wratten gelatin filter a t the center of a square of four 150W tungsten lamps,
Results (i) Toluene. Fluorescence and phosphorescence of toluene are readily identified in the thermoluminescence, after y radiolysis, together with the fluorescence of the benzyl radical: the identity of the latter has been checked by photographing the spectrum, so resolving the vibrational structure.I2 In the case of naphthalene,13 a t high concentrations the monomer fluorescence early in the glow curves is replaced by excimer fluorescence later on: monomer cations, diffusing some distance, dimerize before neutralization. The toluene fluorescence shows a similar shift to longer wavelengths but the excimer emission could not be clearly separated at the low resolution of the photoelectric measurements: this is due in part to the low-fluorescence efficiency of the excimer and the relatively small shift in wavelengthl4 and in part to the distortion of the monomer fluorescence by reabsorption (unusually large here because emission occurs uniformly through a thick sample). Typical glow curves for low doses of y rays are shown in Figure 1: measurements were made a t 280 (fluorescence), 380 (phosphorescence), and 485 nm (benzyl): note the compressed intensity scale. Measurements were made over the concentration range, lO-*-l M . Between &I, benzyl emission can just be detected (intenand sity -1% of the phosphorescence maximum at 5 x M ) at the very end of the glow curve: this is very difficult to study because of the rapid decay of intensity. It may be that the benzyl glow curve is similar to that in Figure l b , but it is masked earlier by the strong phosphorescence (the benzyl glow curve is shifted slightly to later times, probably because of the lower mobility of the benzyl anion, see below). As showri in Figure 1, there i s a sudden change in behavior above M . At first sight, benzyl emission appears to replace phosphorescence; however, this effect is coincidental. The behavior of the toluene emissions will be discussed elsewhere: here it suffices to state that the early part of the glow curve is due to cation-electron pairs with smail separation, the later part to cation-anion recombination; the phosphorescence to fluorescence ratio falls as the temperature rises because of decreasing phosphorescence effi~iency;l5,~6 at high concentrations there is an even sharper fall because neutralization of dimer cations gives triplet excimers which undergo rapid quenching.17 The rise in benzyl intensity is exaggerated by the
0--
Q
X
el
77 0
a5
aoo
95 K 400
5
Glow curves: log (thermoluminescence intensity) vs. time: temperature calibration inserted. Intensities are not corFigure 1.
rected for instrument response, but are comparable between a and b: (0,toluene fluorescence: X , phosphorescence: 0 , benzyl fluorescence: (a) lo-' M, (b\ 5 X I-O-' M toluene in 2 : l mixture of rnethylcyclohexane and isopentane; dose = 1.5 krads. reduction in phosphorescence, but there i s a steady increase up to 0.1 M after which the intensity levels off. Benzyl is not observed in the early part of the glow curve at any concentration. Both above and below 10P2 M sthe relative intensities of the three emissions are independent of dose from 1.5 krads up to a t least 50 krads; benzyl is still increasing with dose at 600 krads while the toluene emissions pass through a maximum around 300 krads. Strong bleaching of the samples after the irradiation eventually removes all the thermoluminescence, but the toluene emissions were removed more. quickly than benzyl. With less bleaching (15 sec, tungsten light above 600 nm) benzyl actually increased ( X 3) while the others were reduced ( X 10) compared with the nonbleached sample. Neither bleaching nor high doses affected the toluene fluorescence to phosphorescence ratios. The addition of sulfur hexafluoride to scavenge electrons had the same effect on the glow curves as in the case of naphtha1ene;Is all intensities were reduced, toluene fluorescence most, benzyl least. Studies of isothermal luminescence and infrared stimulated emission showed only toluene emissions: benzyl could not be detected under any of the conditions used. Fewer studies of photoionization have been made because of the low intensity of thermoluminescence: the intensity quickly levels off with dose because of bleaching. The results given here were obtained with exposures of 5 (14) J. B. Birks, "Photophysics of Aromatic Molecules," Wiley, London, 1970. (15) N. G ,Kilmer and J. D. Spangler,J.Chem. f'hys., 54, 604 (1971). (16) S. Fischer, Chem. Phys. Lett., 10, 397 (1971). (17) R. 5. Cundali and W. Tippett, Trans. Faraday Soc., 68, 350 (1970); R. B. Cundall, L. C. Pereira, and D. A. Robinson, Chem. Phys. Lett., 13, 257 (1972); R. V. Bensasson, J. 1. Richards and J. K. Thomas, ibid., 9, 13 (1971).
The Journal of Physical Chemistry, Vol. 76, No. 25, 1972
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to 60 sec at dose rates up to 1.5 x einstein cm-I sec-l of mercury 254-nm radiation (sample 3.5 cm from the lamp). Qualitatively, the effect of concentration on the glow curves is the same as for y irradiation. Intensities and intensity ratios (benzyl to toluene) were not easily reproducible, varying by a factor of 2 or 3: this may have been due to cracking of the sample and icing of the surface of the tube, affecting the rate of light absorption. M , it is clear that the ratio of benHowever, a t 5 x zyl and toluene intensities increases steadily with dose even at the shortest exposures and, within the considerable experimental error, may be zero initially. The lamp emits light a t longer wavelengths (though in relatively small amountsl8): when a glass filter was used to cut out 254 nm after a few seconds the benzyl intensity and the benzyl to toluene ratio again increased but more slowly. The addition of SF6 reduced intensities but did not affect the ratios. ( 6 1 ) Other Compounds. Emission in the region 480-580 nm has been observed in the thermoluminescence of the following compounds: p-xylene, mesitylene, durene, hexamethylbenzene, indane, and diphenylmethane. Careful spectroscopic studies have not been made but it seems plausible to assign these to the fluorescence of radicals formed by loss of a hydrogen atom from the parent molecule: this has been established in the case of durene.9JO Insofar as studies have been made, these compounds behave like toluene, though the efficiency of radical emission is usually lower and is only easily seen late in the glow curves: indane and mesitylene readily give radical emissions a t low y-ray doses like toluene, while quite large doses (300 kratls) are needed for p-xylene, durene, and kexamethylbenzene. The same differences were observed in photolysis experiments. Phenylacety!ene gives thermoluminescence peaks at about 300 (fluorescence), 430 (phosphorescence), and 480 nm; the latter has been ascribed tentatively to the astyryl radical12 formed by addition of a hydrogen atom (addition of a solvent radical is also possible) as in the case of styrene.ll We have attempted to prepare this radical in other ways without success to date, but the identification remains plausible. The relative intensity of the radical emission increases a t doses of a few hundred kilorads but the effect of concentration is much smaller: the amount of radical emission increases with concentration but the radical to molecule ratio changes only by a factor of 1.5 between and 5 x M Again, the radical is not observed early in the glow curve or in infrared stimulated emissions ( m e ) . Indene gives a srrong fluorescence peak a t 315 nm, very weak emission aroiind 400 nm (possibly the molecular p h o s p h o r e s ~ e n c e ~ and ~ ) , a strong peak at 500 nm. The 315- and 500-nm peaks were scarcely affected by the extensive purification procedures; the latter peak coincides with the long-wavelength emission from indane and is ascribed to the indanyl radical; the photographic methodx2 confirms that the two radicals are identical. Indene then, appears to gain a hydrogen atom rather than lose one. Since indenyl is a nonalternant radical, its fluorescence, if any, would lie in a different spectral region from indanyl. The concentration dependence of thermoluminescence is similar to that of phenylacetylene, but indene differs in that the radical emission can be observed early in the glow curves and in irse (though less strongly). Attempts to photoionize indene and phenylacetylene were unsuccessf ul. The Journal ot Physical Chemistry, Vol. 76, No. 25, 1972
8. Brocklehurst, J. S. Robinson, and D. N. Tawn
Discussion Deniau, et a l . , S have observed emission from the duryl radical in the thermoluminescence of durene solutions after y radiolysis: at doses of 100 krads the radical emission is confined to the second peak of the glow curve which is due to cation-anion recombination (n.b., their “thermal resolution” is better than ours); a t doses > 1 Mrad, radical emission is observed in the first peak and in isothermal luminescence, both due to cation-electron recombination. They suppose that ground state radicals are first formed in the radiolysis, and subsequently trap charges; negative charge is trapped efficiently because of the great differences in electron affinity between durene and duryl; a t high doses there are sufficient radicals to compete for positive charge as well. In our work at low doses, the radical to molecule luminescence ratio is independent of y-ray dose between 50 and 1.5 krads; the latter dose gives concentrations of active species of -10-6 M . so processes involving two radiation-induced steps can be ruled out. The absence at low doses of radical emission when cations recombine with electrons (irse, isothermal luminescence, early part of the glow curves) shows that benzyl cations are not present and that excited benzyl is not produced by dissociation of excited toluene. (This process has been observed in durene, but the efficiency is only 10-3.10) Benzyl cations might be produced by reactions during the diffusion process, but the bleaching studies suggest that benzyl anions are involved. In the highly viscous liquid produced by softening the glass (viscosity lo6 P), catidn-anion neutralization takes place while the two ions are still separate.13 One can regard the electron as tunnelling through the solvent: the highest energy electron will tunnel fastest leaving the anion in its ground state, the cation becoming excited. However, the two molecules may well be close enough for energy (both singlet and triplet) to be transferred back to the former anion; 1 e., toluene cation, benzyl anion recombination can lead exclusively to benzyl excitation. To test this hypothesis, a M toluene solution containing 3 x M naphthalene was irradiated: irse gave mainly toluene emission showing the predominance of toluene cations, while naphthalene emissions were much stronger in thermoluminescence (c7&++ C10H8-).30 The usual mechanism of “bleaching” is to excite electrons from their traps: eventually, this leads to recombination and the loss of thermoluminescence. The enhancement of benzyl emission by continued photolysis might be due to bleaching of toluene anions followed by retrapping on benzyl radicals. However, at the smallest exposures used the concentration of ground state radicals is very M can be estimatedSz1It is difficult to besmall; lieve that the radicals could trap electrons so efficiently; also the effect of SFs would be to reduce benzyl emission much more than toluene, but this i s not observed. It seems necessary to postulate a different effect of the bleaching light.
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C. A. Parker, “Photoluminescence of Solutions,” Elsevier, Amsterdam, 1968, p 162. M. Koyanagi and Y. Kanda, Mem. Fac. Sci., Kyushu Univ., Ser. C., 6, 109 (1968). 8. Brocklehurst and J. P. Guerin. unpublished work. B. Brocklehurst, W. A. Gibbons, F. T. Lang, G. Porter, and M. I. Savadatti, Trans. Faraday Soc., 62, 1793 (1966).
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Aromatic Radical Luminescence The electron affinities of toluene22 and are not known with certainty but their difference is such that light of wavelength 51000 nm would give an exothermic reaction. This mechanism does not immediately explain the radiolysis results: possibly a track or spur effect is involved in which excitation produced locally by the same y ray is transferred between toluenes until it reaches a toluene anion, Such transfer would be efficient at the high toluene concentration. (Transfer of excitation to benzyl after recombination would give benzyl emission during irse, etc.) The reason for the need for high toluene concentration in the photolysis experirnents is not clear. Perhaps electron loss is more efficient than reaction 1, so that efficient re1:rappmg is necessary. At the low temperatures and high concentrations used, considerable aggregation of the toluene molecules to form crystals or local concentrations may occur; Lipsky and Burton24 suggested the existence of “domains” to explain the energy transfer properties of benzene-cyclohexane mixtures a t room temperature. However, triplet states are very short lived in crystals and even in dimers;17 photoionization, a t any rate, must involve isolated molecules. The preceding discussion applies mainly to the lowoses > 1 Mrad, the result of Deniau, et a l , 9 show that radicals are acting as traps for positive charge. While negative charge will be more readily scavenged than posithe, the increase in benzyl to toluene ratio above 100 krads can also be explained if benzyl anions are less readliy bleached than toluene anions during continued y irradiation. Either hypothesis will also explain the appearance of radicals from hexamethylbenzene, durene, and p-xylene in this region of dose, and the rela-
tive increase in other cases such as phenylacetylene. The differences between the methylbenzenes are a little surprising: measurements over a wider range of conditions will be needed to elucidate this. The mechanism of radical excitation (at low doses) when the molecule has gained a hydrogen atom appears to be different from the hydrogen loss case, because of the different concentration dependence. Some, a t least, of the indanyl radicals may have positive ions as precursors since the radical emission is observed weakly in irse; on the other hand, phenylacetylene apparently gives a-styryl anions only. Further measurements are again required before the mechanisms can be discussed further.
Conclusion At high doses, both neutralization of carbonium ions by electrons or molecular anions, and of solute cations by carbanions can produce excited radicals; a t low doses the latter process alone occurs (except, probably, in the case of indene). At high doses, radicals are formed and then act as traps for positive and negative charge: at low doses the carbanions must be formed in other ways: in the case of toluene this probably involves the loss of a hydrogen atom from an excited toluene anion. Acknowledgment. The authors thank the Science .Research Council (U. K.) for the award of maintenance grants to two of them (J. S. R. and D. N. T.). (22) R . N. Cornpton, L. 6.Christophorou, and R .
Lett., 23,656 (1966).
H. Wuebner, Phys.
(23) D. K. Bohrneand L. B. Young, Can. J . Chem,, 49,2918 (1971). (24) S. Lipsky and M. Burton, J . Chem. Phys., 31,1221 (1959).
The Journalof Physical Chemistry, Vol. 76. No. 25, 7972
