- Email: [email protected]
Isothermal luminescence of γ-irradiated 3-methylpentane containing two solutes at 77 K
Radiat. Phys. Chem. Vol. 16, pp. 399-403 Pergamon Press Ltd.. 1980. Printed in Great Britain
ISOTHERMAL LUMINESCENCE OF y-IRRADIATED 3METHYLPENTANE CONTAINING TWO SOLUTES AT 77 K J, MAYER*, M. SZADKOWSKA-N1CZEand J. KROH The Institute of Applied Radiation Chemistry, 93-590, ILod;t,ul. Wr6blewskiego 15, Poland (Received 10 July 1979)
Abstract--A 3-methylpentane glass containing a pair of solutes was y-irradiated at 77 K and the isothermal luminescence was measured. Two pairs of solutes were examined: naphthalene-9,10diphenylanthracene and toluene-naphthalene. For constant concentration of left-hand solutes (SI) the concentration of right-hand solutes ($2) was changed. When the $2 concentration increases, the luminescence intensities measured at the emission bands of $2 increase. The results can be explained in terms of a tunnelling charge transfer model and a competitive charge capture scheme. INTRODUCTION THE MECHANISMof isothermal luminescence (ITL) observed at 7 7 K in y-irradiated hydrocarbon glasses in the presence of aromatic scavengers seems to be understood. "-3) In the case of pure hydrocarbon matrices the origin of the ITL is still open to discussion. There were some suggestions that the radiolysis products--radicals and/or stable, unsaturated end-products formed during the irradiation--may contribute to this phenomenon. "-*) Radicals and olefins (or dienes) having positive electron affinities and lower ionization potential than the matrix molecules<7~ can trap the primary charges. The newly formed cations may recombine with electrons or anions, giving the excited states responsible for ITL. In order to find the reaction paths occurring in such a complicated system, the ITL at 77 K of y-irradicated 3-methylpentane in the presence of two aromatic scavengers has been investigated.
and To-Nph. These two solutes in each pair are not equivalent. The left-hand solutes ( S l ) in each pair have higher ionization potential and higher excited states level than the right-hand ones ($2) whereas the $2 solutes have higher electron affinity than S l. tg~ Figure 1 illustrates the emission spectra of ITL
I 43o~ ~
~
I
EXPERIMENTAL 3-Methylpentane (3MP), Fluka pure grade was chromatographed through a freshly activated silica gel column and kept under argon. Naphthalene (Nph), toluene (To), both analytical grade, and 9,10-diphenylanthracene (DPA), Koch-Light, puriss, were used as received. The 3MP samples, 2.5 cm3 by volume (cylindrical cell, l0 mm i.d.) were outgassed by the freeze-pump-thaw technique and sealed off under a pressure of ca. 10-4 mm Hg. The emission from the y-irradiated samples was analysed at 77 K as described previously. ~s~Irradiations with y-rays of 6°Co were carried out in liquid nitrogen at the dose rate of 1.7x 10~SeVdm-3 s-~. The dose used in experiments was 5.1 x 102°eV drn-~. R E S U L T S AND DISCUSSION In our experiments the following pairs of aromatic scavengers have been used: N p h - D P A
1
J
1
i
600 500450 ~ 0
|
350
a
30Ohm
FIG. !. ITL emission spectra of the y-irradiated glassy systems at 77 K: (a) 3MP + Nph(10-~ tool din-3); (b) 3MP + DPA(5 x 10.5 tool din-3): (c) 3MP + Nph(10-; tool dm -3) + DPA(5 x 10-~ tool din-3). The arrows indicate: the Nph fluorescence ( - 3 2 0 n m ) and phosphorescence (-500rim) bands, and DPA fluorescence band ( ~ 430 nm). The numbers on the right side of each spectrum indicate the sensitivity level of detection.
399
J. MAYERet al.
400
for the following systems 3'-irradiated at 77 K: 3MP + Nph, 3MP + DPA and 3MP + Nph + DPA whereas Fig. 2 exhibits the emission spectra of ITL for the systems: 3MP + Nph, 3MP + T o and 3MP + To + Nph. The emission spectra of 3MP + To and 3MP + Nph matrices are composed of both fluorescence and phosphorescence bands of the solutes (9-1" whereas the ITL emission of the 3,irradiated 3MP + D P A system is due to the D P A fluorescence only. (~) In the systems 3MP + Nph + DPA and 3MP + T o + Nph one can find all the above mentioned emission bands although some of them are superimposed, i.e. the Nph phosphorescence band there is in the long-wavelenght tail of the To phosphorescence emission. Another feature in these spectra is due to the reabsorption effects in the sample. The To fluorescence (Fig. 2) almost disappears as a result of reabsorption by Nph present in the sample. Taking into account all these disturbances, the observations although
/,OOnrrl
-J
500nm
made for all emission bands, could be analysed quantitatively only at the fluorescence band of DPA (~430 rim), Nph phosphorescence band ( - 5 0 0 nm) for the 3"-irradiated 3MP + Nph + D P A system and at Nph fluorescence band ( - 3 2 0 n m ) and To phosphorescence emission ( - 4 0 0 n m ) in the case of the 3MP + To + Nph system. In all experiments the S1 concentration was kept constant (10-:moldm-3), whereas the $2 concentration was changed. The influence of D P A concentration on the spectral response of the 3MP + Nph + D P A system -/-irradiated at 77 K is shown in Fig. 3. For comparison the dependence of D P A fluorescence intensity on [DPA] for a 3 M P + D P A matrix is depicted (Fig. 3). The intensities of Nph emission bands are almost constant in the D P A concentration range up to - 10-4 mol dm -3, For [DPA] > 10-4moldm -3 the Nph luminescence starts to decrease. In the case of Nph fluorescence the reabsorption effect seems to be responsible, to some extent, for this result. The DPA fluorescence intensity, IDp^, increases with D P A concentration. It is interesting to note that the curves of IDpA-f([DPA]) for the 3 M P + D P A and 3 M P +
o
-
/
It)
/ 150-
~
{b) ,,0.3 ,
1o)
I
I D - I D ~ i ~
.-.
//\
I i
Fro. 2. ITL emission spectra of the ?-irradiated glassy systems at 77 K: (a) 3MP + To(10 -I tool din-3); (b) 3MP + Nph(10 -3 tool din-3); (c) 3MP + To(10 -I tool dm -3) + Nph(10 -3 tool din-3). The arrows indicate: the Nph fluorescence and phosphorescence bands ( - 320 and -500nm, respectively) and To fluorescence and phosphorescence bands (~ 400 nm, respectively).
10-5 ~-~ 10-3 bglDPAI ~ FIG. 3. Influence of DPA concentration on the intensity
of DPA emission for ,/-irradiated 3MP + DPA system at 77 K, O and on the spectral response of ,/-irradiated 3MP + Nph(10 -2 mol dm -3) + DPA system: DPA fluorescence, e ; Nph fluorescence, II; Nph phosphorescence, A.
Isothermal luminescence of 3,-irradiated 3-methylpentane containing two solutes at 77 K Nph+DPA systems intersect at ca. 4x 10-" mol dm -3. For higher [DPA] the D P A fluorescence intensity is lower in the presence of Nph than in the 3MP + D P A matrix. The influence of Nph concentration on the spectral response of the ,/-irradiated 3MP + To + Nph system is shown in Fig. 4. For comparison the dependence of Nph fluorescence intensity on [Nph] for the '/-irradiated 3MP + Nph matrix is included (Fig. 4). In the latter case the characteristic hump is observed on the curve as was found before. °'3> It was suggested that there are two different reaction paths responsible for ITL in 3MP glass containing low solute concentration. According to the first one, which explains the maximum, the luminescence is caused by e,-solute cation recombination whereas the residual emission is due to a tunnelling charge transfer between S - and S +. The intensity of To phosphorescence decreases with [Nph] for 3MP + To + Nph system whereas the intensity of Nph fluorescence continuously increases with [Nph]. The decay kinetic of ITL measured at all emission bands can be described by a hyperbolic law. ttFigure 5 demonstrates the representation of our data according to the hyperbolic relation for the "/-irradiated 3MP + Nph + D P A system and for
3.,2-,4~
1 r.u.
050-
401
IO"~.tO
5.0
Ld~/k
lb)
0
I0.0 (o)
5.0 ~
0
I
10
20
Imin]
m_
FIG. 5. ITL decay at 77 K as expressed by a hyperbolic law (I0 denotes the initial ITL intensity at t = ! rain, It denotes the ITL intensity at time t) for the following y-irradiated systems: (a) 3MP + Nph(10 -2 mol dm -3) and 3MP + DPA(5 x 10-S mol dm-3); (b) 3MP + Nph(10 -2 mol dm -3) + DPA(5 x 10-5 mol dm-3). A, A, Nph fluorescence and phosphorescence, respectively; ©, DPA fluorescence. 3MP containing only one scavenger. In the presence of Nph the slope of the DPA fluorescence decay curve ([DPA] = 5 x 10-5 tool dm -3) is very close to the slopes of the Nph emission decay curves ([Nph] = 10-2 tool dm -3) and is bigger than the value found in the one-scavenger system. Similar results were obtained for 3MP + To + Nph mixture '/-irradiated at 77 K (Fig. 6). According to the accepted mechanism of ITL* emission in hydrocarbon matrices containing high scavenger concentration '/-irradiated at 77 K the formation of excited states, S*, is due to electron tunnelling between respective anions, S - and cations S÷. "-3~
(1)
SI- + SI+ ~ SI* + SI
(2)
$ 2 - + $2 + ~ $2" + $2.
One may expect that the cross--recombination reaction, i.e. S I - + $2 + contributes to the I T L in 0 ~-4 ~-3 ~-2 ~ i ~ ) 'our system in particular at high [$2], but we have FIG. 4. Influenceof Nph concentrationon the intensity not found any convincing experimental data supof Nph fluorescencefor y-irradiated3MP+ Nph system porting such a reaction path. Since the concenat 77 K, © and on the spectral response of y-irradiated 3MP +To(10 -2 mol dm -3)+ Nph system: Nph fluores- tration of S I equals ca. 10-2moldm -3 the forcence, O; To phosphorescence, A. mation of S 1 - and S1 ÷ seems to be evidently due
J. MAYER et al.
402
h
10.0
5.0 /
100[ 0
C~C~(~
1~
(b)
~0
Iminl
,/ O/
I 1 10 :20 [min] . FIG. 6. [TL decay at 77 K as expressed by a hyperbolic law for the following y-irradiated systems: (a) 3MP + Nph(10-3 tool dm -3) and 3MP + TO(10-2 tool dm-3); (b) 3MP + To(10-2 mol dm -3) + Nph(10-3 mol dm-3). A, A, fluorescence and phosphorescence of Nph, respectively; (::), 0 , fluorescence and phosphorescence of To, respectively.
to the following reactions. "5"~6~ (3)
3MP ~.- 3MP + + e -
(4)
e - + S I -* S1-
(5)
3MP + + S I -~ S1 + + 3MP.
In the presence of small $2 concentrations, of the order 1 0 - 5 - 5 x 10-4moldm -3 the $ 2 - and $2 ÷ might be formed as a result of the charge transfer reactions (6)
S 1- + $ 2 ~ S 1 + $2-
(7)
S I ++ $ 2 ~ S 1
+ $ 2 +.
For higher $2 concentrations one may expect the additional processes of direct scavenging of charges by $2 in competition with S l (8)
e- + $2 -* $ 2 -
(9)
3MP + + $2 ~ $2 + + 3MP.
Some evidence concerning processes of this type in y-irradiated low temperature matrices is available."7-2°) A tunnelling mechanism was sug-
gested for reaction (6). "7-~9~ The mechanism of the scavenger cation formation, (5), (7), (9) might be also tunnelling although in a butyl chloride matrix Kira et al. ~2°~ suggested the selective capture of different state of matrix holes by each solute. Very recently Arai et al. ~2~ proposed that the primary hole executes zig-zag motion in a rigid matrix and transfers its own charge to the solute molecule on encounter. The different state of the hole arises from the difference in hole velocity. It is very difficult to say whether this model can be applied to the 3MP matrix. Assuming it in reactions (5) and (9) the different states of 3MP ÷ ought to take part. Reactions (6)-(9) explain the increase of $2 luminescence intensity and simultaneous decrease of S I emission as observed in our experiments. Reactions (6) and (7) may be even more effective than the direct scavenging of charges by $2 in the low $2 concentration range giving as a result higher I T L intensity for the 3 M P + N p h + D P A system than in the case of a one-scavenger matrix. The increase of $2 luminescence with [$2] in the two-scavenger system may be due, to some extent, to the energy trasnfer process from $2" to $2 by a radiative mechanism. The reabsorption effects observed by us seem to support this suggestion. Such a mechanism may be particularly important for a [$2] higher than 10- 4 - 5 x 10-4 tool dm -3 making a contribution to the observed Nph fluorescence curve for the 3 M P + T o + Nph system. Kira et al., °9-2°~ using a spectrophotometric method, found some evidence that reactions (3)-(9) took place in y-irradiated, frozen organic matrices containing two charge scavengers. They found an almost linear semilogarithmic plot of S1 anion as well as cation yields vs $2 concentration. Assuming that the I T L intensity at time t is proportional to the concentration of recombining anion--cation pairs, one can expect a similar semilogarithmic dependence for S I ITL intensity vs $2 concentration. The semilogarithmic dependence of To phosphorescence intensity vs Nph concentration shown in Fig. 7 is linear indeed supporting the suggested reaction paths in the investigated systems. The influence of S1 on the kinetic decay of $2 emission in the system containing low $2 concentration is very difficult to explain. According to our previous investigation the slope of the kinetic decay curve described by the hyperbolic law is related to the distribution function of recombining anion-cation pair separations. ~3> At low $2 con-
Isothermal luminescence of y-irradiated 3-methylpentane containing two solutes at 77 K tgl 4.0-
20
20-
1.0-
0
.10-3
5,,.10-3
[NI:~I
FIG. 7. Semilogarithmic dependence of To phosphorescence intensity against of Nph concentration centration the ITL for the 3MP + $2 system arises from the reaction between electrons and 82 +. In the presence of high SI concentrations there are no e- trapped in the matrix and recombination has to occur between S - and S + by tunnelling. Our results suggest that the distribution function of S2 charged pair separations seems to be very similar to the distribution of SI anion-cation pairs in the y-irradiated 3MP + SI + $2 matrix. One may conclude that there are different distribution functions of charged species separations for et--cation (low S concentration) and for anion-cation pairs (high S concentration, both S1 and $2 are present in the matrix).
403
REFERENCES I. J KROti, J. MAYER and M. SZADKOWSKA-NICZE, Faraday Discussions 1977, No. 63, 91. 2. F. KIEFFER, N. V. KLASSEN, and C. LAPERSONNEMEYER, J. Luminescence 1979, 20, 17. 3. J. MAYER, M. SZADKOWSKA-NICZEand J. KRaft, Radiat. Phys. Chem. 1980, 15, 643. 4. B. BROCKLEHURSTand J. S. ROBINSON,Chem. Phys. Lett. 1971, 10, 277. 5. J. KROH, R. LESZCZYNSK1and J. MAYER,Radial. Phys. Chem. 1976, 8, 247. 6. G. H. MORINE and J. E. WILLARD,J. phys. Chem. 1977, 81, 2668. 7. G. R. FREEMAN,Rod. Research Rev. 1968, l, 1. 8. J. MAYER, i . SZADKOWSKAand J. KROH, Nukleonika 1977, 22, 951. 9. J. B. BIRKS, Photophysics of Aromatic Molecules. Wiley-Interscience, London, 1971. 10. C. DENIAU,A. DEROULEDE,F. KIEFFERand J. RIGAUT, J. Luminescence 1971, 3, 325. 1I. J. B. BERLMAN,Handbook of Fluorescence Spectra of Aromatic Molecules. Academic Press, New York 1965. 12. KN. L. BAGDASARi'AN,R. I. MILUTINSKAYAand Yu. V. KOVALEV,Khim. Vysok. Energi 1967, l, 127. 13. P. CORDIER, F. KIEFFER, C. LAPERSONNE-MEYERand J. RIGAUT,Proc. 5th Int. Congr. Radial. Res. (Edited by O. F. Nygzard, H. I. Adler and W. K. Sinclair), p. 426. Academic Press, New York. 14. M. TACHIYAand A. MOZUMDER,Chem. Phys. Lett. 1975, 34, 77. 15. W.H. HAMILL,In Radicallons (Edited by E. T. Kaiser and L. Kevan), Chap. 9. Interscience, New York, 1968. 16. J. B. GALLIVANand W. H. HAMILL,J. chem. Phys. 1966 4,1, 2378. 17. J. R. MILLER, Science, 1975, 189, 221. 18. R. F. KHAIRUTDINOWand K. I. ZAMARAEV, itzt~. Akad. Nauk SSSR, Ser. Khim. 1975, 2782. 19. A. KIRA and M. IMAMURA,.I. phys. Chem. 1978, 82, 1966. 20. A. KIRA, T. NAKAMARAand M. IMAMURA,J. phys. Chem. 1978, 82, 1961. 21. S. AARAIand M. IMAMURA,.I. phys. Chem. 1979, 83, 337.
ISOTHERMAL LUMINESCENCE OF y-IRRADIATED 3METHYLPENTANE CONTAINING TWO SOLUTES AT 77 K J, MAYER*, M. SZADKOWSKA-N1CZEand J. KROH The Institute of Applied Radiation Chemistry, 93-590, ILod;t,ul. Wr6blewskiego 15, Poland (Received 10 July 1979)
Abstract--A 3-methylpentane glass containing a pair of solutes was y-irradiated at 77 K and the isothermal luminescence was measured. Two pairs of solutes were examined: naphthalene-9,10diphenylanthracene and toluene-naphthalene. For constant concentration of left-hand solutes (SI) the concentration of right-hand solutes ($2) was changed. When the $2 concentration increases, the luminescence intensities measured at the emission bands of $2 increase. The results can be explained in terms of a tunnelling charge transfer model and a competitive charge capture scheme. INTRODUCTION THE MECHANISMof isothermal luminescence (ITL) observed at 7 7 K in y-irradiated hydrocarbon glasses in the presence of aromatic scavengers seems to be understood. "-3) In the case of pure hydrocarbon matrices the origin of the ITL is still open to discussion. There were some suggestions that the radiolysis products--radicals and/or stable, unsaturated end-products formed during the irradiation--may contribute to this phenomenon. "-*) Radicals and olefins (or dienes) having positive electron affinities and lower ionization potential than the matrix molecules<7~ can trap the primary charges. The newly formed cations may recombine with electrons or anions, giving the excited states responsible for ITL. In order to find the reaction paths occurring in such a complicated system, the ITL at 77 K of y-irradicated 3-methylpentane in the presence of two aromatic scavengers has been investigated.
and To-Nph. These two solutes in each pair are not equivalent. The left-hand solutes ( S l ) in each pair have higher ionization potential and higher excited states level than the right-hand ones ($2) whereas the $2 solutes have higher electron affinity than S l. tg~ Figure 1 illustrates the emission spectra of ITL
I 43o~ ~
~
I
EXPERIMENTAL 3-Methylpentane (3MP), Fluka pure grade was chromatographed through a freshly activated silica gel column and kept under argon. Naphthalene (Nph), toluene (To), both analytical grade, and 9,10-diphenylanthracene (DPA), Koch-Light, puriss, were used as received. The 3MP samples, 2.5 cm3 by volume (cylindrical cell, l0 mm i.d.) were outgassed by the freeze-pump-thaw technique and sealed off under a pressure of ca. 10-4 mm Hg. The emission from the y-irradiated samples was analysed at 77 K as described previously. ~s~Irradiations with y-rays of 6°Co were carried out in liquid nitrogen at the dose rate of 1.7x 10~SeVdm-3 s-~. The dose used in experiments was 5.1 x 102°eV drn-~. R E S U L T S AND DISCUSSION In our experiments the following pairs of aromatic scavengers have been used: N p h - D P A
1
J
1
i
600 500450 ~ 0
|
350
a
30Ohm
FIG. !. ITL emission spectra of the y-irradiated glassy systems at 77 K: (a) 3MP + Nph(10-~ tool din-3); (b) 3MP + DPA(5 x 10.5 tool din-3): (c) 3MP + Nph(10-; tool dm -3) + DPA(5 x 10-~ tool din-3). The arrows indicate: the Nph fluorescence ( - 3 2 0 n m ) and phosphorescence (-500rim) bands, and DPA fluorescence band ( ~ 430 nm). The numbers on the right side of each spectrum indicate the sensitivity level of detection.
399
J. MAYERet al.
400
for the following systems 3'-irradiated at 77 K: 3MP + Nph, 3MP + DPA and 3MP + Nph + DPA whereas Fig. 2 exhibits the emission spectra of ITL for the systems: 3MP + Nph, 3MP + T o and 3MP + To + Nph. The emission spectra of 3MP + To and 3MP + Nph matrices are composed of both fluorescence and phosphorescence bands of the solutes (9-1" whereas the ITL emission of the 3,irradiated 3MP + D P A system is due to the D P A fluorescence only. (~) In the systems 3MP + Nph + DPA and 3MP + T o + Nph one can find all the above mentioned emission bands although some of them are superimposed, i.e. the Nph phosphorescence band there is in the long-wavelenght tail of the To phosphorescence emission. Another feature in these spectra is due to the reabsorption effects in the sample. The To fluorescence (Fig. 2) almost disappears as a result of reabsorption by Nph present in the sample. Taking into account all these disturbances, the observations although
/,OOnrrl
-J
500nm
made for all emission bands, could be analysed quantitatively only at the fluorescence band of DPA (~430 rim), Nph phosphorescence band ( - 5 0 0 nm) for the 3"-irradiated 3MP + Nph + D P A system and at Nph fluorescence band ( - 3 2 0 n m ) and To phosphorescence emission ( - 4 0 0 n m ) in the case of the 3MP + To + Nph system. In all experiments the S1 concentration was kept constant (10-:moldm-3), whereas the $2 concentration was changed. The influence of D P A concentration on the spectral response of the 3MP + Nph + D P A system -/-irradiated at 77 K is shown in Fig. 3. For comparison the dependence of D P A fluorescence intensity on [DPA] for a 3 M P + D P A matrix is depicted (Fig. 3). The intensities of Nph emission bands are almost constant in the D P A concentration range up to - 10-4 mol dm -3, For [DPA] > 10-4moldm -3 the Nph luminescence starts to decrease. In the case of Nph fluorescence the reabsorption effect seems to be responsible, to some extent, for this result. The DPA fluorescence intensity, IDp^, increases with D P A concentration. It is interesting to note that the curves of IDpA-f([DPA]) for the 3 M P + D P A and 3 M P +
o
-
/
It)
/ 150-
~
{b) ,,0.3 ,
1o)
I
I D - I D ~ i ~
.-.
//\
I i
Fro. 2. ITL emission spectra of the ?-irradiated glassy systems at 77 K: (a) 3MP + To(10 -I tool din-3); (b) 3MP + Nph(10 -3 tool din-3); (c) 3MP + To(10 -I tool dm -3) + Nph(10 -3 tool din-3). The arrows indicate: the Nph fluorescence and phosphorescence bands ( - 320 and -500nm, respectively) and To fluorescence and phosphorescence bands (~ 400 nm, respectively).
10-5 ~-~ 10-3 bglDPAI ~ FIG. 3. Influence of DPA concentration on the intensity
of DPA emission for ,/-irradiated 3MP + DPA system at 77 K, O and on the spectral response of ,/-irradiated 3MP + Nph(10 -2 mol dm -3) + DPA system: DPA fluorescence, e ; Nph fluorescence, II; Nph phosphorescence, A.
Isothermal luminescence of 3,-irradiated 3-methylpentane containing two solutes at 77 K Nph+DPA systems intersect at ca. 4x 10-" mol dm -3. For higher [DPA] the D P A fluorescence intensity is lower in the presence of Nph than in the 3MP + D P A matrix. The influence of Nph concentration on the spectral response of the ,/-irradiated 3MP + To + Nph system is shown in Fig. 4. For comparison the dependence of Nph fluorescence intensity on [Nph] for the '/-irradiated 3MP + Nph matrix is included (Fig. 4). In the latter case the characteristic hump is observed on the curve as was found before. °'3> It was suggested that there are two different reaction paths responsible for ITL in 3MP glass containing low solute concentration. According to the first one, which explains the maximum, the luminescence is caused by e,-solute cation recombination whereas the residual emission is due to a tunnelling charge transfer between S - and S +. The intensity of To phosphorescence decreases with [Nph] for 3MP + To + Nph system whereas the intensity of Nph fluorescence continuously increases with [Nph]. The decay kinetic of ITL measured at all emission bands can be described by a hyperbolic law. ttFigure 5 demonstrates the representation of our data according to the hyperbolic relation for the "/-irradiated 3MP + Nph + D P A system and for
3.,2-,4~
1 r.u.
050-
401
IO"~.tO
5.0
Ld~/k
lb)
0
I0.0 (o)
5.0 ~
0
I
10
20
Imin]
m_
FIG. 5. ITL decay at 77 K as expressed by a hyperbolic law (I0 denotes the initial ITL intensity at t = ! rain, It denotes the ITL intensity at time t) for the following y-irradiated systems: (a) 3MP + Nph(10 -2 mol dm -3) and 3MP + DPA(5 x 10-S mol dm-3); (b) 3MP + Nph(10 -2 mol dm -3) + DPA(5 x 10-5 mol dm-3). A, A, Nph fluorescence and phosphorescence, respectively; ©, DPA fluorescence. 3MP containing only one scavenger. In the presence of Nph the slope of the DPA fluorescence decay curve ([DPA] = 5 x 10-5 tool dm -3) is very close to the slopes of the Nph emission decay curves ([Nph] = 10-2 tool dm -3) and is bigger than the value found in the one-scavenger system. Similar results were obtained for 3MP + To + Nph mixture '/-irradiated at 77 K (Fig. 6). According to the accepted mechanism of ITL* emission in hydrocarbon matrices containing high scavenger concentration '/-irradiated at 77 K the formation of excited states, S*, is due to electron tunnelling between respective anions, S - and cations S÷. "-3~
(1)
SI- + SI+ ~ SI* + SI
(2)
$ 2 - + $2 + ~ $2" + $2.
One may expect that the cross--recombination reaction, i.e. S I - + $2 + contributes to the I T L in 0 ~-4 ~-3 ~-2 ~ i ~ ) 'our system in particular at high [$2], but we have FIG. 4. Influenceof Nph concentrationon the intensity not found any convincing experimental data supof Nph fluorescencefor y-irradiated3MP+ Nph system porting such a reaction path. Since the concenat 77 K, © and on the spectral response of y-irradiated 3MP +To(10 -2 mol dm -3)+ Nph system: Nph fluores- tration of S I equals ca. 10-2moldm -3 the forcence, O; To phosphorescence, A. mation of S 1 - and S1 ÷ seems to be evidently due
J. MAYER et al.
402
h
10.0
5.0 /
100[ 0
C~C~(~
1~
(b)
~0
Iminl
,/ O/
I 1 10 :20 [min] . FIG. 6. [TL decay at 77 K as expressed by a hyperbolic law for the following y-irradiated systems: (a) 3MP + Nph(10-3 tool dm -3) and 3MP + TO(10-2 tool dm-3); (b) 3MP + To(10-2 mol dm -3) + Nph(10-3 mol dm-3). A, A, fluorescence and phosphorescence of Nph, respectively; (::), 0 , fluorescence and phosphorescence of To, respectively.
to the following reactions. "5"~6~ (3)
3MP ~.- 3MP + + e -
(4)
e - + S I -* S1-
(5)
3MP + + S I -~ S1 + + 3MP.
In the presence of small $2 concentrations, of the order 1 0 - 5 - 5 x 10-4moldm -3 the $ 2 - and $2 ÷ might be formed as a result of the charge transfer reactions (6)
S 1- + $ 2 ~ S 1 + $2-
(7)
S I ++ $ 2 ~ S 1
+ $ 2 +.
For higher $2 concentrations one may expect the additional processes of direct scavenging of charges by $2 in competition with S l (8)
e- + $2 -* $ 2 -
(9)
3MP + + $2 ~ $2 + + 3MP.
Some evidence concerning processes of this type in y-irradiated low temperature matrices is available."7-2°) A tunnelling mechanism was sug-
gested for reaction (6). "7-~9~ The mechanism of the scavenger cation formation, (5), (7), (9) might be also tunnelling although in a butyl chloride matrix Kira et al. ~2°~ suggested the selective capture of different state of matrix holes by each solute. Very recently Arai et al. ~2~ proposed that the primary hole executes zig-zag motion in a rigid matrix and transfers its own charge to the solute molecule on encounter. The different state of the hole arises from the difference in hole velocity. It is very difficult to say whether this model can be applied to the 3MP matrix. Assuming it in reactions (5) and (9) the different states of 3MP ÷ ought to take part. Reactions (6)-(9) explain the increase of $2 luminescence intensity and simultaneous decrease of S I emission as observed in our experiments. Reactions (6) and (7) may be even more effective than the direct scavenging of charges by $2 in the low $2 concentration range giving as a result higher I T L intensity for the 3 M P + N p h + D P A system than in the case of a one-scavenger matrix. The increase of $2 luminescence with [$2] in the two-scavenger system may be due, to some extent, to the energy trasnfer process from $2" to $2 by a radiative mechanism. The reabsorption effects observed by us seem to support this suggestion. Such a mechanism may be particularly important for a [$2] higher than 10- 4 - 5 x 10-4 tool dm -3 making a contribution to the observed Nph fluorescence curve for the 3 M P + T o + Nph system. Kira et al., °9-2°~ using a spectrophotometric method, found some evidence that reactions (3)-(9) took place in y-irradiated, frozen organic matrices containing two charge scavengers. They found an almost linear semilogarithmic plot of S1 anion as well as cation yields vs $2 concentration. Assuming that the I T L intensity at time t is proportional to the concentration of recombining anion--cation pairs, one can expect a similar semilogarithmic dependence for S I ITL intensity vs $2 concentration. The semilogarithmic dependence of To phosphorescence intensity vs Nph concentration shown in Fig. 7 is linear indeed supporting the suggested reaction paths in the investigated systems. The influence of S1 on the kinetic decay of $2 emission in the system containing low $2 concentration is very difficult to explain. According to our previous investigation the slope of the kinetic decay curve described by the hyperbolic law is related to the distribution function of recombining anion-cation pair separations. ~3> At low $2 con-
Isothermal luminescence of y-irradiated 3-methylpentane containing two solutes at 77 K tgl 4.0-
20
20-
1.0-
0
.10-3
5,,.10-3
[NI:~I
FIG. 7. Semilogarithmic dependence of To phosphorescence intensity against of Nph concentration centration the ITL for the 3MP + $2 system arises from the reaction between electrons and 82 +. In the presence of high SI concentrations there are no e- trapped in the matrix and recombination has to occur between S - and S + by tunnelling. Our results suggest that the distribution function of S2 charged pair separations seems to be very similar to the distribution of SI anion-cation pairs in the y-irradiated 3MP + SI + $2 matrix. One may conclude that there are different distribution functions of charged species separations for et--cation (low S concentration) and for anion-cation pairs (high S concentration, both S1 and $2 are present in the matrix).
403
REFERENCES I. J KROti, J. MAYER and M. SZADKOWSKA-NICZE, Faraday Discussions 1977, No. 63, 91. 2. F. KIEFFER, N. V. KLASSEN, and C. LAPERSONNEMEYER, J. Luminescence 1979, 20, 17. 3. J. MAYER, M. SZADKOWSKA-NICZEand J. KRaft, Radiat. Phys. Chem. 1980, 15, 643. 4. B. BROCKLEHURSTand J. S. ROBINSON,Chem. Phys. Lett. 1971, 10, 277. 5. J. KROH, R. LESZCZYNSK1and J. MAYER,Radial. Phys. Chem. 1976, 8, 247. 6. G. H. MORINE and J. E. WILLARD,J. phys. Chem. 1977, 81, 2668. 7. G. R. FREEMAN,Rod. Research Rev. 1968, l, 1. 8. J. MAYER, i . SZADKOWSKAand J. KROH, Nukleonika 1977, 22, 951. 9. J. B. BIRKS, Photophysics of Aromatic Molecules. Wiley-Interscience, London, 1971. 10. C. DENIAU,A. DEROULEDE,F. KIEFFERand J. RIGAUT, J. Luminescence 1971, 3, 325. 1I. J. B. BERLMAN,Handbook of Fluorescence Spectra of Aromatic Molecules. Academic Press, New York 1965. 12. KN. L. BAGDASARi'AN,R. I. MILUTINSKAYAand Yu. V. KOVALEV,Khim. Vysok. Energi 1967, l, 127. 13. P. CORDIER, F. KIEFFER, C. LAPERSONNE-MEYERand J. RIGAUT,Proc. 5th Int. Congr. Radial. Res. (Edited by O. F. Nygzard, H. I. Adler and W. K. Sinclair), p. 426. Academic Press, New York. 14. M. TACHIYAand A. MOZUMDER,Chem. Phys. Lett. 1975, 34, 77. 15. W.H. HAMILL,In Radicallons (Edited by E. T. Kaiser and L. Kevan), Chap. 9. Interscience, New York, 1968. 16. J. B. GALLIVANand W. H. HAMILL,J. chem. Phys. 1966 4,1, 2378. 17. J. R. MILLER, Science, 1975, 189, 221. 18. R. F. KHAIRUTDINOWand K. I. ZAMARAEV, itzt~. Akad. Nauk SSSR, Ser. Khim. 1975, 2782. 19. A. KIRA and M. IMAMURA,.I. phys. Chem. 1978, 82, 1966. 20. A. KIRA, T. NAKAMARAand M. IMAMURA,J. phys. Chem. 1978, 82, 1961. 21. S. AARAIand M. IMAMURA,.I. phys. Chem. 1979, 83, 337.