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Excited complexes in triethylamine—ethanol at low temperatures: pulse radiolysis study
Journal of Photochemistry
and Photobiology,
A:
Chemistry,
49 (1989)
325 - 338
325
EXCITED COMPLEXES IN TRIETHYLAMINE-ETHANOL AT LOW TEMPERATURES: PULSE RADIOLYSIS STUDY J. PIEKARSKA-GOEJ$BIOWSKA,
CZ. STRADOWSKI
Institute of Applied Radiation Chemistry, Wrbblewskiego 15, 93-590 66df (Poland) (Received
February
Technical
and M. SZADKOWSKA-NICZE University
(Politechnika),
15, 1989)
Summary
Triethylamine, ethanol and mixtures of the two components were pulse irradiated at 180 - 105 K. The intensity of the luminescence in the mixtures was much greater than in the neat components. The spectrum and lifetime of the luminescence were determined after an electron pulse of 17 ns. It is suggested that the luminescence is due to recombination of dry (unsolvated) electrons with positive charge, leading to the formation of triethylamineethanol excited complexes. The spectral analysis of the luminescence data indicates three excited complexes, in which one triethylsmine molecule is connected with one, (A-T), two (A,T) or three (A,T) molecules of ethanol. Their emission spectra and lifetimes were determined at temperatures in the range 180 - 105 K. Luminescence maxima were observed at 325, 380 and 410 nm with lifetimes of 40, 10 and 16 ns for the excited complexes AT, A2T and and AsT respectively at 180 K.
1. Introduction Recently, we have examined the luminescence properties of y-irradiated triethylamine-ethanol mixed glassy matrices [ 11. It has been demonstrated that the pure components show weak luminescence. The radiothermoluminescence intensity of irradiated triethylamine is more than 80 times higher than for the alcohol. Nevertheless this luminescence is still very small as compared with that observed for the mixtures of 30 - 70 mol.% of triethylamine with ethanol. Intensities of isothermal luminescence (at 77 K) and radiothermoluminescence for the mixtures are one or two orders of magnitude greater than for the pure components. The increase in luminescence is so great that the emitting species formed in the mixtures must be qualitatively different from those formed in pure ethanol and triethylamine. These entities may be excited complexes in which the triethylamine molecule is connected with one or more alcohol molecules. Similar complexes formed kinetically in the excited state (called exciplexes) are considered to be important intermediates in many photochemical lOlO-6030/89/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
reactions [ 21. In some cases, their existence is confirmed by the observation of new emission bands or of longer wavelength tails in the fluorescence spectrum. This type of emission which indicates the generation of exciplexes has been found, for example, in mixtures of ammonia, perylene and anthracene in helium carrier gas [ 31 and in n-hexane containing N, N-dimethylaniline and pyrene [4]. The formation of triethylamine-ethanol exciplexes has been studied by means of steady state and time-resolved fluorescence spectroscopy [ 5, 6 3. The results indicate the appearance of three exciplexes. The aim of this work is to generate these excited complexes via recombination using pulse radiolysis (a radiation-chemical process). The pulse radiolysis measurements enable us to identify three triethylamineethanol complexes and to determine their spectral characteristics and lifetimes. The assignment of these complexes is different from the assignment of the complexes generated via photoexcitation. 2. Experimental details Two solutions were examined in this work: 40 mol.% triethylamine6Omol.% ethanol which, as reported previously [ 11, shows the highest luminescence intensity after y irradiation at 77 K and 3-methylpentane containing 0.3 mM triethylamine and 0.1 M ethanol, which is identical with that studied photochemically by KShler [ 51. Ethanol (POCH, Gliwice, Poland) and triethylamine (Fluka, AG, Buchs) were of spectral grade and were used as received. 3-Methylpentane (Fluka, pure grade) was chromatographed through freshly activated silica gel and kept under argon. The solutions were deaerated in the sample cell by bubbling with argon or helium. The electron pulse from an ELU-6 linear accelerator (U.S.S.R.) was delivered to a Spectrosil cylindrical sample cell. The sample was placed in a home-built cryostat, which enabled the luminescence measurements to be made at low temperatures by letting cold nitrogen pass through. The temperature was measured by means of the copper-constantan thermocouple glued to the optical cell. The electron pulse of 17 ns delivered a dose of approximately 50 Gy. The luminescence was detected with a Hamamatsu K-928 photomultiplier connected with an Iwatsu TS-8123 storage-type oscilloscope. Since the emission was observed during the pulse, Cerenkov radiation was subtracted from the sample luminescence intensity. The Cerenkov light intensities were measured in ethanol and triethylamine at 180 K under the same experimental conditions as those applied for the mixtures. The luminescence intensities obtained from oscilloscope traces were used to construct the emission spectra. The lifetimes of several luminescence components were determined with an accuracy of AO.5 ns. The details of the experimental technique and the detection system are reported elsewhere [ 7, B] .
327
3. Results In neat solvent the emission of light during the pulse is mostly due to cerenkov radiation. A comparison between the luminescence of the mixture
c
320
360
COO
CC0
450
520
560
600
;Inm
(a)
(b) Fig. 1. Luminescence intensities in 40mol.%triethylamine-GOmoI.%ethanol (O), triethylamine (A) and ethanol (m) at 180 K: (a) at the maximum (17 ns after the start of the electron pulse); (b) 8 ns after the end of the pulse.
328
and the intensity of Cerenkov light in triethylamine and ethanol at 180 K is presented in Fig. 1. The intensity of luminescence during the pulse and at the end of the pulse is about six times greater than the cerenkov radiation (see Fig. l(a)) in ethanol or triethylamine. The intensity of light in the neat solvent drops practically to zero (Fig. l(b)) 8 ns after the pulse, while in the mixture the decay of luminescence is observed on a nanosecond time scale. The influence of temperature on the emission spectra is shown in Fig. 2. It is seen that the luminescence intensity decreases at higher temperatures. In addition, the spectrum is shifted slightly towards the red region. The kinetics of luminescence decay after the pulse at 105 K are shown in Fig. 3. The data for kinetic analysis were collected from 8 ns after the end of the pulse in order to minimize the distortions due to the light emitted during the pulse. It is seen that the decay at 400 nm is strictly monoexponential. The full line in Fig. 3(a) represents a monoexponential decay with a lifetime of 47.6 ns. The agreement between simulation and experimental data is very good. The component emitting light at 400 nm is denoted as species I.
300
._
250..
zw..
1st.
loo__
Fig. 2. Luminescence spectra of 40mol.%triethylamine-60mol.%ethanol after the end of the pulse: 1, at 105 K; 2, at 128 K; 3, at 180 K.
observed
8 ns
329
80.. 40..
70
zo
3v
40
30
CO
50
60
70
a0
2,RS
(a)
.j I,a.u. 600..
(b)
50
60
70
I
7,“s
(cl Fig. 3. Kinetics of the luminescence decay after the electron pulse in a triethylamineethanol mixture at 105 K: 0, l, experimental data; full lines, simulated curves. (a) Calculated as monoexponential decay with lifetime of 47.6 ns at 400 nm. (b) Calculated as a superposition of two monoexponential decays with lifetimes of 47.6 and 14.7 ns: 1, at 360 nm; 2, at 380 nm. (c) Calculated as a superposition of two monoexponential decays with lifetimes of 47.6 and 27.8 ns: 1, at 425 nm; 2, at 450 nm.
330
At shorter wavelengths the decay is complex and can be considered as a superposition of two decays, one with the same lifetime as that at 400 nm. Thus, the relative intensity and lifetime of the short wavelength component (species II) can be calculated. The simulated curve and the experimental points for the decay at 380 nm agree well. The agreement for the luminescence at 360 nm is somewhat poorer (Fig. 3(b)). At longer wavelengths there is also a two-component decay. The superposition of two monoexponential decays, Le. species I and species III, gives good agreement with experimental data (see Fig. 3(c)). These studies were performed in the temperature range 105 - 180 K. In all cases three emitting species characterized by three spectra and three lifetimes can be distinguished. The decay at 180 K is relatively fast and the lifetime is almost the same as the pulse length. In addition, the intensity of luminescence decreases. Therefore, the luminescence at higher temperature was not studied. Examples of the kinetics at 180 K are presented in Fig. 4.
Fig. 4. Kinetics of the luminescence decay in triethylamine-ethanol at 180 K: 0, 0, experimental data; full lines, simulated curves. (a) Calculated as monoexponential decay with a lifetime of 15.9 ns at 410 nm. (b) Calculated as a superposition of two monoexponential decays with lifetimes of 15.9 and 10.2 ns: 1, at 350 nm; 2, at 360 nm. (c) Calculated as a superposition of two monoexponential decays with lifetimes of 15.9 and 7.6 ns: 1, at 475 nm; 2, at 500 nm.
331
The agreement between the simulated and experimental data is quite good over the entire temperature range investigated. The kinetic analysis allowed us to calculate the spectra of the three emitting species, which are shown in Fig. 5. The dominant emitting species is species I over the entire temperature range investigated. However, the intensities of species II and III increase with temperature (compare Figs. 5(a) and 5(b)). 400
300
ZOO
340
30
LOO
k 440
_
;I 1 nm 480
520
(a)
I '0,a,u MO ._
120 __
80 __
40..
320
360
400
CC0
480
520
A,“rn
(b)
Fig. 5. Simulated spectra ethylamine-60mol.%ethanol: 180 K.
of the three luminescence components observed in 40mol.%tri0, species I; 0, species II; a, species III. (a) At 105 K; (b) at
332 TABLE
1
The lifetimes and relative intensities are given in parentheses) Temperature
Species
of the luminescence
11 (AzT) III (e- + TEA+)
(their assignments
(K) 180
128
105
1 (A3T)
components
7 (ns)
L,
(a=)
47.6 14.7 27.8
394 (370 nm) 98 (360 nm) 43 (450 nm)
7 (ns)
I,,
(a.u.)
45.5 12.1 19.6
295 (400 87 (390 47 (475
nm) nm) nm)
7 (ns)
Imax (a.u.)
15.9 10.2 7.6
158 (410 142 (380 80 (475
nm) nm) nm)
The lifetimes and relative intensities of these species are collected in Table 1. It is clearly seen that species II and III show a different temperature dependence than species I. The intensities of luminescence of species II and III increase with temperature, while the intensity of species I decreases at higher temperatures. The temperature dependences of the intensity and lifetime of species I, which is dominant over the entire temperature range, were studied in detail. The results are shown in Fig. 6. It is seen that the lifetime of luminescence decreases dramatically with temperature. In addition, the initial intensity of 1 J,a,u
250
200
150
100
20.. 50 IO..
_J
100
110
120
130
lla
150
160
170
180
190
.m
T,K
Fig. 6. Temperature dependence of the intensity (0) and lifetime (0) of species I (ascribed to excited complex of AsT structure). The luminescence intensity was observed at 400 nm 8 ns after the end of the pulse. The lifetimes at the temperatures higher than 180 K were only roughly estimated.
_
.I
360
___
380
120
130
160
150
160
1M
180
TK
2
6
8
M
__^l_“._I_^__“.. _“__” _,_-._,^l._.“...__“”. .
lence of the intensity (0) and lifetime (0) of the main species for 0.3 mM triethylamine iescence intensity was calculated at 325 nm, 8 ns after the end of the pulse.
g W
___“,_.“_ _.______“_____ _,,_ ^._..._ _.”-..-...l- -.--- “.i.“._~.-
and 0.1 M ethanol in
I emitting species in 3-methylpentane matrix containing 0.3 mM triethylamine and 0.1 M ethanol at 180 K: liner species. The assignments of the luminescence bands are given in parentheses.
340
m’;--m
10..
20..
30._
Co..
SO.. -
60..
I,0 u
334
luminescence, after reaching a flat maximum around 140 K, decreases very rapidly with further rise of temperature. In order to compare more directly the present data with photoexcitation studies, an additional experiment with 0.3 mM triethylamine and 0.1 M ethanol in 3-methylpentane solution was carried out at 180 K. The concentrations of triethylamine and alcohol were the same as in ref. 5. The maximum luminescence is observed at 325 nm. The decay kinetics can be treated as a superposition of two decays: the main species with a lifetime of 40 ns and the minor species with a lifetime of 10.2 ns. The spectra of these entities are presented in Fig. 7. The temperature dependences of the intensity and lifetime of the main species (A,,, = 325 nm) are shown in Fig. 8. It is seen that the lifetime of luminescence is only slightly lowered at higher temperatures. In addition, the luminescence intensity reaches a maximum around 170 - 180 K.
4. Discussion The spectral range of luminescence and the lifetimes of the emitting species observed in this work agree well with those observed in the luminescence of triethylamine-ethanol solutions in hydrocarbons on photoexcitation [6]. Therefore, it is reasonable to attribute the luminescence observed in this work to the exciplexes between triethylamine and ethanol. It should be stressed, however, that the excited complexes generated by ionizing radiation may be called “exciplexes” only because of the analogy in structure. There are significant differences between radiation-chemical and photochemical experiments, concerning the mechanism of excitation and temperature. Therefore, only a limited similarity should be expected between these two types of experiments. The strongest emission band, observed at 290 nm by KGhler [6] in a photochemical experiment, was assigned to a loosely bound complex with nitrogen and oxygen as the nearest neighbours. This complex is presumably formed via collision of excited triethylamine with ethanol and subsequent charge separation. Our work was carried out at low temperature, which slows down molecular motions. Therefore, the formation of this type of exciplex is rather unlikely, since the diffusion time exceeds the observation time. _
.
. .
. .
,. .
-
.
1
nl\*
~~~ mr- ____
335
in concentrated solutions. Therefore, the major emitting species observed in the 3-methylpentane solution of triethylamine and ethanol should be ascribed to this species. Thus, it is suggested that the luminescence at 325 nm (see Fig. 7) is due to the exciplex of AT structure. The majority of our experiments were carried out in triethylamineethanol mixtures. Careful analysis of the luminescence data for this system shows that the A-T type of exciplex is not observed in this mixture. Indeed, the luminescence intensity at 325 nm is very low (see Figs. 1, 2 and 5). In addition, the lifetime of the luminescence measured near the maximum (at 410 nm) decreases very steeply with temperature (Fig. 6); but this is not observed in 3-methylpentane glass at 325 nm (Fig. 8). Therefore, we can conclude that the luminescence in the triethylamine-ethanol mixture at low temperatures is due to some other exciplex. Let us consider a possible assignment of the main emitting species observed in triethylamine-ethanol with a maximum at 370 - 410 nm (species I). In order to compare our data with the photoexcitation data at room temperature [6] the lifetime of luminescence measured in this work at 400 nm was extrapolated to 293 K. This extrapolation resulted in a value of 2.3 ns. Both characteristics of species I, i..e. hm8X and lifetime, agree very well with those observed photochemically [6]. Therefore, they are probably identical. It has been suggested that this emission is due to the heterodimer A-T. A significant deuterium isotope effect has indicated that hydrogen bonding is involved in exciplex formation [S]. We believe that a more correct assignment is that the exciplex is of mixed tetramer (A,T) structure. In this exciplex hydrogen bonding is also involved. Furthermore, dielectric relaxation data have shown that mixed tetramers are formed very efficiently, especially in triethylamine-ethanol mixtures with 30 - 70 mol.% triethylamine [9]. Previous work has shown that mixed tetramers are capable of forming clusters, which trap radiation-produced electrons very efficiently [I]. Therefore, charge recombination probably takes place in these clusters, and the probability of formation of an exciplex with the A3T structure is very high. Thus, the main emitting species in triethylamineethanol mixtures is probably the excited complex of A3T structure. The spectral range of luminescence and the lifetime of species II (Figs. 3, 4 and 5) agree well with those of the minor species in Fig. 7. Therefore, we suggest that this is the same species as assigned by KBhler, i.e. the exciplex of mixed trimer (A,T) structure. The only species which remains to be assigned is species III with a h,,, value of about 450 nm. This long-wavelength emission was not observed photochemically [ 5, 61. Therefore, it is probably connected with the recombination of trapped electrons with positive charge. Such entities cannot be generated photochemically, but recombination luminescence is a very common phenomenon in radiation chemistry [12 - 17 3. More extensive considerations of the nature of this luminescence will be published separately. The mechanism of generation of exciplexes by radiation can be entirely different from photochemical generation. Let us consider a possible mech-
anism for the generation of exciplexes by an electron pulse from a linear accelerator. First, triethylamine could be excited by Cerenkov light. However, this possibility can be ruled out on the basis of the low intensity of the cerenkov light compared with the intensity of luminescence (see Fig. 1). Although Cerenkov light is also emitted in the far UV (which was not monitored in this work), the low photochemical yield of exciplex formation [ 51 makes this process less significant. A typical mechanism for the generation of excited species by radiation is the recombination of electrons with positive charge. This is probably the mechanism responsible for the generation of exciplexes in the Methylamine-ethanol system. It has been pointed out that radiothermoluminescence of y-irradiated triethylamine-ethanol glasses is probably caused by the formation of excited complexes during recombination of trapped electrons with positive charge [18]. A similar explanation can be used in our case. However, we should distinguish between the recombination of trapped electrons and the recombination of dry (unsolvated) electrons moving freely through the medium. Electron trapping at low temperatures has been extensively studied [ 19 - 223. It has been demonstrated that the initial localization of electrons is followed by the reorganization of trapped electrons on a nanosecond and microsecond time scale [20,23]. The trapped electrons also decay, most probably via recombination over the same time scale [23]. Since the lifetimes of the exciplexes observed in this work are of the order of nanoseconds, they must be formed in a much shorter time. Thus, it seems unlikely that the recombination of trapped electrons after an electron pulse is the source of luminescence. Therefore, the radiation-produced electrons, which are not trapped and which undergo recombination in the dry (unsolvated) state, are most probably responsible for the instantaneous generation of exciplexes. The positive charge localized on triethylamine (which is associated in the ground state with one, two or three ethanol molecules) captures a dry electron moving through the medium. The electron becomes localized on the ethanol molecules, providing some excess energy to achieve a favourable orientation to form the appropriate exciplex. The electron localization is very fast. The rate-determining process in exciplex formation is the suitable reorientation of the molecular associate to produce the exciplex. The kinetics of luminescence decay are different from those observed in radiation chemistry due to charge recombination [24]. A commonly observed phenomenon at low temperature is electron transfer which occurs over different time scales (from nanoseconds to several days) [20, 23, 241. In this case the rate-limiting step is electron transfer over some range of distances. In this study the lifetimes of the excited complexes are determined by the radiative and radiationless decay. Therefore conclusions concerning the mechanism of exciplex formation are very limited. The kinetics of luminescence decay are monoexponential and thus the formation of exciplexes is much faster than their decay. Thus the generation of exciplexes by radiation is not suitable for the study of their formation, since the amount
337
of excess energy released during recombination cannot be precisely estimated. The yield of exciplexes at various temperatures depends mainly on the concentration of associates and their efficiency of electron localization. The temperature dependences in Figs. 6 and 8 can be qualitatively interpreted in terms of these factors. Thus the steep decrease in the intensity of luminescence of the A3T exciplex above 140 K may be connected with the disintegration of larger associates and the formation of smaller associates. Indeed, the yield of the small associate, i.e. AT, increases at temperatures between 140 and 170 K. In order to evaluate the influence of the efficiency of electron localization, studies of the spectra of trapped electrons over this temperature range are under way in this laboratory. In general, radiation chemistry seems to offer a convenient way of generating excited complexes with identical structures to exciplexes. The mechanism of formation involves the recombination of initially separate charges, in contrast with photochemistry, where charge undergoes separation after photoexcitation. References 1 J. Piekarska-Go$biowska, Cz. Stradowski and J. Kroh, Radiat. Phys. Chem., 30 (1987) 133. 2 J. A. Barltrop and J. D. Coyle, Fotochemia - Podstawy, P.W.N., Warszawa, 1987, p. 113. 3 0. Anner and Y. Haas, J. Phys. Chem., 90 (1986) 4298. 4 K. Kano, M. Yanagimoto and S. Hashimoto, Bull. Chem. Sot. Jpn. 59 (1986) 3451. 5 G_ Kohler, Chem. Phys. Lett. 126 (1986) 260. 6 G. Kohler, J. Photochem., 35 (1986) 189. 7 J. Mayer, M. Szadkowska-Nicze and J. Kroh, J. Radioanal. Nucl. Chem., 101 (1986)
359. 8 S. Karolczak, 177.
K. Hodyr,
R. Lubis and J. Kroh, J. Radioanal.
Nucl.
Chem.,
101 (1986)
9 J. Malecki,
Prace Komisji Matematyczno-Przyrodnictej Poznanskiego Towarzystwa Nauk Wydzialu Matematyczno Przyrodniczego, 1965, XI, p. 97. R. Brandom and R. F. Firestone, J. Phys. Chem.. 78 (1974) 792. L. Sobczyk (ed.), Wiczanie Wodorowe, P.W.N., Warszawa, 1969. P. B. Merkel and W. H. Hamill, J. Chem. Phys. 63 (1970) 3414. K. Funabashi, C. Herbert and J. L. Magee, J. Phys. Chem., 75 (1971) 3221. J. Mayer, M. Szadkowska and J, Kroh, Nukleonika, 22 (1977) 951. F. Kiefer and M. Magat, in M. Haissinsky (ed.), Actions Chimiques Biologiques des Radiations, Vol. 14, Masson, Paris, 1970, p. 135. G. H. Morine and J. E. Willard, J. Phys. Chem., 81 (1977) 2668. J. Mayer, M. Szadkowska-Nicze and J. Kroh, Radiut. Phys. Chem., 23 (1984) 105.
Przyjaciol 10 11 12 13 14 15 16 17 18 19 20 21 22
J. Piekarska-Goebiowska and Cz. Stradowski, unpublished results, 1988. G. V. Buxton, G. A. Salmon, Cz. Stradowski and J. Kroh, Radiat. Phys. Chem , 17 (1981) 361. J. Kroh, J. P. Suwalski, J. Piekarska, E. Hankiewicz, M. Wolszczak and Cz. Stradowski, Radiat. Phys. Chem., 23 (1984) 529. M. Ogasawara, K. Shimizu and H. Yoshida, Radint. Phys. Chem , 17 (1981) 331. N. V. Klassen and G. G. Teather J. Phys. Chem., 87 (1983) 3891.
338 23 24
M. Wolszczak and Cz. Stradowski, Radiat. Phys. Chem., 24 (1984) 213. J. Kroh, E. Romanowska and Cz. Stradowski, Chem. Whys. Lett., 47 (1977) 597. W. M. Bartczak, J. Kroh and Cz. Stradowski, J. Chem. Phys., 66 (1977) 2737.
and Photobiology,
A:
Chemistry,
49 (1989)
325 - 338
325
EXCITED COMPLEXES IN TRIETHYLAMINE-ETHANOL AT LOW TEMPERATURES: PULSE RADIOLYSIS STUDY J. PIEKARSKA-GOEJ$BIOWSKA,
CZ. STRADOWSKI
Institute of Applied Radiation Chemistry, Wrbblewskiego 15, 93-590 66df (Poland) (Received
February
Technical
and M. SZADKOWSKA-NICZE University
(Politechnika),
15, 1989)
Summary
Triethylamine, ethanol and mixtures of the two components were pulse irradiated at 180 - 105 K. The intensity of the luminescence in the mixtures was much greater than in the neat components. The spectrum and lifetime of the luminescence were determined after an electron pulse of 17 ns. It is suggested that the luminescence is due to recombination of dry (unsolvated) electrons with positive charge, leading to the formation of triethylamineethanol excited complexes. The spectral analysis of the luminescence data indicates three excited complexes, in which one triethylsmine molecule is connected with one, (A-T), two (A,T) or three (A,T) molecules of ethanol. Their emission spectra and lifetimes were determined at temperatures in the range 180 - 105 K. Luminescence maxima were observed at 325, 380 and 410 nm with lifetimes of 40, 10 and 16 ns for the excited complexes AT, A2T and and AsT respectively at 180 K.
1. Introduction Recently, we have examined the luminescence properties of y-irradiated triethylamine-ethanol mixed glassy matrices [ 11. It has been demonstrated that the pure components show weak luminescence. The radiothermoluminescence intensity of irradiated triethylamine is more than 80 times higher than for the alcohol. Nevertheless this luminescence is still very small as compared with that observed for the mixtures of 30 - 70 mol.% of triethylamine with ethanol. Intensities of isothermal luminescence (at 77 K) and radiothermoluminescence for the mixtures are one or two orders of magnitude greater than for the pure components. The increase in luminescence is so great that the emitting species formed in the mixtures must be qualitatively different from those formed in pure ethanol and triethylamine. These entities may be excited complexes in which the triethylamine molecule is connected with one or more alcohol molecules. Similar complexes formed kinetically in the excited state (called exciplexes) are considered to be important intermediates in many photochemical lOlO-6030/89/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
reactions [ 21. In some cases, their existence is confirmed by the observation of new emission bands or of longer wavelength tails in the fluorescence spectrum. This type of emission which indicates the generation of exciplexes has been found, for example, in mixtures of ammonia, perylene and anthracene in helium carrier gas [ 31 and in n-hexane containing N, N-dimethylaniline and pyrene [4]. The formation of triethylamine-ethanol exciplexes has been studied by means of steady state and time-resolved fluorescence spectroscopy [ 5, 6 3. The results indicate the appearance of three exciplexes. The aim of this work is to generate these excited complexes via recombination using pulse radiolysis (a radiation-chemical process). The pulse radiolysis measurements enable us to identify three triethylamineethanol complexes and to determine their spectral characteristics and lifetimes. The assignment of these complexes is different from the assignment of the complexes generated via photoexcitation. 2. Experimental details Two solutions were examined in this work: 40 mol.% triethylamine6Omol.% ethanol which, as reported previously [ 11, shows the highest luminescence intensity after y irradiation at 77 K and 3-methylpentane containing 0.3 mM triethylamine and 0.1 M ethanol, which is identical with that studied photochemically by KShler [ 51. Ethanol (POCH, Gliwice, Poland) and triethylamine (Fluka, AG, Buchs) were of spectral grade and were used as received. 3-Methylpentane (Fluka, pure grade) was chromatographed through freshly activated silica gel and kept under argon. The solutions were deaerated in the sample cell by bubbling with argon or helium. The electron pulse from an ELU-6 linear accelerator (U.S.S.R.) was delivered to a Spectrosil cylindrical sample cell. The sample was placed in a home-built cryostat, which enabled the luminescence measurements to be made at low temperatures by letting cold nitrogen pass through. The temperature was measured by means of the copper-constantan thermocouple glued to the optical cell. The electron pulse of 17 ns delivered a dose of approximately 50 Gy. The luminescence was detected with a Hamamatsu K-928 photomultiplier connected with an Iwatsu TS-8123 storage-type oscilloscope. Since the emission was observed during the pulse, Cerenkov radiation was subtracted from the sample luminescence intensity. The Cerenkov light intensities were measured in ethanol and triethylamine at 180 K under the same experimental conditions as those applied for the mixtures. The luminescence intensities obtained from oscilloscope traces were used to construct the emission spectra. The lifetimes of several luminescence components were determined with an accuracy of AO.5 ns. The details of the experimental technique and the detection system are reported elsewhere [ 7, B] .
327
3. Results In neat solvent the emission of light during the pulse is mostly due to cerenkov radiation. A comparison between the luminescence of the mixture
c
320
360
COO
CC0
450
520
560
600
;Inm
(a)
(b) Fig. 1. Luminescence intensities in 40mol.%triethylamine-GOmoI.%ethanol (O), triethylamine (A) and ethanol (m) at 180 K: (a) at the maximum (17 ns after the start of the electron pulse); (b) 8 ns after the end of the pulse.
328
and the intensity of Cerenkov light in triethylamine and ethanol at 180 K is presented in Fig. 1. The intensity of luminescence during the pulse and at the end of the pulse is about six times greater than the cerenkov radiation (see Fig. l(a)) in ethanol or triethylamine. The intensity of light in the neat solvent drops practically to zero (Fig. l(b)) 8 ns after the pulse, while in the mixture the decay of luminescence is observed on a nanosecond time scale. The influence of temperature on the emission spectra is shown in Fig. 2. It is seen that the luminescence intensity decreases at higher temperatures. In addition, the spectrum is shifted slightly towards the red region. The kinetics of luminescence decay after the pulse at 105 K are shown in Fig. 3. The data for kinetic analysis were collected from 8 ns after the end of the pulse in order to minimize the distortions due to the light emitted during the pulse. It is seen that the decay at 400 nm is strictly monoexponential. The full line in Fig. 3(a) represents a monoexponential decay with a lifetime of 47.6 ns. The agreement between simulation and experimental data is very good. The component emitting light at 400 nm is denoted as species I.
300
._
250..
zw..
1st.
loo__
Fig. 2. Luminescence spectra of 40mol.%triethylamine-60mol.%ethanol after the end of the pulse: 1, at 105 K; 2, at 128 K; 3, at 180 K.
observed
8 ns
329
80.. 40..
70
zo
3v
40
30
CO
50
60
70
a0
2,RS
(a)
.j I,a.u. 600..
(b)
50
60
70
I
7,“s
(cl Fig. 3. Kinetics of the luminescence decay after the electron pulse in a triethylamineethanol mixture at 105 K: 0, l, experimental data; full lines, simulated curves. (a) Calculated as monoexponential decay with lifetime of 47.6 ns at 400 nm. (b) Calculated as a superposition of two monoexponential decays with lifetimes of 47.6 and 14.7 ns: 1, at 360 nm; 2, at 380 nm. (c) Calculated as a superposition of two monoexponential decays with lifetimes of 47.6 and 27.8 ns: 1, at 425 nm; 2, at 450 nm.
330
At shorter wavelengths the decay is complex and can be considered as a superposition of two decays, one with the same lifetime as that at 400 nm. Thus, the relative intensity and lifetime of the short wavelength component (species II) can be calculated. The simulated curve and the experimental points for the decay at 380 nm agree well. The agreement for the luminescence at 360 nm is somewhat poorer (Fig. 3(b)). At longer wavelengths there is also a two-component decay. The superposition of two monoexponential decays, Le. species I and species III, gives good agreement with experimental data (see Fig. 3(c)). These studies were performed in the temperature range 105 - 180 K. In all cases three emitting species characterized by three spectra and three lifetimes can be distinguished. The decay at 180 K is relatively fast and the lifetime is almost the same as the pulse length. In addition, the intensity of luminescence decreases. Therefore, the luminescence at higher temperature was not studied. Examples of the kinetics at 180 K are presented in Fig. 4.
Fig. 4. Kinetics of the luminescence decay in triethylamine-ethanol at 180 K: 0, 0, experimental data; full lines, simulated curves. (a) Calculated as monoexponential decay with a lifetime of 15.9 ns at 410 nm. (b) Calculated as a superposition of two monoexponential decays with lifetimes of 15.9 and 10.2 ns: 1, at 350 nm; 2, at 360 nm. (c) Calculated as a superposition of two monoexponential decays with lifetimes of 15.9 and 7.6 ns: 1, at 475 nm; 2, at 500 nm.
331
The agreement between the simulated and experimental data is quite good over the entire temperature range investigated. The kinetic analysis allowed us to calculate the spectra of the three emitting species, which are shown in Fig. 5. The dominant emitting species is species I over the entire temperature range investigated. However, the intensities of species II and III increase with temperature (compare Figs. 5(a) and 5(b)). 400
300
ZOO
340
30
LOO
k 440
_
;I 1 nm 480
520
(a)
I '0,a,u MO ._
120 __
80 __
40..
320
360
400
CC0
480
520
A,“rn
(b)
Fig. 5. Simulated spectra ethylamine-60mol.%ethanol: 180 K.
of the three luminescence components observed in 40mol.%tri0, species I; 0, species II; a, species III. (a) At 105 K; (b) at
332 TABLE
1
The lifetimes and relative intensities are given in parentheses) Temperature
Species
of the luminescence
11 (AzT) III (e- + TEA+)
(their assignments
(K) 180
128
105
1 (A3T)
components
7 (ns)
L,
(a=)
47.6 14.7 27.8
394 (370 nm) 98 (360 nm) 43 (450 nm)
7 (ns)
I,,
(a.u.)
45.5 12.1 19.6
295 (400 87 (390 47 (475
nm) nm) nm)
7 (ns)
Imax (a.u.)
15.9 10.2 7.6
158 (410 142 (380 80 (475
nm) nm) nm)
The lifetimes and relative intensities of these species are collected in Table 1. It is clearly seen that species II and III show a different temperature dependence than species I. The intensities of luminescence of species II and III increase with temperature, while the intensity of species I decreases at higher temperatures. The temperature dependences of the intensity and lifetime of species I, which is dominant over the entire temperature range, were studied in detail. The results are shown in Fig. 6. It is seen that the lifetime of luminescence decreases dramatically with temperature. In addition, the initial intensity of 1 J,a,u
250
200
150
100
20.. 50 IO..
_J
100
110
120
130
lla
150
160
170
180
190
.m
T,K
Fig. 6. Temperature dependence of the intensity (0) and lifetime (0) of species I (ascribed to excited complex of AsT structure). The luminescence intensity was observed at 400 nm 8 ns after the end of the pulse. The lifetimes at the temperatures higher than 180 K were only roughly estimated.
_
.I
360
___
380
120
130
160
150
160
1M
180
TK
2
6
8
M
__^l_“._I_^__“.. _“__” _,_-._,^l._.“...__“”. .
lence of the intensity (0) and lifetime (0) of the main species for 0.3 mM triethylamine iescence intensity was calculated at 325 nm, 8 ns after the end of the pulse.
g W
___“,_.“_ _.______“_____ _,,_ ^._..._ _.”-..-...l- -.--- “.i.“._~.-
and 0.1 M ethanol in
I emitting species in 3-methylpentane matrix containing 0.3 mM triethylamine and 0.1 M ethanol at 180 K: liner species. The assignments of the luminescence bands are given in parentheses.
340
m’;--m
10..
20..
30._
Co..
SO.. -
60..
I,0 u
334
luminescence, after reaching a flat maximum around 140 K, decreases very rapidly with further rise of temperature. In order to compare more directly the present data with photoexcitation studies, an additional experiment with 0.3 mM triethylamine and 0.1 M ethanol in 3-methylpentane solution was carried out at 180 K. The concentrations of triethylamine and alcohol were the same as in ref. 5. The maximum luminescence is observed at 325 nm. The decay kinetics can be treated as a superposition of two decays: the main species with a lifetime of 40 ns and the minor species with a lifetime of 10.2 ns. The spectra of these entities are presented in Fig. 7. The temperature dependences of the intensity and lifetime of the main species (A,,, = 325 nm) are shown in Fig. 8. It is seen that the lifetime of luminescence is only slightly lowered at higher temperatures. In addition, the luminescence intensity reaches a maximum around 170 - 180 K.
4. Discussion The spectral range of luminescence and the lifetimes of the emitting species observed in this work agree well with those observed in the luminescence of triethylamine-ethanol solutions in hydrocarbons on photoexcitation [6]. Therefore, it is reasonable to attribute the luminescence observed in this work to the exciplexes between triethylamine and ethanol. It should be stressed, however, that the excited complexes generated by ionizing radiation may be called “exciplexes” only because of the analogy in structure. There are significant differences between radiation-chemical and photochemical experiments, concerning the mechanism of excitation and temperature. Therefore, only a limited similarity should be expected between these two types of experiments. The strongest emission band, observed at 290 nm by KGhler [6] in a photochemical experiment, was assigned to a loosely bound complex with nitrogen and oxygen as the nearest neighbours. This complex is presumably formed via collision of excited triethylamine with ethanol and subsequent charge separation. Our work was carried out at low temperature, which slows down molecular motions. Therefore, the formation of this type of exciplex is rather unlikely, since the diffusion time exceeds the observation time. _
.
. .
. .
,. .
-
.
1
nl\*
~~~ mr- ____
335
in concentrated solutions. Therefore, the major emitting species observed in the 3-methylpentane solution of triethylamine and ethanol should be ascribed to this species. Thus, it is suggested that the luminescence at 325 nm (see Fig. 7) is due to the exciplex of AT structure. The majority of our experiments were carried out in triethylamineethanol mixtures. Careful analysis of the luminescence data for this system shows that the A-T type of exciplex is not observed in this mixture. Indeed, the luminescence intensity at 325 nm is very low (see Figs. 1, 2 and 5). In addition, the lifetime of the luminescence measured near the maximum (at 410 nm) decreases very steeply with temperature (Fig. 6); but this is not observed in 3-methylpentane glass at 325 nm (Fig. 8). Therefore, we can conclude that the luminescence in the triethylamine-ethanol mixture at low temperatures is due to some other exciplex. Let us consider a possible assignment of the main emitting species observed in triethylamine-ethanol with a maximum at 370 - 410 nm (species I). In order to compare our data with the photoexcitation data at room temperature [6] the lifetime of luminescence measured in this work at 400 nm was extrapolated to 293 K. This extrapolation resulted in a value of 2.3 ns. Both characteristics of species I, i..e. hm8X and lifetime, agree very well with those observed photochemically [6]. Therefore, they are probably identical. It has been suggested that this emission is due to the heterodimer A-T. A significant deuterium isotope effect has indicated that hydrogen bonding is involved in exciplex formation [S]. We believe that a more correct assignment is that the exciplex is of mixed tetramer (A,T) structure. In this exciplex hydrogen bonding is also involved. Furthermore, dielectric relaxation data have shown that mixed tetramers are formed very efficiently, especially in triethylamine-ethanol mixtures with 30 - 70 mol.% triethylamine [9]. Previous work has shown that mixed tetramers are capable of forming clusters, which trap radiation-produced electrons very efficiently [I]. Therefore, charge recombination probably takes place in these clusters, and the probability of formation of an exciplex with the A3T structure is very high. Thus, the main emitting species in triethylamineethanol mixtures is probably the excited complex of A3T structure. The spectral range of luminescence and the lifetime of species II (Figs. 3, 4 and 5) agree well with those of the minor species in Fig. 7. Therefore, we suggest that this is the same species as assigned by KBhler, i.e. the exciplex of mixed trimer (A,T) structure. The only species which remains to be assigned is species III with a h,,, value of about 450 nm. This long-wavelength emission was not observed photochemically [ 5, 61. Therefore, it is probably connected with the recombination of trapped electrons with positive charge. Such entities cannot be generated photochemically, but recombination luminescence is a very common phenomenon in radiation chemistry [12 - 17 3. More extensive considerations of the nature of this luminescence will be published separately. The mechanism of generation of exciplexes by radiation can be entirely different from photochemical generation. Let us consider a possible mech-
anism for the generation of exciplexes by an electron pulse from a linear accelerator. First, triethylamine could be excited by Cerenkov light. However, this possibility can be ruled out on the basis of the low intensity of the cerenkov light compared with the intensity of luminescence (see Fig. 1). Although Cerenkov light is also emitted in the far UV (which was not monitored in this work), the low photochemical yield of exciplex formation [ 51 makes this process less significant. A typical mechanism for the generation of excited species by radiation is the recombination of electrons with positive charge. This is probably the mechanism responsible for the generation of exciplexes in the Methylamine-ethanol system. It has been pointed out that radiothermoluminescence of y-irradiated triethylamine-ethanol glasses is probably caused by the formation of excited complexes during recombination of trapped electrons with positive charge [18]. A similar explanation can be used in our case. However, we should distinguish between the recombination of trapped electrons and the recombination of dry (unsolvated) electrons moving freely through the medium. Electron trapping at low temperatures has been extensively studied [ 19 - 223. It has been demonstrated that the initial localization of electrons is followed by the reorganization of trapped electrons on a nanosecond and microsecond time scale [20,23]. The trapped electrons also decay, most probably via recombination over the same time scale [23]. Since the lifetimes of the exciplexes observed in this work are of the order of nanoseconds, they must be formed in a much shorter time. Thus, it seems unlikely that the recombination of trapped electrons after an electron pulse is the source of luminescence. Therefore, the radiation-produced electrons, which are not trapped and which undergo recombination in the dry (unsolvated) state, are most probably responsible for the instantaneous generation of exciplexes. The positive charge localized on triethylamine (which is associated in the ground state with one, two or three ethanol molecules) captures a dry electron moving through the medium. The electron becomes localized on the ethanol molecules, providing some excess energy to achieve a favourable orientation to form the appropriate exciplex. The electron localization is very fast. The rate-determining process in exciplex formation is the suitable reorientation of the molecular associate to produce the exciplex. The kinetics of luminescence decay are different from those observed in radiation chemistry due to charge recombination [24]. A commonly observed phenomenon at low temperature is electron transfer which occurs over different time scales (from nanoseconds to several days) [20, 23, 241. In this case the rate-limiting step is electron transfer over some range of distances. In this study the lifetimes of the excited complexes are determined by the radiative and radiationless decay. Therefore conclusions concerning the mechanism of exciplex formation are very limited. The kinetics of luminescence decay are monoexponential and thus the formation of exciplexes is much faster than their decay. Thus the generation of exciplexes by radiation is not suitable for the study of their formation, since the amount
337
of excess energy released during recombination cannot be precisely estimated. The yield of exciplexes at various temperatures depends mainly on the concentration of associates and their efficiency of electron localization. The temperature dependences in Figs. 6 and 8 can be qualitatively interpreted in terms of these factors. Thus the steep decrease in the intensity of luminescence of the A3T exciplex above 140 K may be connected with the disintegration of larger associates and the formation of smaller associates. Indeed, the yield of the small associate, i.e. AT, increases at temperatures between 140 and 170 K. In order to evaluate the influence of the efficiency of electron localization, studies of the spectra of trapped electrons over this temperature range are under way in this laboratory. In general, radiation chemistry seems to offer a convenient way of generating excited complexes with identical structures to exciplexes. The mechanism of formation involves the recombination of initially separate charges, in contrast with photochemistry, where charge undergoes separation after photoexcitation. References 1 J. Piekarska-Go$biowska, Cz. Stradowski and J. Kroh, Radiat. Phys. Chem., 30 (1987) 133. 2 J. A. Barltrop and J. D. Coyle, Fotochemia - Podstawy, P.W.N., Warszawa, 1987, p. 113. 3 0. Anner and Y. Haas, J. Phys. Chem., 90 (1986) 4298. 4 K. Kano, M. Yanagimoto and S. Hashimoto, Bull. Chem. Sot. Jpn. 59 (1986) 3451. 5 G_ Kohler, Chem. Phys. Lett. 126 (1986) 260. 6 G. Kohler, J. Photochem., 35 (1986) 189. 7 J. Mayer, M. Szadkowska-Nicze and J. Kroh, J. Radioanal. Nucl. Chem., 101 (1986)
359. 8 S. Karolczak, 177.
K. Hodyr,
R. Lubis and J. Kroh, J. Radioanal.
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9 J. Malecki,
Prace Komisji Matematyczno-Przyrodnictej Poznanskiego Towarzystwa Nauk Wydzialu Matematyczno Przyrodniczego, 1965, XI, p. 97. R. Brandom and R. F. Firestone, J. Phys. Chem.. 78 (1974) 792. L. Sobczyk (ed.), Wiczanie Wodorowe, P.W.N., Warszawa, 1969. P. B. Merkel and W. H. Hamill, J. Chem. Phys. 63 (1970) 3414. K. Funabashi, C. Herbert and J. L. Magee, J. Phys. Chem., 75 (1971) 3221. J. Mayer, M. Szadkowska and J, Kroh, Nukleonika, 22 (1977) 951. F. Kiefer and M. Magat, in M. Haissinsky (ed.), Actions Chimiques Biologiques des Radiations, Vol. 14, Masson, Paris, 1970, p. 135. G. H. Morine and J. E. Willard, J. Phys. Chem., 81 (1977) 2668. J. Mayer, M. Szadkowska-Nicze and J. Kroh, Radiut. Phys. Chem., 23 (1984) 105.
Przyjaciol 10 11 12 13 14 15 16 17 18 19 20 21 22
J. Piekarska-Goebiowska and Cz. Stradowski, unpublished results, 1988. G. V. Buxton, G. A. Salmon, Cz. Stradowski and J. Kroh, Radiat. Phys. Chem , 17 (1981) 361. J. Kroh, J. P. Suwalski, J. Piekarska, E. Hankiewicz, M. Wolszczak and Cz. Stradowski, Radiat. Phys. Chem., 23 (1984) 529. M. Ogasawara, K. Shimizu and H. Yoshida, Radint. Phys. Chem , 17 (1981) 331. N. V. Klassen and G. G. Teather J. Phys. Chem., 87 (1983) 3891.
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M. Wolszczak and Cz. Stradowski, Radiat. Phys. Chem., 24 (1984) 213. J. Kroh, E. Romanowska and Cz. Stradowski, Chem. Whys. Lett., 47 (1977) 597. W. M. Bartczak, J. Kroh and Cz. Stradowski, J. Chem. Phys., 66 (1977) 2737.