A. Kira and J K. Thomas
2094
Formation of Ions and Excited States in the Pulse Radiolysis of Benzonitrile
.Klra and J. K. Thomas* Department Qf Chemistry and the Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received April 17, 1974) Publicafion costs assisted by the U.S. Atomic Energy Commission
Transient ionic and excited species in liquid benzonitrile and solutions of solutes in benzonitrile were investigated using nanosecond pulse radiolysis. The radiolysis of several aromatic solutes gave rise to the cations of these solutes and the yields were determined to be 1.4 ions/100 eV. Anions of the solutes were also detected for all solutes apart from trans-stilbene. The effects of ammonia, sulfur hexafluoride, and nitrous oxide on the yield of ionic species suggest that the precursors of the solute ions are cations and anions of benzonitrile. Solute excited states are also observed, and the G values for the excited singlet and triplet states were determined to be 0.7 and 1.4, respectively. The mechanism of formation of the excited states is discussed.
Introduction The radio1:ysis of liquid benzene gives rise to excited states with G value$, of 5.4-5.8/100 eV.233 The yield of observed ionic species is low, about 0.3 in pulse radiolysis exp e r i m e n t ~ HIigh. . ~ yields of excited states have been measured for toluene and several xylenes.2 The exact mechanism of formation of the excited states has not been elucidated, although charge recombination and direct excitation by secondary electrons have been suggested. The yield of ionic species becomes more marked and the yield of excited state3 decreases in the radiolysis of some derivatives of benzene such as aniline5 and benzyl alcohol6 that contain polar groups. This suggests that in these compounds a t least some of the excited states are formed via ionic processes, i e., by charge recombination. In order to investigate this possibility further, it is necessary to have further data on the relationship between the yields of ions and excited states in other polar arenes. Benzonitrile is one of several compounds which may be used to study this problem, as prior work indicates that the pulse radiolysis of ecdutions of benzonitrile leads to the formation of both ions and excited states of solute molecules The present study was undertaken to obtain quantitative data for the formation of transient ionic and excited species in the nanosecond pulse radiolysis of benzonitrile. ‘s8
Experimental Sect ion The pulse r,sdioly.,isapparatus has been described in detail elsewhere.9 The samples were irradiated with pulses of 5-, IO-, and 15-nsec duration with a total dose of about 1020 eV i.-1/5-nsec pulse. G values were calculated based on the absorptiorP OS hydrated electrons, in the radiolysis of water where (:(eaq-) -- 3.3 and t is 11,700 M-I cm-1 a t A 6000 A, for eaq-. The benzoriatrile used was Kodak’s Spectrograde reagent; distillatior. of the benzonitrile did not affect the data. Zone-refined perylene was purchased from J. Hinton; pyrene was purified by chromatography; trans-stilbene was recrystallized twice; Kodak’s 1,l’-binaphthyl and Fluka’s l,2-benzanthracene were used without further purification. Nitrous oxide, sulfuir hexafluoride, and ammonia were obtained from Matheson Co. All samples were bubbled with The Journal of Phvsical Cnernistry, Vol. 78. No 21, 1974
nitrogen except when saturated with the gases mentioned earlier.
Results Short-Lived Transients in Benzonitrile. The pulse radiolysis of benzonitrile leads to the transitory spectra shown in Figure 1, two typical decay curves are also shown in the figure. A long-lived absorption dominates the spectrum below 400 nm; a typical decay of the species is shown a t 360 nm. A short-lived absorption spreads over the 400-600-nm spectral region, and its decay half-life is 10 nsec, in agreement with that for emission decay which is observed below 380 nm. Another species with a half-life of 30 nsec shows an absorption from 380 to 520 nm. This latter species with a rapid decay dominates the decay curve at 475 nm as shown in Figure 1. If the sample is saturated with ammonia (-0.1 M ) the band at 390 nm is enhanced. Sulfur hexafluoride (SF6) (-0.1 M ) has little effect on the spectrum apart from a slight enhancement of the absorption a t 400 nm. These data are also shown in Figure 1. Pulse Radiolysis of Benzonitrile. The radiolysis of benzonitrile in cyclohexane solution gives rise to the absorption spectrum shown in Figure 2 with a maximum at 400 nm. A similar absorption maximum is observed in the radiolysis of methanolic solutions of benzonitrile. The radiolysis of benzonitrile in benzene solution gives rise to a broad absorption above 400 nm with a half-life of 10 nsec and an absorption with a 450-nm peak and a half-life of 200 nsec. The lifetime of the benzonitrile emission in benzene solution agrees with that of the short-lived 10-nsec absorption. The 400- or 410-nm bands in cyclohexane or methanol solution, respectively, are identified as the benzonitrile anion, a decision which is based on previous studies.1° The longlived species in benzene solution could be ascribed to the triplet state of penzonitrile, since excited states are the dominant species observed in the radiolysis of benzene solutions. The short-lived absorption in benzene solution may be assigned to the excited singlet state or excimer of benzonitrile. The transient spectrum in pure benzonitrile is considered to consist of the above ionic and excited-state components. However, it is difficult to separate and assign each absorption. Another difficulty in the assignment arises
Pulse Radiolysis of Benzonitrile
2095
1
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380 420 Wovelength , nrn
460
I
I
520
'
I
I
I
540
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I
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/-emission I_I__L_u_L_II
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Figure 1. Upper part shows transient spectra in liquid benzonitrile: nitrogen bubbled, 0,amrnonia saturated, A; SFe saturated, 0, at the end of pulse; nitrogen bubbled at 100 nsec, 0 . Lower part shows oscilloscope traces for a nitrogen-bubbled sample.
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560
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580
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Figure 3. Transient spectra of perylene solutions in benzonitrile: nitrogen bubbled (300 nsec), 0;N 2 0 saturated (300 nsec), 0;SF6 saturated (800 nsec), A; ammonia saturated (800 nsec), D. Perylene concentrations are 0.34-0.37 mM.
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I 400 I
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500
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Wovelength , nm
Figure 2. Transient spectra of benzonitrile solutions in benzene (0), cyclohexane (A), and methanol (0) at the end of pulse and in ben-
from the possibility that aggregated ions may be formed in pure liquid benzonilrile. Formation of Solute Ions. Transitory spectra are obse radiolysis of perylene, trans- stilbene, anthracene, pyrene, and 1,2-benzanthracene in benzonitrile solutiori and are sho~nmin Figures 3-5. Solutions of Perylene. In solutions of perylene, saturation with SFe removes an absorption band a t 580 nm while a 545-rim peak remains. Nitrous oxide (N20) (-0.1 M ) does not affect the spectrum; while ammonia intensifies the 580-nm band. These results are consistent with the previous assignment1'J2 that the maxima a t 550 and 580 nm are due to the perylene cation and anion, respectively. Solulions of Anthracene. In the radiolysis of anthracene in benzonitrile, ammonia slightly enhances the absorption near 660 nm and depresses a 740-nm peak due to the anthracene cation. This result indicates that the anthracene anion is also produced in the radiolysis of benzonitrile solutions, contrary to the previous report that only the anthracene cation was observed.s Solutions of Pyrene. A 490-nm absorption is observed in
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480
700
650
Wavelength
1
750
800
, nrn
Figure 4. Transient spectra for aromatic solutes in benzonitriie: pyrene (10 m M in nitrogen-bubbled, 0 , and SF6-SatUrated, x, solutions; transstilbene (10 mnn) in nitrogen-bubbled solution, 0 ; anthracene (10 mnn) in nitrogen-bubbled, A, and ammonia-saturated, A, solutions.
5 -
-2'
4 -
e
2 3-
-a
2-
I -
O' ;4
'
4b0
'
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'
4;O 5;O 5iO Wovelength , nrn
'
6b0
Figure 5. Transient spectra in 30 m M 1,2-benzanthracene solution: nitrogen bubbled, 0, 0 ; ammonia saturated, A, A. Open marks stand for 150 nsec and filled ones for the end of pulse.
the radiolysis of pyrene in benzonitrile and is attributed to the pyrene anion.11J2 The absorption of the pyrene cation The Journal of Physical Chemistry, Voi. 78, No. 2 1 , 1974
A . Kira and J. K. Thomas
2096 15 r
-
.-
€ I .
Figure 6. Growth in
t , 100nsec/div
the transient absorption in a 3 X
M pery-
lene solution. 0
which normally has a peak at 450 nm was not observed, probably because a t this pyrene concentration the cation exists as the dimer cation.13J4 Solutions of trans-Stilbene. The transient spectrum observed in the pulse radiolysis of trans-stilbene in benzonitrile is assigned to the trans-stilbene cation. This decision is based on previous assignmentsg,ll and the fact that the spectrum was not alffected by oxygen and SF6. The transstilbene anion which normally has an absorption peak a t 480 nm was not detected. Solutions of 1,2-13enzanthracene. The pulse radiolysis of l,%benzanthxacene in benzonitrile gives rise to the spectrum shown in Figure 5. The 420-nm absorption peak is assigned to the anion,ll as its intensity is increased by ammonia. The triplet state also has a peak at 430 nm which is about half a6: intense as the main triplet peak a t 490 nm. Thus the observed 420-430-nm band is due to a composite spectrum of the anion and the triplet state of 1,2-benzanthracene. The absorption of the sample itself prohibited measurements below 400 nm, at wavelengths where the cation absorbs strongly. The yield of the trans-stilbene cation extrapolated to infinite solute concentration was determined as 1.4 ions/100 eV; an extinction coefficient for the anion at X 480 nm in acetone solution of 3.55 X lo4 M-l cm-l was used.15 The yields of the ,inions at infinite solute concentration were estimated for pyrene and 1,2-benzanthracene using extinction coefficients of 5.1 X 10412a t h 490 nm and 1.9 X lo4 M - ] cm-l at X 430 nm, respectively. The 490-nm band of the pyrene anion is overlapped by the absorption of the dimer cation and tlie triplet state. If the optical density a t 490 nm is regarded as being solely due to the anion, a G (anion) of 0.7 is obtained. The contribution of other species to the optical density at 490 nm is estimated to be about 30% based on the effect of SF6 on reducing the anion yield. Accordingly,
