Role of the low-energy excited states in the radiolysis of aromatic liquids

Article abstract of DOI:10.1021/jp202802a

The contribution of the low-energy excited states to the overall product formation in the radiolysis of simple aromatic liquids-benzene, pyridine, toluene, and aniline-has been examined by comparison of product yields obtained in UV-photolysis and in γ-radiolysis. In photolysis, these electronic excited states were selectively populated using UV-light excitation sources with various energies. Yields of molecular hydrogen and of "dimers" (biphenyl, bibenzyl, dipyridyl for benzene, toluene, pyridine, respectively, and of ammonia and diphenylamine for aniline) have been determined, since they are the most abundant radiolytic products. Negligibly small production of molecular hydrogen in the UV-photolysis of aromatic liquids with excitation to energies of 4.88, 5.41, 5.79, and 6.70 eV and the lack of a scavenger effect suggest that this product originates from short-lived high-energy singlet states. A significant reduction in "dimer" radiation-chemical yields in the presence of scavengers such as anthracene or naphthalene indicates that the triplet excited states are important precursors to these products. The results for toluene and aniline suggest that efficient dissociation from the lowest-energy excited triplet state leads to noticeable "dimer" production. For benzene and pyridine, the lowest-energy triplet excited states are not likely to fragment into radicals because of the relatively large energy gap between the excited state level and corresponding bond dissociation energy. The "dimer" formation in the radiolysis of benzene and pyridine is likely to involve short-lived high-energy triplet states.

Full text of DOI:10.1021/jp202802a

ARTICLE
pubs.acs.org/JPCA
Role of the Low-Energy Excited States in the Radiolysis
of Aromatic Liquids
Aliaksandr Baidak,† Matthew Badali,† and Jay A. LaVerne*,†,‡
†Radiation Laboratory and ‡Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, United States
ABSTRACT: The contribution of the low-energy excited states to the
overall product formation in the radiolysis of simple aromatic liquids;
benzene, pyridine, toluene, and aniline;has been examined by compar-
ison of product yields obtained in UV-photolysis and in γ-radiolysis. In
photolysis, these electronic excited states were selectively populated using
UV-light excitation sources with various energies. Yields of molecular
hydrogen and of “dimers” (biphenyl, bibenzyl, dipyridyl for benzene,
toluene, pyridine, respectively, and of ammonia and diphenylamine for
aniline) have been determined, since they are the most abundant radiolytic
products. Negligibly small production of molecular hydrogen in the UV-
photolysis of aromatic liquids with excitation to energies of 4.88, 5.41, 5.79,
and 6.70 eV and the lack of a scavenger effect suggest that this product
originates from short-lived high-energy singlet states. A significant reduc-
tion in “dimer” radiation-chemical yields in the presence of scavengers such as anthracene or naphthalene indicates that the triplet
excited states are important precursors to these products. The results for toluene and aniline suggest that efficient dissociation from
the lowest-energy excited triplet state leads to noticeable “dimer” production. For benzene and pyridine, the lowest-energy triplet
excited states are not likely to fragment into radicals because of the relatively large energy gap between the excited state level and
corresponding bond dissociation energy. The “dimer” formation in the radiolysis of benzene and pyridine is likely to involve short-
lived high-energy triplet states.
’ INTRODUCTION
example, a singlet excited state of benzene is believed to be the
precursor to molecular hydrogen and acetylene,8,9 while a triplet
excited state has been shown to give rise to a phenyl radical and
hydrogen atom.10 Scavenging studies conclude that most of the
dipyridyl production in the γ-radiolysis of pyridine is due to the
decay of a pyridine triplet excited state.11 However, little is
known about which singlet or triplet excited state leads to
product formation. Photochemical studies of aromatic hydro-
carbons have shown that internal conversion from the higher to
lower-energy excited states is very fast.12 The generally accepted
concept is that higher energy states initiate little chemistry.13
Early studies on the relaxation of benzene suggested that, in
addition to fluorescence and intersystem crossing, a third path-
way exists for the decay of higher-energy excited states.14 This
process has been proposed to occur in pyridine to account for
radiationless decay and for the production of H2.15,16 A similar
phenomenon has also been proposed for the dissociation of the
higher-energy singlet excited states of benzene and similar
compounds,17 but the overall significance of higher-energy
excited states in the general scheme for decomposition of
aromatic liquids has not been established.
Selective excitation to a specific molecular state and monitor-
ing its subsequent decay is of obvious importance in typical
photochemical studies of hydrocarbon liquids, but ionizing
radiation can lead to the population of a wide variety of energetic
molecular states whose role in the overall radiolytic process is not
very well understood. The passage of high-energy radiation in
simple aromatic liquids is known to result in the formation of
excited states and ionized species.1,2 For example, the average
energy required to produce an ion pair (W value) of benzene is
26.9 eV per ion pair, whereas its ionization potential is only 9.2
eV.3,4 In other words, for every ion pair produced in benzene,
17.7 eV or more than 65% percent of the energy ends up in
electronic, vibrational, and rotational modes of excitation that do
not lead to ionization. Neutralization reactions in media of low
relative static permittivity (dielectric constant) are very efficient.
The radiation-chemical yield (G value) for free ion pair produc-
tion in the γ-radiolysis of liquid benzene is only 5.5 nmol/J
(0.053 ions per 100 eV).4 The extremely fast ion-recombination
reactions populate molecular excited states with energies ranging
from the lowest-energy excited state all the way to the ionization
continuum. Each of these molecular states can initiate a chemical
pathway for the decomposition of the medium.
Excited states are known to play an important role in the
radiolysis of aromatic compounds. Both molecular products and
radicals are thought to originate from excited states.1,5ꢀ7 For
Received: March 25, 2011
Revised:
June 1, 2011
Published: June 02, 2011
r
2011 American Chemical Society
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The purpose of the present study is to determine the role of
selected excited states in the formation of some of the observed
products in the radiolysis and photolysis of simple aromatic
liquids. The response of these media to specific UV excitation
energies are compared to that observed with γ-radiolysis. Ben-
zene, pyridine, aniline, and toluene were chosen as representative
of simple aromatic liquids. The molecules have sufficient varia-
tion that the effects due to heteroatom substitution and the
addition of side chains can be evaluated.
’ EXPERIMENTAL SECTION
Benzene, pyridine, toluene and aniline in the highest purity
available were obtained from Sigma Aldrich and used as received,
with the exception of aniline. Rapid oxidation of aniline required
it to be distilled under vacuum immediately prior to irradiations.
A spinning band distillation apparatus from B/R Instrument
Corporation was used for this purpose. Biphenyl, bibenzyl, and
2,20-dipyridyl were also obtained from Sigma Aldrich and used
without further purification. Diphenylamine from Fisher Scien-
tific Company was recrystallized twice from isopropanol before
use. Napthalene and N,N,N0,N0-tetramethyl-p-phenylenedia-
mine (TMPD) used as triple state scavengers in aniline were
obtained from Aldrich. The ferrioxalate actinometer was synthe-
sized using potassium oxalate monohydride and anhydrous ferric
chloride, both from Alfa Aesar with purity of at least 98%. 1,10-
phenanthroline used for coloring the ferrioxalate actinometer
was of 99þ% purity from Sigma Aldrich.
Figure 1. UVꢀvisible absorption spectra of benzene, toluene, pyridine,
and aniline.
were determined by dividing the quantum yields by the energy
per photon and conversion to the radiation chemical unit of
nanomoles per Joule (nmol/J).
Molecular hydrogen was determined using an inline technique
as previously described.15 Gas chromatographic analysis was
performed with a SRI 8610 apparatus equipped with a thermal
conductivity detector. The chromatographic column was a
6.4 mm diameter 13X molecular sieve 3 m long, maintained at
40 °C. Calibration was performed by injection of pure H2 at
normal conditions.
Biphenyl, dipyridyl isomers, bibenzyl, and diphenylamine
were analyzed with a Thermo Finnigan Trace gas chromato-
graphꢀmass spectrometer functioning in the selective ion mon-
itoring mode. Biphenyl was detected at mass 154, dipyridyl
isomers at mass 156, diphenylamine at mass 169, and bibenzyl
at mass 182. A 25 m Chrompack CP-Sil-8-CB capillary column
(0.32 mm i.d.) was used for these separations. A 1.0 μL aliquot of
the studied samples was analyzed in a splitless injection mode.
Chromatographic analysis of bibenzyl and dipyridyl isomers
consisted of maintaining an initial column temperature at
145 °C for 30 min. The temperature was then raised to 250 °C
at a rate of 30 °C/min and held for 10 min. The separation of the
dipyridyl isomers could not be fully achieved, and the sum of
their yields is given. Experimental conditions of diphenylamine
detection included maintaining of initial column temperature at
160 °C for 30 min and subsequently heating to 250 °C at a rate of
30 °C/min and then held for 10 min. Biphenyl analysis was
performed with an initial column temperature held at 100 °C for
15 min and then increased to 145 °C at a rate of 30 °C/min and
held for 14 min followed by heating to 250 at 30 °C/min and held
for 10 min. The relative retention times of the compounds of
interest were confirmed by comparison with corresponding
standards and their fragmentation patterns. Sets of standard
solutions were also used for concentration calibration.
Irradiations were carried out using a Shepherd 109 ꢀ 68 60Co
γ-source in the Radiation Laboratory at the University of
Notre Dame. Radiolysis of all compounds was performed under
inert atmosphere (ultrahigh purity argon or helium). The dose
rate was 67.4 Gy/min as measured by the Fricke dosimeter.18
The absorbed dose in irradiated aromatics was assumed to
be proportional to its electron density relative to that of the
dosimeter. Samples for H2 analysis were irradiated in a cell made
from a quartz cuvette containing an entrance and exit port for
inline connection to the gas chromatograph. Samples for analysis
of other products were purged and flame-sealed in Pyrex tubes of
1 cm diameter by about 10 cm long.
Excitation with UV was obtained by using several different
discharge lamps. A low pressure mercury lamp was used at
254 nm (4.88 eV) and a similar lamp with a high purity quartz
window was used at 185 nm (6.70 eV). Both lamps were from
Atlantic Ultraviolet. Cadmium and zinc lamps of 229 nm (5.41
eV) and 214 nm (5.79 eV), respectively, from Ealing were also
used. The sample cell was a modified quartz cuvette containing a
four way valve for connection to the gas chromatograph. The
3.5 mL sample was purged with ultrahigh purity argon before
irradiation and continuously stirred throughout the irradiation.
Actinometry was performed in the same configuration as used
with the aromatics. The actinometer used at 185 nm was an
aqueous solution of ethanol.19 The actinometer used for the
other lamps was the ferrioxolate.20,21 At 254 nm, the actinometry
based on [Co(NH3)5Br]Br2 complex was used as well.22 The
actinometer solutions were stirred during irradiation in the same
fashion as the aromatic samples. Absorbance spectra of the
aromatics as presented in Figure 1 show that virtually all of the
incident photons are absorbed by the sample. Photolytic quan-
tum yields were determined by dividing the amount of products
formed by the number of photons absorbed. Since all photon
energy is absorbed by the media the absolute photolytic yields
The production of ammonia was determined using an analytic
AMMONIA CHEMets kit from CHEMetrics, Inc., which employs
the salicylate method of ammonia determination. Following irra-
diation, aniline was purged with a stream of helium gas, which then
passed through two traps. Aniline vapors were captured in the first
trap that was filled with toluene. The resulting heliumꢀammonia
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ARTICLE
These latter two lamps were only used for H2 determination and
were found to give identical results as with the 254 nm lamp.
The high-energy singlet excited states of aromatic compounds
generated in γ-radiolysis are rather short-lived species. In gen-
eral, they are expected to undergo internal conversion to the
lower levels within about 10ꢀ11 s.4 In UV-photolysis, internal
conversion often brings the aromatic system into a highly
vibrationally excited ground electronic state, which is commonly
called a “hot molecule”. The rate of internal conversion increases
rapidly as the energy of the excited state increases. For instance,
the S2 state of benzene and its analogues are found to undergo
internal conversion in the subpicosecond time scale.12,17 The
energy possessed by the “hot molecule” is high enough to cause
the bond rupture. However, the dissociation process is rather
slow and may take several microseconds to occur.27
Photoexcitation of benzene and its derivatives results in
variety of reactions.28 Part of the excited state molecules under-
goes photoisomerization reactions forming the fulvenes, benzva-
lenes, Dewar-isomers, prismanes, as well as isomerization of
the alkyl substituent in the aromatic ring and fragmentation
reactions.29 These photochemical processes account for the
reduced internal conversion efficiency from the S2 and S3 to S1
state of benzene and its derivatives.30 Isomerization products are
not observed in the γ-radiolysis of aromatics. The bulk of the
primary excitations in γ-radiolysis is to the higher energy range of
20ꢀ40 eV, i.e., much higher levels than S2 or S3 are initially
excited.31 These high-energy excited states undergo internal
conversion directly to the lowest energy singlet level, which is
manifested by the increased fluorescence efficiency in γ-radi-
olysis compared to UV-excitation.32 On another hand, the
reaction pathways originating from the high-energy excited states
are accessible only in γ-radiolysis. This variation in the accessible
excited states must be kept in mind while contemplating
the molecular product patterns observed in the continuous
UV-photolysis experiments with those found in γ-radiolysis.
All electronically excited states may dissociate when the
excitation energy exceeds the energy of the weakest chemical
bond. However, in the liquid phase, the excess energy of the
excited state is promptly dissipated if the dissociation constants
are less than typical vibrational relaxation rates in fluids, i.e.,
Figure 2. Energies of the three lowest-energy singlet excited states of
benzene, pyridine, toluene, and aniline and the corresponding energies
of the UV excitation sources.
mixture was bubbled through a water trap where the ammonia was
captured and analyzed according to the AMMONIA CHEMets
procedure. A cross check of this procedure was made using a
UVꢀvisible spectrometer to get accurate absorbance values at λmax
=
700 nm. The concentration of ammonia was calculated on the
basis of calibration performed with ammonia standard solutions
in the concentration range between 0.5 ppm and 2.0 ppm of NH3
nitrogen. Stock standard solutions of ammonia (50 ppm NH3
nitrogen) were obtained from the Hach Company.
’ RESULTS AND DISCUSSION
Two γ-rays of 1.17 and 1.33 MeV energy are emitted by the
decay of 60Co.2 These γ-rays lose energy by interactions with
electrons of the medium in Compton processes to produce
secondary electrons with hundreds of kiloelectron volts. The
mean energy loss by high-energy electrons is on the order of tens
of electron volts, which is much higher than the energy required
for most common transitions of a molecule and leads to
ionization of the medium molecules with unit efficiency.23 In
aromatic liquids, the initially formed radical cations and electrons
undergo an extremely fast recombination. For instance, in liquid
benzene, the combination reaction in ion pairs happens within
1.2 ps.24 Similarly fast recombination is known to occur in
toluene and pyridine. Only the aniline radical cation has a life-
time long enough to be easily observed by nanosecond pulse
radiolysis.25 Neutralization reactions in γ-radiolysis produces
excited states of the medium ranging in energy from the lowest to
superexcited states lying above the ionization continuum.26
In contrast, low-energy photons of the few electron volts used
here are completely absorbed (cf. Figure 1) to excite the medium
to the corresponding molecular excited states. The compounds
examined in this study are in the liquid phase so the energy levels
are not discrete, but broad bands. Figure 2 depicts the lowest
three energy levels of excited singlet states for the aromatic
compounds chosen for this study and the corresponding energies
of the excitation sources. Excitation with a mercury vapor lamp at
254 nm (4.88 eV) will excite all of the compounds to at least the
lowest-energy singlet excited state. The other mercury vapor line
at 185 nm (6.70 eV) is capable of exciting the S3 level of all four
aromatic compounds. Various energy levels can be reached by
the cadmium 229 nm (5.41 eV) and zinc 214 nm (5.79) lamps.
1012ꢀ1013
s .
ꢀ1 26 Taking into account very high rates of internal
conversion processes between excitation levels in aromatics, it
follows that the major part of high-energy excited states even-
tually dissipate their energy and populate the lowest-energy
excited state. The lowest energy singlet and triplet states (more
precisely, their excimers, which is a dimer of the excited molecule
with a ground state molecule) of aromatic liquids are formed in
high yields in γ-radiolysis. For instance, radiation-chemical yields
of the lowest-energy excited states in γ-radiolysis of benzene
equal to G(S1) = 0.17 μmol Jꢀ1 and G(T1) = 0.44 μmol Jꢀ1 1
.
The lowest energy singlet and triplet states of aromatics are
relatively long-lived species with lifetimes on the nanosecond
time scale. So, if allowed thermodynamically, they can initiate
chemical reactions. Dissociation processes such as ArH*fAr• þ
H• are the dominant primary sources of radicals formed in
radiolysis. In turn, the formation of “dimer” products in the γ-
radiolysis of aromatics is related to a larger or lesser extent to
radical combination reactions. The energetics of the process
must be analyzed to establish the feasibility of the direct
dissociation from the lowest-energy excited state. Thus, the
energies of the lowest singlet and triplet states and the bond
dissociation energies for the dissociation processArH*fAr• þ H•
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Table 1. Energies of the Lowest S1 and T1 States and Bond Dissociation Energy
benzene
pyridine
toluene
aniline
bond dissociation energy*, kJ molꢀ1
vertical energy of the lowest singlet excited state S1, kJ molꢀ1
vertical energy of the lowest triplet excited state T1, kJ molꢀ1
47533,34
47338
35438
43933,35
45739
39639
37836
44838
37138
37037
41838
36138
* For benzene PhH* f Ph• þ H•; pyridine PyrH* f 2-Pyr• þ H•; toluene PhCH2H* f PhCH2• þ H•; aniline PhNH2* f PhNH• þ H•.
Table 2. Experimental Yields of Some Products Formed in Photolysis and γ-Radiolysis of Aromatics
product yield, nmol Jꢀ1
185 nm (6.70 eV)
quantum yield
aromatic compound
benzene
product
254 nm (4.88 eV)
γ-Co60
254 nm (4.88 eV)
185 nm (6.70 eV)
hydrogen
biphenyl
0.21
0.039
∼0
0.069
0.098
0.65
0.17
22
1.4
0.25
0.22
1.5
3.9
7.8
9.1 ꢁ 10ꢀ5
2.2 ꢁ 10ꢀ5
∼0
9.4 ꢁ 10ꢀ4
1.7 ꢁ 10ꢀ4
8.5 ꢁ 10ꢀ5
9.9 ꢁ 10ꢀ4
4.8 ꢁ 10ꢀ4
4.0 ꢁ 10ꢀ3
1.2 ꢁ 10ꢀ3
4.0 ꢁ 10ꢀ2
3.6 ꢁ 10ꢀ2
pyridine
toluene
aniline
hydrogen
dipyridyl
2.8
130
14.5
6.8
3.3 ꢁ 10ꢀ5
9.1 ꢁ 10ꢀ5
3.2 ꢁ 10ꢀ4
8.3 ꢁ 10ꢀ5
9.1 ꢁ 10ꢀ3
1.5 ꢁ 10ꢀ2
hydrogen
bibenzyl
0.82
7.1
hydrogen
diphenylamine
ammonia
1.9
12.4
4.15
52
61
31
40
are presented in Table 1 for the molecules examined here.33ꢀ39
On the basis of the data presented in Table 1, the thermodynamic
feasibility of CꢀH bond dissociation exists for the lowest-energy
singlet excited state of the studied aromatics. The excess energy is
especially large for toluene and aniline. In the case of toluene and
aniline, even the lowest-energy triplet excited state possesses
enough energy to allow for CꢀH bond dissociation. In contrast,
the dissociation from the T1 state of benzene and pyridine has a
low probability based on energetic reasons.
In this work, the measured yields of H2 and of “dimers’’ of the
parent molecules are used as probes of the effects caused by the
primary excitation to the different energy levels. The molecular
products investigated here are typically the most abundant and
should give a good representation of the overall medium decay.
The results of the γ-radiolysis and UV-photolysis of benzene,
pyridine, toluene, and aniline with various excitation energies are
presented in Table 2. The yields are expressed in the radiation
chemical unit of nanomoles per joule (104 nmol Jꢀ1 = 1
molecule/100 eV). The equivalent unit can be obtained from
the photolytic quantum yields by the number of photons
absorbed and the energy per photon.
H2 Production. The H2 yields for the UV-photolysis at 254
and 185 nm and for γ-radiolysis are arranged in graphic form in
Figure 3. Higher yields of H2 are observed in the γ-radiolysis of
all the studied aromatics compared to those values obtained in
photolysis. The γ-radiolysis of toluene and aniline produces
considerably more H2 compared to benzene and pyridine due
to the presence of the H• atom containing substituents (ꢀCH3
and ꢀNH2, respectively), which are expected to be actively
involved in fragmentation processes. Radiolysis of aliphatic
compounds is well-known to produce H2 in much larger
quantities than found with aromatic compounds.1
Molecular hydrogen yields obtained in the UV-photolysis are
negligibly small in all cases compared to the γ-radiolysis results.
Nevertheless, H2 yields increase when the photon excitation
energy is increased. This observation suggests that high-energy
excited states are the main precursors of H2. The formation of H2
from the high-energy excited states must be due to very rapid
processes since they have to compete with internal conversion,
which typically occurs on the picosecond time scales.
The yield of H2 in the radiolysis of liquid benzene and pyridine
varies significantly with the type of radiation and on the linear
energy transfer (LET = ꢀdE/dx).15,40 In general, G(H2) in-
creases with LET due to the increased significance of the intra-
track bimolecular processes. Burns has suggested that H2 is
produced in the radiolysis of benzene as a result of the reaction
between two singlet excited state molecules.8,41 The implied
intermediate in this process is a high-energy singlet state that is
more likely to decompose to H2 than the S1 excited state. A study
of the fluorescence intensity dependence on LET in the heavy
ion radiolysis of benzene has provided strong support for the role
of the excited singlet state in H2 production.9,40 A logical
assumption of this observation is that reaction between singlet
excited states is facilitated in the radiolysis of all the aromatic
compounds studied here because of the nonhomogeneous
nature of energy deposition in radiolysis as compared to the
homogeneous deposition of energy in photolysis. Excited states
produced by the latter are generated homogeneously in the
bulk of the solution, and the chances of their encounter are
rather small.
One more experimental evidence corroborating the impor-
tance of the bimolecular mechanism of molecular hydrogen
formation comes from isotopic composition studies. Total
hydrogen yield G(H2 þ HD þ D2) generated from benze-
neꢀbenzene-d6 equimolar mixtures clearly shows a large con-
tribution of HD, which is the product of bimolecular reaction.42
Identical isotopic composition of molecular hydrogen is ob-
served for the different types of radiation, although the total yield
changes markedly.43 Again, this result is evidence for the
bimolecular mechanism of molecular hydrogen formation. Only
a small number of H• atoms contribute to the H2 yield in the case
of benzene and its derivatives, since H• atoms readily add to the
aromatic ring.44 Irradiation of toluene and its deuterated analo-
gues demonstrates that the H2 is formed from the H• atoms of
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Figure 3. Molecular hydrogen production in photoexcitation at 254
and 185 nm and in the γ-radiolysis of benzene, pyridine, toluene, and
aniline: left to right in each panel.
Figure 4. Biphenyl, dipyridyl, and bibenzyl production in photoexcita-
tion at 254 and 185 nm and in the γ-radiolysis of benzene, pyridine, and
toluene, respectively: left to right in each panel.
both the side-chain and the aromatic ring.45 However, the
contribution of the toluene methyl group to H2 formation is
greater than that of the aromatic moiety. Unimolecular processes
account for about 38% of H2 formed from the ring, while 62% is
due to bimolecular reactions.
solutions of 0.5 M chloroform in benzene, toluene, and aniline
were irradiated, and H2 yields were determined. The yields of H2
were reduced by 10ꢀ15% compared to the neat aromatics.
However, a more detailed study of a wide range of chloroform
concentrations in toluene revealed that the reduction in H2 yield
is proportional to the molar fraction of the scavenging additive.
The result indicates that the decrease in the amount of H2 comes
simply from a dilution effect since the radiolysis of chloroform
itself is not known to produce H2.55
The conclusion based on the presented data is that H2 formed
in the radiolysis of these aromatic liquids originates from short-
lived high-energy excited states. Available literature data also
strongly suggest that molecular hydrogen comes from a bimole-
cular process. These excited states are formed and decompose
at the earliest stages after irradiation in the subpicosecond
time range. Higher levels of excitation than used in this work
(>6.7 eV) would be necessary to identify the excitation energy
limit required to achieve H2 yields comparable with those in
γ-radiolysis.
Biphenyl Formation from Benzene. The main simple hydro-
carbon product observed in the γ-radiolysis of neat liquid
benzene is biphenyl with a small yield of 7.8 nmol Jꢀ1.7 The
initial decomposition of benzene following the absorption of
energy from ionizing radiation results in the generation of the
phenyl radical, C6H5•, and a H• atom. One might assume that
biphenyl is formed by the recombination of phenyl radicals by
analogy to the dimer formation in the radiolysis of cyclic
alkanes.47 However, aromatic compounds are good radical
scavengers, and studies with I2 have shown that both C6H5•
and the H• atom combine with parent benzene molecules to
initiate formation of phenylcyclohexadienyl and cyclohexadienyl
radicals, respectively.7 Subsequent reactions of the phenylcyclo-
hexadienyl radical result in biphenyl formation, but most radicals
generated in radiolysis contribute to polymer production.56
The total radiation-chemical yield of biphenyl in neat benzene
is due to several contributing processes. Even at the highest
concentrations of I2, when virtually all phenyl radicals are
scavenged, the yield of biphenyl does not drop to zero.40 The
fact that biphenyl yield does not depend on LET further suggests
that precursors of this dimer do not participate in bimolecular
Experimental efforts to elucidate the precursor of H2 produced
in the radiolysis of liquid aromatics have used quenchers to
selectively scavenge radicals, electrons, radical cations or excited
states. The addition of typical radical scavengers, like iodine,
shows no effect on the formation of H2 in the radiolysis of
benzene,46 pyridine,47 and toluene.1 Unfortunately, radical sca-
vengers I2 and 1,1-diphenyl-2-picrylhydrazyl (DPPH) cannot be
used with aniline because of their direct chemical interaction.48
One can conclude from the available information that radical
processes do not contribute noticeably to the radiolytic forma-
tion of H2. Experiments on benzene solutions with electron
scavengers (COS, N2O) demonstrate that there is significant
formation of electrons in the system as reflected by the produc-
tion of CO and N2, respectively, but the H2 yields are not
affected.49 In this work, similar results were observed with
saturated solution of N2O in toluene; that is, no effect on H2
yields was noticed with added quencher. C2H5OH and C2H5Br
have been used as radical cation and electron scavengers,
respectively, in the radiolysis of toluene, and no influence on
H2 yield is noticed.50 Finally, the H2 yields show no dependence
on the addition of the excited state scavengers anthracene and
biphenyl in the radiolysis of pyridine and aniline.47,51 The
formation of H2 in the γ-radiolysis of aniline is not affected by
the addition of naphthalene and TMPD either.51 Apparently,
none of the conventional quenchers are capable of scavenging
any appreciable fraction of H2 formed in the radiolysis of liquid
aromatics. The only possible precursor to H2 remaining are the
high-energy excited states of these aromatics. These high-energy
excited states must be very short-lived, since they can not be
scavenged by the quenchers even at high concentration.
There are reports in the literature that the addition of chloro-
form leads to the reduction of the relative fluorescence efficiency
of the high-energy excited states.52ꢀ54 These excited states can be
viewed as charge-transfer (CT) complexes.32 Chloroform acts as
an extremely efficient electron acceptor and quenches such CT
complexes. An attempt was made to observe whether the
presence of chloroform would affect the H2 formation in the
γ-radiolysis of aromatic hydrocarbons. For this purpose, the
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reactions of reactive species, and part of this product is formed
due to a nonradical mechanism.40 Biphenyl may be formed by the
dissociation of high-energy excited states. These excited states
are short-lived, so their decomposition may account for minor
processes such as the formation of biphenyl.
with the increased concentrations of triplet excited states. The
result of the annihilation is a repopulation of high-energy singlet
excited states. Photolysis measurements show that the lowest T1
state is unlikely to produce pyridyl radical. This dissociation is
also not favored energetically (cf. Table 1). The high observed
yield of dipyridyl in the radiolysis of pyridine suggests that the
high-energy excited states might be responsible for “dimer”
formation.
Bibenzyl Formation from Toluene. Analysis of the published
data shows that most of the photolytic studies on toluene were
conducted in the vapor phase.58,60ꢀ64 Collision-free conditions
are created in the gaseous state and the dissipation of energy by
the excited state molecules through interaction with medium can
be prevented. Hence, the formation of various photoproducts is
expected in such conditions. The photolytical research on
toluene performed in the liquid phase reveals only the impor-
tance of photophysical relaxation processes and of photoisome-
rization reactions.17,65ꢀ67 Nevertheless, the formation of bibenzyl
in the Hg-sensitized photolysis of toluene vapor has been
reported. Bibenzyl is formed in relatively high yield compared
to other stable molecular products.68 In this work, bibenzyl is also
detected as a dominant “simple” product among other con-
densed-phase products generated both in steady-state photolysis
and γ-radiolysis. Another contemporary work corroborates our
observation.69
The yield of biphenyl in the 254 and 185 nm photolysis of
benzene corresponds only to 0.5% and 3.2% of the radiation
chemical yield obtained in γ-radiolysis (cf. Table 2 and Figure 4).
Excitation at 254 nm produces the lowest energy singlet state S1
of benzene, while 185 nm excitation generates the S3 excited
state. Subsequently, the S3 state undergoes internal conversion to
the S1 level with an efficiency of 0.45.57 The lowest-energy triplet
excited state T1 is formed from S1 with an intersystem crossing
efficiency equal to 0.56.1 The formation of phenyl radical due to
the dissociation of the excited molecule of benzene in the T1 state
is improbable due to the unfavorable energetics of the process
(cf. Table 1). The remaining alternative for biphenyl production
in photolysis is a direct dissociation process. Such dissociation is
thought to take place not from the S3 state directly, but from the
ground state via a “hot” benzene mechanism.58 Very small yields
of biphenyl obtained in UV-photolysis, as shown in Figure 4,
seem to reflect this low probability of the excited molecule
dissociation.
Radiation-chemical yield of biphenyl in the radiolysis of
benzene is strongly reduced in the presence of naphthalene
or anthracene, well-known scavengers of the triplet excited
states.1,10,59 The reduction of biphenyl yield in their presence
suggests that a high-energy triplet state of benzene is a precursor
to the phenyl radical and H• atom. Nevertheless, both naphtha-
lene and anthracene were shown not only to quench the triplet
states of benzene, but also to add readily to phenyl radicals.59
Biphenyl production is obviously affected by additives at different
stages of its formation: quenching of high-energy triplet excited
states of benzene and reactions of already formed phenyl radical.
In other words, the triplet scavenger effect on the “dimer”
formation in the radiolysis of benzene is complex. This observa-
tion must be borne in mind when considering the scavenger
effect on the production of “dimers” of other studied aromatics
as well.
Dipyridyl Formation from Pyridine. The photolytic yields of
dipyridyl are negligibly small (<1%) in comparison to its radia-
tion chemical yield (cf. Figure 4 and Table 2). These results
suggest the lower-energy excited states of pyridine give only a
very minor contribution to the dimer formation. Iodine scaven-
ging studies demonstrate that most of dipyridyl has a radical
precursor, but only about 10% of it is due to the pyridyl radical.15
In close analogy to the biphenyl formation in benzene radiolysis,
the dipyridyl yield is the result of several processes: direct
recombination of the pyridyl radicals, dehydrogenation of the
“pyridyl radicalꢀpyridine” (analogous to the phenylcyclohexa-
dienyl radical), deprotonation of the chargeꢀtransfer complex of
pyridiniumyl radical cation and neutral pyridine (minor channel,
only a few percent).15 The production of dipyridyl can almost
completely be thwarted by the presence of scavengers of the
triplet excited states like anthracene.11 As in the case of benzene,
the quencher effect may be complex too, that is, the high-energy
triplet excited states can be scavenged as well as the low-energy
states or pyridyl radicals may add to the quencher molecules.
In contrast to biphenyl, the value of G(dipyridyl) is LET
dependent. The dimer yield decreases with increasing track
average LET.15 This result is attributed to intratrack tripletꢀ
triplet annihilation processes that gain importance at high LET
The yield of bibenzyl in the γ-radiolysis of liquid toluene is
reduced to 5% of the initial value in the presence of radical
scavengers.70 Hence, bibenzyl produced upon irradiation of
toluene derives almost entirely from a radical precursor, the
benzyl radical. Benzyl radical, in turn, is thought to be formed due
to the decay of excited state molecules of toluene.71 Irradiation of
toluene under collision-free conditions at 193 nm showed that R-
H• atom elimination and CH3 elimination are the major
dissociation channels (cf. reactions 1ꢀ2):
ꢂ
C6H5CH3 f C6H5CH2• þ H•
ð1Þ
ꢂ
C6H5CH3 f C6H5• þ CH3•
ð2Þ
The dissociation rate for process (1) was found to be 2 ꢁ 106 sꢀ1
.
Direct CꢀH and CꢀC bond cleavage is believed to be the
dissociation mechanism, proceeding through the vibrationally ex-
cited ground electronic state of toluene (“hot toluene”).72 Rapid
internal conversion to S0 and S1 states was observed for the S2 state
of toluene produced by 200 nm excitation. This state has a lifetime
of ∼50 fs.73 The “hot molecule” reaction mechanism is justified by
the fast internal conversion from initial high-energy excited states to
lower-energy excited states and the slow dissociation rate of toluene.
These considerations are based on the gas phase studies, whereas in
liquid solutions the dissociation process was shown to be rather
inefficient due to the fast dissipation of excess energy through
intermolecular collisions. Indeed, the quantum yields of bibenzyl,
the product of benzyl radical combination reactions, obtained in this
work are small: 3.2 ꢁ 10ꢀ4 for 254 nm irradiation and 4.0 ꢁ 10ꢀ3
for 185 nm. However, as can be seen from Figure 4 and Table 2, the
yield of bibenzyl at 185 nm photolysis is about the same as in
γ-radiolysis. This result gives clear evidence that even 6.70 eV
excitation of liquid toluene leads to results similar to that in
γ-radiolysis where a large range of excitation is accessible.
There was a distinct difference in temperature regimes while
performing 185 nm photolysis and γ-radiolysis of toluene. The
exposure of the toluene sample to the 185 nm source heats it to
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Figure 6. Production of diphenylamine and ammonia (left to right in
each panel) in photoexcitation at 254 and 185 nm and in the γ-radiolysis
of aniline.
level.78 The fluorescence lifetime was found to be about a few
nanoseconds. Among the nonradiative processes, the intersys-
tem crossing from the S1 state appears to be the major one. The
triplet excited state decays rapidly, so that no phosphorescence is
observed. The rate of the triplet excited state decay increases as
the energy of the state increases.79
Photolysis of aniline with high energy excitation, such as the
6.7 eV photons employed in this work, involves excitation of the
electrons of the aromatic ring (π f π*). Internal conversion of
these excited states occurs very fast and dissociation of the
molecule proceeds from the ground state through the channels
with low dissociation barriers (N ꢀ H and C ꢀ N bond
rupture). Nevertheless, about 75% of H• atom elimination
occurs in the electronic excited state.80 Four major dissociation
channels are presented in Figure 5.80 As a peculiarity of aniline
photolysis, the release of closed shell molecules (H2 and NH3)
in the primary dissociation processes is observed. Even the
lowest excitation energy of 254 nm photons used in this study is
sufficient enough to overcome the activation energies of all four
channels (cf. Figure 5).
Experimental yields of ammonia and diphenylamine in the
photolysis and γ-radiolysis of aniline are shown in Figure 6.
According to the scheme in Figure 5, ammonia can be formed by
the direct dissociation of aniline (branch 4) or by the NH2 radical
intermediate (branch 3). Formation of ammonia increases with
increasing photon energy and the highest yield of NH3 is
achieved with γ-rays. Nevertheless, even 254 nm irradiation
generates NH3 in significant amounts. This result is not surpris-
ing since the decomposition branch 4 (cf. Figure 5) giving
ammonia directly has the lowest dissociation barrier, which is
below all the excitation energies applied in this work.
Diphenylamine is most likely to be formed in radical combina-
tion reactions. Unfortunately, the typical radical scavenger I2 was
found to directly react with aniline and could not be used to
identify specific radical precursors to diphenylamine. Two pos-
sible pathways for diphenylamine formation are shown in
Figure 7: the first one is the recombination of aminyl and phenyl
radical, and the second one is the reaction between 1,2-dehy-
drobenzene and aniline. Dehydrobenzene has been reported to
react readily with amines,81 and it could be formed in quantities
similar to that for ammonia by the decomposition of low-energy
excited states (branch 4 of Figure 5). At least 3 out of 4 initial
Figure 5. Four possible dissociation channels of the aniline excited
state80 (254 nm = 4.88 eV and 185 nm =6.70 eV).
40ꢀ45 °C during long-time irradiations, whereas γ-radiolytic sam-
ples were at room temperature. Radiolysis studies have shown that
the yield of bibenzyl exhibits complex temperature dependence.74 For
example, G(bibenzyl) at 25 °C is reported to be 4.1 nmol Jꢀ1,
whereas at 50 °C it is almost doubled to 7.3 nmol Jꢀ1. In this work,
the radiation-chemical yield of bibenzyl in γ-radiolysis was repeatedly
found to be 6.8 nmol Jꢀ1 at room temperature. A temperature effect
in photolysis would suggest a slightly lower yield at room temperature
than the observed value of 7.1 nmol Jꢀ1 at 185 nm. Nevertheless, the
results show that the 185 nm excitation of toluene generates excited
states that are capable of photodissociation in the liquid phase with
yields approaching or equal to that in γ-radiolysis.
Iodine scavenging studies in toluene show that about 95% of
bibenzyl has benzyl radical as a precursor.70 The energy of the
lowest triplet excited state T1 of toluene is comparable to the
bond dissociation energy for the process: PhCH3* f PhCH2• þ
H• (cf. Table 1). Therefore, even the 254 nm excitation source
employed in this work provides the toluene molecule with
enough energy for dissociation either from S1 or from T1 state.
An excitation energy of 6.4 eV is sufficient for direct CꢀC
bond and CꢀH bond dissociation in toluene.72 Therefore, the
185 nm (6.70 eV) excitation of toluene can lead directly to the
production of benzyl radicals. This bond rupture occurs
through the vibrationally excited ground electronic state
(“hot toluene” mechanism).
Diphenylamine and Ammonia Formation from Aniline.
The γ-radiolysis of aniline is known to generate a large variety of
products including the isomeric aminobiphenyls, aminodiphe-
nylamines, and diaminobiphenyls with ammonia, H2, benzene,
and diphenylamine having the greater yields.51 UV photochem-
istry of aniline has been studied in detail with special emphasis
given to the photoionization and photodissociation processes
and how the medium influences them.75ꢀ77 Polar solvents such
as water were shown to facilitate the photoionization from aniline
singlet excited states. Fluorescence spectroscopy of the first
singlet excited state of aniline S1 has delivered information on
the fluorescence lifetime and quantum yield of each vibronic
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energy singlet states decompose to give H2. Population of the
higher-energy singlet excited states can occur only for γ-radi-
olysis in our studies, which justifies the big difference in H2 yield
between radiolysis and photolysis of aniline (cf. Figure 3). Once
the higher-energy excited state fragments, it is not capable of
populating the lower-energy excited states through internal
conversion. However, photolysis directly populates the lower-
energy excited states, which tend to dissociate into radical
fragments that are the precursors of diphenylamine. Dissocia-
tion is especially efficient from the triplet excited state of
aniline.82 Taking into account the high yield of intersystem
crossing (ΦISC = 0.68)76 for the process S1 f T1 in aniline,
the 254 and 185 nm excitation used in this work should populate
well the lowest-energy triplet excited state of aniline with some
additional vibrational excitation, which would lead to efficient
dissociation and eventually to the formation of diphenylamine in
high yields.
Figure 7. Diphenylamine formation from aniline.
Aniline has high viscosity (3.71 cP), which is 4ꢀ6 times
greater than the viscosity of other aromatic liquids studied here.
Such high viscosity of the medium could influence the efficiency
of solvent stirring and, therefore, the homogeneity of the
products in the photolysis of aniline. High local concentrations
of products could lead to new reactive pathways with the more
transient species, or those products themselves could be photo-
lytically degraded.
Figure 8. Yield of diphenylamine in the γ-radiolysis of aniline as a
function of added (9) naphthalene and (b) TMPD.
’ CONCLUSIONS
The role of the lower-energy excited states in the radiolysis of
simple aromatics;benzene, toluene, pyridine, and aniline;has
been examined using UV-light excitation sources with various ener-
gies. The contribution of the excited states to the overall product
formation has been evaluated by comparison of product yields
obtained in UV-photolysis and in γ-radiolysis. Yields of H2 and of
“dimers” (biphenyl, bibenzyl, dipyridyl, and diphenylamine for
benzene, toluene, pyridine, and aniline, respectively) have been
determined since they are the most abundant radiolytic products.
Ammonia production in aniline has also been determined.
Negligibly small H2 production in the UV-photolysis of aromatic
liquids (4.9, 5.4, 5.8, and 6.7 eV excitation energies) and the lack
of any scavenger effect suggests that H2 originates from very
short-lived higher-energy excited states. A significant reduction
in “dimer” radiation-chemical yield in the presence of anthracene
or naphthalene demonstrates that triplet excited states are
important precursors in “dimer” formation of the studied aro-
matics. Triplet excited state molecules dissociate to give radical
fragments: H• atom and the corresponding phenyl, benzyl,
pyridyl, or aniline radical. However, attachment of the radical
to the parent molecule is strongly favored, and the resulting
product does not always lead to “dimer” formations, such as in
the case of benzene. The probability of dissociation into radical
fragments is governed by the energetic proximity of the lowest-
energy excited state level and corresponding bond dissociation
energy. In case of benzene, the energetic gap between the lowest
triplet state and CꢀH bond dissociation energy is large, and
phenyl radicals are formed in low yields. In contrast, for toluene
and aniline, the energies of the lowest triplet states lie very close
to the respective bond dissociation energies, which results in
effective radical production. The recombination of radicals
subsequently leads to the dimeric molecular products. For this
reason, even low excitation energies of 4.9 and 6.7 eV give yields
dissociation channels of excited aniline (Figure 5) generate
precursors of diphenylamine. These diverse sources make it
quite difficult to track down the origin of diphenylamine to any
particular excited state.
Diphenylamine yields as shown in Figure 6 are significantly
higher in photolysis than in radiolysis. The γ-radiolysis studies of
liquid aniline revealed that H2 production is not quenched in the
presence of various scavengers of excited states, whereas the yield
of ammonia and diphenylamine is reduced approximately by
half.51 Hence, H2 is produced in very fast processes that are most
probably due to the direct decay of the higher-energy excited
singlet states, which have lifetimes in the picosecond time
domain. In contrast, at least half of the yield of ammonia and
diphenylamine is derived from a precursor that can be scavenged
by quenchers of triplet excited states, like naphthalene, in a
diffusion-controlled reaction (cf. Figure 8). The conclusion is
that the triplet excited state is a precursor to at least part of the
ammonia and diphenylamine. Work elucidating the role of the
lower-energy excited states in photoreactions of aromatic amines
has suggested that the photodissociation (RNH2 f RNH• þ H•)
reaction takes place from both lowest-energy singlet and triplet
excited states of aniline, but the process occurs more efficiently
from the triplet excited state.82 This photodissociation reaction
creates the aminyl radical, which is one of diphenylamine
precursors.
Time-resolved photoacoustic and multiphoton ionization
spectroscopic studies of aniline show that the energy deposited
into aniline can be selectively directed either into ionization or
dissociation.83 The initial yield of the first singlet excited state in
the pulse radiolysis of aniline is very low. This state is populated
through the internal conversion from the higher-energy singlet
excited states over a period of ∼10 ns.25 Part of these higher
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of “dimers”, bibenzyl and diphenylamine, that are comparable or
even higher than those observed in γ-radiolysis.
(34) Barckholtz, C.; Barckholtz, T. A.; Hadad, C. M. J. Am. Chem.
Soc. 1999, 121, 491.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: laverne.1@nd.edu.
2006, 758, 149.
(38) Hou, X.-J.; Quan, P.; Hoeltzl, T.; Veszpremi, T.; Nguyen, M. T.
J. Phys. Chem. A 2005, 109, 10396.
’ ACKNOWLEDGMENT
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The authors thank Dr. G. J. Ferraudi for assistance with the
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the Notre Dame Radiation Laboratory.
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