Spectroscopic studies on photochemical formation of o-xylylene in solution

Article abstract of DOI:10.1021/jp970655v

The photochemical formation of o-xylylene is studied by two-pulse fluorescence and transient absorption spectroscopy. o-Xylylene is produced by photolysis of α,α′-dichloro-o-xylene, α,α′-dibromo-o-xylene, α,α′-bis(trimethylstannyl)-o-xylene, and 2-indanon

Full text of DOI:10.1021/jp970655v

4912
J. Phys. Chem. A 1997, 101, 4912-4915
Spectroscopic Studies on Photochemical Formation of o-Xylylene in Solution
M. Fujiwara,* K. Mishima, K. Tamai, and Y. Tanimoto*
Department of Chemistry, Faculty of Science, Hiroshima UniVersity, Kagamiyama,
Higashi-Hiroshima 739, Japan
K. Mizuno and Y. Ishii
Department of Applied Chemistry, College of Engineering, Osaka Prefecture UniVersity,
Gakuen-cho, Sakai 593, Japan
ReceiVed: February 24, 1997; In Final Form: May 8, 1997X
The photochemical formation of o-xylylene is studied by two-pulse fluorescence and transient absorption
spectroscopy. o-Xylylene is produced by photolysis of R,R′-dichloro-o-xylene, R,R′-dibromo-o-xylene, R,R′-
bis(trimethylstannyl)-o-xylene, and 2-indanone at 266 nm in room-temperature cyclohexane solution. On
excitation at 308 nm, the fluorescence of o-xylylene is observed with a maximum at 460 nm and a lifetime
of 9.3 ( 0.5 ns. The absorption of o-xylylene is obtained with a maximum at 370 nm. The formation of
o-xylylene from R,R′-dichloro-o-xylene occurs, by 266-nm two-photon excitation, with an apparent yield of
0.30 ( 0.08 at a fluence of 40 mJ cm-2. Its formation from 2-indanone proceeds, by 266-nm one-photon
excitation, with a rate of (1.4 ( 0.1) × 107 s-1 and a yield of 0.90 ( 0.08.
1. Introduction
For the two-pulse fluorescence measurement, a pulse of the
fourth harmonic (266 nm, 4-5 ns) of a Nd:YAG laser (Quanta-
Ray GCR-11) was used as the photolysis light. The fluores-
cence was induced by a second pulse (308 nm, 8-12 ns) from
a XeCl excimer laser (Lumonics 500). The delay time was
adjusted with a digital delay generator (EG&G PAR 9650). The
laser beams were focused on the cell coaxially. The fluores-
cence was detected with a monochromator (Ritsu MC-10N), a
photomultiplier (Hamamatsu R636), and a digital oscilloscope
(Tektronix 2440). Signals were accumulated with a personal
computer (NEC PC-9801UV) to improve an S/N ratio.
o-Xylylene is a reactive molecule that has been studied over
the past 30 years. Spectroscopic investigation on o-xylylene
has given direct evidence for its existence. The absorption,1-5
fluorescence,1-4,6 IR,4,5 and Raman4 spectra of o-xylylene have
been observed in low-temperature matrices. The absorption
spectrum of o-xylylene has been obtained in solution by flash
photolysis7,8 and stopped flow9 techniques. A rate constant for
o-xylylene bimolecular decay, where [4+2] and [4+4] dimers
are produced, has been measured to be 9.9 × 103 dm3 mol-1
s-1 7,9
However, the fluorescence spectrum of o-xylylene in
.
For the transient absorption measurement, the photolysis light
was the 266-nm laser pulse. The laser power was attenuated
with neutral density filters (Sigma FNDU-50C02-10, -20, -50,
-70, -80) and monitored with a thermopile detector (Ophir 03A-
P, DGX). The white light was provided by a Xe flash lamp
(Ushio UXL-500D-O). The Xe lamp beam was collimated on
the cell at a right angle with the laser beam. The Xe lamp beam
passed the region close (0.0-1.6 mm) to the laser irradiation
inner surface in the cell to minimize decrease of the laser power
by sample absorption. The detection apparatus for the white
light was the same as for the fluorescence. The UV filter
(Toshiba UV-D33S, 240-400 nm) was inserted in front of the
monochromator to reject visible light for the measurement in
the short wavelength region (<400 nm). All experiments were
performed at room temperature (293 K).
solution has not been reported yet. The mechanisms of the
formation of o-xylylene are not well understood.
The present paper deals with the formation of o-xylylene by
photolysis of several precursors at 266 nm in cyclohexane
solution at room temperature. The fluorescence, which is
induced by excitation at 308 nm, and the absorption are recorded
for o-xylylene. The number of photons necessary for its
formation is determined. A quantum yield and kinetics for its
formation are measured. The mechanisms of the photochemical
formation of o-xylylene are discussed.
2. Experimental Details
R,R′-Dichloro-o-xylene (CX, Tokyo Chemical, >95%) and
R,R′-dibromo-o-xylene (BX, Kanto Chemical, >97%) were
recrystallized from n-hexane. R,R′-Bis(trimethylstannyl)-o-
xylene (MSX) was synthesized by the method described
elsewhere.10 2-Indanone (IN, Aldrich, >98%) was sublimed
under vacuum. Naphthalene (Nacalai Tesque) was recrystallized
from ethanol. Cyclohexane (Dojindo, spectrosol) was used as
received. The sample solutions, which contained CX (6.2 ×
10-3 mol dm-3), BX (1.1 × 10-3 mol dm-3), MSX (∼10-3
mol dm-3), and IN (2.8 × 10-3 mol dm-3) with cyclohexane,
were degassed by freeze-pump-thaw cycles and sealed in a
10 × 10-mm quartz cell under vacuum. The molar extinction
coefficient was measured with an absorption spectrophotometer
(Hitachi U-3210).
3. Results and Discussion
3.1. Two-Pulse Fluorescence and Transient Absorption
Spectra. The two-pulse fluorescence spectra, observed by
excitation with the second pulse at 308 nm after photolysis of
CX, BX, MSX, and IN with the first pulse at 266 nm, are given
in Figure 1. Without the 266-nm pulse, the 308-nm pulse does
not induce the fluorescence. The four spectra are identical
within an experimental error and exhibit a band at 460 nm. The
fluorescence lifetimes, obtained from photolysis of CX, BX,
MSX, and IN, agree well and are of a value of 9.3 ( 0.5 ns.
The fluorescence is assigned to o-xylylene, since it is a common
intermediate species that is produced from photolysis of CX,
X Abstract published in AdVance ACS Abstracts, June 15, 1997.
S1089-5639(97)00655-5 CCC: $14.00 © 1997 American Chemical Society

Photochemical Formation of o-Xylylene
J. Phys. Chem. A, Vol. 101, No. 27, 1997 4913
Figure 1. Two-pulse fluorescence spectra of o-xylylene observed by
excitation with a 308-nm pulse at 1 µs after photolysis of (a) CX, (b)
BX, (c) MSX, and (d) IN with a 266-nm pulse. Upper curves in (a-
d) are generated with both 266- and 308-nm pulses, whereas lower
curves with only a 308-nm pulse. Fluences of 266- and 308-nm
pulses: (a) 5.2 and 88; (b) 5.2 and 76; (c) 96 and 110; (d) 6.4 and 7.6
Figure 2. Transient absorption spectra of o-xylylene and an o-
(chloromethyl)benzyl radical recorded after photolysis of (a, b) CX
and (c) IN with a 266-nm pulse. Fluences of a 266-nm pulse: (a) 94;
(b) 32; (c) 32 mJ cm-2
.
at 330 nm is assigned to the o-(chloromethyl)benzyl radical, in
comparison with the band of the o-methylbenzyl radical in
solution.11 The o-(chloromethyl)benzyl band disappears at a
100-µs delay. A band at 370 nm is assigned to o-xylylene,
since it is similar to the bands observed in low-temperature
matrices.2-5 The band position is in accordance with the ones
obtained in solution.7,9 The band shape, however, differs from
the one reported by Scaiano et al;8 the long wavelength tail
ends at 410 nm in the present spectra, whereas it extends to
500 nm in their spectrum. The o-xylylene band decays a little
at a 20-µs delay. The intensity ratio of the o-xylylene to the
o-(chloromethyl)benzyl band increases when the 266-nm pulse
fluence increases.
mJ cm-2
.
SCHEME 1
The transient absorption spectrum, recorded by photolysis of
IN with the 266-nm pulse, is given in Figure 2c. A band at
370 nm is assigned to o-xylylene, since it is identical with that
in Figure 2a,b. The o-xylylene band does not decay at a 20-µs
delay. Observation of the absorption in the 280-340-nm region
is interrupted by the fluorescence of an impurity within a 0.2-
µs delay. No absorption assignable to the benzyl radical is
obtained in longer delays (<20 µs).
A broad absorption band at 340 nm is observed by photolysis
of BX. It is assigned to the o-(bromomethyl)benzyl radical,
compared with the band of the o-methylbenzyl radical in
solution.11 A broad band at 380 nm is obtained by photolysis
of MSX. It is assignable to the trimethylstannyl radical, in
comparison with the band of the tri-n-butylstannyl radical in
solution.12 The absorption due to o-xylylene is obscured by
that of the o-(bromomethyl)benzyl and trimethylstannyl radicals
for photolysis of BX and MSX, respectively.
BX, MSX, and IN (see Scheme 1). The spectra in Figure 1 are
similar to those observed in low-temperature matrices.2-4,6 The
fluorescence intensities do not decrease in 0.2-20-µs delays
from the 266- to the 308-nm pulse.
The shape of the fluorescence spectra does not change with
variation of the 266-nm pulse fluence (5-100 mJ cm-2) or the
delay (0.2-100 µs) between the 266- and 308-nm pulses. The
fluorescence decay can be fitted by a single-exponential function
(9.3 ( 0.5-ns lifetime) in the 420-520-nm region. These
observations deny any contribution from the fluorescence of
the o-(Z-methyl)benzyl radicals (Z ) Cl, Br, Sn(CH3)3, CdO),
which are generated from the precursors by cleavage of one
C-Z bond with the 266-nm pulse, though the fluorescence of
the o-methylbenzyl radical has been reported to appear at 500
nm with a lifetime of 4.1 ( 1.0 ns in solution.11 The
fluorescence intensity of the o-(Z-methyl)benzyl radicals may
be much weaker than that of o-xylylene.
The transient absorption spectra, observed by photolysis of
CX with the 266-nm pulse, are given in Figure 2a,b. A band
The S2 r S0 and S1 r S0 absorption bands of IN appear in
the 240-280-nm (∼102 dm3 mol-1 cm-1) and 290-330-nm

4914 J. Phys. Chem. A, Vol. 101, No. 27, 1997
Fujiwara et al.
(∼100 dm3 mol-1 cm-1) ranges, respectively. Excitation with
the 308-nm pulse at 1 µs after excitation of IN to the S2(π,π*)
state with the 266-nm pulse produces the o-xylylene fluores-
cence. Excitation of IN to the S1(n,π*) state with the 308-nm
pulse alone does not induce the o-xylylene fluorescence, since
o-xylylene is formed from IN so slowly ((1.4 ( 0.1) × 107
s-1, see Section 3.3) that it cannot be excited to its fluorescent
state by the second photon within the 10-ns duration of the 308-
nm pulse. The o-xylylene absorption is observed at 1 µs after
excitation of IN to the S2(π,π*) state with the 266-nm pulse.
The identical absorption is obtained at 1 µs after excitation of
IN to the S1(n,π*) state with the 308-nm pulse.
3.2. Photolysis Pulse Fluence Dependence and Quantum
Yields. The number of photons required for the formation of
o-xylylene and the o-(chloromethyl)benzyl radical by dissocia-
tion of CX and IN has been determined from the dependence
of their absorbances on the photolysis pulse fluence. For
dissociation of CX, the observed absorbance must be separated
into the o-xylylene and o-(chloromethyl)benzyl components,
because of overlap of the corresponding bands in the 300-
400-nm region. o-Xylylene does not decay within a 20-µs delay,
as shown by the delay dependence of its fluorescence intensity.
The o-(chloromethyl)benzyl radical decreases by a bimolecular
process in the same delay. Thus, the observed absorbance A(λ,t)
at the wavelength λ and the delay t is represented, using the
initial absorbances of o-xylylene AX(λ,0) and the o-(chloro-
methyl)benzyl radical AB(λ,0), by the following equation:
Figure 3. Laser power dependence of absorption intensities of
o-xylylene (circles) and an o-(chloromethyl)benzyl radical (squares)
observed by photolysis of (a) CX and (b) IN at 266 nm. Monitoring
wavelengths: 370 (circles); 330 (squares) nm.
A(λ,t) ) AX(λ,0) + 1/(1/AB(λ,0) + KBt)
(1)
KB is a parameter defined as KB ) kB/ꢀB(λ)l, where kB is the
bimolecular decay rate constant of the o-(chloromethyl)benzyl
radical, ꢀB is its extinction coefficient, and l is the path length
of the white light. For dissociation of CX, the initial absor-
bances of o-xylylene and the o-(chloromethyl)benzyl radical are
obtained by fitting the absorption time profiles at 370 and 330
nm within a 20-µs delay according to eq 1. For dissociation of
IN, only the o-xylylene absorption band is observed, and the
initial absorbance of o-xylylene is read directly from the
absorption time profile at 370 nm.
For dissociation of CX, the logarithmic plots of the initial
absorbances of the o-(chloromethyl)benzyl radical and o-
xylylene versus the photolysis pulse fluence are given in Figure
3a. The formation process of the o-(chloromethyl)benzyl radical
is represented by a straight line with a slope of 0.93 ( 0.10.
Dissociation of one C-Cl bond in CX is shown to occur by
266-nm one-photon excitation. The formation process of
o-xylylene is described by linear fitting with a slope of 2.05 (
0.20. Dissociation of two C-Cl bonds in CX is proved to
require 266-nm two-photon excitation.
For dissociation of IN, the plot of the initial absorbance of
o-xylylene as a function of the photolysis pulse fluence is given
in Figure 3b. Linear fitting results in a slope of 1.04 ( 0.06 to
represent the formation process of o-xylylene. Elimination of
a CdO group in IN is found to proceed by 266-nm one-photon
excitation.
The quantum yields for the formation of the o-(chloromethyl)-
benzyl radical and o-xylylene from dissociation of CX and IN
have been measured by comparing their initial absorbances with
the triplet naphthalene one. The transient absorption signals
(0.02-0.2 absorbance) are obtained by excitation of the sample
solutions (2.0 cm-1 absorbance (266 nm)) of CX, IN, and
naphthalene with the 266-nm pulse (2-40 mJ cm-2). The
extinction coefficient for the o-(chloromethyl)benzyl radical is
assumed to be 1.0 × 104 dm3 mol-1 cm-1 (330 nm) from the
value for the o-methylbenzyl radical.13 The extinction coef-
ficients used for o-xylylene and triplet naphthalene are 3.0 ×
103 dm3 mol-1 cm-1 (370 nm)9 and 1.4 × 104 dm3 mol-1
cm-1 (415 nm),14 respectively. The intersystem crossing yield
for naphthalene is 0.68.14 For dissociation of CX, the o-
(chloromethyl)benzyl yield is determined to be 0.19 ( 0.03.
The apparent o-xylylene yield, which is directly proportional
to the photolysis pulse fluence, is estimated to be 0.30 ( 0.08
at a 40 mJ cm-2 fluence. For dissociation of IN, an estimate
for the o-xylylene yield of 0.90 ( 0.08 is made.
As it is found that the formation of o-xylylene from
dissociation of CX proceeds via a 266-nm two-photon process,
a question arises about an intermediate species that absorbs the
second 266-nm photon. For the intermediate species, three
possibilities can be considered: (1) the S1 state of CX, (2) the
T1 state of CX, and (3) the o-(chloromethyl)benzyl radical. The
fluorescence of CX is not observed, showing that the S1 state
of CX decays rapidly (<1-ns lifetime). If so, the S1 state of
CX does not absorb the second photon within the 5-ns duration
of the photolysis pulse. The apparent o-xylylene yield becomes
higher than the o-(chloromethyl)benzyl yield in fluences of >50
mJ cm-2, as seen from Figure 3a with consideration of the
extinction coefficients for o-xylylene (3.0 × 103 dm3 mol-1
cm-1)9 and the o-(chloromethyl)benzyl radical (1.0 × 104 dm3
mol-1 cm-1).13 This means that o-xylylene is not produced by
excitation of the o-(chloromethyl)benzyl radical. Therefore, the
T1 state of CX is suggested to be the intermediate species. To
check this point, the fraction of the T1 state absorbing the second
photon for CX has been evaluated. The extinction coefficient
of 1.1 × 104 dm3 mol-1 cm-1 (266 nm) for the Tn r T1
absorption of benzene15 is used. An assumption is made that
the S1 f T1 intersystem crossing occurs rapidly. The fraction
of the T1 state excited to the Tn state for CX is calculated to be
0.85 at a 40 mJ cm-2 fluence. The high photolysis pulse fluence
seems to be sufficient to account for the formation of o-xylylene

Photochemical Formation of o-Xylylene
J. Phys. Chem. A, Vol. 101, No. 27, 1997 4915
a 0.02-µs delay is interrupted by the fluorescence of an impurity.
The rise rates of the fluorescence and absorption intensities agree
well and are of a value of (1.4 ( 0.1) × 107 s-1
.
It is known that dibenzyl ketone causes rapid R-cleavage
(>1010 s-1) from the T1(n,π*) state to form the phenylacetyl
radical,16 which subsequently undergoes decarbonylation to the
benzyl radical.17-19 Decarbonylation rates of 106-107 s-1 have
been reported for the phenylacetyl radical.17-19 From this
viewpoint, the present result on the formation of o-xylylene with
the rate of (1.4 ( 0.1) × 107 s-1 may be interpreted in terms of
decarbonylation of the acyl benzyl biradical to o-xylylene.
4. Conclusion
Upon photolysis at 266 nm, CX, BX, MSX, and IN generate
o-xylylene. CX dissociates two Cl atoms, by two-photon
absorption, with an apparent yield of 0.30 ( 0.08 at a fluence
of 40 mJ cm-2. Dissociation of a C-Sn bond occurs in MSX.
IN eliminates a CdO group, by one-photon absorption, with a
rate of (1.4 ( 0.1) × 107 s-1 and a yield of 0.90 ( 0.08.
Acknowledgment. This work was supported by Grants-in-
Aid for Scientific Research (Nos. 08218243, 08245246) from
the Ministry of Education, Science, Sports, and Culture.
Figure 4. Time evolution of (a) fluorescence and (b) absorption
intensities of o-xylylene produced by photolysis of IN at 266 nm.
Fluorescence is induced by excitation at 308 nm, of which a delay
time is an abscissa for (a). Monitoring wavelengths: (a) 470; (b) 370
nm.
References and Notes
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(10) Mizuno, K.; Ishii, Y. To be published.
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(12) Chatgilialoglu, C.; Ingold, K. U.; Lusztyk, J.; Nazran, A. S.;
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(13) Claridge, R. F. C.; Fischer, H. J. Phys. Chem. 1983, 87, 1960.
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istry; Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. 1, Chapter
16.
by dissociation from the Tn state of CX. The analysis suggests
that the second 266-nm photon is absorbed by the T1 state of
CX to dissociate two Cl atoms.
3.3. Formation Kinetics. The fluorescence intensity of
o-xylylene, from dissociation of CX and BX, is observed to
rise within the 5-ns duration of the photolysis pulse. The
fluorescence intensity of o-xylylene, from dissociation of MSX,
is detected to be weak, reflecting a low dissociation yield. Exact
kinetics has not been analyzed for dissociation of CX, BX, or
MSX.
Although photolysis of IN may produce the acyl benzyl
•
biradical •(CO)CH2C6H4CH2 , the fluorescence and absorption
only assignable to o-xylylene are observed in 0.02-100-µs de-
lays. No absorption due to the acyl benzyl biradical is obtained,
because of interruption of the fluorescence of an impurity in
the 280-340-nm region. The rise of the fluorescence and
absorption intensities of o-xylylene, generated by photolysis of
IN with the 266-nm pulse, is given in Figure 4. The fluores-
cence intensity is monitored by variation of the delay of the
308-nm pulse. Measurement of the absorption intensity within
(15) Nakashima, N.; Sumitani, M.; Ohmine, I.; Yoshihara, K. J. Chem.
Phys. 1980, 72, 2226.
(16) Turro, N. J. Modern Molecular Photochemistry; University Science
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