High resolution electronic absorption spectra of anisole and phenoxyl radical

Article abstract of DOI:10.1016/S1386-1425(02)00143-9

High resolution absorption spectra of anisole and phenoxyl radical has been recorded in the vapor phase at room temperature by flash photolysis technique, and subsequent reactions have been investigated by kinetic spectroscopy. It was possible to follow t

Full text of DOI:10.1016/S1386-1425(02)00143-9

Spectrochimica Acta Part A 59 (2003) 135Á138
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www.elsevier.com/locate/saa
High resolution electronic absorption spectra of anisole and
phenoxyl radical
1
Fuat Bayrakceken *, Selin Aktas , Melek Toptan, Asli Unlugedik
¨
¸
¸
¨
Department of Electronics Engineering, Yeditepe University, Kay sdag˘ , 81120 Istanbul, Turkey
ı¸ ı
Received 19 February 2002; received in revised form 22 March 2002; accepted 9 April 2002
Abstract
High resolution absorption spectra of anisole and phenoxyl radical has been recorded in the vapor phase at room
temperature by flash photolysis technique, and subsequent reactions have been investigated by kinetic spectroscopy. It
was possible to follow the kinetics of the radical’s decay which occurred predominantly by bimolecular recombination.
The concentration of the phenoxyl radical calculated from the concentration of diphenoxyl molecules formed in the
reaction cell during the optical pumping. The absolute extinction coefficient of this radical was measured from the
absorption band at 291 nm, and found to be 0.95ꢀ
/
10ꢁ4 molꢂ1 l cmꢂl
.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Phenoxyl radical; Flash photolysis; UV-absorption; Extinction coefficient; Biomolecular decay
1. Introduction
matic molecule results in the formation of an
unstable excited state. This excited state becomes
deactivated by either a photophysical process or a
photochemical process, dissociation into radicals
or ionization. In most cases, although practically
most of the energy is lost by a photophysical
process, a definite action of exciting energy results
in neutral radical or ion-radical formation. The
photolytic method is often a good way of produ-
cing specific aromatic radicals as one can irradiate
with light of energy capable of breaking only the
required bond.
Radicals can be prepared by either oxidation or
reduction of aromatic compounds. Such transfor-
mations may be carried out by a variety of
methods photolysis, radiolysis, chemical oxidation
or reduction, pyrolysis, electrolysis, and electrical
discharge. The first three methods are the most
commonly used. Absorption of light by an aro-
* Corresponding author. Tel.: ꢁ
/
90-216-578-0000; fax: ꢁ90-
/
216-578-0400
E-mail address: bayrakf@yeditepe.edu.tr (F. Bayrakceken).
Phenolic compounds show antioxidant activity
mainly by reacting with the free radicals, that are
known to demage biomolecules. However, their
¸
Present address: Medical school, Istanbul University,
1
Istanbul, Turkey.
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 1 4 3 - 9

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Fig. 1. (a) Transient absorption spectrum of phenoxyl radical, anisole (1 Torr) and nitrogen (500 Torr) in the vapor phase, laꢃ
nm; (b) absorption spectrum of anisole (1 Torr) and nitrogen (500 Torr), in the vapor phase; (c) absorption spectrum of empty cell; (d)
wavelength scale (220Á300 nm).
/291
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various competing reactions and the mechanism of
action as antioxidants are still a matter of debate.
In recent time, several natural phenols from plants
like curcumin from turmeric and other plant
derived phenols such as dehydrozingerone, euge-
nol and isoeugenol are found to show antioxidant
behaviour. It possesses many structural similarities
with curcumin and shows a wide range of phar-
macological activities. Organic free radicals are
generally short-lived, high reactive species, and the
means of experimentally determining their struc-
ture and properties are, therefore, fairly limited.
The radicals are decaying rapidly by dimerisa-
tion, disproportionation or reaction with solvent.
In such cases, special steps have to be taken in
2. Experimental
Flash photolysis apparatus: This employed two
air filled photolysis lamps in series, with a flash
duration of 2 ms, at a discharge energy of 845 J.
Spectra were recorded on a Hilger medium quartz
spectrograph, slitwidth 0.025 mm. The Ilford, HP-
3 plates used were developed in Ilford PQ uni-
versal developer. Plate photometry: the spectra
were photometered on a JoyceÁ
beam recording microdensitometer
/
Loeble double
model
MK1IB. Calibrated optical densities on the photo-
graphic plate were obtained with a seven step filter
(Hilger, F-1273). Pressure measurements: low
pressures of parent molecules (0.5 mTorrÁ3
/
order to observe their absorptions [1Á8].
/
The first satisfactory method of observing the
spectra of aromatic radicals was by the flash
photolysis technique of Porter [9], by this method
momentarily high concentrations of radicals are
produced photochemically by a short-duration
high energy light flash. The transient radical
absorption in then monitored in one of two
ways: either by photographing over a wide wave-
length range with a second lower-intensity light
flash, triggered a few microseconds after the main
flash, or at a single wavelength using a continuous
monitoring light source and photoelectric detec-
tion.The former is the most useful for rapidly
measuring complete absorption spectra [9,10].
Fig. 2. (a) Background substracted absorption spectrum of the
phenoxyl radical in the vapor phase; (b) microdensitometer
trace of the phenoxyl radical in the vapor phase; (c) micro-
densitometer background trace.

F. Bayrakceken et al. / Spectrochimica Acta Part A 59 (2003) 135Á
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Torr) were flashed after mixing with excess of
nitrogen (at pressures up to 700 Torr), to maintain
isothermal conditions. Pressures were measured
with a McLoad gauge (up to 0.1 Torr), and above
50 Torr, with a mercury manometer. End product
analysis: radical concentration was measured from
the concentration by diphenoxyl molecule formed
in the reaction cell during the photolysis flash.
Face to face collision of two phenoxyl radicals
produce diphenoxyl molecule which is in the
steady state. Diphenoxyl molecule dissolved in
ethyl alcohol, by using this solution, the concen-
tration of diphenoxyl molecule measured by using
Varian 3700/111 chromatography. Therefore, the
concentration of the phenoxyl radical will be the
twice of the concentration of the diphenoxyl
molecule. The lifetime of the phenoxyl free radical
was measured from the decay of the radical at 291
nm, which is the maximum location of the
absorption band of the radical.
Fig. 3. Flash photolysis of anisole (1 and 500 Torr) showing the
absorption spectrum of phenoxyl free radical and its decay,
3. Results and discussion
(flash energyꢃ1125 J).
/
Phenoxyl radical spectrum: radical was observed
on flash photolysis of anisole and phenol. With
phenol the radical absorption was just detected,
due to the low vapor pressure at room temperature
(i.e. phenol is in the solid form) but with anisole
strong phenoxyl absorption spectra were obtioned.
The free radical spectrum from the phenol is
identical but the absorption bands of the radical
spectra obtained by using phenol as a parent
molecule do not appear clearly in photographic
reproductions due to the low concentrations of the
radicals formed during the vapor phase photolysis
of phenol. Fig. 1, shows the high resolution
absorption spectrum of the phenoxyl and anisole
in the vapor phase at room temperature. Kinetics:
a typical example of the decay of phenoxyl radical
from anisole is shown in Figs. 3 and 4. The
qualitative indications were that the bimolecular
recombination of the phenoxyl radicals was an
important reaction in the decay of the radical in
experiments with both parent molecules. The life-
time of the radical is about 200 ms calculated from
the decay parameters of Figs. 3 and 4. The primary
photochemical processes (side chain fission) are as
Fig. 4. (a) Decay of the phenoxyl radical at 291 nm; (b) second
order plot for the decay of the phenoxyl radical at 291 nm.

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138
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follows:
LambertÁ
/
Beer Law we also calculated the extinc-
tion coefficient at 291 nm.
The rate constant for the disappearance of
C6H5OHꢁhg (UV) 0 C6H5O+ ꢁH+
phenoxyl free radical was: kꢃ
/
3.65ꢀ
104
/
C7H8Oꢁhg (UV) 0 C6H5O+ ꢁCH3
absorbanceꢀ
/
sꢂ1 for anisole.
+
A more detailed investigation of these and
similar systems would be facilitated by an appa-
ratus with which the ambient temperature could be
varied, and with a more sensitive analytical
technique. This would make possible the extention
of the method to analogous compounds with lower
pressure, phenol in particular, and probably result
in a more complete kinetic analysis.
Fig. 2(a), shows the background substracted
absorption spectrum of the phenoxyl radical in the
vapor phase, (b) microdensitometer trace of the
phenoxyl radical, and (c) microdensitometer back-
ground trace. Fig. 3, shows the typical absorption
spectrum and decay of the phenoxyl radical in the
vapor phase. Fig. 4(a) shows the decay of the
phenoxyl radical at 291 nm, and (b) shows the
second order plot for the decay of the phenoxyl
radical at 291 nm (peak location). The decay of the
phenoxyl radical in phenol system was rather
faster than in anisole and second order plots
were somewhat more erratic suggesting a possible
contribution from the recombination of phenoxyl
radicals with CH3+, From Fig. 1, the ionization
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potential of anisole is found to be 5.4 eV (lꢃ229
/
nm).The continuous absorption starts at 229 nm
for anisole.
/
/
C6H5O+ ꢁC6H5O+ 0 C6H5O
: OH5C6 (Diphenoxyl)
/5959.
/
/
Face-to-face collision of two radicals, produce
diphenoxyl molecule in the steady-state, therefore,
we can calculate the concentration of phenoxyl
radical by using the concentration of diphenoxyl
molecules formed in the reaction cell. By using
/300.
¸
[10] F. Bayrakceken, METU-J Pure Appl. Sci. 9 (1) (1976) 51Á
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61.