Photocontrolled oscillatory dynamics in the bromate-1,4-cyclohexanedione reaction

Article abstract of DOI:10.1063/1.1809111

We report observations of photocontrolled oscillatory behavior in the 1,4-cyclohexanedione-bromate reaction (CHD - cyclohexanedione). Experiments in a batch reactor show that illumination may exhibit qualitatively different effects on the reaction dynamics, where illumination with a moderate intensity favors oscillations while strong illumination quenches spontaneous oscillations. A transition from light-quenched to light-induced oscillations during the course of the reaction has also been observed. Investigations in a continuous flow stirred tank reactor further illustrate that the influence of light in the 1,4-CHD-bromate reaction depends not only on the intensity of the illumination but also on the phase at which the illumination is switched on. Mechanistic investigations suggest that 1,4-benzoquinone, a final product in the 1,4-CHD-bromate reaction system, plays a significant role in the occurrence of photoinduced oscillations.

Full text of DOI:10.1063/1.1809111

Photocontrolled oscillatory dynamics in the bromate-1,4-cyclohexanedione reaction
Jichang Wang, Krishan Yadav, Bei Zhao, QingYu Gao, and Do Sung Huh
Citation: The Journal of Chemical Physics 121, 10138 (2004); doi: 10.1063/1.1809111
View online: http://dx.doi.org/10.1063/1.1809111
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 121, NUMBER 20
22 NOVEMBER 2004
Photocontrolled oscillatory dynamics in the bromate-1,4-cyclohexanedione
reaction
Jichang Wang,a) Krishan Yadav, Bei Zhao, QingYu Gao, and Do Sung Huh
Department of Chemistry and Biochemistry, University of Windsor, Ontario N9B 3P4, Canada
͑
Received 22 March 2004; accepted 31 August 2004͒
We report observations of photocontrolled oscillatory behavior in the 1,4-cyclohexanedione-
bromate reaction ͑CHD—cyclohexanedione͒. Experiments in a batch reactor show that illumination
may exhibit qualitatively different effects on the reaction dynamics, where illumination with a
moderate intensity favors oscillations while strong illumination quenches spontaneous oscillations.
A transition from light-quenched to light-induced oscillations during the course of the reaction has
also been observed. Investigations in a continuous flow stirred tank reactor further illustrate that the
influence of light in the 1,4-CHD-bromate reaction depends not only on the intensity of the
illumination but also on the phase at which the illumination is switched on. Mechanistic
investigations suggest that 1,4-benzoquinone, a final product in the 1,4-CHD-bromate reaction
system, plays a significant role in the occurrence of photoinduced oscillations. © 2004 American
Institute of Physics. ͓DOI: 10.1063/1.1809111͔
I. INTRODUCTION
reaction dynamics of the uncatalyzed 1,4-CHD-bromate sys-
tem. In addition to transitions between light-enhanced and
light-inhibited oscillatory phenomena, an evolution from
light-quenched to light-induced oscillatory behavior has also
been observed at a constant light intensity. Mechanistic in-
vestigations illustrate that 1,4-benzoquinone, a product in the
above system, plays an important role in the observed photo-
induced oscillations. Illumination-induced production of bro-
mide from BrCHD, on the other hand, is the culprit for caus-
ing quenching phenomena.
Perturbed nonlinear dynamics had attracted increasing
attention in the last two decades.1–8 Among existing studies,
the photosensitive Belousov–Zhabotinsky ͑BZ͒ reaction has
played an important role because photosensitivity provides a
convenient approach to implement a perturbation.9
–21
A
number of intriguing behaviors such as resonance phenom-
ena had been observed when the photosensitive BZ system
was subjected to an external periodic or irregular
9
–21
illumination.
In general, under the same reaction condi-
tions illumination is expected to quench or support the oscil-
latory behavior and such an effect shall be independent of the
intensity of applied light. However, in recent reports Kurin-
Cs o¨ rgei and co-workers observed that chemical waves in the
II. EXPERIMENT
All reactions in a closed system were carried out in a
thermostated cylindrical glass vessel, in which the tempera-
ture was maintained at ͑25Ϯ0.1͒ °C by a circulating water
bath ͑Thermo NesLab RTE 10͒. No temperature change has
been observed when the reaction mixture was illuminated
with light. Reactions were followed by a platinum electrode
coupled with a double junction reference electrode which
1,4-cyclohexanedione-bromate-ferroin system exhibited op-
posite responses when the applied light intensity was ad-
justed ͑CHD—cyclohexanedione͒.2
2,23
A similar behavior
has also been reported recently in the stirred 1,4-CHD-
bromate-ferroin medium, in which light-induced and light-
inhibited oscillations were observed under the same reaction
used saturated KCl as the inner solution and 1M KNO as
3
2
4
conditions. The subtle response of its reaction dynamics to
illumination, together with the absence of production of
gases, makes the 1,4-CHD-bromate reaction an attractive
model system to explore novel spatial and temporal
the outer solution. The outer electrolyte was replaced every
Ϫ
1
2 h to avoid Cl contamination from the inner electrolyte.
The redox potential was recorded with a multichannel Can-
lab recorder through a Corning Pinnacle pH meter. A bro-
mide ion-selective electrode ͑Orion, 9435BN͒ coupled with
the double junction reference electrode was also employed to
follow the response of the reaction to illumination, which
produced the same behavior as recorded by the platinum
electrode. However, because the signal recorded by the bro-
mide electrode ͑Ͻ30 mv͒ was weaker than the Pt potential
2
2–30
behaviors.
Despite the 1,4-CHD-bromate-ferroin reaction has re-
ceived a lot of attention recently and a number of interesting
photocontrolled behaviors have been observed there, its un-
derlying photoreaction mechanism remains to be understood.
Earlier investigations had demonstrated that ferroin could be
excited by light to induce temporal oscillations or pattern
formation in the ferroin-bromate-malonic acid reaction.21,31
To discriminate whether ferroin plays a similar role in the
͑Ͼ100 mv͒, time series presented here were the redox poten-
tials recorded by the platinum electrode couple. A fiber optic
halogen lamp ͑Fisher Scientific, Model DLS-100HD, 150 W͒
with continuous variable light level was used as the light
1,4-CHD-bromate system, this paper investigated the photo-
source and the light intensity was adjusted between 0% and
a͒FAX: ϩ1 519 9737098; Electronic mail: jwang@uwindsor.ca
100% of the maximum light power, I ͑100 mW/cm ͒. The
2
0
0
021-9606/2004/121(20)/10138/7/$22.00
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10138
© 2004 American Institute of Physics
1

J. Chem. Phys., Vol. 121, No. 20, 22 November 2004
Photocontrolled oscillatory reaction dynamics
10139
absolute light intensity was measured with an optical pho-
tometer from Newport. The optical fiber was positioned at
about 2 cm above the solution surface.
Experiments in a continuous flow stirred tank reactor
͑CSTR͒ were followed with a platinum electrode ͑from Lon-
don Scientific͒ coupled with a Hg͉Hg SO4 K SO reference
͉
2
2
4
electrode. The Pt potential signal was recorded through a
REC 80 Servograph recorder. The reaction temperature was
maintained at ͑25Ϯ0.1͒ °C by a circulating water bath
͑Thermo NesLab RTE 7͒. Here, light perturbation was ap-
plied by using two bifurcated optic fibers to illuminate the
glass chamber from opposite directions. Stock solutions
NaBrO3 ͑Aldrich, 99%͒, 0.45M, and 1,4-CHD ͑Aldrich,
98%͒, 0.144M, were dissolved, respectively, in 1.5M sulfu-
ric acid solution. Hydroquinone ͑Aldrich, 99%͒, 0.06M was
directly dissolved in doubly distilled water. All three stock
solutions were pumped into the CSTR by a peristaltic pump
at equal flow rates. All chemicals were used in their commer-
cial grade without further purification.
III. RESULTS IN A BATCH REACTOR
The 1,4-CHD-bromate reaction was first investigated
over a broad range of conditions in order to identify a set of
suitable conditions for characterizing the influence of light. A
desired condition to characterize the inhibitory effect of light
is that under which spontaneous oscillations last for a long
time period so that multiple perturbations could be examined
in one run. Our experiments illustrated that increasing the
initial concentration of bromate prolonged the oscillatory
window; however, the induction time of spontaneous oscil-
lations was also significantly prolonged. Our further explo-
ration showed that the addition of hydroquinone (H Q)
2
FIG. 1. Time series showing the light-inhibited oscillatory behavior at dif-
could greatly shorten the induction period. More signifi-
cantly, as shown in the following, the initial concentration of
hydroquinone could also be employed as an effective dy-
namic control parameter ͑see Figs. 1 and 2͒ to alter the os-
cillatory behavior. Therefore, hydroquinone was employed
throughout this study as a means of manipulating the reac-
tion dynamics and reducing the induction period of the 1,4-
CHD-bromate oscillations.
ferent initial concentrations of H Q: ͑a͒ 0.02M and ͑b͒ 0.03M. Initial com-
2
Ϫ
positions of other reagents are ͓BrO ͔ϭ0.15M, ͓H SO ͔ϭ1.0M, and
3
2
4
͓1,4-CHD͔ϭ0.048M.
managed to raise the potential significantly in both attempts,
but only resulted in the decrease in the amplitudes of these
oscillations. There was a noticeable difference in the rise in
2
Figure 1 presents two examples of quenching phenom-
ena. Here spontaneous oscillations occur at about 1 h after
the beginning of the reaction, i.e., after mixing all reactants
together. Initial compositions of the reaction solution
were ͓H SO ͔ϭ1.0M, ͓NaBrO ͔ϭ0.15M, ͓1,4-CHD͔
potential when 50% I ͑50 mW/cm ͒ was used on both oc-
0
casions. This difference may be attributed to the stage of the
2
reaction. The second application of 50% I0 ͑50 mW/cm ͒
2
4
3
ϭ0.048M, and ͓H Q͔ϭ͑a͒ 0.02M and ͑b͒ 0.03M. As shown
2
in Fig. 1͑a͒, during the earlier oscillatory stage, 25% I ͑25
0
2
mW/cm ͒ successfully quenched the oscillation and caused
the Pt potential to return to the same level as before the
occurrence of spontaneous oscillations. After the light was
turned off to allow the spontaneous oscillations to resume,
2
another application of 25% I ͑25 mW/cm ͒ could not suc-
0
cessfully quench the system as it did in the earlier trail. In-
stead, the overall potential was shifted upwards while the
oscillation amplitude did not appear to be affected. However,
2
the application of 50% I ͑50 mW/cm ͒ resulted in quench-
0
ing of the system in two separate attempts. In Fig. 1͑b͒, 25%
FIG. 2. Time series showing light-induced oscillations. Initial compositions
2
I ͑25 mW/cm ͒ only managed to shift the potential upwards
Ϫ
0
of reaction solution are ͓BrO ͔ϭ0.15M, ͓H SO ͔ϭ1.0M, ͓H Q͔
3
2
4
2
2
and reduced the amplitude slightly. 50% I0 ͑50 mW/cm ͒
ϭ0.04M, and ͓1,4-CHD͔ϭ0.048M.
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10140
J. Chem. Phys., Vol. 121, No. 20, 22 November 2004
Wang et al.
TABLE II. Photocontrolled reaction behavior under different concentrations
of hydroquinone (H Q). Other initial compositions are ͓1,4-CHD͔
2
Ϫ
3
ϭ0.048M, ͓H SO ͔ϭ1.0M, and ͓BrO ͔ϭ0.15M.
2
4
Spontaneous/
No oscillation
͓
H Q͔(M)
Light induced/inhibited
2
0
No oscillations
No oscillations
Spontaneous
No activity
No activity
Inhibited; quenching at above 25% I0
early on, 50% at later stage.
Inhibited, quenching at above
62.5% I0
0
0
.01
.02
FIG. 3. Time series showing a transition from light-quenched to light-
induced oscillations. The initial compositions of the reaction mixture are
0.03
Spontaneous
Ϫ
3
͓
BrO ͔ϭ0.15M, ͓H SO ͔ϭ1.0M, ͓H Q͔ϭ0.03M, and ͓1,4-CHD͔
0.04
0.05
No oscillations
No oscillations
No oscillations
No oscillations
Induced; oscillations at above 30% I0
Induced; oscillations at above 50% I0
Induced; oscillations at above 63% I0
Induced; oscillations at above 75% I0
2
4
2
ϭ0.04M.
0
0
.06
.07
was during the latter stage of the oscillatory part of the re-
action, and the system seemed to have had more difficulty in
raising the potential. Successful quenching was achieved af-
oscillations’’ were bound to occur for some time after the
light supply was cut off.
2
ter the light intensity was raised to 63% I ͑63 mW/cm ͒.
0
The above results illustrate that an increase in the initial con-
centration of hydroquinone reduces the negative influence of
light on spontaneous oscillations. Such an effect could well
be resulted from the fact that hydroquinone, a photosensitive
species, absorbs increasing amounts of light to shield the
influence of light on other photosensitive species.
Figure 2 presents the time series of light-induced oscil-
latory behavior in the uncatalyzed 1,4-CHD-bromate reac-
tion. As shown in Fig. 2͑a͒, the system did not oscillate in the
absence of illumination. This nonoscillatory behavior was
obtained by increasing the initial concentration of hydro-
quinone to 0.04M. Together with the results shown in Fig. 1,
it illustrated that the concentration of hydroquinone was an
effective bifurcation control parameter. Although no sponta-
neous oscillations occurred here, there was still a sharp drop
in the Pt potential after a short time period ͑Ͻ30 min͒ and
light-induced oscillations could be achieved after the abrupt
drop in the Pt potential took place. Later, the redox potential
returned to its initial level ͑i.e., before the potential drop͒ and
Time series shown in Fig. 3 illustrates a transition from
light-inhibited ͑at the earlier stage of the reaction͒ to light-
induced oscillatory behavior ͑at the later stage of the batch
reaction͒. Here the initial concentration of 1,4-CHD is
0.04M, slightly lower than that in Fig. 1͑b͒, while other re-
action conditions are kept constant. The use of light ͑25 and
2
50 mW/cm ͒ resulted in an upward shift in the potential and
the frequency of oscillations was also increased dramatically
2
upon the application of light. Use of 63% I ͑63 mW/cm ͒
0
was required to quench oscillations. Surprisingly, application
2
of 50% I ͑50 mW/cm ͒ after spontaneous oscillations dis-
0
appeared resulted in a ‘‘revival’’ of oscillations. Such a quali-
tative change in responding to illumination provides a set of
unique information for deciphering the underlying photore-
action mechanisms. We would like to note that the above
revival phenomena could also be achieved under other initial
conditions. However, the time period over which revival os-
cillations can be achieved is quite narrow ͑Ͻ5 min͒.
To gain more insight into the appearance of the above
observed light-induced and light-quenched oscillations, ex-
periments were carried over a broad range of 1,4-CHD and
hydroquinone concentrations. The results were summarized
in Tables I and II, where initial concentrations of sulfuric
acid and bromate were held constant at 1.0M and 0.15M,
respectively. As listed in Table I, an increase in the initial
concentration of 1,4-CHD favored the occurrence of sponta-
neous oscillations and the system also required stronger illu-
mination for a successful quenching. When hydroquinone
no light-induced oscillations could be obtained afterwards.
2
As shown in Fig. 2͑b͒, 50% I ͑50 mW/cm ͒ was able to
0
induce oscillations most effectively. What is interesting to
note is that when the light was turned off after the first ap-
2
plication of 50% I ͑50 mW/cm ͒, smaller amplitude ͑and
0
lower frequency͒ oscillations continued at a lower potential
for several minutes. This implies that the species created
through use of light would eventually be used up in the re-
action some time after the light was stopped; thus ‘‘residual
TABLE I. Photocontrolled reaction behavior at different initial concentrations of 1,4-CHD. Other initial con-
ditions are ͓H Q͔ϭ0.03M, ͓H SO ͔ϭ1.0M, and ͓BrO ͔ϭ0.15M.
2
2
4
3
͓
1,4-CHD͔(M)
Spontaneous/No oscillation
Light induced/inhibited
No activity at all
0.01
0.02
0.03
0.04
0.05
0.06
0.07
No oscillations
No oscillations
No oscillations
Spontaneous
Spontaneous
Spontaneous
Spontaneous
No activity at all
Induced; oscillations at above 25% I0
Inhibited; quenching at 63% I0
Inhibited; quenching at 50% I0
Inhibited; quenching at 63% I0
Inhibited; quenching at 75% I0
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1

J. Chem. Phys., Vol. 121, No. 20, 22 November 2004
Photocontrolled oscillatory reaction dynamics
10141
TABLE III. Photocontrolled reaction behavior at different initial concentra-
tions of H SO . Other initial conditions are ͓H Q͔ϭ0.03M, ͓1,4-CHD͔
2
4
2
Ϫ
3
ϭ0.06M, and ͓BrO ͔ϭ0.15M.
͓
H SO ͔
Spontaneous/No scillations
Light induced/inhibited
2
4
0
.4M
No oscillations
No oscillations
No oscillations
Spontaneous
Spontaneous
Spontaneous
Spontaneous
Spontaneous
No oscillations
N/A
0.5M
0.6M
0.7M
0.8M
0.9M
1.0M
1.1M
1.2M
Induced at 25% I0
Induced at 25% I0
Quenching at 50% I0
Quenching at 50% I0
Quenching at 50% I0
Quenching at 50% I0
Quenching at 50% I0
No response to light
FIG. 4. Time series showing the influence of light on the 1,4-CHD-bromate
oscillation in a CSTR. Concentrations of the stock solutions are provided in
the experimental section. The volume of the reactor is 40 ml and the mean
resident time of the reaction solution is 90 min. The light intensity I
ϭ100 mW/cm .
0
2
was adjusted as a variable, its overall effects on the oscilla-
tory dynamics were not monotonic, where spontaneous os-
cillations were obtained only within a narrow parameter win-
dow. Also shown in Table II was that light of higher intensity
was required to initiate oscillations when the concentration
of hydroquinone was increased.
the bifurcation control parameter in a CSTR. The result that
spontaneous oscillations in Fig. 1 do not appear until 60 min
after mixing all reactants together suggests that the mean
resident time of the reaction solution in the CSTR should be
larger than 60 min in order to obtain oscillations. In addition,
the short-lived spontaneous oscillations in a batch reactor
indicate that spontaneous oscillations could only be obtained
within a narrow range of flow rate in the CSTR when the
same reaction conditions are employed. Our following ex-
perimental results are consistent with the above analysis.
Figure 4 presents an example of quenching behavior in
the CSTR. Here, the mean resident time of the reaction so-
How the photocontrolled reaction dynamics response to
the variation of the concentration of sulfuric acid has also
been examined here. Experiments illustrated that spontane-
ous oscillations could be achieved when ͓H SO ͔ was be-
2
4
tween 0.7M and 1.2M while initial concentrations of other
reactants were ͓1,4-CHD͔ϭ0.060M, ͓H Q͔ϭ0.030M, and
2
͓
BrO ͔ϭ0.15M. When the concentration of sulfuric acid
3
was below 0.7M, no spontaneous oscillations could be
achieved and the reaction exhibited qualitatively the same
type of Pt potential curve as that shown in Fig. 2͑a͒. Under
these nonoscillatory reaction conditions, if the intensity of
3
lution in the reactor (Vϭ40 cm ) is 90 min, which is larger
than the induction time period observed in the batch system.
Spontaneous one peak per period oscillations were observed
under the above conditions. When a constant illumination of
2
the applied illumination was above 25 mW/cm but below 50
2
mW/cm , light-induced oscillations could be obtained. Suc-
2
the intensity of 25 mW/cm was applied, the oscillation
cessful quenching behavior was observed when the light in-
2
merely shifted upward while the frequency of oscillation be-
comes a bit smaller. No quenching was obtained under the
tensity was above 50 mW/cm . However, same as observed
in Fig. 1͑b͒, illuminations of lower intensity only resulted in
an increase in the frequency of oscillations, exhibiting con-
structive effects on the oscillatory dynamics. In the case that
2
perturbation of 25 mW/cm . Successful quenching was ob-
served after the intensity of the illumination was increased to
2
5
0 mW/cm . Note that the system was driven toward bottom
͓
H SO ͔Ͼ1.2M, the system became nonoscillatory again;
2 4
2
by the perturbation of 50 mW/cm . The illumination was
kept on until the Pt potential became flat. Surprisingly, the
system remained at the low Pt potential state even after the
illumination was turned off. Indeed, the system could return
to the oscillatory state only after the flow rate was briefly
changed from 0.44 to 0.3 ml/min for about 1.0 s. This sce-
nario indicates that the 1,4-CHD-bromate system actually
has two coexisting steady states, i.e., a limit cycle and a
stable steady state. The observed quenching behavior is be-
cause illumination pushed the system from the limit cycle to
the steady state.
but, the system only stayed at the low potential state of the
U-shaped curve for a very short time period and no light-
induced oscillations were obtained there. Table III provided a
summary of the effects of the initial concentration of sulfuric
acid on the photoreaction dynamics. Thresholds for light-
induced and light-quenched oscillations seemed to remain
constant within the concentration range investigated here.
Such a result implied that hydrogen ions were not the limit-
ing reagent here or were not involved in the photoreaction
processes.
Figure 5 presents quenching phenomena achieved when
the same illumination ͑50 mW/cm ͒ was applied at different
IV. RESULTS IN A CSTR
2
The influence of light on the 1,4-CHD-bromate reaction
is also characterized in a CSTR, in which fresh reactants are
continuously pumped into the reaction chamber to keep the
reaction at a stable yet far from thermodynamic equilibrium
state. As opposed to that the slowly but continuously de-
creasing concentration of reagents is the bifurcation param-
eter in a batch system, flow rate, which can be kept steady, is
phases of oscillation. Significantly, qualitatively different be-
haviors were observed here: When the illumination was
switched on at around the top of the oscillation, the pertur-
bation quenched these oscillations and drove the system up-
ward. Notably, as soon as the illumination was turned off,
spontaneous oscillations were restored. In contrast, if the
light was switched on at around the bottom part of the oscil-
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10142
J. Chem. Phys., Vol. 121, No. 20, 22 November 2004
Wang et al.
FIG. 5. Time series showing the dependence of the influence of light on the
phase of oscillation. Reaction conditions here are the same as that in Fig. 4.
2
The intensity of the applied light, which is 50 mW/cm , is kept constant
through out this experiment.
FIG. 6. Time series showing the influence of illumination on the production
of Br from BrCHD. Compositions of the reaction solution consist of
Ϫ
0
.5M H SO and 0.03M BrCHD, prepared by mixing 0.01M BrO3 ,
2 4
Ϫ
lation, spontaneous oscillations were also quenched; how-
ever, the system would then be driven downward and, more
significantly, the system continuously evolved toward the
lower potential state even after the illumination was turned
off. The oscillatory behavior could be restored only after a
perturbation was applied to the flow rate. The above phase
dependence of the influence of light has not been observed in
a batch reactor, which is presumably because no bistability
has been achieved there. So far, no light-induced oscillations
were obtained in the CSTR even though the same reaction
conditions as used in the batch reactor had been employed.
0.02M Br , and 0.03M 1,4-CHD in a sealed beaker. The light was kept on
between 5 and 10, 15 and 20, and 25 and 30 min. The intensity of the
applied light is 90 mW/cm .
2
with the logarithm of the concentration of bromide͒. We also
used light to illuminate a diluted bromine solution ͑ca.
0
position of bromine played any significant role here. Our
bromide selective electrode did not record any change, im-
plying that such a factor is not as important as light-induced
decay of BrCHD.
.001M) in order to examine whether light-induced decom-
V. MECHANISTIC INVESTIGATIONS
To shed light on the mechanism of photoinduced oscil-
lations, in Fig. 7 we investigated the reaction between acidic
bromate and hydroquinone with or without light illumina-
tion. The solid line is the result obtained in the absence of
light, whereas the dashed curve ͑plotted by hand after over-
lapping the two results͒ represents the reaction taken place
Same as in the BZ reaction,32–34 oscillations studied here
could also be quenched by adding small amounts of bromide.
It prompts us to wonder whether quenching phenomena in
the bromate-1,4-CHD reaction is also resulted from light-
Ϫ
induced additional production of Br . To examine the above
2
under a constant illumination ͑90 mW/cm ͒. According to the
hypothesis, we first investigated the influence of illumination
on the decomposition of BrCHD. According to the mecha-
nism proposed by K o¨ r o¨ s and co-workers, BrCHD is the
major source of the reproduction of Br in the 1,4-CHD-
2
9
mechanism proposed by K o¨ r o¨ s and co-workers, following
2
9
reactions are expected to take place:
Ϫ
Ϫ
3
H QϩBrO →QϩHBrO ,
͑1͒
͑2͒
͑3͒
͑4͒
͑5͒
2
2
Ϫ
3
bromate oscillator. To prepare the BrCHD solution, BrO ,
Ϫ
ϩ
•
2
Ϫ
BrO ϩHBrO ϩH →2BrO ϩH O,
Br , and 1,4-CHD acidic solutions are mixed in a sealed
3
2
2
beaker according to the 1:2:3 stoichiometric relationship.
The bromination process took about 35 min to complete,
indicated by the disappearance of the orange color ͑i.e., bro-
mine͒. Once the reaction mixture becomes transparent, a bro-
mide ion-selective electrode coupled with a double junction
reference electrode is inserted into the beaker to follow the
•
2
•
H QϩBrO →HQ ϩHBrO ,
2
2
•
•
2
HQ ϩBrO →QϩHBrO ,
2
•
2
HQ →H QϩQ.
2
Ϫ
variation of Br concentration. Continuous decrease in the
bromide potential is recorded, which is due to the production
of bromide ions. In Fig. 6, the directly measured potentials
were converted to the concentration of bromide according to
a standard curve obtained earlier. In this experiment, illumi-
nation was switched on and off periodically ͑every 5 min͒.
As shown in the figure, the variation in the concentration of
bromide was indeed slightly accelerated under illumination,
although this effect became less pronounced in time. We
have also tried to make BrCHD directly from 1,4-CHD and
bromine; however, because of the presence of larger amounts
Ϫ
FIG. 7. Times series showing the influence of illumination on the reaction
between bromate and hydroquinone. Initial compositions of the reaction
of Br , a byproduct in the above reaction, the potential mea-
surement became less sensitive ͑i.e., our calibration illus-
trates that the potential reading has a linear relationship
Ϫ
mixture are ͓H SO ͔ϭ0.1M, ͓H Q͔ϭ0.01M, and ͓BrO ͔ϭ0.06M. The
2
4
2
3
2
intensity of the applied light is 90 mW/cm .
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1

J. Chem. Phys., Vol. 121, No. 20, 22 November 2004
Photocontrolled oscillatory reaction dynamics
10143
sponse of the 1,4-benzoquinone-bromate reaction to illumi-
nation is the same as that observed in the 1,4-CHD-bromate
system, suggesting that 1,4-benzoquinone could be respon-
sible for light-induced oscillations.
VI. CONCLUSIONS
When the 1,4-CHD-bromate reaction is exposed to light,
a variety of interesting dynamical behaviors have been ob-
served, which include both light-induced and light-inhibited
oscillations and transitions from light-inhibited to light-
induced oscillatory dynamics. Significantly, under the same
reaction conditions, light could either enhance or quench the
oscillating dynamics, depending on its intensity. Since the
ferroin-bromate-1,4-CHD reaction exhibits similar effects of
FIG. 8. Times series showing the influence of illumination on the reaction
between bromate and 1,4-benzoquinone. Initial compositions of the reaction
light,2
3,24
this study suggests that ferroin does not play a
Ϫ
mixture are ͓H SO ͔ϭ0.5M, ͓Q͔ϭ0.01M, and ͓BrO ͔ϭ0.06M. Illumina-
significant role in the photosensitivity of the ferroin-bromate-
,4-CHD reaction. Our preliminary mechanistic investiga-
2
4
3
2
tion of the intensity of 90 mW/cm was turned on at about 600 s and then off
at 750 s. The same illumination was switched on again at about 900 s.
1
tions further suggest that light-enhanced production of bro-
mide from BrCHD is likely to be responsible for the
quenching phenomena. Meanwhile, although hydroquinone
is a well-known photosensitive reagent, it does not play an
active role in the observed photoinduced oscillations. In ac-
cord with a recent report by G o¨ rner, which suggests that
Consistent with the above mechanism, time series shown
in Fig. 7 illustrates that the reaction between hydroquinone
and bromate is autocatalytic. Remarkably, there is no change
in the induction time of the autocatalytic reaction when the
system is exposed to light. Such a result suggests the follow-
ing: ͑1͒ Unlike in the BZ reaction, here there is no light-
1
,4-benzoquinone could be excited by light to produce
35
hydroquinone, result shown in Fig. 8 illustrates that 1,4-
benzoquinone plays a significant role in the onset of photo-
induced oscillations in the 1,4-CHD-bromate reaction.
induced production of HBrO as otherwise the induction pe-
2
riod should be shortened by light; ͑2͒ hydroquinone, despite
being a photosensitive reagent, is not involved in the above
light-induced oscillations. Note that the concentration of sul-
furic acid is decreased to 0.1M here in order to have a longer
ACKNOWLEDGMENT
This work was supported by the Natural Science and
Engineering Research Council, Canada ͑NSERC͒.
induction period. If ͓H SO ͔ϭ1.0M, the autocatalytic reac-
2
4
tion would take place within a few seconds after mixing
bromate and hydroquinone solutions together.
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1