Study of electron transfer reactions associated with benzyl viologen-β-cyclodextrin complexation in buffer solution of pH 7: Equilibrium and kinetic aspects

Article abstract of DOI:10.14233/ajchem.2020.22736

In this study, complex formation of benzyl viologen dication (BzV²⁺) with β-cyclodextrin (β-CD), in its different oxidation states, had been studied in buffer solution of pH 7, through cyclic voltammetry. In buffer solution of pH 7, extensive d

Full text of DOI:10.14233/ajchem.2020.22736

Asian Journal of Chemistry; Vol. 32, No. 8 (2020), 1981-1985
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2020.22736
Study of Electron Transfer Reactions Associated with Benzyl Viologen-β-Cyclodextrin
Complexation in Buffer Solution of pH 7: Equilibrium and Kinetic Aspects
1,*,
2
2
SHAZIA PERVEEN
, IFTIKHAR IMAM NAQVI and SUMMYIA MASOOD
1
2
Department of Chemistry, NED University of Engineering and Technology, Karachi-75270, Pakistan
Department of Chemistry, University of Karachi, Karachi-75270, Pakistan
*
Corresponding author: E-mail: shaziaperveen@neduet.edu.pk
Received: 23 March 2020; Accepted: 11 May 2020;
Published online: 27 July 2020;
AJC-19977
2+
In this study, complex formation of benzyl viologen dication (BzV ) with β-cyclodextrin (β-CD), in its different oxidation states, had
been studied in buffer solution of pH 7, through cyclic voltammetry. In buffer solution of pH 7, extensive deposition of benzyl viologen
2+
neutral (BzVº) has been found. Further to that first redox process of BzV was found reversible in buffer solution of pH 7. Benzyl viologen
+
•
+•
mono-cation (BzV ) was found to interact with β-CD reversibly according to EC mechanism. Equilibrium constant of BzV -nβ-CD
-n
complex in buffer solution of pH 7, has been calculated as 5.49 M . The value of n was found to be 0.2. The bimolecular rate constant for
+•
-n −
the complex formation of BzV with β-CD was found to be 6.44 M s in buffer solution of pH 7. This study is based upon an electrochemical
approach to obtain the bonding equilibrium constant, the rate constants of formation and the association numbers of the complex formed
between reduced forms of viologen and β-CD. This study explore the electron transfer reaction mechanism of benzyl viologen with β-CD
to extend their applications at neutral pH.
Keywords: Benzyl viologen, β-Cyclodextrin, Cyclic voltammetry, Equilibrium constant, rate constant.
and this problem has been resolved by the complexation of
reduced form of viologen with β-cyclodextrin (β-CD) [16,17].
Most of the previous studies [14,15,18] are based on electro-
chromic properties of non-alkyl group substituted viologen,
these reports however do not emphasize on quantitative deter-
mination of thermodynamic and kinetic parameters of that
systems. Murugavel [19] have reported the biological appli-
cations of benzylic viologen dendrimers as an antiviral, anti-
bacterial and antifungal agent. Benzyl viologen dication and
β-CD, both have potential applications in the field of medicine
for drug delivery, food, agriculture and textile, etc.
INTRODUCTION
Applications of viologen [1] (1,1′-disubstituted 4,4′-bipyri-
dinium ion) are diversified.Viologen have gained considerable
attention as redox catalyst [2-5], redox indicators in biological
studies [6] and electron mediators [7-9]. Some of the members
of viologen family (paraquat and benzyl homologue) possess
herbicidal activity [10,11]. Viologen have versatile redox
2+
+
system with three oxidation states: V , V and Vº and have
tendency to change their colour reversibly many times upon
reduction and oxidation. Owing to this property of viologen
they have significant importance in electrochromic display
systems [12].
To extend the applications of viologen and cyclodextrins,
thermodynamic and kinetics data on the reaction of stated viologen
is necessary. This study is aimed to understand the equilibrium,
Cyclodextrins (CD) are defined as the class of cyclic oligo-
saccharide having tendency to form inclusion complexes [13].
Electrochromic properties of viologen and that further reaction
where viologen participates are affected by the dimer formation
and precipitation of reduced forms of viologen [14,15]. That′s
why practical use of viologen is limited due to the appearance
of residues. Short alkyl group viologen deposits on the electrode
kinetic and mechanistic aspects of benzyl viologen dication
2+
(
BzV )-β-CD system in buffer solution of pH 7 by cyclic voltam-
metry. This report presents the quantitative determinations of
equilibrium constant, rate constants for the BzV and β-CD
system.
2+
This is an open access journal, and articles are distributed under the terms of the Attribution 4.0 International (CC BY 4.0) License. This
license lets others distribute, remix, tweak, and build upon your work, even commercially, as long as they credit the author for the original
creation. You must give appropriate credit, provide a link to the license, and indicate if changes were made.

1
982 Perveen et al.
Asian J. Chem.
0
0
0
0
.8
.6
.4
.2
0
EXPERIMENTAL
β-Cyclodextrin, benzyl viologen dichloride (1,1′-disubsti-
tuted 4,4′-bipyridinium) were purchased from Merck. Buffer
solution of pH 7 (phosphate buffer, Merck) was used as solvent.
Deionized water was used throughout the experiment. White
spot nitrogen gas (99.9% pure) was provided by Pakistan
Oxygen Company Ltd., Karachi, Pakistan.
Electrochemical experiments: Electrochemical measure-
ments were carried out on the ElectrochemicalAnalyzer (Model
CHI600C Series). For cyclic voltammogram (CV), a glassy
carbon electrode 3 mm in diameter, a platinum wire auxiliary
electrode and a saturated calomel reference electrode (SCE)
were used. Before taking each run the glassy carbon electrode
was polished with 0.05 µm alumina on a felt surface and rinsed
with deionized water. Benzyl viologen solution (0.116 mM)
was prepared by dissolving an appropriate amount in buffer
solution of pH 7. This solution was purged with nitrogen for
-
0.2
0.4
0.6
0.8
1.0
1.2
1.4
a
-
b
-
-
c
-
d
-
-
-0.40 -0.50 -0.60 -0.70 -0.80 -0.90 -1.00 -1.10 -1.20 -1.30
Potential (V vs. SCE)
Fig. 2. Cyclic voltammograms of benzyl viologen Di-cation in buffer
solution of pH 7 at different scan rates; (a-d) are different scan
rates. a = 0.03 V/s, b = 0.05 V/s, c = 0.1 V/s, d = 0.8 V/s
The electrochemical parameters for the first reduction of
1
5 min and then CVs of benzyl viologen solution were taken
2+
BzV in buffer solution of pH 7 are given in Table-1. For
checking the reversibility of redox process (BzV /BzV ), ratio
of peak currents and peak to peak separations were calcu-lated
in the absence and presence of varied concentration of β-CD
at different scan rates. For blank correction, CVs of buffer solu-
tion of pH 7 were taken at different scan rates. Cyclic voltam-
mogram (CV) of blank buffer solution did not show presence
of any electroactive species in the solution.
2
+
+•
in the absence of β-CD host. The ratio of peak currents |Ipa/Ipc
|
2
+
+•
for BzV /BzV redox process was calculated as 1.02. On the
other hand, cathodic to anodic peak separation for this redox
process was found to be -0.049 V (Table-2). These calculated
values were found close to the requirements of a reversible
RESULTS AND DISCUSSION
Fig. 1 shows the cyclic voltammogram of benzyl viologen
system. Half peak potentials (Ep/2) and half wave potentials (E1/2
)
2+
dication (BzV ) in buffer solution of pH 7 on a scan rate of
.05 V/s. The second reduction peak was remarkably sharp
for the first redox process of benzyl viologen dication were
also calculated in the absence of β-CD host. When diagnostic
tests were applied to these electrochemical parameters of
benzyl viologen dication, the values of |E
for the first redox process (BzV /BzV ) were also found close
to the reversible system (Table-2). It suggests that first reduc-
0
due to extensive deposition of benzyl viologen neutral (BzVº).
This neutral deposition gets oxidized back to produce a large
anodic peak which however, well removed from its cathodic
partner. A small closely spaced anodic peak appeared with the
large anodic peak. This peak may have emerged due to oxida-
p
− Ep/2| and |E − E1/2|
p
2+
+•
2+
tion process of BzV is reversible.
+•
tion of the benzyl viologen mono-cation (BzV ) in solution.
2+
BzV+•
2+
BzV + e
(1)
Cyclic voltammograms (CVs) of BzV in buffer solution of
pH 7 were also recorded at different scan rates (Fig. 2). It was
found that by varying the scan rate between 0.03V/s to 0.8V/s,
the second reduction peak remained sharp even at higher scan
rates. Against that both anodic peaks were found to get comp-
letely merged.
Inclusion complexation of benzyl viologen mono-cation
with β-CD host in buffer solution of pH 7: Inclusion comple-
xation of benzyl viologen with β-CD host in buffer solution of
pH 7 was studied by recording cyclic voltammograms of BzV
in the presence of different concentrations of β-CD. In case of
viologen with short alkyl chain the deposition problem of neutral
2
+
3
2
1
.0
.0
.0
0
TABLE-1
ELECTROCHEMICAL PARAMETERS: REDUCTIONS OF
2+
BzV IN THE ABSENCE AND PRESENCE OF β-CD AT
0.05 V/s SCAN RATE IN BUFFER SOLUTION OF pH 7
-
1.0
2.0
3.0
4.0
5.0
6.0
7.0
e
1
f
-
Ipc ×
E (A)
6.683
7.458
7.374
6.517
Ipa ×
c
1
d
1
Substance/s (mM)
Epc (V) Epa1 (V)
-6
-6
E (A)
-
a
b
b
b
b
0
0
0
.116 + 0.000
.116 + 0.397
.116 + 1.492
-0.591
-0.591
-0.584
-0.573
-0.542
-0.539
-0.531
-0.516
-6.821
-6.958
-6.654
-6.271
-
a
-
a
a
-
0.116 + 10.00
a
b
-
Concentration of benzyl viologen solution; Addition of β-CD host in
.116 mM BzV solution; Peak potential for the first reduction step;
Peak potential for the first oxidation step; Cathodic peak currents
measured for the first redox process; Anodic peak current measured
-0.40 -0.50 -0.60 -0.70 -0.80 -0.90 -1.00 -1.10 -1.20 -1.30
2+
c
0
d
e
Potential (V vs. SCE)
f
Fig. 1. Cyclic voltammogram of benzyl viologen Di-cation in buffer
solution of pH 7 at 0.05 V/s scan rate
for first redox process.

Vol. 32, No. 8 (2020) Electron Transfer ReactionsAssociated with BenzylViologen-β-Cyclodextrin Complexation in Buffer Solution 1983
TABLE-2
+
2
ELECTROCHEMICAL PARAMETERS MEASURED FOR BzV IN THE ABSENCE AND PRESENCE OF β-CD AT 0.05 V/s SCAN RATE
Substance/s (mM)
|Ipa /Ipc |
Ep =Epc -Epa (V)
(E -Epc/2)1 (V)
(E -Epa/2)1 (V)
(E -E )c (V)
(E - E ) (V)
1
1
1
1
1
pc
pa
p
1/2
1
p
1/2 a1
0.021
0.022
0.024
0.025
a
b
b
b
b
0
0
0
0
.116 + 0.000
1.02
0.93
0.90
0.96
-0.049
-0.052
-0.053
-0.057
-0.041
-0.044
-0.046
-0.049
0.041
0.043
0.047
0.050
-0.021
-0.022
-0.023
-0.025
a
.116 + 0.397
a
.116 + 1.492
a
.116 + 10.00
Ipa /Ipc , is the anodic to cathodic peak current ratios measured for the first redox process. Epc – Epa is the cathodic to anodic peak separation
1
1
1
1
measured for the first redox process. (EpC/2)1 and(Ep ) are the first cathodic and anodic half peak potentials respectively. (E1/2)c and(E1/2)a are
a/2
1
1
1
the first cathodic and anodic half wave potentials respectively.
Concentration of benzyl viologen solution; Addition of β-CD host in 0.116 mM BzV solution.
a
b
2+
viologen was removed by the use of β-CD [16]. For the case
of benzyl viologen it was observed that even on the addition
of 85 times concentrated β-CD host, the electrodeposition of
BzVº couldn′t be removed and the peak of neutral benzyl
viologen remained sharp at different scan rates (Figs. 3-6). On
0.8
0.6
0.4
0.2
0
c
a b
-0.2
2+
the other hand, first reduction peak of BzV was shifted to positive
-0.4
direction by the successive addition of β-CD host. Electrochemical
-0.6
-0.8
-1.0
2+
parameters for the first reduction of BzV in the presence of
c
β-CD are given in Table-1. The calculated values of |Ipa/Ipc|, ∆E ,
p
-
1.2
1.4
1.6
1.8
0.40 -0.50 -0.60 -0.70 -0.80 -0.90 -1.00 -1.10 -1.20 -1.30
Potential (V vs. SCE)
Fig. 5. Effect of the additions of β-CD on cyclic voltammogram of BzV
in buffer solution of pH 7 at 0.8V/s scan rate; Cyclic voltammograms
b
-
a
3
2
1
.0
.0
.0
0
-
-
-
-
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
2+
-
2
+
2+
of BzV in the absence (a = 0.116 mM BzV ) and the presence of
-
c
b
a
different concentrations (b,c) of β-CD. Where b = 1.492 mM, c =
-
0
.01 M
-
-
1.5
1
.2
.9
-
0
a
-
b
c
-
0.40 -0.50 -0.60 -0.70 -0.80 -0.90 -1.00 -1.10 -1.20 -1.30
Potential (V vs. SCE)
Fig. 3. Effect of the additions of β-CD on cyclic voltammogram of BzV
in buffer solution of pH 7 at 0.05 V/s scan rate; Cyclic voltammo-
0.6
0.3
0
2
+
-0.3
-0.6
-0.9
2
+
2+
grams of BzV in the absence (a = 0.116 mM BzV ) and the
presence of different concentrations (b,c) of β-CD. Where b = 1.492
mM, c = 0.01 M
-
1.2
1.5
1.8
2.1
2.4
0.40 -0.50 -0.60 -0.70 -0.80 -0.90 -1.00 -1.10 -1.20 -1.30
Potential (V vs. SCE)
Fig. 6. Effect of the additions of β-CD on cyclic voltammogram of BzV
in buffer solution of pH 7 at 3 V/s scan rate; Cyclic voltammograms
c
-
4
3
2
1
.0
.0
.0
.0
-
b
-
a
-
-
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
-
-
-
-
-
-
-
-
-
2
+
2
+
2+
of BzV in the absence (a = 0.116 mM BzV ) and the presence of
c
different concentrations (b,c) of β-CD. Where b = 1.492 mM, c =
0
.01 M
b
a
E
p
− Ep/2 and E
p
− E1/2 for the first redox process were closed to
-
10.0
the reversible system (Table-2). It means that the reaction of
BzV with β-CD is reversible.
-0.40 -0.50 -0.60 -0.70 -0.80 -0.90 -1.00 -1.10 -1.20 -1.30
+
•
Potential (V vs. SCE)
Fig. 4. Effect of the additions of β-CD on cyclic voltammogram of BzV
in buffer solution of pH 7 at 0.1V/s scan rate; Cyclic voltammograms
2
+
Quantitative determination of inclusion complex of benzyl
viologen with β-cyclodextrin
2
+
2+
of BzV in the absence (a = 0.116 mM BzV ) and the presence of
different concentrations (b,c) of β-CD. Where b = 1.492 mM, c =
Equilibrium constants: Fig. 7 simply demonstrates the
positive shifting of first cathodic peak potential in the presence
0
.01 M

1
984 Perveen et al.
Asian J. Chem.
1
0.0
0.020
0.018
a
8.0
6.0
4.0
2.0
0
b
c
0.016
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0
-2.0
-4.0
-6.0
-8.0
y = 0.0129x + 0.0437
2
R = 0.9994
-
0.42 -0.45 -0.48 -0.51 -0.54 -0.57 -0.60 -0.63 -0.66 -0.69 -0.72
Potential (V vs. SCE)
Fig. 7. Effect of the additions of β-CD on the first cathodic peak of BzV
in buffer solution of pH 7 at 0.05 V/s scan rate; Scan of the first
2
+
0
-1.000
-2.000
log -CD
Fig. 8. Plot of first cathodic peak potentials against log β-CD
-3.000
-4.000
2
+
cathodic peak of BzV in the presence of different concentrations
a-c) of β-CD. Where a = 0.397 mM, b = 1.492 mM, c = 0.01 M
β
(
of β-CD host. It was noted that as concentration of β-CD host
2
+
BzV+•
BzV + e
(re. 1)
(3)
was increased the peak current ratios for the first redox process
2+
+•
of BzV decreased (Table-2). It indicates the binding of BzV
+•
+•
BzV + n[β-CD]
BzV – nβ-CD
with β-CD. The shift in the peak potential and change in the
peak current ratios confirmed the coupling of chemical reaction
and the electron transfer reaction [20,21]. Positive shifting of
peak also confirms the presence of EC mechanism. On the basis
of these observations, the reaction scheme is developed as
given below:
β-CD was in excess so reaction (eqn. 3) was assumed to
be a pseudo-first order one. To this effect, following equation
was used for determination of k
as determined for EVº − β-CD complex previously.
b
, the first order reverse reaction,

RT 
1
2

E = E
−
0.78 + lnKN.S
nF 
+
{
lna − lnk (1+ K ) − ln(1+ K
N.S  (4)

}
)
p
1/2

b
N.S

BzV2+
BzV
+•
where E
p
is the peak potential of benzyl viologen in the presence
of β-CD, E1/2 is the half wave potential of benzyl viologen in
the absence of host E is the peak potential of benzyl viologen
+
nβ-CD
Keq
p
–
nβ-CD
in the presence of β-CD, E1/2 is the half wave potential of benzyl
n
viologen in the absence of host, KN.S = K × [β-CD] and a =
-1
BzV+•(β-CD)n
nFν/RT (ν is scan rate inV s ). By putting these values in above
-1
equation, k
′ (pseudo-first order rate constant) can be calculated using
the following eqn. 5.
b
was calculated as 1.17 s . From this value of k
b
,
Following eqn. 2 [22] was used for determination of equili-
brium constant (Keq) and n (number of β-CD associated with
k
f
+
•
BzV ).
kf
KN.S
=
o
P

2.3RT 
 2.3RT 
logK + (n)
(5)
kb
E − E =
log[βCD] (2)

P


eq

F

F

+•
The value of kf′ for the reaction of BzV with β-CD (3)
In this equation, E
benzyl viologen obtained in the absence of β-CD and E
potential of the same obtained in the presence of β-CD.
Different values of E were obtained by addition of increa-
sing amounts of β-CD (Table-1). Experimentally determined
values of E − E º for the first reduction peak were plotted against
P
º is the cathodic peak potential of
-1
was obtained as 2.57 s .
P
is the
From this pseudo-first order rate constant, second order
rate constant k
2
was calculated using eqn. 6.
P
kf′
k2 =
n
(6)
[β-CD]
was found to be 6.44 M s .
P
P
-n
-1
log β-CD (Fig. 8).Values of slope and intercept, as obtained from
the graph, were 0.0129 and 0.0437, respectively. The ‘n’ can
be obtained from the slope and Keq from the intercept. Thus n,
the fraction of β-CD associated with BzV was found to be
.2 and Keq obtained for BzV -nβ-CD complex in buffer
The value of k
Conclusion
2
+•
In this study, an interaction of non-alkyl group substi-
tuted viologen with β-cyclodextrin (β-CD) has been explored
by means of cyclic voltammetry. This study is based upon a
simple electrochemical approach, which is useful for evaluating
the equilibrium constants and kinetic parameters, like the rate
constants which have not been reported by earlier workers for
non-alkyl group substituted viologen-CD systems in buffer of
+
•
0
-n
solution of pH 7 was 5.49 M .
+
•
Rate constants:As the complex formation of BzV with
β-CD is the case of reversible chemical reaction following a
reversible charge transfer, Nicholson-Shain theory [21] was
used for the evaluation of rate constants.

Vol. 32, No. 8 (2020) Electron Transfer ReactionsAssociated with BenzylViologen-β-Cyclodextrin Complexation in Buffer Solution 1985
1
1
1
1
0. L.A. Summers, The Bipyridinium Herbicides,Academic Press: NewYork,
p. 449 (1980).
1. J.H. Ross, L.O. Lim and R.I. Krieger, Drug Chem. Toxicol., 2, 193 (1979);
https://doi.org/10.3109/01480547908998242
2. R.J. Mortimer, D.R. Rosseinsky and P.M.S. Monk, Electrochromic
Materials and Devices, Wiley-VCHVerlag GmbH & Co. KGaA (2015).
3. N. Sharma and A. Baldi, Drug Deliv., 23, 739 (2016);
https://doi.org/10.3109/10717544.2014.938839
pH 7. This electrochemical study of benzyl viologen may extend
its application for mediating/triggering electron transfer reaction
in the field of biotechnology.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests
regarding the publication of this article.
14. A. Yasuda and J. Seto, J. Appl. Electrochem., 18, 333 (1988);
https://doi.org/10.1007/BF01093745
1
5. A. Yasuda, H. Kondo, M. Itabashi and J. Seto, J. Electroanal. Chem.,
10, 265 (1986);
https://doi.org/10.1016/0022-0728(86)80579-4
REFERENCES
2
1
2
.
.
L. Michaelis and E.S. Hill, J. Gen. Physiol., 16, 859 (1933);
https://doi.org/10.1085/jgp.16.6.859
G.D. Watt, D. Hansen, D. Dodson, M. Andrus and D. Wheeler, Renew.
Energy, 36, 1523 (2011);
16. A.E. Kaifer, P.A. Quintela and J.M. Schuette, J. Incl. Phenom. Mol.
Recognit. Chem., 7, 107 (1989);
https://doi.org/10.1007/BF01112788
https://doi.org/10.1016/j.renene.2010.10.016
D.R. Wheeler, J. Nichols, D. Hansen, M. Andrus, S. Choi and G.D. Watt,
J. Electrochem. Soc., 156, B1201 (2009);
17. S. Akbar, I.I. Naqvi and M. Mohammad, J. Iran. Chem. Soc., 11, 615
(2014);
3
4
.
.
https://doi.org/10.1007/s13738-013-0331-4
https://doi.org/10.1149/1.3183815
K.K. Park, C.W. Lee, S.Y. Oh and J.W. Park, J. Chem. Soc. Perkin Trans.
I, 2356 (1990);
18. A. Yasuda, H. Mori and J. Seto, J. Appl. Electrochem., 17, 567 (1987);
https://doi.org/10.1007/BF01084131
19. K. Murugavel, Polym. Chem., 5, 5873 (2014);
https://doi.org/10.1039/C4PY00718B
https://doi.org/10.1039/P19900002356
5
6
7
8
9
.
.
.
.
.
D. Mandler and I. Willner, J. Am. Chem. Soc., 106, 5352 (1984);
https://doi.org/10.1021/ja00330a053
P.M.S. Monk, TheViologens: Physicochemical Properties, Synthesis and
Applications of the Salts of 4,4′-Bipyridine, Wiley: Chichester (1998).
D.G. Whitten, Acc. Chem. Res., 13, 83 (1980);
https://doi.org/10.1021/ar50147a004
K. Kalyanasundaram, Coord. Chem. Rev., 46, 159 (1982);
https://doi.org/10.1016/0010-8545(82)85003-0
E. Adar, Y. Degani, Z. Goren and I. Willner, J. Am. Chem. Soc., 108, 4696
20. T. Fiaz, S. Perveen and N. Fatima, J. Iran. Chem. Soc., 16, 2211 (2019);
https://doi.org/10.1007/s13738-019-01688-1
21. R.S. Nicholson and I. Shain, Anal. Chem., 36, 706 (1964);
https://doi.org/10.1021/ac60210a007
22. N. Gupta and H.L. Linschitz, J. Am. Chem. Soc., 119, 6384 (1997);
https://doi.org/10.1021/ja970028j
(
1986);
https://doi.org/10.1021/ja00276a002