Reversible electrochemical pH-switching of luminescence in a p-sulfonatothiacalix[4]arene- terbium(3+) system

Article abstract of DOI:10.1007/s11172-010-0275-0

A quinine-hydroquinone system was used as a reversible electrochemical pH-switcher of luminescence in the p-sulfonatothiacalix[4]arene-terbium(3+) complex in an aqueous medium.

Full text of DOI:10.1007/s11172-010-0275-0

1538
Russian Chemical Bulletin, International Edition, Vol. 59, No. 8, pp. 1538—1542, August, 2010
Reversible electrochemical pHꢀswitching of luminescence
in a pꢀsulfonatothiacalix[4]arene—terbium(3+) system
A. S. Stepanov, V. V. Yanilkin, A. R. Mustafina, V. A. Burilov, S. E. Solov´eva,
I. S. Antipin, and A. I. Konovalov
A. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Research Center of the Russian Academy of Sciences,
8
ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843) 275 2253. Eꢀmail: yan@iopc.knc.ru
A quinone—hydroquinone system was used as a reversible electrochemical pHꢀswitcher
of luminescence in the pꢀsulfonatothiacalix[4]arene—terbium(3+) complex in an aqueous
medium.
Key words: pꢀsulfonatothiacalix[4]arene, terbium(3+) complexes, luminescence, electroꢀ
chemical pHꢀswitching, cyclic voltammetry.
Supramolecular systems with photoꢀ1—9, redox,10—21
quinone redox system as an electrochemical pHꢀswitcher
and pHꢀswitchable2
2—25
properties, in particular, devices
of luminescence in the complex of pꢀsulfonatothiacalixꢀ
[4]arene (TCAS) with the Tb3 ion.
+
with switchable binding and switchable color and lumiꢀ
nescence, are widely studied. In the earlier created
pHꢀswitching devices, switching occurs by the addition of
acid and base. This results in an increase in the salt conꢀ
tent with each cycle. Therefore, switching occurs only
restrictedly and, hence, this variant of pHꢀswitching canꢀ
not be used for the creation of practically useful devices.
In this connection, it is important to develop another
method of pHꢀswitching when the reversible change in
pH would be accompanied by the reversible change in the
composition of the solution and functioning of such
a system during unrestricted quantity of cycles.
It is known26 that the oxidation of organic compounds
is often associated with proton elimination, whereas
the reduction is accompanied by the accumulation of
protons. At the same time, there are organic redox pairs in
which redox processes involving protons are chemically
reversible, for instance, pairs quinone—hydroquinone or
nitroso compound—hydroxylamine. Therefore, reversible
electrochemical switching of the pH of the medium with
the reversible change in the composition of the solution
during electrolysis is principally possible. The only probꢀ
lem is the choice of the pH of the switcher with a set of
properties required for each particular case.
The Tb3+ ion itself does not luminesce, and its comꢀ
plex with TCAS, in which the metal ion is bound at the
lower phenolate rim, possesses high luminescence. Bindꢀ
ing and, therefore, luminescence, are pHꢀdependent. At
pH 7 the Tb3 ion is bound and the complex luminesces,
+
whereas at pH 3 the metal ion is bound to a lesser extent
and the luminescence is quenched.3
,27
Taking into
account these data, we stated a problem of electrochemiꢀ
cal pHꢀswitching from 7 to 3 and back in a nonꢀbuffer
aqueous solution containing 0.1 М NaClO as a supportꢀ
4
–
4
–4
ing electrolyte and a system 5•10 М TCAS + 5•10
Tb using the quinone—hydroquinone redox pair.
М
3
+
Waterꢀsoluble pꢀsulfonatothiacalixarenes bind effiꢀ
ciently bulky complex cations at the upper sulfonate rim,
The cyclic voltammograms (CV) of quinone reducꢀ
tion and hydroquinone oxidation at pH 3.04 and 7.01 in
the absence and in the presence of TCAS were prelimiꢀ
narily detected to determine the region of oxidation and
reduction potentials. As it was expected, one peak of
2
,3
whereas metal ions are bound at the lower phenolate rim.
Complexes of this type are used in the creation of devices
with redox and pHꢀswitching luminescence.2 Now we
report the results of investigation of the quinone—hydroꢀ
,3
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1502—1505, August, 2010.
066ꢀ5285/10/5908ꢀ1538 © 2010 Springer Science+Business Media, Inc.
1

Electrochemical pHꢀswitching of luminescence
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
1539
i
a
i
c
[
0.25 μA
[
0.25 μA
b
d
–
0.2
0
0.2 0.4 0.6 0.8 V/V (vs Ag/AgCl)
–0.2
0
0.2
0.4
0.6 V/V (vs Ag/AgCl)
Fig. 1. Cyclic voltammograms at pH = 3.04 (a, b) and 7.01 (c, d) of quinone in the presence (a, c) and in the absence of TCAS (b, d)
= 1.5•10– mol L , СTCAS = 2.0•10 mol L , υ = 100 mV s
3
–1
–3
–1
–1
.
at the Pt electrode in an H O/0.1 М NaClO medium; С
2
4
quinone
quinone reduction and one peak of hydroquinone reoxidaꢀ
tion were detected on each CV of quinone (Fig. 1).
Typical dependences of the potentials of the reducꢀ
tion and oxidation peaks on pH are observed for the
quinone—hydroquinone system. Especially high sensitivꢀ
ity is characteristic of the reoxidation peak of hydroꢀ
quinone: in a highly acidic medium the peaks are shifted
to anodic values. In the both solutions the introduction of
TCAS decreases noticeably the current of the quinone
Scheme 1
reduction peak (i ), which is controlled by the diffusion
p
Table 1. CV data at the Pt electrode in an H O/0.1 М NaClO
for quinone and hydroquinone in the absence and presence of
TCAS*
2
4
rate in all cases. This is indicated by the linear depenꢀ
0
.5
dence of i on υ . This decrease in the current in the
p
presence of TCAS indicates, most likely, the binding of
quinone with TCAS, which does not almost affect the
reduction and reoxidation potentials (Table 1).
System
studied
pH
Ep,red** Ep,ox** Ip,red Ip,ox
V
μA
The CV morphology of hydroquinone almost correꢀ
sponds to the CV curves of quinone with the only distincꢀ
tion that the peaks of hydroquinone oxidation and quinone
rereduction are detected in this case and, correspondꢀ
ingly, the ratio of peak heights changes. Another distincꢀ
tion is that TCAS exerts no effect on the electrochemical
behavior o hydroquinone. Therefore, unlike quinone, hyꢀ
droquinone is not bound with TCAS.
Thus, in this pH range the chemically reversible reꢀ
duction of quinone occurs with the transfer of two elecꢀ
trons, consumption of two protons, and formation of
hydroquinone (Scheme 1). The reduction and oxidaꢀ
Quinone
3.04 –0.02
+0.57
+0.37
+0.59
+0.38
+0.54
+0.56
+0.55
+0.55
1.65 0.65
2.25 0.43
1.00 0.53
1.70 0.40
0.43 1.63
7
.01 –0.06
Quinone—TCAS
Hydroquinone
3.04 –0.03
7.01 –0.07
3.04 +0.09
7.01 ꢀ0.11
3.04 +0.07
7.01 ꢀ0.11
—
1.23
0.50 1.63
1.10
Hydroquinone—
TCAS
—
*
1
The concentrations of quinone and hydroquinone are
.5•10 mol L , СTCAS = 2.0•10 mol L , υ = 100 mV s .
–3
–1
–3
–1
–1
** Relative to Ag/AgCl.

1540
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
Stepanov et al.
tion peaks are observed in the same potential region (see
i
a
3
+
Table 1), where TCAS and Tb are electrochemically
inactive. From the viewpoint of oxidation and reduction
potentials and pHꢀswitching, the quinone—hydroquinone
redox system is ideally suitable for electrochemical
pHꢀswitching in the TCAS—Tb3+ system.
[0.25 μA
Then we studied a possibility of electrochemical pHꢀ
b
c
–
4
switching of luminescence in the system 5.0•10
М
–
4
3+
–3
TCAS + 5.0•10 М Tb + 2.0•10 М hydroquinone +
–
4
2
.0•10 М quinone vs 0.1 М NaClO . This initial soluꢀ
4
tion, whose pH was adjusted with NaOH to 7.05, lumiꢀ
nesces (Fig. 2). The concentration of protons equal to
–3
–1
1
•10 mol L should be created in the solution to change
pH from 7.0 to 3.0. This number of protons can be obꢀ
tained by the oxidation of 5.0•10–4 М hydroquinone.
However, TCAS possesses some buffer capacity and,
therefore, a high concentration of hydroquinone should
be introduced into the solution. In addition, the diffusion
current during electrolysis decreases with time according
to the dependence i ∼ τ– and, hence, prolonged elecꢀ
trolysis is needed for the complete processing of the subꢀ
strate. In the next experiments, we took a fourfold amount
of hydroquinone (C = 2•10 mol L ) and some amount
of quinone (C = 2•10–4 mol L–1) was added to the soluꢀ
tion, although it is not necessary in the first oxidation step
in the case of the initial solution.
The CV of the initial solution exhibits the peaks
of both quinone reduction and hydroquinone oxidation
1/2
–
0.2
0
0.2
0.4
0.6 V/V (vs Ag/AgCl)
–
3
–1
Fig. 3. Cyclic voltammograms of an aqueous solution containꢀ
ing 5.0•10– М TCAS + 5.0•10 М Tb + 2.0•10 М hydꢀ
4
–4
3+
–3
roquinone + 2.0•10– М quinone + 0.1 М NaClO before elecꢀ
4
4
trolysis, pH 7.05 (a); after electrochemical oxidation, pH 3.01 (b);
after reversible electrochemical reduction, pH 7.14 (c).
(
ii) a change in the pH to 3.01, and (iii) expected lumiꢀ
(
Fig. 3). The electrooxidation of the solution in a divided
cell at the controlled oxidation potential of hydroquinone
E < +0.4 V) with the charge 1.37 times exceeding
the theoretical value based on the proton concentration
nescence quenching (see Fig. 2). Therefore, the change
in the pH and luminescence is due just to the oxidation of
hydroquinone. The reverse reduction at the controlled
potential of quinone reduction (Е > –0.1 V), at the same
current, and with the same charge returns the initial
pH 7.14, luminescence (see Fig. 2), and the starting
CV shape (see Fig. 3), indicating the entire reduction
of the initial composition of the solution.
It should be mentioned that luminescence quenching
after the electrochemical pH change from 7.05 to 3.01 is
the same as for the chemical oxidation of the solution
with hydrochloric acid in the system 5.0•10 М TCAS +
5.0•10–4 М Tb (Fig. 4).
(
–
3
–1
С = 1.0•10 mol L results in (i) an increase in
the quinone concentration with simultaneous
a
decrease in the hydroquinone concentration (see Fig. 3),
I (at. units)
3
4
3
3
2
2
1
1
00000
50000
00000
50000
00000
50000
00000
–
4
1
3+
+
Thus, using the reversible redox system quinone—
2
hydroquinone, we succeeded to switch luminescence in a
TCAS—Tb3 system adequate to switching with the
chemical change of pH. However, unlike the chemical
variant, the electrochemical variant reversibly changes the
composition of the solution. This result provides chalꢀ
lenges for search for electrochemical pHꢀswitchers for
other systems and devices.
+
5
0000
0
–
50000
4
50
500
550
600
λ/nm
–
4
Fig. 2. Luminescence spectra of an aqueous solution 5.0•10
М
–
4
3+
–3
Experimental
TCAS + 5.0•10 М Tb + 2.0•10 М hydroquinone +
.0•10– М quinone + 0.1 М NaClO at pH = 7.05 to (1) and
4
2
4
after the cycle of electrochemical oxidation (2) and reversible
reduction (3) (λexc = 330 nm).
pꢀSulfonatothiacalix[4]arene tetrasodium salt (Na TCAS)
in the cone conformation was synthesized according to a
4

Electrochemical pHꢀswitching of luminescence
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
1541
I (at. units)
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