Syntheses and electrochemical characterization of new water soluble octaarylthiosubstituted manganese phthalocyanines

Article abstract of DOI:10.1016/j.dyepig.2010.09.012

This paper reports on the synthesis and characterization of new manganese phthalocyanine (MnPc) complexes: 2,3-octakis-[(2-mercaptopyridine) phthalocyaninato] acetato manganese (III) (1) and its quaternized (hence water soluble) derivative: 2,3-octakis-{[

Full text of DOI:10.1016/j.dyepig.2010.09.012

Dyes and Pigments 89 (2011) 111e119
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Syntheses and electrochemical characterization of new water soluble
octaarylthiosubstituted manganese phthalocyanines
a
a
b
a,
*
Irvin Booysen , Fungisai Matemadombo , Mahmut Durmu s¸ , Tebello Nyokong
a
Chemistry of Department, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa
Gebze Institute of Technology, Department of Chemistry, P.O. Box: 141, 41400 Gebze-Kocaeli, Turkey
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 May 2010
Received in revised form
This paper reports on the synthesis and characterization of new manganese phthalocyanine (MnPc)
complexes: 2,3-octakis-[(2-mercaptopyridine) phthalocyaninato] acetato manganese (III) (1) and its
quaternized (hence water soluble) derivative: 2,3-octakis-{[(N-methyl-2-mercaptopyridine) phthalo-
cyaninato] acetato manganese (III)} sulphate (2). The complexes were used to form self assembled
monolayers (SAMs). Voltammetry proved that both of the SAMs are well packed, strongly passivating and
act as selective and efficient barriers to ion permeability. Furthermore, surface coverage studies
confirmed that the MPc macrocycles adsorb onto the gold electrode as monolayers. Both MPc SAMs were
successfully used as electrochemical sensors of nitrite.
1
September 2010
Accepted 3 September 2010
Available online 25 October 2010
Keywords:
Octaarylthio-substituted phthalocyanine
Manganese
Ó 2010 Elsevier Ltd. All rights reserved.
Water-soluble
Quaternization
Spectroelectrochemistry
1
. Introduction
MnPc complexes show interesting electrochemical behaviour
I
with oxidation states of the central Mn ion ranging from Mn to
IV
II
III
Metallophthalocyanines (MPcs) are macrocyclic
p
-electron
Mn [8e15]. The spectra of Mn Pc and Mn Pc complexes are now
conjugated molecules which exhibit a series of electrochemical
processes and can consequently be used as efficient electron
mediators, for example, in electrocatalysis [1e6].
well known. For MnPc complexes tetra substituted with 2-mer-
captopyridine at the peripheral positions (3
b) and then quaternized
to give 4 , spectroelectrochemistry showed that the reduction of
b
II
I
The presence of electron donating sulfur groups will result in the
shift of the Q-band to longer wavelengths compared to unsubstituted
MPc. Pc complexes containing Mn as a central metal, in particular,
show a highly red-shifted Q-band [7]. The presence of sulfur will also
make oxidation of these complexes favourable. Hence, in this work,
we present the synthesis and electrochemistry of new sulphur
substituted MnPc complexes (1 and 2, Scheme 1) and we compare
their electrochemical behaviour with those of tetrasubstituted
counterparts: manganese (III) 2,(3)-tetra-(2-mercaptopyridine)
Mn Pc to Mn Pc occurs only when the complex is in its quaternized
form (4b). The reduction (to Mn Pc ) of the quaternized form
occurs at a lower potential than that of the unquaternized form (to
I
ꢀ2
II ꢀ3
I
ꢀ2
Mn Pc ), showing that metal (to Mn Pc ) versus ligand reduction
II ꢀ3
II
(to Mn Pc ) in Mn Pc complexes may depend on the nature of the
ring substituents. This work explores the effect of increasing
the number of substituents on the electrochemical behaviour of the
mercaptopyridine substituted MnPc derivatives. Furthermore,
arylthio derivatised MnPcs in this work are attached to gold elec-
trodes to form self-assembled monolayers (SAMs). Alkyl or arylthio
MPcs are known to form SAMs without cleavage of the aryl or alkyl
group [16]. The SAMs are employed for the analyses of nitrite.
phthalocyanine (
tive manganese (III) 2,(3)-tetra-N-methyl{(2-mercaptopyridine)
phthalocyanine ( -(OH)Mn-Q-TMPyPc, 4 )}, Fig.1. The syntheses and
characterization of complexes 3 and 4 have been previously reported
8]. The water solubility of MnPc complexes is desirable despite the
b-(OH)MnTMPyPc, 3b) and its quaternized deriva-
b
b
[
2. Experimental
fact that the aggregation tendency in such a polar medium is very
high. Hence, we present here the synthesis of a water soluble octa-
substituted MnPc derivative (2).
2.1. Materials
Acetone was provided by Protea Chemicals and distilled before
use. Dimethylformamide (DMF), methanol (MeOH), n-pentanol,
*
Corresponding author. Tel.: þ27 46 6038260; fax: þ27 46 6225109.
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.09.012

112
I. Booysen et al. / Dyes and Pigments 89 (2011) 111e119
Scheme 1. Synthesis route of 2-mercaptopyridine substituted manganese (III) phthalocyanine complexes (1 and 2).
n-hexane, tetrahydrofuran (THF), chloroform, dichloromethane
DCM), ethanol, ethyl acetate, nitric acid (55%), H SO , buffer (pH 4
nitrite was purchased from Merck. Column chromatography was
performed on silica gel 60 (Merck, 0.04e0.063 mm).
(
2
4
and 7) tablets were procured from Saarchem. Deionised water was
obtained from a Millipore-Q system. Nitrogen gas was purchased
2.2. Equipment
from Afrox. Manganese (II) acetate, K
2
CO
3
, dimethylsulphate
) and 1,8-
All electrochemical experiments were performed using Autolab
(
DMS), tetrabutylammonium tetrafluoroborate (TBABF
4
potentiostat PGSTAT 302 (Eco Chemie, Utrecht, The Netherlands)
driven by the general purpose Electrochemical data processing
software (GPES, software version 4.9). Square wave voltammetric
analysis was carried out at a frequency of 10 Hz, amplitude: 50 mV
and step potential: 5 mV. A conventional three electrode system
consisting of a glassy carbon working electrode (1.5 mm radius),
AgjAgCl pseudo reference electrode and a platinum wire counter
electrode were employed. The potential response of AgjAgCl
pseudo reference electrode was less than the AgjAgCl (3M KCl) by
diazabicyclo-[5.4.0]-undec-7-ene (DBU) were purchased from
Aldrich. 2-Mercaptopyridine was purchased from Fluka. 4,5-
Dichlorophthalonitrile was synthesized and purified according to
the literature procedure [17]. All solvents were dried as described
by Perrin and Armarego [18] before use. All other reagents were
obtained from commercial suppliers and used as received. Sodium
0
.015 ꢁ 0.003 V. All electrochemical experiments were performed
ꢂ
at 25.0 ꢁ 1.0 C using freshly distilled solvents.
UV-visible spectra were recorded on a Shimadzu 2001 UVeVis
spectrophotometer or on a Cary 500 UV-visible/NIR spectropho-
tometer. Spectroelectrochemical data were recorded using an
optically transparent thin-layer electrochemical (OTTLE) cell con-
nected to a BioAnalytical System (BAS) 27 voltammogram. FT-IR
spectra (KBr pellets) were recorded on a Bio-Rad FTS 175C FTIR
spectrometer. The mass spectra were acquired on a Bruker Dal-
tonics (Bremen, Germany) MicrOTOF mass spectrometer equipped
with an electronspray ionization (ESI) source. The instrument was
operated in positive ion mode using a m/z range of 50e3000. The
capillary voltage of the ion source was set at 6000 V and the
capillary exit at 190 V. The nebulizer gas flow was 1 bar and drying
ꢂ
gas flow 8 mL/min. The drying temperature was set at 200 C for
synthesized phthalonitrile compound and positive ion and linear
mode MALDI-MS spectrum of phthalocyanine complexes was
obtained in 3,5-dimethoxy-4-hydroxycinnamic acid MALDI matrix
using nitrogen laser accumulating 50 laser shots using a Bruker
1
Microflex LT MALDI-TOF mass spectrometer. H spectra were
recorded in CDCl
00 MHz spectrometer. Elemental analyses were obtained with
a Thermo Finnigan Flash 1112 Instrument.
3
solution for phthalonitrile compound on a Varian
Fig. 1. Molecular structure of tetra substituted complexes 3 and 4. 3
b
and 4
b are
5
peripherally substituted as shown in this figure. 3a and 4a are non-peripherally
substituted [8].

I. Booysen et al. / Dyes and Pigments 89 (2011) 111e119
113
2
2
.3. Syntheses
amounts of ultrapure Millipore water and acetone to remove
residual alumina particles trapped at the surface.
.3.1. Synthesis of 4,5-bis(2-mercaptopyridine)phthalonitrile,
This pre-treatment was followed by placing the Au electrode in
a DMF solution of 1 mM of solutions of complexes 1 or 2 for 24 h at
room temperature to form the SAM. Upon removal from the MnPc
solution, the electrode was rinsed with buffer before any electro-
chemical analysis.
Scheme 1
In a stream of nitrogen, 2-mercaptopyridine (6.74 g, 60.8 mmol)
and 4,5-dichlorophthalonitrile (6.00 g, 30.4 mmol) were dissolved
in DMF (100 mL) and the mixture stirred at room temperature for
3
2 3
0 min. Thereafter, finely ground K CO (15 g, 108 mmol) was
added portion-wise over a period of 4 h and the reaction mixture
left to stir for a further 12 h at room temperature. The mixture was
added to water (200 mL) and stirred for 30 min. The resulting
precipitate was filtered off, thoroughly washed with water, dried
3
. Results and discussion
3.1. Syntheses and characterization
ꢂ
and recrystallised from chloroform:ethanol (1:4). M.p.: 220 C
The metallophthalocyanine complex (1) was prepared by the
ꢀ1
Yield: 5.79 g (55%). IR [(KBr)
(
n
max/cm ]: 3037 (Ar-CH), 2230
C^N), 1574 (C]C). H-NMR (CDCl ): , ppm 8.55 (d, 2H, Pyridyl-
H), 7.83 (s, 2H, Phthalonitrile-H), 7.70 (t, 2H, Pyridyl-H), 7.38 (d, 2H,
Pyridyl-H), 7.28 (t, 2H, Pyridyl-H). Calc. for C18 : C 62.41, H
template reaction of thiol-substituted phthalonitrile precursor with
anhydrous manganese (II) acetate in the presence of DBU using n-
pentanol as solvent. Column chromatography with silica gel was
employed to obtain the pure products from the reaction mixture
resulting in a 59 % yield for complex 1. Quaternization of the
1
3
d
10 4 2
H N S
2
3
.91, N 16.17; Found: C 62.20, H 2.80, N 16.17. MS (ESI-MS) m/z: Calc.
46; Found: 347.1 [M þ H] .
þ
ꢂ
manganese (III) phthalocyanine compound was achieved at 120 C.
Yield of the product was 79 % for complex 2.
2.3.2. 2,3-Octakis(2-mercaptopyridine)phthalocyaninato
Generally, phthalocyanine compounds are insoluble in most
organic solvents; however, introduction of substituents on the ring
increases the solubility. The quaternized manganese (III) phthalo-
cyanine complex (2) exhibited excellent solubility in water. The new
compounds were characterized by UV-vis, IR and NMR spectros-
copies (the latter for the phthalonitrile only), ESI-MS (for the
phthalonitrile compound), MALDI-TOF mass spectra and elemental
analysis. The analyses are consistent with the predicted structures as
shown in the experimental section. The sharp peak in the IR spectra
for the C^N vibrations of phthalonitrile at 2230 cm disappeared
after conversion into manganese (III) phthalocyanine complex,
indicative of metallophthalocyanine formation. The characteristic
vibrations corresponding to C]C groups at w1570 cm , aromatic
CH stretching at 3037e3047 cm were observed for all compounds.
manganese (III) acetate (1, Scheme 1)
mixture of anhydrous manganese (II) acetate (0.50 g,
.88 mmol), 4,5-bis(2-mercaptopyridine)phthalonitrile (1.0 g,
A
2
2
.88 mmol), DBU (0.44 mL, 3 mmol) and n-pentanol (25 mL) was
ꢂ
stirred at 160 C for 12 h under nitrogen atmosphere. After cooling,
the solution was dropped into n-hexane. The brown solid product
was precipitated and collected by filtration and washed with n-
hexane. The crude product was washed with water, ethanol,
acetone, THF, CHCl
product was purified by passing through a silica gel column with
firstly ethyl acetate and then CH Cl :MeOH (10:1) elution. Yield:
max nm (log ) 355 (4.49), 586 (3.86),
max/cm ]: 3041 (Ar-CH),1724 (C]
3
, n-hexane and diethyl ether. The brown crude
ꢀ1
2
2
0
6
.63 g (59 %). UVeVis (DMF):
47 (4.75), 703 (4.00). IR [(KBr)
l
e
ꢀ1
ꢀ
1
n
ꢀ1
O),1571(C]C),1556,1445,1415,1321,1278,1119,1070, 984, 956, 895,
ꢀ1
Aliphatic CH stretches were observed at w2951 cm for quater-
nized phthalocyanine complex. S]O stretching at w1216 and
w1161 cm and SeO stretching at 618 cm for the quaternized
phthalocyanine complex (2) is indicative of quaternization.
7
5
43 16 8 2
55. Calc. for C74H N S O Mn: C 59.27, H 2.89, N 14.94; Found: C
9.61, H 2.96, N14.34. MALDI-TOF-MS m/z:Calc.1498; Found: 1439.7
ꢀ1
ꢀ1
þ
[M-COOCH
3
] .
In addition to the elemental analysis results, the mass spectra
data for the newly synthesized phthalonitrile and manganese (III)
phthalocyanine complexes (1 and 2) were consistent with the
assigned formulations. The molecular ion peak of phthalonitrile
2
.3.3. 2,3-Octakis[(N-methyl-2-mercaptopyridine)
phthalocyaninato manganese (III) acetate] sulphate (2, Scheme 1)
ꢂ
Compound 1 (0.20 g, 0.13 mmol) was heated to 120 C in freshly
distilled DMF (10 mL) and dimethyl sulphate (0.2 mL) was added
dropwise. The mixture was stirred at 120 C for 12 h, cooled to
þ
was observed at m/z 347.1[M þ H] . The molecular ion peak of the
ꢂ
non-quaternized phthalocyanine complex (1) was found at m/z
room temperature and the product was precipitated with hot
acetone and collected by filtration. The greenish brown solid
product was washed successively with hot ethanol, ethyl acetate,
THF, chloroform, n-hexane and diethylether. The resulting hygro-
scopic product was dried over phosphorous pentoxide. Yield: 0.21 g
þ
1439.7 [M-COOCH
3
] . The molecular ion peaks of quaternized
phthalocyanine complex (2) showing parent ions was found at m/z
þ
1
891.3 [M-COOCH
3
-Mn] . MALDI-TOF-MS spectra of quaternized
phthalocyanine complex (2) show the base peak cleavage of axial
ligand and manganese atom. We did not observe the molecular ion
peak for quaternized complex 2.
(
(
1
1
79 %). UVeVis (H
4.27), 697 (5.08). IR [(KBr)
732 (C]O), 1567 (C]C), 1490, 1216 (S]O), 1161 (S]O), 1056,
043, 846, 765, 741, 618 (SeO). Calc. for C82 12Mn (3H O):
2
O): lmax nm (log e) 376 (4.54), 577 (3.92), 625
ꢀ1
n
max/cm ]: 3047 (Ar-CH), 2951 (CH),
3.2. UVeVisible spectra
H
73
N
16
O
21
S
2
C 47.85, H 3.57, N 10.89; Found: C 48.12, H 3.96, N 10.29. MALDI-
Complex 1 readily dissolves in most organic solvents but is
þ
TOF-MS m/z: Calc. 2004.2; Found 1891.3 [M-COOCH
3
-Mn] .
moderately soluble in DCM. Complex 2 did not dissolve in THF,
CHCl , DCM, toluene, acetone (or their combinations) and dissolved
3
2.4. The preparations of SAMs
with great difficulty in DMF and DMSO (complex 2 should be left
overnight in these solutions to dissolve completely). Fig. 2(a) and
The gold electrode was cleaned by immersing it for approxi-
mately 2 minutes in a hot solution of 1:3 (v:v) nitric acid (55%) and
concentrated H SO . This procedure removes organic impurities on
the electrode. This was then followed by polishing the gold elec-
trode, using aqueous slurries of alumina (<10 micron), on a SiC-
emery paper (type 2400 grit), and then to a mirror finish on
a Buehler-felt pad. Finally the Au electrode was rinsed in copious
(
b) shows the spectra of complexes 1 and 2 in various solvents,
respectively.
It has been suggested [19,20] that an equilibrium exists between
2
4
MnPc species in DMF under air as shown by equations (1)e(5).
II
% PcMnIII (O
PcMn þ O
2
2
)
(1)

114
I. Booysen et al. / Dyes and Pigments 89 (2011) 111e119
complex 1. In water, complex 2 showed no presence of
m-oxo
complexes. The spectrum of the quaternized complex 2 were blue
shifted compared to the corresponding MnPc species of unquater-
nized derivative (1), Fig. 2. This is due to the lowering of the electron
donating ability of the nitrogen groups upon quaternization.
3.3. Voltammetric and spectroelectrochemical studies
3.3.1. 2,3-Octakis-[(2-mercaptopyridine) phthalocyaninato] acetato
manganese (III) (1)
Solution redox properties of 1 were studied using cyclic vol-
tammetry (CV) and square wave voltammetry (SWV) in freshly
distilled DMF. Argon was bubbled through the solution and an
argon atmosphere was maintained throughout the cyclic voltam-
metry scans, hence the species being analysed is not expected to
contain
m-oxo species, though its presence cannot be completely
ruled out.
Three redox processes (IeIII) were observed for complex 1 in
DMF, Fig. 3a. All processes were reversible to quasi-reversible with
cathodic to anodic peak potential (
the E for the ferrocene standard was 65 mV). In comparison with
literature [8,22], Table 1, processes III and II for complex 1 in DMF
DE) ranging from 60 to 100 mV
(
D
III ꢀ2
II ꢀ2
(
Fig. 3a) are assigned to Mn Pc /Mn Pc
(E
¼ ꢀ0.69 V), respectively. Process
¼ ꢀ1.09 V) is then assigned as a further ring reduction process.
Comparing complexes 1 and 3 (structure shown in Fig.1), confirms
that process II is ring based Mn Pc /Mn Pc rather than metal
½
¼ 0.023 V) and
II ꢀ3
II ꢀ2
Mn Pc /Mn Pc
(E
½
(E
½
I
b
II ꢀ2
II ꢀ3
II ꢀ2
I
ꢀ2
based Mn Pc /Mn Pc . The observed broadening of redox couple
II is ascribed to the aggregation as a result of the high concentrations
ꢀ
3
(
1 ꢃ10 M) used for voltammetry analysis. This is affirmed by the
square wave voltammogram showing a split redox couple II where
one is due to the monomer and other to the aggregate [23]. Table 1
shows that complex 1 is easier to reduce than the corresponding
Fig. 2. UV-visible spectra of: (a) 1 in DMF, DCM and CHCl
3
and (b) 2 in DMF and H
2
O.
complexes 3a and 3b. The presence of electron donating sulphur
groups is expected to make oxidation easier and reduction more
difficult. However when comparing with other MnPc complexes
such as manganese tetraamino phthalocyanine derivatives (with
PcMnIII (O
II
III
III
2
) þ PcMn % PcMn -O
2
- Mn Pc
(2)
(3)
(4)
(5)
III
II
Mn /Mn process occurring at w ꢀ0.3 V vs AgjAgCl for the same
III
III
IV
PcMn -O
2
- Mn Pc % 2PcMn O
electrolyte/solvent system) [24], complex 1 is easier to reduce.
3.3.2. 2,3-Octakis-{[(N-methyl-2-mercaptopyridine)
PcMn O þ 2PcMn % 2PcMnIII-O- MnIIIPc
IV
II
2
4
phthalocyaninato] acetato manganese (III)} sulphate (2)
Fig. 3b shows that three redox processes (labeled IeIII) recorded
II
% 2PcMnIII-O- Mn Pc (Net equation)
III
PcMn þ O
2
4
in DMF containing TBABF were obtained for complex 2. Couple II
shows splitting due to aggregation. The redox couples III and II are
III ꢀ2
II ꢀ2
II ꢀ2
Thus, in general, the electronic absorption spectra in the visible
assigned to Mn Pc /Mn Pc
(E
¼ ꢀ0.38 V) in comparison with the corresponding tet-
rasubstituted derivatives, Table 1. Couple I (E
¼ ꢀ0.64 V) is
assigned to ring based reduction, Table 1. Complex 2 is easier to
reduce than complexes 4 and 4 , which are tetra substituted at
½
¼ ꢀ0.038 V) and Mn Pc
/
III
II
I
ꢀ2
region for MnPc complexes may be attributed to Mn Pc, Mn Pc
and -oxo MnPc species in DMF under air. The presence of the
Mn Pc (E
½
m
m-
½
oxo MnPc species may be confirmed by monitoring the spectral
transformations of an MPc complex DMF solution when not de-
a
b
aerated and when de-aerated with dry N
work. Thus in Fig. 2a (complex 1), the spectra in DMF is mainly that
of the -oxo MnPc species with a strong band at 651 nm [15,21] and
the Mn Pc peak at 708 nm. The Mn Pc peak is not evident in
Fig. 2a. This peak would have been expected to be red shifted as
observed in the other solvents in Fig. 2a. The intensity of the
2
gas; this was done in this
different positions. All complexes contain sulphur groups, which are
electron donating and expected to result in a more difficult reduc-
tion. It would be expected that complex 2 would be more difficult to
reduce than both 4a and 4b due to the former having more sulphur
groups, however, the presence and plurality of positive charges will
m
II
III
affect the ease of reduction. In aqueous media the couples were
II ꢀ2
6
51 nm band (due to
m-oxo MnPc species) decreased on bubbling
observed at E
½
¼ ꢀ0.52 V (ring reduction), E
½
¼ ꢀ0.41 V (Mn Pc
/
I
ꢀ2
III ꢀ2
II ꢀ2
argon through the solution and there was an increase in the
Mn Pc ), E
½
¼ 0.053 V (Mn Pc /Mn Pc ). Comparison in Table 1
II
intensity of the band at 708 nm due to Mn Pc species. The spectra
with literatureshows that the quaternized derivative (2)exhibits the
II ꢀ2
I
ꢀ2
of complex 1 in chloroform and DCM is typical of Mn(III)Pc species,
Mn Pc /Mn Pc couple, while the unquaternized derivative (1)
II ꢀ2
II ꢀ3
showing a red shifted Q band.
shows the Mn Pc /Mn Pc couple [8].
II
Complex 2 in DMF (Fig. 2b) showedthe Mn Pc absorption band at
III
6
88 nm, Mn Pc as a broad peak near 730 nm, the peak due to
m-oxo
3.3.3. Spectroelectrochemical studies
MnPc species was not well defined, showing that the quaternized
derivative (2) has a less tendency of forming -oxo complexes than
Fig. 4 shows the application of potentials more negative than
couple III for complex 2 in DMF. The spectrum before electrolysis in
m

I. Booysen et al. / Dyes and Pigments 89 (2011) 111e119
115
Fig 4. UV-visible spectral changes of 2 in DMF containing 0.1 M TBABF
tion at potentials of process III (wꢀ0.3 V).
4
upon reduc-
species, only 60 % could be obtained which is ascribed to the
II
reduction of Mn concentration by molecular oxygen. These spec-
tral observations thus confirmed the assignment of redox couple III
III ꢀ2
II ꢀ2
to the metal reduction process Mn Pc /Mn Pc . Due to the
presence of the -oxo MnPc species further reduction (at process II)
m
was not possible. Spectral changes similar to those observed in DMF
were also observed in water upon reduction of 2 at potentials of
process III.
3.4. Self assembled monolayer (SAM) characterization
Voltammetry techniques for the characterization of SAMs are
based on the principle that SAMs block a number of Faradic
processes [25].
Fig. 3. Cyclic and square wave (insert) voltammograms on a GCE recorded in DMF
ꢀ
1
containing 0.1 M TBABF
for (a) shows the peak due to the aggregate.
4
for (a) 1 and (b) 2. Scan rate ¼ 100 mV s . The arrow in insert
3
.4.1. Ion barrier factor (Gibf
The ion barrier factor (Gibf) is obtained by comparison of the
total charge under the gold redox peak of a SAM modified electrode
)
DMF shows monomeric behaviour and the presence of mainly
III
Mn Pc species, when compared to Fig. 2b. This again shows the
(QSAM) with that of an un-modified electrode (QBare), Eq. (6):
strong effects of the presence of oxygen in solution. Solutions for
spectroelectrochemistry were extensively deaerated, but the
¼ 1 ꢀ Q
(6)
SAM
presence of
m-oxo MnPc cannot be completely ruled out as stated
G
ibf
Q
Bare
above. Upon reduction at potentials of couple III, a blue shift in the
Q-band was observed from 729 nm to a weak peak at w 685 nm
An ion barrier factor of one indicates that there are no pinholes in
the SAM and that it is an ideal barrier to ion and solvent perme-
ability [25,26].
III
II
which is typical of Mn reduction to Mn [15], and a stronger
m-oxo
MnPc peak was observed at 648 nm. The -oxo MnPc would be
m
a result of the presence of residual oxygen in solution, even after
Fig. 5(a) shows the cyclic voltammograms of SAMs of complexes
1 and 2 (labeled 1 and 2 respectively) and unmodified gold elec-
trode (labeled 3) recorded in 1 M Na SO in pH 4 buffer solution.
deoxygenating with argon. Concurrently, a charge transfer band at
III
5
41 nm also increased in intensity. Upon regeneration of the Mn
2
4
Table 1
Redox potentials (V vs. AgjAgCl) for MnPc derivatives.
Complexa
Solvent/Electrolytec
Mn Pc /Mn Pc
II ꢀ3
II ꢀ4
Mn Pc /Mn Pc
II ꢀ2
II ꢀ3
Mn Pc /Mn Pc
II ꢀ2 ꢀ2
I
MnIIIPc /Mn Pc
ꢀ2
II ꢀ2
Ref.
1
DMF/TBABF
4
ꢀ1.15
ꢀ0.69
0.023
This work
ꢀ
0.60b
b
-(OH)MnTMPyPc (3
b
)
DMF/TBABF
DMF/TBABF
4
ꢀ0.71
ꢀ0.057
ꢀ0.038
8
c
)b
2
4
ꢀ0.64
ꢀ0.31(E
0.38
p
This work
ꢀ
O/TBAPd
ꢀ0.52
ꢀ0.76
c
ꢀ0.41
0.053
ꢀ0.063
ꢀ0.051
ꢀ0.056
This work
2
b
a
a
H
2
-(OH)Mn-Q-TMPyPc (4
-(OH)MnTMPyPc (3
-(OH)Mn-Q-TMPyPc (4
b
)
)
DMF/TBABF
DMF/TBABF
DMF/TBABF
4
4
4
ꢀ0.56
8
8
8
a
)
a
ꢀ0.59
a
TMPyPc ¼ tetra-(2-mercaptopyridine)phthalocyanine, Q-TMPyPc ¼ quaternized tetra-(2-mercaptopyridine)phthalocyanine;
b
¼ peripheral substitution,
a
¼ non-periph-
eral substitution.
b
Shoulder.
c
I
ꢀ2
I
ꢀ3
Tentatively assigned to Mn Pc /Mn Pc .
d
TBAP ¼ tetrabutyl ammonium perchlorate.

116
I. Booysen et al. / Dyes and Pigments 89 (2011) 111e119
The peaks observed on the un-modified gold electrode (curve 3) are
due to the gold oxide redox reaction. Upon modification, the peaks
disappear completely for both the complexes (curves 1 and 2). From
Fig. 5(a), ion barrier factors of 0.9955 and 0.9952 for 1 and 2
respectively were obtained (using Eq. (6)), Table 2. These results
show that only w1 % of the gold surface is not covered by the
complex SAMs and that the SAMs are compact and virtually defect
free. Furthermore, the obtained values compare favourably to those
of similarly structured molecules that have recently been published
in the literature [27,28].
3.4.2. Interfacial capacitance (C
s
)
A defined potential window of a cyclic voltammogram, in which
no peaks are observed for both unmodified and SAM modified gold
electrodes, will have a charging current (ich), whose value may be
s
used to calculate the interfacial capacitance (C ) using Eq. (7):
ch
C
s
¼ i
(7)
is the scan rate (V sꢀ1) and A is the electrode surface area
n
A
where
n
2
(cm ).
The lower the C
s
value, the fewer are the defects present in the
SAM and so the less permeable is the electrode surface to electro-
lyte ions [26,29]. A SAM modified gold electrode should therefore
display a lower C
s
value than an unmodified gold electrode.
values which were found to
F cm for 1 and 2 respectively (using Eq. (7)),
values are lower than for an un-modified gold elec-
Fig. 5(a) was used to estimate the C
s
ꢀ
2
be of 391 and 577
Table 2. The C
trode (C
¼ 1500
is attributed to closely packed, relatively defect free SAM layers.
m
s
ꢀ
2
s
mF cm ). The decrease in C on formation of SAMs
s
3.4.3. Underpotential deposition (UPD) of copper
Fig. 5(b) shows howthe SAMs of complexes 1 and 2 (curve 1 and 2
respectively) inhibit the Faradic process of Cu metal deposition onto
a gold electrode relative to an unmodified gold electrode (curve 3).
The bulk deposition of copper began at approximately þ0.1 V vs.
AgjAgCl during the negative going scan of the un-modified gold
electrode, Fig. 5(b). A large underpotential deposition (UPD) strip-
ping peak for the Cu metal is observed between 0 and 0.15 V on the
return scan (Fig. 5(b), curve 3). This peak has a characteristic shape
and occurs in a similar potential window to that previously pub-
lished [29,30]. Negligible current responses are observed on the
cyclic voltammogram of complex 1 SAM (curve 1) relative to that of
the un-modified gold electrode (curve 3), Fig. 5(b). Lower intensity
peaks were observed for 1 relative to 2, Fig. 5(b). Therefore, from
Fig. 5(b), the SAM of 1 covers the gold surface better than 2 because
the gold surface is no longer fully accessible to the Cu solution and
hence the redox reaction is not clearly observed for 1. Complex 2 is
less effective in covering the gold electrode.
III
II
Fig. 5. Cyclic voltammograms (1st scans) of SAM complexes 1 and 2 (labeled 1 and 2
respectively) and unmodified gold electrode (labeled 3) recorded in: (a) 1 M Na SO in
pH buffer solution; (b) 1 mM CuSO in pH buffer solution (c) 1 mM Fe
NH (SO $6H O in 1 M HClO
solution. Scan rate ¼ 100 mV s
3
.4.4. Inhibition of Fe /Fe redox processes
2
4
Fig. 5(c) shows the cyclic voltammograms of an unmodified gold
electrode (curve 3) and gold electrodes modified with 1 and 2 SAMs
curves 1 and 2 respectively) recorded in [Fe(H
in 1 M HClO solution. Fe(NH )SO in HClO
analyzing electrolyte because its redox couple [Fe(H
4
4
4
ꢀ
1
(
4
)
2
4
)
2
2
4
.
3
þ 2þ
2 6
/[Fe(H O) ]
(
2
O)
6
]
4
4
4
4
was used as the
3
þ
/[Fe
2
O)
6
]
2
þ
2 6
(H O) ]
has a lower electron transfer rate constant than [Fe
Table 2
Characterization parameters of SAMs.
Parameter
1
2
3
b
4b
3a
4a
Ion Barrier Factor, Gibf
0.9955
391
2.40 ꢃ 10
0.9952
577
1.14 ꢃ 10
0.9997
452
1.73 ꢃ 10
0.9994
539
2.77 ꢃ 10
0.9998
303
2.80 ꢃ 10
0.9996
302
2.67 ꢃ 10
s
(m
F cmꢀ2)
Interfacial Capacitance, C
Surface Coverage, (mol cm
ꢀ
2
ꢀ10
ꢀ10
ꢀ10
ꢀ10
ꢀ10
ꢀ10
G
)

I. Booysen et al. / Dyes and Pigments 89 (2011) 111e119
117
ꢀ
]3 /[Fe(CN)
4ꢀ
(
CN)
6
6
]
and hence mass transport does not determine
Table 3
Electrochemical parameters for nitrite determination.
the reaction rate, even at small overpotentials [31]. The Faradic
3
þ
2þ
6
couple [Fe(H
2
O)
6
]
/[Fe(H
2
O)
]
occurring in Fig. 5(c) in the ꢀ0.1 to
MPc LOD (M)
Ep (V) vs. Tafel Slope
a
nt
Sensitivity
(A M cm
Ref.
AgjAgCl (mV decadeꢀ1)
ꢀ1
ꢀ2
)
þ0.4 V region on the un-modified gold electrode (curve 3) is largely
2.94 ꢃ 10ꢀ 0.74
7
134
0.56 1.78 0.28
This
inhibited for both of the SAM modified gold electrodes. Complex 1
1
2
work
This
work
27
27
27
(
curve 1) SAM displayed the best inhibition of the redox processes
1.97 ꢃ 10ꢀ 0.74
7
174
0.66 1.81 0.42
showing that this film is more compact and acts as better barrier to
3
þ
2þ
6
O) ]
the transport of [Fe(H
2
O)
6
]
/[Fe(H
2
ions relative to 2.
ꢀ7
3
4
3
4
b
b
a
a
2.69 ꢃ 10
2.72 ꢃ 10
3.02 ꢃ 10
0.72
0.74
0.74
157
167
152
94
0.62 2.06 0.31
0.65 1.99 0.46
0.61 2.01 0.27
0.37 2.00 0.37
ꢀ
ꢀ
7
7
7
3.4.5. Surface coverage (G)
1.78 ꢃ 10ꢀ 0.76
27
A linear plot of background corrected peak current vs. the scan
rate for the peaks of adsorbed SAM were obtained (not shown) and
was used to calculate surface coverage (G) using Eq. (8) [25]:
nitrite detection potentials for the MnPc SAMs compare well with
the literature, Table 3 [27,35,36].
2
2
n F A
G
ðvÞ
Linear relationships between the peak current and the square
root of the scan rate were obtained for complexes 1 and 2 SAMs
indicating diffusion controlled electrocatalytic oxidation of nitrite.
The Tafel slopes were determined using the standard equation
(Eq. 9) for a totally irreversible process [37]:
i
p
¼
(8)
4RT
where i
is the Faraday constant (F ¼ 96,485 C mol ), A is the real surface
p
is the peak current (amps), n is the number of electrons, F
ꢀ
1
2
ꢀ1
area of the electrode (0.021 cm ),
n is the scan rate (V s ), R is the
ꢀ
1
ꢀ1
gas constant (R ¼ 8.314 J K mol ), and T is the temperature in
ꢀ10
2:3RT
Kelvin. The G values of the MnPc SAMs, Table 2, are 2.40 ꢃ 10
and
E
p
¼
ÞnFlog
y
þ K
(9)
ꢀ10
ꢀ2
2ð1 ꢀ
a
1.14 ꢃ10
mol cm for 1 and 2 respectively. The G value for 2 the
ꢀ
10
ꢀ2
value is within the range (1 ꢃ10
mol cm ) reported for adsor-
where
a
is the transfer coefficient,
n
is the scan rate, n is the number
bed monolayers of other metallophthalocyanines lying flat on the
electrode [32e34]. The value for 1 is higher than reported for
molecules lying flat on the electrode, hence may suggest a different
of electrons involved in the rate determining step, and K is
a constant. Tafel slopes were obtained from plots of log scan rate
versus potential, Fig. 7a (for 1 as an example). Tafel slopes were 134
orientation on the electrode. This also applies to complexes 3b, 4b,
3
a and 4a in Table 2.
3.5. Detection of nitrite on SAMs
Nitrite was employed in this work to probe the effectiveness of
SAMs in electrocatalysis. Fig. 6 shows the cyclic voltammograms of
unmodified (curve 1) and SAMs of complexes 1 (curve 2) and 2
(
curve 3) recorded in 1 mM nitrite (in pH 7 buffer solution). The
disproportionation of nitrite to NO is insignificant at the pH used,
thus the observed responses are for the oxidation of nitrite, not
nitric oxide. A weak peak was observed for nitrite on an unmodified
gold electrode (Fig. 6, curve 1). Nitrite oxidation peaks were
observed at 0.74 V vs. AgjAgCl for SAMs of the MnPc complexes,
Table 3. Nitrite oxidation occurs on the un-modified gold electrode
at approximately 0.77 V vs. AgjAgCl, but the peak current is
enhanced upon modification with all of the SAMs. The values of the
Fig. 6. Cyclic voltammograms of: unmodified Au electrode (1) and SAMs of complexes
p
Fig. 7. (a) plot of E vs. log y for 1 mM nitrite in pH 7 buffer solution on SAM of complex
1
(2) and
2
(3) recorded in 1 mM nitrite (in pH
.
7
buffer solution). Scan
1. (b) Plot of the peak current vs. nitrite (in pH 7 buffer solution) concentration for
SAMs of complexes 1 and 2.
ꢀ
1
rate ¼ 100 mV s

118
I. Booysen et al. / Dyes and Pigments 89 (2011) 111e119
and 174 mV decadeꢀ1 for 1 and 2 respectively, suggesting that the
first one electron transfer is rate determining. These values are
higher than the normal 30 to 120 values indicating either chemical
reactions coupled to electrochemical steps or interaction between
nitrite and catalyst [34,38], Table 3.
carrying pendant bulky units. Journal of Porphyrins and Phthalocyanines
009;1(3):669e80.
4] Ozoemena KI, Stefan-van Staden RI, Nyokong T. Metallophthalocyanine based
2
[
[
0
0
carbon paste electrodes for the determination of 2 3 -dideoxyinosine. Elec-
troanalysis 2009;21:1651e4.
5] Koc I, Camur M, Bulut M, Özkaya AR. Electrocatalytic performances of carbon
supported metallophthalocyanine and Pt/metallophthalocyanine catalysts
towards dioxygen reduction in acidic medium. Catalysis Letters
2009;131:370e80.
There is a greater probability of forming products in the reaction
transition state by both SAMs of this work because they had
[
6] Koç I, Ozer M, Ozkaya AR, Bekaro gꢀ lu O. Electrocatalytic activity, methanol
a
values greater than 0.5, Table 3. Again, since most of the Tafel
tolerance and stability of perfluoroalkyl-substituted mononuclear, and ball-
type dinuclear cobalt phthalocyanines for oxygen reduction in acidic medium.
Dalton Transactions; 2009:6368e76.
slopes are large and do not fall within the usual 30 to
ꢀ
1
1
20 mV decade region, the
The total number of electrons (n
catalytic oxidation of nitrite by the SAMs was calculated using Eq.
10), valid for a totally irreversible electrode process [37]:
a
values are tentative.
[
7] Nyokong T, Isago H. The renaissance in optical spectroscopy of phthalocya-
nines and other tetraazaporphyrins. Journal of Porphyrins and Phthalocya-
nines 2004;8:1083e90.
[8] Sehlotho N, Durmu s¸ M, Ahsen V, Nyokong T. The synthesis and electro-
chemical behaviour of water soluble manganese phthalocyanines: anion
radical versus Mn(I) species. Inorganic Chemistry Communications
t
) involved in the electro-
(
1
1 1
D2y2
5
2
i
p
¼ 2:99 ꢃ 10 n
t
½ð1 ꢀ
a
Þnꢄ AC
o
(10)
2008;11:479e83.
[
9] Zhu J, Gu F, Zhag J. Preparation and photovoltaic properties of near-infrared
absorbing manganese(II) phthalocyanine polymer films. Materials Letters
2007;61:1296e8.
2
where A is the area of the electrode (cm ), C
o
is the concentration of
ꢀ
3
nitrite (mol cm ), and D is the diffusion coefficient of nitrite
D ¼ 2.1 ꢃ10 cm2 s
ꢀ5
ꢀ1
[10] Posiuk-Bronikoswska W, Krajewska M, Pfis-Kabulska I. Transformations of
[39,40]). The total number of electrons
manganesetetrasulphophthalocyanine in oxidative conditions. Polyhedron
transferred was calculated to be 1.78 and 1.81 for complexes 1 and 2
SAMs respectively, Table 3. The mechanism with two electrons
transferred will be the same as reported before [41].
1998;18:561e70.
[11] Knecht S, Dürr K, Schmid G, Subramanian LR, Hanack M. Synthesis and
properties of soluble phthalocyaninatomanganese(III) Complexes. Journal of
Porphyrins and Phthalocyanines 1999;3:292e8.
12] Rittenberg DK, Baarrs-Hibbe L, Böhm A, Miller JS. Manganese(II) octabutox-
ynaphthalocyanine and its ferrimagnetic electron-transfer salt with TCNE.
Journal of Materials Chemistry 2000;10:241e4.
13] Mbambisa G, Tau P, Antunes E, Nyokong T. Synthesis and electrochemical
properties of purple manganese(III) and red titanium(IV) phthalocyanine
complexes octa-substituted at non-peripheral positions with pentylthio
groups. Polyhedron 2007;26:5355e64.
[14] Leznoff CC, Black LS, Heibert A, Causey PW, Christendat D, Lever ABP. Red
manganese phthalocyanines from highly hindered hexadecaalkoxyph-
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Fig. 7b shows that there is a linear relationship between the peak
[
ꢀ
4
ꢀ2
currentandthenitriteionconcentration(10 e10 Mrange)forthe
SAMs of this work making the electrodes useful for analyses of nitrite
concentrations within the shown range. The sensitiveness of the
[
ꢀ
1
ꢀ2
ꢀ1
ꢀ2
complexes 1 and 2 SAMs were 0.28 A M cm and 0.42 A M cm
ꢀ7
respectively, Table 3. Detection limits of 2.95 ꢃ10
M
and
ꢀ7
1
.97 ꢃ 10 M (3
s criterion) for complexes 1 and 2 SAMs, respec-
tively, were observed. These values are comparable to those of the
corresponding tetrasubstituted derivatives in Table 3. The low
detection suggests that the SAMs have good potential for use as
nitrite sensors. Both SAMs showed good short term stability as there
was less than 10% decay in current output after 35 continuous cyclic
voltammetry recordings.
[
[
4
. Conclusion
[
MnPc complexes substituted with thio groups (2,3-octakis-(2-
[
19] Obirai J, Nyokong T. Synthesis, spectral and electrochemical characterization
of mercaptopyrimidine-substituted cobalt, manganese and Zn (II) phthalo-
cyanine complexes. Electrochimica Acta 2005;50:3296e304.
20] Lever ABP, Wilshire JP, Quan SK. Oxidation of manganese(II) phthalocyanine
by molecular oxygen. Inorganic Chemistry 1981;20:761e8.
[21] Janczak J, Kubiak R, Sled ꢁz M, Borrmann H, Grin Y. Synthesis, structural
mercaptopyridine) phthalocyaninato manganese (III) acetate (1)
and 2,3-octakis-[(N-methyl-2-mercaptopyridine) phthalocyaninato
manganese (III) acetate] sulphate (2)) were synthesized and char-
acterized via spectroscopic and electrochemical methods. The new
complexes were used to form self assembled monolayers (SAMs).
Both MPc SAMs were successfully used as electrochemical sensors
of nitrite.
[
investigations and magnetic properties of dipyridinated manganese phtha-
locyanine, MnPc(py)
2
. Polyhedron 2003;22:2689e97.
[
22] Nyokong T. Electronic spectral and electrochemical behavior of near infrared
absorbing metallophthalocyanines. Structure and Bonding 2010;135:45e87.
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cyanines: properties and applications, vol. 3. New York: VCH Publishers; 1993.
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[
Acknowledgements
[24] Nombona N, Tau P, Sehlotho N, Nyokong T. Electrochemical and electro-
catalytic properties of
nines. Electrochimica Acta 2008;53:3139e48.
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3. New York: Marcel Dekker; 1996.
26] Pilloud DL, Chen X, Dutton PL, Moser CC. Electrochemistry of self-assembled
monolayers of iron protoporphyrin IX attached to modified gold electrodes
a-substituted manganese and titanium phthalocya-
This work was supported by the Department of Science and
Technology (DST) and National Research Foundation (NRF) of South
Africa through DST/NRF South African Research Chairs Initiative for
Professor of Medicinal Chemistry and Nanotechnology and Rhodes
University and Scientific Research Project of Gebze Institute of
Technology (BAP-2007-A-01). FM thanks Rhodes University for
a graduate bursary.
[
[
through
thioether
linkage.
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of
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