Synthesis and electrochemical investigation of phthalocyanines with dendritic bulky ethereal substituents

Article abstract of DOI:10.1016/j.poly.2012.05.025

Metal-free and metallophthalocyanines carrying four dendritic 3,4-bis(benzyloxy)benzyloxy groups on the periphery were prepared by cyclotetramerization of a new precursor, namely 4-[3,4-bis(benzyloxy)benzyloxy] phthalonitrile (2) in the presence of the corresponding divalent metal salts (Zn(II), Co(II)). The novel phthalonitrile derivative 2 was synthesized by the reaction of 4-nitrophthalonitrile and 3,4-dibenzyloxybenzyl alcohol (1), which was prepared by the catalytic reduction of 3,4-dibenzyloxybenzaldehyde with NaBH₄ in ethanol. The new compounds were characterized by elemental analysis, together with FT-IR, ¹H NMR and UV-Vis spectra. The electrochemical and in situ spectroelectrochemical measurements showed that while the cobalt phthalocyanine complex (5) gives both metal-based and ring-based redox processes, the metal-free and zinc phthalocyanine complexes (3 and 4) give only ring-based electron transfer processes. An in situ electrocolorimetric method was applied to investigate the colour of the electrogenerated anionic and cationic forms of the novel phthalocyanine complexes carrying bulky ethereal aryloxy substituents.

Full text of DOI:10.1016/j.poly.2012.05.025

Polyhedron 42 (2012) 227–235
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Polyhedron
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Synthesis and electrochemical investigation of phthalocyanines with
dendritic bulky ethereal substituents
Sßennur Özçelik a, Atıf Koca b, Ahmet Gül a,
⇑
a Department of Chemistry, Technical University of Istanbul, Maslak, Istanbul 34469, Turkey
b Department of Chemical Engineering, Faculty of Engineering, Marmara University, Göztepe, Istanbul 34722, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Metal-free and metallophthalocyanines carrying four dendritic 3,4-bis(benzyloxy)benzyloxy groups on
the periphery were prepared by cyclotetramerization of a new precursor, namely 4-[3,4-bis(benzyl-
oxy)benzyloxy]phthalonitrile (2) in the presence of the corresponding divalent metal salts (Zn(II), Co(II)).
The novel phthalonitrile derivative 2 was synthesized by the reaction of 4-nitrophthalonitrile and 3,4-
dibenzyloxybenzyl alcohol (1), which was prepared by the catalytic reduction of 3,4-dibenzyloxybenzal-
dehyde with NaBH4 in ethanol. The new compounds were characterized by elemental analysis, together
with FT-IR, 1H NMR and UV–Vis spectra. The electrochemical and in situ spectroelectrochemical measure-
ments showed that while the cobalt phthalocyanine complex (5) gives both metal-based and ring-based
redox processes, the metal-free and zinc phthalocyanine complexes (3 and 4) give only ring-based elec-
tron transfer processes. An in situ electrocolorimetric method was applied to investigate the colour of the
electrogenerated anionic and cationic forms of the novel phthalocyanine complexes carrying bulky ethe-
real aryloxy substituents.
Received 30 March 2012
Accepted 17 May 2012
Available online 29 May 2012
Keywords:
Phthalocyanine
Electrochemistry
Spectroelectrochemistry
Benzyloxy groups
Synthesis
Dendritic ethereal groups
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
allowing fine tuning of the physical properties in order to meet
specific qualities, suitable for their applications [16].
Phthalocyanines (Pcs) are planar aromatic macrocycles consist-
ing of four isoindole units. These units combine to form an 18
electron aromatic system where the -electrons are delocalized
Due to strong intermolecular interactions between the macro-
cycles, most of the peripherally unsubstituted phthalocyanines
are insoluble in common organic solvents and in aqueous media;
this property minimizes their applications and causes problems
in the synthesis and purification steps. However the appropriate
substitution pattern (peripheral and/or axial) helps to overcome
the solubility problem [17,18]. Adding different kinds of substitu-
ents, such as crown ethers, bulky or long chain alkyl, alkylthio or
alkoxy groups, at the peripheral/non-peripheral positions of the
phthalocyanine ring increases their solubility [19–22]. The change
in symmetry due to the presence of substituents in different posi-
tions is also important for the solubility of the phthalocyanines
[23]. Bulky dendritic wedges attached to the periphery of a phtha-
locyanine significantly decreases self-association of these chro-
mophores in solution and in the condensed state [24].
p
-
p
over an arrangement of alternating carbon and nitrogen atoms
[1]. As a consequence of their dark green–blue colour, phthalocya-
nines are an important class of chemicals for commercial use in
inks, dyestuff for textiles and colorants for metals and plastics
[2]. In addition to this traditional application, some recent focuses
have been put on their application as optical recording materials
[3,4], semi-conductors for organic field effect transistors [5–7], cat-
alysts for oxidative degradation of pollutants [8,9] and sensitizers
for photovoltaics [10,11] and photodynamic therapy [12–14].
The extensive research interest in phthalocyanines as advanced
materials stems from their remarkable properties that include flex-
ibility, chemical and thermal stability, semi-conductivity and
photoconductivity [1,15]. Another remarkable feature is their ver-
satility; the two hydrogen atoms of the central cavity can be re-
placed by more than 70 metallic or even some non-metallic ions
and a variety of substituents can be incorporated, both at the
periphery of the macrocycle and/or at the axial positions, thus
The present study concerns the preparation and characteriza-
tion of metal-free and metallophthalocyanines [M = Co(II), Zn(II)]
bearing four dendritic 3,4-bis(benzyloxy)benzyloxy groups on the
periphery. As the functions of metallophthalocyanine derivatives
are mostly based on their electron transfer reactions, because of
the p-electron-conjugated ring system, it is necessary to examine
the electron transfer behaviour of newly synthesized phthalocya-
nine complexes in solution [16]. Additionally, an electrochemical
reaction also involves various processes, such as adsorption of
⇑
Corresponding author. Tel.: +90 212 285 68 27; fax: +90 212 285 63 86.
E-mail address: ahmetg@itu.edu.tr (A. Gül).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.05.025

228
Sß. Özçelik et al. / Polyhedron 42 (2012) 227–235
electro-active species on the electrode, aggregation and electro-
polymerization of the species and the contribution of coupled
chemical reactions. Our previous papers on the electrochemistry
of MPcs showed that the metal centre and the substituents on
the periphery play an important role on the redox properties of
MPcs and their possible application in the field of the electrochem-
ical technologies [25–32]. Therefore, in the present paper, we have
also reported on the voltammetric, in situ spectroelectrochemical
and in situ electrocolorimetric responses of newly synthesized me-
tal-free, Zn(II) and Co(II) phthalocyanines bearing bulky ethereal
aryloxy substituents. The results of this study will be essential
for opening a new way to the possible technological applications
of these novel MPcs, especially in various electrochemical technol-
ogies such as electrosensor, electrochromic and electrocatalytic
applications.
chemical cell. The standard illuminant A with a 2-degree observer
at constant temperature in a light booth designed to exclude exter-
nal light was used. Prior to each set of measurements, background
colour coordinates (x, y and z values) were taken at open-circuit,
using the electrolyte solution without the MPc under study. During
the measurements, readings were taken as a function of time un-
der kinetic control, however the colour coordinates at the begin-
ning and end of each redox processes were also reported.
2.3. Synthesis
2.3.1. Synthesis of 3,4-dibenzyloxybenzyl alcohol (1)
To
a
solution of 3,4-dibenzyloxybenzaldehyde (3.18 g,
10.0 mmol) in 40 mL of ethanol at 0 °C, (NH4)2CO3 (0.96 g,
10.0 mmol) and NaBH4 (0.38 g 10.0 mmol) were added. The mix-
ture was stirred vigorously at 0 °C until the reaction was com-
pleted, based on TLC monitoring. After completion of the
reaction, the reaction mixture was warmed up to room tempera-
ture and filtered through a pad of Celite. The solvent was evapo-
rated under reduced pressure. The crude product was dissolved
in 25 mL of THF. The reaction mixture was filtered in order to re-
move the insoluble by products. After removal of the solvent, a
white solid product was obtained. Yield: 2.59 g (81%); M.p.:
2. Experimental
2.1. Chemicals and instrumentation
All reagents and solvents were of reagent grade quality, ob-
tained from commercial suppliers. The solvents were stored over
molecular sieves (4 Å). 4-Nitrophthalonitrile was synthesized as
reported in the literature [33]. 3,4-Dibenzyloxybenzaldehyde was
used as supplied commercially. The progress of the reactions was
monitored by TLC (SiO2). IR spectra were recorded on a Perkin El-
mer Spectrum One FTIR (ATR sampling accessory) spectrophotom-
eter, electronic spectra in the UV–Vis region were recorded with a
Scinco S-3100 single beam UV/Vis PDA spectrophotometer. 1H
NMR spectra were recorded on a Varian UNITY INOVA 500 MHz
spectrophotometer using TMS as an internal reference. Elemental
analyses were performed on a Thermo Flash EA 1112.
68 °C. FTIR
m
max/cmꢀ1: 3282 (broad, O–H), 3063–3034 (Ar-H),
2918–2871 (Aliph-H), 1226–1216 (C–O–C); 1H NMR (CDCl3) d,
ppm: 7.48–7.32 (m, 10H, Ar-H), 7.09 (s, 1H, Ar-H), 6.93–6.90 (d,
1H, Ar-H), 6.85–6.82 (d, 1H, Ar-H), 5.15 (s, 4H, O–CH2), 4.49 (s,
2H, O–CH2), 2.84 (s, 1H, OH); Anal. Calc. for C21H20O3 (320.38 g/
mol): C, 78.73; H, 6.29. Found: C, 78.62; H, 6.21%.
2.3.2. Synthesis of 4-[3,4-bis(benzyloxy)benzyloxy]phthalonitrile (2)
3,4-Dibenzyloxybenzyl alcohol (1.85 g, 5.78 mmol) and 4-
nitrophthalonitrile (1.00 g, 5.78 mmol) was dissolved in dry DMF
(15 mL) under nitrogen. Finely ground anhydrous potassium car-
bonate (1.20 g, 8.67 mmol) was added portion-wise over 2 h. The
reaction mixture was stirred vigorously at room temperature un-
der nitrogen for 48 h, then the mixture was poured into a stirred
mixture of ice water (150 mL). The aqueous phase was extracted
with chloroform (3 ꢁ 15 ml). The combined extracts were dried
over MgSO4, and the solvent was removed at reduced pressure.
Purification of the product was achieved by column chromatogra-
phy with silica gel using chloroform as the eluent. Yield: 1.76 g
2.2. Electrochemical in situ spectroelectrochemical and in situ
electrocolorimetric measurements
The electrochemical and spectroelectrochemical measurements
were carried out with a Gamry Reference 600 potentiostat/galva-
nostat utilizing a three-electrode cell configuration at 25 °C. For
cyclic voltammetry (CV) and square wave voltammetry (SWV)
measurements, the working electrode was a Pt disc with a surface
area of 0.071 cm2. The surface of the working electrode was pol-
ished with a diamond suspension before each run. A Pt wire served
as the counter electrode. A saturated calomel electrode (SCE) was
employed as the reference electrode and separated from bulk of
the solution by a double bridge. Ferrocene was used as an internal
reference. Tetrabutylammonium perchlorate (TBAP) in dimethyl-
sulfoxide (DMSO) and/or dichloromethane (DCM) was employed
(68%); M.p.: 78 °C. FTIR
m
max/cmꢀ1: 3090–3034 (Ar-H), 2946–
2863 (Aliph-H), 2228 (C„N), 1263–1251 (C–O–C); 1H NMR (CDCl3)
d, ppm: 7.66–7.63 (d, 1H, Ar-H), 7.44–7.24 (m, 12H, Ar-H), 7.17–
7.12 (s, 1H, Ar-H), 6.95–6.86 (q, 2H, Ar-H), 5.16 (s, 4H, O–CH2),
5.01 (s, 2H, O–CH2); Anal. Calc. for C29H22N2O3 (446.50 g/mol): C,
78.01; H, 4.97; N, 6.27. Found: C, 78.07; H, 4.89; N, 6.34%.
as the supporting electrolyte at a concentration of 0.10 mol dmꢀ3
.
2.3.3. Synthesis of 2,9(10),16(17),23(24)-tetrakis-[3,4-
High purity N2 was used to remove dissolved O2 for at least
15 min prior to each run and to maintain a nitrogen blanket during
the measurements. IR compensation was applied to the CV and
SWV scans to minimize the potential control error. UV/Vis absorp-
tion spectra and chromaticity diagrams were measured by an
OceanOptics QE65000 diode array spectrophotometer. In situ spec-
troelectrochemical measurements were carried out utilizing a
three-electrode configuration of a thin-layer quartz spectroelectro-
chemical cell at 25 °C. The working electrode was a Pt semi-trans-
parent electrode. A Pt wire counter electrode separated by a glass
bridge and a SCE reference electrode separated from bulk of the
solution by a double bridge were used.
bis(benzyloxy)benzyloxy]-phthalocyanine (3)
A mixture of 0.5 mmol (0.223 g) 4-[3,4-bis(benzyloxy)benzyl-
oxy]phthalonitrile and an excess amount of lithium metal
(0.035 g, 5 mmol) in 2 mL of pentanol was heated and stirred at
140 °C for 3 h under N2 in a sealed tube. The reaction mixture
was cooled to ambient temperature and poured into 100 mL of
ice-water mixture with the addition of 0.1 N HCl (1 mL). The
resulting green precipitate was filtered off, washed with water,
methanol and hexane, and then dried in vacuo. Further purification
was accomplished by precipitation with methanol from its solution
in THF and by column chromatography on silica gel using chloro-
form as the eluent. Yield: 0.038 g (17%); M.p.: >200 °C. FTIR
m
In situ electrocolorimetric measurements under potentiostatic
control were obtained using an OceanOptics QE65000 diode array
spectrophotometer in the colour measurement mode by utilizing a
three-electrode configuration of a thin-layer quartz spectroelectro-
max/cmꢀ1: 3290 (N–H), 3086–3030 (Ar-H), 2947–2865 (Aliph-H),
1258–1226 (C–O–C); 1H NMR (CDCl3) d, ppm: 7.76–7.16 (m, 52H,
Ar-H), 6.98 (d, 8H, Ar-H), 6.72 (s, 4H, Ar-H), 5.02 (s, 24H, O–CH2),
ꢀ4.32 (br s, N–H); UV–Vis (THF): k max/nm (log
e
, L molꢀ1 cmꢀ1):

Sß. Özçelik et al. / Polyhedron 42 (2012) 227–235
229
705 (4.76), 668 (4.72), 342 (4.69), 285 (4.57); Anal. Calc. for
116H90N8O12 (1788.00 g/mol): C, 77.92; H, 5.07; N, 6.27. Found:
C
C, 77.87; H, 5.02; N, 6.21%.
2.3.4. Synthesis of 2,9(10),16(17),23(24)-tetrakis-[3,4-
bis(benzyloxy)benzyloxy]-phthalocyaninatozinc (II) (4)
A mixture of 0.5 mmol (0.223 g) phthalonitrile derivative (2),
0.125 mmol (0.023 g) of anhydrous Zn(CH3COO)2 and a catalytic
amount of (8 lL) 1.8-diazabicyclo [5.4.0] undec-7-ene (DBU) in
1.5 mL of anhydrous pentanol was stirred and heated at 140 °C
for 24 h under N2 in a sealed tube. The reaction temperature was
cooled to the room temperature and the crude product was precip-
itated by addition of methanol. After filtration, the resulting green
product was first washed with water, in order to remove any unre-
acted metal salt, and then with methanol and diethyl ether. Purifi-
cation of the product was achieved by column chromatography on
silica gel, first with methanol and then with THF/hexane (2:1) as
the eluent. Yield: 0.056 g (24%); M.p.: >200 °C. FTIR
m :
max/cmꢀ1
3088–3030 (Ar-H), 2944–2866 (Aliph-H), 1262–1229 (C–O–C);
1H NMR (CDCl3) d, ppm: 7.79–7.68 (d, 4H, Ar-H), 7.56–7.21 (m,
48H, Ar-H), 6.97–6.85 (d, 8H, Ar-H), 6.55 (s, 4H, Ar-H), 5.16 (s,
16H, O–CH2), 5.01 (s, 8H, O–CH2); UV–Vis (THF): k max/nm (log
L molꢀ1 cmꢀ1): 679 (5.21), 347 (4.65), 284 (4.39); Anal. Calc. for
116H88N8O12Zn (1851.37 g/mol): C, 75.25; H, 4.79; N, 6.05. Found:
e,
C
C, 75.31; H, 4.88; N, 6.12%.
2.3.5. Synthesis of 2,9(10),16(17),23(24)-tetrakis-[3,4-
bis(benzyloxy)benzyloxy]-phthalocyaninatocobalt (II) (5)
Scheme 1. Synthetic route for compounds 1–5. (i) NaBH4, (NH4)2CO3, ethanol, 0 °C,
(ii) K2CO3, DMF, room temperature, (iii) Metal salts, reflux, 1-pentanol (for
compounds 3 and 4), 2-(dimethylamino)ethanol (for compound 5).
A mixture of 2 (0.223 g, 0.5 mmol), anhydrous CoCl2 (0.016 g,
0.125 mmol) and dry 2-(dimethylamino) ethanol (2 mL) was
heated and stirred at 135 °C in a sealed glass tube for 24 h under
nitrogen. After cooling to room temperature, the blue-green reac-
tion mixture was poured into methanol (10 mL). The precipitate
was filtered and washed several times with water and methanol.
The resulting solid was precipitated from its THF solution by addi-
tion of hexane. The precipitate was filtered and washed with hex-
ane and diethyl ether, and then dried. Finally, the green precipitate
was chromatographed on silica gel using THF as the eluent. Yield:
0.049 g (21%); M.p.: >200 °C. FTIR
2948–2865 (Aliph-H), 1263–1227 (C–O–C); UV–Vis (THF): k max/
nm (log
, L molꢀ1 cmꢀ1): 666 (4.88), 328 (4.80), 282 (4.89); Anal.
m
max/cmꢀ1: 3086–3029 (Ar-H),
e
Calc. for C116H88N8O12Co (1844.99 g/mol): C, 75.52; H, 4.81; N,
6.07. Found: C, 75.63; H, 4.90; N, 6.14%.
3. Results and discussion
Fig. 1. UV spectra of H2Pc (3), ZnPc (4) and CoPc (5) (6 ꢁ 10ꢀ6 M in THF).
3.1. Synthesis and characterization
Zinc (II), cobalt (II) and metal-free phthalocyanines carrying
four bulky ethereal aryloxy substituents at peripheral positions
were prepared according to the route shown in Scheme 1. In the
first step, compound 1 was prepared according to a reported pro-
cedure with some modifications [34]. Compound 1 was obtained
from the reduction of 3,4-dibenzyloxy-benzaldehyde with sodium
borohydride in the presence of ammonium carbonate. As the yield
of the reduction reaction at room temperature was low, it was per-
formed at 0 °C in ethanol. After filtration of the reaction mixture
through a pad of Celite and removal the solvent under reduced
pressure, the crude product was dissolved in THF. The insoluble
by-products were removed by filtration. This simple, environmen-
tally safe and inexpensive method gave rise to compound 1 with a
relatively high yield (81%). The dinitrile derivative (2) was obtained
by the aromatic nucleophilic substitution reaction between 4-
nitrophthalonitrile, prepared by the method given in the literature
[33], and 3,4-dibenzyloxybenzylalcohol (1). The base of choice in
this reaction was K2CO3 and dry DMF was the preferred solvent.
A practical method to prepare metal-free phthalocyanine in-
volves the treatment of phthalonitrile derivatives with lithium alk-
oxide and the subsequent demetallization of the resulting lithium
phthalocyanine with a mineral acid. In this study, the metal-free
phthalocyanine 3 was prepared by the cyclotetramerization of
compound 2 using lithium metal dissolved in refluxing pentanol
and acidification with HCl solution [35]. The preparation of the zinc
(II) phthalocyanine complex (4) was accomplished by the reaction
of the phthalonitrile derivative 2 with zinc(II) acetate in the pres-
ence of 1,8-diazabicyclo[5.4.0]undec-7-ene in refluxing pentanol.
For the preparation of complex 5, the cyclotetramerization reaction
was carried out in 2-(dimethylamino)ethanol at 135 °C, and Co(II)
chloride was used as the metal salt.

230
Sß. Özçelik et al. / Polyhedron 42 (2012) 227–235
Table 1
Voltammetric data of the complexes.
d
Complex
Ring oxidation
MIII/MII
MII/MI
Ring reductions
DE1/2
a
H2Pc
E1/2 vs. SCE
0.80(0.60)
–
–
0.52
93
0.90
0.51
67
0.98
0.77
–
ꢀ0.86
90
0.95
ꢀ1.08
97
0.87
ꢀ1.62
120
0.90
ꢀ1.63
100
ꢀ1.20
110
0.91
ꢀ1.18
112
–
–
–
–
–
–
1.46
1.60
1.00
0.85
b
D
Ep (mV)
c
Ipa/Ipc
a
ZnPc
E1/2 vs. SCE
b
D
Ep (mV)
c
Ipa/Ipc
0.85
CoPc (in DCM)
CoPc (in DMSO)
E1/2 vs. SCE
1.46e
–
–
0.24
63
0.94
ꢀ0.49
80
0.95
ꢀ0.61
150
a
b
D
Ep (mV) vs. SCE
c
Ipa/Ipc
a
E1/2 vs. SCE
b
D
Ep (mV) vs. SCE
c
Ipa/Ipc
–
0.88
–
a
b
c
E1/2 = (Epa + Epc)/2 at 0.100 V sꢀ1
.
D
Ep = |Epa ꢀ Epc|.
Ipa/Ipc for reduction, Ipc/Ipa for oxidation processes at 0.100 V sꢀ1 scan rate.
d
e
D
E1/2 = E1/2 (first oxidation) ꢀ E1/2 (first reduction) = HOMO–LUMO gap for metallophthalocyanines having an electro-inactive metal centre.
The process is recorded with SWV.
Fig. 2. (a) CVs of H2Pc at various scan rates in the DCM/TBAP electrolyte system on
a Pt working electrode. (b) SWV of H2Pc, SWV parameters: pulse size = 100 mV;
step size: 5 mV; frequency: 25 Hz.
Fig. 3. (a) CVs of CoPc at various scan rates in the DCM/TBAP electrolyte system on
a Pt working electrode. (b) SWV of CoPc, SWV parameters: pulse size = 100 mV;
step size: 5 mV; frequency: 25 Hz.
The novel phthalocyanines 3–5 are highly soluble in THF,
dichloromethane, chloroform, DMF and DMSO. The enhancement
in solubility on tetra substitution is probably based on it being a
mixture of positional isomers rather than a single product and
the presence of bulky ethereal aryloxy groups causing substantial
disruption in the order of the parent phthalocyanine rings [36].
All the new compounds obtained in this study have been charac-
terized by spectroscopic techniques, such as 1H NMR, FT-IR, UV–
Vis and elemental analysis. The spectroscopic data of compounds
1–5 confirmed the proposed structures.
confirmed the proposed structure by the broad peak at 2.84 ppm,
assignable to the proton of the –OH group. The aliphatic protons
neighbouring to the –OH appeared at 4.49 ppm as a singlet and
the etheric protons came out at 5.15 ppm as a singlet, as expected.
The aromatic protons of the benzyloxy groups appeared at 7.48–
7.32 ppm as a multiplet. The aromatic protons of the benzyl group
were observed at 7.09, 6.93–6.90 and 6.85–6.82 ppm as a singlet,
doublet and doublet, respectively.
IR spectrum of 2 indicated CN groups by the presence of an in-
tense band at 2228 cmꢀ1 and etheric (Ar C–O–C) units at 1263–
1251 cmꢀ1. Moreover, the aromatic and aliphatic C–H peaks ap-
peared at 3090–3034 and 2946–2863 cmꢀ1, respectively. In the
1H NMR spectrum of the phthalonitrile derivative 2, the aromatic
protons were observed at 7.66–7.63, 7.44–7.24, 7.17–7.12 and
6.95–6.86 ppm as a doublet, multiplet, singlet and quartet, respec-
The IR spectrum of compound 1 clearly indicates the formation
of proposed alcohol structure by the broad –OH stretching band at
around 3282 cmꢀ1 and the disappearance of the intense CO
stretching band at 1675 cmꢀ1. The aromatic CH and the aliphatic
CH stretching bands appeared at 3063–3034 and 2918–
2871 cmꢀ1 respectively. The 1H NMR of compound
1 also

Sß. Özçelik et al. / Polyhedron 42 (2012) 227–235
231
3.2. Electrochemical measurements
Solution redox properties of the complexes were studied using
CV and SWV measurements in DCM and/or DMSO on a platinum
electrode. Table 1 lists the assignments of the redox couples and
the electrochemical parameters, including the half-wave peak
potentials (E1/2), anodic to cathodic peak potential separation
(
D
Ep), ratio of anodic to cathodic peak currents (Ipa/Ipc) and the dif-
ference between the first oxidation and reduction potentials ( E1/
2). The peak to peak separations, E1/2 and E1/2 values are in agree-
D
D
ment with the reported data for redox processes of metallopht-
halocyanine complexes [28–32,44–48].
Metal-free phthalocyanine compound (3) and the zinc phthalo-
cyanine compound (4) indicate similar redox reactions. When the
Pc ring centres of compound 3 are coordinated with zinc (4), the
redox potentials shift to negative potentials due to the different
effective nuclear charge of the Pc ring centres of the complexes.
Based on the well-known electrochemical behaviours of these
phthalocyanine complexes, all the redox couples of the complexes
are assigned to the phthalocyanine ring [28–32,44–48].
Fig. 2 shows the CV and SWV of the metal-free compound 3,
which indicates two one-electron reduction couples and a broad
split oxidation peak. For the reduction couples, the
DEp values
change from 60 to 110 mV with increasing scan rate, suggesting
electrochemical reversibility of the electron transfer process [49].
Theoretically, for a system that is both electrochemically and
Fig. 4. (a) CVs of CoPc at various scan rates in the DMSO/TBAP electrolyte system on
a Pt working electrode. (b) SWV of CoPc, SWV parameters: pulse size = 100 mV;
step size: 5 mV; frequency: 25 Hz.
chemically reversible,
rate. In practical applications, the
pared with that of the universal indicator ferrocene/ferrocenium
couple. In our system, the Ep values changed from 60 to
D
Ep should be 0.059/n, independent of scan
D
Ep value of a couple is com-
tively. The alkyl protons of the etheric groups appeared as a singlet
at 5.16 and 5.01 ppm. Phthalocyanines 3–5 have very similar IR
absorptions to compound 2; the major difference is the disappear-
ance of the C„N band at 2228 cmꢀ1, indicating the cyclotetramer-
ization of the dinitrile.
D
110 mV for ferrocene on increasing the scan rates from 0.010 to
1.00 V sꢀ1. The effect of coupled chemical reactions on the electron
transfer reactions is illustrated by the Ip,a/Ip,c ratio change as a func-
tion of the scan rate, and Ip changes as a function of the square root
The data obtained from the 1H NMR investigations of the met-
allophthalocyanines have provided the characteristic chemical
shifts for the structures, as expected. As tetra substituted phthalo-
cyanines obtained from 4-substituted phthalonitriles are naturally
a mixture of isomers, the 1H NMR spectra of phthalocyanines 3 and
4 are somewhat broader than the corresponding signals in the
dinitrile derivative 2 [37]. The aromatic protons of phthalocyanine
3 were observed at 7.76–6.72 ppm and those of phthalocyanine 4
appeared at 7.79–6.55 ppm. There is a slight difference between
the chemical shift values of the alkyl protons belonging to the
etheric groups for 3 (5.02 ppm) and 4 (5.16 and 5.01 ppm).
Moreover, the N–H protons of the metal-free phthalocyanine 3
were identified in the 1H NMR spectrum as a broad peak at
ꢀ4.32 ppm, presenting the typical shielding of the inner core pro-
tons, which is a common feature of 1H NMR spectra of metal-free
phthalocyanines [38,39].
of the scan rate ((Ipc
ꢀ
m
1/2). The Ip,a/Ip,c ratios of the reduction pro-
cesses are unity at all scan rates, which indicates a diffusion con-
trolled mass transfer of the species during the electrochemically
reversible reduction processes. Electrochemical analysis of the
complex does not give a well defined oxidation process due to
the aggregation of the complex in this electrolyte system. The oxi-
dation process split due to the presence of different aggregated
species. Spectroelectrochemical studies (given below) indicated
there is only one Pc based electron transfer process during the oxi-
dation process under applied potential at 1.00 V.
The cobalt phthalocyanine complex (5) undergoes a one-elec-
tron oxidation labelled as O1 at 0.51 V and two one-electron reduc-
tion couples labelled as R1 at ꢀ0.49 V and R2 at ꢀ1.62 V during the
CV measurements within the electrochemical window of TBAP/
DCM (Fig. 3). A second irreversible oxidation process was recorded
at 1.46 V with SWV. While the first oxidation couple is electro-
chemically and chemically reversible, the reduction couples have
Electronic spectra are especially important to establish the
structure of phthalocyanines. The UV–Vis spectra of the phthalocy-
anine core is dominated by two intense bands, the B band in the UV
region at about 300–400 nm, arising from the deeper
transition, and the Q band in the visible part of the spectrum
around 600–700 nm, attributable to the
p⁄ transitions from
p
levels – p⁄
quasi-reversible character with respect to the
DEp and Ip,a/Ip,c val-
ues. First-row transition metal Pcs differ from main-group metal
Pcs because the energy of the metal d-orbitals may be positioned
between the energies of the HOMO and LUMO of the phthalocya-
nine (Pc2ꢀ) ligand. The first oxidation and first reduction processes
occur on the metal centre in the MPc complexes only for Mn, Fe
and Co derivatives in polar solvents such as DMF and DMSO, while
the first reduction and second oxidation processes are metal-based
in non-polar solvents such as DCM [46]. Thus, the first reduction
and second oxidation processes of compound 5 could be assigned
easily to the CoII/CoI and CoII/CoIII redox couples respectively, and
the remaining processes to the phthalocyanine ring [46]. Assign-
ments of the redox couples according to the in situ spectroelectro-
chemical measurements are given in Table 1.
p–
the highest occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO) of the Pcꢀ2 ring [40–42].
The UV–Vis spectra of metallophthalocyanine complexes (4 and
5) in THF exhibited an intense single Q band absorption of the
p–
p⁄ transitions at 679 and 666 nm, respectively. Due to its D2h
symmetry, the metal-free phthalocyanine (3) gave a split Q-band
at 668 and 705 nm [36,43]. The B bands of these phthalocyanines
appeared in the UV region at around 328–347 nm. Additionally,
an intense absorption due to the
p–
p⁄ transition of the benzene
groups appeared for all the phthalocyanine derivatives 3–5 in the
UV region at about 285 nm (Fig. 1).

232
Sß. Özçelik et al. / Polyhedron 42 (2012) 227–235
Fig. 5. In situ UV–Vis spectral changes of H2Pc. (a) Eapp = ꢀ1.00 V. (b) Eapp = ꢀ1.40 V. (c) former Eapp = 1.00 V. (d) Chromaticity diagram of H2Pc; each symbol represents the
+1
colour of the electro-generated species, h, [H2Pcꢀ2]; s, [H2Pcꢀ3
]
ꢀ1; 4, [H2Pcꢀ4
] ] .
ꢀ2; q, [H2Pcꢀ1
To investigate the effect of solvent, the redox properties of co-
balt compound 5 were also investigated in the TBAP/DMSO electro-
lyte system. Two reductions (labelled R1 at ꢀ0.86 V and R2 at
ꢀ1.18 V) and two oxidation processes (labelled O1 at 0.74 V and
O2 at 1.41 V) are recorded with compound 5 within the potential
window of the DMSO/TBAP electrolyte system (Fig. 4). When com-
pared with the redox responses in DCM, all the redox peaks shift to
negative potentials due to the polarization effect of DMSO. Espe-
cially, the first oxidation process shifts to a more positive potential
due to the change of the electron transfer centre of the complex.
While CoPc gives a ring based redox process in DCM, it gives a me-
tal based oxidation process in DMSO due to the stabilization of the
CoIII oxidation state on coordination of DMSO to the CoIII metal
centre. These assignments of electron transfer processes were
proved with the in situ spectroelectrochemical measurements.
potentials are given in Fig. 5. As the metal-free phthalocyanines
have D2h symmetry, two sharp bands (split Q band) appeared at
670 and 704 nm [36,40–43]. When the working electrode is polar-
ized at ꢀ1.00 V, [H2Pc2ꢀ] is one-electron reduced to form the
[H2Pc3ꢀ 1ꢀ anion. During the reduction of [H2Pc2ꢀ] at constant po-
]
tential application, instead of the split Q-band, a sharp single band
increases at 677 nm. At the same time, B1 at 280 nm and B2 at
348 nm increase in intensity (Fig. 5a). During this process, well-de-
fined isosbestic points, recorded at 460, 590 and 708 nm, demon-
strate that the reduction proceeds clearly in deoxygenated DCM
to give a single, reduced species. Rollmann and Iwamoto [50] and
Koca et al. [29] also reported similar spectroscopic changes for
the one-electron reduction of H2Pc complexes. It was mentioned
that there might be a symmetry change from D2h to D4h because
of the substitution of hydrogens in the pc core by the cations of
supporting electrolyte. It was shown that the pair of excited states,
3.3. Spectroelectrochemical studies
which usually gives rise to Q-band doubling in [H2Pc3ꢀ 1ꢀ
]
species,
is still present but the states are too close to be resolved.
Spectroelectrochemical studies were employed in order to con-
firm the assignments in the CVs of the complexes. To represent the
in situ spectroelectrochemical response of the metal-free phthalo-
cyanines, spectral changes of the metal-free phthalocyanine com-
pound 3 during controlled potential application at the redox
During the potential application at ꢀ1.40 V, [H2Pc3ꢀ 1ꢀ
]
is one-
electron reduced to form the [H2Pc4ꢀ 2ꢀ
]
dianion (Fig. 5b). Spectro-
scopic changes show that while two new bands at 580 and 786 nm
increase in intensity, the Q band decreases in intensity. Clear
isosbestic points are recorded at 645 and 708 nm in the spectra.

Sß. Özçelik et al. / Polyhedron 42 (2012) 227–235
233
Fig. 6. In situ UV–Vis spectral changes of CoPc in DCM/TBAP electrolyte system. (a) Eapp = ꢀ0.70 V. (b) Eapp = ꢀ1.70 V). (c) Eapp = 0.70 V. (d) Chromaticity diagram of CoPc; each
+1
symbol represents the colour of the electro-generated species after the potential applications, h, [CoIIPcꢀ2]; s, [CoIPcꢀ2
]
ꢀ1; 4, [CoIPcꢀ3
] ] .
ꢀ2; q, [CoIIPcꢀ1
During the oxidation of compound 3 at 1.00 V applied potential,
the intensity of the Q bands decrease in intensity while a new band
is recorded at 783 nm (Fig. 5c). At the same time, the intensity of
the region at around 500 nm increases and the B2 band shifts from
341 to 328 nm, while the B1 band increases. Clear isosbestic points
are recorded at 360, 580 and 728 nm in the spectra which
demonstrate that the oxidation proceeds clearly in deoxygenated
DCM to give a single type oxidized species. These spectroscopic
is very important in deciding their possible electrochromic
application.
In situ UV–Vis spectral changes and the in situ recorded chroma-
ticity diagram of cobalt compound 5 in TBAP/DCM are given in
Fig. 6. Before any potential application, due to the aggregated spe-
cies a broad Q band is recorded at 675 nm. During the potential
application at ꢀ0.70 V, this Q band at 675 nm shifts completely
to 708 nm, while a new band at 470 nm increases in intensity.
An increasing band between 400 and 500 nm and a decreasing Q
band without shift characterize the formation of CoIPc2ꢀ species.
The final spectrum represents the presence of aggregation at the
end of the first reduction reaction. These spectral changes indi-
cated the reduction of the aggregated compound 5 species [51–
57] (Fig. 6a). A colour change from light blue (x = 0.2658 and
y = 0.3307) to light green (x = 0.3488 and y = 0.403) was recorded
during the first reduction process. Further reduction of the com-
plex at ꢀ1.70 V indicates a ring based redox process, because the
Q band at 708 nm decreases without shift while a new broad band
is recorded at 537 nm. During this process, the presence of a band
at 470 nm without change indicates the CoI oxidation state of the
central metal (Fig. 6b). The colour of the complex changes slightly
and a greenish yellow colour (x = 0.3486 and y = 0.3841) was re-
corded. Fig. 6c represents the spectral changes during the oxidation
processes of the complex. During 0.70 V potential application, the
changes are characteristics for the [H2Pc2ꢀ]/[H2Pc1ꢀ 1+
process [29,46,50].
]
redox
The colour change of the solution of the complexes during the
redox processes were recorded using in situ colorimetric measure-
ments. The chromaticity diagrams of compound 3 recorded simul-
taneously during the spectroelectrochemical measurements are
given in Fig. 5d. Without any potential application, the solution
of compound 3 is bluish green (x = 0.2981 and y = 0.3474). As the
potential is stepped from 0 to ꢀ1.00 V, the colour of neutral 3 starts
to change and a light green colour (x = 0.2783 and y = 0.384) of the
monoanionic form of metal-free compound 3 was obtained at the
end of the first reduction. Similarly the colour of the dianionic
species was recorded as blue (x = 0.2662 and y = 0.2541) and for
monocationic species the colour was recorded as light green
(x = 0.3297 and y = 0.3502). Measurement of the xyz coordinates
allows the quantification of each colour of redox species, which

234
Sß. Özçelik et al. / Polyhedron 42 (2012) 227–235
Fig. 7. In situ UV–Vis spectral changes of CoPc in the DCM/TBAP electrolyte system. (a) Eapp = ꢀ0.70 V. (b) Eapp = ꢀ1.60 V. (c) Eapp = 0.40 V (inset: Eapp = 1.00 V). (d)
Chromaticity diagram of CoPc, each symbol represents the colour of the electro-generated species after the potential applications: h, [CoIIPcꢀ2]; s, [CoIPcꢀ2
]
ꢀ1; 4,
+1
[CoIPcꢀ3 ꢀ2; q, [CoIIIPcꢀ2
] ] .
Q band decreases without a shift, while two new bands are re-
corded at 467 and 940 nm (Fig. 6c). Observation of a new band
at 467 nm and the decrease of the Q band without a shift indicate
recorded in DMSO have the same trend with those recorded in
DCM. These spectral changes indicates that [CoIIPcꢀ2] is reduced
to [CoIPcꢀ2 ꢀ1
]
at ꢀ0.60 V and then to [CoIPcꢀ3 ꢀ2
at ꢀ1.70 V [51–
]
a ring-based process, assigned to the [CoIIPcꢀ2]/[CoIIPcꢀ1 1+
]
process
57]. During the potential application at 0.60 V, the Q band at
667 nm shifts completely to 678 nm with a slight increase. These
spectral changes indicate a metal based oxidation processes
(Fig. 7c). Spectral changes recorded during the further oxidation
process are typical for a ring based oxidation reaction [51–57].
The colour changes of the complex during electron transfer reac-
tions are given in the chromaticity diagrams in Fig. 7d.
[51–57]. These in situ spectral changes are in agreement with the
literature, which clearly state that ring oxidation occurs before me-
tal oxidation in CoPc complexes in non-coordinating solvents [46].
Further oxidation of the species showed the oxidation of the metal
centre. The colour changes from blue to light blue (x = 0.3123 and
y = 0.334) and then light yellow (x = 0.339 and y = 0.321) were re-
corded during the oxidation processes (Fig. 6d).
Fig. 7 represents the in situ UV–Vis spectral changes and in situ
recorded chromaticity diagram of compound 5 in TBAP/DMSO.
When the reduction processes were compared with those recorded
in the DCM, it is clear that there is no assignment difference of
CoPc in both of these solvents. However spectral changes during
the oxidation processes are different in these solvents. Whilst
phthalocyanine complex 5 gives a ring based oxidation reaction
before the metal based oxidation process in DCM, a metal based
electron transfer process is recorded during the first oxidation
reaction in DMSO. Fig. 7a and b show the reduction of the complex
at ꢀ0.60 and ꢀ1.70 V respectively. These spectral changes
4. Conclusion
In this study, novel metallo and metal-free phthalocyanines
substituted with bulky dendritic ethereal aryloxy groups were
synthesized and their structures were elucidated with several
spectroscopic techniques. Voltammetric and spectroelectrochemi-
cal studies show that while the metal-free and zinc phthalocyanine
complexes give ring-based, multi-electron and reversible/quasi-
reversible redox processes, the cobalt phthalocyanine complex
gives both metal and ring-based, diffusion controlled, multi-elec-

Sß. Özçelik et al. / Polyhedron 42 (2012) 227–235
235
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tron and reversible/quasi-reversible reduction processes. The col-
ours of the electrogenerated anionic and cationic forms of the com-
plexes were also studied as definite colour determination is
important for deciding the possible electrochromic application of
the complexes.
Acknowledgement
[28] A. Koca, H.A. Dinçer, E. Gonca, A. Gül, J. Porphyrins Phthalocyanines 13 (2009)
This work was supported by the Research Fund of the Istanbul
Technical University.
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