Synthesis, characterization, OFET and electrochemical properties of novel dimeric metallophthalocyanines

Article abstract of DOI:10.1039/c3dt50212k

The synthesis of 4,4′-[6,6′-methylenebis (2-(2-(3,4- dicyanophenoxy)-5-methylbenzyl)-4-methyl-6,1-phenylene)] bis (oxy) diphthalonitrile 1 was achieved starting from 4-nitrophthalonitrile and 6,6′-methylenebis(2-(2-hydroxy-5-methylbenzyl)-4-methylphenol in DMF at 50 °C by the catalysis of K₂CO₃ under argon. The corresponding dimeric metallophthalocyanines (Zn₂Pc₂2 and Co₂Pc₂3) were tetramerized in dimethylaminoethanol with the appropriate metal salt. Newly synthesized compounds were characterized by elemental analysis, UV-vis, FT-IR (ATR), MALDI-TOF mass and ¹H-NMR spectroscopy techniques. The electrochemical properties of the complexes were examined by cyclic voltammetry, differential pulse voltammetry, controlled potential coulometry and in situ spectroelectrochemistry in nonaqueous media. The results showed that while there is considerable weak interactions between the two metal phthalocyanine units in dimeric zinc phthalocyanine, these interactions in dimeric cobalt phthalocyanine is remarkable. The catalytic performances of dimeric cobalt phthalocyanine in the reduction of oxygen in a medium similar to the working conditions of the polymer electrolyte membrane fuel-cells were found to be much higher than that of dinuclear zinc phthalocyanine. Solution-processed films of the complexes were utilized as an active semiconducting layer in the fabrication of organic field-effect transistors (OFETs) in the bottom-gate configurations. The output characteristics of the resulting p-type OFET devices were investigated to evaluate the performances such as the field effect mobility (μ_F). A relatively high field effect mobility of 7.3 × 10^-3 cm ² V^-1 s^-1 was observed for dimeric cobalt phthalocyanine.

Full text of DOI:10.1039/c3dt50212k

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Synthesis, characterization, OFET and electrochemical
properties of novel dimeric metallophthalocyanines
Cite this: Dalton Trans., 2013, 42, 6633
Lale Meyancı Özer,a Metin Özer,a Ahmet Altındal,*b Ali Rıza Özkaya,a Bekir Salihc
and Özer Bekaroğlu*d
The synthesis of 4,4’-[6,6’-methylenebis (2-(2-(3,4-dicyanophenoxy)-5-methylbenzyl)-4-methyl-6,1-pheny-
lene)] bis (oxy) diphthalonitrile 1 was achieved starting from 4-nitrophthalonitrile and 6,6’-methylenebis-
(2-(2-hydroxy-5-methylbenzyl)-4-methylphenol in DMF at 50 °C by the catalysis of K2CO3 under argon.
The corresponding dimeric metallophthalocyanines (Zn2Pc2 2 and Co2Pc2 3) were tetramerized in
dimethylaminoethanol with the appropriate metal salt. Newly synthesized compounds were character-
ized by elemental analysis, UV-vis, FT-IR (ATR), MALDI-TOF mass and 1H-NMR spectroscopy techniques.
The electrochemical properties of the complexes were examined by cyclic voltammetry, differential pulse
voltammetry, controlled potential coulometry and in situ spectroelectrochemistry in nonaqueous media.
The results showed that while there is considerable weak interactions between the two metal phthalo-
cyanine units in dimeric zinc phthalocyanine, these interactions in dimeric cobalt phthalocyanine is
remarkable. The catalytic performances of dimeric cobalt phthalocyanine in the reduction of oxygen in a
medium similar to the working conditions of the polymer electrolyte membrane fuel-cells were found to
be much higher than that of dinuclear zinc phthalocyanine. Solution-processed films of the complexes
were utilized as an active semiconducting layer in the fabrication of organic field-effect transistors
(OFETs) in the bottom-gate configurations. The output characteristics of the resulting p-type OFET
devices were investigated to evaluate the performances such as the field effect mobility (µF). A relatively
high field effect mobility of 7.3 × 10−3 cm2 V−1 s−1 was observed for dimeric cobalt phthalocyanine.
Received 21st January 2013,
Accepted 18th February 2013
DOI: 10.1039/c3dt50212k
www.rsc.org/dalton
its different derivatives,8 1a,8b-dihydronaphtho[b]naphthofuro-
[3,2-d]-furan-7,10-diol,9 phenolphthalein,10 9,9′-bis(4-hydroxyphe-
1. Introduction
For a couple of decades, mononuclear phthalocyanines (Pcs) nyl)fluorine,11 4,4′-(octahydro-4,7-methano-5H-inden-5-ylidene)-/
have been the issue of an intensive study in multidisciplinary bisphenol,12 and dicumarol,13 syntheses of various ball-type
fields.1 Cofacial sandwich type Pc dimers have been known since metallophthalocyanines (MPcs) were achieved.
the first quarter of twentieth century, though the first planar
Dinuclear Pcs show a wide range of interactions between
dimer was reported at the end of the aforementioned century.2 the Pc rings, mainly depending on the metal center, bridging
Since then, many papers have been published on both cofacial links and the presence or absence of axial ligands. The dimers
Pc dimmers3 and planar homodimers of Pc and Pc derivatives.4 having flexible bridging units usually do not exhibit any mea-
Furthermore, a large number of a new class of dimeric com- surable interaction, while rigid systems such as the anthracene
plexes, ball type Pcs which are covalently bonded with each other and naphthalene bridged species do, however, exhibit strong
were reported. Owing to different bridges such as tert-butylcalix interactions.14 We also detected a strong interaction between
[4]arene,5 1,1′-methylenedinaphthalen-2-ol,6 pentaerythritol7 and the face-to-face Pc rings or two metal centers in ball-type com-
plexes with spectroscopic and electrochemical evidence.15
These novel compounds showed interesting mixed-valence be-
haviour, electrical and optical properties due to these inter-
actions. The fact that even small modifications may lead to
important changes in Pc characteristics and our continuing
efforts in the design of novel macrocycles with potential appli-
cability in various technological areas prompted us to syn-
thesize new examples of dinuclear MPcs.
aDepartment of Chemistry, Marmara University, 34722 Göztepe-Istanbul, Turkey.
E-mail: lmeyanci@marmara.edu.tr, metinozer@marmara.edu.tr,
aliozkaya@marmara.edu.tr; Fax: +90 216 3478783; Tel: +90 216 3479641
bDepartment of Physics, Davutpaşa Caddesi, 34220, Esenler, Istanbul, Turkey.
E-mail: altindal@yildiz.edu.tr; Fax: +90 212 383 42 34; Tel: +90 212 383 42 31
cDepartment of Chemistry, Hacettepe University, 06800, Beytepe, Ankara, Turkey.
E-mail: bekir@hacettepe.edu.tr; Fax: +90 312 288 2163; Tel: +90 312 297 7975
dDepartment of Chemistry, Technical University of Istanbul, 34469 Maslak-Istanbul,
Turkey. E-mail: obek@itu.edu.tr; Fax: +90 216 3860824; Tel: +90 216 3479641
Organic field effect transistors (OFETs) act as key com-
ponents for the next generation of cheap and flexible organic
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based circuits such as, displays, radio frequency identification
tags, microelectronic logic, and some other small-scale inte-
grated circuits.16 Although the achieved performance of OFET
thus far still cannot compete with the widely utilized inorganic
analogues, i.e., the amorphous hydrogenated silicon (a-Si:H)
field effect transistors (FETs), significant progress has been
made in the past decade toward understanding OFETs and rea-
lizing their industrial applications.17 In order to further increase
the number of modifiable physicochemical parameters of the
subunit, we tried to obtain novel dimeric MPc, in which two Pc
units are connected to each other with two –CH2– bridges.
We now report the synthesis and characterization of new
dimeric zinc(II) and cobalt(II) Pc complexes, and also their
applications as electrocatalytic materials for dioxygen
reduction and FET.
2. Results and discussion
2.1. Synthesis and characterization
The synthesis of 1 was performed beginning from 4-nitrophtha-
lonitrile and 6,6′-methylenebis(2-(2-hydroxy-5-methylbenzyl)-4-
methylphenol in DMF at 50 °C by the catalysis of K2CO3 under
argon. The suitable dimeric MPcs 2 and 3 were prepared in
dimethylaminoethanol with the appropriate metal salt. Both
complexes are not soluble in any pure solvent. However, the
addition of organic or mineral acid to the heterogeneous
mixture of the MPc and solvent makes the compound soluble.
The formation of 1 was confirmed by the disappearance of
the NO2 band of 4-nitrophthalonitrile at 1350 cm−1 and the
appearance of new intensive absorption band at 1247 cm−1
belonging to Ar–O–Ar in the IR spectra. In the IR spectrum of
1, the strong and characteristic CuN band at 2232 cm−1 dis-
appeared after conversion to MPcs 2 and 3. The intensive bands
at 1713 and 1715 cm−1 showed the CvN stretching vibrations
for 2 and 3, respectively. Furthermore, the peaks observed at
1587–1483 cm−1, 1602–1467 cm−1 and 1604–1403 cm−1 were
attributed to unsaturated aromatic CvC double bond stretch-
ing frequencies for 1, 2 and 3 respectively. Besides, the stron-
gest peaks appearing at ca. 1247 for 1, 1219 for 2 and
1225 cm−1 for 3 in the spectra of the complexes were attributed
to the aromatic ether bands (C–O–C intense stretching). The
general shift trend of the complexes and metal sensitivity can
also be found easily in this region. The peak at 1086 cm−1 for
2 shifts to 1103 cm−1 for 3 indicating that this vibration mode
is metal sensitive.
Fig. 1 shows the UV-vis spectra of 2 and 3 in DMF and
DMSO at varying concentrations within the range of 1.73 ×
10−5–5.78 × 10−7 mol dm−3. These spectra showed character-
istic Q-band absorptions between 686 and 672 nm which were
attributed to the π → π* transitions from the highest occupied
molecular orbital (HOMO) to the lowest unoccupied molecular
orbital (LUMO) of the Pc ring. The other bands (B) in the UV
region within the range of 355–327 nm were observed due to
the transitions from the deeper π levels to the LUMO. The
spectrum of 3 in DMF or DMSO involved a broad Q band
Fig. 1 UV-vis spectra of (A) 2 in DMF, (B) 2 in DMSO, (C) 3 in DMF and (D) 3 in
DMSO within the range of 1.73 × 10−5–5.78 × 10−7 mol dm−3
.
absorption with a shoulder at the higher energy side, which is
quite typical for intermolecularly interacting or aggregated
metallo Pcs. The trend of Pcs to self-aggregate through
6634 | Dalton Trans., 2013, 42, 6633–6644
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coplanar association of the Pc rings to form dimers and higher
order aggregates is well known.18 In general, Pc aggregation
results in a decrease in intensity and broadening of the Q
band, and increasing intensity of the Soret band correspond-
ing to the monomeric species. However, As shown in Fig. 1C
and 1D, while the solutions of 3 in DMF and DMSO were
diluted from 1.73 × 10−5 mol dm−3 to 5.78 × 10−7 mol dm−3
,
the absorption ratio of Q and satellite bands did not change as
in the literature19 and the broadness of the spectrum did not
decrease. This observation strongly suggests that the broad Q
band absorption in the spectrum of 3 is due to the intramole-
cular interactions between the two CoPc units in the ball-type
dinuclear cobalt Pc, Co2Pc2. On the other hand, the electronic
absorption spectra of 2 in DMF and DMSO displayed a sharp
Q band absorption with its vibrational component at the
higher energy side Fig. 1A and 1B which is characteristic not
only for non-aggregated or intermolecularly interacting, but
also for intramolecularly non-interacting MPcs.
Fig. 3 Positive ion and linear mode MALDI-TOF mass spectrum of 3 in dithra-
nol MALDI matrix using nitrogen laser (at 337 nm wavelength) accumulating
100 laser shots.
MALDI-TOF mass spectra of 1 and 3 are given in Fig. 2 and
3, respectively. These spectra were recorded in dithranol with
better intensity than the other conventional MALDI matrices.
Although the high resolution spectra could be accumulated for
1, it was not possible to obtain reasonable peak intensities for
the cobalt complex 3 because of the short life times of the pro-
tonated molecular ion in reflectron mode mass spectrometer.
For 1, sodiated and potasiated ion signals were observed in the
case of the protonated ion signal due to the low affinity of the
ligand to the proton compared to the sodium and potassium.
It was concluded that compound 1 was synthesized and puri-
fied perfectly. The MALDI-TOF mass spectra of 3 suggested
that many fragmentation occurred and the fragment ions
mainly followed each other with 224 Da masses representing
side chain fragmentation and also high intense dimer and
trimer of the complex (data not shown). The fragmentation
may be attributed to dominant aggregation in the crystal form
of the MALDI sample.
Fig. 4 Positive ion and linear mode MALDI-TOF mass spectrum of
C
126H80N16O8Zn2 2 in 2,5-dihydroxybenzoic acid MALDI matrix using nitrogen
laser (at 337 nm wavelength) accumulating 100 laser shots.
protonated ion signal, water adduct signal also was observed at
reasonably high intensity. However, methane loss from the
protonated molecular ion signal was observed at high intensity
because of the possibly available methane leaving group of the
complex. On the other hand, the peaks for doubly charged
ions of all water adducts, protonated molecular ion and major
fragment ions after methane elimination were observed with
really high intensity. Mass of the protonated ion signal con-
firmed that the complex was synthesized perfectly. It was also
concluded from the clean MALDI-TOF mass spectrum that the
complex was purified completely.
Fig. 4 shows MALDI-TOF mass spectra of 2 in 2,5-dihydroxy-
benzoic acid which had better intensity than those recorded
with other conventional MALDI matrices. Beside the
1
The H NMR spectrum of 1 displayed characteristic signals
for –CH3 substituted to phenyl as two singlets at 2.11 and
2.19 ppm, and –CH2– protons binding of two phenyls as
singlet at 3.42 ppm. The peak at 6.70–8.27 ppm showed the
presence of aromatic protons.
2.2. Electrochemistry
The voltammetric measurements were carried out on platinum
in DMSO for 2 and 3. The relevant data are given in Table 1.
Typical cyclic and differential pulse voltammograms of 2 are
shown in Fig. 5A. It displayed two reductions at E1/2 = −0.72 V
and E1/2 = −1.15 V versus SCE, and an oxidation at E1/2 = 0.78 V
Fig. 2 Positive ion and reflectron mode MALDI-TOF mass spectrum of
C63H40N8O4 1 in dithranol MALDI matrix using nitrogen laser (at 337 nm wave-
length) accumulating 100 laser shots. Inset spectrum shows expanded molecu-
lar mass region of the sodium adduct of the ligand.
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Table 1 The electrochemical data for 2 and 3 in DMSOa
Complex
Redox processes
E1/2 (V)
ΔEp (V)
ΔE1/2 (V)
ΔEs (V)
2
[Zn(II)Pc(-1)]22+/[Zn(II)Pc(-2)]2 (O1)
0.78
−0.72
−1.15
0.52
−0.28
−0.62
−1.07
−1.35
0.04
0.03
0.20
—
0.08
0.08
0.10
0.10
1.50
[Zn(II)Pc(-2)]2/[Zn(II)Pc(-3)]22− (R1)
[Zn(II)Pc(-3)]22−/[Zn(II)Pc(-4)]24− (R2)
3
[Co(II)Pc(-2)·Co(II)Pc(-1)]+/[Co(II)Pc(-2)]2 (O1)
[Co(II)Pc(-2)]2/[Co(II)Pc(-2)·Co(I)Pc(-2)]− (R1)
[Co(II)Pc(-2)·Co(I)Pc(-2)]−/[Co(I)Pc(-2)]22− (R2)
[Co(I)Pc(-2)]22−/[Co(I)Pc(-2)·Co(I)Pc(-3)]3− (R3)
[Co(I)Pc(-2)·Co(I)Pc(-3)]3−/[Co(I)Pc(-3)]24− (R4)
0.80
0.34
0.28
a Potentials are reported with respect to the SCE. E1/2 values were measured by cyclic voltammetry [E1/2 = (Epa + Epc)/2] and/or differential pulse
voltammetry. The peak separation values (ΔEp = Epa − Epc) are reported at 0.100 V s−1 except R2 at 0.500 V s−1 for 2. ΔE1/2 is the potential
difference between the first oxidation and first reduction potentials. It represents the HOMO–LUMO gap for 2, but the charge transfer transition
for 3 involving redox-active metal centers. ΔEs indicates mixed-valence splitting energy for the relevant split redox couple.
Fig. 5 (A) Cyclic and differential pulse (inset) voltammograms of 2 on Pt and in situ UV-vis spectral changes during the controlled-potential electrolysis of the solu-
tion of 2 at (B) −0.85 V (two groups of spectral changes were observed; inset shows the first group) (C) −1.30 V and (D) 0.95 V versus SCE in DMSO/TBAP.
versus SCE which could be detected only by differential pulse process also involves the transfer of two electrons per the
voltammetry. The CPC studies showed that each reduction molecule. The transfer of two electrons for each redox process
process of 2 involves the transfer of two electrons per the mole- suggests that there is not considerable interaction between two
cule. The comparison of the area under each peak in differen- Pc rings and/or two metal centers in this complex, and thus
tial pulse voltammogram of 2 implies that the oxidation two MPc units electrochemically react at the same potentials.
6636 | Dalton Trans., 2013, 42, 6633–6644
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The similarity of the redox behaviour of 2 to mononuclear Pcs −0.85 V, two groups of spectral changes with different isosbes-
supports this idea.20 However, the peaks in the voltammo- tic points were recorded. During the first group of spectral
grams are somewhat broad, which can be attributed to the changes, the absorptions of the bands at 640, 351 and 258 nm
presence of weak intramolecular interactions. The overall split- decrease while that of the main Q-band at 680 nm increases (the
ting of the molecular orbitals with a high degree on going inset in Fig. 5B). These spectral changes are associated by the
from the monomeric to dimeric, trimeric or oligomeric Pcs formation well-defined isosbestic points at 670, 534 and 384 nm.
depending on the intramolecular interactions between the two The disappearance of the band at 640 nm may be due to the
MPc units are expected to occur only in the case that these decrease in the intramolecular interactions, probably as a result
units are positioned in face to face orientation with a small of the conformational changes in the molecule before the elec-
distance between them. Thus, it may be concluded that two tron transfer process upon the applied electrical potential.
MPc units in 2 are approximately planar in nature or far to
During the second group of spectral changes, the absorptions
each other if they are cofacial. As shown in (Fig. 5A), the of the main Q-band around 680 nm and the B-band at 360 nm
anodic component of the R2 couple is not observed at low decrease whereas the absorption within the range of 418–660 nm
scan rates suggesting that the electrochemically produced increases and a new band at 576 nm forms (Fig. 5B). These spec-
species during the second reduction process are not stable. In tral changes especially decrease in the Q-band without shift and
situ spectroelectrochemical measurements were also carried the formation of a new band between 500 and 600 nm are
out during the electrolysis of the solution of 2 in DMSO/TBAP characteristic for ligand-based electron transfer processes.21 Simi-
at suitable constant potentials corresponding to its redox pro- larly, the formation of a new band at 542 nm and decrease in the
cesses. The spectrum monitored at the beginning of the elec- Q-band without shift during the second reduction R2 at −1.30 V
trolysis at −0.85 V versus SCE corresponding to R1 involves a versus SCE corresponds to a ligand-based process (Fig. 5C).
band at 640 nm at the blue side of the main Q band at 680 nm Although the first oxidation of 2 could be monitored by differen-
(the inset in Fig. 5B). In dimeric Pcs, intramolecular coupling tial pulse voltammetry, the electrolysis at the constant potential
between the rings causes a band at the red side of the charac- of 0.95 V versus SCE led to its decomposition, evidenced by
teristic Q band, at around 630 nm. Thus, the band at 640 nm decrease in absorption of all bands (Fig. 5D).
can be attributed to the intramolecular interactions between
The voltammetric behaviour of 3 was considerably different
the two MPc units in 2. Upon the application the potential of from that of 2. This compound showed one-electron redox
Fig. 6 (A) Cyclic voltammogram of 3 at 0.100 V s−1 on Pt and in situ UV-vis spectral changes during the controlled-potential electrolysis of the solution of 3 at (B)
−0.80 V (insets a and b show the formation of various isosbestic points within the ranges of 560–580 nm and 680–710 nm, respectively, rather than a well-defined
single isosbestic point (C) −1.50 V and (D) 0.65 V versus SCE in DMSO/TBAP.
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Chart 1 Electrode reactions for 3. M: Co.
processes. A typical cyclic voltammogram for this compound is reduction.22 The formation of various isosbestic points within
shown in Fig. 6A. the ranges of 580–590 nm rather than a single one should also
The transfer of one electron in each step indicates that the be due to the formation of short-lived mixed-valence species of
splitting of the molecular orbitals occurs as a result of the [Co(I)Pc(-2)·Co(I)Pc(-3)]3− (the inset in (Fig. 6C). Fig. 6D shows
remarkable interaction between the two Pc rings and/or two the spectral changes recorded during the electrolysis at 0.65 V
metal centers in 3, and thus the redox processes of each Pc versus SCE. The red-shifting of the main Q-band at 665 nm to
ring and/or metal center in this complex occur at different 677 nm with the increase the absorption is characteristic for
potentials. This behaviour implies that two MPc units in 3 are metal-based Co(II)/Co(III) oxidation22,23 and can be attributed
planar, at least partially, rather than being planar since the to [Co(II)Pc(-2)]2/[Co(III)Pc(-2)·Co(II)Pc(-2)]+ process since O1
remarkable interaction between the molecular orbitals of two involves the transfer of one electron per the molecule. The for-
Pcs and/or two metal centers, in a dimer cause considerable mation of well-defined isosbestic points at 294, 350 and
changes in redox potentials, compared with corresponding 660 nm supports the oxidation of [Co(II)Pc(-2)]2 species to only/
mono Pcs and non-interacting planar dimers. The electrode [Co(III)Pc(-2)·Co(II)Pc(-2)]+ ones. Some studies in the literature
reactions and the mixed-valence species formed by these reac- showed that while some binuclear and tetranuclear Pcs
tions are schematically illustrated in (Chart 1) for 3. The formed metal-centered type or ring-centered type mixed-
mixed-valence splitting, ΔEs, values in V for the redox pro- valence species, some of them did not.14b,5–7,9,15,23 The differ-
cesses of 3 are presented in (Table 1). These values give evi- ence was attributed mainly to the presence or absence of axial
dence of the delocalization of charge among the two Pc rings ligands, respectively inhibiting or facilitating the close
and/or two metal centers in 3, and thus the formation of elec- approach of the rings, and to the other steric factors. The least
trochemically stable oxidized and reduced mixed-valence strongly coupled systems included some zinc complexes of
species. Although the splitting of the Co(II)/Co(I) redox couple binuclear Pcs. Thus, weak intramolecular interactions
into two waves as [Co(II)Pc(-2)]2/[Co(II)Pc(-2)Co(I)Pc(-2)]- and observed for 2 in this study are consistent with the litera-
[Co(II)Pc(-2)Co(I)Pc(-2)]-/[Co(I)Pc(-2)]22−, and the Pc(-2)/Pc(-3) ture.14b,23 Mixed-valence ring reduction, Pc(-2)·Pc(-3), was
couple as [Co(I)Pc(-2)]22−/[Co(I)Pc(-2)·Co(I)Pc(-3)]3− and [Co(I)Pc- observed previously only in strongly coupled silicon species,23c
(-2)·Co(I)Pc(-3)]3− in 3 was detected clearly by cyclic voltamme- but was not unequivocally observed with the other bridged
try measurements, it was not possible to monitor the spectra binuclear species. The presence of an extra π-electron usually
of the mixed-valence reduced species the during spectroelec- repels the π-electron density in the other ring and therefore
trochemical measurements under the conditions of constant- inhibits formation of these species unless they are constrained
potential electrolysis. However, upon reduction at −0.80 V to lie close together. The large splitting values for the silicon
versus SCE, the formation of various isosbestic points within species was attributed to the shorter Pc…Pc contacts than in
the ranges of 560–580 nm and 680–710 nm rather than a well- the bridged systems. The splitting of the redox processes of
defined single isosbestic point imply the formation of short- the Pc ring and metal center for 3 in this study may be attribu-
lived mixed-valence species of [Co(II)Pc(-2)Co(I)Pc(-2)]- (the ted to the deviation from planarity due to flexible –CH2– group
insets in Fig. 6B). The remarkably broad band and especially in the bridging units.
the shoulder at its blue side in the first spectrum in (Fig. 6B),
The electrocatalytic activities of the ball-type Pc complexes
monitored at the beginning of the electrolysis of the solution 2 and 3 towards oxygen reduction reaction (ORR) was also
of 3 in DMSO/TBAP, are probably due to the intramolecular tested since some articles reported by our group showed that
interactions between the two MPc units. The decrease in the electrocatalytic activities of ball-type dinuclearmetallo Pcs for
absorptions of the main Q-band at 665 nm with the red-shift ORR are encouraging in general for applicability in fuel
to 677 nm and in its shoulder around 625 nm is accompanied cells.8b,c,12,13,23 The ring and disk polarization curves of a rotat-
by the formation of the new bands at 473 and 706 nm. These ing ring disk electrode (RRDE), disk of which was modified by
spectral changes are characteristic for Co(II)/Co(I) reduc- a mixture of the relevant Pc complex with Vulcan XC-72 (VC)
tion.21a–c,22 Upon reduction at −1.50 V versus SCE, the main and Nafion (Nf), in O2-saturated 0.5 M H2SO4 aqueous electro-
Q-band nearly disappears and the intensity of the new band at lyte solution (Fig. 7), have been monitored and then the cataly-
473 nm increases (Fig. 6C). These spectral changes are tic performance has been characterized the by the onset
accompanied by considerable increase in absorption within potential (Eo) where the oxygen reduction current begins to
the range of 400–500 nm which is typical for Pc ring- increase, the limiting diffusion disk current density (JL) and
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Table 2 Electrocatalytic properties of 2 and 3, and the comparison with pre-
viously reported8b,c,12,13,23a,b dinuclear ball-type cobalt phthalocyanines
Complex Eo for ORRa/V JL for ORRb/mA cm−2 E1/2 for ORR/V Ref.
Zn2Pc2 2 −0.15
1.06
2.54
1.66
2.63
0.92
4.66
3.30
4.27
2.17
4.50
4.84
−0.30
−0.07
−0.21
0.35
−0.20
−0.15
−0.14
0.41
twc
twc
Co2Pc2 3
0.06
d
Co2Pc2
0.45
13
e
Co2Pc2f
0.11
0.10
0.68
12
23a
23b
Co2Pc2g
Co2Pc2
−0.19
−0.09
0.05
h
Co2Pc2i
0.25
0.51
8c
8c
Co2Pc2
a The potential at which the current density reaches to 0.100 mA cm−2
was taken as the onset potential. b The limiting diffusion current density
at 2500 rpm. c This work. d Dicumarol bridged. e 4,4′-(Octahydro-4,7-
methano-5H-inden-5-ylidene)bisphenylbridged. f Dithioerythritol-bridged.
g Cyclopentyldisilanoxy-polyhedral oligomeric silsesquioxanes bridged.
h Pentaerythritol-bridged.
i Pentaerythritol-bridged
with
hepta-
decafluorodecyl substituents.
peroxide produced at a given potential was determined by the
following equations:24
nt ¼ 4ID½ID þ ðIR=NÞꢀ
ð1Þ
ð2Þ
% H2O2 ¼ 100ð4 ꢁ ntÞ=2
where N, ID, and IR are the collection efficiency, disk current
and ring current, respectively. Fig. 7B shows the nt and percen-
tage of hydrogen peroxide and water generated at different
potentials. The occurrence of two waves during ORR in
(Fig. 7A) and the nt values higher than 3 for the first wave in
(Fig. 7B) for 2-catalyzed ORR implies that it probably occurs
via peroxo species, producing water as the main product and
some hydrogen peroxide as the side product. The second wave,
appearing at considerably negative potentials probably corres-
ponds to the reduction of hydrogen peroxide to water. The
absolute ring current decreases simultaneously while the disk
current increases during the appearance of the second wave
(Fig. 7A), which confirms that hydrogen peroxide produced at
the first step as the side product is further reduced to water
during the second wave.
According to some authors, electrocatalytic reduction of
oxygen on MPcs takes place through a redox-catalysis type of
process where oxygen binding ability and the redox potential
of the central metal ions play a critical role.25 The metal ion in
the center of Pc ring is oxidized by oxygen molecule during its
adsorption and thus reduces it. The metal centers in 3 are
redox-active while those in 2 are redox-inactive, and thus
neither oxidized nor reduced under the conditions of
electrocatalytic measurement, as judged from their in situ
spectroelectrochemical measurements and well-known electro-
chemical behaviour.20 So, the higher catalytic activity of 3 as
compared to 2 can be attributed to its redox-active metal
centers and their ability to bind dioxygen molecule due to dis-
tinctive coordinating properties. This type of improved cataly-
tic activity towards ORR was also suggested for various
Fig. 7 (A) RRDE polarization curves for the electrocatalytic ORR at 0.005 V s−1
on VC/Nf/M2Pc2 modified rotating (2500 rpm) glassy carbon disk electrode and
platinum ring electrode in O2-saturated 0.5 M H2SO4. (B) Number of total elec-
trons exchanged for VC/Nf/M2Pc2 modified electrodes (inset shows %H2O2 pro-
duced in ORR as a function of disk potential (Ering = 0.95 V vs. SCE)).
the half-wave potential (E1/2). The disk potential at which the
current density reaches to 0.100 mA cm−2 was taken as Eo.
Table 2 lists the electrocatalytic performances of 2 and 3. It
also includes the catalytic activities of some previously
reported ball-type dinuclear cobalt Pcs with dicumarol,13
4,4′-(octahydro-4,7-methano-5H-inden-5-ylidene)bisphenol,12
dithioerytritol,23a cyclopentyldisilanoxy-polyhedral oligomeric
silsesquioxanes,23b and pentaerythritol,8b,c bridging units for
comparison. The ORR catalyzed by 3 displays two reduction
waves (Fig. 7A). The limit current densities for 3 is much
higher than that of 2. In addition, Eo value for 3 is much more
positive than that of 2. Therefore, the catalytic performance of
ball-type dinuclear cobalt Pc2 towards ORR is much better
than that of ball-type dinuclear zinc Pc 2. ORR in acidic
medium can be either a four-electron process to form water or
a two-electron reduction to form hydrogen peroxide. The ring
currents in RRDE voltammetry measurements, carried out
with a VC/Nf/M2Pc2 modified glassy carbon disk electrode and
a platinum ring electrode polarized at 0.95 V versus SCE were
used to determine the number of electrons peroxide and thus
water selectivity. The nt and the percentage of hydrogen
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covalently linked porphyrine dimers in the literature.26 decreases from VGS at the source end to VGS–VDS at the drain
However, it should be noted that the catalytic performance of end. Since the channel is no longer of uniform depth; rather,
previously reported ball-type cobalt Pcs with dicumarol,13 the channel take the tempered form being deepest at the
4,4′-(octahydro-4,7-methano-5H-inden-5-ylidene)bisphenol,12 source end and shallowest at the drain end. As VDS is increased
dithioerytritol,23a cyclopentyl disilanoxy-polyhedral, oligo- more, the channel becomes more tempered and its resistance
meric silsesquioxanes,23a and pentaerythritol,8b,c bridging increases correspondingly. Thus, the output characteristic
units for ORR was higher than that of 3. This can be attributed (IDS–VDS plot) does not continue as a straight line. This situ-
to the difference in the distance between the two CoPc units ation is known as pinch-off of the conductive channel of the
and the flexibility of the molecules.
transistor. Therefore it can be said that the observed saturation
in IDS is due to pinch-off of the channel. Fig. 8 also shows that
the drain–source current is effectively modulated by gate
voltage, the drain current of both the 2-based OFET and
3-based OFET increases with negative gate voltages. These data
show that the devices exhibit a field effect with operation in a
p-type accumulation mode.
The relation between the IDS and the VDS and VGS in the
linear and saturation regions is given by eqn (3) and (4),
respectively.27
2.3. Characteristics of OFETs
In Fig. 8, the drain–source current IDS is plotted as a function
of the drain–source voltage VDS for different gate–source vol-
tages VGS applied to ITO electrode, which acted as a gate. From
the plots, the linear and saturation regions can be seen clearly
with the increase of drain–source voltage. At low drain–source
voltages, the drain–source current follows Ohm’s law, and is
proportional to drain–source voltage. As the drain–source
voltage increased, the voltage, measured relative to the source,
along the channel increases from the 0 to VDS. Thus the
voltage between the gate and points along the channel
W
L
1
2
IDS
¼
CiμFðVGS ꢁ VtÞVDS
ꢁ
VD2S
ð3Þ
ð4Þ
W
2
IDS
¼
CiμFðVGS ꢁ VtÞ
2L
where μF is the field-effect mobility, W and L are the channel
width and length, respectively, Ci is the capacitance of the
dielectric layer, and Vt is the threshold voltage.
The most important parameter describing the performance
of an OFET is the field-effect mobility that indicates how easy
the charge carriers can drift under the influence of electric
field, and most efforts to date have been devoted to its
enhancement. The mobility of the transistors can be obtained
in different ways. In this work, the values of the mobility for
both transistors were extracted from the saturation region
operation of the devices. For this purpose, the IDS currents at
constant VDS = −55 V were obtained from the response curves
(Fig. 8). The square roots of these currents were then plotted
as function of the gate voltage. Fig. 9 shows this plot for 2/PVA
OFET. With the aid of eqn (2), from the slope of the linear
portion of Fig. 9 the μF values were found as 2.8 × 10−3 cm2
V−1 s−1 and 7.3 × 10−3 cm2 V−1 s−1 for 2 and 3, respectively.
It was observed that the obtained mobility values of the
studied Pc-based OFETs are higher than the mobilities of
CuPc-based FET using PVA as gate insulator28 with the value of
2.2 × 10−4 cm2 V−1 s−1 and CuPc-OFET using polymethyl
methacrylate (PMMA) dielectric layer with the mobility of
0.2 × 10−4 cm2 V−1 s−1 29
.
3. Experimental
3.1. Materials
All chemicals used were of reagent grade quality. 4-Nitrophtha-
lonitrile and 6,6′-methylenebis(2-(2-hydroxy-5-methylbenzyl)-4-
methylphenol were purchased from TCI and used as received.
All reactions were carried out under a dry argon atmosphere
Fig. 8 The output characteristics (IDS–VDS) of (A) 2 (80 nm)/PVA (300 nm) OFET
and (B) 3 (80 nm)/PVA (300 nm) OFET at different VGS from 0 V to −70 V.
unless otherwise noted. The target compound
1 was
6640 | Dalton Trans., 2013, 42, 6633–6644
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CHCl3, THF, DMF and DMSO. Mp: 253 °C. Yield: 1.94 g
(79.34%).
Anal. calculated for C63H40N8O4 C, 77.76; H, 4.14; N,
11.52%; found C, 77.34; H, 4.16; N, 11.67%. IR (ATR),
νmax(cm−1): 3104, 3075, 3040, 2924, 2854, 2232, 1587, 1568,
1
1483, 1247. H NMR (400 MHz, DMSO-d6) δ ppm; 2.11(s, 6H),
2.19(s, 6H), 3.42(s, 6H), 6.70, 6.78, 6.81, 6.83, 6.908(s, 8H), 7.02
(m, 6H), 7.33(d, J = 2.46 Hz, 2H), 7.82(m, 2H), 7.90(d, J = 8.77,
2H), 8.27(s, 2H). MALDI-TOF-MS: m/z 995.32 (M + Na)+
(Scheme 1).
3.3.2. 2′,10′,16′,24′-{6,6′-Methylenebis[2(2-hydroxy)-5-methyl-
benzyl]-4-methyl}-bis(oxydiphthalocyaninato-dizinc(II)
2. A
mixture of 1 (0.48 g, 0.50 mmol) and Zn(OAc)2·2H2O (0.11 g,
0.50 mmol) was heated with 2 mL of dry DMA in a sealed tube.
The mixture was kept at 180 °C for 3 h under oxygen free
atmosphere. After cooling to room temperature, the reaction
mixture was treated with methanol and filtered. The raw green
product was washed by several solvents to purify and extracted
in Soxhlet with acetone for 12 h. The crude product was puri-
fied by column chromatography on silica gel eluting with THF
and AcOH of 5% in DMF. The complex 2 is not soluble in any
common solvent. However, it is soluble in DMF or DMSO after
Fig. 9 Dependence of IDS current and its square root on the VGS voltage of the
3-based device.
synthesized and purified according to the methods described
previously in literature,30 and was isolated by column chrom-
atography on silica gel 60. The Pc compounds were purified
successively by washing with hot acetic acid, water and metha-
nol, respectively, and extracted in the Soxhlet with acetone.
The melting points of the Pc compounds were found to be
higher than 300 °C.
3.2. Equipment
IR spectra were recorded on a Perkin Elmer Spectrum One
FTIR (ATR sampling accessory) spectrophotometer. UV-vis
spectra were recorded on a Shimadzu UV-1601 UV-vis spec-
trometer. 1H-NMR spectra were recorded on a Varian UNITY
INOVA 500 MHz spectrometer. Elemental analyses were per-
formed by the Instrumental Analysis Laboratory of the
TUBITAK, Marmara Research Centre. Mass spectra were
acquired on a Voyager-DE™ PRO MALDI-TOF mass spec-
trometer (Applied Biosystems, USA) equipped with a nitrogen
UV-Laser operating at 337 nm. Spectra were recorded in reflec-
tron mode for 1 and linear mode for 2 and 3 with average of
100 shots.
3.3. Synthesis
3.3.1. 4,4′-[6,6′-Methylenebis(2-(2-(3,4-dicyanophenoxy)-5-
methylbenzyl)-4-methyl-6,1-phenylene)]bis(oxy)diphthalonitrile
1. A mixture of 6,6′-methylenebis(2-(2-hydroxy-5-methylben-
zyl)-4-methylphenol) (0.93 g, 2 mmol), 4-nitrophthalonitrile
(1.37 g, 8 mmol) and anhydrous K2CO3 (1.65 g, 12 mmol) in
40 mL of anhydrous dimethylformamide (DMF) in a flask was
sealed after removal of O2. After the reaction mixture was
stirred at 50 °C for 72 h, the mixture was poured into icy water
and the formed precipitate was filtered off, washed by using
water and dried under the vacuum at 50 °C for 48 h. The crude
product was purified by column chromatography with silica
Scheme 1 (i) K2CO3, DMF, 50 °C, 72 h; (ii) (a) Co(OAc)2·4H2O, DMA, 180 °C,
gel eluting with CH2Cl2. Compound 1 is soluble in CH2Cl2, 20 min, (b) Zn(OAc)2·2H2O, DMA, 180 °C, 20 min.
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dropping acetic acid (about 1%) to the solvent. Mp > 300 °C. least 20 min prior to running the experiment, and the solution
Yield: 214.08 mg (41.30%). was protected from air by a blanket of nitrogen during the
Anal. calculated for C126H80N16O8Zn2: C, 73.32; H, 3.91; N, experiment. For the controlled-potential coulometry (CPC)
10.83%; found C, 73.56; H, 3.96; N, 10.78%. IR (ATR), studies, a platinum gauze working electrode, a platinum wire
νmax(cm−1): 2919, 1713, 1602, 1467, 1220, 1086, 1039, 746, 438. counter electrode separated with a bridge, a SCE as reference
MALDI-TOF-MS: m/z (M − CH4 + H)+, 2073.49 (M + H)+, (M + electrode, and a model 377/12 synchronous stirrer were used.
H2O + H)+. UV–vis(DMF): λmax (nm), (log ε): 355 (5.10), 623
The spectroelectrochemical measurements were carried out
(4.76), 680 (5.33) (Scheme 1).
by an Agilent Model 8453 diode array spectrophotometer
3.3.3. 2′,10′,16′,24′-{6,6′-Methylenebis[2(2-hydroxy)-5-methyl- equipped with the potentiostat/galvanostat and utilizing a
benzyl]-4-methyl}-bis(oxydiphthalocyaninato-dicobalt(II) 3. The three-electrode configuration of thin layer quartz spectroelec-
synthesis and the purification of 3 was achieved by the same trochemical cell at 25 °C. The working electrode was transpar-
route used for 2. A mixture of 1 (0.48 g, 0.50 mmol) and Co- ent Pt gauze. Pt wire counter electrode separated by a glass
(OAc)2·4H2O (0.088 g, 0.50 mmol) was heated with 2 mL of dry bridge and a SCE reference electrode separated from the bulk
DMA in a sealed tube. The solubility of blue complex 3 is the of the solution by a double bridge were used.
same of 2. Mp > 300 °C. Yield: 240.14 mg (46.55%).
Ultra pure water and extra pure sulphuric acid (Merck) were
Anal. calculated for C126H80Co2N16O8: C, 72.87; H, 3.88; N, used to prepare the electrolyte solution in electrocatalytic
10.79%; found C, 72.59; H, 3.91; N, 10.64%. IR (ATR), measurements. VC (Cabot Co.), 5% Nf solution (Aldrich), extra
νmax(cm−1): 2921, 1715, 1604, 1554, 1470, 1403, 1225, 1103, pure ethyl alcohol (Merck), VC supported platinum particles
1056, 752, 443. MALDI-TOF-MS: m/z 2063.50 (M + H)+, UV–vis (ElectroChem, Inc.) and the Pc compounds were used in cata-
(DMF): λmax (nm), (log ε): 320 (4.32), 612 (3.89), 668 (4.26) lyst preparation. Glassy carbon disk electrode, glassy carbon
(Scheme 1).
disk-platinum ring electrode and a polishing kid for these elec-
trodes were purchased from Pine Instruments. Two Gamry
Reference 600 potentiostats, controlled by an external PC and
3.3.4. MALDI sample preparation
MALDI matrix, dithranol was prepared in ACN : H2O : DMSO utilizing a four electrode configuration at 25 °C, were con-
mixture in 1 : 1 : 1 v/v/v ratio at a concentration of 10 mg mL−1
nected as a bipotentiostat to control the ring and disk poten-
.
MALDI samples for 1 and 3 were prepared by mixing sample tials and to collect the respective currents during RRDE
solutions (2 mg mL−1 in ACN : H2O : DMSO mixture in 1 : 1 : 1, experiments. A Pine Instrument Company AFMSRCE modu-
v/v/v ratio) with the matrix solution (1 : 10 v/v) in a 0.5 mL lator speed rotator was employed in the experiments. The
Eppendorf® micro tube. Finally 0.5 μL of this mixture was working electrode was a glassy carbon disc (5.61 mm dia) and
deposited on the sample plate, dried at room temperature and a platinum ring leading to a collection efficiency, N = 37%.
then analyzed. On the other hand MALDI matrix, 2,5-dihydroxy- The disk potential was swept at 0.005 V s−1, the ring potential
benzoic acid was prepared in ACN : H2O mixture in 1 : 1 v/v was held at 0.95 V versus SCE and the rotation rate of the elec-
ratio at a concentration of 10 mg mL−1 and acidified with tri- trode was 2500 rpm. These experiments were carried out in
fluoroacetic acid (0.1%). MALDI samples for 2 were prepared 0.5 M H2SO4 oxygen saturated electrolyte at 25 °C. In order to
by mixing sample solutions (2 mg mL−1 in water : DMSO in disperse the catalysts on a carbon support, a mixture of the Pc
1 : 1 ratio by v/v containing 0.1% trifluoroacetic acid) with the compound, VC and 5 wt% Nf solution in absolute ethanol was
matrix solution (1 : 10 v/v) in a 0.5 mL Eppendorf® micro tube. prepared and ultrasonically homogenized for half an hour. A
Finally 0.5 μL of this mixture was deposited on the sample given volume of this ink was deposited using a micropipette
plate, dried at room temperature and then analyzed.
onto a freshly polished glassy carbon electrode leading to a cata-
lyst loading of 106 µg cm−2. In order to obtain a total coverage
of the glassy carbon surface, the deposited drop of ink was
3.3.5. Electrochemistry
Cyclic voltammetry and differential pulse voltammetry dried with pulsed air at ambient temperature to evaporate the
measurements were carried out with a PAR Model VersoStat II solvent quickly. Each voltammogram was recorded with a
Potentiostat/Galvanostat controlled by an external PC, and uti- freshly prepared electrode due to the possible catalyst degra-
lizing a three electrode configuration at 25 °C. Tetrabutyl- dation in acid medium.
ammonium perchlorate (TBAP) (Electrochemical grade, Fluka
Chemical Co.) was used as the supporting electrolyte in extra
3.3.6. OFET fabrication
pure DMSO. A saturated calomel electrode (SCE) was employed We fabricated OFETs having a bottom-gate and top source–
as the reference electrode and separated from the bulk of the drain contact geometry. To fabricate the OFETs used in this
solution by a fritted glass bridge filled with the solvent–sup- work, an indium tin oxide (ITO)-coated glass substrate was
porting electrolyte mixture. The ferrocene/ferrocenium couple used as gate electrode. Before using, the ITO coated substrates
(Fc/Fc+) was used as an internal standard, but potentials were were cleaned by ultrasonic treatment in acetone, isopropyl
reported with respect to SCE. The working electrode was plati- alcohol, and deionized water. After being dried by Ar gas, ITO
num in the measurements. A platinum spiral wire was used as surfaces were treated by UV-ozone for 20 min. Then, on the
the auxillary electrode. Solutions containing the complexes top of ITO surface, polyvinyl alcohol (PVA) was dissolved in dis-
were deoxygenated by a stream of high-purity nitrogen for at tilled water, with thicknesses of 300 nm was spin coated at
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1500 rpm. After drying the PVA film at 115 °C for 1 h in a
vacuum oven, a solution of Pcs in DMF, with a concentration
of 3 × 10−3 M, was spun on top of PVA film rotating at
2500 rpm. Elipsometric technique was used to measure the thick-
ness of the dielectric layer and the Pc films. Subsequently, the
deposited films (80 nm) were thermally annealed at 140 °C for
30 min to evaporate the solvent. Finally, on the top of Pc layer,
gold source and drain contacts with thickness of 110 nm was
evaporated by thermal evaporation at the pressure below
10−5 mbar. The source and drain electrodes were formed
through the shadow mask. The electrical characteristics of these
devices were measured in air. For electrical measurements a
Keithley 617 programmable electrometer and a Keithley 2400
sourcemeter were used.
2 C. C. Leznoff, H. Lam, S. M. Marcuccio, W. A. Nevin,
P. Janda, N. Kobayashi and A. B. P. Lever, J. Chem. Soc.,
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Conclusions
8 (a) M. Özer, A. Altındal, B. Salih, M. Bulut and
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A. Altındal, A. R. Özkaya and Ö. Bekaroğlu, Dalton Trans.,
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In this work, the novel dimeric MPcs, 2 and 3 have been
obtained from tetrakis-phthalonitrile in dimethylaminoetha-
nol with the proper metal salts. Electrochemical and in situ
spectroelectrochemical measurements suggested that while
complex 2 displays only Pc-based two-electron redox processes,
compound 3 shows both metal-based and Pc-based one-elec-
9 Z. Odabas, A. Altındal, A. R. Özkaya, M. Bulut, B. Salih and
Ö. Bekaroğlu, Polyhedron, 2007, 26, 3505.
tron processes. It was concluded from the electrocatalytic 10 S. Altun, A. Altındal, A. R. Özkaya, M. Bulut and
measurements suggested that the catalytic performance of Ö. Bekaroğlu, Tetrahedron Lett., 2008, 49, 4483.
Co2Pc2 3 towards ORR is much higher than that of Zn2Pc2 2. 11 M. S. Ağırtaş, A. Altındal, B. Salih, S. Saydam and
The catalytic measurements displayed that the nature of the Ö. Bekaroğlu, Dalton Trans., 2011, 40, 3315.
central metals in a dinuclear ball-type Pc complex has a 12 A. Sengül, H. Z. Doğan, A. Altındal, A. R. Özkaya, B. Salih
remarkable influence on its catalytic activity towards ORR. and Ö. Bekaroğlu, Dalton Trans., 2012, 41, 7559.
Compounds 2 and 3 have been applied as semiconducting 13 Ü. Salan, A. Altındal, A. R. Özkaya, B. Salih and
layer in OFETs. The results proved that spin coated films of Ö. Bekaroğlu, Dalton Trans., 2012, 41, 5177.
these compounds can be used as the semiconducting layers of 14 (a) N. Kobayashi, Y. Yanagisawa, T. Osa, H. Lam and
OFETs functioned as p-channel accumulation devices. From
the analysis of the electrical characteristics of the OFETs, it
was observed that the 2-based and 3-based OFETs with poly-
meric gate insulator presented satisfactory p-type electrical
characteristics with high mobilities. Such high performance
OFETs are a great impetus for realizing large-area, low-cost
and flexible displays.
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