Synthesis and self-assembly of phthalocyanines bearing sulfur-containing substituents

Article abstract of DOI:10.1016/j.cclet.2013.10.034

Asymmetrically substituted phthalocyanines with sulfur-containing substituents for fabrication of self-assembled monolayers were synthesized. Phthalocyanine 7, bearing a disulfide group, was synthesized from phthalocyanine with a hydroxyl group, which was prepared via mixed condensation of the corresponding substituted phthalonitriles. Phthalocyanine 10, bearing an acetyl protected thiol group, was synthesized through the Pd-catalyzed coupling reaction of an iodophthalocyanine. Their self-assembling behavior on gold substrates was further studied by UV-vis spectroscopy.

Full text of DOI:10.1016/j.cclet.2013.10.034

Chinese Chemical Letters 25 (2014) 46–50
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Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Synthesis and self-assembly of phthalocyanines bearing
sulfur-containing substituents
*
Yan Zhao, Jie Ding, Xue-Bin Huang
School of Chemistry, Beijing Institute of Technology, Beijing 100081, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Asymmetrically substituted phthalocyanines with sulfur-containing substituents for fabrication of self-
assembled monolayers were synthesized. Phthalocyanine 7, bearing a disulfide group, was synthesized
from phthalocyanine with a hydroxyl group, which was prepared via mixed condensation of the
corresponding substituted phthalonitriles. Phthalocyanine 10, bearing an acetyl protected thiol group,
was synthesized through the Pd-catalyzed coupling reaction of an iodophthalocyanine. Their self-
assembling behavior on gold substrates was further studied by UV–vis spectroscopy.
ß 2013 Xue-Bin Huang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
Received 29 July 2013
Received in revised form 10 October 2013
Accepted 22 October 2013
Available online 23 November 2013
Keywords:
Phthalocyanine
Sulfur-containing substituents
Self-assembly
1. Introduction
of substituents can be introduced onto the periphery of the
phthalocyanine rings and their properties can be finely tuned as
With their outstanding electronic and optical properties,
phthalocyanines and their metallic derivatives have become a
promising family of materials in recent years having already been
used in laser printers, photo-copiers, light-emitting diodes (LEDs),
data storage systems and photovoltaic cells. Their potential
applications in optical limiting, gas sensors, field-effect transistors
(FETs) and photodynamic therapy have attracted even more
research interests. In the above research, phthalocyanine thin films
have always required fabrication. Consequently, there are several
ways to fabricate the thin films [1]: Langmuir–Blodgett technique,
solvent evaporation, thermal evaporation, electrochemical depo-
sition, stamping and self-assembly method. Among them, the self-
assembly method seems to become even more attractive with the
growth of the research field in molecular electronics. This method
has already been used in fabricating organic electronic devices [2].
Compared with thin films prepared by other methods, the self-
assembled monolayers (SAMs) are simple to fabricate, very
thermally and mechanically stable and can be grown on substrates
of all sizes. The self-assembling behavior of small molecules
bearing sulfur-containing substituents, such as disulfide, S-acetyl
and mercapto-groups, has been studied extensively [3]. They have
been shown to form self-assembled monolayers on gold, silver and
copper substrates. Compared to these small molecules, the self-
assembly of phthalocyanines with excellent optical and electrical
properties may be of more importance. As is well known, a variety
such. Many substituted phthalocyanines have been synthesized,
but only limited work has been done on the synthesis and the self-
assembly of phthalocyanines bearing sulfur-containing substitu-
ents [4–6]. We report here on the synthesis and self-assembly of
two phthalocyanines bearing sulfur-containing substituents in-
cluding disulfide and S-acetyl groups.
2. Experimental
The precursors, 4-nitrophthalonitrile and 4-tert-butylphthalo-
nitrile, were purchased from Tokyo Kasei Kogyo Co., Ltd. and used
as received without further purification. Phthalocyanines 8, 9 and
S-(4-iodophenyl)ethanethioate were prepared according to litera-
ture methods. Tetrahydrofuran (THF) was distilled from Na-
˚
benzophenone under nitrogen. Other solvents were dried over 4 A
molecular sieve and distilled prior to use. The 1H NMR spectra were
obtained on a Varian Unity 200 NMR Spectrometer, or Bruker
DMX 300 NMR Spectrometer. UV–vis spectra were measured
using a Hitachi Model U-3010 Spectrophotometer. MS spectra
(MALDI-TOF-MS) were obtained on a Bruker BIFLEX III Mass
Spectrometer.
2.1. Synthesis of 1,6-hexanediol monotetrahydropyranyl ether (2)
To an ice-cooled solution of 1,6-hexanediol (59 g, 500 mmol), p-
toluene sulfuric acid monohydrate (1.6 g, 8.4 mmol) and dihy-
dropyran (4.2 g, 50 mmol) were added with stirring. The mixture
was then allowed to warm to room temperature and stirred for
24 h. The resulting mixture was diluted with ether, washed with
*
Corresponding author.
E-mail address: huangxb@bit.edu.cn (X.-B. Huang).
1001-8417/$ – see front matter ß 2013 Xue-Bin Huang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.10.034

Y. Zhao et al. / Chinese Chemical Letters 25 (2014) 46–50
47
saturated aqueous potassium carbonate solution and dried with
2.3. Synthesis of phthalocyanine disulfide (7)
A solution of phthalocyanine 6 (90 mg, 103
anhydrous potassium carbonate. After solvent evaporation, the
remaining white solid was extracted with petroleum ether (60–
90 8C). Upon removal of the solvent, 6.4 g (63% yield) of 2 was
m
mol) in THF
(20 mL) and ethanol (6 mL) was brought to reflux. Then thiourea
(60 mg, 0.79 mmol) was added. The reaction was monitored by
TLC. After all of the starting material was consumed, aqueous
sodium hydroxide solution (20%, 12 mL) was added. When the
reaction monitored by TLC was complete, the resulting mixture
was poured into a mixture of dilute hydrochloride acid and ice and
extracted with dichloromethane. The organic phase was separated
and dried with anhydrous magnesium sulfate. After removal of
the solvent, the residue was purified by column chromatography
using dichloromethane as eluant. Phthalocyanine disulfide 7 was
obtained as a purple solid (36 mg, 43% yield). Data for 7: 1H NMR
obtained as colorless oil. Data for 2: 1H NMR (200 MHz, CDCl3):
d
1.41 (m, 4H, CH2), 1.55 (m, 8H, CH2), 1.87 (m, 2H, CH2), 3.31–3.57
(m, 2H, OCH2), 3.64 (t, 2H, J = 7.1 Hz, OCH2), 3.70–3.95 (m, 2H,
OCH2), 4.58 (t, 1H, J = 7.3 Hz, OCHO).
2.1.1. Synthesis of 4-[6-(tetrahydro-pyran-2-yloxy)
hexyloxy]phthalonitrile (3)
A
mixture of 4-nitrophthalonitrile (5.0 g, 29 mmol), 1,6-
hexanediol monotetrahydropyranyl ether 2 (5.8 g, 29 mmol) and
anhydrous potassium carbonate (12 g, 87 mmol) in DMF (150 mL)
was stirred overnight at 60 8C. The solvent was removed under
reduced pressure. The residue was dissolved with dichloro-
methane, filtered and purified by column chromatography (silica
gel) using dichloromethane as eluant. The product 3 was obtained
as yellow oil (7.5 g, 79% yield). Data for 3: 1H NMR (200 MHz,
(300 MHz, C6D6):
d
3.64 (br, 2H, NH), ꢀ3.33 (br, 2H, NH), 0.54–1.95
(m, 70H), 2.98 (m, 4H, SCH2), 4.35 (m, 4H, OCH2), 7.00–9.40
(m, 24H, aryl). MALDI-TOF-MS: m/z calcd. for C100H106N16O2S2:
1626.81, found: 1627.1 (M+). UV–vis (in chloroform):
701 (0.37), 664 (0.54), 641 (0.52), 338 (0.60).
lmax (log e)
acetone-d6):
d 1.48 (m, 12H, CH2), 1.80 (m, 2H, CH2), 3.40 (m, 2H,
OCH2), 3.75 (m, 2H, OCH2), 4.23 (t, 2H, J = 7.1 Hz, OCH2), 4.54 (m,
1H, OCHO), 7.45 (dd, 1H, phenyl), 7.60 (d, 1H, phenyl), 7.94 (d, 1H,
phenyl).
2.4. Synthesis of phthalocyanine (10)
To a solution of phthalocyanine 9 (0.2 g, 0.26 mmol) and S-(4-
iodophenyl) ethanethioate (72 mg, 0.26 mmol) in triethylamine
(10 mL), dichlorobis(triphenylphosphine)palladium(II) (9 mg,
2.2. Synthesis of 4-(6-hydroxyhexyloxy)phthalonitrile (4)
13
mmol) and copper (I) iodide (5 mg, 26 mmol) were added.
Phthalonitrile 3 (7.5 g, 23 mmol) and p-toluenesulfonic acid
(PTSA) (0.43 g, 2.3 mmol) were dissolved in ethanol (150 mL). The
resulting solution was stirred at 50 8C for 10 h at 50 8C. After
removal of the solvent, the residue was purified by column
chromatography (silica gel) using dichloromethane as eluant.
Phthalonitrile 4 (3.5 g, 63% yield) was obtained as a light yellow
The mixture was stirred at room temperature for 23 h. The solvent
was evaporated under reduced pressure. The residue was purified
by column chromatography using dichloromethane/methanol
(50:1) as eluent. The product was obtained as a black solid
(83 mg, 35% yield). Data for 10: 1H NMR (300 MHz, C6D6):
d 1.67
(m, 27H, t-butyl), 2.42 (s, 3H, COCH3), 6.54–8.70 (m, 16H, aryl).
MALDI-TOF-MS: m/z calcd. for C54H46N8OSZn: 918.28, found:
solid. Data for 4: 1H NMR (300 MHz, acetone-d6):
d 1.84 (m, 4H,
CH2), 1.50 (m, 4H, CH2), 3.65 (t, 2H, J = 7.1 Hz, OCH2), 4.05 (t, 2H,
J = 7.1 Hz, OCH2), 4.58 (br, 1H, OH), 7.18 (dd, 1H, phenyl), 7.25 (d,
1H, phenyl), 7.71 (d, 1H, phenyl).
918.4 (M+). UV–vis (in chloroform): lmax (log
e) 692 (1.6), 617
(0.38), 350 (0.93).
Quartz slides were washed with chloroform and immersed into
a solution of potassium hydroxide in 100 mL of deionized water
and 250 mL methanol for 12 h. Then the quartz slides were washed
thoroughly with deionized water and dried in a steam of nitrogen.
The gold substrates were prepared by thermal evaporation of a
layer of gold onto freshly cleaned quartz slides that had been
precoated with a chromium adhesion layer. For UV–vis characteri-
zation, 5 nm of chromium was deposited on cleaned quartz slides
followed by an 8 nm gold layer. Phthalocyanine SAMs were
prepared by immersing gold substrates into phthalocyanine
solutions in chloroform (6.7 ꢁ 10ꢀ5 mol/L and 2.9 ꢁ 10ꢀ5 mol/L
2.2.1. Synthesis of 2,9,16-tri(tert-butyl)-23-(6-hydroxyhexyloxy)
phthalocyanine (5)
To n-pentanol (70 mL), 4-tert-butylphthalonitrile (6.1 g,
33 mmol) and phthalonitrile 4 (2.7 g, 11 mmol) were added. The
mixture was brought to reflux. Then lithium metal (1.0 g,
0.14 mmol) was added in small portions. The reaction was
continued for 6 h. The solvent was removed under reduced
pressure. The black residue was washed with methanol and further
purified by column chromatography (silica gel) using chloroform
as eluant. The first band proved to be tetra-tert-butyl phthalocya-
nine. Then the second band was collected. Upon removal of the
solvents, the desired product was obtained as a shining purple
for
7 and 10, respectively) for 24 h at room temperature.
References for UV–vis measurements were prepared by immersing
corresponding substrates into pure chloroform for 24 h. Then the
substrates were washed thoroughly with chloroform and dried in a
stream of nitrogen.
solid (1.7 g, 19% yield). Data for 6: 1H NMR (300 MHz, C6D6):
d 2.34
(br, 1H, NH), ꢀ2.79 (br, 1H, NH), 1.71 (m, 2H), 1.86 (m, 31H), 2.16
(m, 2H), 3.84 (m, 2H, OCH2), 4.46 (m, 2H, OCH2), 7.20–9.20 (m, 12H,
aryl). MALDI-TOF-MS: m/z calcd. for C50H54N8O2: 798.44, found:
3. Results and discussion
798.3 (M+). UV–vis (in chloroform): lmax (log
e) 701 (0.49), 665
(0.41), 648 (0.17), 603 (0.09), 343 (0.25).
Several methods have been developed to synthesize substituted
phthalocyanines. Among these methods, condensation of the
corresponding phthalonitriles is the method usually referenced
and appears to be the most successful one. A variety of uniformly
substituted phthalocyanines have been synthesized through this
method. There are also several reports with details to synthesize
non-uniformly substitutedphthalocyanines [7,8], however, it seems
that the above method is the most satisfactory one at present. Two
kinds of phthalonitriles bearing different substituents were used
in the condensation. The product is a mixture of substituted
phthalocyanines. The desired substituted phthalocyanine is then
2.2.2. Synthesis of 2,9,16-tri(tert-butyl)-23-(6-methane
sulfonyloxyhexyloxy)phthalocyanine(6)
To an ice-cooled solution of
5 (120 mg, 150 mmol) in
dichloromethane (20 mL), triethylamine (60 drops) and methane
sulfonyl chloride (30 drops) were added with stirring. The mixture
was allowed to warm to room temperature and stirring continued
for 30 min. Then the resulting solution was washed with water and
dried with anhydrous magnesium sulfate. The product was
purified by column chromotography (silica gel) using chloroform
as eluant to yield 6 as a black blue solid (90 mg, 68% yield).
obtained after following
a careful separation procedure. 4-

48
Y. Zhao et al. / Chinese Chemical Letters 25 (2014) 46–50
NC
NC
DHP
PTSA, THF
4-nitrophthlonitrile
NC
ethanol,
PTSA
K CO , DMF
2
3
HO(CH2)6OTHP
HO(CH2)6OH
O(CH2)6OH
NC
O(CH2)6OTHP
1
2
4
3
C(CH3)3
C(CH3)3
N
N
N
N
N
N
N
4-tert-butylphthalonitrile
O(CH2)6OSO2CH3
O(CH2)6OH
Li, pentanol
(H3C)3C
NH
HN
(H3C)3C
NH
HN
Et N, CH Cl
2
MeSO Cl
2
3
2
N
N
N
N
N
C(CH3)3
C(CH3)3
5
6
C(CH3)3
1) Thiourea
2) NaOH
3) H+
N
N
N
N
(H3C)3C
NH
HN
O(CH2)6S
N
N
2
C(CH3)3
7
Scheme 1. Synthesis of phthalocyanine 7.
Nitrophthalonitrile was usually used as a precursor for many other
phthalonitriles through a nucleophilic substitution reaction of the
nitro-group [9].
phthalocyanine 5 bearing a hydroxyl group. Structural isomers
are possible for phthalocyanine 5 [10]. An established method was
used to convert the hydroxyl phthalocyanine to the phthalocya-
nine disulfide [11]. The hydroxyl phthalocyanine was first
mesylated, then reacted with thiourea forming an isothiuronium
salt. Phthalocyanine disulfide 7 was obtained when hydrolysis of
the isothiuronium salt was carried out in air. Phthalocyanine
disulfide was synthesized instead of mercaptophthalocyanine.
both of which can form SAM on gold substrates [4,12]. However,
the thiol can change into the disulfide in air, which makes the latter
one unstable for use in an air envirnoment.
Scheme 2 shows the synthetic route to phthalocyanine 10. The
monoiodophthalocyanine 8 was synthesized according to the
literature method [5]. Several reagents, including 2-methyl-3-
butyn-2-ol, and trimethylsilylacetylene, were used to introduce
the acetylenyl group into the monoiodophthalocyanine [13]. The
second one is preferable because the reaction conditions are milder
and can be carried out at room temperature. Tetrabutylammonium
fluoride and sodium hydroxide are always used as the reagents for
the desilylation reaction [13]. The method using sodium hydroxide
is preferred because the reaction condition is more moderate and
The synthetic route to phthalocyanines 7 is shown in Scheme 1.
Attempts to synthesize 4-(6-hydroxyhexyloxy)phthalonitrile
through the reaction of 1,6-hexanediol with 4-nitrophthalonitrile
proved to be unsuccessful due to the two reactive hydroxyl groups
in each diol molecule, which may react with two phthalonitrile
molecules. In order to prevent this possibility, one hydroxyl group
of the 1,6-hexanediol was protected by a tetrahydropyranyl group.
We used a large excess of diol to react with dihydropyran, then
extracted with petroleum ether to obtain 1,6-hexanediol mono-
tetrahydropyranyl ether 2. The protected diol was then reacted
with 4-nitrophthalonitrile to afford 4-(6-tetrahydropyranylhex-
yloxy)phthalonitrile 3. After deprotection of the hydroxy group,
one of the phthalonitrile precursors 4 was obtained. Additionally,
4-tert-butylphthalonitrile was selected as another phthalonitrile to
ensure that the phthalocyanines synthesized have good solubility
in organic solvents. The mixed condensation of phthalonitrile 4
and 4-tert-butylphthalonitrile (1:3 ratio) was carried out in
n-pentanol in the presence of lithium pentoxide to afford
C(CH )
3 3
C(CH )
3 3
C(CH )
3 3
N
N
N
Zn
N
N
N
N
N
N
Zn
N
1) (Ph3P)2PdCl2, CuI, Et3N
trimethylsilylacetylene
N
Zn
N
(Ph3P)2PdCl2, CuI, Et3N
(H C) C
N
N
3
3
(H C) C
N
N
3
3
(H C) C
N
N
3
3
S-(4-iodophenyl)ethanethioate
2) NaOH
I
N
N
N
N
N
N
SAc
C(CH )
3 3
C(CH )
3 3
C(CH )
3 3
10
8
9
Scheme 2. Synthesis of phthalocyanine 10.

Y. Zhao et al. / Chinese Chemical Letters 25 (2014) 46–50
49
vis spectra of these solutions (Fig. 1), we can see that the
aggregation tendency of the two compounds is very high. They
appear primarily as dimers as shown in the Q-band of their UV–vis
spectra. UV–vis spectra of the self-assembled monolayers were
measured using references prepared by immersing gold sub-
strates on quartz slides in pure solvent. Characteristic Q-bands
associated with phthalocyanines can be seen in the spectra
(Fig. 2). It is obvious that the Q-bands of the SAMs are broadened
and blue-shifted relative to those observed in solution (Fig. 1).
Such phenomena are also observed by other authors [17,18] and
can be explained by the existence of phthalocyanine molecules in
more aggregated states in self-assembled monolayers than in
solutions.
4. Conclusion
Asymmetrically substituted phthalocyanines with sulfur-con-
taining substituents for the fabrication of self-assembled mono-
Fig. 1. UV–vis spectra of phthalocyanine 7 (solid line) and 10 (dashed line) in
layers were synthesized and characterized by MALDI-TOF-MS, 1
H
chloroform.
NMR and UV–vis spectrometry. Phthalocyanine 7, bearing a
disulfide group, was synthesized from phthalocyanine with a
hydroxyl group via a mixed condensation of the corresponding
substituted phthalonitriles. Phthalocyanine 10, bearing an acetyl
protected thiol group, was synthesized through the Pd-catalysed
coupling reaction of an iodophthalocyanine. Both 7 and 10 can
form self-assembled monolayers on gold substrates based on the
UV–vis spectra data. It was found that the Q-bands of the self-
assembled monolayers were broadened and blue-shifted com-
pared to their corresponding spectra in solutions.
the reagents are less expensive [14]. Phthalocyanine 9 was further
reacted with S-(4-iodophenyl)ethanethioate [15] in the presence
of dichlorobis(triphenylphosphine)palladium(II) and copper (I)
iodide to afford the target phthalocyanine 10. In this procedure, the
yield was only 35% because the acetyl-protected thiol group may
cause Pd-catalyst poisoning. Fortunately, we can use other groups
to protect the thiol group to improve the yield [16]. These
phthalocyanine compounds were characterized by MALDI-TOF-
MS, 1H NMR and UV–vis spectrometry. The results are consistent
with their structures.
Acknowledgment
It is well known that thiols, acetyl-protected thiols, and
disulfides can form self-assembled monolayers on gold substrates
[3]. We studied the self-assembly behavior of phthalocyanine
derivatives 7 and 10. Freshly prepared gold substrates on quartz
slides were immersed in solution of compounds 7 or 10 in
chloroform (6.7 ꢁ 10ꢀ5 mol/L and 2.9 ꢁ 10ꢀ5 mol/L for 7 and 10,
respectively) for 24 h at room temperature. Then the substrates
were washed thoroughly with chloroform to remove unbonded
phthalocyanine molecules and dried in a stream of nitrogen.
UV–vis spectra of solutions and self-assembled monolayers of
phthalocyanine 7 and 10 are shown in Figs. 1 and 2. From the UV–
This work was supported by the NSFC (Nos. 50973011 and
20702004).
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