Octathienyl/phenyl-substituted zinc phthalocyanines J-aggregated through conformational planarization

Article abstract of DOI:10.1039/c0dt01066a

A novel family of thiophene-decorated phthalocyanines (M-TPcs) was efficiently synthesized, during which the key intermediate of these compounds was purified through a chemical approach. In the optimized geometries of M-TPcs, the peripherally linked thiophene rings are tilted from the Pc core, which oppose aggregation considering the mutual steric hindrance. However, Zn-TPc formed J-aggregates in many solvents while Ni-TPc and Cu-TPc did not. Octaphenyl-substituted zinc Pc (Zn-PPc) showed similar J-aggregation behavior as Zn-TPc. This unusual J-aggregation was attributed to the conformational planarization of the corresponding molecules.

Full text of DOI:10.1039/c0dt01066a

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 393
www.rsc.org/dalton
PAPER
Octathienyl/phenyl-substituted zinc phthalocyanines J-aggregated through
conformational planarization†
Zihui Chen,*a,b Lihong Niu,a Yuanhua Cheng,a Xinyu Zhou,a Cheng Zhonga and Fushi Zhang*a
Received 19th August 2010, Accepted 6th October 2010
DOI: 10.1039/c0dt01066a
A novel family of thiophene-decorated phthalocyanines (M-TPcs) was efficiently synthesized, during
which the key intermediate of these compounds was purified through a chemical approach. In the
optimized geometries of M-TPcs, the peripherally linked thiophene rings are tilted from the Pc core,
which oppose aggregation considering the mutual steric hindrance. However, Zn-TPc formed
J-aggregates in many solvents while Ni-TPc and Cu-TPc did not. Octaphenyl-substituted zinc Pc
(Zn-PPc) showed similar J-aggregation behavior as Zn-TPc. This unusual J-aggregation was attributed
to the conformational planarization of the corresponding molecules.
aggregates, but J-aggregates are promising for various technolog-
ical applications such as spectral sensitization, optical storage,
Introduction
and nonlinear optics.4 In light of such considerations, it is not
surprising that methods to assemble J-aggregates of Pcs are being
actively pursued. To date, most reported J-aggregates of Pc were
achieved either by utilizing the coordination of the side substituent
from one Pc molecule to the central metal ion in a neighbor,5 or by
interfacial engineering.6 Isago7 reported a novel antimony(III)-Pc
complex that could J-aggregate in dichloromethane. However, the
J-aggregation mechanism of this compound is as yet unknown.
During our exploration of photo-controllable self-assemblies
of functional dyes, we synthesized a novel family of thiophene-
decorated Pcs, namely 2,3,9,10,16,17,24,25-octakis(2,5-dimethyl-
3-thienyl) metal Pcs (abbreviated as M-TPc), and unexpectedly
discovered that Zn-TPc was able to J-aggregate in many solvents
in an unprecedented manner.
Besides the intriguing J-aggregation behavior, M-TPcs may ex-
hibit some other unusual physicochemical properties, considering
the unique optical and redox properties of thiophene rings, as well
as their self-assembly properties on solid surfaces or in the bulk.8
The high polarizability of the sulfur atom in thiophene ring leads to
a low aromatic stabilization energy, which is the most important
reason for it being widely used as constructing units of many
functional materials, such as charge-transporting molecules9 and
thermally irreversible photochromic compounds.10 In addition,
the non-bonding sulfur–sulfur interaction of neighboring thio-
phenes is also very interesting to scientists.11 Tian and co-workers
designed a series of bis-thienylethene-based tetraazoporphyrin
and Pc hybrids as photo-switching molecules.12 These thiophene-
Pc hybrids showed photochromism in chloroform and the fluo-
rescence intensity of these compounds could be photo-regulated.
It is well-known that the unique physico-chemical properties of
Pcs generally arise from their large flat aromatic surface. Any
change to this surface can exert unpredictable influences on the
special electronic and optical properties of Pcs, as well as their
Being a versatile class of functional dyes, the importance and
potential of phthalocyanines (Pcs) are rapidly growing in modern
science. This is mainly because of their high degree of aromaticity,
their intriguing electronic and optical characteristics, their excep-
tionally high chemical and thermal stability, and the flexibility
involved in the synthesis of these compounds.1 Moreover, different
metal atoms in the center and substituents on the sides of the large
Pc ring endow Pcs with various functionalities. Synthetic chemists
are able to rationally design the structure of Pc according to their
own needs.2
It has been well-established that Pcs can form two types of one-
dimensional aggregates, namely face-to-face H-aggregates and
head-to-tail J-aggregates.3 The J-aggregated Pcs are characterized
by a remarkable bathochromic shift of the Q-band absorption,
whereas the H-aggregated counterparts show a hypsochromic
shift from the isolated monomers. Pcs form preferentially the H-
aThe Key Lab of Organic Photoelectrons & Molecular Engineering of
Ministry of Education, Department of Chemistry, Tsinghua University,
Beijing, 100084, PR China
bDepartment of Physics & Tsinghua-Foxconn Nanotechnology Research Cen-
tre, Tsinghua University, Beijing, China. E-mail: zhchen@tsinghua.edu.cn,
zhangfs@tsinghua.edu.cn; Fax: + 86 10 62770304; Tel: +86 10 62782596
† Electronic supplementary information (ESI) available: Fig. S1: (a) The
TLC experiment of intermediate 2 and the product obtained from the
Suzuki cross-coupling reaction, with a 2 : 1 mixed solvent of petroleum
1
ether and ethyl acetate as eluent. (b) H NMR spectra of the product in
CDCl3. Fig. S2: TLC experiments in the synthesis of phthalonitrile 7 from
the mixture obtained from the Suzuki cross-coupling reaction. Fig. S3: UV-
Vis spectrum of Cu-TPc and Ni-TPc in THF (c = 1 ¥ 10-4 mol L-1). Fig.
S4: 1H NMR spectra of Ni-TPc and Zn-TPc. Fig. S5: UV-Vis spectrum of
Zn-TPc in THF (c = 2 ¥10-5 mol L-1). Fig. S6: UV-Vis spectral changes of
Zn-TPc (c = 2 ¥10-5 mol L-1) in chloroform upon the addition of a drop of
pyridine. Fig. S7: Plot of lg(c - (A700/e700)) vs. lg(A700/e700) of Zn-TPc in the
concentration range of 1.25 ¥ 10-6 to 2 ¥ 10-5 mol L-1. Fig. S8: Computer
optimized conformation of Zn-PPc by energy minimization method. See
DOI: 10.1039/c0dt01066a
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 393–401 | 393
©

View Online
self-assembly behavior. Therefore, thiophene-decorated Pc in
which the basic skeleton of Pc is kept intact, may be a better
prototype to combine the respective advantages of thiophenes
and Pc than corresponding hybrids. However, such compounds
are as yet rarely reported. Muto et al.13 first prepared a Pc
derivative bearing eight 2-thienyl substituents at the b-postions.
By electrochemical oxidation of this Pc, they constructed a
p-conjugated polymer film which has potential electrochromic
material applications.
Taking into account all the significant factors mentioned above,
we report here the synthesis and J-aggregation properties of
M-TPcs with specific discussion on the possible J-aggregation
mechanism.
even after prolonged reaction time, probably as a result of the steric
bulkiness of the system.14 To make matters worse, the Rf value of
the product is exactly the same as that of reagent 1, which means
that complete purification of compound 2 by chromatography is
probably very difficult. Column chromatography on silica gel with
petroleum–ethyl acetate (5 : 1, volume ratio) afforded a mixture
1
containing compound 2 as a yellow ropy liquid, and H NMR
experiments showed that the mixture was composed of reagent
1, intermediate 2 and the mono-substituted byproduct 8 with
a molar ratio of about 1 : 1.75 : 1 (Fig. S1, ESI†). These results
apparently ruled out recrystallization as an alternative method for
the purification of compound 2.
Since the products obtained from Suzuki cross-coupling reac-
tion cannot be purified directly with simple physical methods,
the exploration of effective chemical approaches in purification
of intermediate 2 is of vital importance to the synthetic route in
Scheme 1.
Results and discussion
Synthesis
According to our experience, the reactions involved in the
synthesis of phthalonitrile 7 from compound 2 are very reliable
(Scheme 1) with conversions close to quantitative. Besides, the
molecular polarity of desired product is quite different from
that of reagents in each step, which is excellent for purification.
With the synthesis route discussed above, compounds 8 and 1
were converted to the corresponding phthalonitriles 9 and 10
(structures shown in Scheme 2). Note that the bromo-groups in
these compounds are extremely mobile leaving groups in SNAr
reactions and thus, can be substituted with various nucleophiles.
Based on this observation, a cascade synthetic strategy of M-TPcs
was designed.
As shown in Scheme 1, the synthesis of M-TPcs started with
the Suzuki cross-coupling of 2,5-dimethylthien-3-ylboronic acid
with 4,5-dibromophthalic acid dimethyl ester in the presence of
Pd(PPh3)4 to give the key intermediate 2. However, coupling of
the second equivalent of boronic acid was proven to be inefficient
Scheme 1 The structures and synthetic route for Cu-TPc, Zn-TPc and
Ni-TPc.
Scheme 2 The synthesis and purification of intermediate 7.
394 | Dalton Trans., 2011, 40, 393–401
This journal is
The Royal Society of Chemistry 2011
©

View Online
The mixture obtained from the Suzuki cross-coupling reaction
was directly used for the preparation of phthalonitrile 7 according
to predetermined route in Scheme 1. For each step in this
route, only TLC was employed to monitor the synthesis process
(Fig. S2, ESI†). The crude products obtained were used directly
in the next steps without purification. As shown in Scheme 2,
after compounds 1/2/8 were converted to corresponding ph-
thalonitriles, the resulting mixtures were treated with an excess
of hydroquinone and K2CO3 in N,N-dimethylformamide (DMF)
at room temperature. Phthalonitriles 9 and 10 transformed to
compound 11 and 12 completely, while phthalonitrile 7 remained
unchanged. Compounds 11 and 12 could then react with sodium
hydroxide to form water-soluble compounds. Therefore, a mixture
of compounds 11, 12 and 7 was added to an aqueous solution of
sodium hydroxide. After stirring for a few minutes, the insoluble
substance was mostly phthalonitrile 7. Further purification with
column chromatography over silica gel produced pure phthaloni-
trile 7 with a total yield of 19%.
The excellent dispersibility of Ni-TPc was further demonstrated
by H NMR (Fig. S4A, ESI†). The resonance signals of Ni-TPc
1
in d-chloroform (c = 1.5 ¥ 10-3 mol L-1) are well defined. The
hydrogens atom on the benzene ring is located at 9.29 ppm and
that on the thiophene ring is located at 6.69 ppm. These single
peaks are well consistent with the symmetrical structure of Ni-TPc,
indicating that this compound exists as monomers in chloroform
even at such high concentration.
Zn-TPc showed very different UV-Vis absorption behavior.
Fig. 2 displays the concentration-dependent UV-Vis spectra of
Zn-TPc in THF. It can be seen that in this concentration range,
Zn-TPc exhibited typical monomeric absorption behavior, which
suggested a homogeneous dispersion in solution. However, when
the concentration of Zn-TPc exceeded 2 ¥ 10-5 mol L-1, in addition
to the normal Q-band at 694 nm, a red-shifted band at 740 nm
was observed (Fig. S5, ESI†).
Having completed the preparation of intermediate 7, we now
intended to complete the synthesis of M-TPcs. A mixture of
phthalonitrile 7 and salts of appropriate metals in ethoxyethanol
were refluxed for 3 h under nitrogen to yield the target products
Cu-TPc, Zn-TPc and Ni-TPc. These products were characterized
by IR, MS, UV-Vis spectroscopy and elemental analysis.
Aggregation abilities
Typical UV-Vis spectra of monomeric Pcs feature two distinct
absorption bands, namely the B- and Q-band, which can be
readily interpreted by using of Gouterman’s highly simplified
four-orbital model.15 The UV-Vis spectra of Cu-TPc and Ni-TPc
in tetrahydrofuran (THF) are depicted in Fig. 1. Both of the
samples showed sharp, intense Q bands, which were at 694 nm
(Cu-TPc) and 692 nm (Ni-TPc), respectively. The absorptions of
these complexes obey the Beer–Lambert law in a concentration
range from 1.25 ¥ 10-6 to 1 ¥ 10-4 mol L-1 (Fig. S3, ESI†). The UV-
Vis absorption properties of Cu-TPc and Ni–Pc in chloroform,
toluene and DMSO were quite similar to those in THF, suggesting
that these two Pcs were essentially evenly dispersed in these
solvents.
Fig. 2 Concentration-dependent UV-Vis spectra of Zn-TPc in THF.
This unusual red-shifted band was much more remarkable
in chloroform. As shown in Fig. 3, in addition to the typical
Q-absorption at 700 nm, the absorption of Zn-TPc in chloroform
exhibited a broad red-shifted band at 750 nm. After adding
small amounts of ethyl acetate (EA) to the chloroform solution
Fig. 1 UV-Vis spectra of Cu-TPc and Ni-TPc in THF (c = 5 ¥
Fig. 3 UV-Vis spectra of Zn-TPc in chloroform (-·-) and in a 20 : 1 mixed
solvent of chloroform and EA (—); c = 5 ¥ 10-6 mol L-1.
10-6 mol L-1).
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 393–401 | 395
©

View Online
of Zn-TPc, the absorbance at 700 nm increased from 0.89 to
1.01, whereas the red-shifted peak at 750 nm disappeared. As a
consequence, a typical monomeric spectrum was obtained. It was
noted that some other solvents such as pyridine and DMF show
a similar but more effective role relative to that of EA (Fig. S6,
ESI†).
Generally, there are three possibilities that can produce the
unusual red-shifted absorption in Fig. 3: (1) oxidation of Pc
ring; (2) monoprotonation of Pc ring; (3) J-aggregation of Pc
molecules. The first possibility can be readily ruled out by the
lack of the typical broad absorption of an oxidized Pc ring in
the region of 520 nm. Aforementioned titration experiments can
also rule out the oxidation mechanism since EA, pyridine and
DMF are all far from good reductants. Moreover, compared with
the basic nitrogen atoms in the Pc ring, EA is a much weaker
proton acceptor. It is unlikely therefore that EA can deprotonate
the protonated Pc molecules. Therefore, the spectral changes of
Zn-TPc in chloroform upon the addition of EA also make the
second possibility unlikely.
The concentration-dependent absorptions of Zn-TPc in chloro-
form are shown in Fig. 5. With an increase in concentration, the
apparent molar extinction coefficient at 700 nm became smaller. In
turn, the intensity of the band around 750 nm grew. In a solid film,
the relative intensity of the lower energy band became comparable
to that of the higher energy counterpart. These concentration-
dependent spectral changes of Zn-TPc are typical of molecular
J-aggregation in an equilibrium process.
To thoroughly rule out the second possibility, a study of the
UV-Vis spectra of monoprotonated Zn-TPc is helpful. The mono-
protonation of Pc ring was achieved by adding trifluoroacetic acid
(TFA) to a 20 : 1 chloroform–EA solution of Zn-TPc. As shown
in Fig. 4, with increasing TFA concentration, the absorption at
700 nm decreased sharply, while a new red-shifted absorption band
at 746 nm emerged with four isobestic points at 329, 408, 571 and
720 nm. The intensity of this new band at 746 nm reached its
maximum when the TFA concentration exceeded 0.45 mol L-1;
use of an excess of TFA resulted in the degradation of Zn-TPc.
Compared with Fig. 3, not only the shape of the red-shifted
Q-band, but also the positions of isobestic points shown in
Fig. 4 were apparently different, suggesting that the red-shifted
absorption of Zn-TPc in chloroform was not originated from
protonation.
Fig. 5 Concentration dependent UV-Vis spectra (in Q-region) of Zn-TPc
in chloroform.
It is well known that the efficiencies of processes such as energy
transfer and electron transfer are sensitive to the aggregation
state of dyes,16 and the formation of J-aggregates often lead to a
decrease offluorescence yieldofPcs. Fig. 6 showedthe fluorescence
emission spectra of Zn-TPc in THF and chloroform when excited
at 650 nm. Compared with that in THF, the fluorescence quantum
yield of Zn-TPc in chloroform was decreased from 0.21 to 0.09.
Together with above-discussed UV-Vis absorption studies, the
different fluorescence spectra of Zn-TPc in different solvents can
simply be ascribed to that Zn-TPc showed a stronger tendency to
J-aggregate in chloroform than in THF.
Fig. 4 Absorption spectral changes upon the addition of TFA to Zn-TPc
(5 ¥ 10-6 mol L-1) in 20 : 1 mixed solvent of chloroform and EA.
Therefore it seems that the unusual red-shifted band of Zn-TPc
in Fig. 3 might be attributed to the formation of Pc J-aggregates.
This assumption was first demonstrated by the concentration-
dependent UV-Vis spectra of Zn-TPc in chloroform and further
corroborated with fluorescence spectra.
Fig. 6 Fluorescence spectra of Zn-TPc in THF and chloroform when
excited at 650 nm (c = 5 ¥ 10-6 mol L-1).
396 | Dalton Trans., 2011, 40, 393–401
This journal is
The Royal Society of Chemistry 2011
©

View Online
1H NMR experiments provided another powerful line of
evidence for the J-aggregation of Zn-TPc. In a 5 : 1 mixed solvent of
d-chloroform and d5-pyridine, as shown in Fig. 7(a), the resonance
signals of Zn-TPc (c = 1.5 ¥ 10-3 mol L-1) are well-defined (the
peaks at 8.62, 7.65, 7.55 and 7.26 ppm are ascribed to the solvents
(Fig. S4C, ESI†)). The single peak at 9.53 ppm belongs to the
hydrogen atoms on the benzene rings. The hydrogen atoms on the
thiophene rings showed a signal at 6.79 ppm. Additionally, there
are two single peaks located at 2.51 and 2.37 ppm, respectively,
which belong to the two types of methyl groups on the thiophene
rings. The ratio of the integral areas of these four peaks is 1 : 1 : 3 : 3.
and 7(c) deviated from theoretical values, which can be readily
explained by the shielding effect of aggregation.
The average aggregation number (n) and the aggregation
equilibrium constant (k) of Zn-TPc in chloroform can be estimated
according to the following procedures. This procedure assumes
single-step equilibrium between the monomers and the aggregate
(eqn (1)):
(1)
n M ꢀ Mn
where M is the monomer of Zn-TPc, and Mn is the J-aggregate of
Zn-TPc with aggregation number n.
1
All the H NMR signals shown in Fig. 7(a) are consistent with
Defining the overall concentration of Zn-TPc as c and the
concentration of monomer is c1 when the equilibrium is estab-
lished, then the concentration of aggregates will be (c - c1)/n at
equilibrium. Then, the aggregation equilibrium constant (k) is
given by eqn (2):
the D4h symmetrical structure of monomeric Zn-TPc, clearly
demonstrating the purity of Zn-TPc. In d-chloroform (c = 1.5 ¥
1
10-3 mol L-1), as shown in Fig. 7(b), the H NMR spectrum of
Zn-TPc became much more complex. Compared with Fig. 7(a),
the intensity of the signal at 9.53 dropped significantly and the
signal at 6.79 disappeared, accompanied by the appearance of
more than eight new signals. When the concentration of Zn-TPc
n
k = (c - c1)/nc1
(2)
In our experiments, the Q-absorbance of momomeric phthalo-
cyanine at a certain concentration is almost constant in different
solvents, e.g. toluene, THF, etc. Then e700 (the molar extinction
coefficient of monomeric Zn-TPc at 700 nm) can be calculated
from Fig. 3 to be 1.97 ¥ 105, and A700 at different concentrations
can be read from Fig. 5.
1
increased to 1.5 ¥ 10-2 mol L-1, the H NMR spectrum of Zn-
TPc was remarkably different from that at low concentration. As
shown in Fig. 7(c), the resonance signals of Zn-TPc observed in
Fig. 7(b) almost vanished, concomitant with the appearance of
two new peaks at 6.98 and 5.01 ppm. These signals in Fig. 7(b)
and 7(c) are no longer consistent with a D4h symmetrical structure
of Zn-TPc.
Then c1 can be calculated by eqn (3):
(3)
(4)
(5)
A
700/e700
Based on eqn (2) and eqn (3), we obtain k from eqn (4):
c − A700 /e700
(
)
k =
n
)
n A700 /e700
(
This can be rewritten as:
lg(c - (A700/e700)) = lg nk + n lg(A700/e700
)
At fixed temperature, n, k and e700 are constant. Therefore, if lg(c
- (A700/e700)) is plotted against lg(A700/e700), a straight line should
be obtained. The slope of this line is n and the intercept is at lg nk.
Such a plot is shown in Fig. 8. Accordingly, we calculated
n = 1.78 and k = 1060 when the concentration of Zn-TPc varied
Fig. 7 1H NMR spectra of (a) 1.5 ¥ 10-3 mol L-1 Zn-TPc solution in a
5 : 1 mixed solvent of d-chloroform and d5-pyridine; (b) 1.5 ¥ 10-3 mol L-1
Zn-TPc solution in d-chloroform; (c) 1.5 ¥ 10-2 mol L-1 Zn-TPc solution
in d-chloroform. The signals marked with asterisks are due to solvents.
Both the aggregation and monoprotonation of Pc ring could
lower the symmetry of Zn-TPc. However, for a dynamic
protonation–deprotonation equilibrium of Pcs, the variation in
concentration can only change the relative intensity of signals
but not their position. Therefore, the concentration-dependent
1H NMR spectra of Zn-TPc in chloroform also have eliminated
the possibility of a protonation mechanism. Besides, it was also
observed that the relative integral areas of the signals in Fig. 7(b)
Fig. 8 Plot of lg(c - (A700/e700)) vs. lg(A700/e700) with the concentration of
Zn-TPc varied from 5 ¥ 10-6 to 7 ¥ 10-5 mol L-1.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 393–401 | 397
©

View Online
from 5 ¥ 10-6 to 7 ¥ 10-5 mol L-1. However, in the concentration
range of 1.25 ¥ 10-6 to 2 ¥ 10-5 mol L-1, n and k of Zn-TPc in
chloroform were calculated as 1.10 and 2.32, respectively (Fig. S7,
ESI†), which indicated that the concentration of Zn-TPc exerted
significant influence on its aggregation behavior. The larger the
concentration, the easier it is for Zn-TPc to aggregate.
In order to gain more insights into the J-aggregation of
Zn-TPc, molecular geometry optimization was performed in
energy minimization mode. As shown in Fig. 10(a), the thiophene
rings linked to the Pc core are tilted from the Pc core, and the
dihedral ◦angles between each thiophene ring and the Pc core are
about 60 . Mckeown et al.19 reported a series of Pcs with out-of-
plane alkyl substituents, and found that these substituents served
as a “picket-fence” to prevent the macrocycles from getting close
to each other owing to mutual steric hindrance. The lowest energy
conformation of Zn-TPc possesses similar characteristics to the
“picket-fence” Pcs, which opposes aggregation.
In recent years, studies on several compounds20 have revealed
that a more planar conformation is favored in aggregates, even
though the twist conformation is preferred in the isolated state.
The J-aggregation behavior of Zn-TPc is thus thought to originate
from its higher energy conformers. A possible mechanism is
depicted in Fig. 10(a). Though the “picket-fence” conformation is
dominant when Zn-TPc is molecularly dissolved in a good solvent,
the rotation of thiophene rings generates other conformations
with different coplanarization, in dynamic equilibrium. The
coplanar conformers of Zn-TPc are theoretically advantageous
to aggregate in terms of both electronic structure and steric
hindrance. However, Zn-TPc can not assume a perfect coplanar
conformation due to the consequent steric crowdedness, which
precludes parallel face-to-face intermolecular interactions. In
contrast, these coplanar conformers are arranged in head-to-
tail direction, forming J-aggregates of Zn-TPc. This, in turn,
induced more “picket fence” conformers to transform to coplanar
conformers until new equilibria were established. Moreover, it is
assumed that there are several different coplanar conformers that
could J-aggregate, leading to various degrees of intermolecular
p–p stackings with different bathochromic shifts of the Q-band
absorption. This can explain why the J-band of Zn-TPc in
chloroform was broad and not sharp and intense as previously
reported.5
Abaham et al.17 reported Zn(II) porphyrins which could form
aggregates in chloroform through a mechanism involving p–p
1
stacking, and found that accompanying the aggregation, the H
NMR signals corresponding to the hydrogen atoms on the por-
phyrin ring were remarkably upfield shifted. From Fig. 7, it can be
seen that Zn-TPc showed a similar tendency as those porphyrins.
Therefore, we deduced that intermolecular p–p interaction also
was the dominating driving force for the J-aggregation of Zn-TPc.
To test this assumption, we subsequently measured the
UV-Vis spectrum of Zn-TPc in toluene. Unlike pyridine, toluene
can not coordinate with ZnPc, however, it can disrupt the
p–p interaction.18 Therefore, if it is true that Zn-TPc underwent
J-aggregation through intermolecular p–p stacking, the aggrega-
tion tendency of Zn-TPc in toluene will be weaker than that in
chloroform. As shown in Fig. 9, the experimental result indeed
matched the above prediction. For a 5 ¥ 10-6 mol L-1 solution of
Zn-TPc in toluene, the value of A700/A750 in toluene is about 36,
which is 2.3 times larger than that in chloroform.
The different aggregation abilities between Ni-TPc and
Zn-TPc can be explained by the intramolecular polarization
between the phthalocyanine ring and its central metal. Abraham
et al. investigated the p–p interaction of a series of porphyrins with
different metal cores and found that the greater the intramolecular
polarization between the porphyrin and the metal, the stronger is
the p–p interaction between two porphyrins, thereby exhibiting
larger tendency to aggregate.21
Fig. 9 UV-Vis spectra of Zn-TPc in toluene (c = 5 ¥ 10-6 mol L-1).
Fig. 10 (a) Computer optimized conformation of Zn-TPc by energy minimization method; (b) the mechanism for J-aggregation of Zn-TPc.
398 | Dalton Trans., 2011, 40, 393–401 This journal is The Royal Society of Chemistry 2011
©

View Online
Fig. 11 (a) The chemical structure of Zn-PPc and (b) the UV-Vis spectra of Zn-PPc (5 ¥ 10-6 mol L-1) in THF and chloroform.
It is noteworthy that aforementioned “conformational pla-
narization induced J-aggregation behavior” is not exclusive to Zn-
TPc. We have observed that the octaphenyl-substituted zinc Pc
(abbreviated as Zn-PPc), which possesses similar conformational
characteristics to Zn-TPc (Fig. S8, ESI†), could also J-aggregate
through the same mechanism. As an example, the solvent-
dependent UV-Vis spectra of Zn-PPc (Fig. 11) demonstrated its
J-aggregation properties. In THF, Zn-PPc exhibited an intense
Q-band absorption at 698 nm, which is typical of monomer
dispersion. When Zn-PPc was dissolved in chloroform, the absorp-
tion at 698 nm sharply decreased from 1.06 to 0.18. Meanwhile,
a remarkably broadened and red shifted Q-band appeared,
suggesting the formation of J-aggregates. The comparison between
Fig. 11 and Fig. 3 revealed that Zn-PPc is more subject to
J-aggregation than Zn-TPc. This result is in excellent agreement
with the conformational analysis: the steric crowdedness involved
in Zn-PPc is smaller than that in Zn-TPc. As a result, Zn-PPc
forms coplanar conformers more easily. Besides, the p–p stacking
ability of the phenyl substituent is larger than that of the thienyl
substituent. These two reasons explain why ZnPPc showed much
stronger J-aggregation tendency than Zn-TPc.
optical properties. we believe this study will be helpful in both
supramolecular chemistry and photonics.
Experimental
General remarks
Absorption spectra were measured by a diode array spectropho-
tometer HP8452A; IR spectra were obtained on a Nicolet Avatar
360 FT-IR spectroscopy; NMR spectra were recorded on a
JOEL JNM-ECA300 spectrometer with TMS as the internal
reference. Mass spectroscopy was recorded on a Bruker LC-
MS/MS (ESQUIRE-LC) 1100; TOF-MS spectra were obtained
with BEFLEX III. Elemental analyses were performed on a Carlo-
Erba-1106 elementary analyzer. The fluorescence spectra were
recorded with a Hitachi F-4500 fluorescence spectrophotometer.
Melting points were measured using a XT4A microscopic melting
point apparatus (Beijing Keyi Photo-electronic Corporation) and
were uncorrected.
Except for the synthetic intermediates, reagents and solvents
used in the present research are commercial products from
Beijing Chemical Reagent Company. Solvents for spectroscopy
measurements were refined by standard methods, while others
were used as received. The solid film of Zn-TPc for UV-Vis study
was prepared from a chloroform solution of Zn-TPc by spin-
coating onto a glass plate and evaporation of the solvent.
Conclusions
A novel family of thiophene-bearing phthalocyanines (M-TPcs)
was synthesized, during which the key intermediate of these
phthalocyanines was purified through a chemical approach.
The synthetic route seems long and some intermediates were
hard to purify, but in reality, the synthesis of target materials
was facile thanks to the simplified separation procedures. In
the lowest energy conformations of M-TPcs, the peripherally
linked thiophene rings are tilted from the Pc core, which op-
pose aggregation theoretically. However, Zn-TPc tends to form
J-aggregates through intermolecular p–p stacking and this ten-
dency is especially remarkable in chloroform, whereas Ni-TPc and
Cu-TPc showed excellent dispersibility. The unusual J-aggregation
behavior of Zn-TPc was attributed its coplanar conformers.
Octaphenyl-substituted zinc Pc (Zn-PPc) exhibited a similar
J-aggregation behavior as Zn-TPc. We have thus discovered a
new method to construct J-aggregates of Pcs. It is well-known
that the J-aggregates of Pcs are highly desirable to maximize their
Synthesis and charaterization
4,5-Dibromophthalic acid dimethyl ester (1), 2,5-dimethylthien-
3-ylboronic acid and Zn-PPc were synthesized according to
literature,14,22,23 other steps are discussed below.
Compound 2
A mixture of 2,5-dimethylthien-3-ylboronic acid (3.91 g), 4,5-
dibromophthalic acid dimethyl ester (2.96 g) and finely grounded
dry potassium carbonate powder (9.17 g) was added into dioxane
(30 mL) under argon flux. After being stirred for 15 min at
room temperature, Pd(PPh3)4 (0.45 g) was added into the argon
flow. The mixture was then heated to 90 ◦C for 9 h with TLC
monitoring. After being cooled to room temperature, H2O (50 mL)
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 393–401 | 399
©

View Online
and Et2O (50 mL) were added, and the organic and aqueous
phases were separated. The aqueous layer was then extracted
twice with Et2O. The combined organic phase was washed with
NaOH solution (2 mol L-1) and pure water in turn, dried over
anhydride Na2SO4 and concentrated. Purification by column
chromatography on silica gel (eluent: 1 : 3 petroleum–ethyl acetate)
gave the crude product as a yellow dope. TLC showed that the Rf
value of product is exactly the same as that of reagent 1. 1H NMR
showed that the yellow dope is a mixture of compounds 1, 2 and
mono-substituted byproduct 8 with a molar ratio of 1 : 1.75 : 1.
Compound 7
The solid obtained in the above reaction (2.32 g) was dissolved in
DMF (10 mL) and cooled to 0 ◦C under argon flow and a solution
of SOCl2 (9.5 mL) in DMF (7 mL) was slowly added. The solution
was kept stirring for 2 h and then slowly poured into ice-water
(40 mL). A faint yellow precipitate appeared and was collected.
This solid was washed with saturated NaHCO3 aqueous solution
and pure water. The crude product (1.82 g) was dried in vacuum.
TLC results showed that diamide was completely converted.
Purification of intermediate 7
Compound 3
The crude product (1.82 g) obtained in the reaction 6 → 7 and
hydroquinone (3.00 g) was dissolved in N,N-dimethylformamide
(10 ml). After being stirred for 15 min at room temperature,
finely powdered dry potassium carbonate (2.20 g) was added.
The mixture was kept stirring for 24 h at room temperature
and then poured into 60 ml ice-water. After precipitation was
complete, the pale yellow solid was filtered off and washed with
water. The mixture of compound 11, 12 and 7 was poured into an
aqueous solution of sodium hydroxide and stirred for 5 min. The
insoluble substance was collected and further purified by column
chromatography over silica. The first band collected proved to
be phthalonitrile 7 (0.72 g). The effectiveness of this approach
was tested by various characterizations. 1H NMR (300 MHz,
CDCl3, 25 ◦C): 2.03 (s, 6H, CH3), 2.35 (s, 6H, CH3), 6.25 (s, 2H,
thiophene ring), 7.72 (s, 2H, benzene ring) ppm; 13C NMR: 13.77,
15.11, 113.63, 115.59, 126.47, 134.24, 135.16, 135.82, 136.80,
141.94 ppm; IR/cm-1: 3067.70, 2949.39, 2915.50, 2856.64, 2231.50
(CN), 1739.16, 1650.68, 1592.32, 1502.37, 1440.11, 1249.32,
1143.84, 830.97; ESI-MS: 371.07 (M + Na+). Elemental analysis
for C20H16N2S2: calc.: C 68.93, H 4.63, N 8.04; found: C 68.92, H
4.61, N 8.05%. Mp 121–122 ◦C.
The mixture obtained from the Suzuki cross-coupling reaction
(3.80 g) was dissolved in THF (10 mL) and slowly added to a
NaOH saturated mixed solution with water (10 mL) and methanol
(50 mL). This was followed by stirring for 5 h at 40 ◦C. The
white precipitate which formed was filtered off and washed with
methanol. Then, it was dissolved in water and slowly acidified
◦
by dilute HCl (4 M) at 0 C. The yellow precipitate (2.78 g) was
collected and dried in vacuum. TLC showed that this reaction
occurred quantitatively. The crude product was directly used in
next step without further purifications.
Compound 4
The crude product obtained in the above step (2.78 g) was added
to Ac2O (12 mL) and the mixture was stirred at 80 C for 2.5 h.
◦
AcOH generated in the reaction and excess Ac2O were removed
by azeotropic evaporation with toluene several times, and then
dried in vacuum. TLC results showed that the starting materials
completely disappeared. The yellow solid obtained was again
directly used in the next step without any purification.
The second band collected was p◦roved to be pure intermediate
5. 1H NMR (300 MHz, CDCl3, 25 C), 2.05 (s, 6H), 2.44 (s, 6H),
6.27 (s, 2H), 7.80 (s, 2H), 7.85 (s, 1H) ppm; 13C NMR: 13.81,
15.14, 126.00, 127.06, 130.90, 134.27, 135.90, 136.12, 167.98 ppm;
IR/cm-1: 3215.99, 3069.83, 2953.26, 2917.24, 2853.91, 11774.34,
1715.16, 1614.44, 1494,44, 1438.60, 1372.95, 1327.43, 834.53,
749.49; ESI-MS: 366.17 (M - H+). Elemental analysis for
C20H17NO2S2: calc.: C 65.37, H 4.66, N 3.81; found: C 65.34,
H 4.62, N 3.82%. Mp 180–182 ◦C.
Compound 5
The product obtained above was placed in a round-bottomed flask
containing 16 g urea under argon flow. The solid mixture was then
heated to 140 C, forming a clear, yellow melt. It was stirred for
15 min until bubbles ceased and then was allowed to cool to room
temperature. Subsequently, water (100 mL) was added and the
solid mixture was ultrasonically dispersed. The precipitate was
collected and dried in vacuum. The reliability of this reaction
was proved by TLC analysis. Without further purification, the
precipitate was directly used in the next step.
◦
Zn-TPc
A mixture of phthalonitrile 7 (0.17 g), zinc acetate (37 mg), and
DBU (80 mg) in 2-ethoxyethanol (8 ml) was refluxed for 3 h under
argon flow. The resulting green suspension was cooled to ambient
temperature and poured into 20 ml methanol. The precipitate
was filtered off and washed with methanol. Finally, pure Zn-TPc
(18 mg, yield: 10.1%) was obtained by chromatography on silica
gel with 1 : 20 mixed solvent of ethyl acetate and chloroform as the
eluent. MALDI-TOF: 1458.0 (calc: 1459.33); IR/cm-1: 2913.83
(CH3), 2852.25 (CH3), 1644.05 (C N), 1615.53 (Ben), 1495.37
(Ben), 1439.98 (Ben), 1347.64 (CH3), 1094.04, 905.14 (Ben), 692.00
(Thio); UV-Vis (THF): lmax/nm (log e): 356 (4.75), 624 (4.51), 694
(5.24); 1H NMR (TMS, 300 MHz, 5 : 1 mixed solvent of CDCl3 and
Compound 6
The precipitate obtained in above reaction was dissolved in THF
(20 mL) and mixed with concentrated ammonia (20 mL). This
mixture was stirred for 48 h at room temperature and then allowed
to stand for a few minutes. The organic phase was isolated and
the aqueous layer extracted twice with THF (20 mL ¥ 2). The
organic phases were combined and concentrated under reduced
pressure. The crude product (2.32 g) was dried in vacuum. TLC
experiment showed that only 60% of the imides were converted to
diamides. This crude was also directly used in next step without
further purification.
◦
d5-pyridine, 25 C): 2.37 (s, 24H, CH3), 2.51 (s, 24H, CH3), 6.79
(s, 8H, thiophene ring), 9.57 (s, 8H, benzene ring) ppm. Elemental
400 | Dalton Trans., 2011, 40, 393–401
This journal is
The Royal Society of Chemistry 2011
©

View Online
analysis for C80H64N8S8Zn: calc.: C 65.84, H 4.42, N 7.68; found:
C 65.82, H 4.43, N 7.65%.
4 M. Matsumoto, T. Nakazawa, R. Azumi, H. Tachibana, Y.
Yaman- aka, H. Sakai and M. Abe, J. Phys. Chem. B, 2002, 106, 11487.
5 K. Kameyama, M. Morisue, A. Satake and Y. Kobuke, Angew. Chem.,
Int. Ed., 2005, 44, 4763; J. L. Sessler, J. Jayawickramara-jah, A.
Gouloumis, G. D. Pantos, T. Torres and D. M. Guldi, Tetrahedron,
2006, 62, 2123–2131; X. Huang, F. Q. Zhao, Z. Y. Li, L. Huang, Y. W.
Tang, F. Zhang and C-H. Tung, Chem. Lett., 2007, 36, 108; X Huang,
F. Q. Zhao, Z. Y. Li, Y. W. Tang, F. S. Zhang and C-H. Tung, Langmuir,
2007, 23, 5167; M. Morisue and Y. Kobuke, Chem.–Eur. J., 2008, 14,
4993–4700.
6 K. Adachi and H. Watarai, J. Mater. Chem., 2005, 15, 4701; K. Adachi,
K. Chayama and H. Watarai, Soft Matter, 2005, 1, 292; K. Adachi, K.
Chayama and H. Watarai, Langmuir, 2006, 22, 1630.
7 H. Isago, Chem. Commun., 2003, 1864.
8 A. Mishra, C-Q. Ma and P. Ba¨uerle, Chem. Rev., 2009, 109,
1141.
Ni-TPc
Pure Ni-TPc (28 mg, 15.8% yield) was obtained through a similar
procedure as in the synthesis of Zn-TPc with NiCl2 used in place of
Zn(OAc)2 and the eluent changed to 1 : 3 petroleum–chloroform.
MALDI-TOF: 1452.1 (calc: 1452.63); IR/cm-1: 2915.57 (CH3),
2850.08 (CH3), 1652.51 (C N), 1615.94 (Ben), 1496.23 (Ben),
1436.53 (Ben), 1375.56 (CH3), 1099.36, 904.13 (Ben), 750.72
(Thio); UV-Vis (THF): lmax/nm (log e): 312 (4.81), 356 (4.75), 618
(4.46), 694 (5.16); 1H NMR (TMS, 300 MHz, CDCl3, 25 ◦C): 2.29
(s, 24H, CH3), 2.48 (s, 24H. CH3), 6.69 (s, 8H, thiophene ring), 9.29
(s, 8H, benzene ring) ppm. Elemental analysis for C80H64N8NiS8:
calc.: C 65.15, H 4.44, N 7.71; found: C 65.17, H 4.42, N 7.73%.
9 For some recent examples, see: P. Gao, D. Berkmann, H. N. Tsao,
X. L. Feng, V. Enkelmann, M. Baumgarten, W. Pisula and K. Mu¨llen,
Adv. Mater., 2009, 21, 213; T. Yasuda, H. Ooi, J. Morita, Y. Akama,
K. Minoura, M. Funahashi, T. Shimomura and T. Kato, Adv. Funct.
Mater., 2009, 19, 411; Y. Liu, Y. Wang, W. P. Wu, Y. Q. Liu, H. X. Xi,
L. M. Wang, W. F. Qiu, K. Lu, C. Y. Du and G. Yu, Adv. Mater., 2009,
19, 772.
Cu-TPc
10 M. Irie, Chem. Rev., 2000, 100, 1685.
Pure Ni-TPc (26 mg, 20.9% yield) was obtained through a similar
procedure as in the synthesis of Zn-TPc with CuCl used in place of
Zn(OAc)2 and the eluent changed to chloroform. MALDI-TOF:
1457.2 (calc: 1457.48); IR/cm-1: 2913.87 (CH3), 2815.98 (CH3),
1643.96 (C N), 1611.83 (Ben), 1502.15 (Ben), 1436.41 (Ben),
1337 (CH3), 1094, 900.28 (Ben), 693.91 (Thio); UV-Vis (THF):
11 M. Q. He and F. X. Zhang, J. Org. Chem., 2007, 72, 442; X. C. Li,
H. Sirringhaus, F. Garnier, A. B. Holmes, S. C. Moratti, N. Feeder, W.
Clegg, S. J. Teat and R. H. Friend, J. Am. Chem. Soc., 1998, 120, 2206.
12 B. Z. Chen, M. Z. Wang, Y. Q. Wu and H. Tian, Chem. Commun., 2002,
1060; H. Tian, B. Z. Chen, H. Y. Tu and K. Mu¨llen, Adv. Mater., 2002,
14, 918; Q. F. Luo, B. Z. Chen, M. Z. Wang and H. Tian, Adv. Funct.
Mater., 2003, 13, 233.
l
max/nm (log e): 350 (4.71), 622 (4.45), 692 (5.21). Elemental
13 T. Muto, T. Temma, M. Kimura, K. Hanabusa and H. Shirai, Chem.
Commun., 2000, 1649.
analysis for C80H64CuN8S8: calc. C 65.93, H 4.43, N 7.69; found:
C 65.91, H 4.45, N 7.67%.
14 C. C. Ko, W. M. Kwok, V. W. W. Yam and D. L. Phillips, Chem.–Eur. J.,
2006, 12, 5840.
15 M. Gouterman, in The Porphyrins, ed. D. Dolphin, Academic Press,
NY 1978, Vol. III, p. 1–165.
Acknowledgements
16 K. P. Seefeld, D. Mobius and H. Kuhn, Helv. Chim. Acta, 1977, 60,
2608.
This work was supported by National Natural Science General
Foundation of China (20773077), the Postdoctoral Science Fund-
ing (20090460297) and the National Key Fundamental Research
Program (2007CB808000).
17 R. J. Abraham, F. Eivazi, H. Pearson and K. M. Smith, J. Chem. Soc.,
Chem. Commun., 1976, 698.
18 G. M. Sander, M. van Dijk, A. Veldhuizen, H. C. Van der Plas, U.
Hofstra and T. J. Schaafsma, J. Org. Chem., 1988, 53, 5272.
19 N. B. McKeown, M. Helliwell, B. M. Hassan, D. Hayhurst, H. Li, N.
Thompson and S. J. Teat, Chem.–Eur. J., 2007, 13, 228.
20 M. Levitus, K. Schmieder, H. Ricks, K. D. Schimizu, U. H. F. Bunz and
M. A. Garcia-Garibay, J. Am. Chem. Soc., 2001, 123, 4259; J. D. Luo,
Z. L. Xie, J. W. Y. Lam, L. Cheng, H. Y. Chen, C. F. Qiu, H. S. Kwok,
X. W. Zhan, Y. Q. Li, D. B. Zhu and B. Z. Tang, Chem. Commun.,
2001, 1740; B-K. An, S-K. Kwon, S-D Jung and S-Y. Park, J. Am.
Chem. Soc., 2002, 124, 14410; K. N. Baker, A. V. Fratini, T. Resch, H. C.
Knachel, W. W. Adams, E. P. Socci and B. L. Farmer, Polymer, 1993, 34,
1571.
Notes and references
1 N. B. Mckeown, Phthalocyanine Materials: Synthesis, Structure and
Function, Cambridge University Press, Cambridge, UK, 1998; K. M.
Kadish, K. M. Smith, R. Guilard, The Porphyrin Handbook, Academic
Press,San Diego, CA, 2003, vol 15: Phthalocyanines: Synthesis; H. Ali
and J. E. Van Lier, Chem. Rev., 1999, 99, 2379; H. Yaku, T. Murashima,
D. Miyoshi and N. Sugimoto, Chem. Commun., 2010, 46, 5740.
2 J. Rusanova, M. Pilkington and S. Decurtins, Chem. Commun., 2002,
2236; Z. H. Chen, C. Zhong, Z. Zhang, Z. Y. Li, L. H. Niu, Y. J. Bin and
F. S. Zhang, J. Phys. Chem. B, 2008, 112, 7387; X. B. Leng, C-F. Choi,
P-C. Lo and D. K. P. Ng, Org. Lett., 2007, 9, 231–234; T. Fukuda, I.
Sugita and N. Kobayashi, Chem. Commun., 2009, 2449.
21 R. J. Abraham, F. Eivazi, H. Pearson and K. M. Smith, J. Chem. Soc.,
Chem. Commun., 1976, 699.
22 Z. H. Chen, S. D. Dong, C. Zhong, Z. Zhang, L. H. Niu, Z. Y. Li and
F. S. Zhang, J. Photochem. Photobiol., A, 2009, 206, 213.
23 T. Sugimori, M. Torikata, J. Nojima, S. Tominaka, K. Tobikawa, M.
Handa and K. Kasuga, Inorg. Chem. Commun., 2002, 5, 1031; S. A.
Mikhalenko, L. A. Yagodina and E. A. Ink’ yanets, Zh. Obshch. Khim.,
1976, 46, 1598.
3 K. M. Kadish, K. M. Smith, R. Guilard, The Porphyrin Handbook,
Academic Press, San Diego, CA, 2003, Vol 17: Phthalocyanines:
Properties and Materials.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 393–401 | 401
©