Tetrabrominated naphthalocyaninatozinc complex with terminal carboxylate functionalities

Article abstract of DOI:10.1246/cl.2012.1012

Novel hydrophilic naphthalocyanine was synthesized and characterized, which could be solublized in water as H-aggregates. It was found that this novel naphthalocyanine showed potential of application as optical limiting material.

Full text of DOI:10.1246/cl.2012.1012

doi:10.1246/cl.2012.1012
1012
Published on the web September 8, 2012
Tetrabrominated Naphthalocyaninatozinc Complex
with Terminal Carboxylate Functionalities
Liqiang Luan,1,2 Jingwei Chen,3 Lanlan Ding,1,2 Junjie Wang,1,2 Xiufeng Cheng,1,2 Quan Ren,3 and Wei Liu*1,2
1State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China
2Institute for Crystal Materials, Shandong University, Jinan 250100, P. R. China
3Department of Optics, Shandong University, Jinan 250100, P. R. China
(Received May 22, 2012; CL-120442; E-mail: weiliu@sdu.edu.cn)
Novel hydrophilic naphthalocyanine was synthesized
and characterized, which could be solublized in water as
H-aggregates. It was found that this novel naphthalocyanine
showed potential of application as optical limiting material.
Recently, naphthalocyanine (Nc) is of particular interest in
optical materials for its expanded transmission window and
red-shifted absorption compared to phthalocyanine (Pc).1-7 The
extension of the ³-system results in Nc with remarkably high
extinction coefficients in the near-infrared region, imparting
superior photophysical properties to Nc. Among all nonlinear
optical (NLO) materials that attract considerable studies at
present,8-12 Nc is one of the most promising optical limiters not
only on the basis of its strong reverse saturable absorption
(RSA) behavior and picosecond response time,2,13,14 but also for
its high thermal stability allowing harsh treatment for device
process. On the other hand, it has been shown that Br-substituted
Ncs exhibit better optical limiting properties than their counter-
parts, in which Br atoms are either on the peripheral of the
macrocycle or as axial ligands.15-17 However, there are fewer
reports on Nc comparing to Pc, mainly because of its
complicated synthetic procedures and poor solubility in common
organic solvents.
Scheme 1. Synthetic route for naphthalocyanine.
As a matter of fact, the solubility of Nc is crucial during
fabrication and deposition of this functional material into
devices via solution-phase processability. Hence, the employ-
ment of substituents on Nc core allows for lowering its tendency
to form stacked aggregates, while at the same time increasing
its solubility in common organic solvents. Focusing on the
structure-property-activity relationship, water-soluble moiety
can be incorporated to endow Nc with hydrophilicity, thus
enabling its processing under environmentally friendly condi-
tions and fabrication of organic-inorganic materials to overcome
the phase separation. To the best of our knowledge, few water-
soluble Ncs have been reported so far.
Herein, we report the synthesis and characterization of novel
tetrabrominated naphthalocyanine with terminal carboxylate
functionalities. Scheme 1 shows the synthetic route used for
the targeted zinc(II) naphthalocyanines.33 Naphthalocyanine 5
was prepared starting from 6,7-dibromo-2,3-dicyanonaphthalene
(1) that was synthesized following reported procedures.4,18
Alkylation of 1 by commercially available methyl p-hydroxy-
benzoate (2) was asynchronous at the two adjacent Br atoms,
with the monoalkylation product 3 predominating at 80 °C even
in the presence of excess of 2, in 74% yield. In comparison, the
dialkylated product 4 was only formed at elevated temperatures.
At 120 °C, using two equivalents of 2 to 1, 4 was obtained in
15-20% yield accompanied by the monoalkylation product of 3.
Figure 1. X-ray structure of 4.
The molecular structure of 4 was established by single-
crystal X-ray diffraction analysis. As shown in Figure 1, the
two benzyl ester units are located at opposite sides of the
dicyanonaphthalene plane. The dihedral angles formed by the
dicyanonaphthalene ring and the benzene ring are 72.24(5) and
71.35(5)°, respectively.
Cyclization of 3 in the presence of zinc(II) acetate and a
catalytic amount of DBU in n-heptanol at 160 °C led to the
formation of Nc 5 with concomitant transesterification of the
ester groups to give the corresponding heptyl esters; this has also
been previously observed by us.19,20 Due to its good solubility in
Chem. Lett. 2012, 41, 1012-1014
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1013
Figure 2. UV-vis absorption of 5 (solid, 2.20 © 10¹6 M) and
6 (dash, 2.13 © 10¹6 M) in DMF.
Figure 3. The electronic absorption spectra of 6 in DMF-H2O
mixed solvent with different volume ratios. The concentration of
6 was kept constant.
common organic solvents, 5 could be submitted to chromatog-
raphic purification using chloroform/pyridine (20:1) as eluent,
which not only decreases the molecular aggregation but also
reduces the adsorption of Nc on silica. Hydrolysis of the ester
groups of 5 using NaOH in a mixed solvent system (THF/
MeOH/H2O), according to published procedure,20 gave the Nc 7
bearing four sodium carboxylate groups. Upon addition of 1 M
HCl to an aqueous solution of 7 until pH µ3, a green precipitate
of 6 was obtained in high yield. Compounds 5 and 6 are soluble
in most organic solvents, such as THF, pyridine, DMF, CHCl3,
benzene, and DMSO, but show stronger tendency to aggregate
in nonpolar solvents. In contrast, cyclization of 4 failed under
the same conditions we used for formation of 5; even at elevated
temperature in trichlorobenzene, no corresponding Nc could
be observed. The mass spectra (MALDI-TOF) of compounds
5 and 6 appeared at m/z 2029.97 (M+) and 1636.96 (M+)
comparing to their theoretical m/z 2029.28 and 1636.84,
respectively.
Figure 4. Experimental setup for optical limiting measure-
ment. D1 and D2: reference and signal detectors; L: lens that
focuses the laser onto the sample; BS: beam splitter; A: pinhole
used to make the laser collimating; Glan: Glan prism that is used
as a polarizer.
Both of 5 and 6 show similar UV-vis absorption upon
dissolved in DMF as shown in Figure 2, with a highly resolved
Q-band centered at 763 nm assigned to the monomeric species.
The UV-vis spectra show almost no influence of peripheral
chain length on their Q-band positions.
It is remarkable that compound 6 could be soluble in water.
To date, no report can be found on the aggregation behavior of
hydrophilic Nc in water yet, although a number of Pcs bearing
hydrophilic moieties have been studied so far.21-27 Original
molecular aggregation studies of water-soluble Nc were con-
ducted in solution as a function of increasing volume fraction of
H2O in H2O-DMF solution of 6. Upon increasing H2O volume
fraction (0 ¼ 50%), aggregation of 6 was remarkably confirmed
by a marked decrease in the Q band absorption at 763 nm
with simultaneously rising of a hypsochromic peak at 710 nm
(Figure 3), indicating that aggregation under these solvent
conditions was predominantly cofical H-aggregation.
The thermal stabilities of these Ncs were evaluated by the
thermogravimetric analysis of TG/DTA. Compounds 5 and 6
start to decompose directly at 305 and 311 °C, respectively,
without melting points. These decompose temperatures indicate
that such Nc derivatives are as highly thermally stable as
Pcs.28-32
Figure 5. The optical-limiting properties 6 in DMF at the
concentration of [C] = 1 © 10¹2 M (cell thickness: 2 mm) upon
excitation with 18 ns pulse at 1053 nm (The solid line gives the
transmission if there were no limiting).
center of the sample cell, and the other was used as the reference
beam. Nonlinear transmittance responses versus incident energy
spanning from 25 to 400 ¯J at 1053 nm with 18 ns pulse for Nc 6
in DMF ([C] = 1 © 10¹2 M; cell thickness: 2 mm) are shown in
Figure 5. The response of the Nc is linear to the input energy
intensity at the beginning, but as input energy intensity
increases, the transmittance of the solution decreases and a
nonlinear relationship is observed between the output and input
energy intensity, revealing the occurrence of reverse saturable
absorption (RSA). The output energy of the limiter is restricted
to 35 ¯J for input energies over the range of our study, with less
The nonlinear optical experiments were conducted on a
Sunlight 200 ns Nd:YLF laser system as shown in Figure 4. The
laser beam was divided into two beams: one was focused on the
Chem. Lett. 2012, 41, 1012-1014
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1014
than 8% transmission as the incident energy is increased up to
400 ¯J. The optical limiting threshold, which is defined as the
input fluence at which the output transmittance is 50% of the
linear transmittance is ca. 200 ¯J for 6. At energies that above
the threshold, the ratio of the transmission of the Nc 6 limiter to
that predicted by the linear line in Figure 5 is ca. 1/4.
In conclusion, novel hydrophilic tetrabrominated zinc(II)
naphthalocyanine 6 has been successfully synthesized and
characterized. This compound displays positive nonlinear
optical absorption with a laser pulse. Obviously, this compound
can further be applied to fabrication organic-inorganic hybrid
materials to overcome the phase separation.
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This work was supported by the grant from Natural Science
Foundation of China (No. 21071090) and the Excellent Young
Scientist Fund of Shandong Province (No. BS2010CL032). The
authors also acknowledge the financial supports from the
Research Fund for the Doctoral Program of Higher Education
of China (No. 20100131120026).
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© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/