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Chemistry Letters Vol.36, No.1 (2007)
A Novel Self-aggregates of Phthalocyanine Based on Zn–O Coordination
Xin Huang,1 Fuqun Zhao,1 Zhongyu Li,1;2 Lei Huang,1 Yingwu Tang,1
Fushi Zhang,ꢀ1 and Chen-Ho Tungꢀ1
1Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China
2Department of Chemical Engineering, Jilin Institute of Chemical Technology, Jilin 132022, P. R. China
(Received September 14, 2006; CL-061066; E-mail: zhangfs@mail.tsinghua.edu.cn, chtung@mail.ipc.ac.cn)
RO
ꢀ-Aryl/alkoxy-substituted phthalocyanines (Pcs) were
2.0
a
b
synthesized and the formation of J-type self-aggregation for zinc
phthalocyanines has been observed in organic non-coordinating
solvents. The mechanism of the formation of this self-assembly
was studied by UV–vis spectroscopy, fluorescence spectroscopy
and MALDI-TOF MS, which revealed that it was stabilized by
Zn–O self-coordination.
N
M
N
RO
N
N
N
N
1.5
1.0
0.5
0.0
1
2
3
4
N
N
OR
OR
1 R = Ph, M = Zn
2 R = Ph, M = 2H
3 R = CH3 , M = Zn
4 R = CH3, M = 2H
600
650
700
750
800
Wavelength /nm
The aromatic macrocycle phthalocyanines (Pcs) are one of
the best known synthetic porphyrin analogues showing a wide
range of applications in material science, medicine, catalysis,
and photonics because of their exceptionally high thermal and
chemical stability.1 Molecular assemblies of chromophores play
crucial roles in nature’s most important systems, including light
harvesting in photosynthesis,2 and are important in technological
applications.3 The organization of phthalocyanines into control-
led structures, in particular, has been attracting considerable at-
tention. The main problem encountered during the application of
molecular assemblies and aggregates is a difficulty to control the
arrangement and orientation of monomers in the aggregates. The
J-aggregates are one of specific molecular assemblies discovered
by Jelley and Scheibe.4,5 Intermolecular interactions based on a
side-by-side arrangement lead to a characteristic narrow and in-
tense absorption band (J-band) that shows a bathochromic shift
compared to the relevant monomer band.5 Recently, making
self-organising J-aggregates of large aromatic chromophores
for optoelectronic applications have also attracted much interest
by a number of researchers.6 However, there are only few reports
of the Pcs J-aggregates so far. Farren et al.7a observed the strong
aggregation of tetra-solketal-substituted zinc Pcs at low temper-
ature. Kaneko et al.7b and Isago7c reported the observation of
Pcs Q-band at longer wavelength in aqueous and non-aqueous
media, respectively. Some of other cases of Pc J-aggregates
are also formed under specific conditions in the solid state.7d
Here, we report the synthesis of aryl/alkoxy substituents Pcs,
and the first observation of self-assembled aggregation of zinc
Pcs in organic non-coordinating solvents at room tempetature
based on Zn–O self-coordination.
Figure 1. (a) Structures of the Pcs 1–4. (b) Absorption spectra
of Pcs 1–4 in CHCl3: 1 (1:4 ꢁ 10ꢂ5 M), 2 (1:4 ꢁ 10ꢂ6 M), 3
(1:6 ꢁ 10ꢂ5 M), and 4 (2:4 ꢁ 10ꢂ5 M) at room temperature
(25 ꢃC).
720 nm and 692, 727 nm, respectively. However, for zinc(II)
Pcs 1 and 3, in addition to normal Q band (710 and 706 nm, re-
spectively), an unusual red-shifted sharp band was observed at
741 and 746 nm, respectively. The formation of Pc aggregates
causes significant spectral perturbations, owing to the coupling
between the electronic states of individual monomeric Pc units.
Therefore, the bands shifted to longer wavelength at 741 and
746 nm are ascribed to the absorption of J-aggregates. In order
to understand the nature of the aggregation and test the stability
of the self-aggregate, UV–vis spectra of ZnPc 1 was also meas-
ured in other solvents. Interestingly, the J-band formation is
close related to the coordination behavior of solvents, i.e. it
was observed in non-coordinating solvents, such as toluene,
1-chloronaphthalene, and n-hexane, but did not appear in
coordinating solvents, such as DMF, pyridine, and methanol in
which the Q band shows nomal absorption peak at ca. 690 nm
(Figure 2a).
The effect of concentration on the absorption spectra of
ZnPc 1 in non-coordinating solvent of CHCl3 was also studied.8
It was found that the absorption spectra almost did not change
with concentration (scope from 6:4 ꢁ 10ꢂ7 to 1:5 ꢁ 10ꢂ5 M)
of zinc phthalocyanine in solution. The constant of self-associa-
2.0
1.5
1.0
0.5
0.0
2.0
1.5
1.0
0.5
0.0
1.0
0.8
0.6
0.4
0.2
0.0
DMF
Pyridine
Methanol
Toluene
1-chloronaphthalene
Chloroform
a
b
Aryl/alkoxy-substituted Pcs 1–4 (Figure 1a) were prepared
according to the typical procedure by treating corresponding
phthalonitriles in refluxing dimethylaminoethanol with or with-
out Zn(OAC)2. The products obtained were purified by column
chromatography on silica gel and were fully characterized by
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
-1
Concentration of methanol /mol L
300
400
500
600
700
800
550
600
650
700
750
800
1
UV–vis, MS spectra, H NMR, and elemental analysis.8 Figure
Wavelength /nm
Wavelength /nm
1b shows the electronic absorption spectra of Pcs 1–4 in chloro-
form (CHCl3). In general the Q band of UV–vis spectrum shows
splitted peaks for metal-free Pcs and one single peak for metal
Pcs because of the different molecular symmetry.9 As expected,
metal-free Pcs 2 and 4 shows two splitted Q bands at 687,
Figure 2. (a) Absorption spectra of ZnPc 1 in different solvents
at room temperature (7:0 ꢁ 10ꢂ6 M). (b) Changes in the absorp-
tion spectra of ZnPc 1 in CHCl3 upon addition of methanol.
Inset: A plot illustrating the change in ZnPc 1 absorption at
741 nm versus increasing concentration of methanol in CHCl3.
Copyright Ó 2007 The Chemical Society of Japan
Huang, Xin
