A series of asymmetrical phthalocyanines: Synthesis and near infrared properties

Article abstract of DOI:10.3390/molecules18044628

We report here the preparation of asymmetrical phthalocyanine dimers 1a-3a, which are endowed with novel charge transfer bands at 1,151-1,154 nm and strong NIR luminescences at 840-860 nm and 1,600-1,650 nm. Through H-bonding interaction, 1a-3a are inclined to self-assemble into hexrod nanotubes at the interface of CHCl3 and CH3OH. Our results provide further insights into the interaction in molecular dimers, and suggest that 1a-3a have potential application in magnets and supramolecular architectures.

Full text of DOI:10.3390/molecules18044628

Molecules 2013, 18, 4628-4639; doi:10.3390/molecules18044628
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Communication
A Series of Asymmetrical Phthalocyanines: Synthesis and Near
Infrared Properties
Guoqing Huang 1, Jianxi Li 2, Fangdi Cong 2, Chao Li 2, Xixi Chu 2, Yanyan Meng 2, Guotong Du 1,*
and Xiguang Du 2,*
1
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering,
Jilin University, Changchun 130012, China
2
Department of Chemistry, Northeast Normal University, Changchun 130024, China
* Authors to whom correspondence should be addressed; E-Mails: dugt@jlu.edu.cn (X.G.);
xgdu@nenu.edu.cn (X.D.); Tel.: +86-138-4083-8794 (X.G.); +86-138-4401-8023 (X.D.).
Received: 30 January 2013; in revised form: 7 April 2013 / Accepted: 8 April 2013 /
Published: 19 April 2013
Abstract: We report here the preparation of asymmetrical phthalocyanine dimers 1a–3a,
which are endowed with novel charge transfer bands at 1,151–1,154 nm and strong NIR
luminescences at 840–860 nm and 1,600–1,650 nm. Through H-bonding interaction, 1a–3a
are inclined to self-assemble into hexrod nanotubes at the interface of CHCl3 and CH3OH.
Our results provide further insights into the interaction in molecular dimers, and suggest
that 1a–3a have potential application in magnets and supramolecular architectures.
Keywords: organic green dyes; syethesis; NIR-absorption; NIR luminescence; 1-D hexrod
nanotubes
1. Introduction
Considerable efforts have been made to develop various types of supramolecular architectures
formed by self-organization between functional chromophores, because these supramolecular
architectures have better photophysical and electronic properties as a result of excitonic interactions [1–8].
Phthalocyanines (Pcs), as a type of functional chromophores, have potential application in
supermolecular assemblies, charge transfer properties, and magnets [9], such as NIR-absorbing Pc
supermolecules with some unique electronic spectrum and electrochemical behaviors [10–16]. Among
them, double-decker Pc dimers have been of specially interest in numerous contexts because they are

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inclined to generate intermolecular π-π interactions [17,18]. Especially those with new CT bands in the
range of 1,100–1,280 nm (in oxidized state) have potential applications in emerging photoelectric
technologies [19,20]. Some self-organization structures of Pcs were reported by other groups [21].
Of particular interest in a number of contexts are Pcs which self-assemble and self-organize into 1D
nanostructures. Recently, Pcs that self-organize by H-bonding, electrostatic, labile metal-ligand bonds
and flat π-π interaction into highly ordered supramolecular structures with controlled dimensions and
size, have attracted great interest, such as Nolte and co-workers’ reports of a Pc with four crown ether
moieties, which are self-organized into helical, micrometer-long fibers in chloroform solution [22].
Ultralong nanowires with new crystal structured are formed by CuPc [23]. Although scientists
studying Pcs have reported some 1D nanostructures [24,25], 1D nano-tubular structures produced by
self-assembly methods have seldom been reported.
Our previous research work on the design and synthesis of metallo-Pcs and Pc 1D tubular
structures [26–28], helped us find and obtain A3B asymmetrical Ni (II) Pcs 1a–3a, which are
connected with three phenoxy groups and one benzyloxy group. It is found that 1a–3a are
self-assembled into hexo-nanotubes by H-bonding interactions between Pc molecules. The mechanism
of formation of the hexotubular structures is proposed.
2. Results and Discussion
2.1. Synthesis
The synthesis of 1a–3a is summarized in Scheme 1. Only the cyclic tetramerization of single Pn1-3
(Scheme S1 and Figure S1) in 1-octanol in the presence of 1,8-diaza-bicyclo[5.4.0] undec-7-ene
(DBU) and at higher temperature of 170 °C (the traditional method use 140 °C), yields 1a–3a and 1b–3b
(A3B-type Pcs and A4-type Pcs).
Scheme 1. Synthesis of 1a–3a.
Reagents and Conditions: 1-octanol (5 mL), DBU (2 mL), 170 °C, NiCl2·6H2O, 8 h.

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The yield of 1a is 15%, and the 2a and 3a is 13% and 8% respectively. This method overcomes
many of the separation difficulties typically encountered in the traditional syntheses of asymmetrical
Pcs [29] and only uses a single substituted phthalonitrile to synthesize asymmetrical Pcs
(the traditional methods use two substituted phthalonitriles). A possible mechanism for the synthesis of
1a–3a is provided in the Supporting Information (Scheme S2). Compounds 1a–3a were characterized
1
by UV-Vis-NIR (Figure S2), H-13C-NMR (Figure S3), high resolution MS and MALDA-TOF MS
(Figure S4), IR (Figure S5). Characterization of 1a, as an example of 1a–3a, is discussed below. The
data of 2a–3a are provided in the Supporting Information.
2.2. High Resolution MS
Figure 1A shows the clear m/z peaks ([M+H+] or M+) of 1a obtained in high resolution MS. It is
found that the molecular weight of 1a is 1,148.42169, which is in line with the real molecular weight.
The m/z peak at 2,297.3 provides direct evidence that 1a tends to form a 1a dimer (Figure 1B).
Figure 1. (A) Hi-Res MS spectrum of 1a; (B) MALDI-TOF MS spectrum of 1a dimer.

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2.3. 1H-13C-NMR
Because of the magnetism of 1a, the peak area is influenced. The structure of 1a is characterized by
1H-13C-NMR (in CDCl3) in Figure 2. We can see that label 1 is assigned to the C and H atoms in the
benzyloxy group, label 2 is assigned to the C and H atoms in the isopropyl group, label 3 is the C and
H atoms of the methyl group linked to the benzene ring, label 4 is the C and H atoms of the
hypo-methyl group in the benzyloxy structure and the label 5 is the C and H atom of methyl group in
isopropyl. The structure of 1a was confirmed by 1H-13C-NMR and high resolution MS.
Figure 2. The 1H-13C-NMR of 1a.
2.4. UV-Vis-NIR Spectroscopy
The same conclusion concerning the aggregation properties of the molecule were also obtained
using UV-Vis-NIR spectroscopy (Figure 3). The B, Q and shoulder bands of 1a in CHCl3 (1 × 10−4 mol/L)
are observed around 385–390, 610 and 677 nm, respectively. It is noteworthy that the Q band of 1a
broadens considerably relative to the symmetrical one (Figure S6), but its molar extinction coefficient
decreases dramatically from log  5.35 to log  2.89 (Figure 3A), which are just the characteristic
absorptions of Pc dimers and oligomers [30]. Additionally, a broad intensity-strong band at around
1,154 nm is attributed to 1a dimer, which is assignable to a CT consequence of electronic coupling due
to strong H-bonding interaction in-between the 1a dimer units (Figure 4). Compared with 2a, the Q
band and the NIR band of 3a is red shifted from 1,148 nm to 1,230 nm, because the donating electron
ability of phenylthiol group is better than that of a phenoxy group. The solid state UV-Vis-NIR
spectrum of 1a dimer shows that the molecule strongly NIR-absorbs light from 900 to 2,000 nm
(Figure 3B), which hints that 1a may exist in a face-to-tail J-type aggregates in the solid state and 1a
film might be improved as an available NIR material in optical communication field in the future.

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Figure 3. (A) UV-Vis/NIR spectrum of 1a in CHCl3 (1.0 × 10−4 mol/dm3) and (B) Solid
state UV-Vis/NIR Spectrum of 1a.
Figure 4. 1a dimer in H-bonding interaction.
O
O
N
N
N
N
N
N
N
N
O
H
O
Ni
Ni
N
N
N
N
O
O
H
N
N
N
N
O
O
the H-bonding interaction
2.5. NIR Luminescence
To further confirm the CT capacity of dimer, we tested the NIR luminescence properties of 1a by a
325 nm He-Cd laser as the excitation source and Ge detector (Figure 5). When 1a is excited in KBr, it
showed strong NIR luminescence at 840–860 nm and 1,600–1,650 nm, respectively. The NIR
luminescence at 840–860 nm results from the fluorescent radiation transition (S1→S0) in the monomer,
while the luminescence at 1,600–1,650 nm comes from the fluorescent radiation monomer-to-monomer
transition. The results provide direct evidence of CT action in 1a dimer.

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Figure 5. Emission spectrum of 1a at room temperature.
2.6. 1-D Molecular Morphology and XRD
Figure 6 shows 1-D molecular morphology of 1a, which was tested by scanning electron
microscope (SEM, Figure 6A,B) and high resolution transmission electron microscope (HR-TEM,
Figure 6C,D). The SEM images show 1a is self-assembled into hexrod nanotubes at the CHCl3 and
CH3OH interface, and the nanotubes differ in length and thickness. The HR-TEM images show the
hexrod-nanotubes are hollow, and the black lines in the HR-TEM images is the edges of nanotubes.
These nanotubes indicate strong intermolecular aggregation in-between 1a units.
Figure 6. (A,B) SEM images of 1a nanotubes and (C,D) HR-TEM micrographs of 1a nanotubes.

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The XRD spectrum of 1a (Figure 7) shows two clear diffraction peaks at 4.33° and 27.5°, whose
plane separations are approximately 21.45 Å (d1) and 3.098 Å (d2), respectively. The two peaks of the
nanotubes shows that the 1a molecules have been stacked in highly long-range ordered lamellar
structure [31], d1 and d2 are the interlayer spacing of the nanotubes [32].
Figure 7. XRD data of 1a.
8000
6000
4000
2000
0
1a
4.33
27.5
0
5
10
15
20
25
30
2-Theta scale
Connecting the molecular morphology with the XRD data of 1a, the possible formation of 1a
nanotubes can be described as shown in Figure 8: (1) the H-bonding interaction in-between 1a is self-
assembled into 1a dimer. The angle of 1a dimer is 120°, which is similar to the H-bonding interaction
of H2O; (2) according to “proximity compatibility principle”, the 1a dimer is self-assembled into a
hexacyclic compound; (3) through π-π interaction in J-type aggregates [33], the hexacyclic compound
forms nanotubes differing in length and thickness.
Figure 8. The possible formation of 1a hexrod nanotubes.
2.7. Magnetic Susceptibility
Magnetic susceptibility data of 1a and 1b were recorded using a Quantum Design SQUID MPMS
XL-5 magnetometer. Variable temperature susceptibility measurements were carried out in the

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temperature range of 2–300 K at a magnetic field of 1,000 Oe on polycrystalline samples. Since
Ni(II)Pc is diamagnetic due to the electronic configuration (b2g)2(eg)4(a1g)2, the paramagnetism of 1a
and 1b should arise from the spin polarization of electrons on the aromatic macrocycle [34,35]. Both
1a and 1b with Ni(II) ion appear as planar molecules with no axial ligand, so their crystals are formed
from various stacking columnar structures with different tilting angles and interplanar distances. As
seen in Figure 9, upon continuously lowering temperature from 300 K to 2 K, the χmT values of 1a and
1b smoothly decreases to a zero value; this is because the antiferromagnetic exchange interaction
occurs between the radical aromatic macrocycle complexes in the solid state.
Figure 9. χmT curves of magnetic measurements for 1a and 1b.
3. Experimental
3.1. Experimental Materials and Equipment
Chloroform was distilled from CaH2 under nitrogen. DMSO was distilled from molecular sieves.
The other reagents were purchased from commercial sources and used without further treatment.
1H-NMR and 13C-NMR spectra were recorded on a Varian 500 MHz instrument at 25 °C. Mass spectra
were recorded on a Hi-Res MALDI and a LDI-1700 MALDI-TOF MS spectrometer. IR spectra (KBr)
were recorded on a Magna-560 FT-IR spectrophotometer in the 400~4,000 cm−1 range. UV/VIS/NIR
spectra were recorded on a Cary 500UV-VIS-NIR spectrophotometer.
3.2. Synthesis of Substrates and Pc Derivatives
3.2.1. General Procedure for the Synthesis of Substrates Pn1–Pn3
Lithium hydroxide hydrate (4.20 g, 0.10 mol) was interfused at room temperature into a stirred
mixture of the appropriate phenol/thiophenol (2-isopropyl-5-methylphenol, 6.08 g, 0.04 mol for Pn1,
2-isopropylphenol, 5.44 g, 0.04 mol for Pn-2 or 2-isopropylphenylthiol, 6.08 g, 0.04 mol for Pn-3) and
4-nitrophthalonitrile (6.92 g, 0.04 mol) in dimethylsulfoxide (80 mL) over a 2 h period. The mixture
was stirred unceasingly and the reaction was monitored by thin-layer chromatography. After 24 h, the
reactant was poured into NaCl solution (400 mL, 10%) and stirred till precipitate appeared. The
product was collected by vacuum filtration and purified by column chromatography (silica
gel/petroleum ether–ethyl ether 1:1) to afford Pn-1-Pn-3.

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4-(2-Isopropyl-5-methylphenoxy)phthalonitrile (Pn1). Yellow solid (8.77 g, 0.032 mol, 79% yield).
1H-NMR (CDCl3): δ = 7.71 (d, J = 8.5 Hz, 1 H; Ar-H), 7.30 (d, J = 8.5 Hz, 1 H; Ar-H), 7.22 (s, 1 H;
Ar-H), 7.19 (d, J = 8 Hz, 1 H; Ar-H), 7.11 (d, J = 8 Hz, 1 H; Ar-H), 6.75 (s, 1 H; Ar-H), 2.96 (m, 1 H; C-H),
2.33 (s, 3 H; ArCH3), 1.15 (d, J = 6.5 Hz, 6 H; CH(CH3)2). TOF-MS: m/z calcd: 299.1 [M+Na]+, 315.1
[M+K]+; found: 299.1, 314.9. IR (KBr): C-O-C 1242 cm−1, CN 2230 cm−1. Elemental analysis calcd
(%) for C18H16N2O [M = 276.13 g·mol−1]: C 78.24, H 5.84, N 10.14; found: C 78.28%, H 5.91%, N 10.02%.
4-(2-Isopropylphenoxy)phthalonitrile (Pn2). Light yellow solid (8.38 g, 0.032 mol, 79% yield). 1H-NMR
(CDCl3): δ =7.54 (q, J = 8.5 Hz, 1 H; Ar-H), 7.42 (t, J = 8.5 Hz, 1 H; Ar-H), 7.286 (t, J = 8.5 Hz, 2 H;
Ar-H), 7.07 (t, J = 8 Hz, 1 H; Ar-H), 7.05 (t, J = 8 Hz, 2 H; Ar-H), 2.95 (m, J = 7 Hz, 1 H; C–H), 1.27
(d, 6H; J = 7 Hz, CH(CH3)2). TOF-MS: m/z calcd: 299.1 [M+Na]+, 315.1 [M+K]+; found: 299.1,
314.9. IR (KBr): C–O–C 1242 cm−1, –CN 2230 cm−1. Elemental analysis calcd (%) for C17H14N2O
[M = 262.1 g·mol−1]: C 77.84, H 5.38, N 10.68; found: C 78.80%, H 5.42%, N 10.66%.
4-(2-Isopropylphenylthio)phthalonitrile (Pn3). Light yellow solid (1.01 g, 0.036 mol, 89% yield). 1H-NMR
(CDCl3): δ = 7.54 (q, J = 8.5 Hz, 1 H; Ar-H), 7.42(t, J = 8.5 Hz, 1 H; Ar-H), 7.286 (t, J = 8.5 Hz, 2 H;
Ar-H), 7.07 (t, J = 8 Hz, 1 H; Ar-H), 7.05 (t, J = 8 Hz, 2 H; Ar-H), 2.95 (m, J = 7 Hz, 1 H; C–H), 1.27
(d, J =7 Hz, 6 H; CH(CH3)2). TOF-MS: m/z calcd: 301.38 [M+Na]+, 315.1 [M+K]+; found: 299.1,
314.9. IR (KBr): C–S–C 1240 cm−1, –CN 2230 cm−1. Elemental analysis calcd (%) for C17H14N2O
[M = 262.1 g·mol−1]: C 67.75, H 4.68, N 10.64; found: C 67.73%, H 4.68%, N 10.66%.
3.2.2. Synthesis of 1a and 1b
NiCl2·6H2O (95 mg, 0.4 mmol) and Pn1 (440 mg, 1.6 mmol) were added to 1-octanol (5 mL) and
stirred for 0.5 h in succession. After DBU (2 mL) was added, the mixture was sequentially stirred for
21 h and then heated to 160 °C over 8 h under a nitrogen atmosphere. After cooling down, 1-octanol
was removed under reduced pressure. The collected solid was purified by column chromatography
(silica gel/CHCl3–MeOH 20:1) to afford pure 1a as a green powder in 15% yield (70 mg, 0.05 mmol) and
1b as a blue powder in 37% yield (172 mg, 0.15 mmol), respectively.
1a: MS(CHCl3): found: m/z = 1162.9 (M); FT-IR (KBr) cm−1: 3080, 3050, 2961, 2924, 2869, 1770,
1735, 1699, 1650, 1613, 1573, 1559, 1535, 1502, 1471, 1412, 1337, 1248 (C–O–C), UV-Vis B band:
299, 332nm; Q band: 609, 675nm. Anal. Calcd: C 73.68%; H 5.73 %; N 9.48%; calculated: C 74.29%;
H 5.54%; N 9.63%.
1b: 1H-NMR (500 MHz, CDCl3): δ = 7.87 (brs, 12 H; Pc-Ar-H), 7.41 (brs, 4 H; O-Ar-H), 7.10 (brs,
8 H; O-Ar-H), 3.58 (brs, 4 H; tert-C-H), 2.37 (brs, 12 H; Ar-CH3), 1.41 (brs, 24 H; isopropyl-CH3).
TOF-MS: m/z calcd: 1162.4 [M]+; found: 1162.9. UV/Vis (CHCl3): λmax (logε) = 677 nm (5.35). IR
(KBr): C–O–C 1249 cm−1. Elemental analysis calcd (%) for C72H64N8O4Ni [M = 1162.44 g·mol−1]: C
74.29%, H 5.54%, N 9.63%; found: C 74.68%, H 5.73%, N 9.48%.
The synthesis of Pcs 2a–3a are the same as above; the yields of 2a and 3a were 14% and 11%,
respectively.

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3.3. Synthesis of 1a–3a Nanotubes
The 1a–3a molecules are self-organized into nanotubes via a simple solvent diffusion method [36,37].
When methanol was added dropwise into 1a–3a chloroform solution, the 1a–3a molecules are
self-assembled into nanotubes. The tubes were then washed carefully by ethanol to remove chloroform.
4. Conclusions
In this paper, through H-bonding interaction a series of 1a–3a dimers were constructed by
self-assembly as a result of surface accumulation. The dimers are endowed with a novel CT band at
around 1d154 nm. Images of 1a nanotubes are observed by electron microscopy. Our results provide
insights into the interaction of molecular dimers and the direct possibility to synthesize materials
having potential application in magnets and supramolecular architectures.
Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/18/4/4628/s1.
Acknowledgments
The work at the Jilin University was supported by National Nature Science Foundation of China
(Nos. 20472014) and the Fundamental Research Funds for the Central Universities (No. DUT11LK40).
Conflicts of Interest
The authors declare no conflict of interest.
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