Derivatizable phthalocyanine with single carboxyl group: Synthesis and purification

Article abstract of DOI:10.1016/j.inoche.2005.12.002

Phthalocyanine zinc (ZnPc) is an important class of photosensitizers. Derivatization of phthalocyanine ring with chemical functional groups provides anchor points for covalent attachment of biological targeting agents, thus increasing the selectivity of the photosensitizers. ZnPc with symmetric and multiple functional groups, e.g., tetracarboxyphthalocyanine zinc, is easy to be prepared but is unfavorable for linking targeting agents due to many reasons. On the other hand, single-substituted ZnPc, is more challenging to synthesize and, especially, to purify. Here we report the synthesis of an unsymmetrical phthalocyanine, 2-carboxyphthalocyanine zinc 1, with a single carboxylic group on phthalocyanine ring, and the design of its chromatographic purification.

Full text of DOI:10.1016/j.inoche.2005.12.002

Inorganic Chemistry Communications 9 (2006) 313–315
www.elsevier.com/locate/inoche
Derivatizable phthalocyanine with single carboxyl group: Synthesis
and purification
a
b
b
b,
a,
*
Jincan Chen , Naisheng Chen , Jinling Huang , Jundong Wang *, Mingdong Huang
a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, 155 Yang Qiao Xi Lu, Fuzhou, Fujian 350002, China
b
Institute of Functional Materials, College of Chemistry and Chemical Engineering, Fuzhou University, Fujian 350002, China
Received 19 July 2005; accepted 2 December 2005
Available online 18 January 2006
Abstract
Phthalocyanine zinc (ZnPc) is an important class of photosensitizers. Derivatization of phthalocyanine ring with chemical functional
groups provides anchor points for covalent attachment of biological targeting agents, thus increasing the selectivity of the photosensi-
tizers. ZnPc with symmetric and multiple functional groups, e.g., tetracarboxyphthalocyanine zinc, is easy to be prepared but is unfa-
vorable for linking targeting agents due to many reasons. On the other hand, single-substituted ZnPc, is more challenging to
synthesize and, especially, to purify. Here we report the synthesis of an unsymmetrical phthalocyanine, 2-carboxyphthalocyanine zinc
1, with a single carboxylic group on phthalocyanine ring, and the design of its chromatographic purification.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Unsymmetrical phthalocyanine; Carboxyl phthalocyanine; Photodynamic therapy; Biological targeting agents
In photodynamic therapy [1], infrared light with specific
wavelength is shined onto tissue/tumor pre-administrated
with photosensitizing agent. In the presence of physiologi-
cal oxygen, the light-absorbing photosensitizer produces
reactive oxygen species (ROS) and/or free radicals in its
immediate locale, rendering biological toxicity. Phthalocy-
anine zincs, and several other metallophthalocyines, have
been successfully used as a photosensitizing agents in
photodynamic therapy at both cellular and animal levels
[2–6]. Conjugation of sensitizer with biological targeting
agents, e.g., monoclonal antibodies or lipoproteins,
increases the concentration of sensitizer on targeted tissue
over normal tissue, providing additional level of selectivity
and possibility to reduce effective sensitizer concentration,
and thus phototoxicity to normal tissue [7,8].
but is often unfavorable for linking targeting agents. For
example, once conjugated to antibodies, tetracarboxypht-
halocyanine zinc will become insoluble polymer. Thus,
conjugation of ZnPc with targeting agents prefers a single
functionality in the phthalocyanine ring. The synthesis of
such unsymmetrical phthalocyanine is typically through
the expansion of subphthalocyanine that is composed of
three isoindole units [9,10], or through statistical condensa-
tion. Purification is always the key to obtain intended
products for all these synthetic methods. Here we report
a two step synthesis of an unsymmetrical phthalocyanine,
2-carboxyphthalocyanine zinc 1, with a single carboxylic
group on phthalocyanine ring. We also reported the devel-
opment of a simple column-based purification scheme of 1.
Condensation of trimellitic anhydride or phthalic anhy-
dride in the presence of urea and catalyst at high
temperature will lead to formation of tetra-formamido-
phathalocyanine or phthalocyanine, respectively. Using a
mixture of trimellitic anhydride and phthalic anhydride at
a ratio of 1:7 for condensation at 170 °C and in the
presence of urea, zinc acetate, and ammonium molybdate,
we have made a mixture of 2-formamidophthalocyanine
Phthalocyanine zinc (ZnPc) with multiple and symmetri-
cal functionalities in the phthalocyanine ring, e.g.,
tetracarboxyphthalocyanine zinc, is easy to be prepared
*
Corresponding author. Tel.: +86 591 83795550 (J. Wang); Tel.: +86
591 83704996; fax: +86 591 83714946 (M. Huang).
E-mail address: mhuang@fjirsm.ac.cn (M. Huang).
1387-7003/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2005.12.002

314
J. Chen et al. / Inorganic Chemistry Communications 9 (2006) 313–315
zinc 2 and phthalocyanine zinc (Scheme 1). The mixture
was then hydrolyzed under alkaline condition, yielding 2-
carboxyphthalocyanine zinc 1 (Scheme 2). The final prod-
uct of 1 was then purified from the hydrolysis mixture.
Its purity was confirmed based on a major elution band
on an analytical C18 HPLC column (Fig. 1b). At our
molar ratio of starting materials (1:7) for condensation,
only trace amount presumably polycarboxy substituted
phthalocyanine zinc was found (small peaks eluted ahead
of 1 in Fig. 1b). Compound 1 was further characterized
by UV/VIS, IR, proton NMR, and mass spectra.
Large amount of phthalocyanine zinc byproduct exists
in this synthetic scheme. This phthalocyanine zinc in our
reaction mixture was soluble in DMF (Fig. 1), presumably
due to axial coordination to zinc ion. Hence, the separation
of desired product from reaction mixture was necessary for
the success of the preparation of 1. We performed extensive
analysis of reaction mixture by analytical HPLC (see Sup-
plementary data) to guide our design of the purification
scheme. We chose not to carry out purification after con-
densation reaction (Scheme 1) but after hydrolysis reaction
(Scheme 2), even though this seems added another level of
complexity (i.e., incomplete hydrolysis) to the separation.
This is because the HPLC analysis indicated that com-
pound 2 from condensation reaction was quite similar to
phthalocyanine zinc (Fig. 1a) on a C18 HPLC column,
and thus its separation from phthalocyanine zinc will be
difficult. On the other hand, compound 1 is quite distinct
from both 2 and phthalocyanine zinc on a HPLC column
(Fig. 1b), which indicates larger polarity of compound 1.
Indeed, we were able to work out a convenient protocol
Fig. 1. Analytical HPLC monitored at 670 nm of (a) the mixture after
condensation, indicating the presence of 2-formamido-phthalocyanine
zinc 2 and phthalocyanine zinc at about 1:4 ratio; and (b) 2-carboxypht-
halocyanine zinc 1 purified twice over a silica gel column. See Supple-
mentary Data for detailed HPLC conditions. Basically, a C18 column was
gradient eluted from water to DMF (5–100%) at a flow rate of 1 ml/min.
to separate 1 from both 2 and phthalocyanine zinc on a sil-
ica gel column eluted with a mixed elution solvent
(DMF:acetone at 3:1 ratio). Compound 2 and phthalocya-
nine zinc migrated as a leading band and compound 1 was
retained as a well-separated second band due to its larger
polarity shown on HPLC column (Fig. 1b).
The existence of carboxylic group on phthalocyanine
ring was confirmed by IR spectrum (1719 cmÀ1, COOH
stretching, and 1655 cmÀ1, C@O stretching). On ESI Mass
spectrum, molecular ion of 1 appears as major peak (Fig. 2.
obs: 621.8, expected: 621.92). Upon second ionization
(Fig. 2 inset), this ion lost a COOH group and fragmented
into phthalocyanine zinc (obs: 576.4, expected: 576.90). A
strong mass peak at 1242.9 corresponded to the dimer of
2-carboxyphthalocyanine zinc, which was further frag-
mented into a peak with m/z of 1119.1, presumably due
to the lost of two zinc atoms. Trace amount of trimer of
compound 1 (1864.5) was also detected on mass spectrum.
The existence of the mono-substitution of carboxy group
on phthalocyanine ring is also confirmed by proton
Scheme 1. Condensation.
Fig. 2. ESI Mass spectrum of purified 2-carboxyphthalocyanine zinc.
Molecular ion appears at 621.8 (expected: 621.92). Upon second ioniza-
tion (inset), this ion lost a COOH group and fragmented into a peak at
576.4, corresponding to phthalocyanine zinc (576.9).
Scheme 2. Hydrolysis.

J. Chen et al. / Inorganic Chemistry Communications 9 (2006) 313–315
315
Table 1
Solubility of compound 1 in common organic solvent
Solvent
Solubility (g/L)
N,N-Dimethylformamide
Tetrahydrofuran
Ethanol
11.25
3.60
4.65
5.55
Acetone
In summary, we have synthesized a new unsymmetrical
phthalocyanine, 2-carboxyphthalocyanine zinc 1, and
designed a convenient column based purification scheme
for the product. The carboxylic acid functional group on
phthalocyanine allows site specific covalent attachment of
biological markers to enhance the target specificity of
phthalocyanine zinc as photosensitizer in photodynamic
therapy.
Fig. 3. Proton NMR of 2-carboxyphthalocyanine zinc in deuterated
DMSO and the tentative assignment of proton chemical shifts.
Acknowledgements
We thank Prof. Yu Zheng for helpful discussion on the
Infrared spectrum of phthalocyanines, and financial
support from a Fujian NSF grant (C0320003) and from
Chinese Academy of Sciences (Bairen grant to MH).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.inoche.2005.
12.002.
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Fig. 4. UV–VIS spectra of 2-carboxyphthalocyanine zinc 1 (dotted line)
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