photobleaching than 3a both with and without surfactants,
which means it is more suitable to be a photosensitizer in PDT
than 3a.
4. Conclusion
Four amphiphilic phthalocyanine carboxyl oligomers, 3a (zinc as
centric atom), 3b (aluminum), 3c (ytterbium) and 3d (metal-free)
have been synthesized, and their photophysical and photochemical
properties investigated.
The existed researches usually illustrated Triton X 100 can
increases both of the UF and UD by dissociating the aggregation of
phthalocyanines. In this study, it was inserting that the Triton
X 100 only enhanced the UF and UD of polymeric Pcs, but
had puny effect on their aggregation; while the behaviour of
CTAB was similar to what has been reported in the literature.9,36
It indicated that the two surfactants worked in quite differ-
ent ways on these unique polymeric phthalocyanine carboxyl
molecules.
In summary, 3a and 3b had higher singlet oxygen quantum
yields; while 3c and 3d were better at fluorescence. The electronic
spectra showed that 3b had a lower aggregation tendency than
the other three in pH 7.4 buffer solution. The anionic surfactant
CTAB can dissociate the aggregates of 3a, 3c and 3d, but the
non-ionic surfactant Triton X 100 could not. The photophysical
and photochemical properties of the four compounds were more
sensitive to Triton X100. The quantum yield of fluorescence was
observed from 0.23 to 0.87 for 3c; with an increasing of UD (0.55
to 0.76) being recorded for 3a as well, both in the presence of
Triton X 100. Additionally, 3b showed a stronger photobleaching
resistance than 3a.
The experimental results indicate that polymeric carboxyl
phthalocyanines can be used as a photosensitizer with more
specific advantages than the monomeric phthalocyanines. 3a
showed rapid photobleaching and a high UD, which may be used
as photocatalyst in co-degradation of homogeneous systems. The
non-aggregation of 3b was considered as a valuable amphiphilic
photosensitizer in PDT applications. The fluorescence emission of
3c can be enhanced by Triton X 100 and quenched by methanol
in water solution, which was desired as fluorescent probe or
photodiagnostic imaging materials.
As shown in Fig. 5, the carboxyl groups of the polymeric
Pcs mainly exist as carboxylic anion in pH 7.4 solution; while
CTAB has a quaternary ammonium ion. We suggested that
the cationic CTAB may wrap the oligomer molecules by an
electrostatic effect between quaternary ammonium cation and
carboxylic anion. This enveloping function of CTAB resulted in
the effective disaggregation of 3a, 3c and 3d (shown in Fig. 5).
While non-ionic Triton X 100 was only functional by being
inserted into the interspace between the adjacent molecules as
Ng37 reported, thus the aggregates of 3a, 3c and 3d still existed.
Moreover, the oxygen atoms provided the long chain of Triton
X 100 with a high electronic density distribution; alternatively it
was low density alkyl chain for CTAB at the similar position.
The singlet excited state of these polymeric molecules had larger
polarity than the ground state (Table 2, Stoke shift). Triton X 100
had better capacity to disperse the polarity and stabilize the singlet
state through conjugated effect. We propose that the enhancement
of UF and UD by the stable singlet state was stronger than the
reducing of them by dissipation effect from aggregation; while
CTAB only reduced the aggregation and so affected the excited
state less effective. Thus, CTAB presented a weaker effect than
that of Triton X 100. The above hypothesis was partly proved by
the long lifetime of fluorescence (tF = 130.1 ns) for 3c recorded
in presence of Triton X 100 but not for CTAB (tF = 12.7 ns,
Table 4). In addition, 3b had a lower aggregation and a weaker
response to the surfactants than that of 3a, 3c and 3d. In common
phthalocyanines, AlPc also behave more monomeric than other
metallic Pc in solution.
Acknowledgements
We are pleased to acknowledge the financial support provided
by the National Science Foundation of China (no. 20773077 and
50872149) and the National Basic Research Program of China
(no. 2007CB808000). The author also thank Dr Cheng Zhong
(Chem. Dept., Tsinghua University) for his contribution to the
work.
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Fig. 5 A schematic diagram showing the interaction of 3a, 3c and 3d with
different surfactants. (The HOMO density isolated value is 0.1, calculated
and mapped by HyperChem 7.5.)
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 6327–6334 | 6333
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