Comparative studies of photophysical and photochemical properties of solketal substituted platinum(II) and zinc(II) phthalocyanine sets

Article abstract of DOI:10.1016/j.tet.2010.02.079

A complete set of platinum(II) solketal substituted phthalocyanines has been synthesized and characterized. To evaluate their potential as Type II photosensitizers for photodynamic therapy, comparative studies of their photophysical and photochemical properties with analogous zinc(II) series have been achieved: electronic absorption, fluorescence quantum yields, lifetimes, and fluorescence quenching by benzoquinone, as well as singlet oxygen generation and photodegradation. It appears that platinum(II) phthalocyanines are worth being used as Type II photosensitizers, as they exhibit good singlet oxygen generation and appropriate photodegradation.

Full text of DOI:10.1016/j.tet.2010.02.079

Tetrahedron 66 (2010) 3248–3258
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Comparative studies of photophysical and photochemical properties of solketal
substituted platinum(II) and zinc(II) phthalocyanine sets
,
a
a,b,
*
Yunus Zorlu a, Fabienne Dumoulin a , Mahmut Durmus¸ , Vefa Ahsen
*
a Gebze Institute of Technology, Department of Chemistry, PO Box 141, Gebze 41400, Kocaeli, Turkey
b TUBITAKdMarmara Research Centre, Materials Institute, PO Box 21, Gebze 41470, Kocaeli, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
A complete set of platinum(II) solketal substituted phthalocyanines has been synthesized and charac-
terized. To evaluate their potential as Type II photosensitizers for photodynamic therapy, comparative
studies of their photophysical and photochemical properties with analogous zinc(II) series have been
achieved: electronic absorption, fluorescence quantum yields, lifetimes, and fluorescence quenching by
benzoquinone, as well as singlet oxygen generation and photodegradation. It appears that platinum(II)
phthalocyanines are worth being used as Type II photosensitizers, as they exhibit good singlet oxygen
generation and appropriate photodegradation.
Received 3 August 2009
Received in revised form
26 January 2010
Accepted 22 February 2010
Available online 1 March 2010
Keywords:
Phthalocyanine
Platinum
Ó 2010 Elsevier Ltd. All rights reserved.
Zinc
Photosensitizer
Quantum yields
Singlet oxygen generation
Photodegradation
1. Introduction
enhance fluorescence and intersystem crossing (ISC), enhancing sin-
glet oxygen generation, a key propertyof efficient photosensitizers.12–16
Phthalocyanines are among the best photosensitizers currently
used in photodynamic therapy.1–10 Their photosensitizing proper-
ties are related to the nature of the coordinating central metal or
pseudo-metal,4 giving a diamagnetic complex. Zn(II), Al(III), Ga(III),
Si(IV) phthalocyanines are among the most commonly used.
Different molecular mechanisms exist in photodynamic therapy,
depending on the type of photosensitizer. Phthalocyanines are Type II
photosensitizers: upon irradiation, they induce the conversion of
oxygen from its basic state to its excited state, i.e., singlet oxygen.
Phthalocyanines are said to be second-generation photosensitizers,
offering multiple advantages, mainly absorption at long wavelengths
compatible with the biological window, biocompatibility and photo-
stability.1–10 Their efficiency in generating singlet oxygen is strongly
related to their central metal. Transition metals, such as palladium or
platinum, despite being rarely introduced in photodynamic therapy
agents, are worthy of more study, as available results are promising:
palladium(II) and platinum(II) have recently been introduced at the
centre of chlorin derivatives with a very positive effect on singlet
oxygen generation, hence on tumour cell elimination.11 The effect of
heavy metals, and more generally heavy atoms, is indeed known to
Platinum phthalocyanines have been described to generate
singlet oxygen effectively when used as oxidation catalysts.17
This prompted us to prepare a series of platinum(II) phthalocy-
anines, substituted by solketal moieties, in order to evaluate the
contribution of platinum to the photosensitizing properties of the
phthalocyanine macrocycle. Solketal is the protected form of gly-
cerol, a cheap and versatile chemical.18 Solketal substituted phtha-
locyanines are easily convertible into the corresponding glycerol
substituted phthalocyanines, offering the advantage of being pos-
sibly water soluble.19 This will be the next step of our research, as the
results of the present studies confirm the interest of using plati-
num(II) phthalocyanines as photodynamic therapy agents.
We therefore prepared a complete set of platinum(II) phthalo-
cyanines, designated hereafter as Pt-set: unsubstituted 1-Pt, pe-
ripherally and non-peripherally tetra- and octa-substituted,
respectively, 2-Pt, 3-Pt, 4-Pt and 5-Pt, presented in Figure 1.
Our final future aim, after obtaining water-soluble platinum
phthalocyanines, is to benefit from the combined advantages of the
heavy nature of platinum, of platinum’s innocuousness and even
its anti-tumour effect,20–24 associated with the invaluable
efficiency of phthalocyanine macrocycles for the required photo-
physical and photochemical effects implied in photodynamic
therapy treatments. The present investigations may be considered
* Corresponding authors. Tel.: þ90 262 6053123; fax: þ90 262 6053101; e-mail
addresses: fdumoulin@gyte.edu.tr (F. Dumoulin), ahsen@gyte.edu.tr (V. Ahsen).
0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.02.079

Y. Zorlu et al. / Tetrahedron 66 (2010) 3248–3258
3249
O
O
O
O
O
O
O
O
O
O
O
N
Pt
N
N
Pt
N
N
Pt
N
O
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
O
O
N
O
O
O
O
O
O
O
O
O
O
O
1-Pt
2-Pt
3-Pt
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
N
Pt
N
N
Pt
N
O
O
N
N
N
N
N
N
N
N
N
N
N
O
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
4-Pt
5-Pt
Figure 1. Representation of the platinum phthalocyanines (Pt-set).
as preliminary studies in the quest for more efficient photodynamic
therapy agents, based on platinum(II) phthalocyanines. The pho-
tosensing properties of the Pt-set have been compared with those
of an analogous series of Zn(II) phthalocyanines, designated here-
after as Zn-set. These were chosen as reference because Zn is one of
the most common and efficient metals in photosensitizing phtha-
locyanines used in photodynamic therapy.
diiminoisoindoline has been described to give satisfactory yields
under ammoniac atmosphere: yields were similar to those obtained
with previously purified diiminoisoindoline 1038 under an inert
atmosphere.37 We chose to work from isolated diiminoisoindoline,
as compounds 11 and 12 are new and had to be fully characterized.
Several experiments on 2-Pt synthesis proved that, at least with
our substituents, yields of isolated phthalocyanines are better
when using diiminoisoindoline 10 in N,N0-dimethylaminoethanol
under an ammoniacal atmosphere than under argon atmosphere. It
probably enhances the reactivity of the diiminoisoindoline and
prevents its eventual decomposition.37 Except for the octa non-
peripheral derivative 5-Pt, all the platinum phthalocyanines have
been prepared using diiminoisoindoline as a precursor, under an
ammoniacal atmosphere, with a ratio of 2:1 phthalonitrile/PtCl2, in
10–20% yields, satisfactory regarding the simplicity of the pro-
cedure and purification. Under these conditions, no free base
phthalocyanine was detected by mass spectroscopy and UV–vis
spectrophotometry, saving us from the tedious separations of
a metallated phthalocyanine from the analogous free one. The
phthalonitrile 9 couldn’t be converted to the corresponding dii-
minoisoindoline, probably due to steric hindrance, and 5-Pt was
therefore prepared directly from 9.
2. Results and discussions
2.1. Syntheses and characterizations
Compound 1-Zn was purchased from Aldrich and used as re-
ceived. The synthesis of 2-Zn,19,25,26 3-Zn,19 4-Zn27–29 and 5-Zn29
have been performed following the reported procedures.
The very few descriptions of substituted platinum phthalocya-
nine syntheses report the difficulty in synthesis and purification.30
Most of the reported data about platinum phthalocyanines con-
cerned so far their crystallography.31 Their electrochemical prop-
erties also aroused strong interest.32 Syntheses of unsubstituted
platinum phthalocyanines are described in harsh conditions,32–34
conditions not applicable for substituted derivatives. Use of mi-
crowaves recently optimized the yields, starting from platinum
tetrachloride to yield platinum(II) phthalocyanines.35 It appeared in
addition that the insertion of platinum in the centre of a free-based
phthalocyanine remains unsuccessful, the metal-assisted tetrame-
rization being more efficient.36 We therefore chose this strategy to
optimize the synthesis of 2-Pt.
Among the available platinum salts, PtCl2 was the most suitable,
being more reactive than Pt(acac)2 or Pt(OAc)2. It has been reported
that dimethylaminoethanol is the best solvent for platinum(II)
phthalocyanine synthesis.36 The use of phthalonitrile 6 following
described methods37 lead anyway to 2-Pt in poor yields (<2%), even
under an ammoniacal atmosphere. Use of in situ prepared
Compounds 10,38 11 and 12 have been, respectively, prepared
from phthalonitriles 6,28 729 and 828 (Scheme 1), following a pro-
cedure previously described by us,38 in nearly quantitative yields.
All the substituted phthalocyanines of the Pt-set have been
characterized by NMR, mass spectrometry (MALDI) and high per-
formance liquid chromatography (HPLC). Mass spectroscopy anal-
yses of phthalocyanines have previously been challenging,
especially for unsubstituted ones.39,40 In our case, MALDI experi-
ments, using dihydroxybenzoic acid as the matrix, allowed us to
observe the molecular ion (see Supplementary data). The purity of
the four substituted Pt-set’s phthalocyanines: 2-Pt, 3-Pt, 4-Pt and
5-Pt, has been established by HPLC experiments, on a silica column

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Y. Zorlu et al. / Tetrahedron 66 (2010) 3248–3258
O
O
O
O
O
O
O
O
O
CN
CN
CN
CN
CN
CN
CN
CN
O
O
O
6
7
8
O
O
O
O
O
O
O
NH
NH
NH
O
NH
O
O
O
O
O
CN
CN
NH
NH
NH
NH
NH
NH
NH
NH
O
O
O
O
O
13
10
11
12
9
1-Pt
2-Pt
3-Pt
4-Pt
5-Pt
Scheme 1. Preparation of platinum phthalocyanines.
eluted by a mixture of dichloromethane/isopropanol (detection
from 190 to 950 nm). Respective retention times were 16.78, 17.44,
14.59 and 31.35 min. The unique peak on the profile certifies to the
purity of the phthalocyanines (see profiles in Supplementary data).
13C NMR spectra have been recorded (see Supplementary data).
They are similar to those of similar solketal substituted phthalo-
cyanines.29 In the case of 4-Pt and 5-Pt, the effect of the platinum
on chemical shifts can be compared to the similar Ni, Zn and metal
free phthalocyanines:29 the peak corresponding to the internal
carbon of the phthalocyanine macrocycle is observed at 138.70 for
4-Pt and at 140.48 for 5-Pt. Compared to analogous zinc(II) and
free-based phthalocyanines, this peak is shifted to higher fields, as
are those of analogous nickel derivatives. This is coherent with the
fact that nickel and platinum belong to the same group on the
periodic table. Due to its heavy atom character, the effect of plati-
num is even more pronounced than for nickel (5 ppm more shiel-
ded for 4-Pt and 5-Pt compared with the analogous nickel
phthalocyanines).
for 1-Pt, which is insoluble in common organic solvents. Reported
electronic absorption spectra of 1-Pt were recorded in strong in-
organic acids such as concentrated sulfuric acid,35 not suitable for
the other molecules of this study. The insolubility of 1-Pt un-
fortunately prevented us from achieving further photophysical and
photochemical investigations, despite the interest it would have
presented.
Electronic absorption spectra of the Zn-set phthalocyanines
showed non-aggregated monomeric behaviour, as evidenced by
a single and narrow Q band, typical of metallated phthalocya-
nines41 (Fig. 2a).
The maximum absorptions in the Q band region and extinc-
tion coefficients for Zn-set and Pt-set are summarized in Table 1.
The B band region was observed between 300 and 350 nm for all
the phthalocyanines. The maximum absorption in the Q band
area for the substituted zinc(II) phthalocyanines (2-Zn to 5-Zn)
are red-shifted relative to that of the unsubstituted 1-Zn, im-
plying that the Highest Occupied Molecular Orbital (HOMO)–
Lowest Unoccupied Molecular Orbital (LUMO) energy gap of the
phthalocyanine ring is reduced when introducing electron-
donating substituents. As said above, we couldn’t check if simi-
lar effects appear in the Pt-set, due to the insolubility of 1-Pt in
DMF, while the solketal substituents can’t survive in acidic
media, prohibiting measurements in the inorganic acids in which
1-Pt is soluble.
2.2. Photophysical and photochemical parameters
2.2.1. Ground state electronic absorption. Comparative studies of
the electronic absorption behaviour of Zn-set and Pt-set have been
performed in N,N0-dimethylformamide (DMF), a solvent that offers
convenient solubility for all of the studied phthalocyanines, except
Figure 2. Absorption spectra of tetra- and octa-substituted (a) zinc (2-Zn, 3-Zn, 4-Zn and 5-Zn) and (b) platinum (2-Pt, 3-Pt, 4-Pt and 5-Pt) phthalocyanines in DMF.
Concentration¼1ꢀ10ꢁ5 mol dmꢁ3
.

Y. Zorlu et al. / Tetrahedron 66 (2010) 3248–3258
3251
Table 1
hypsochromic shift of the Pt-set’s Q-band compared to the Zn-set’s
ones. It must be noted in addition that extinction coefficients of the
Pt-set are lower than those of the Zn-set.
Maximum electronic absorption and extinction coefficient for the Zn-set and the
Pt-set in the Q band area, in DMF: Q band (lmax, nm, log 3)
Compound
Zn
1
2
3
4
5
The effect of peripheral substitution compared with non-
peripheral is the same for the two sets, with a red-shift of the Q
band for non-peripheral substituted phthalocyanines compared to
the peripheral ones. These red spectral shifts are typical of phtha-
locyanines with substituents at the non-peripheral positions: they
are due to the linear combinations of the atomic orbitals (LCAO)
coefficients at the non-peripheral positions of the HOMO. These
coefficients are greater than those for peripherally substituted
670 (5.37) 677 (5.09)
696 (5.30) 672 (5.28)
729 (4.99)
Pt
d
654 (4.34), 676 (4.80) 650 (4.30), 711 (4.89)
602 (4.40) 601 (4.24)
Platinum (II), when introduced in the centre of a phthalocyanine
macrocycle, is known to strongly enhance aggregation.42 On the
other hand, non-peripheral substitution is known to inhibit ag-
gregation.42 Substituents in non-peripheral positions prevent
macrocycles from getting too close to each other, thanks to an
intercalative steric hindrance. The fact that peripherally substituted
2-Pt and 4-Pt derivatives present cofacial aggregation, as evi-
denced by the presence of two non-vibrational peaks in the Q band
region, when the non-peripheral derivatives 3-Pt and 5-Pt show
minimal aggregation as judged by the little broadening of their Q
band, is in accordance with this hypothesis. The lower energy (red-
shifted) bands at 654 nm for 2-Pt and 650 nm for 4-Pt are due to
the monomeric species, while the higher energy (blue-shifted)
bands at 602 nm for 2-Pt, and at 601 nm for 4-Pt are due to the
aggregated species (Fig. 2b). Compared to analogous substituted
phthalocyanines of the Zn-set, those of the Pt-set are all more
aggregated, offering one more illustration of the aggregation ten-
dencies induced by platinum.
phthalocyanines.43,44 The energy gap (
DE) between the HOMO and
LUMO becomes smaller, resulting in bathochromic shifts. The value
of these red-shifts depends on the peripheral or non-peripheral
position of the substituents and is the same for the two sets with
no influence of the metal. Red-shifts are observed of w20 nm for
tetra-substituted phthalocyanines (2-Zn and 2-Pt vs 3-Zn and 3-Pt)
and of w60 nm for octa-substituted phthalocyanines (4-Zn and 4-
Pt vs 5-Zn and 5-Pt).
2.2.2. Aggregation behaviour of the Pt-set compared to the Zn-
set. Aggregation results from the coplanar association of the
phthalocyanine rings, progressing from monomer to dimer then to
complexes of higher order. It depends on several parameters:
temperature, concentration, nature of the solvent, nature, central
complexed metal ions, number and position of the substituents.45
In the aggregated state, the electronic structure of the phthalocy-
anine rings is perturbed, resulting in an alternation of the ground
and excited state electronic structures.46 These perturbations
modify the electronic absorption events and are therefore visible in
the UV–vis spectra: the peak due to aggregation is blue-shifted
with respect to the monomer’s one.
The same comparison between the two sets about the maxi-
mum absorption in the Q band area results in the general obser-
vation of a blue-shift of approximately 20 nm for the Pt-set
compared to the Zn-set (Table 1). This could be due to the larger
size and heavy nature of the platinum atom. A
p
–
p transition upon
*
photon irradiation occurs at the Q band energy levels and implies
the d orbitals of the molecule. The electronic structures of zinc(II)
and platinum(II) are of different energy, explaining the
Keeping in mind that compared to zinc, platinum is a metal
strongly enhancing aggregation,37,42 we chose to investigate the
Figure 3. Comparative aggregation behaviour of (a) 2-Zn and 2-Pt in DMF, (b) 3-Zn and 3-Pt in tetrahydrofuran, (c) 4-Zn and 4-Pt in toluene, (d) 5-Zn and 5-Pt in chloroform. Zn
derivatives are in pink, Pt derivatives in blue.

3252
Y. Zorlu et al. / Tetrahedron 66 (2010) 3248–3258
The inhibiting effect of non-peripheral substituents on aggre-
gation is strong enough to limit the aggregation of 3-Zn, 5-Zn, 3-Pt
and 5-Pt in all the solvents: 3-Zn and 5-Zn are in their monomeric
form in all the solvents, as are 3-Pt and 5-Pt in chloroform, DMF,
THF and toluene. Peripheral 2-Pt and 4-Pt exhibit aggregation in
chloroform, DMF, THF and toluene.
The addition of surfactants such as Triton X-100 can reduce
aggregation. The aggregated molecules are dissociated thanks to
the intercalation of the surfactant. Addition of Triton X-100 to so-
lutions of 2-Zn and 4-Zn in methanol, a solvent in which they are
aggregated, resulted in their dissociation, as evidenced by the in-
creased intensity and red-shift of the monomer peak, while
the peak for the aggregated species decreases in intensity (Fig. 4 for
2-Zn).
The aggregation behaviour of the substituted Pt(II) phthalocy-
anines (3-Pt and 5-Pt) and Zn(II) phthalocyanines (2-Zn to 5-Zn)
were also investigated at different concentrations in DMF (Fig. 5,
case of 4-Zn and 5-Pt). The intensity of the Q band increased with
the concentrations, obeying the Beer–Lambert law at concentra-
Figure 4. Absorption spectral changes for 2-Zn in methanol upon the addition of
Triton X-100.
aggregation behaviour of the substituted phthalocyanines of the
Pt-set and of the Zn-set in the most common organic solvents:
chloroform, DMF, methanol, tetrahydrofuran, toluene and DMSO.
Non-peripheral substituted phthalocyanines are known to show
less aggregation than peripheral substituted ones:47 these trends
were followed by all the molecules in the two sets.
tions ranging from 1.4ꢀ10
ꢁ5 to 4ꢀ10ꢁ6 mol dmꢁ3
.
2.2.3. Fluorescence studies. Fluorescence measurements have been
recorded in DMF, the solvent offering the best average solubility for
all the molecules.
While all the substituted Zn-set’s phthalocyanines are soluble in
the selected solvents, the substituted Pt-set‘s phthalocyanine aren’t
soluble in methanol and DMSO, and much more aggregated in
other solvents than the corresponding zinc derivatives (Fig. 3).
2.2.3.1. Absorption, excitation and emission experiments. The
non-peripheral octa-substituted phthalocyanines 5-Zn and 5-Pt
showed less fluorescence emission than the other complexes. In the
case of 5-Pt, the emission of the complex was even not detectable.
Figure 5. Aggregation behaviour of (a) 4-Zn and (b) 5-Pt in DMF at different concentrations: 14ꢀ10ꢁ6 (A), 12ꢀ10ꢁ6 (B), 10ꢀ10ꢁ6 (C), 8ꢀ10ꢁ6 (D), 6ꢀ10ꢁ6 (E), 4ꢀ10ꢁ6 mol dmꢁ3 (F).
(Inset: plot of absorbance vs concentration).

Y. Zorlu et al. / Tetrahedron 66 (2010) 3248–3258
3253
Figure 6 shows the fluorescence emission, absorption and excita-
tion spectra for 2-Zn and 2-Pt. Fluorescence emission peak data are
summarized in Table 2. The Stokes shifts are of 13, 14, 10 and 17 nm,
respectively, for 2-Zn, 3-Zn, 4-Zn and 5-Zn, and of 22, 24 and
28 nm, respectively, for 2-Pt, 3-Pt and 4-Pt (Table 2).
5-Zn (Table 3). The FF value for 5-Zn is lower compared to other
substituted Zn(II) complexes (2-Zn, 3-Zn and 4-Zn). As 5-Zn is less
aggregated than 2-Zn, 3-Zn and 4-Zn, it enhances the energy
transfer via the intersystem crossing (ISC), thus limiting FF. The FF
values of substituted Pt-set’s phthalocyanines were lower than
those of the analogous Zn-set’s phthalocyanines (Table 3). This is
due to intersystem crossing enhanced by the presence of the heavy
atom: platinum.48
Table 3
Photophysical and photochemical parameters of Zn-set and Pt-set phthalocyanines
in DMF
Compound
FF
sF (ns)
s0 (ns)
akF (sꢁ1
(ꢀ107)
)
Fd (ꢀ10ꢁ4
)
FD
1-Zn
2-Zn
3-Zn
4-Zn
5-Zn
2-Pt
3-Pt
4-Pt
5-Pt
0.17
0.20
0.13
0.17
0.085
0.0051
0.0086
0.013
d
1.03
2.10
1.09
1.69
1.33
0.18
0.17
0.55
d
6.05
10.31
8.68
16.53
9.69
11.50
10.53
6.38
2.77
5.07
2.37
d
0.23
1.38
1.58
0.87
1.19
1.32
0.62
8.58
0.87
0.56
0.50
0.57
0.54
0.48
0.35
0.63
0.65
0.35
9.50
15.66
36.09
19.79
42.22
d
Fluorescence lifetime (sF) refers to the average time a molecule
remains in its excited state before returning to its ground state. This
value is related to that of FF: the longer the fluorescence lifetime is,
the higher the quantum yield of fluorescence. Any factor shortening
the fluorescence lifetime reduces the value of FF. Among such
factors are internal conversion and intersystem crossing events,
influenced by the nature and the environment of the fluorophore.
The sF values (Table 3) were calculated using the Strickler–Berg
equation. The sF values of the Zn-set’s phthalocyanines are higher
compared to those of Pt-set, as the natural radiative lifetime (s0
)
Figure 6. Absorption, excitation and emission spectra of (a) 2-Zn and (b) 2-Pt in DMF.
Excitation wavelength: 650 nm.
and the rate constants for fluorescence (kF) values also given in
Table 3. Once again, this is due to the heavy atom effect.48
Table 2
2.2.3.3. Fluorescence quenching studies by benzoquinone
[BQ]. Fluorescence quenching by benzoquinone of the Zn-set was
investigated. The Pt-set’s behaviour was not studied because of
the very low fluorescence emission. In the presence of
a quencher (BQ), energy transfer occurs between the fluorophore
(the excited Zn(II) phthalocyanine) and the quencher. The fluo-
rescence quenching of Zn(II) phthalocyanine by BQ obeyed
Stern–Volmer kinetics. This is consistent with diffusion-
controlled bimolecular reactions. Figure 7 shows the quenching
Absorption, excitation and emission spectral data for Zn-set and Pt-set in DMF
Compound
Q band lmax
(nm)
Excitation lEx
(nm)
Emission lEm
(nm)
Stokes shift
DStokes (nm)
1-Zn
2-Zn
3-Zn
4-Zn
5-Zn
2-Pt
3-Pt
4-Pt
5-Pt
670
677
696
672
729
670
680
700
673
736
671
676
673
d
676
690
710
682
746
676
700
678
d
6
13
14
10
17
22
24
28
d
654, 602
676
650, 601
711
The shape of the excitation spectra is similar to the absorption
spectra’s shape for all the Zn-set phthalocyanines (2-Zn to 5-Zn).
This suggests that the nuclear configurations of the ground and
excited states are not affected by excitation. In the case of Pt-set,
the shape of the excitation spectra appear to be different from the
absorption spectra (Fig. 6b, example of 2-Pt). This is probably due to
modifications such as loss of the symmetry of the molecule upon
excitation. The difference in the excitation behaviour of Pt-set
phthalocyanines from Zn-set phthalocyanines could be due to the
fact that the larger platinum atom is out of the plane of the
phthalocyanine ring, inducing a loss of symmetry.31
2.2.3.2. Fluorescence quantum yields and lifetimes. The fluores-
cence quantum yields (FF) are given in Table 3. The fluorescence
quantum yields (FF) of 2-Zn, 3-Zn and 4-Zn are typical of metal-
lated phthalocyanines, except for non-peripheral octa-substituted
Figure 7. Fluorescence emission spectral changes of 4-Zn (1.00ꢀ10ꢁ5 mol dmꢁ3) on
addition of different concentrations of BQ in DMF. [BQ]¼0, 0.008, 0.016, 0.024, 0.032,
0.040 mol dmꢁ3
.

3254
Y. Zorlu et al. / Tetrahedron 66 (2010) 3248–3258
of 4-Zn by BQ. The slope of the plots shown at Figure 8 gave the
Stern–Volmer constant (KSV) values. The KSV and bimolecular
quenching constant (kq) values for the BQ quenching of the Zn-
set are listed in Table 4. The KSV values of the substituted 2-Zn to
5-Zn are lower than those of unsubstituted 1-Zn. The KSV values
decreased as follows: 3-Zn>2-Zn>4-Zn>5-Zn. The bimolecular
quenching constant (kq) values of the substituted phthalocya-
nines (2-Zn to 5-Zn) are also lower than those of unsubstituted
1-Zn. The bimolecular quenching rate constants were found to be
close to the diffusion-controlled limits, w1010 Mꢁ1 sꢁ1. The kq
values for the substituted complexes decreased as follows:
3-Zn>4-Zn>2-Zn>5-Zn (Table 4).
triplet state of a photosensitizer (here the metallated phthalocy-
anine) and the ground state of molecular oxygen leads to its
conversion into singlet oxygen. This transfer must be as much
efficient as possible to generate large amounts of singlet oxygen.
This is quantified by the singlet oxygen quantum yield (FD),
a parameter giving an indication of the potential of molecules to
be used as photosensitizers in applications where singlet oxygen
is required (e.g., for Type II mechanism). The FD values were
determined in DMF using a chemical method and 1,3-diphenyl-
isobenzofuran (DPBF) as a quencher. The disappearance of DPBF
was monitored using UV–vis spectrophotometer (Fig. 9: example
of 3-Pt).
Many factors are responsible for the magnitude of the de-
termined quantum yield of singlet oxygen, including: the
triplet excited state energy, the ability of substituents and
solvents to quench singlet oxygen, the triplet excited state
lifetime and the efficiency of the energy transfer between the
triplet excited state and the ground state of oxygen. There was
no decrease in the Q band and no appearance of new bands
during FD determinations (Fig. 9): this indicates that the
phthalocyanines were not damaged by the generated singlet
oxygen. It is believed that during photosensitization, the
phthalocyanine is firstly excited to the singlet state, then rea-
ches the triplet state through intersystem crossing, then
P
3
transfers its energy to ground state oxygen, O2
(
g), itself
converted into its excited state (singlet oxygen), O2
(
1Dg). This
singlet oxygen is the chief cytotoxic species, which sub-
sequently oxidizes the surrounding substrates: this oxidation is
the key of the Type II mechanism.
Figure 8. Stern–Volmer plots for benzoquinone (BQ) quenching of 2-Zn, 3-Zn, 4-Zn
and 5-Zn in DMF. [ZnPc] w1.00ꢀ10ꢁ5 mol dmꢁ3. [BQ]¼0, 0.008, 0.016, 0.024, 0.032,
Compound 1-Zn was used as the reference and the measure-
ments were performed in DMF. Table 3 shows that the all
substituted Zn(II) phthalocyanines have roughly the same FD
values than the unsubstituted 1-Zn, the number and position of
substituents having a very moderate influence.
While the FD values of 3-Pt and 4-Pt are higher than those
of 1-Zn, 2-Pt and 5-Pt exhibit lower FD values than 1-Zn in
DMF. Although the non-peripheral 3-Zn and 3-Pt showed
higher FD values than peripheral 2-Zn and 2-Pt, the 5-Zn and
5-Pt showed lower FD values than peripheral 4-Zn and 4-Pt
(Table 3).
0.040 mol dmꢁ3
.
Table 4
Fluorescence quenching data of the Zn-set in DMF
Compound
KSV (Mꢁ1
)
kq/1010 (dm3 molꢁ1 sꢁ1
)
1-Zn
2-Zn
3-Zn
4-Zn
5-Zn
57.60
48.68
52.46
42.83
26.65
5.59
2.31
4.79
2.53
2.00
The differences in the observed values are due to the different
nature of zinc and platinum, which belong to different groups on
the periodic classification table and have different outermost
electronic configuration.
2.2.4. Singlet oxygen quantum yields. A good photosensitizer must
be very efficient in generating singlet oxygen, the active species of
a photodynamic therapy treatment. Energy transfer between the
Figure 9. A typical spectrum for the determination of singlet oxygen quantum yield of 3-Pt in DMF using DPBF as the singlet oxygen quencher. Concentration¼1ꢀ10ꢁ5 mol dmꢁ3
(Inset: plots of DPBF absorbance vs time.).

Y. Zorlu et al. / Tetrahedron 66 (2010) 3248–3258
3255
We at least established the efficiency of platinum(II) phthalo-
3. Conclusions
cyanines to generate singlet oxygen. This confirms that further
investigations are worth to be achieved.
We prepared a full set of solketal substituted platinum(II)
phthalocyanines, precursors of potentially water-soluble plati-
num(II) phthalocyanines utilizable as photodynamic therapy
agents. Preliminary studies of their interest as Type II photosensi-
tizers, and comparison with analogous zinc(II) phthalocyanines’
photophysical and photochemical properties, demonstrated that
these complexes are efficient singlet oxygen generators. In addi-
tion, their photodegradation parameters are suitable for potential
photodynamic therapy agents.
These promising results confirm the interest of platinum(II)
phthalocyanines as photodynamic therapy agents. The next steps of
this work: obtaining water-soluble platinum(II) phthalocyanines is
currently being achieved by our team.
2.2.5. Photodegradation studies. Degradation of the molecules
upon irradiation lowers their availability, thus their photo-
sensitizing efficiency, and is expressed by their photodegradation
quantum yield values. The loss of the absorption peaks without
distortion of their shape corresponds to clean photodegradation
events, not associated with phototransformation. Photo-
degradation measurements have been recorded in DMF. Examples
of spectral changes observed during irradiation for all the
substituted phthalocyanines (2-Zn to 5-Zn and 2-Pt to 5-Pt) are
given in Figure 10 (example of 5-Zn and 5-Pt) and hence confirms
that photodegradation occurred without phototransformation.
Figure 10. The photodegradation of (a) 5-Zn and (b) 5-Pt in DMF showing the disappearance of the Q band and the appearance of the reduction band at 1 min intervals for 5-Zn
and 5 min intervals for 5-Pt (Inset: plot of Q band absorbance versus time).
All the substituted complexes, 2-Zn to 5-Zn and 2-Pt to 5-Pt,
have roughly similar stability with photodegradation quantum
yield (Fd) of the order of 10ꢁ4 (Table 3). Stable complexes show Fd
values as low as 10ꢁ6. Molecules are considered unstable with
4. Experimental
4.1. Materials
values around 10ꢁ3 49
.
Our two sets of molecules have an in-
Unsubstituted zinc phthalocyanine (1-Zn) was purchased
from Aldrich and used as received. Zinc(II) phthalocyanines (2-
Zn to 5-Zn), phthalonitriles 6, 7, 8 and 9, diiminoisoindolines
10 and 13 have been prepared following described
procedures.19,25,26,28,29,38
termediate photostability. This is particularly required for photo-
dynamic therapy applications: very unstable molecules degrade
before having time to act, but on the other hand, highly stable
molecules aren’t easily eliminated by the host organism.

3256
Y. Zorlu et al. / Tetrahedron 66 (2010) 3248–3258
4.2. Equipments
(CDCl3): d, ppm 158.55,135.17, 126.92, 121.29,115.64,109.64,102.32,
73.98, 68.55, 66.90, 26.96, 25.45. MALDI-TOF-MS m/z: calcd
1227.32; found 1228.33 [MþH]. Retention time: 16.78 min. UV–vis,
HPLC profiles have been recorded using an HPLC system Agilent
1100 series (ChemStation software) equipped with a G 1311A pump,
a G1315B diode array detector monitoring the 190–950 nm range and
a normal phase column Lichrosorb-SI-60 (250ꢀ4.6 mm) from All-
tech. Associates, Inc. LC–ESI mass spectra were recorded with
a Bruker microTOF spectrometer equipped with electronspray ioni-
zation (ESI) source. Matrix-assisted laser desorption/ionization time-
of-flight mass spectrometry (MALDI-TOF-MS) measurements were
performed on a Bruker Daltonics microTOF. 1H and 13C NMR spectra
were recorded in DMSO-d6 and CDCl3 solutions on a Varian 500 MHz
spectrometer. Absorption spectra in the UV–visible region were
recorded with a Shimadzu 2001 UV spectrophotometer using a 1 cm
pathlengthcuvette at room temperature. Fluorescence excitationand
emission spectra were recorded on a Varian Eclipse spectrofluo-
rometer using 1 cm pathlength cuvettes at room temperature.
Photo-irradiations were done using a General Electric quartz
line lamp (300 W). A 600 nm glass cut off filter (Schott) and a water
filter were used to filter off ultraviolet and infrared radiations, re-
spectively. An interference filter (Intor, 670 nm with a band width
of 40 nm) was additionally placed in the light path before the
sample. Light intensities were measured with a POWER MAX5100
(Molelectron detector incorporated) power meter.
lmax nm (log 3): CHCl3, 654 (4.84), 605 (4.49); THF 654 (4.77), 602
(4.57); DMF 654 (4.34), 602 (4.40). HRMS (ESI) m/z: calcd for
C56H57N8O12Pt: 1228.3738; found: 1228.3700.
4.3.3.3. 1,8(11),15(18),22(25)-Tetra((2,2-dimethyl-1,3-dioxolan-4-
yl)methoxy)phthalocyaninato platinum (3-Pt). Obtained following
the above procedure, from 11 (1.82 mmol, 0.5 g). Yield 10%
(55.8 mg). 1H NMR (CDCl3):
d
, ppm 6.36–8.45 (m, 12H, Ar), 3.65–
5.20 (m, 20H, 4CH2CHCH2), 1.57, 1.68 (br m, 24H, 8CH3). 13C NMR
(CDCl3): , ppm 156.05, 139.33,129.98, 125.00,121.61, 117.06, 114.04,
d
111.34, 110.02, 74.95, 74.21, 67.45, 27.51, 25.98. MALDI-TOF-MS m/z:
calcd 1227.32; found 1228.46 [MþH]. Retention time: 17.44 min.
UV–vis, lmax nm (log 3): CHCl3, 675 (5.23), 607 (4.58); THF, 674
(5.27), 608 (4.76); DMF, 676 (4.80). HRMS (ESI) m/z: calcd for
C56H57N8O12Pt: 1228.3744; found: 1228.3746.
4.3.3.4. 2,3,9,10,16,17,23,24-Octa((2,2-dimethyl-1,3-dioxolan-4-
yl)methoxy)phthalocyaninato platinum (4-Pt). Obtained following
the above procedure, from 12 (1.98 mmol, 0.8 g). Yield 35% (302 mg).
1H NMR (CDCl3):
d
, ppm 7.98 (s, 8H, Ar), 4.29–4.90 (m, 40H,
8CH2CHCH2), 1.62, 1.71 (br m, 48H, 16CH3). 13C NMR (CDCl3):
d, ppm
150.61,138.70,129.92,109.83,106.03, 74.38, 70.44, 67.29, 27.03, 25.68.
4.3. Syntheses
MALDI-TOF-MS m/z: calcd 1747.62; found 1749.16 [Mþ2H]. Retention
time: 14.59 min. UV–vis, lmax nm (log 3): CHCl3, 651 (5.09), 589
4.3.1. 3-(2,2-Dimethyl-1,3-dioxolan-4-yl)-methoxy-1,3-dihydro-1,3-
diiminoisoindole (11). Prepared following described procedure for
10,38 from 7 (1 g, 3.88 mmol). Yield 85% (900 mg). Greenish solid.
1H NMR (DMSO-d6): 7.64, 7.62, 7.35, 7.34 (m, 3H, aromatics),
4.02–4.60 (m, 5H, CH2CHCH2), 3.55 (br s, NH), 1.36, 1.42 (2br s,
6H, 2CH3). 13C NMR (DMSO-d6): 154.30, 137.60, 133.87, 120.16,
116.02, 114.79, 108.90, 73.28, 69.49, 65.34, 26.35, 24.86. ESI–LC–
MS m/z: calcd 275.1; found 276.2 [MþH]. Anal. Calcd for
C14H17N3O3: C, 61.08; H, 6.22; N, 15.26. Found: C, 61.11; H, 6.24;
N, 15.18.
(4.46); THF, 650 (4.81), 602 (4.55); DMF, 650 (4.30), 601 (4.24). HRMS
(ESI) m/z: calcd for C80H97N8NaO24Pt: 1771.6156; found: 1771.6100.
4.3.3.5. 1,4,8,11,15,18,22,25-Octa((2,2-dimethyl-1,3-dioxolan-4-
yl)methoxy)phthalocyaninato platinum (5-Pt). Obtained following
the above procedure, from 9 (0.52 mmol, 0.2 g). Yield 8% (18 mg).
1H NMR (CDCl3):
d
, ppm 7.71 (s, 8H, Ar), 4.20–5.08 (m, 40H,
8CH2CHCH2), 1.41, 1.47 (br m, 48H, 16CH3). 13C NMR (CDCl3):
d
, ppm
152.09, 140.58, 127.24, 120.62, 109.59, 74.52, 73.68, 67.21, 27.00,
25.42. MALDI-TOF-MS m/z: calcd 1747.62; found 1748.93 [MþH].
Retention time: 31.35 min. UV–vis, lmax nm (log 3): CHCl3, 705
4.3.2. 5,6-Di-[(2,2-dimethyl-1,3-dioxolan-4-yl)-methoxy]-1,3-di-
(5.11); THF 706 (5.11); DMF, 711 (4.89). HRMS (ESI) m/z: calcd for
hydro-1,3-diiminoisoindole (12). Prepared following described
C80H97N8O24Pt: 1748.6258; found: 1748.6321.
procedure for 10,38 from
8 (400 mg, 1.03 mmol). Yield 96%
(400 mg). Greenish solid. 1H NMR (CDCl3): 7.20 (s, 2H, aromatics),
4.62 (br s, 1H, CH), 3.92–4.16 (4H, m, (OCH2)2), 1.37, 1.42 (2br s, 6H,
2CH3). 13C NMR (CDCl3): 165.05, 151.49, 127.80, 109.65, 105.84,
73.66, 66.64, 66.39, 26.54, 25.27. ESI–LC–MS m/z: calcd 405.2;
found 406.2 [MþH]. Anal. Calcd for C20H27N3O6: C, 59.25; H, 6.71;
N, 10.36. Found C, 59.22; H, 6.72; N, 10.33.
4.4. Photophysical and photochemical parameters
4.4.1. Fluorescence quantum yields and lifetimes. Fluorescence
quantum yields (FF) were determined by the comparative method
(Eq. 1).50,51
F$AStd
$
h2
h2
4.3.3. Procedure for the synthesis of Pt-set phthalocyanines. Pre-
cursor and PtCl2 (0.5 equiv) were refluxed in N,N-dimethylami-
noethanol under ammonia atmosphere for 24 h. The cooled
reaction mixture was then poured into hexane. The solid product
was filtered, dissolved in a minimum amount of dichloromethane
and precipitated in hexane (several times). On silica gel column
chromatography, initial elution by hexane/ethyl acetate (2:1)
eliminated the remaining organic impurities. The platinum
phthalocyanine was then eluted by tetrahydrofuran.
FF
¼
FFðStdÞ
F$AStd
(1)
$
Std
where F and FStd are, respectively, the areas under the fluorescence
curves of the samples (2-Zn to 5-Zn and 2-Pt to 5-Pt) and the standard.
A and AStd are the respective absorbances of the sample and of the
standard at the excitation wavelengths.
h and hStd are the refractive
indices of solvents used, respectively, for the sample and the standard.
Compound 1-Zn in DMSO (FF¼0.18)52 was employed as the standard.
Natural radiative lifetimes were (s0) were determined using
PhotochemCAD Program,53 which uses the Strickler–Berg equation,
4.3.3.1. Phthalocyaninato platinum (1-Pt). Obtained following
the above procedure, from 13 (1.73 mmol/250 mg). Yield 32% (98 mg).
and fluorescence lifetimes (sF) are given by Eq. 2.
sF
s0
FF
¼
(2)
4.3.3.2. 2,9(10),16(17),23(24)-Tetra((2,2-dimethyl-1,3-dioxolan-
4-yl)methoxy)phthalocyaninato platinum (2-Pt). Obtained follow-
ing the above procedure, from 10 (3.64 mmol, 1 g). Yield 35%
Using the sF values, rate constants for fluorescence (kF), in-
tersystem crossing (kISC), internal conversion (kIC) and photo-
degradation (kd) were estimated.
(392 mg). 1H NMR (CDCl3):
d
, ppm 5.99–6.85 (m, 12H, Ar), 3.48–
4.38 (m, 20H, 4CH2CHCH2), 1.51, 1.46 (br m, 24H, 8CH3). 13C NMR

Y. Zorlu et al. / Tetrahedron 66 (2010) 3248–3258
3257
4.4.2. Singlet oxygen quantum yields. Singlet oxygen (FD) quantum
Supplementary data
yield determinations were carried out using the experimental set-
up described in literature.54,55 Typically, a 2 mL portion of the
phthalocyanine solutions (absorbance w1 at the irradiation
wavelength) containing the singlet oxygen quencher was irradiated
in the Q band region. FD values were determined in air using the
relative method with 1,3-diphenylisobenzofuran (DPBF) as a singlet
oxygen chemical quencher in DMF (Eq. 3):
HLPC profiles, 13C NMR and mass spectra of 2-Pt, 3-Pt, 4-Pt and
5-Pt are available. Supplementary data associated with this article
can be found in the online version, at doi:10.1016/j.tet.2010.02.079.
References and notes
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Std
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(3)
abs
FD
¼
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(4)
I
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¼ 1 þ KSV ½BQꢂ
(5)
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the fluorescence lifetime sF, i.e.,
KSV ¼ kq$sF
(6)
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Acknowledgements
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The authors wish to thank Ilker Un and Go¨khan Bilsel for NMR
and high resolution mass spectroscopy measurements.

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