Iodinated zinc phthalocyanine – The novel visible-light activated photosensitizer for efficient generation of singlet oxygen

Article abstract of DOI:10.1016/j.jphotochem.2018.03.030

Phthalocyanines are promising photosensitizers for photodynamic therapy (PDT). A novel zinc 1,8(11),15(18),22(25)-tetraiodo-3,10(9),17(16),24(23)-tetra-tert-butylphthalocyanine has been synthesized and characterized. The synthesis of iodinated phthalocyan

Full text of DOI:10.1016/j.jphotochem.2018.03.030

Accepted Manuscript
Title: Iodinated zinc phthalocyanine – The novel visible-light
activated photosensitizer for efficient generation of singlet
oxygen
Authors: Małgorzata Cyza, Arkadiusz Gut, Łukasz Łapok,
˛
˛
´
Jedrzej Solarski, Valeri Knyukshto, Mariusz Kepczynski,
Maria Nowakowska
PII:
S1010-6030(17)31673-8
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2018.03.030
JPC 11199
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
3-1-2018
17-3-2018
18-3-2018
˛
Please cite this article as: Małgorzata Cyza, Arkadiusz Gut, Łukasz Łapok, Jedrzej
˛
´
Solarski, Valeri Knyukshto, Mariusz Kepczynski, Maria Nowakowska, Iodinated
zinc phthalocyanine – The novel visible-light activated photosensitizer for efficient
generation of singlet oxygen, Journal of Photochemistry and Photobiology A:
Chemistry https://doi.org/10.1016/j.jphotochem.2018.03.030
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Dr. Łukasz Łapok
Kraków, 17/11/2017
Jagiellonian University,
Faculty of Chemistry,
Gronostajowa 2,
Kraków 30-387, Poland,
lapok@chemia.uj.edu.pl
phone: 0048-12-686-27-50
For the attention of:
Mir Wais Hosseini,
Editor of New Journal of Chemistry,
Please find enclosed the manuscript entitled “Iodinated zinc phthalocyanine – the novel
visible-light activated photosensitizer for efficient generation of singlet oxygen” by Łapok et
al., which is submitted to be considered for publication in your journal.
The manuscript describes the method of preparation of a novel photosensitizer from the
class of phthalocyanines. The synthesis is accompanied by an in-depth photophysical
characterization with special attention paid to the ability of this new phthalocyanine to generate
singlet oxygen. Also, the possibility of excited triplet state formation of this new photosensitizer
is investigated by both transient absorption spectroscopy and phosphorescence measurement.
We recently developed an interest in the synthesis of phthalocyanines as useful
photosensitizers in photodynamic cancer therapy (PDT). There is a quest for new
photosensitizers for PDT with improved properties compared to the photosensitizers that have
currently been used in clinical practice.
We believe that this paper and the results presented within will be of interest to
chemists who are involved in the synthesis of new photosensitizers and are interested in
developing new therapies for cancer treatment.
Respectfully yours,
Dr. Łukasz Łapok

Table-of-contents
Abstract
1. Introduction
2. Results and discussion
2.1.
2.2.
2.3.
2.4.
2.5.
Synthesis and characterization
Optical properties
Excited triplet state decay dynamics and phosphorescence
Singlet oxygen generation
Photostability
3. Experimental
3.1.
3.2.
3.3.
Materials and methods
Synthesis
Measurements
4. Conclusions
5. Acknowledgements
6. References

Iodinated zinc phthalocyanine – the novel visible-light activated photosensitizer
for efficient generation of singlet oxygen
Małgorzata Cyza1, Arkadiusz Gut1, Łukasz Łapok1*, Jędrzej Solarski2, Valeri Knyukshto3, Mariusz
Kępczyński1, Maria Nowakowska1
1Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Kraków, Poland.
2Institute of Physical Chemistry, Polish Academy of Science, 44/52 Kasprzaka, 01-224 Warsaw, Poland.
3B. I. Stepanov Institute of Physics, National Academy of Science, Independence Avenue 68, 220072,
Minsk, Belarus.
*Corresponding author: lapok@chemia.uj.edu.pl, Tel.: +48 12 686 25 31, Fax: +48 12 686 27 50.
Graphical abstract
Highly desirable photophysical and photochemical properties were induced in novel non-
aggregating zinc phthalocyanine by iodination of the aromatic rings.

Highlights






Iodinated zinc phthalocyanine is described with the purpose of applying it as a
photosensitizer to eradicate cancerous cells.
The photosensitizer exhibits absorption band within the so-called “therapeutic window”
with a very high molar absorption coefficient of 295 000 [M-1cm-1].
Iodinated zinc phthalocyanine produces singlet oxygen with an excellent quantum yield
of 0.76 via Type II mechanism.
Iodinate zinc phthalocyanine exhibits moderate stability in the presence of oxygen with
quantum yield of photodegradation (d) equal 4.01 x 10-4
Moderate photostability of the dye is advantageous for the PDT to promote rapid
breakdown and clearance after treatment.
The dye is phosphorescent with λP 1103 nm and it exhibits excited triplet state lifetime
of τT 3.3 μs.
Abstract
Phthalocyanines are promising photosensitizers for photodynamic therapy (PDT). A novel zinc
1,8(11),15(18),22(25)-tetraiodo-3,10(9),17(16),24(23)-tetra-tert-butylphthalocyanine has been
synthesized and characterized. The synthesis of iodinated phthalocyanine was achieved through
templated cyclotetramerization of 6-tert-butyl-4-iodo-1,3-diiminoisoindoline around zinc ions
(Zn2+). A six step route leading to 6-tert-butyl-4-iodo-1,3-diiminoisoindoline was proposed. The
synthesis started with a regiospecific iodination of 4-tert-butyl-o-xylene to 5-tert-butyl-2,3-
dimethyl-iodobenzene, followed by oxidation of the methyl groups to phthalic acid, conversion
of phthalic acid to phthalimide, hydrolysis of phthalimide to phthalamide, conversion of
phthalamide to phthalonitrile, followed by conversion of phthalonitrile to a more reactive 6-
tert-butyl-4-iodo-1,3-diiminoisoindoline. Highly desirable photophysical and photochemical
properties were induced in zinc phthalocyanine by iodination of the aromatic rings, while the
presence of tert-butyl groups rendered the compound non-aggregating in solution. The
photosensitizer studied exhibits absorption band within the so-called “therapeutic window”,
with λmax at 689 nm and very high molar absorption coefficient of 295 000 [M-1cm-1] in DMF.
Transient absorption experiment revealed T1 → Tn absorption spanning from ca. 400 nm to ca.

600 nm and allowed determination of the excited triplet state lifetime (τT). The estimated τT in
THF equaled 3.3 μs. The phosphorescence maximum was found to be located at λP 1103 nm and
the excited triplet state energy (ET) equaled 1.12 eV in 2-methyltetrahydrofuran at 77 K. The
photosensitizer studied is characterized by a high value of singlet oxygen quantum yield (ΦΔ) of
ca. 0.76 and the photooxygenation process proceeds via Type II mechanism under the
experimental conditions applied. The photodegradation studies revealed that the iodinated
phthalocyanine was moderately stable in the presence of oxygen with quantum yield of
photodegradation (d) equaled 4.01 x 10-4. Moderate photostability of photosensitizer is
advantageous from a PDT point of view where rapid breakdown and fast clearance from the
patient’s system is necessary to avoid prolonged skin photosensitivity following treatment.
Keywords: phthalocyanine, photosensitizer, singlet oxygen, heavy atom effect, PDT, triplet
sensitizer,
1. Introduction
Photodynamic therapy (PDT) is a well-established treatment for certain forms of cancer and skin
diseases [1]. It is a noninvasive alternative to conventional treatments like surgical tumor
resection, chemo- and radiotherapy [2]. Cancerous and other pathologically changed cells are
eradicated by the combined action of light, oxygen present in the tissues and intravenously or
topically administered dye molecule termed as photosensitizer [3]. Photosensitizers, due to
their ability to transfer the energy acquired by absorption of light to molecules of oxygen
resulting in the formation of highly cytotoxic reactive oxygen species (ROS), have proven to be
useful tools in photodynamic cancer therapy (PDT) [4,5].
For a given molecule in order to be called an efficient photosensitizer the following criteria must
be met: (1) possibly high absorption coefficient in the spectral region suitable for PDT; (2)
sufficient energy of the triplet state (ET ≥ 0.985 eV) to facilitate energy transfer to the triplet
state oxygen (ground state oxygen); (3) high quantum yield of triplet state formation (ΦT > 0.4)
accompanied by a long triplet state lifetime (τT > 1 μs); (4) reasonable photostability [4,5].

When all these criteria are met high values of the quantum yield of singlet oxygen formation
(ΦΔ) and other ROS are observed. Another key factor, often discussed in the literature, is the
location of maximum absorption of a photosensitizer [6]. Studies have shown that tissue-
penetration depth of light is strongly dependent on the light wavelength and it increases with
the increase of the light wavelength. Light characterized by the wavelength shorter than 550 nm
is unsuitable for PDT. However, ability of light to penetrate tissue doubles from 550 nm to 650
nm and it doubles further as the wavelength moves to 700 nm. Between 700 nm and 800 nm
light penetration further increases of 10%. This observations have some practical implications,
viz. in order to treat more deeply seated tumors one needs photosensitizers possessing long-
wavelength absorption. The “therapeutic window” for PDT was defined as 600-1100 nm [6]. A
great deal of attention has been paid by researchers to develop next generation
photosensitizers that would possess ability to absorb light from the visible and near-IR region of
the spectrum while maintaining other parameters important for PDT, such as photostability,
lack of dark cytotoxicity or high singlet oxygen and other ROS quantum yield.
The first photosensitizer to win approval by regulatory agencies in many countries was
Photofrin, a complex mixture of hematoporphyrin oligomers. Although Photofrin is successfully
used to treat certain forms of cancer it possesses severe limitations and these include poor
absorption in the “therapeutic window”, compositional complexity, tendency to be retained by
the patient’s body and thus to produce a prolonged skin photosensitivity [7]. These limitations
have inspired a world-wide research aimed at developing a second-generation photosensitizers.
These new photosensitizers usually share a tetrapyrrole, or less often tripyrrole (e.g. Lutrin),
framework and belong to the class of porphyrins, chlorins (e.g. Foscan, Purlytin, NPe6),
bacteriochlorins (Tookad), porphycenes or phthalocyanines [5,6,7]. Among all aforementioned
classes of photosensitizers a considerable attention has been paid over past decades to
phthalocyanines; a blue/green dyes closely related to the porphyrins and tetraazaporphyrins
[8,9].
Given the indisputable advantages of the phthalocyanine dyes over conventional first
generation photosensitizers in terms of range of light absorption, very high absorption
coefficient in the “therapeutic window” and photophysical properties, a tremendous effort has

been made by researchers to develop novel PDT drugs based on this class of dyes [10].
Currently, one of the silicon containing phthalocyanine, known as Pc 4, has been approved in
several countries (the USA, Japan, the Netherlands and France) as a photosensitizer for
photodynamic treatment of certain forms of cancer [5,11,12,13]. Pc 4 was first invented at Case
Western Reserve University followed by its clinical trials at University Hospitals Case Medical
Center.
At the core of development of the next generation photosensitizers is a thorough understanding
of the photophysical and photochemical characteristics of the dye molecule that govern its PDT
efficacy [4,7]. We report herein the synthesis, photophysical and photochemical
characterization of novel zinc phthalocyanine complex 9 (Scheme 2). Substitution of the
peripheral positions of the phthalocyanine, Pc, with four bulky tert-butyl groups, through the
steric hindrance, protects the Pc organic ligand from aggregation while greatly enhancing its
solubility in organic solvents, facilitating purification and spectroscopic characterization. At the
same time, replacement of four aromatic H atoms at the positions closer to the organic ligand
core with four iodine atoms, leads, through the heavy atom effect, to the formation of a dye
with increased singlet oxygen formation capacity [14,15]. The presence of heavy iodine atoms in
the Pc molecule reflects on both intersystem crossing quantum yield (ΦISC) and triplet lifetime
(τT) in both cases by increasing them [16].
2. Results and discussion
2.1. Synthesis and characterization
The starting point for the synthesis of 9 was the phthalonitrile 7 (Scheme 1). The stepwise
synthesis of 7 started with regiospecific electrophilic introduction of iodine substituent into 4-
tert-butyl-o-xylene (1) to give 5-tert-butyl-2,3-dimethyl-iodobenzene (2) in 81 % yield. A
protocol described elsewhere was adopted [17]. In the next step the methyl groups in 2 were
oxidized to give the corresponding phthalic acid 3. This was achieved by using potassium
permanganate in water/pyridine mixture at 90 °C. After 16 hours reaction time the phthalic acid
3 was obtained in 80 % yield. The IR spectrum of 3 was dominated by a sharp absorption band
at 1713 cm-1 (C=O) and a broad absorption band spanning 3300 cm-1 to 2250 cm-1 (OH), while

1
the H-NMR spectrum revealed a broad signal centered at 13.26 ppm accounting for the two
protons of the carboxylic groups.
Initially, it was planned to follow the classical protocol known for the preparation of
phthalonitriles, which requires the phthalic acid to be converted into phthalic anhydride. Thus,
phthalic acid 3 was reacted with either acetyl chloride or acetic acid anhydride as dehydrating
agents to give the corresponding phthalic anhydride 4 in 74 % yield upon crystallization from
hexane. Strong bands at 1858 cm-1, 1837 cm-1, 1815 cm-1 and a very strong band at 1776 cm-1
observed in the IR spectrum, along with the disappearance of the previously observed broad
absorption band characteristic of the OH group of the carboxylic acid, corroborated
unambiguously the formation of cyclic anhydride 4. However, in the course of our experiments
we found that the reaction pathway leading to the phthalonitrile 7 could be shorten by omitting
the step in which phthalic anhydride intermediate was formed. It was found that simple heating
of the phthalic acid in formamide at 180–190°C for 45 minutes allowed obtaining of the
phthalimide 5 in 95 % yield. The reaction was straightforward and afforded a high purity
product, meaning, no chromatographic separation was needed. A strong absorption in the IR
spectrum at 3207 cm-1 indicated the presence of an N-H stretching vibration, while two strong
absorption bands at 1769 cm-1 and 1713 cm-1 were assigned to C=O stretching vibrations. In
addition, in the 1H NMR spectrum a broad peak centered at 7.92 ppm was observed for the N-H
hydrogen of phthalimide 5. The classical protocol used to obtained phthalonitriles requires the
phthalimide to be transformed to phthalamide which is then converted to phthalonitrile. It was
found that a strict control of the temperature and more importantly reaction time of the
ammonolysis was crucial for the reaction yield of the phthalonitrile 7. A prolonged reaction time
favours gradual hydrolysis of the phthalamide 6 into ammonium carboxylate (diammonium
phthalate or/and ammonium 1-(aminocarbonyl)benzoate or/and 2-(aminocarbonyl)benzoate).
This in turn significantly lowers overall reaction yield of the phthalonitrile 7. To provide the key
precursor 7 in the highest possible yield we studied the scope and limitations of the
phthalamide 6 formation. Thus, when the ammonolysis of phthalimide was carried out at 80 °C
for 1 h and 45 min it was found that the phthalonitrile 7 was obtained in a 24 % yield. Lowering
of the reaction time to 1 h and 15 min allowed the phthalonitrile 7 be formed in 28 % yield.

Further lowering of the reaction time of the ammonolysis caused drop of the reaction yield of
the phthalonitrile 7. Treatment of phthalamide 6 with a phosphorus oxychloride POCl3 in
anhydrous pyridine afforded 5-tert-butyl-3-iodophthalonitrile (7) in 28 % yield (reaction yield
1
calculated based on the phthalimide 5 used). The H NMR spectrum confirmed unambiguously
1
the assigned structure of 7. In the H NMR spectrum, two doublets were observed in the
aromatic region of the spectrum with equal intensities and coupling constants of ca. 1.80 Hz.
The measured coupling constant clearly indicated that both aromatic protons were located in
meta positions, thereby the 1,2,3,5-substitution pattern was unambiguously corroborated. The
integration of the proton signals observed for the tert-butyl group and aromatic ring gave, as
expected, a 9:2 ratio. The IR spectrum of the product showed a sharp peak at 2231 cm-1 clearly
indicating the presence of the CN groups (aromatic nitriles). Finally, we tested the possibility of
transforming phthalonitrile 7 into its more reactive form, viz. 1,3-diiminoisoindoline 8. This
intermediate is widely used whenever phthalocyanines with less reactive metals or non-metals
are to be obtained. Treating of a methanolic solution of 7 with an ammonia gas in the presence
of sodium methoxide at room temperature afforded 1,3-diiminoisoindoline 8 with a
quantitative yield. The ESI-MS clearly confirmed that 8 was formed as two peaks were
discernible in the mass spectrum, viz. m/z 328 [M+H]+ and m/z 350 [M+Na]+.
An attempt to use phthalic acid 3, phthalic anhydride 4, phthalimide 5 or phthalamide 6 with
urea as a nitrogen source and ammonium molybdate as a catalyst did not allow obtaining of
phthalocyanine 9. Thus, only phthalonitrile 7 and 1,3-diiminoisoindoline 8 proved to be reactive
enough to form zinc phthalocyanine complex 9. The rearrangement of 1,3-diiminoisoindoline 8
into phthalocyanine 9 was performed in boiling 2-dimethylaminoethanol in the presence of zinc
acetate (Scheme 2). Phthalocyanine 9, after chromatographic purification, was obtained in 42 %
yield. Due to the symmetry of the phthalonitrile 7, phthalocyanine 9 is formed as a mixture of
four regioisomers with C4h, C2v, Cs and D2h symmetry, respectively. Phthalocyanine 9 is a deep
blue/green compound that thanks to the tert-butyl groups, possesses an excellent solubility in
nearly every organic solvent. The formation of phthalocyanine 9 was unambiguously
corroborated by mass spectrometry, IR and 1H NMR spectroscopy (see Supporting Information).

MALDI-TOF analysis revealed a molecular ion peak at m/z 1304.936 [M+H]+ with isotope pattern
1
matching the calculated isotope pattern. In the H NMR spectrum, a set of peaks was observed
in the aromatic region of the spectrum, viz. between 8 ppm and 9.5 ppm, due to non-equivalent
aromatic protons and another set of peaks between 1.6 ppm and 1.9 ppm due to non-
equivalent aliphatic protons of the different regioisomers of 9. The integration of the signals
observed for the tert-butyl group and aromatic ring gave, as expected, a 9:2 ratio.
2.2. Optical properties
The normalized UV-Vis absorption and emission spectra of 9 in DMF are displayed in Figure 1
and the data are collected in Table 1. The UV-Vis absorption spectrum of 9 shows typical for
non-aggregating phthalocyanines features, viz. very intense, sharp Q(0,0) band with λmax 689
nm, accompanied by a markedly weaker B band with λmax 348 nm. The compound presented in
this paper possess a structural feature, viz. presence of bulky tert-butyl substituents, that
renders this phthalocyanine non-aggregating in solution. Consequently, phthalocyanine 9 obeys
the Lambert-Beer law in a wide range of concentrations. Compound 9 is a very good absorber of
light from the so-called “therapeutic window” with high molar absorption coefficient (ε) of 295
000 [M-1cm-1] in DMF. This value is slightly higher compared to that of the unsubstituted ZnPc,
viz. 235 000 [M-1cm-1] at 670 nm in DMF [18]. Phthalocyanine 9 is emissive at room temperature
and the fluorescence maximum was found at λf 696 nm. However, the heavy atom effect
exerted by the iodine atoms present in the structure of 9 lowers the fluorescence quantum yield
(Φf) to merely the value of 0.015 [19]. A mono-exponential fluorescence decay was found for 9
with time constant of 0.42 ns in DMF. Based on the fluorescence spectrum the excited singlet
state (ES) energy for 9 was calculated: ES = 1.78 eV.
2.3. Excited triplet state decay dynamics and phosphorescence
We have investigated the excited triplet state decay dynamic of 9 by nanosecond time-resolved
transient absorption spectra. The transient absorption spectra, transient decays and triplet state
lifetimes are presented in Figure 2 and Table 1. The transient absorption spectrum of 9 recorded
in deaerated THF, feature broad positive triplet-triplet absorption spanning from ca. 400 nm to

ca. 600 nm. The decay of the transient T1 → Tn absorption and the recovery of the absorption S0
→ S1 in the singlet state (bleaching) were used to calculate triplet state lifetimes (τT) of 9. Both
methods gave similar values of triplet lifetimes. A mono-exponential decays of the transient
absorption and recovery of the ground state absorption were observed for 9. Phthalocyanine 9
was found to have the triplet lifetime (τT) of 3.3 ± 0.005 µs.
Phosphorescence of phthalocyanines, due to the hurdles in detecting such signals and the spin-
forbidden nature of such transition, is rarely reported [20,21]. Phthalocyanines for which
phosphorescence spectra and triplet state energies (ET) are reported are either palladium or
platinum complexes [22-25]. The normalized phosphorescence spectrum of 9 is displayed in
Figure 2. Phthalocyanine 9 exhibit strong NIR emission in oxygen-free, glass forming 2-
methyltetrahydrofuran at 77 K. The phosphorescence maximum was found to be located at λP
1103 nm. The value of the lowest triplet excited state (ET) was calculated directly from the
phosphorescence spectrum. Phthalocyanine 9 was found to have its lowest triplet state energy
(ET) at 1.12 eV.
2.4. Singlet oxygen generation
One of the most important features of a given photosensitizer, whether it be for photocatalytic
or biomedical applications, is its ability to form singlet oxygen (1O2) and other reactive forms of
•
•-
oxygen (viz. OH•, HO2 , O2 ). Singlet oxygen is formed via energy transfer between the
photosensitizer in the excited triplet state (3Sens*) and the molecular oxygen (3O2) itself in the
•-
triplet ground state (Type II mechanism). Formation of superoxide radical anions (O2 ), hydroxyl
•
radicals (OH•), hydroperoxyl radicals (HO2 ) and alkoxy radicals (RO•) is the result of an electron
transfer (Type III mechanism). The measure of the efficacy of a given photosensitizer is known
as singlet oxygen quantum yield (φΔ). The singlet oxygen quantum yield (φΔ) of phthalocyanine 9
was measured in DMF and we employed two methods to find the value of φΔ. First method rely
on the chemical reaction between singlet oxygen and/or other reactive forms of oxygen with
the acceptor molecule, viz. 1,3-diphenylisobenzofuan (DPBF). The decay of the absorption band
at λmax 417 nm of the acceptor molecule is monitored upon irradiation with light absorbed by

the investigated compound and by the standard molecule with known value of singlet oxygen
quantum yield. Second method is based on the observation of the phosphorescence of singlet
oxygen (1O2) at λP 1270 nm. The intensity of the singlet oxygen emission at 1270 nm as a
function of energy of the exciting laser pulses is observed and compared for the investigated
compound and for the standard molecule with known value of φΔ. This method allows exclusive
detection of singlet oxygen, while the chemical quencher, that is DPBF, reacts indistinguishable
with all reactive oxygen species. High values of singlet oxygen quantum yield were found for
compound 9. The method that utilizes the chemical quencher gave the value of φΔ 0.76 ± 0.02,
while the method that is based on the singlet oxygen luminescence gave the value of φΔ 0.74 ±
0.03. Both methods gave similar values of singlet oxygen quantum yield (φΔ), meaning that
phthalocyanine 9 sensitizes formation of singlet oxygen via energy transfer and that no radicals
were formed (Type II process) [4].
We indicated in the Introduction of this paper that a good photosensitizer should possess
sufficient energy of the excited triplet state, viz. ET ≥ 0.985 eV, triplet state lifetimes (τT) longer
than 1 μs and high absorption coefficient in the spectral region of the excitation [4]. The
phosphorescence measurements revealed that phthalocyanine 9 fulfilled the first requirement
set for an efficient photosensitizer; the ET equals 112 kJ/mol (1.12 eV). The energy separation
between phthalocyanine 9 excited triplet state and singlet oxygen (1Δg) is roughly 0.14 eV (13.06
kJ/mol). Also, compound 9 fulfills second requirement, viz. its triplet lifetime (τT) equals 3.3 μs,
facilitating the energy transfer between the photosensitizer (3Sens*) and ground state oxygen
(3O2). Moreover, phthalocyanine 9 is a very good visible light absorber with high molar
absorption coefficient of 295 000 [M-1cm-1]. Combination of these photophysical parameters
resulted in high singlet oxygen quantum yield (φΔ) measured for compound 9.
2.5. Photostability
We studied the light driven degradation processes of photosensitizer 9 in an effort to
understand its potential in biomedical applications. In general, this process can proceed either
via phototransformation or clean photodegradation. In the case of a phototransformation a

decrease of the intensity of the absorption spectrum of the photosensitizer is observed with
concomitant evolution of new absorption bands, while in the case of photodegradation a
gradual depletion of the spectrum is discernible over time without formation of new absorption
bands in the UV-Vis spectrum [26]. Figure 3, left panel, presents the change of the intensity of
the absorption spectrum of phthalocyanine 9 over time in an oxygen saturated DMF solution.
Prolonged exposure of the DMF solution of 9 to the light resulted in gradual decrease of the
absorption spectrum without appearance of new absorption bands, meaning that the
photobleaching of 9 proceeds via clean photodegradation and not via phototransformation
(both in an oxygen and oxygen-free atmosphere). This observation is in agreement with the
results presented by other researchers indicating that photobleaching of phthalocyanines tends
to proceed via clean photodegradation without phototransformation [27-29].
Monitoring the decay of the Q band at λ 689 nm in DMF over time allowed to study the kinetics
of the photodegradation of 9 and establish the exact values of both the photodegradation
quantum yield (d) and photobleaching constant (Kd) in the presence of oxygen and in an
oxygen-free atmosphere (Figure 3, right panel). The photodegradation of 9 proved to be a two
stage process with the initial step proceeding faster followed by a second slower stage. First-
order kinetics were fitted to both stages and the values of Kd and d were summarized in Table
2. Moreover, we noticed that the presence of an oxygen accelerates the photodegradation
process of 9, meaning that singlet oxygen (1O2) and other ROS participate in the decomposition
of this photosensitizer [30].
In general compounds having quantum yields of photodegradation (d) higher than 10-3 are
considered to be unstable. Phthalocyanine 9 is characterized by a d equal 4.01 x 10-4, thus it
could be regarded as a moderately stable photosensitizer. Moderate photostability of 9 can be
advantageous from a PDT point of view where rapid breakdown and fast clearance from the
patient’s system is necessary to avoid prolonged skin photosensitivity following treatment.
Photosensitizer used for PDT with high photostability (e.g. Photofrin) are known to be
problematic in clinical application as they may cause skin photosensitivity that last up to several
weeks [31-33].

3. Experimental
4.1. Materials and methods
1H-NMR and 13C-NMR spectra were recorded on Bruker Avance II 300 and 75 MHz, respectively,
in deuterated solvents. Electrospray mass spectrometer microTOF II (Bruker Daltonics,
Bremen/Germany) with a resolution of > 16 500 FWHM and a mass accuracy of < 2 ppm was
used to obtain the high resolution mass spectra. FT-IR spectra were recorded using a Bruker
Equinox 55. UV-Vis absorption spectra at 298 K were measured using a Hewlett-Packard 8452A
diode-array spectrophotometer equipped with a HP 89090A Peltier temperature control
accessory. Steady-state fluorescence spectra of the samples were recorded on an SLM-AMINCO
8100 Instruments spectrofluorimeter at room temperature. Emission spectra were corrected for
the wavelength dependency of the detector response by using an internal correction function
provided by the manufacturer. For the elemental micro analysis the CHNS Vario Micro Cube
analyzer combined with the electronic microbalance was used. Reaction progress was
controlled using TLC on silica gel (Supelco). All reagents were obtained from commercial sources
(Sigma Aldrich and TCI) and solvents were used without prior purification, unless stated
otherwise. Pyridine was dried with NaOH and distilled under inert gas atmosphere.
4.2. Synthesis
4-tert-butyl-o-xylene (1): see references [34,35]. Yield: 85%.
5-tert-butyl-2,3-dimethyl-iodobenzene (2): see reference [36]. Compound 1 (25 g; 154.2 mmol),
periodic acid (7.13 g; 31.28 mmol) and iodine (15.69 g; 61.82 mmol) were added to a mixture of
glacial acetic acid (79 mL), concentrated sulphuric acid (2.8 mL) and water (14 mL). The reaction
mixture was stirred at 60-65 °C. The iodine was consumed after 6 hours and a heavy oil
collected at the bottom of the flask. Water (200 mL) and sodium thiosulfate (1 g; 6.32 mmol)
were added to the reaction mixture. The product was extracted with diethyl ether (3 x 80 mL),
and the ethereal solution was washed with water (3 x 100 mL). Ether was removed on a rotary
evaporator. The crude product was finally purified by distillation under reduced pressure (92 °C,
10-2 mbar). Everything that distilled off before the main fraction was discarded. The main

fraction was collected and identified as product. Yield: 36 g (81%), colourless oily liquid. 1H NMR
(300 MHz, CDCl3) δ [ppm]: 1.28 (s, 9H, t-Bu), 2.35 (s, 3H, CH3), 2.38 (s, 3H, CH3) 7.13 (d, 1H, 4Jmeta
4
1.9 Hz), 7.68 (d, 1H, Jmeta 2.0 Hz). 13C NMR (75 MHz, CDCl3) δ [ppm]: 22.39 (CH3), 24.85 (CH3),
31.40 (t-Bu), 34.24 (t-Bu), 102.97, 127.38, 134.27, 136.57, 136.99, 150.61. GC-MS (electron
impact): m/z 288 [M]+, 273 [M-CH3]+. FT-IR (film): ν = 3020, 2964, 2907, 2866, 1598, 1543, 1478,
1465, 1442, 1395, 1362, 1276, 1250, 1222, 1202, 1161, 999, 922, 869, 817, 785, 705, 686, 614
cm-1.
5-tert-butyl-3-iodophthalic acid (3): KMnO4 (51.93 g; 328.61 mmol) was added in 4 portions to
a solution of 2 (20 g; 69.41 mmol) in pyridine (200 mL) and water (25 mL). Reaction mixture was
kept at 90 °C for 16 h. A water (100 mL) solution of NaOH (8 g) was added to the reaction
mixture and stirred. Reaction mixture was diluted with ethanol (80 mL) and the formed MnO2
was filtered off and washed with ethanol. On a rotary evaporator pyridine and ethanol were
removed. The remaining water solution was concentrated to around 50 mL and acidified with
concentrated HCl (until pH 1-2) and left for 0.5 h under vigorous stirring to well crush the
solidifying phthalic acid 3. The formed precipitate was filtered off (G-4) and washed thoroughly
with water until neutral pH. The product was washed out with acetone leaving behind the rest
of MnO2. Acetone was removed and the product was dried. Yield: 19.23 g (80%) of white
1
4
powder. H NMR (300 MHz, d6-DMSO) δ [ppm]: 1.29 (s, 9H, t-Bu), 7.89 (d, 1H, Jmeta 1.89 Hz),
8.03 (d, 1H, 4Jmeta 1.89 Hz), 13.26 (bs, 2H, COOH). 13C NMR (75 MHz, d6-DMSO) δ [ppm]: 30.56 (t-
Bu), 34.36 (t-Bu), 94.13, 126.04, 129.48, 139.47, 139.57, 153.00, 165.88 (COOH), 169.35 (COOH).
ESI-MS (positive mode): m/z = 333 [M-CH3]+, 349 [M+H]+, 371 [M+Na]+. ESI-MS (negative mode):
m/z = 348 [M]-. FT-IR (KBr): ν = 3300-2250 broad vs (OH), 2966 vs, 2905 vs, 2870 vs, 2660 s, 2564
s, 1779 m, 1713 (C=O) vs, 1696 (C=O) vs, 1594 s, 1543 m, 1475 s, 1462 s, 1424 s, 1388 s, 1365 s,
1296 vs, 1276 vs, 1257 vs, 1165 m, 1138 m, 924 s, 888 s, 806 m, 795 s, 698 vs cm-1. Anal. Calcd
for C12H13O4I: C, 41.40; H , 3.76. Found: C, 42.63; H, 3.96.
5-tert-butyl-3-iodophthalic anhydride (4): Phthalic acid 3 (2 g; 5.75 mmol) was refluxed with
acetyl chloride (4 mL; 4.42 g; 56.25 mmol) for 1 h. The unreacted acetyl chloride was removed
on a rotary evaporator under vacuum. Crude product was dissolved in hot hexane and
crystallized to give pure product. Yield: 1.39 g (74%), white solid. Alternatively, acetic acid

anhydride can be used to obtain 4. It requires heating of the starting material 3 (2 g, 5.75 mmol)
with acetic acid anhydride (6 mL, 63.53 mmol) at 100 °C for 18 h. The crude product, after
removal of the unreacted acetic acid anhydride, was purified by crystallization from hexane.
Yield: 1.39 g (74 %), white solid. ESI-MS (positive mode): m/z = 331 [M+H]+. FT-IR (KBr): ν = 3421
w, 3078 w, 2965 s, 2928 s, 2872 s, 1858 (C=O) s, 1837 (C=O) s, 1815 (C=O) m, 1776 (C=O) vs,
1605 s, 1573 m, 1478 m, 1456 m, 1399 m, 1367 m, 1328 m, 1276 m, 1243 s, 1232 s, 1148 m,
1130 m, 924 s, 898 s, 824 s, 740 s, 717 s, 683 s, 655 m cm-1.
5-tert-butyl-3-iodophthalimide (5): Method I: Phthalic acid 3 (15.73 g, 45.18 mmol) was heated
in formamide (7.5 mL, 188.8 mmol) at 180 - 190 °C for 45 min. Upon cooling to room
temperature the product solidified. Reaction mixture was vigorously stirred while water was
added dropwise until reaction mixture was diluted to the volume of 100 mL. White precipitate
was filtered off, washed thoroughly with water and dried in vacuum to give the product. Yield:
14.13 g (95 %). No further purification was necessary as the compound was obtained with a very
good purity as judged by TLC (acetone/hexane, 2:3). However, in case the purification is needed,
a column chromatography can be used to purify the material (silica gel; acetone/hexane, 1:4
v/v). Yield: 76 %. Method II: Phthalic anhydride 4 (0.5 g; 1.51 mmol) was mixed with urea (2 g;
33.32 mmol) and placed in a 50 mL round-bottom flask. Reaction mixture was immersed in a
preheated oil bath. The melt was kept at 160 °C for 15 min. After that time starting material was
no longer present in the reaction mixture as judged by TLC (toluene/hexane, 1:2 v/v). The
reaction mixture was cooled followed by a dropwise addition of water (50 mL) and vigorous
stirring for 30 minutes to crush the solidified product. The precipitate formed was filtered and
washed with copious volume of water to remove unreacted urea. The product was dried under
1
vacuum and crystallized from CHCl3. Yield: 0.44 g (89 %), white solid. H NMR (300 MHz, CDCl3)
δ [ppm]: 1.37 (s, 9H, t-Bu), 7.88 (d, 1H, 4Jmeta 1.6 Hz), 7.92 (s, 1H, NH), 8.11 (d, 1H, 4Jmeta 1.6 Hz).
13CNMR (75 MHz, CDCl3) δ [ppm]: 31.15 (t-Bu), 35.82 (t-Bu), 89.08, 121.10, 134.55, 142.84,
160.27, 161.88, 166.32 (CO), 166.67 (CO). ESI-MS (negative mode): m/z = 329 [M]-. FT-IR (KBr): ν
= 3207 (N-H) s, 3061 m, 2962 vs, 2872 s, 1769 (C=O) s, 1731 (C=O) s, 1713 (C=O) s, 1607 m, 1566
m, 1479 m, 1457 m, 1404 m, 1368 m, 1339 s, 1301 s, 1266 m, 1235 m, 1169 m, 1093 s, 1043 s,

725 s, 699 s, 684 s, 660 s, 638 m cm-1. Anal. Calcd for C12H12O2NI: C, 43.79; H, 3.67; N, 4.26.
Found: C, 44.39; H, 3.78; N, 4.09.
5-tert-butyl-3-iodophthalamide (6): Method I: Phthalimide 5 (0.8 g; 2.43 mmol) was dissolved in
methanol (150 mL). Ammonia gas was passed through the solution at 0 °C for 2 hours. No amide
6 was formed under this reaction conditions. Method II: Phthalimide 5 (250 mg; 0.76 mmol) was
dissolved in methanol (50 mL) followed by addition of 25% ammonia solution (30 mL). The
formed mixture was stirred for 1 hour and 15 minutes at 80 °C. The course of the reaction was
followed by TLC (acetone/hexane, 1:4). The reaction was stopped immediately after the
unreacted phthalimide 5 was consumed. Caution: prolonged reaction time should be avoided as
it promotes hydrolysis of the amide to the ammonium carboxylic salt (COONH4). Water and
methanol were removed on a rotary evaporator under vacuum at 60 °C. The white solid
obtained, despite being contaminated with a major admixture of ammonium carboxylic salt,
1
was used for the next reaction without additional purification. H NMR (300 MHz, d6-DMSO) δ
4
4
[ppm]: 1.25, 1.26 (s, t-Bu, salt and amide), 7.26 (br s, NH2), 7.63 (d, Jmeta 2.0 Hz), 7.76 (d, Jmeta
4
4
2.0 Hz), 7.79 (d, Jmeta 2.0 Hz), 7.80 (d, Jmeta 2.0 Hz), 8.35 (br s, NH2). ESI-MS (negative mode):
m/z = 330 [M-NH2]-. FT-IR (KBr): v = 3394 cm-1 (N-H) vs, 3203 (N-H) vs, 2960 (t-Bu) vs, 2867 s,
1665 (C=O) vs, 1611 vs, 1569 (N-H) vs, 1458 vs, 1367 vs, 1351 vs, 1264 s, 1068 w, 880 m, 817 m,
911 m cm-1.
5-tert-butyl-3-iodophthalonitrile (7): Anhydrous pyridine (10 mL) was added to the
phthalamide 6 (obtained from 250 mg of phthalimide 5) followed by injecting of POCl3 (0.25 mL;
2.68 mmol). The reaction mixture was stirred under an inert gas atmosphere for 1 h at 0 °C, and
then for 1 h at room temperature. The reaction mixture was quenched by pouring it onto
crushed ice and stirred for 10 min, followed by extraction with CH2Cl2 (3 x 30 mL). The organic
phase was washed with 5 % HCl solution (2 x 50 mL) to remove the pyridine and then with water
until neutral pH was obtained. After solvent evaporation the residue was purified by column
chromatography (silica gel; toluene). The first colorless fraction was collected and identified as
1
the product. Yield: 66 mg, (28%). H NMR (300 MHz, CDCl3) δ [ppm]: 1.34 (s, 9H, t-Bu), 7.77 (d,
4
4
1H, Jmeta 1.80 Hz), 8.13 (d, 1H, Jmeta 1.80 Hz). 13C NMR (75 MHz, CDCl3) δ [ppm]: 30.74 (t-Bu),
35.75 (t-Bu), 100.00, 115.22 (CN), 116.70 (CN), 117.64, 120.59, 130.47, 141.01, 158.52. ESI-MS:

m/z = 311 [M+H]+, 333 [M+Na]+. MALDI-TOF-MS: m/z = 333.075 [M+Na]+. FT-IR (KBr): v = 3422,
3073 (Ar-H), 2971 (t-Bu), 2873, 2231 (CN), 1581, 1532, 1478, 1467, 1395, 1367, 1275, 1242,
1210, 1158, 1086, 887, 818, 778, 724 cm-1. Anal. Calcd for C12H11N2I: C, 46.40; H, 3.50; N, 9.00.
Found: C, 48.58; H, 3.87; N, 8.52.
6-tert-butyl-4-iodo-1,3-diiminoisoindoline (8): Phthalonitrile 7 (200 mg; 0.645 mmol) was
dissolved in methanol (9 mL). A catalytic amount of sodium methoxide MeONa was added to
the reaction mixture. A stream of ammonia gas was passed through the solution until
saturation. The ammonia saturated solution was then left to stand for two days. After this time
the solution was saturated again with a stream of ammonia gas and left to stand for additional
two days. Reaction progress was followed by TLC (silica gel;, toluene). After the substrate was
completely consumed solvent was removed under reduced pressure at 55 °C. The substrate was
quantitatively converted into 1,3-diiminoisoindoline and could be used as a precursor for
phthalocyanine without prior purification. ESI-MS: m/z = 328 [M+H]+, 350 [M+Na]+, 655
[2M+H]+, 677 [2M+Na]+.
Zinc 1,8(11),15(18),22(25)-tetraiodo-3,10(9),17(16),24(23)-tetra-tert-butylphthalocyanine (9)
(a mixture of four regioisomers): Diiminoisoindoline 8 (obtained from 200 mg of phthalonitrile
.
7; 0.645 mmol) and Zn(CH3COO)2 2H2O (42 mg; 0,192 mmol) were reacted in DMAE (6 mL).
Reaction mixture was brought slowly to boiling and then it was kept refluxing for 24 hrs. Upon
heating formation of green product was observed. Reaction mixture was cooled down, water
was added dropwise under stirring and the precipitate formed was filtered off. The solid residue
was washed with a mixture of acetonitrile and water (7:3) in order to remove yellow impurities.
Finally, product was purified by column chromatography on silica gel. Toluene with 1 % of
acetone (v/v) was used as an eluent. The green fraction was collected. Upon solvent removal 88
mg (42 %) of dark-green product was obtained. FT-IR (KBr): ν = 2954 (t-Bu), 2901, 2864, 1602
(CAr-H), 1543, 1474 (CAr-H), 1360, 1349, 1330, 1302, 1259, 1201, 1099, 1048, 1022, 929, 874,
831, 768, 744, 667 cm-1. UV/Vis (DMF): λ = 689 (ε = 295 200 [M-1 cm-1]), 346 nm. MALDI-TOF
1
MS: m/z 1304.924 (calculated); 1304.936 [M+H]+ (found). H NMR (300 MHz, CDCl3) δ [ppm]:
1.65 – 1.84 (m, 9H, t-Bu), 8.22 – 9.35 (m, 2H). Anal. Calcd for C48H44N8I4Zn 1305.92 g/mol: C,
44.15; H, 3.4; N, 8.58. Found: C, 45.15; H, 3.77; N, 8.72.

4.3. Measurements
Determination of fluorescence quantum yield (ΦF): The fluorescence quantum yield (ΦF) of
phthalocyanine ZnPc(t-Bu)4I4 9 in DMF was determined using the comparative method
according to the equation
F A n2
F  SFtd
,
Std
FStd A nS2td
where F and FStd are the areas under the fluorescence emission curves of the sample and the
standard, A and AStd are the absorbance of the sample and the standard and n and nStd are the
refractive indices of the solvent used for the sample and the standard, respectively [37].
Std
Unsubstituted zinc phthalocyanine ZnPc (ΦF = 0.2 in DMSO) was employed as the standard
[38]. The absorbance of the solutions at the excitation wavelength was in the range of 0.02 -
0.04.
Fluorescence lifetimes: Time-resolved fluorescence decays in the nanosecond range were
obtained using a modular time-correlated single-photon-counting (TCSPC) system (HORIBA
Jobin Yvon Inc.), consisting of a Nano LED pulsed diode (340 nm, <1.2 ns pulse), a picosecond
photon counting detector (HORIBA Jobin Yvon Inc.), and Fluoro Hub TCSPC module.
Fluorescence decays were analyzed using Data Station v2.3 software.
Transient triplet-triplet absorption spectra and triplet state lifetimes: A home-built laser flash
photolysis setup has been used. The pump pulses of 25 ns duration and energy of 5 mJ are
provided by a home-build dye laser, pumped by excimer laser (Labda Physik LPX100) working at
1-10 Hz. The continuous light of laser-driven Xe lamp (Energetiq EQ-99-Plus-EU) and pump
pulses of Xe flash lamp (PerkinElmer 1100 Series) have been involved for kinetic and spectral
data acquisitions, respectively. The setup is equipped with the two-channel CCD spectrometer
(Avantes AvaSpec-ULS2048-2-USB2) for spectral measurements and allows registration of time-
resolved spectra from 1 ms to seconds. The acquisition of kinetic data has been performed by
home-built monochromator equipped with fast photomultiplier tube (Hamamatsu H6780) and

oscilloscope (Tektronix TDS 3032). Prior to the measurement all sample solutions were
deoxygenated using the freeze-pump-thaw technique (five cycles). Samples were irradiated in 1
cm path-length quartz cell. To ensure accuracy of the obtained data upon collecting the first set
of transient triplet-triplet absorption spectra and triplet state lifetimes the solutions were
diluted down to half of the initial concentration and all measurements were repeated.
Phosphorescence spectra: Corrected phosphorescence spectra, as well as phosphorescence
excitation spectra were recorded by a two-disk phosphoroscope driven by two synchronous
motors. The operating spectral region was from 200 nm to 1100 nm, and the exciting light
sources were xenon and tungsten lamps or several different photodiodes (Thorlabs, Inc.). The
experimental set-up was equipped with a thermocooling system for the photomultiplier (FEU-83
or FEU-100). The reproducibility of the system is 5%. The apparatus has been described in
details elsewhere [39].
Determination of singlet oxygen quantum yield (ΦΔ): Chemical method. The quantum yield of
singlet oxygen (ΦΔ) of phthalocyanine 9 in DMF solution was measured indirectly according to
the procedure described elsewhere [40-42]. DPBF was applied as acceptors of singlet oxygen
and zinc phthalocyanine (ΦΔ = 0.56 in DMF) was used as a reference [43,44]. Typically, 2 mL of
the DMF solution containing the sensitizer (2.8 × 10-7 M), and the acceptor (5 × 10-5 M) was
introduced into the quartz cuvette and irradiated using an Osram® xenon lamp (XBO 75W/2). A
630 nm glass cut-off filter was used to filter the ultraviolet radiation. The value of ΦΔ was
measured in the air and the following equation was employed for the calculations:
Std
abs
R  I
  Std

,
RStd  Iabs
where ΦΔStd is the quantum yield of singlet oxygen formation for the standard, R and RStd are the
DPBF photobleaching rates in the presence of the phthalocyanine 9 and in the presence of the
Std
reference molecule, viz. zinc phthalocyanine, respectively. Iabs and Iabs are the rates of light

absorption by the measured sample and the respective standard. The mean intensity of the light
absorbed by the samples (Iabs, Iabsstd) was calculated according to the following equation:

Iabs



I



110A

d
  T

where T(λ) is the transmittance of the filter, A(λ) is the absorbance of the sensitizer and I(λ) is
the light intensity emitted by the lamp. The photosensitizer absorbance was maintained at
about 0.1. Each experiment was repeated at least four times.
Direct method. The photogeneration of singlet oxygen was measured directly by detecting its
phosphorescence at 1270 nm, as described elsewhere [45]. Photosensitizer absorbance was
maintained at about 0.3.
Photostability: The kinetics and quantum yield (ΦD) of photodegradation of the phthalocyanine
9 were measured both in an atmosphere of oxygen and argon. Before the irradiations, the
solution containing phthalocyanine 9 (4.1 × 10-6 M) was purged with the appropriate gas and
tightly sealed. The sample was irradiated with a xenon lamp. A 630 nm glass cut-off filter was
used for this experiment. The kinetics of the dye degradation was followed with absorption
spectroscopy. Photobleaching constants were calculated in accordance with first order reaction
kinetics. The intensities of the light absorbed by the samples were determined using a
potassium reineckate actinometer K[Cr(NH3)2(SCN)4], using a procedure described in detail
elsewhere [46,47].
4. Conclusions
In conclusion, we have outlined a synthesis pathway leading to a novel photosensitizer from the
class of phthalocyanines. The synthesis of iodinated phthalocyanine was achieved through
templated cyclotetramerization of 6-tert-butyl-4-iodo-1,3-diiminoisoindoline around zinc ions
(Zn2+). A six step route leading to 6-tert-butyl-4-iodo-1,3-diiminoisoindoline was proposed. The

synthesis started with a regiospecific iodination of 4-tert-butyl-o-xylene to 5-tert-butyl-2,3-
dimethyl-iodobenzene. The methyl groups of the latter were oxidized by potassium
permanganate to phthalic acid. The phthalic acid was directly and conveniently transformed to
phthalimide, followed by its hydrolysis to phthalamide. Conversion of phthalamide to
phthalonitrile was achieved by treating the phthalimide with POCl3 in dry pyridine. Finally, the
phthalonitrile was transformed to a more reactive 6-tert-butyl-4-iodo-1,3-diiminoisoindoline.
Due to the combination of iodine substituents, tert-butyl groups and diamagnetic zinc ion
incorporated into the phthalocyanine ligand, the novel photosensitizer possesses outstanding
photophysical and photochemical properties important from the PDT point of view. The
phthalocyanine studied is an extremely efficient absorber of visible light, as evidenced by the
very high molar absorption coefficient (ε) equaled 295 000 [M-1cm-1] at 689 nm. The compound
studied exhibits absorption band within the so-called “therapeutic window”, with λmax located at
689 nm. By recording the time-resolved transient absorption spectra we determined the excited
triplet state lifetime (τT) of the iodinated zinc phthalocyanine. The excited triplet state lifetime
equaled 3.3 μs. The phosphorescence maximum was found to be located at λP 1103 nm and it
was used to calculate the lowest triplet excited state energy (ET). The iodinated phthalocyanine
was found to have its lowest triplet state energy (ET) at 1.12 eV. Upon excitation the molecules
of the photosensitizer studied efficiently transfer their excited triplet state energy (ET) to the
ground state oxygen (3O2), resulting in high values of quantum yields of singlet oxygen
formation (ΦΔ) equaled to 0.76. To determine the value of ΦΔ we resorted to two methods, viz.
direct and indirect method. A comparison of the results obtained by both methods indicate that
the photooxygenation process induced by iodinated zinc phthalocyanine proceeds via Type II
mechanism, with singlet oxygen being the only reactive oxygen species formed under the
experimental conditions applied. The compound studied is moderately photostable in the
presence of oxygen (d = 4.01 x 10-4). A moderate stability can be advantageous in PDT of
cancer as rapid breakdown and clearance from the patient’s system after treatment is desirable.
In summary, the triplet state properties of iodinated zinc phthalocyanine, lack of aggregation in
solution and its ability to generate singlet oxygen with high ΦΔ combined with moderate

photostability, render this novel photosensitizer a promising candidate for a photosensitizer for
biomedical applications.
Acknowledgements
This work was supported by the NCN grant, DEC-2011/03/D/ST5/05910. The research was
carried out with the equipment purchased thanks to the financial support of the European
Regional Development Fund in the framework of the Polish Innovation Economy Operational
Program (contract no. POIG.02.01.00-12-023/08).

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Figure 1. Left: absorption (blue) and fluorescence (read) normalized spectra of 9 measured in DMF. Right:
concentration dependent spectra of 9 in DMF (concentration range: 5.9 x 10 -6 – 6.3 x 10-7 M).
Figure 2. Left: transient absorption spectrum of phthalocyanine 9 measured in THF at room temperature, excitation
wavelength 643 nm. Transient decay is shown in the inset. Right: phosphorescence spectrum recorded in 2-
methyltetrahydrofuran (2-mTHF) at 77 K.

Figure 3. Left panel: UV-Vis absorption spectra of phthalocyanine 9 recorded after different irradiation times in
oxygen saturated DMF. Right panel: the photodegradation kinetics of phthalocyanine 9 in DMF measured as a
decrease of the Q-band intensity at a 5 min intervals (A0 stands for the initial absorption intensity and A stands for
the intensity of the Q-band after a given irradiation time).