Facile Synthesis, Triplet-State Properties, and Electrochemistry of Hexaiodo-Subphthalocyanine

Article abstract of DOI:10.1002/chem.201803316

In view of the ever-growing demand for efficient triplet photosensitizers and photoactive components of various optoelectronic devices, we herein report the synthesis and properties of hexaiodo-subphthalocyanines (I₆SubPcs). The improved five-s

Full text of DOI:10.1002/chem.201803316

A Journal of
Accepted Article
Title: The facile synthesis, triplet state properties and electrochemistry
of hexaiodo-subphthalocyanine
Authors: Łukasz Łapok, Magdalena Obłoza, Jędrzej Solarski, Tomasz
Pędziński, and Maria Nowakowska
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Graphical abstract
The facile synthesis, triplet state properties and electrochemistry of
hexaiodo-subphthalocyanine
Magdalena Obłoza1, Łukasz Łapok1*, Jędrzej Solarski2, Tomasz Pędziński3, 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.
3Faculty of Chemistry, Adam Mickiewicz University in Poznań, 89b Umultowska, 61-614 Poznań, Poland.
*Corresponding author: LukaszLapok@chemia.uj.edu.pl, Tel.: + 48 12 686 25 31, Fax: + 48 12 686 27 50.
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Highlights


I6SubPcs efficiently absorb visible light (λmax ca. 580 nm).
Efficient transfer of energy upon excitation with light to excited triplet state (T1) occurs
for I6SubPcs.


I6SubPcs exhibit long-living excited triplet states allowing sensitization of ground state
oxygen to a singlet oxygen with very high singlet oxygen quantum yields (φΔ), ca. 0.9.
I6SubPcs are phosphorescent with λP equal ca. 970 nm and triplet state energy (ET) equal
ca. 1.3 eV.
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Table-of-contents
Abstract
1. Introduction
2. Results and discussion
2.1. Synthesis and characterization
2.2. Optical properties
2.3. Phosphorescence and triplet state energies
2.4. Time-resolved spectroscopy
2.5. Singlet oxygen formation
2.6. Photostability
2.7. Redox potentials and HOMO/LUMO energy levels
3. Discussion
4. Experimental
4.1. Materials and methods
4.2. Synthesis
4.3. Measurements
5. Conclusions
6. Acknowledgements
7. References
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The facile synthesis, triplet state properties and electrochemistry of
hexaiodo-subphthalocyanine
Magdalena Obłoza1, Łukasz Łapok1*, Jędrzej Solarski2, Tomasz Pędzińsk3, 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.
3Faculty of Chemistry, Adam Mickiewicz University in Poznań, 89b Umultowska, 61-614 Poznań, Poland.
*Corresponding author: LukaszLapok@chemia.uj.edu.pl, Tel.: + 48 12 686 25 31, Fax: + 48 12 686 27 50.
Keywords: subphthalocyanine; phosphorescence; singlet oxygen; photosensitizer; excited-
triplet state.
Abstract: In view of the ever growing demand for efficient triplet photosensitizers and
photoactive components of various optoelectronic devices, we herein report the synthesis and
properties of hexaiodo-subphthalocyanines (I6SubPcs) . The improved five-step route to 4,5-
diiodophthalonitrile, which serves as a precursor for the synthesis of I6SubPcs, is reported. The
improved synthesis required merely one chromatographic separation to afford a high purity
target product. Highly desirable photophysical and photochemical properties were induced in
I6SubPcs due to the presence of six heavy iodine atoms. In particular, high values of singlet
oxygen quantum yields (φΔ) ranging from 0.83 to 0.9 were measured. The I6SubPcs investigated
proved to be phosphorescent at 77 K in 2-MeTHF with emission band maximum (λP) located at
957 nm for 7 and 970 nm for 8. The excited triplet state energies (ET) were estimated to be ca.
1.30 eV , while the triplet lifetimes (τT) were found to be 27.7 μs for 7 and 30.1 μs for 8,
respectively. The CV/DPV measurements indicated that both I6SubPcs exhibited one irreversible
oxidation and one quasi-reversible reduction. The spectroelectrochemical measurements
pointed to relative stability and reversibility of the electrochemically formed anion radical, viz.
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I6SubPc•-, and instability of the species formed upon one-electron oxidation, viz. I6SubPc•+.
Estimation of the EHOMO gave the value of ca. – 5.8 eV while the ELUMO was found to be located at
ca. – 3.8 eV.
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1. Introduction
Compounds that can be efficiently excited (with high quantum yield, φT) to the triplet excited
state (T1) are termed triplet photosensitizers [1]. Consequently triplet photosensitizers are
utilized as triplet energy donors to initiate either photochemical reactions or induce
photophysical processes in another molecular entity [2]. Triplet photosensitizers have been
considered for a variety of applications such as phosphorescent materials [1,3-4],
phosphorescent bioimaging [5-6], chemosensors [7-8], photoinitiators of polymerization [9],
photocatalysts (e.g. for photoinduced hydrogen production from water) [10-12], triplet-triplet
annihilation based upconversion (TTAU) [13-14], oxygen sensors [15] and initiators of
photoredox organic reactions [16]. Furthermore, triplet photosensitizers have found use in
oncological and antibacterial photodynamic therapy (PDT) to eradicate cancerous cells or
pathogens, respectively [17-19]. Strong absorption of light (large molar extinction coefficient, ε),
high yield of intersystem crossing (φISC) allowing to efficiently populate the T1 excited state and
long triplet lifetime (τT) are the prerequisites for a good photosensitizer [1]. Synthesis of
inexpensive yet efficient and stable triplet photosensitizers with tailored photophysical and
electrochemical properties remains a challenging field of chemistry with the portfolio of
photosensitizers rapidly expanding every year. It is well-established that incorporation of certain
transition metals (e.g. d10 Cu(I), d6 Re(I), d4 Re(III), d6 Ru(II), d6 Os(II), d6 Ir(III), d8 Pt(II), d8 Pd(II),
d10 Au(I) or d8 Au(III)) into the organic ligands results in formation of highly populated triplet
states upon electronic excitation [1,3,20]. Similarly, functionalization of the organic molecule
with atoms with high atomic numbers, such as I, Br, Ir, Pt, Ru, Os, Pb, Tl, etc., can induce spin
orbit coupling due to the heavy atom effect, thus facilitating intersystem crossing (ISC) [21-23].
Finally, an effort has been dedicated to developing heavy atom-free organic triplet
photosensitizers with C60-choromophore hybrids being an example of the later [1].
Subphthalocyanines (SubPcs), a lower homologues of better-known phthalocyanines (Pcs), are
an interesting class of organic chromophores that can be utilized as triplet photosensitizers [24].
This rare example of a nonplanar, bowl-shaped , 14 π-electron aromatic system was first
reported in 1972 by Meller and Ossko as a product formed during an unsuccessful attempt to
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prepare a boron phthalocyanines [25]. Three iminoisoindole subunits N-fused around boron
atom and a halogen atom at the axial position form a subphthalocyanine macrocycle [26]. The
recent surge of interest in this class of compounds stems from their unique and tunable optical
and electrochemical properties, such as strong absorption of light in the visible part of the
spectrum, tunable HOMO/LUMO energy levels and triplet state characteristics. Currently,
subphthalocyanines are tested mainly as functional materials in applications such as
photoactive components of solar cells, organic light emitting diodes (OLED), organic field-effect
transistors (OFETs) and NLO materials [24,27-31].
In this report, we wish to introduce the facile synthesis, photochemical and electrochemical
characterization of two examples of halogenated subphthalocyanines (I6SubPcs). The peripheral
positions of the aromatic rings were substituted with six iodine atoms in order to induce highly
desirable photophysical properties via inducing an efficient spin orbit coupling and facilitate
intersystem crossing (ISC). In one of the SubPcs studied the axial chlorine substituent was
replaced with 3-hydroxypyridine to improve solubility of the I6SubPcs and facilitate its
purification and spectroscopic characterization. Furthermore, 3-pyridyloxy group offers the
possibility of converting this novel photosensitizer into a water soluble derivative through
quaternization reaction. Research dedicated to hydrophilic photosensitizers for PDT purposes
and photocatalysts operating in aqueous media are an ongoing issues in our laboratory. So far
the following halogen-substituted subphthalocyanines have been reported in the literature:
dodecafluoro-subphthalocyanine (F12SubPcCl), dodecachloro-subphthalocyanine (F12SubPcCl),
hexachloro-subphthalocyanine (Cl6SubPcCl), triiodo-subphthalocyanie (I3SubPcCl), hexaiodo-
subphthalocyanine (I6SubPcBr) as well as various derivatives of these [32-36]. Understanding
the relationship between the photosensitizer’s structure and the triplet state energies (ET) and
lifetimes (τT) is essential for developing next generation photosensitizers for biomedical
application as well as for designing the artificial light-harvesting systems. This paper aims at
exploring the possibility of triplet state formation in hexaiodo-subphthalocyanines (I6SubPcs),
and in particular, at establishing the triplet-state energies (ET) via measuring the
phosphorescence (λP) and triplet-state lifetimes (τT) via recording the decay of the transient T1
→ Tn absorption in nanosecond time-resolved transient absorption spectra. Furthermore, via
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the CV and DPV measurements, we determine the redox properties and we establish the
alignment of the HOMO/LUMO energy levels of these novel halogenated subphthalocyanines. In
order to get an insight into the stability of the anion radicals (I6SubPc•-) and cation radicals
(I6SubPc•+) formed, reversibility of these processes and to spectroscopically characterize the
electrochemically
generated
intermediates,
we
performed
the
thin-layer
spectroelectrochemistry. Knowledge of the stability of the electrochemically generated radicals
and reversibility of the redox processes is of prime importance for evaluating the potential
application of these materials in organic electronics.
2. Results and discussion
2.1. Synthesis and characterization
Scheme 1. Synthesis of 4,5-diiodophthalonitrile 6 proposed by us and the original synthesis proposed by
Leznoff [37].
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The synthesis of the hexaiodo-subphthalocyanines (I6SubPcs) under question started with the
preparation of the precursor compound, viz. 4,5-diiodophthalonitrile (6) (Scheme 1). 4,5-
Diiodophthalonitrile (6) was first reported in 1996 by Leznoff et al., who proposed a synthetic
pathway starting from electrophilic aromatic iodination of phthalimide, followed by
ammonolysis of phthalimide (4) to phthalamide (5) [37]. In the last step the 4,5-
diiodophthalamide (5), upon treatment with trifluoroacetic anhydride in pyridine gave 4,5-
diiodophthalonitrile (6) (Scheme 1). Unfortunately, this method required a harsh iodination in
refluxing 30 % fuming sulfuric acid at 80 °C – 90 °C and yielded a complex mixture of 4,5-
diiodophthalimide, 3,4-diiodophthalimide and, at slightly lower reaction temperature, 4,5-
diiodophthalic acid. The procedures reported involved a tedious and multistep, poorly
reproducible separations (column chromatographies and crystallizations) at either the
phthalimide step or phthalonitrile step. Inspired by the inconvenience and poor reproducibility
of this method we propose a novel, elegant, and significantly improved protocol for the
synthesis of 4,5-diiodophthalonitrile (6) [38]. Importantly, our procedure requires only one
straightforward chromatography at the last step out of five steps (Scheme 1).
Oxidation of the methyl groups in 4,5-diiodo-o-xylene (2) with KMnO4 in water/pyridine solution
at 40 °C gave phthalic acid 3 in excellent yield of 82 %. The isolation procedure involved simple
precipitation of the reaction product and required no chromatography or crystallization to
afford high purity product 3. The classical protocol used to obtained phthalonitriles requires
conversion of phthalic acids to phthalic anhydrides. However, in the course of the experiments,
we found that the phthalic acid 3 could be directly converted into the phthalimide 4, an
observation which allowed us to skip one reaction step, thereby simplifying the whole
procedure and improving the total reaction yield. We applied the procedure first disclosed in a
Japanese patent and then successfully adopted by Hanack et al. [39-40]. Treatment of phthalic
acid 3 with formamide at elevated temperature (180 °C) afforded phthalimide 4 in a good 89 %
yield. Simple precipitation of the reaction product was sufficient to obtain a product with good
purity and no chromatographic separation was needed. Treatment of phthalimide 4 with water
solution of ammonia gave the corresponding 4,5-diiodophthalamide (5) in a satisfactory 78 %
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yield. Conversion of phthalamide 5 into the corresponding 4,5-diiodophthalonitrile (6) was
carried out with a phosphorus oxychloride as the dehydrating agent in dry pyridine/dioxane
mixture. Finally, after chromatographic separation, pure phthalonitrile 6 was obtained in 84 %
yield.
Scheme 2. Synthesis of hexaiodo-subphthalocyanines 7 and 8.
I6SubPc 7 (Scheme 2) containing chlorine as the axial substituent was obtained by reacting the
phthalonitrile 6 with BCl3 in o-dichlorobenzene under microwave heating with a very good 63%
yield. Refluxing of compound 7 in o-dichlorobenzene in the presence of 3-hydroxypyridine with
the utilization of microwaves led to compound I6SubPc 8 with a moderate 37% yield. The
exchange of chlorine into 3-hydroxypyridine resulted in a significant increase of solubility of 8,
1
which greatly facilitated both the purification and spectroscopic analysis of this compound. H
NMR, 13C NMR, FT-IR and HR-ESI-MS were used to corroborate the structure of the precursor
compounds 2-6 and the subphthalocyanines 7 and 8 (see Supporting Information, Figure S1-
S26). Exemplarily, the aromatic protons of the subphthalocyanine 8 give rise to a singlet at δ =
9.09 ppm, while the resonance signal of the 3-hydroxypyridyl group appeared as a series of
signals at δ = 7.91 ppm (dd), δ = 6.93 – 6.82 ppm (m) and 5.88 ppm (ddd), respectively. In the
high resolution ESI mass spectrum of 8 peak corresponding to the molecular ion [M+H+]+
appeared at m/z = 1245.5347.
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2.2. Optical properties
Figure 1. Normalized absorption (left panel) and fluorescence (right panel, λex = 521 nm) spectra of
hexaiodo-subphthalocyanines 7 and 8 recorded in THF (inset: THF solutions of I6SubPcs 7 and 8).
The electronic absorption spectra of hexaiodo-subphthalocyanies 7 and 8 are outlined in Figure
1 and summarized in Table 1. The solutions of I6SubPcs 7 and 8 exhibit a deep magenta color.
Both 7 and 8 absorb strongly in the visible region with λmax = 584 nm for 7 and λmax = 581 nm for
8 in THF. The following molar absorption coefficients (εmax) were determined: 92 000 [M-1cm-1]
for 7 and 89 000 [M-1cm-1] for 8 in THF. I6SubPcs 7 and 8 proved to have a negligible room-
temperature fluorescence with a fluorescence quantum yield below 1% and the fluorescence
maxima observed at λf = 596 nm (Stokes shift of 12 nm) for 7 and λf = 594 nm (Stokes shift of 13
nm) for 8, respectively. Based on the fluorescence spectra the following singlet excited state (S1)
energies were calculated: ES = 2.08 eV for 7 and ES = 2.09 eV for 8, respectively. The
fluorescence lifetimes (τS) for both 7 and 8 were extremely short, viz. τS for 7 equaled 0.18 ns
and for 8 it was found to be 0.17 ns. It is due to the efficient intersystem crossing (ISC) induced
by the presence of iodine substituents that efficiently channels the excitation energy into the
triplet excited state (T1) and makes the I6SubPcs nearly non-fluorescent at room temperature.
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Table 1. Summary of photophysical data for hexaiodo-subphthalocyanines 7 and 8 in THF and 2-MeTHF.
Compound
λabs
λf
Φf
ES
ε
τS
λP
ET
τT
ΦΔ
[nm]
[nm]
[eV] [M-1cm-1]
[ns]
[nm]
[eV]
[μs]
7
8
584
581
596
594
0.0052
0.0063
2.08
2.09
92 000
89 000
0.18
957
970
1.30
1.28
27.7 ± 0.1 0.90 ± 0.03
30.1 ± 0.1 0.83 ± 0.03
0.17
λabs: absorption maximum in THF, λf: fluorescence maximum in THF (λex = 521 nm), Φf: fluorescence quantum yield
(λex = 521 nm, rose Bengal was used as a reference compound with Φf = 0.05 in ethanol), Es: singlet excited state
energy in THF, ε: molar absorption coefficient at λabs in THF, τS: excited singlet state lifetime in THF measured based
on fluorescence decay, λP: phosphorescence maximum in 2-MeTHF (λex = 530 nm) at 77 K, ET: triplet excited state
energy in 2-MeTHF at 77 K, τT: excited triplet state lifetime in THF, ΦΔ: singlet oxygen quantum yield in THF
(luminescence method, λex = 345 nm, as a reference compound zinc phthalocyanine with ΦΔ = 0.53 in THF was
used).
2.3. Phosphorescence and triplet state energy
Figure 2. The normalized wavelength-corrected phosphorescence spectra of I6SubPcs 7 and 8 recorded in
2-MeTHF at 77 K upon excitation at λex = 530 nm.
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The most direct way to measure the energy of the triplet excited state (ET) is by recording the
phosphorescence spectrum. To the best of our knowledge there is only one paper reporting the
phosphorescence of subphthalocyanine [41]. In 2004 Echegoyen et al. reported triplet-state
properties of subphthalocyanine derivative equipped with three iodine substituents [41]. This
compound is claimed to have the phosphorescence maximum at λP 835 nm, and accordingly,
triplet-state energy (ET) of 1.45 eV, S1-T1 energy gap of 0.65 eV and triplet lifetime (τT) of 32 μs in
a mixture of methylcyclohexane, 2-MeTHF and ethyliodide at 77 K. The normalized wavelength-
corrected phosphorescence spectra of I6SubPcs 7 and 8 are displayed in Figure 2 and the triplet
state properties are summarized in Table 1. The I6SubPcs 7 and 8 represent a class of NIR
emitting dyes that exhibit detectable phosphorescence (viz. transition from T1 to S0) in a glass-
forming 2-MeTHF solvent at 77 K upon excitation with light of λex 530 nm. I6SubPc 7 was found
to have the phosphorescence maximum at ca. λP 957 nm, while I6SubPc 8 was found to
phosphoresce at ca. λP 970 nm. The value of the lowest triplet excited state (ET) for I6SubPcs 7
and 8 was calculated directly from the phosphorescence spectrum and it was found to be 1.30
eV for 7 and 1.28 eV for 8 at 77 K.
2.4. Time-resolved spectroscopy
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Figure 3. Nanosecond transient absorption spectra of hexaiodo-subphthalocyanines 7 (left) and 8 (right)
measured in THF at room temperature, excitation wavelength λex 355 nm. Transient decays monitored at
490 nm with fits shown in the insets.
Recording of the nanosecond time-resolved transient absorption spectra allowed studying of
the triplet excited state dynamics of hexaiodo-subphthalocyanines 7 and 8 (Figure 3). The
transient absorption spectra were collected upon 3rd harmonic of the Nd:YAG laser excitation
(λmax 355 nm) of THF solutions, argon saturated prior to the measurement. The transient
absorption spectra of I6SubPcs 7 and 8 recorded in THF, feature a very broad positive T1 → Tn
absorption band, superimposed with a negative signals due to the ground-state bleaching. For
I6SubPcs 7 and 8, the triplet-triplet absorption spanned from ca. 380 to ca. 750 nm. The decay of
the transient T1 → Tn absorption and the recovery of the absorption S0 → S1 in the singlet state
were used to calculate triplet lifetimes (τT) of the investigated hexaiodo-subphthalocyanies.
Both methods gave similar values of triplet lifetimes (τT). For I6SubPcs 7 and 8, mono-
exponential decays of the transient absorption and recovery of the ground-state absorption
were observed. The mono-exponential decay found for I6SubPc 7 was fitted with a time
constant of 27.7 ± 0.1 µs, while I6SubPc 8 was found to have a triplet state lifetime (τT) of 30.1 ±
0.1 µs.
Figure 4. Femtosecond flash photolysis of 7 and 8 in THF at room temperature, excitation wavelength λex
350 nm, laser energy 0.15 μJ/pulse. Transient decays monitored at 603 nm for 7 nm and 601 nm for 8
with fits shown in the insets.
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Changing the time domain of the measurements from nanosecond flash photolysis to
femtosecond flash photolysis allowed observation of excited singlet-state transient absorption
spectra and dynamics (S1 → Sn) (Figure 4). Hexaiodo-subphthalocyanies 7 and 8 proved to have
very short-living excited singlet-state (τS) in a picosecond range. Compound 7 was found to have
an excited singlet lifetime of 200 ± 13 ps, while compound 8 had the excited singlet lifetime of
226 ± 16 ps. These excited singlet lifetime results are in excellent agreement with the excited
singlet state lifetime (τS) calculated from the fluorescence decay (see Table 1).
2.5. Singlet oxygen formation
The ability of I6SubPcs 7 and 8 to sensitize the formation of singlet oxygen (1O2) was considered
and quantitatively assessed as a singlet oxygen quantum yield (φΔ). We resorted to the direct
method 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 was compared for the investigated
compounds 7 and 8 and for the standard molecule with known value of φΔ in THF (zinc
phthalocyanine, φΔ = 0.53 in THF) [42]. A particularly high quantum yield of singlet oxygen was
found for I6SubPc 7, viz. φΔ = 0.90 ± 0.03, while I6SubPc 8 produced singlet oxygen with φΔ of
0.83 ± 0.03. High singlet oxygen quantum yields measured for 7 and 8 are an immediate
consequence of the triplet state properties of these photosensitizers (viz. ET, τT and the energy
3
separation between I6SubPc3* and O2). The phosphorescence measurements showed that it is
energetically favorable for each of the hexaiodo-subphthalocyanines studied to sensitize the
formation of singlet oxygen (1O2). It is well-known that the singlet oxygen (1Δg) exhibits a
phosphorescence at λP 1270 nm which means that the energy separation between the ground
-
state oxygen (triplet state, 3Σg ) and its singlet state form (1Δg) equals 0.97 eV. It implies that for
any molecule to act as a photosensitizer (Sens) capable of exciting the ground state oxygen to its
higher energy form, the energy separation between the triplet excited state (3Sens*) and the
ground state sensitizer (1Sens) has to be higher than 0.97 eV. In other words the photosensitizer
in its triplet excited state (3Sens*) has to act as “an energy reservoir” for the whole process.
Triplet state energies of I6SubPcs 7 and 8 are well-above that of singlet oxygen (1Δg). The energy
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separation between triplet state I6SubPcs 7 and singlet oxygen (1Δg) is roughly 0.33 eV, while the
energy difference between triplet state I6SubPcs 8 and singlet oxygen (1Δg) is roughly 0.31 eV.
Furthermore, relatively long excited triplet state lifetimes (τT) of 7 and 8, ca. τT 30 μs, increase
the probability of the energy transfer between the excited state photosensitizer (I6SubPc3*) and
ground state molecular oxygen (3O2). Furthermore, very high values of singlet oxygen quantum
yields (φΔ) for 7 and 8 are an indirect evidence of the triplet quantum yield (φT) approaching
unity, pointing to a very good triplet sensitizing properties of these compounds.
2.6. Photostability
Figure 5. UV/Vis monitored photostability of 7 and 8 exposed to irradiation at λ > 473 nm and intensity
1000±50 W/m2 in THF in the presence of oxygen.
The THF solutions of 7 and 8 were exposed to the bright light of xenon lamp (1000±50 W/m2)
separated from the samples with a 473 nm cut-off filter under aerobic conditions (Figure 5).
UV/Vis spectra were taken over time in an effort to assess the relative photostability of the
hexaiodo-subphthalocyanines studied. After 10 h of irradiation 98% of the initial intensity of the
absorption band was preserved for compound 7 and 97% for compound 8, respectively. These
findings point to a very good photostability of 7 and 8 under the experimental conditions
applied. Prolonged exposure of the THF of 7 and 8 to the light resulted in a gradual decrease of
the absorption spectrum without appearance of new absorption bands, meaning that the
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photobleaching of hexaiodo-subphthalocyanines proceeds via clean photodegradation and not
via phototransformation.
2.7. Redox potentials and HOMO/LUMO energy levels
Figure 6. Differential pulse voltammetry (DPV) and cyclic voltammetry (CV) of 7 and 8. Data collected on
glassy carbon electrode in 1,2-dichlorobenzene with 0.1 M nBu4NBF4 as the supporting electrolyte, scan
rate 100 V/s.
The oxidation (Eox)and reduction (Ered) potentials of 7 and 8 were studied by cyclic voltammetry
(CV) and differential pulse voltammetry (DPV) on a glassy carbon working electrode in 1,2-
dichlorobenzene and were referenced to the ferrocene/ferrocenium redox couple and
summarized in Table 2. Both iodinated I6SubPcs exhibit one reduction within the potential range
1
1
applied (Figure 6), viz. Ered = -1.26 V for 7 and Ered = -1.30 V for 8. I6SubPc 7 exhibited an
1
oxidation peak at Eox = +0.78 V, while for 8 an oxidation potential of Eox1 = +0.76 V was found.
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The DPV measurement of 8 revealed an additional oxidation (shoulder at Eox = +0.64 V)
overlapped with the main peak and it was not observed for the chlorine substituted 7 that we
attribute to the oxidation of the 3-pirydyloxy axial substituent. The oxidations observed were
irreversible while the reductions were quasi-reversible in nature.
Table 2. The frontier energy levels [eV] and redox potentials [V] vs. Fc/Fc+ for 7 and 8 obtained from
differential pulse voltammetry (DPV). Glassy carbon electrode was used as a working electrode. Data
collected in 1,2-dichlorobenzene with 0.1 M tetrabutylammonium tetrafluoroborate as supporting
electrolyte.
1
1
a
b
Compound
Eox
Ered
ELUMO
[eV]
EHOMO
[eV]
Eg (electro) c
[eV]
[V]
[V]
7
+ 0.78
- 1.26
- 3.84
- 3.80
- 5.88
- 5.86
2.04
2.06
8
+ 0.76
- 1.30
+
+
a ELUMO= - (E[first reduction vs. Fc/Fc ] + 5.1) [eV], b EHOMO= - (E[first oxidation vs. Fc/Fc ] + 5.1) [eV], c E(electro) = electrochemical gap.
1
With the first reduction (Ered1) and oxidation (Eox
) potentials in hand, we calculated the
respective LUMO (ELUMO) and HOMO (EHOMO) energy levels, assuming that the value of the
formal potential of the Fc/Fc+ redox couple equals – 5.1 eV in the Fermi scale (Table 2) [43].
Estimation of the EHOMO gave the value of – 5.88 eV for 7 and - 5.86 eV for 8, respectively. The
ELUMO was found to be located at – 3.84 eV for 7 and – 3.80 eV for 8, respectively. These data
suggest that axial substituents have virtually no influence on the alignment of the HOMO and
LUMO energy levels in subphthalocyanines.
For assignment of the redox events observed, spectroelectrochemical measurements in 1,2-
dichlorobenzene of I6SubPcs was performed (Figure 7). Exemplarily, when a potential negative
enough for the first reduction to occur was applied (i.e. - 1.1 V vs Ag/Ag+, see Figure 7), new,
red shifted the broad absorption band at ca. λmax 700 nm appeared at the expense of the
absorption bands initially observed at λmax 581 nm and 335 nm for compound 8. These spectral
changes are associated with the formation of an anion radical, viz. [I6SubPc]•-. When the applied
potential was changed to more positive, the original spectrum was fully recovered in its initial
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shape and intensity, pointing to both reversibility of the first reduction and relative stability of
the anion radical formed, viz. [I6SubPc]•-, in the time frame of the experiment. Shifting the
controlled electrolysis potential from -1.1 V to + 0.9 V vs Ag/Ag+, where first oxidation occurs
(I6SubPc → [I6SubPc]•+), resulted in a swift depletion of the absorption bands characteristic of
the neutral form of 8 without evolution of new absorption bands. Furthermore, upon applying a
more negative potential no recovery of the initial absorption spectra of 8 was observed,
pointing to irreversible decomposition of the species formed as a result of the oxidation. We
failed to performed the spectroelectrochemistry for compound 7 due to its limited solubility in
1,2-dichlorobenzene. Recently, a comprehensive study on spectroelectrochemistry of 14
differently substituted SubPcs was published by Bender et al. [44]. Our observations are in
agreement with the findings reported by Bender et al. For all 14 tested compounds upon one-
electron reduction the main absorption band decreased in intensity and was replaced by a red-
shifted broad absorption band, while the one-electron oxidation resulted in flattening of the
whole absorption spectrum. Also, reversibility of the first reduction and irreversibility of the
oxidation was noted by these authors, again, an observation which substantiate our own
findings.
Figure 7. UV-Vis spectra of I6SubPc 8 collected during electrolysis at two different potentials, viz. - 1.1 V
and + 0.9 V vs Ag/Ag+. Data collected in 1,2-dichlorobenzene with 0.1 M tetrabutylammonium
tetrafluoroborate as supporting electrolyte with Pt working electrode.
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2. Discussion
We demonstrated that the hexaiodo-subphthalocyanines 7 and 8 produce highly populated
triplet states upon excitation with light from the visible spectral region. This was evidenced by
the observation of transient absorption spectra (T1 → Tn), negligible emission from the singlet
excited state (S1→S0) and most of all near unity singlet oxygen quantum yields (φΔ). In addition,
hexaiodo-subphthalocyanines 7 and 8 have the capability to efficiently harvest visible light (λmax
ca. 580 nm) , as evidenced by high molar absorption coefficients (ε), viz. from 89 000 M-1cm-1 to
92 000 M-1cm-1, and are capable to withstand exposure to intense light over an extended
period. The singlet oxygen quantum yields (φΔ) measured for hexaiodo-subphthalocyanines 7
and 8 are among the highest ever reported for this class of chromophores, viz. φΔ = 0.83 - 0.90.
A compound closely related to the hexaiodo-subphthalocyanines 7 and 8 is the well-known
triodo-subphthalocyanine (I3SubPc-Cl) with the following photophysical characteristics: φΔ =
0.74, φT = 0.77, τT = 56 μs and φF = 0.013 [45]. Given the triplet state properties of the hexaiodo-
subphthalocyanines described in this work we anticipate that these compounds may be useful
when highly efficient triplet sensitizers are required. One example of such an application could
be photodynamic therapy (PDT). The following criteria have been defined for a good
photosensitizer: a) absorption of light from the so called “therapeutic window” (from 600 nm to
1100 nm), b) a high absorption coefficient, c) a long-living triplet state (longer than 1 μs) and d)
the ability to generate reactive forms of oxygen (singlet oxygen and radicals). The hexaiodo-
subphthalocyanines 7 and 8 meet all but one of these criteria – they do not absorb light from
the “therapeutic window”. However, one has to bear in mind that anti-microbial photodynamic
therapy (aPDT) does not require the photosensitizer to be active in the “therapeutic window”
suggesting that hexaiodo-subphthalocyanines developed here could be considered as
photosensitizers for aPDT.
Other applications of triplet photosensitizers include TTA upconversion, hydrogen (H2)
production and photoredox catalytic organic reactions. In particular, photon upconversion
based on sensitized triplet-triplet annihilation (TTA) holds promise in applications like dye-
sensitized solar cells, photocatalysis and artificial photosynthetic systems. All these applications
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require dyes with highly populated, long-living triplet states. We therefore believe that
hexaiodo-subphthalocyanines presented in this work are natural candidates for these
applications.
3. Experimental
3.6. Materials and Methods
1H-NMR and 13C-NMR spectra were recorded on Bruker Avance II 300 MHz, Bruker Avance III
400 MHz and Bruker Avance III 600 MHz 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.
MALDI MS spectra were recorded in positive reflectron mode with the use of a MALDI-TOF/TOF
mass spectrometer ultrafleXtreme (Bruker Daltonik). Cesium triiodide clusters were utilized for
calibration of the m/z scale. FT-ATR-IR spectra were recorded using a Bruker Equinox 55 and
Thermo Scientific Nicolet iS10 spectrometers. 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. 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). 1,2-Dichlorobenzene was dried with CaH2 and
freshly distilled prior to use. Compound 1 was purchased from TCI Europe and used as supplied.
3.7. Synthesis
4,5-diiodo-o-xylene (2): see reference [46]. To a mixture of o-xylene (20 g, 189 mmol), iodine
(42 g, 165 mmol) and periodic acid H5IO6 (17.2 g, 75.45 mmol) a mixture of glacial acetic acid
(100 ml), sulfuric acid (3 ml) and water (20 ml) was added. The reaction mixture was kept at 60-
65 °C for 24 h. The reaction mixture was quenched with a saturated aqueous solution of sodium
carbonate until all acid was neutralized and pH ca. 7 was reached. Then, a saturated aqueous
solution of sodium thiosulfate was added to consume all unreacted iodine. The crude product
was extracted with dichloromethane (3 x 60 ml) and the combined organic extracts were
washed with water( 3 x 100 ml). The product was purified by a repetitive crystallization from
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1
hexane. Yield: 40.7 g (54 %), white crystals. H NMR (600 MHz, CDCl3) δ (ppm) = 7.61 (s, 2 H),
2.15 (s, 6 H). 13C NMR (151 MHz, CDCl3) δ (ppm) = 140.1, 138.7, 103.9, 19.1. HR-ESI-MS: m/z
381.2981 [M+Na+]+, 375.2488 [M+H2O]+. ATR-FT-IR: ν = 2975 (m), 2912 (m), 2849 (m), 2723 (w),
1719 (w), 1458 (vs), 1437 (vs), 1375 (s), 1328 (s), 1263 (m), 1152 (m), 1098 (w), 1016 (m), 994
(w), 860 (vs) cm-1. Anal. Calcd for C8H8I2: C 26.84%, H 2.25%. Found: C, 26.97%, H, 2.31%
4,5-diiodophthalic acid (3): 4,5-diiodo-o-xylene (2, 30.06 g, 75.51 mmol) and KMnO4 (51.5 g,
0.326 mol) were suspended in a mixture of pyridine (300 ml) and water (60 ml). The reaction
mixture was stirred at 40 °C for 8 days. After the reaction was completed, a solution of NaOH
(6.85 g, 0.171 mol) in water (60 ml) was added to the reaction mixture. Next, ethanol (100 ml)
was added to the reaction mixture to consume the unreacted KMnO4. The MnO2 formed was
filtered off and washed with water. Solvents were evaporated under vacuum and the solid
obtained was dissolved in 100 ml of water and placed in an ice bath. Hydrochloric acid (35 %)
was added dropwise with vigorous stirring until an acidic pH was attained. The precipitate
formed was filtered and followed by washing with water and acetone. Yield: 26.48 g (82%),
white powder. 1H NMR (300 MHz, DMSO-d6): δ (ppm) = 8.37 (s, 2H). 13C NMR (101 MHz, DMSO-
d6): δ (ppm) = 166.3, 140.9, 134.2, 112.1. HR-ESI-MS: m/z 440.8091 (calcd for [M+Na+]+:
440.8091). FT-ATR-IR: ν = 3550 (m), 3079 (m), 2115 (w), 1909 (w), 1711 (vs), 1607 (m), 1560 (m),
1535 (s), 1362 (m), 1228 (vs), 1150 (m), 1074 (s), 904 (w), 852 (m), 770 (m) cm-1. Anal. Calcd for
C8H4I2O4: C, 22.99%, H, 0.96%. Found: C, 22.32%, H, 1.06%.
4,5-diiodophthalimide (4): 4,5-diiodophthalic acid (3, 9.8 g, 23 mmol) was heated in 50 ml of
formamide at 180 °C for 35 min. The reaction progress was monitored by TLC (silica gel,
acetone/hexane, 4:6). The reaction mixture was cooled to RT followed by a dropwise addition of
water under vigorous stirring. The white precipitate formed was filtered off and washed
thoroughly with water and dried. Yield: 9.37 g (89%), white solid. 1H NMR (300 MHz, DMSO-d6):
δ (ppm) = 11.43 (bs, 1H), 8.24 (s, 2H). 13C NMR (75 MHz, DMSO-d6): δ (ppm) = 167.8, 133.1,
132.7, 116.5. HR-ESI-MS: m/z 421.8145 (calcd for [M+Na+]+: 421.8145). FT-ATR-IR: ν = 3466 (s),
3192 (m), 3075 (m), 3002 (m), 2723 (m), 1770 (vs), 1716 (vs), 1598 (w), 1388 (w), 1346 (m), 1230
(w), 1140 (w), 1057 (w), 849 (w), 760 (w), 737 (w) cm-1. Anal. Calcd for C8H3I2NO2: C, 24.09%, N,
3.51%, H, 0.76%. Found: C, 24.40%, N, 4.63%, H, 0.97%
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4,5-diiodophthalamide (5): 4,5-diiodophtalimide (4, 2.15 g, 5.4 mmol) was suspended in 200 ml
of ammonia water solution (25%). The reaction mixture was stirred for 1.5 h at 60 °C. The crude
product was filtered off and washed carefully with water and methanol. Yield: 1.74 g (78%),
1
white solid. H NMR (300 MHz, DMSO-d6): δ (ppm) = 7.93 (s, 2H), 7.81 (s, 2H), 7.40 (s, 2H). 13C
NMR (75 MHz, DMSO-d6): δ (ppm) = 167.8, 137.6, 137.0, 109.8. HR-ESI-MS: m/z 438.8411 (calcd
for [M+Na+]+: 438.08411). FT-ATR-IR: ν =3425 (s), 3292 (s), 3158 (s), 1691 (s), 1649 (vs), 1600
(vs), 1562 (m), 1532 (s), 1380 (s), 1303(w), 1290 (m), 1259 (m), 1096 (s), 895 (s), 842 (s), 763 (m),
659 (m) cm-1. Anal. Calcd for C8H6I2N2O2: C, 23.10%, N, 6.73%, H, 1.45%) Found: C, 23.49%, N,
6.20%, H, 1.30%.
4,5-diiodophthalonitrile (6): 4,5-diiodophthalamide (5, 0.450 g, 0.12 mmol) was dissolved in a
mixture of anhydrous pyridine and dioxane (28 ml, 5:2 v/v). The mixture was placed in an ice
bath and POCl3 (1.2 ml, 12.8 mmol) was added dropwise via a syringe. The ice bath was
removed and the mixture was stirred under an inert gas atmosphere overnight. The mixture was
quenched by pouring it onto crushed ice, followed by extraction with ethyl acetate (3 x 90 ml).
The organic phase was washed with water (3 x 50 ml). After solvent evaporation the residue
was purified by column chromatography (silica gel, toluene). Yield: 0.344 g (84%), yellow-white
1
solid. H NMR (300 MHz, CDCl3): δ (ppm) = 8.22 (s, 2H). 13C NMR (75 MHz, CDCl3) δ (ppm) =
142.9, 115.6, 115.2, 113.7. HR-ESI-MS: m/z 402.8201 (calcd for [M+Na+]+: 402.8200).
FT-ATR-IR: ν = 3069 (m), 2230 (m), 1555 (m), 1450 (m), 1319 (m), 1260 (m)1096 (w), 803 (m),
885 (m), 862 (m) cm-1. Anal. Calcd for C8H2I2N2: C, 25.29%, N, 7.37%, H, 0.53%. Found: C,
25.57%, N, 7.25%, H, 0.53%.
I6SubPcBCl (7): A mixture of heptane solution of BCl3 (1 M, 2 ml, 2.00 mmol), 4,5-
diiodophthalonitrile (6, 0.254 g, 0.67 mmol) and dry 1,2-dichlorobenzene (5 ml) was placed in a
glass tube. The glass tube was closed with a septa cap, inserted into the CEM microwave reactor
and heated to 150°C for 10 min. The reaction mixture was poured onto a vigorously stirred cold
mixture of EtOH and hexane (20 ml, v/v, 1:1). The shiny violet precipitate formed was filtered
and washed thoroughly with water and ethanol, followed by drying. Yield: 165 mg (63%), shiny
1
violet solid. H NMR (300 MHz, py-D5) δ = 9.72 (s, 6H). 13C NMR – not recorded due to low
solubility of 7 in deuterated solvents. MALDI-MS: m/z = 1185.504 [M+H+]+, 1150.536 [M-Cl-]+.
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UV-Vis (THF): nm (log ε) = 584 (4.96), 334.5 (4.53). FT-ATR-IR: ν (cm-1) = 3055 (w), 2162 (w),
1978 (w), 1725 (w), 1591 (m), 1524 (m), 1452 (m), 1419 (m), 1399 (s), 1361 (m), 1268 (m), 1215
(m), 1177 (s), 1101 (m), 1061 (s), 1022 (w), 968 (s), 925 (m), 880 (s), 792 (s), 756 (m), 702 (s).
Anal. Calcd for C24H6BClI6N6: C, 24.30%, N, 7.09%, H, 0.51%. Found: C, 24.83%, N, 6.79%, H,
0.58%.
I6SubPcBOpy (8): Subphthalocyanine 7 (251 mg, 0.212 mmol), 3-hydroxypyridine (100 mg, 1.05
mmol) and dry 1,2-dichlorobenzene (0.8 ml) were placed in a glass tube filled with an inert gas.
The glass tube was closed with a septa cap, inserted into the CEM microwave reactor and
heated to 160°C for 1 h. The crude product was precipitated with hexane, filtered through a
cotton plug and washed with a mixture of MeOH and water (v/v 4:1, 50 ml). The product was
then dissolved in THF and preadsorbed on silica gel and purified by column chromatography
1
(SiO2; toluene/acetone 20:1) to give 96 mg (37%) of pure product as a red solid. H NMR (300
MHz, DMSO – d6) δ (ppm) = 9.09 (s, 6H), 7.91 (dd, J = 4.6, 1.3 Hz, 1H), 6.93 – 6.82 (m, 2H), 5.88
(ddd, J = 8.1, 2.8, 1.4 Hz, 1H). 13C NMR – not recorded due to low solubility of 8 in deuterated
solvents. HR-ESI-MS: m/z 1245.5347 (calcd for [M+H+]+: 1245.5381), 1267.5140 (calcd for
[M+Na+]+: 1267.5200). UV-Vis (THF): nm (log ε) = 581 (4.95), 334 (4.57). FT-ATR-IR: ν (cm-1) =
3053 (w), 2920 (w), 2850 (w), 1723 (w), 1590 (m), 1523 (w), 1453 (m), 1397 (s), 1260 (m), 1166
(s), 1055 (s), 923 (m), 879 (m), 757 (m), 701 (m). Anal. Calcd for C29H10BI6N7O (adduct with
toluene): C, 32.35%, N, 7.33%, H, 1.36%). Found: C, 32.35%, N, 6.96%, H, 1.54%.
3.8. Measurements
Nanosecond Laser Flash Photolysis. Samples for time-resolved photolysis were excited using
(the 3rd harmonic of the Nd:YAG laser(λex 355 nm). Nd:YAG laser pulses (Spectra Physics
Mountain View, CA, USA, model INDI 40–10) of 6–8 ns duration were employed. The energy of
the excitation pulses equaled 3.5 mJ. The decay (T1 → Tn) at λ = 490 nm was followed to
determine the excited triplet lifetime (τT). The monitoring system consisted of a 150 W Xe lamp
with the lamp pulser (Applied Photophysics, Surrey, UK), a monochro-mator (Princeton
Instruments, model Spectra Pro SP-2357, Acton, MA, USA), and a R955 photomultiplier
(Hamamatsu, Japan), powered by PS-310 Power Supply (Stanford Research System, Sunnyvale,
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CA, USA). Data processing system consisted of the real- time acquisition using the digital
oscilloscope (WaveRunner 6100A, LeCroy, Chestnut Ridge, New York, USA) which was triggered
by a fast photodiode (Thorlabs, DET10M, ca. 1 ns rise time) and transferred to the computer
equipped with software based on LabView 8.0 (National Instruments, Austin, Texas, USA) which
controls the timing and acquisition functions of the system. The experiments were carried out in
rectangular 1 x 1 cm quartz cells at room temperature. All samples were purged with high purity
Ar for 20 min before the photolysis experiments. More details of the setup can be found
elsewhere [46].
Femtosecond Transient Absorption. Femtosecond transient absorption spectra and kinetics
were recorded on a setup from Spectra-Physics (USA) and Ultrafast Systems (USA). The laser
system consisted of a one-box short-pulse titanium–sapphire oscillator with a high-energy
titanium–sapphire regenerative amplifier (Solstice, Spectra-Physics, USA, 80 fs at 1 kHz). The  =
800 nm beam is divided into two beams to generate pump and probe pulses; 95% of the beam
was directed to the automated optical parametric amplifier (TOPAS Prime, Light Conversion,
Lithuania) and allowed generation of pump pulses in the wavelength range of  = 280 nm to 2.5
m. The remaining 5 % of the beam was used for white-light continuum generation in a CaF2
plate (HELIOS, Ultrafast Systems, USA). The two beams were combined in the spectrometer
(HELIOS) equipped with a computer-controlled delay line (up to 3 ns). The setup allowed
measurements of events as short as 200 fs. The pump pulse energy in the experiment was set to
about 0.15 J. The absorbance of the solutions studied was set to about 0.6 at the excitation
wavelength (= 350 nm, excitation wavelength) in a 2 mm quartz flow-through cell. Transient
absorption kinetic traces were analyzed and fitted by using commercial Surface Xplorer 2.2
software from Ultrafast Systems. The transient absorption experiments were performed at
room temperature.
Singlet oxygen quantum yield (ΦΔ). The photogeneration of singlet oxygen was measured
directly by detecting its phosphorescence at 1270 nm using a FluoTime 300 fluorescence
spectrometer (PicoQuant, Germany). Samples were excited using a CW Xe lamp (λ 345 nm). A
luminescence at 1270 nm was measured perpendicularly to the excitation beam in a photon
counting mode using a thermoelectric cooled NIR PMT module (model H10330-45, Hamamatsu)
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equipped with a 1100 nm cut-off filter and additional selected narrow-band filters. Data were
collected using a computer-mounted PCI-board multichannel scaler (NanoHarp 250, PicoQuant).
Photosensitizer absorbance was maintained at about 0.12. Unsubstitited zinc phthalocyanines
was used as a reference compound (ΦΔ = 0.53) [42].
4. Conclusions
In conclusion, we have outlined a new and very convenient method to obtain 4,5-
diidophthalonitrile (6). In contrast to the previously reported protocol, our procedure required
only one straightforward chromatography at the last step - out of five steps proposed. 4,5-
Diiodophthalonitrile (6) served as a precursor for the synthesis of hexaiodinated
subphthalocyanine 7. The nucleophilic replacement of the chlorine substituent at the boron
atom in 7 by a 3-hydroxypyridine resulted in a derivative with increased solubility in organic
solvents. Highly desirable photophysical and photochemical properties were induced in these
compounds by introducing six iodine atoms in the aromatic rings of the subphthalocyanine.
Both I6SubPcs exhibited a noticeable NIR phosphorescence at 77K in glass forming 2-MeTHF,
with λP located at 957 nm for 7 and λP located at 970 nm for 8. Based on the phosphorescence
measurements the lowest triplet excited state energies (ET) were unambiguously determined to
be 1.30 eV for 7 and 1.28 eV for 8, respectively. Upon excitation, both I6SubPcs efficiently
transferred their triplet-state energy (ET) to the ground-state oxygen, resulting in high values of
singlet oxygen quantum yields (ΦΔ), ranging from 0.83 to 0.90 – one of the highest ever
reported for this class of compounds. Interestingly, although most subphthalocyanine dyes tend
to be susceptible to photodegradation, the photostability studies of hexaiodinated
subphthalocyanines 7 and 8 pointed to a very good photostability of these derivatives under
aerobic conditions in THF. High singlet oxygen quantum yields (ΦΔ) in combination with good
photochemical stability recommends these compounds as efficient photocatalysts and
photosensitizers for biomedical applications. Combination of both nano- and femto-second
transient absorption techniques allowed detailed studies of ultrafast dynamics of the
compounds (τS and τT) of 7 and 8. Both compounds have very short-living excited singlet lifetime
(τS). For subphthalocyanine 7 the following excited singlet lifetime was measured: τS 200 ± 13
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ps, while compound 8 had the excited singlet lifetime of 226 ± 16 ps. The excited triplet
lifetimes (τT) measured were in the range of 27.71 µs to 30.15 µs at room temperature. Both
I6SubPcs are very good absorbers of visible light with λabs ca. 580 nm and high molar absorption
coefficients (ε) ranging from 89 000 to 92 000 [M-1cm-1]. CV/DPV measurements revealed that
both I6SubPcs undergo one irreversible oxidation and one quasi-reversible reduction within the
potential range applied. The spectroelectrochemical measurements pointed to relative stability
and reversibility of the electrochemically formed anion radical, viz. I6SubPc•-, and instability of
the species formed upon one-electron oxidation, viz. I6SubPc•+. Estimation of the EHOMO gave the
value of ca. – 5.8 eV while the ELUMO was found to be located at ca. – 3.8 eV, respectively.
5. Acknowledgements
This work was supported by the NCN grant number DEC-2011/03/D/ST5/05910 and NCN grant
number 2016/23/N/ST5/01284. 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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