Synthesis, characteristics and photochemical studies of novel porphyrazines possessing peripheral 2,5-dimethylpyrrol-1-yl and dimethylamino groups

Article abstract of DOI:10.1016/j.tetlet.2012.02.010

The synthesis and physicochemical properties of novel porphyrazines possessing an alternate system of two peripheral substituents, 2,5-dimethylpyrrol-1-yl and dimethylamino, are presented. All the macrocycles were subjected to HPLC purity studies. Spectroscopic studies of magnesium(II) porphyrazine encompassed steady state absorption, emission measurements, including fluorescence decays, transient absorption spectra, and thermoluminescence. Additionally, magnesium(II) porphyrazine was found to be a moderate photosensitizer with singlet oxygen generation values of 0.12 and 0.14 in DMF and DMSO, respectively. Comparison of the quantum yields of singlet oxygen generation before and after deoxygenation showed that the photodynamic effect of magnesium(II) porphyrazine is governed by the photosensitization mechanism II. Magnesium(II) and manganese(III) porphyrazines were characterized using X-ray crystallography.

Full text of DOI:10.1016/j.tetlet.2012.02.010

Tetrahedron Letters 53 (2012) 2040–2044
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis, characteristics and photochemical studies of novel porphyrazines
possessing peripheral 2,5-dimethylpyrrol-1-yl and dimethylamino groups
Wojciech Szczolko a, Lukasz Sobotta b, Piotr Fita c, Tomasz Koczorowski a, Michal Mikus a, Maria Gdaniec d,
Aleksandra Orzechowska e, Kvetoslava Burda e, Stanislaw Sobiak a, Marcin Wierzchowski a, Jadwiga
a,
Mielcarek b, Ewa Tykarska a, , Tomasz Goslinski
⇑
⇑
a Department of Chemical Technology of Drugs, Poznan University of Medical Sciences, Grunwaldzka 6, 60-780 Poznan, Poland
b Department of Inorganic and Analytical Chemistry, Poznan University of Medical Sciences, Grunwaldzka 6, 60-780 Poznan, Poland
c Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Hoza 69, 00-681 Warsaw, Poland
d Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
e AGH University of Science and Technology in Krakow, Faculty of Physics and Applied Computer Science, al. Mickiewicza 30, 30-059 Krakow, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and physicochemical properties of novel porphyrazines possessing an alternate system of two
peripheral substituents, 2,5-dimethylpyrrol-1-yl and dimethylamino, are presented. All the macrocycles were
subjected to HPLC purity studies. Spectroscopic studies of magnesium(II) porphyrazine encompassed steady
state absorption, emission measurements, including fluorescence decays, transient absorption spectra, and
thermoluminescence. Additionally, magnesium(II) porphyrazine was found to be a moderate photosensitizer
with singlet oxygen generation values of 0.12 and 0.14 in DMF and DMSO, respectively. Comparison of the
quantum yields of singlet oxygen generation before and after deoxygenation showed that the photodynamic
effect of magnesium(II) porphyrazine is governed by the photosensitization mechanism II. Magnesium(II)
and manganese(III) porphyrazines were characterized using X-ray crystallography.
Received 24 November 2011
Revised 22 January 2012
Accepted 3 February 2012
Available online 11 February 2012
This Letter is dedicated to Professor
Anthony G. M. Barrett on the occasion of his
60th birthday and for his contributions to
porphyrazine chemistry.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Linstead macrocyclization
Porphyrazine
Singlet oxygen
Photochemistry
X-ray
Porphyrazines (Pzs) form a distinct class of macrocycles, along
with the functionalized porphyrins, phthalocyanines and corroles.
Pzs can be modified by the introduction of a metal cation into
the core and/or by peripheral substitutions. For the last 30 years,
Pzs substituted at their b-positions with nitrogen, oxygen or sulfur
residues, or rings bound to both b,b-positions have been synthe-
sized and subjected to physicochemical as well as biological stud-
ies.1–9 Novel porphyrazine-based macrocyclic systems have been
investigated as photosensitizers for biomedical (photodynamic
therapy),10 analytical applications (metal ion probes),11–13 and as
compounds of potential use in materials chemistry (nanotechnol-
ogy, electrophotography, optical data recording systems, electronic
devices, photovoltaic cells, fuel cells, and electrochromic dis-
plays).14–17
2,3-thiophene, 1,2,5-selenodiazole), and showed their broad appli-
cations.3 A synthetic approach to use 1,2,5-selenodiazole rings
fused to the Pz core as synthetic precursors was later developed
by Barrett and Hoffman.2,5,14 There are limited data on Pzs substi-
tuted directly at their b-position with five-membered heteroaro-
matic rings. One example,
a two-core porphyrazine system,
possessing 12 trimethylthienyl groups on the periphery, synthe-
sized by Luo et al.,18 has been considered as a compound of poten-
tial use in information storage systems and molecular switches.
Begland et al. found that 2,5-hexanedione on reaction with
diaminomaleonitrile undergoes the Paal–Knorr reaction to give
2,5-dimethylpyrrol-1-yl-functionalized maleonitrile.19 Our study
on the reactivity of its derivative possessing methyl(3-pyridyl-
methyl)amino and 2,5-dimethylpyrrol-1-yl groups, in macrocycli-
zation reactions led to novel Pzs, which were found to contain a
peripheral alternate set of 2,5-dimethylpyrrol-1-yl and methyl(3-
pyridylmethyl)amino groups. These novel Pzs exhibited interesting
optical, electrochemical and metal-sensor properties.20 In the
course of this research, we recently reported syntheses and
discussed the physicochemical properties of novel Pzs possessing
Donzello, Ercolani and Stuzhin first synthesized Pzs possessing
peripherally annulated five-membered heteroarene rings (4,5-thiazole,
⇑
Corresponding authors. Fax: +48 61 854 6639.
E-mail addresses: etykarsk@ump.edu.pl (E. Tykarska), tomasz_goslinski@tlen.pl
(T. Goslinski).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.02.010

W. Szczolko et al. / Tetrahedron Letters 53 (2012) 2040–2044
2041
a peripheral non-alternate system of two 2,5-di(2-thienyl)pyrrol-
1-yl and dimethylamino substituents.21
In the present Letter we report the synthesis, characterization
and photochemical properties of novel Pzs possessing peripheral
2,5-dimethylpyrrol-1-yl and dimethylamino groups.
2,5-Hexanedione was subjected to the condensation reaction
with diaminomaleonitrile (1) in benzene with oxalic acid to give
maleonitrile derivative 2 following Begland’s procedure.19 Next, 2
was methylated to give derivative 3 using the conditions [NaH,
(CH3)2SO4 in THF] reported by Baum et al. (Scheme 1).22
consists of the [PzÁMgÁH2O] complex and solvent molecules, two
ethanol molecules in 4a and pyridine in 4b (see Supplementary
data). The coordination geometry of the magnesium center is
square-pyramidal as expected, the Mg ion being bonded to the por-
phyrazine core, and one water molecule located in the apical
position (Fig. 1a and b). The Mg cation is displaced from the
best plane of the Pz core by 0.716(1) Å in 4a and by 0.586(2) Å in
4b. The Mg–N(Pz) distance ranges are 2.035(3)–2.049(2) and
2.025(4)–2.039(4) Å in 4a and 4b, respectively. The Mg–O bond is
significantly shorter in 4a [1.970(2) Å] than in 4b [2.038(4) Å]
(see Supplementary data). In manganese(III) Pz 7 the metal ion is
located in the plane of the Pz core and has distorted octahedral
geometry with the chloride ion and ethanol molecule placed at ax-
ial positions (Fig. 1c). The Mn–N(Pz) distances are within the range
1.936(2)–1.950(2) Å and the Mn–Cl and Mn–O bonds are 2.5147(8)
and 2.409(2) Å, respectively.
In compounds 4a, 4b and 7 the orientation of the peripheral
substituents at the b,b-positions is similar. The outer pyrrole rings
are almost perpendicular to the porphyrazine core formed by 24
atoms, whereas the dihedral angle of the dimethylamino frag-
ments with the macrocyclic platform is 10°–30°, except for the
dimethylamino group of the unit C in 4a where this angle is
48.8(2)o. This amine N-atom acts as a hydrogen bond acceptor
(see Supplementary data) from one of the ethanol molecules and
is pyramidal (the sum of the bond angles around the N atom is
342.1°), in contrast to the other amine nitrogen atoms where the
sum of the bond angles is closer to 360° which indicates their
sp2 hybridization. An average CPz–Namine distance for sp2-hybrid-
ized N atoms is 1.360(5) Å (the range of 1.355(4)–1.371(4)
Å, 1.339(6)–1.372(6) Å and 1.344(4)–1.365(4) Å for 4a, 4b and 7,
respectively) indicating some degree of conjugation of the amine
Derivative 3 was used in the Linstead macrocyclization with
Mg(n-BuO)2 in n-butanol to give novel porphyrazine 4, possessing
an alternate system of peripheral substituents, in 25% overall
yield.2,23 Pz 4 was carefully purified by column chromatography
and further analyzed using HPLC. Interestingly, other isomers as ob-
served in our previous studies, possessing non-alternate systems of
peripheral substituents, were not detected by TLC.20,21 Demetalla-
tion of 4 was performed in trifluoroacetic acid (TFA) using a standard
procedure2,21 which led to novel free base porphyrazine 5, and other
demetallated side-products. Difficulties in separating 5 from the
side-products prompted us to use another macrocyclization method
on 3. The macrocyclization of 3 in dimethylaminoethanol (DMAE)
was found to be superior to that in n-pentanol with 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU) as the base, mainly due to the short
reaction time (2 h) and lack of side-products.24,25 Moreover, this free
base Pz 5 was isolated in a moderate yield of 35%.
Maleonitrile 3 was also used in macrocyclization reactions
in n-pentanol with DBU and various metal salts: Zn(OAc)2,
MnCl2 Â 4H2O, and CuCl2 to give novel metallated porphyrazines
6–8. Only in the macrocyclization reaction leading to copper por-
phyrazine 8 was an additional isomeric side-product, characterized
by UV–vis, FT IR and MS, found (see Supplementary data). Porphyr-
azines 4–6 and 8 were found to be pure according to HPLC analyses
(see Supplementary data). Unfortunately, porphyrazine 7 presented
separation problems. Magnesium, free base and zinc porphyrazines
4–6 were analyzed using various NMR techniques (see Supplemen-
tary data).
lone pair with the
p system of Pz. The Pz molecules in 4a and 7
associate to form back-to-face stacks with coordinated and solvent
molecules located between adjacent Pz macrocycles. In 4b the por-
phyrazine platforms form edge-to-edge zig–zag chains with pyri-
dine molecules H-bonded to the coordinated water molecules
(see Supplementary data).
Additionally, porphyrazines 4 and 7 were subjected to crystalli-
zation that fortunately resulted in single crystals suitable for X-ray
analyses. In magnesium(II) Pzs 4a and 4b the asymmetric unit
The absorption spectrum of 4 (Fig. 2a) has a structure typical of
porphyrinoids, and consists of two bands: the Q band, between 600
and 800 nm, and the Soret band, between 300 and 400 nm.
Scheme 1. Reagents and conditions: (i) 2,5-Hexanedione, benzene, oxalic acid, reflux, 10 h; (ii) NaH, (CH3)2SO4, THF, À15 °C to rt, 2.5 h; (iii) Mg(n-BuO)2, n-BuOH, reflux,
20 h; (iv) TFA, 20 min, rt; (v) metal salt, DBU, n-pentanol, reflux, 24 h; (vi) DMAE, reflux, 2 h; TFA—trifluoroacetic acid, DMAE—dimethylaminoethanol,
DBU—1,8-diazabicyclo[5.4.0]undec-7-ene.

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W. Szczolko et al. / Tetrahedron Letters 53 (2012) 2040–2044
Figure 2. (a) Steady-state absorption (dashed) and fluorescence (solid line) spectra
of 4 in DMF, (b) fluorescence decay shown together with the time-correlated single
photon-counting (TCSPC) instrument response function (IRF) in DMF, (c) selected
transient absorption spectra recorded with 400 nm excitation of 4 in DMF, and
(d) thermoluminescence spectrum of 4 illuminated with a single flash in THF.
Figure 1. The crystal structures of 4a, 4b and 7 showing the metal coordination and
solvent molecules connected by O–HÁ Á ÁO and O–HÁ Á ÁN hydrogen bonds (dashed
lines). H atoms are omitted for clarity except those involved in H-bonds. Only the
major (84%) occupancy of the disordered fragment in 4b has been shown. The atom
labeling scheme is the same for the A, B, C and D units of the porphyrazine
macrocycles in 4a, 4b and 7.
monoexponential with the decay time being equal to sFl
=
3.36 0.01 ns. Figure 2c presents selected transient absorption
spectra of 4 in DMF recorded for the excitation in the Soret band,
at 400 nm. Only two bands can be clearly distinguished: a broad
excited state absorption (ESA) for wavelengths shorter than
600 nm and a strong negative band for longer wavelengths due
to overlapped bleaching and stimulated emission.
Fluorescence is observed for wavelengths longer than 700 nm and
its spectrum does not depend on the excitation wavelength in the
range 290–670 nm. Therefore, it can be assumed that the observed
emission originates exclusively from the first excited singlet state, S1.
The fluorescence spectrum has mirror-image symmetry with
respect to the absorption spectrum. The Stokes shift in DMF,
(480 50) cmÀ1 is small, as expected for a molecule without a
permanent dipole moment. The fluorescence decay (Fig. 2b) is
Transient absorption decays in time at all wavelengths within
the spectral window of the experiment. The data have been fitted
globally using the matrix reconstruction algorithm26 (see Supple-
mentary data), and two decay times turned out to be necessary
to fit the temporal evolution of the spectra. The first decay time,
s
1 = 8.8 1.0 ps, corresponds to rapid initial decay, whereas the
second, 2 = 3.4 1.0 ns, is the excited state lifetime equal to the
fluorescence decay time Fl. A constant component is required to
s
s

W. Szczolko et al. / Tetrahedron Letters 53 (2012) 2040–2044
2043
fit the data satisfactorily, which means that a fraction of molecules
in the excited singlet state undergo intersystem crossing and pop-
ulate the lowest triplet state. The triplet state lifetime is much
longer than the temporal window of the experiment. Therefore,
its decay cannot be observed.
(U DU) of 4 before (0.1233 0.0035 and 0.1427 0.0075) and
after deoxygenation (0.1137 0.0024 and 0.1184 0.0049) in
DMF and DMSO, respectively, indicated that the photodynamic ef-
fect was governed by the photosensitization mechanism II (ZnPc
U
in DMF 0.56, in DMSO 0.67).28,29
Additional transient absorption measurements were carried out
In conclusion, the synthesis of novel porphyrazines possessing
a peripheral alternate system of 2,5-dimethylpyrrol-1-yl and
in order to identify the nature of the process behind
s1, in particular
to test whether this process is related to the internal conversion of
the molecule from higher electronic excited states into S1. It turned
out, however, that the rapid decay was still present in transient
absorption data even after direct excitation to the S1 state without
excess energy, at 700 nm. Both, the amplitude and the time constant
of this decay, 6.2 1.0 ps, are practically the same as that after the
excitation at 400 nm (see Supplementary data). It can therefore be
excluded that this process is related to the internal conversion.
The lack of significant differences between the spectral dynamics re-
corded after the excitation in the Q and Soret bands means that the
internal conversion for the excitation at 400 nm is completed within
a time shorter or comparable to the temporal resolution of the
experimental set-up (300 fs). Thus, the process corresponding to
dimethylamino substituents on the periphery has been de-
scribed. Broad physicochemical, especially photochemical studies
performed on magnesium(II) porphyrazine as a representative
revealed its interesting optical properties. In addition, other por-
phyrazine analogues, such as demetallated Pz, zinc(II), copper(II)
and manganese(III) Pzs were obtained. Zinc(II), magnesium(II)
and demetallated porphyrazines were characterized using vari-
ous NMR techniques. Moreover, the magnesium(II) and manga-
nese(III) porphyrazines were characterized using X-ray
crystallography. All the macrocycles were subjected to HPLC pur-
ity analyses. Spectroscopic studies performed on magnesium(II)
porphyrazine encompassed steady state absorption and emission
measurements. Its photophysical properties were typical of a
molecule with a small or vanishing dipole moment. Steady-state
absorption and emission spectra possess mirror-image symmetry
and only emission from the lowest excited singlet state S1 was
observed, regardless of the excitation wavelength. Femtosecond
transient absorption studies revealed an ultrafast process most
likely related to energy redistribution in the S1 state, but no evi-
dence of internal conversion from higher excited states was
found for pump–probe delay times longer than 300 fs. Interest-
ingly, a single flash resulted in the appearance of two bands in
the thermoluminescence spectrum. Most probably, they are re-
lated to recombination of two different charge pairs. Also, mag-
s
1 occurs in the lowest excited singlet state and is most likely related
to energy redistribution through solute–solvent interactions.
Thermoluminescence (TL) is a weak light emission stimulated
by warming of irradiated samples. Photochemically separated
charge pairs recombine due to increased temperature and en-
hanced vibrational energy which allows activation energy barriers
to be overcome. TL spectra obtained for porphyrazine 4 in THF ex-
posed to a single flash show two TL bands at about 42 °C and
106 °C (Fig. 2d). These bands can be ascribed to two different types
of charge pairs as they are not observed without previous illumina-
tion. Further experiments are planned to elucidate their origin.
The singlet oxygen quantum yield determination is crucial for
the potential application of novel photosensitizers in photody-
namic therapy. The potential photosensitizing activity of novel
magnesium(II) porphyrazine 4 was evaluated by measuring its
ability for singlet oxygen production, which is the result of interac-
tion between the activated photosensitizer and oxygen. 1,3-Diph-
nesium(II) porphyrazine was found to be
a
moderate
photosensitizer with singlet oxygen generation values of 0.12
and 0.14 in DMF and DMSO, respectively. Comparison of the
quantum yields of singlet oxygen generation before and after
deoxygenation has provided evidence for the fact that the photo-
dynamic effect of magnesium(II) porphyrazine was governed by
photosensitization mechanism II.
enylisobenzofuran (DPBF) was used as
a chemical quencher
which undergoes a cycloaddition reaction with singlet oxygen to
produce an endoperoxide.27 Measurements were assessed before
and after deoxygenation by comparing with a reference zinc(II)
phthalocyanine (ZnPc). According to these results, 4 was found to
be a moderate photosensitizer with singlet oxygen generation val-
ues of 0.12 and 0.14 in DMF and DMSO, respectively (Fig. 3). Com-
parison of the quantum yields of singlet oxygen generation
Acknowledgments
This study was supported by the National Science Centre under
Grant No. N N404 069440 and by the Polish Ministry of Science and
Higher Education under Grant No. IP2010 009 270. Time-resolved
studies were carried out at the University of Geneva and were
financially supported by the European Social Fund, under the pro-
gramme Human Capital 2007–2013. The authors thank Beata
Kwiatkowska for excellent technical assistance and Professor Eric
Vauthey for providing access to the research equipment of the
Ultrafast Photochemistry Group.
Supplementary data
The supplementary crystallographic data for 4a, 4b and 7 (CCDC
854143, 854144 and 854145, respectively) can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge, CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk.30
Supplementary data (the experimental details, synthetic,
spectroscopic data and X-ray data (for 4 and 7)) associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2012.02.010. These data include MOL files and InChiKeys
of the most important compounds described in this article.
Figure 3. Changes in the UV–vis spectrum during irradiation of 4 and DPBF in DMF
under aerobic conditions over the 540 s time (irradiation at 719 nm). Changes in the
Q band absorption during the experiment did not exceed 2%.

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17. Vesper, B. J.; Salaita, K.; Zong, H.; Mirkin, C. A.; Barrett, A. G. M.; Hoffman, B. M.
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