Synthesis and mesomorphism of cationic derivatives of meso-aryl-substituted porphyrins and their metal complexes

Article abstract of DOI:10.1016/j.mencom.2010.06.020

The water-soluble derivatives of 5,10,15,20-tetra-meso-aryl-substituted porphyrins and their metal complexes have been synthesised, and the liquid crystal properties of mesogenic compounds have been studied.

Full text of DOI:10.1016/j.mencom.2010.06.020

Mendeleev
Communications
Mendeleev Commun., 2010, 20, 239–241
Synthesis and mesomorphism of cationic derivatives
of meso-aryl-substituted porphyrins and their metal complexes
Nikita V. Novikov,*a Kirill A. Formirovsky,a Natalya A. Bragina,a Andrey F. Mironov,a
Galina A. Anan’eva,b Venera V. Bykovab and Nadezhda V. Usol’tsevab
a M. V. Lomonosov Moscow State Academy of Fine Chemical Technology, 119571 Moscow, Russian Federation.
Fax: +7 495 936 8902; e-mail: nikshabash@yandex.ru
b Research Institute of Nanomaterials, Ivanovo State University, 153025 Ivanovo, Russian Federation.
Fax: +7 4932 32 4677; e-mail: nadezhda_usoltseva@yahoo.com
DOI: 10.1016/j.mencom.2010.06.020
The water-soluble derivatives of 5,10,15,20-tetra-meso-aryl-substituted porphyrins and their metal complexes have been
synthesised, and the liquid crystal properties of mesogenic compounds have been studied.
Amphiphilic porphyrin derivatives capable of forming ordered
aggregates are of considerable current interest.1–3 The reason is
that porphyrins find use in studies on super-fast photoprocesses
in supramolecular systems simulating fundamental natural pro-
cesses, as well as in the creation of photoconductive organic
materials. Mesogenic tetrapyrrole compounds as self-organizing
supramolecular structures are of particular interest for opto-
electronics and for devices intended for displaying and storing
information.4–7 Liquid crystals based on porphyrins belong to
the class of discotic mesogens that can form three main types
of thermotropic mesophases: columnar (Col), discotic nematic
(ND) and lamellar (L).8
The functionalisation of meso-aryl-substituted porphyrins by
long chain aliphatic substituents makes it possible to success-
fully perform goal-oriented searches for mesomorphic lipophilic
derivatives.5,7,9–12 The incorporation of metals generally changes
mesomorphic properties.13–16 It has been shown that amphiphilic
porphyrin derivatives containing sulfonic groups possess lyotropic
mesomorphism in solutions;17,18 thermotropic mesomorphism
of amphiphilic porphyrins was not reported.
In order to attain thermotropic mesomorphism, we modified
the structure of porphyrins by incorporating spacer hydrocarbon
bridges (CH2)n (n = 5 or 10) that separate cationic pyridinium
groups from the porphyrin macrocycle. Obviously, positively
charged meso-substituents can affect the charge distribution
(electron density) in the porphyrin ring via induction effects
only. Therefore, as such substituents become farther from the
macrocycle, the porphyrin basicity increases, which weakens
the electrostatic repulsion between porphyrin macrocycles when
ordered structures are formed.
Porphyrins 3 and 4 were synthesised by monopyrrole con-
densation according to Lindsey19 followed by modification of
substituents at the aromatic rings.† meso-Tetrakis[4-(6-bromo-
hexanoyloxyphenyl)]- and meso-tetrakis[4-(11-bromoundecan-
oyloxyphenyl)]porphyrins obtained from pyrrole and the re-
spective substituted 4-hydroxybenzaldehydes in 35–40% yields
according to the procedure developed for synthesising lipophilic
porphyrins20 were used as the precursors of target porphyrins.
Zinc, cobalt, nickel and copper metal complexes7–14 were obtained
from ligands 3 and 4 using standard techniques.21 Cationic
porphyrins 5, 6 and 15–22 were obtained by the quaternisation
of pyridine with bromo-substituted precursors in 93–95% yields.
The authenticity and structures of the compounds were con-
firmed by elemental analyses, TLC, UV, 1H NMR spectroscopy
and mass spectrometry.‡
The aim of this work was to synthesise amphiphilic cationic
meso-aryl-substituted porphyrins and their metal complexes
and to study their mesomorphic properties. We synthesised
cationic porphyrins 5 and 6 and their complexes with Zn, Cu,
Ni and Co metals 15–22 (Scheme 1). Incorporation of hydro-
philic functional groups allowed us to obtain amphiphilic com-
pounds soluble in water.
The liquid crystal properties of compounds 5, 6 and 15–22
were studied by optical polarisation microscopy in two modes:
R
R
R
R
Porphyrin
n
M
O
H
5
6
5
10
5
10
5
10
5
10
5
10
2H
2H
Zn
Zn
Cu
Cu
Ni
Ni
Co
Co
N
M
N
N
N
15
16
17
18
19
20
21
22
i
iii
N
N
NH
HN
Br
(CH2)n
O
O
1, 2
R
R
R
R
3, 4 R = OCO(CH2)nBr
7–14 R = OCO(CH2)nBr
ii
ii
Br–
15–22 R =
Br–
+
+
5, 6 R =
OCO(CH2)nN
OCO(CH2)nN
Scheme 1 Reagents and conditions: i, pyrrole, BF3·Et2O, CHCl3, DDQ; ii, pyridine, reflux; iii, acetate of the corresponding metal, CHCl3, MeOH.
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© 2010 Mendeleev Communications. All rights reserved.

Mendeleev Commun., 2010, 20, 239–241
heating to 300 °C (Figure 1) and cooling to room temperature
(Figure 2).
Isotropic liquid
Mesophase
Vitrified state
Ligands 5 and 6 and their metal complexes 15–22 possess
thermotropic mesomorphism. Porphyrins with cationic pyridi-
nium groups, both with 5 carbon atoms in the spacer 5 and with
10 carbon atoms 6, were found to form liquid-crystal phases.
Porphyrin metal complexes with cobalt 21, 22, copper 17, 18
and nickel 19, 20 form a mesophase either upon heating or
cooling (enantiotropic mesomorphism), whereas ligand 5 forms
a mesophase only upon cooling (monotropic mesomorphism).
A common feature of all of the compounds is that the phase
5
6
15
16
17
18
19
20
21
22
†
The NMR spectra were recorded on a Bruker MSL-300 instrument
(Germany) at a working frequency of 300 MHz; measurements were carried
out in the d scale using TMS as the internal reference. Electronic spectra
were recorded using a Jasko UV-7800 spectro-photometer (Japan) in
dichloromethane. Elemental analyses were performed on a FLASH EA
112 C, H, N, S analyser (Termo Finnigan, Italy).
Mesomorphic properties were studied by optical polarisation microscopy
using a Leitz Laborlux 12 Pol optical thermopolarisation microscope
equipped with a Mettler FP 82 heating device for thermotropic meso-
morphism, whereas lyotropic mesomorphism was studied in water, toluene,
benzene, chloroform, DMF and other solvents.
25
75
125
175
T/°C
225
275
Figure 1 Results of optical polarisation microscopy: phase transition
temperatures in the case of heating.
Isotropic liquid
Mesophase
Vitrified state
5
6
General procedure for the preparation of substituted benzaldehydes 1, 2.
A solution (20 ml) of 6-bromohexanoyl chloride (2.52 g, 11.8 mmol) or
11-bromoundecanoyl chloride (3.35 g, 11.8 mmol) in dichloromethane
(10 ml) was added dropwise for 30 min to a solution (25 ml) of p-hydroxy-
benzaldehyde (1.20 g, 9.83 mmol) and DMAP (0.90 g, 7.37 mmol) in
dichloromethane. The components were stirred for 24 h at room tempera-
ture. The excess solvent was removed in vacuo. To purify the compound,
the reaction mixture was treated by column chromatography on G 60
silica gel. Elution was carried out using chloroform–hexane (4:1).
4-(6-Bromohexanoyloxy)benzaldehyde 1: yield 2.35 g (80%), Rf 0.3,
(chloroform–hexane, 7:3). 1H NMR (CDCl3) d: 1.59 [m, 2H, OCO(CH2)2-
CH2(CH2)2Br], 1.81 (m, 2H, OCOCH2CH2), 1.92 (m, 2H, CH2CH2Br),
2.62 (t, 2H, OCOCH2), 3.44 (t, 2H, CH2CH2Br), 7.26 (d, 2H, 2,6-HAr),
7.92 (d, 2H, 3,5-HAr), 10.00 (s, H, CHO). IR (n/cm–1): 2926, 2848
[(CH2)5], 1676 (C=O), 1467 (Ar), 1160, 1138 (C–O), 642, 550 (C–Br).
4-(11-Bromoundecanoyloxy)benzaldehyde 2: yield 3.41 g (94%), Rf 0.4
(chloroform–hexane, 7:3). 1H NMR (CDCl3) d: 1.32 [br. m, 12H, OCO-
(CH2)2(CH2)6(CH2)2Br], 1.75 (m, 2H, OCOCH2CH2), 1.83 (m, 2H,
CH2CH2Br), 2.58 (t, 2H, OCOCH2), 3.38 (t, 2H, CH2CH2Br), 7.28 (d, 2H,
2,6-HAr), 7.92 (d, 2H, 3,5-HAr), 9.99 (s, H, CHO). IR (n/cm–1): 2926, 2848
[(CH2)5], 1676 (C=O), 1467 (Ar), 1160, 1138 (C–O), 642, 550 (C–Br).
General procedure for the preparation of porphyrins 3, 4. Pyrrole
(100 mg, 1.42 mmol) and 4-(6-bromohexanoyloxy)benzaldehyde 1
(448 mg, 1.5 mmol) or 4-(11-bromoundecanoyloxy)benzaldehyde 2
(554 mg, 1.5 mmol) were dissolved in chloroform (50 ml). The reaction
mixture was saturated with an inert gas under continuous stirring at
room temperature for 5 min, and then BF3·Et2O (20 μl, 0.15 mmol) and
anhydrous ethanol (20 μl) were added. The reaction mixture was stirred
for 2 h at room temperature in a stream of an inert gas. After that, the
inert gas stream was discontinued, DDQ (300 mg, 1.35 mmol) was added
and the mixture was stirred for one more hour at room temperature. The
reaction mixture was concentrated in vacuo. Oligomeric products were
separated by flash chromatography on G 60 silica gel and eluted with
chloroform. The target product was purified by column chromatography
on G 60 silica gel and eluted with chloroform–hexane (4:1).
15
16
17
18
19
20
21
22
25
75
125
175
T/°C
225
275
Figure 2 Results of optical polarisation microscopy: phase transition
temperatures in the case of cooling.
‡
General procedure for the preparation of porphyrins 5, 6. A starting
porphyrin (3 or 4) was dissolved in pyridine and refluxed for 3 h. The
reaction product was isolated by recrystallisation from diethyl ether.
Yields 93–97%.
5,10,15,20-Tetra[4-(6-pyridylhexanoyl)oxyphenyl]porphyrin tetra-
bromide 5. Electronic spectrum (lmax/nm): 415, 525, 580, 600, 640 (peak
ratio: 51, 2.8, 1.45, 1.0, 0.5, respectively). 1H NMR ([2H6]DMSO) d:
1.53 [br. m, 8H, OCO(CH2)2CH2(CH2)2Py], 1.86 (m, 8H, OCOCH2CH2),
2.10 (m, 8H, CH2CH2Py), 2.82 (t, 8H, OCOCH2), 4.72 (m, 8H,
CH2CH2Py), 7.58 (d, 8H, 2,6-HAr), 8.23 (t, 8H, 3,5-Py), 8.28 (d, 8H,
3,5-HAr), 8.66 (t, 4H, 4-Py), 8.88 (br. s, 8H, pyrrole), 9.22 (d, 8H,
2,6-Py).
5,10,15,20-Tetra[4-(11-pyridylundecanoyl)oxyphenyl]porphyrin tetra-
bromide 6. Electronic spectrum (lmax/nm): 413, 512, 545, 589, 642 (peak
ratio: 72.9, 3.2, 1.6, 1.0, 0.7, respectively). H NMR (CD3OD) d: 1.26
[br. m, 48H, OCO(CH2)2(CH2)6(CH2)2Py], 1.72 (m, 8H, OCOCH2CH2),
1.93 (m, 8H, CH2CH2Py), 2.61 (t, 8H, OCOCH2), 4.53 (m, 8H,
CH2CH2Py), 7.21 (d, 8H, 2,6-HAr), 7.88 (d, 8H, 3,5-HAr), 8.01 (t, 8H,
3,5-Py), 8.48 (t, 4H, 4-Py), 8.69 (br. s, 8H, pyrrole), 8.91 (d, 8H,
2,6-Py).
1
5,10,15,20-Tetra[4-(6-bromohexanoyl)oxyphenyl]porphyrin 3: yield
148 mg (31%), mp 184 °C, Rf 0.8 (CHCl3). Electronic spectrum
(lmax/nm): 419.8, 514.4, 549.8, 589, 645.6 (peak ratio: 63.4, 1.7, 1.4, 1.0,
1
0.9, respectively). H NMR (CDCl3) d: 1.62 [m, 8H, OCO(CH2)2CH2-
General procedure for the preparation of metal complexes of cationic
porphyrins 15–22. A metal acetate (chloride) (10 equiv.) in methanol
was added to porphyrin 3 or 4 (1 equiv.) in methanol and the mixture was
stirred for 3 h. The reaction completion was determined using spectral
data. The reaction mixture was concentrated; the residue was dissolved
in chloroform, filtered from inorganic salts and crystallised from diethyl
ether to give compounds 7–14. Metal complexes 7–14 were dissolved in
an excess of pyridine and refluxed for 3 h. The reaction completion was
determined using TLC data in chloroform–methanol (1:1). The product
was isolated from Et2O. The yields of complexes 15–22 are 60–95%.
(CH2)2Br], 1.77 (m, 8H, OCOCH2CH2), 1.94 (m, 8H, CH2CH2Br), 2.38
(t, 8H, OCOCH2), 4.17 (t, 8H, CH2CH2Br), 7.18 (d, 8H, 2,6-HAr), 8.03
(d, 8H, 3,5-HAr), 8.81 (br. s, 8H, pyrrole).
5,10,15,20-Tetra[4-(11-bromoundecanoyl)oxyphenyl]porphyrin 4: yield
208 mg (33%), mp 75 °C, Rf 0.8 (CHCl3). Electronic spectrum (lmax/nm):
418, 514.6, 549.8, 590.2, 645.8 (peak ratio: 68.4, 2.4, 1.7, 1.0, 0.86,
respectively). 1H NMR (CDCl3) d: 1.38 [m, 48H, OCO(CH2)2(CH2)6-
(CH2)2Br], 1.89 (m, 8H, OCOCH2CH2), 1.90 (m, 8H, CH2CH2Br), 2.76
(t, 8H, OCOCH2), 3.43 (t, 8H, CH2CH2Br), 7.52 (d, 8H, 2,6-HAr), 8.22
(d, 8H, 3,5-HAr), 8.88 (s, 8H, pyrrole).
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Mendeleev Commun., 2010, 20, 239–241
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phase–crystal transition temperature) than in the heating mode
(crystal–mesophase transition temperature). Insignificant changes
in the electron shell of the complex-forming metal result in
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Incorporation of a metal decreases the thermal stability of the
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On cooling, ligands 5 and 6 and their metal complexes with
zinc (15, 16) and copper (17) undergo vitrification, which
maintains the texture of the preceding liquid-crystal mesophase.
Such compounds show promise for the development of mate-
rials for contemporary photonics.17
12 N. V. Usoltseva, V. V. Bykova and N. V. Zharnikova and M. A. Zharova,
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morphism and form a mesophase in binary mixtures with water
and organic solvents, such as benzene, toluene, DMF and CHCl3
at room temperature.
Thus, our study of the liquid crystal properties of cationic
amphiphilic derivatives of 5,10,15,20-tetra-meso-aryl-substituted
porphyrins and their metal complexes allowed us to find out
that these compounds possess thermotropic mesomorphism that
is considerably affected by the metal nature and the size of the
aliphatic spacer.
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The study was supported by the Russian Foundation for Basic
Research (grant no. 07-03-00427) and Analytical Departmental
Goal-Oriented Programme ‘Development of the Scientific Potential
of Higher School’ no. 2.1.1/2889.
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