Synthesis and liquid-crystal properties of new amphiphilic long-chain derivatives of meso-arylporphyrins with terminal polar groups

Article abstract of DOI:10.1016/j.mencom.2012.09.018

New amphiphilic alkoxyarylporphyrins with long-chain substituents bearing carboxy, methoxycarbonyl and hydroxy groups, and their metal complexes, were synthesized. Those of them having mesogenic properties were identified by optical polarisation microscop

Full text of DOI:10.1016/j.mencom.2012.09.018

Mendeleev
Communications
Mendeleev Commun., 2012, 22, 278–280
Synthesis and liquid-crystal properties of new amphiphilic long-chain
derivatives of meso-arylporphyrins with terminal polar groups
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 University of Fine Chemical Technology, 119571 Moscow, Russian Federation.
Fax: +7 495 936 8902; e-mail: former.ka@gmail.com
b Research Institute of Nanomaterials, Ivanovo State University, 153025 Ivanovo, Russian Federation.
Fax: +7 4932 32 4677; e-mail: nv_usoltseva@mail.ru
DOI: 10.1016/j.mencom.2012.09.018
New amphiphilic alkoxyarylporphyrins with long-chain substituents bearing carboxy, methoxycarbonyl and hydroxy groups, and their
metal complexes, were synthesized. Those of them having mesogenic properties were identified by optical polarisation microscopy,
and their liquid-crystal properties were studied.
Synthesis of analogues of natural tetrapyrrole compounds and
design of new functional structures based on these compounds
by means of self-assembly and self-organization of molecules are
currently a field of intense studies.1,2 The amphiphilic structure
of molecules is the determining factor in the self-assembly of
supramolecular ensembles driven by noncovalent interactions.1,3
Of particular interest in this respect are meso-arylporphyrins
containing both hydrophobic substituents that favour surface
immobilisation of molecules and give them mesogenic properties,4
and polar groups that can be subjected to various chemical trans-
formations and used as sites for binding with various ligands,
including nanoparticles.5 Polar hydroxy and carboxy groups in
which hydrogen bonds act as the driving force are used most
commonly for this purpose.1,3,6 The liquid-crystal state is also
considered as a method for the structural self-organisation of
molecules,1 and porphyrin liquid-crystal ensembles are promising
compounds for optoelectronics and for creating data display and
storage devices.7–9
Previously, we have synthesized amphiphilic cationic 5,10,15,20-
tetra-meso-aryl-substituted porphyrins bearing long-chain sub-
stituents with terminal pyridinium groups.10 These compounds
and their metal complexes possessed thermotropic mesomorphism,
whereas the length of the aliphatic spacer and the metal nature
affected considerably their liquid-crystal properties.
Herein, we studied new structural analogues, namely, amphi-
philic meso-aryl-substituted porphyrins with terminal carboxy,
methoxycarbonyl and hydroxy groups, as well as their metal
complexes (Scheme 1).
Alkaline hydrolysis in a two-phase aqueous–organic system
quantitatively gave amphiphilic porphyrins 3a,b with terminal
carboxy groups. Reduction of terminal methoxycarbonyl groups
in intermediates 2a,b with lithium aluminium hydride afforded
alcohol porphyrins 4a,b.‡ Complexes with zinc, cobalt and copper
5,10,15,20-Tetrakis[4-(5-methoxycarbonylpentyloxy)phenyl]porphyrin
2a: yield 0.14 g (31%). Rf 0.9 (CH2Cl2). UV [lmax/nm (e×10–3)]: 419.5
(365), 515.8 (14.3), 554.8 (6.3), 589.6 (4.89), 649.2 (3). 1H NMR, d: –2.81
(s, 2H, NH), 1.63 [m, 8H, O(CH2)3], 1.78 [m, 8H, O(CH2)4COOMe],
1.95 (m, 8H, OCH2CH2), 2.37 (t, 8H, CH2COOMe, J 7 Hz), 2.47 (t, 8H,
OCH2, J 7 Hz), 3.67 (s, 12H, COOMe), 7.21 (d, 4H, HAr, J 7 Hz), 8.18
(d, 4H, HAr, J 7 Hz), 8.82 (s, 8H, pyrrole). MS, m/z: 1193 [M+ +1].
5,10,15,20-Tetrakis[4-(10-methoxycarbonyldecyloxy)phenyl]porphyrin
2b: yield 0.19 g (34%). Rf 0.9 (CH2Cl2). UV [lmax/nm (e×10–3)]: 421.5
(360), 518.6 (15), 556.2 (6.2), 592 (4.49), 650.4 (3.1). 1H NMR, d: –2.72
(s, 2H, NH), 1.31 [br.m, 40H, (CH2)5], 1.60 [m, 8H, O(CH2)8], 1.93 [m,
8H, (CH2)7COOMe], 2.01 [m, 8H, O(CH2)8], 2.75 (t, 8H, CH2COOMe,
J 7 Hz), 3.91 (s, 12H, COOMe), 4.27 [t, 8H, O(CH2)8, J 7 Hz], 7.46
(d, 8H, HAr, J 2.6 Hz), 8.22 (d, 8H, HAr, J 3.5 Hz), 8.9 (s, 8H, pyrrole).
MS, m/z: 1473 [M+ +1].
‡
General procedure for the preparation of porphyrins 3a,b. Aqueous
sodium hydroxide (50%, 7 ml) was added to a solution of ester porphyrin
2a,b (90 mg) in THF (10 ml). The reaction mixture was stirred for 16 h
at 60°C, acidified with conc. HCl until the product was completely
transferred to the organic phase, and washed with water. The organic
phase was dried with anhydrous Na2SO4 and concentrated in vacuo. The
product was recrystallised from diethyl ether. Rf 0.2 (CH2Cl2–MeOH, 9:1).
5,10,15,20-Tetrakis[4-(5-carboxypentyloxy)phenyl]porphyrin 3a: yield
1
75 mg (87%). H NMR, d: –2.81 (s, 2H, NH), 1.61 [m, 8H, O(CH2)3],
1.76 [m, 8H, O(CH2)4COOH, J 7 Hz], 1.95 (m, 8H, OCH2CH2, J 7 Hz),
2.39 [t, 8H, O(CH2)4COOMe], 4.16 (t, 8H, OCH2, J 7 Hz), 7.18 (d, 8H,
HAr, J 7 Hz), 8.04 (d, 8H, HAr, J 7 Hz), 8.8 (s, 8H, pyrrole). UV [lmax/nm,
(e×10–3)]: 420 (345), 517 (10.2), 552 (4.80), 592 (3.96), 649 (2.3). MS,
m/z: 987 [M+ +1].
5,10,15,20-Tetrakis[4-(6-hydroxyhexyloxy)phenyl]porphyrin 4a was
obtained by reduction of compound 2a (65 mg, 0.31 mmol) with LiAlH4
(25 mg, 0.66 mmol) in THF. The mixture was stirred for 20 min, then
water (10 ml) was added. The mixture was extracted with CH2Cl2, filtered,
concentrated and crystallised from diethyl ether. Yield, 60 mg (96%).
Rf 0.14 (CH2Cl2–MeOH, 9:1). UV [lmax/nm (e×10–3)]: 418.2 (455.8),
519 (14.4), 556 (7.30), 591 (4.52), 646 (3.79). 1H NMR, d: –2.81 (s, 2H,
NH), 8.68–8.51 (s, 8H, pyrrole), 7.86–7.69 (d, 8H, HAr), 6.98–6.86 (d,
8H, HAr), 3.88 (t, 8H, CH2OH, J 6.4 Hz), 3.41 (t, 8H, OCH2, J 6.6 Hz), 1.69
(m, 8H, CH2CH2OH, J 6.2 Hz), 1.53–1.18 [m, 24H, HOCH2CH2(CH2)3].
MS, m/z: 1078.6 [M+].
Porphyrins 2a,b were synthesized from substituted benz-
aldehydes 1a,b in 35–40% yields using reported techniques.†,11,12
†
For synthesis of compounds 1a,b, see Online Supplementary Materials.
General procedure for the preparation of porphyrins 2a,b. Pyrrole
(100 mg, 1.42 mmol) and 4-(5-methoxycarbonylpentyloxy)benzaldehyde 1a
(0.38 g, 1.5 mmol) were dissolved in dichloromethane (50 ml). The mixture
was bubbled with argon for 5 min under continuous stirring at room tem-
perature, then boron trifluoride etherate (20 ml, 0.15 mmol) and anhydrous
ethanol (20 ml) were added, and the mixture was stirred for 30 min at
room temperature in a stream of argon. DDQ (300 mg, 1.35 mmol) was then
added, and stirring was continued for another 1 h at room temperature.
The reaction mixture was concentrated in vacuo. The oligomeric products
were separated by flash chromatography on G60 silica gel on eluting with
dichloromethane. The target product was finally purified by column chro-
matography on G60 silica gel on eluting with dichloromethane–hexane (6:1).
For characteristics of compounds 3b (yield, 88%) and 4b (yield, 83%),
see Online Supplementary Materials.
© 2012 Mendeleev Communications. All rights reserved.
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Mendeleev Commun., 2012, 22, 278–280
(CH2)nCOOMe
(CH2)nY
O
O
O
H
NH
N
N
NH
N
N
MeOOC(CH2)n
Y(CH2)n
i
ii or iii
O
O
O
O
(CH2)nCOOMe
(CH2)nY
HN
HN
O
(CH2)nCOOMe
1a,b
a n = 5
b n = 10
O
O
MeOOC(CH2)n
Y(CH2)n
2a,b
3a,b Y = COOH
4a,b Y = CH2OH
iv
v
(CH2)nCOOMe
(CH2)nY
O
O
N
N
N
N
N
N
N
N
MeOOC(CH2)n
Y(CH2)n
M
M
O
O
O
O
(CH2)nCOOMe
(CH2)nY
O
O
MeOOC(CH2)n
Y(CH2)n
5a,b
M = Zn2+, Cu2+, Co2+
6a,b Y = COOH
7a,b Y = CH2OH
Scheme 1 Synthesis of porphyrins 2–7. Reagents and conditions: i, pyrrole, Et2O·BF3, CH2Cl2, DDQ; ii, 50% NaOH, heating; iii, LiAlH4, THF; iv, M(OAc)2,
CH2Cl2, MeOH; v, M(OAc)2, DMF, heating.
were obtained from free ligands 2–4 in 75–90% yields routinely.§,13
The structures of the compounds were confirmed by TLC, UV, 1H
NMRspectroscopyandliquidchromatography–massspectrometry
(LCMS). The obtained products are crystalline compounds soluble
in organic solvents; porphyrins containing carboxy groups colourize
aqueous alkaline solutions.
The liquid-crystal properties of compounds 2–7 were studied
by optical polarization microscopy in two modes: heating to 300 °C
(Figure 1) and cooling to room temperature. In comparison with
the previously studied cationic analogues,10 only a limited number
of compounds that we obtained had mesomorphic properties,
namely five compounds: Co-5a, Zn-5a and Co-7b were enantio-
tropic, whereas 3b and Zn-5b were monotropic (Figure 1).
Note that on cooling, all the compounds formed a glassy state
with preservation of the mesophase texture. A study by the method
of contact agents with organic solvents showed that mesogenic
300
275
Crystal phase
Mesophase
225
175
125
75
Isotropic liquid
Glass state
§
General procedure for the synthesis of porphyrin metal complexes
5a,b. Zinc, cobalt or copper acetate (10 equiv.) in methanol was added
to porphyrin 2a or 2b (1 equiv.) in methanol and the mixture was stirred
for 3 h. The reaction course was monitored by UV spectroscopy. The
reaction mixture was concentrated in vacuo; the residue was dissolved in
CH2Cl2, filtered and crystallised from diethyl ether.
General procedure for the preparation of porphyrin metal complexes
6 and 7. A metal acetate (10 equiv.) was added to porphyrin 3 or 4
(1 equiv.) in DMF and the mixture was refluxed for 6 h with stirring. The
reaction completion was monitored by UV spectroscopy. The reaction
mixture was concentrated; the residue was dissolved in CH2Cl2, filtered
and crystallised from diethyl ether. The yields were 60–95%.
25
Co-5a
Zn-5a
Co-7b
3b
Zn-5b
Figure 1 Optical polarisation microscopy results: phase transition tem-
peratures of mesogenic compounds in the course of heating (left columns)
and cooling (right columns). Observation limit for Co-7b is 139°C.
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Mendeleev Commun., 2012, 22, 278–280
compounds Co-5a and Co-7b manifest liotropic mesomorphism;
inducing mesomorphism in five non-mesogenic compounds (2a,
2b, Cu-5b, Co-5b and Zn-7b) by means of creating charge
transfer complexes, as well as in systems with organic solvents.
furthermore, non-mesogenic compounds 2b, Co-5b and Zn-7b
manifested liotropic mesomorphism in binary systems with chloro-
form, DMF and DMSO. Along with induction of mesomorphism
due to solvents, mesomorphism was also induced in charge
transfer complexes of compounds 2a,b with an electron acceptor,
2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)propionic acid
[(–)-TAPA], as well as in the complex of compound Cu-5b with
an electron acceptor, trinitrofluorenone (TNF).
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.mencom.2012.09.018.
References
Similarly to lipophilic porphyrin derivatives,14 incorporation
of coordination-unsaturated metals (Zn or Co) changed con-
siderably the properties of non-mesomorphic free ligands 2a,b,
4b. Metal complexes of porphyrins with terminal methoxy-
carbonyl groups (Zn-5a, Co-5a, Zn-5b) were found to be most
liable to manifest liquid-crystal properties. Of free bases, meso-
morphism was observed for 5,10,15,20-tetrakis[4-(10-carboxy-
decyloxyphenyl)]porphyrin 3b, whereas its lipophilic analogues
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C5 spacer, does not possess liquid-crystal properties; in this
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liquid-crystal properties on heating. Due to its decomposition
on heating to 160°C (isotrope region), heating and observation
of the mesophase were limited to 139°C.
In summary, 24 new amphiphilic porphyrins with terminal polar
groups have been obtained in this work. Five of them manifest
thermotropic mesomorphism, like their cationic analogues.10
The porphyrins containing 10 carbon atoms in the aliphatic chain,
both free ligands and their metal complexes, can spontaneously
form liquid-crystal phases, whereas their structural analogues with
shorter spacers manifest mesomorphic properties only in metal
complexes. All mesogenic compounds of this series can undergo
glass transition while maintaining the texture of the parent meso-
phase. Manifestation of liquid-crystal properties is affected by the
length of the aliphatic moiety, the nature of terminal polar groups,
and the presence of a complex-forming metal. We succeeded in
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Received: 12th March 2012; Com. 12/3895
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