Synthesis and characterization of azobenzene-porphyrins

Article abstract of DOI:10.1002/jhet.1004

A new series of novel covalently connected meso-tetrakis(3-azophenyl-4- hydroxy-5-methoxyphenyl)porphyrins were synthesized by linking azobenzene unit at the meta-position of the meso-phenyl group. These are characterized by UV-vis, IR, ¹H-NMR, CHN, and FABMS spectroscopic techniques. All the porphyrin compounds showed a typical high energy Soret band at around 435 nm and azobenzene absorption at around 350 nm in UV-vis spectra. Fluorescence intensity of meso-tetrakis(3-(4-methoxyazophenyl)-4-hydroxy-5-methoxyphenyl) porphyrin (2c) has been observed to be maximum compared with other azobenzene porphyrins.

Full text of DOI:10.1002/jhet.1004

268
Synthesis and Characterization of Azobenzene–Porphyrins
Vol 50
a
a
a
*
M. Amaravathi, S. Kanakaraju, and G. V. P. Chandramouli
Department of Chemistry, National Institute of Technology, Warangal 506004, India
*
E-mail: ammi_reddy@hotmail.com
Received April 6, 2011
DOI 10.1002/jhet.1004
Published online 20 March 2013 in Wiley Online Library (wileyonlinelibrary.com).
A new series of novel covalently connected meso-tetrakis(3-azophenyl-4-hydroxy-5-methoxyphenyl)por-
phyrins were synthesized by linking azobenzene unit at the meta-position of the meso-phenyl group. These
1
are characterized by UV–vis, IR, H-NMR, CHN, and FABMS spectroscopic techniques. All the porphyrin
compounds showed a typical high energy Soret band at around 435nm and azobenzene absorption at around
350 nm in UV–vis spectra. Fluorescence intensity of meso-tetrakis(3-(4-methoxyazophenyl)-4-hydroxy-5-meth-
oxyphenyl)porphyrin (2c) has been observed to be maximum compared with other azobenzene porphyrins.
J. Heterocyclic Chem., 50, 268 (2013).
INTRODUCTION
excitation because of the favorable spectral and lumines-
cence properties of these extensively p-conjugated macrocy-
clic systems. However, many attempts were made to build
functionally active porphyrin-based photoswitches with
switching action being effected by the azobenzene moiety
[8]. Feng and coworkers reported a novel class of covalently
connected azonaphthalene porphyrin derivatives, and they
studied tautomerism in these novel classes of compounds
[9]. Most of the azobenzene containing porphyrins carry
azobenzene group unit either in the para-position of the
meso-phenyl group [10], via a flexible linker [11], or by
axial coordination [12]. Recently, Peters et al. reported
azobenzene-confined porphyrins containing two azobenzene
units on two ortho-positions of one meso-phenyl group to
limit the number of possible switching gates [13]. However,
the incorporation of azobenzene unit at the meta-position of
the meso-phenyl group in meso-tetraphenylporphyrins
has not been reported in the literature. In this manuscript,
the synthesis of meso-tetrakis(3-azophenyl-4-hydroxy-5-
methoxyphenyl)porphyrins (2a–b) was reported, and
azobenzene units were incorporated in the meta-position of
the meso-phenyl group of these porphyrins.
Porphyrins and their derivatives have become the
current focus of research because of their outstanding
physicochemical properties and applications in diverse
fields [1]. The elegant porphyrin derivatives can act as
models for natural photosynthesis [2], sensitizers in dye-
sensitized solar cells [3], nonlinear optical materials [4],
and photoswitches in the fabrication of molecular
electronic/optical devices [5]. In these fields, the investiga-
tion of photoinduced electron transfer and cis–trans
isomerization reactions is essential in understanding the
mechanism and processes of these molecular scale photo-
chromic compounds. Azobenzene and some azobenzene
derivatives could perform cis–trans isomerization under
photochemical reactions and might undergo energy
transfer [6]. A number of molecular assemblies, which
can potentially exhibit modulation of their fluorescence/
redox properties that is induced by cis–trans isomerization
of the appended azobenzene units, are being developed by
many researchers [7]. Porphyrin-based systems are useful
in the design of devices involving long-wavelength
© 2013 HeteroCorporation

March 2013
Synthesis and Characterization of Azobenzene–Porphyrins
269
RESULTS AND DISCUSSIONS
shown in Scheme 2. Further, the reaction mixture was
oxidized with DDQ. The solvent was removed, and the
residue was subjected to flash silica gel column chromatog-
raphy to give the corresponding porphyrins (2a–c) up to
22% yield. The yield was only 10% in the case of the
nitro-substituted porphyrin (2d). The decrease in yield may
be due to the electron withdrawing nature of the nitro group.
Ultraviolet–visible spectra of all the porphyrin com-
pounds show a Soret band at around 435 nm and four low
intense Q bands between 500 and 660nm. In addition to
these bands, each porphyrin also shows typical azobenzene
absorption at around 350 nm. The absorption peak at
350 nm indicates that the azo unit in these porphyrins
exists in trans form. The fluorescence spectra of these
azobenzene-substituted porphyrins in trans form were
recorded in dichloromethane solvent. Fluorescence emission
spectra of these porphyrins 2a–d in dichloromethane solvent
The synthesis of azobenzene aldehydes and azobenzene–
porphyrins is outlined in Schemes 1 and 2. To the best of
our knowledge, this is the first reported synthesis of
azobenzene–porphyrins where the azobenzene units are
incorporated in the meta-position of the meso-phenyl group
in meso-tetraphenylporphyrins. meso-Tetrakis(3-azophenyl-
4-hydroxy-5-methoxyphenyl)porphyrins (2a–b) were
prepared by treating substituted 3-azophenyl-4-hydroxy-
5-methoxybenzaldehydes (1a–d) with pyrrole by adopting
the Lindsey method. Substituted 3-azophenyl-4-hydroxy-
5-methoxybenzaldehydes (1a–d) had been synthesized
by treating 3-methoxy-4-hydroxybenzaldehyde with
diazonium salts which were obtained by diazotization of
corresponding p-substituted anilines (Scheme 1). The
substituted azobenzene aldehydes (1a–c) were obtained
in around 35% yield and 3-(4-nitroazophenyl)-4-hydroxy-
5-methoxybenzaldehyde (1d) in 56% yield.
The azobenzene-substituted aldehydes (1a–d) were
treated with pyrrole in the presence of trifluoroacetic acid
and dichloromethane solvent at room temperature as
Scheme 1
CHO
CHO
i) R-NH2 /HCl,NaNO2, 0-5 o
C
ii) Na2CO3, 0-5 0
C
H3CO
N N R
H3CO
OH
OH
1a-d
1a
R=
1b
Me
Figure 1. Fluorescence emission spectra were recorded for all the
porphyrins (2a–d) at the concentration of 5 Â 10À6 mol/L and lex 550 nm.
[Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
OMe
NO2
1c
1d
Scheme 2
R
N
N
OCH3
OH
OCH3
HO
N
R
N
CHO
OH
N
HN
NH
TFA /DCM
DDQ
+
H3CO
N N R
N
N
H
N
N R
H3CO
HO
OH
1(a-d)
H3CO
N
N R
2a
R=
2(a-d)
2b
Me
OMe
NO2
2c
2d
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

270
M. Amaravathi, S. Kanakaraju, and G. V. P. Chandramouli
Vol 50
3-(4-Methoxyazophenyl)-4-hydroxy-5-methoxybenzaldehyde
(1c). Yield 34.96% (2.5 g); mp 148ꢀC; IR (potassium bromide):
3428 (b, O-H stretch), 1688 (s, -CHO stretch), 1600, 1484,
excited at 550nm are shown in Figure 1. Emission intensity
of 5.2 Â 104 au has been observed for 2c and 3 Â 104 and
2.8 Â 104 au for 2b and 2a, respectively, at around 730 nm.
The emission maxima of 2d were quenched in comparison
with other azobenzene-substituted porphyrins. The quench-
ing may be due to the intramolecular electron transfer from
porphyrin to nitro units in the case of 2d, and emission
intensity is increased because of intramolecular transfer of
electrons from the methoxyl group to porphyrin in the case
of 2b. The detailed cis–trans isomerism and electrochemical
and emission studies are in progress.
1438, 1418 (m, aromatic C═C, N═N stretch)cmÀ1 1H-NMR
;
(deuteriochloroform) d: 14.21 (s, 1H, OH), 9.95 (s, 1H, CHO),
8.05 (s, 1H, ArC2-H), 7.90 (d, 2H, C2 and C6-H of azophenyl),
7.51 (s, 1H, ArC6-H), 7.06 (d, 2H, C3 and C5-H of azophenyl),
4.02 (s, 3H, O-CH3), 3.95 (s, 3H, O-CH3); MS: m/z 287 (M + H)+
requires 286. Anal. Calcd for C15H14N2O4: C, 62.93; H, 4.89; N,
9.79. Found: C, 63.08; H, 4.72; N, 9.61%.
3-(4-Nitroazophenyl)-4-hydroxy-5-methoxybenzaldehyde
(1d).
Yield 55.8% (4.2 g); mp 142–143ꢀC; IR (potassium
bromide): 3428 (b, O–H stretch), 1686 (s, -CHO stretch), 1600,
1486, 1438, 1416 (m, aromatic C═C, N═N stretch) cmÀ1; 1H-NMR
(deuteriochloroform): d 13.81 (s, 1H, OH), 9.95 (s, 1H, CHO), 8.52
(d, 2H, C3 and C5-H of azophenyl), 8.2 (m, 3H, Ar C2-H and C2
and C6-H of azophenyl), 7.52 (s, 1H, ArC6-H), 4.02 (s, 3H, O-CH3);
MS: m/z 302 (M + H)+ requires 301. Anal. Calcd for C14H11N3O5:
C, 55.81; H, 3.65; N, 13.95. Found: C, 55.68; H, 3.73; N, 13.81%.
EXPERIMENTAL
Ultraviolet–visible spectra were recorded on a SHIMADZU UV
160 A UV–vis–NIR spectrophotometer (Kyoto, Japan), using chlo-
roform as solvent. IR spectra were recorded as KBr pellets using a
SHIMADZU 8010 FTIR spectrophotometer. 1H-NMR spectra were
recorded on Varian FT 400MHz instrument (USA) using CDCl3 as
solvent and TMS as internal reference. FAB mass spectra were
recorded on a VG Micromass 7070H (F or CI) auto spectrometer
(USA). The C, H, and N analyses of the compounds were
performed on a Carlo Erba Model EA 1108 CHNS–O elemental
analyzer (Milan, Italy). Porphyrins were purified by flash column
chromatography (Aldrich) using 230–400 mesh silica gel.
Fluorescence spectra were obtained using Fluoromax-P spec-
trometer (Kyoto, Japan).
General procedure. Synthesis of meso-tetrakis(3-azophenyl-
4-hydroxy-5-methoxyphenyl)porphyrin (2a).
A mixture of
3-azophenyl-4-hydroxy-5-methoxybenzaldehyde 1a (0.512 g,
2 mmol) and pyrrole (0.134 g, 2mmol) was dissolved in 150 mL
of dichloromethane under nitrogen atmosphere in the dark. To
this, trifluoroacetic acid (2 mmol) was added while stirring at
room temperature. The reaction mixture was stirred at
room temperature for 2 h under nitrogen atmosphere. Then,
DDQ (2 mmol) was added and refluxed for 2 h to oxidize
the porphyrinogen to the corresponding porphyrin. The
dichloromethane was removed under reduced pressure, and the
obtained solid material was subjected flash silica gel column
chromatography using chloroform : methanol (99:1) as eluent
to give a purple color solid 2a (150mg, 25%); mp > 300ꢀC;
UV–vis: lmax nm (deuteriochloroform) (log x): 345 (5.25), 435
(5.82), 525 (4.69), 560 (4.41), 600 (4.3), 655 (4.08); IR
(potassium bromide): 3443 (broad, N-H and O-H str of
porphyrin), 1599, 1503, 1462 (C═C, C═N in plane bend), 981,
926 (porphyrin macrocyclic bend), 835, 796 (s, aromatic C═C-
Synthesis of 3-azophenyl-4-hydroxy-5-methoxybenzaldehyde
(1a). A mixture of aniline (2.5g, 27mmol), conc. HCl (8 mL),
and water (8mL) was taken in a conical flask and cooled to 0ꢀC.
To this, a cold sodium nitrite solution (2g in 10 mL of water) was
added slowly below 5ꢀC while stirring. Meanwhile, 3-methoxy-4-
hydroxybenzaldehyde (3.8 g, 25mmol) was dissolved in 30mL
of 10% sodium carbonate solution and cooled in freezing
mixture. This diazonium solution was added to the cold
aldehyde solution in a period of 30min without raising the
temperature above 5ꢀC. After the complete addition, the
reaction mixture was left at room temperature for 1 h and
filtered yielding a dark red colored product. The crude product
was passed through column chromatography using the mixture
of chloroform: hexane (60:40) as eluent to give 1a. Yield
37.5% (2.4g); mp: 129–130ꢀC; IR (potassium bromide): 3425
(b, O-H stretch), 1688 (s, -CHO stretch), 1596, 1483, 1434,
H out-of-plane bend) cmÀ1 1H-NMR (deuteriochloroform): d
;
13.7 (s, 4H, 4Â OH), 9.06 (s, 8H, pyrrole C-H), 8.48 (s, 4H,
C2-H), 7.98–7.96 (d, 8H, C2 and C6 azophenyl-H), 7.91 (s, 4H,
C6-H), 7.51–7.56 (m, 12H, azophenyl-H), 4.08 (s, 12H, 4Â O-CH3),
2.61 (s, 2H, porphyrin N-H); FABMS: m/z 1216 (M++2) requires
1214. Anal. Calcd for C72H54O8N12: C, 71.17; H, 4.44; N, 13.83.
Found: C, 71.26; H, 4.36; N, 13.74%.
1402 (m, aromatic C═C, N═N stretch)cmÀ1
;
1H-NMR
meso-Tetrakis{3-(4-methylazophenyl)-4-hydroxy-5-methoxy-
(deuteriochloroform): d 14.25 (s, 1H, OH), 9.90 (s, 1H, CHO),
8.15 (s, 1H, ArC2-H), 7.91 (d, 2H, C2 and C6-H of
azophenyl), 7.50 (m, 4H, ArC6-H and azophenyl-H), 4.06 (s,
3H, O-CH3); MS: m/z 257 (M+ H)+ requires 256. Anal. Calcd
for C14H12N2O3: C, 65.62; H, 4.68; N, 10.93. Found: C,
65.58; H, 4.72; N, 10. 96%.
phenyl}porphyrin (2b).
Yield 22%; mp > 300ꢀC; UV–vis:
lmax nm (deuteriochloroform) (log x): 350 (5.53), 435 (5.89),
525 (4.90), 560 (4.68), 600 (4.41), 655 (4.58); IR (potassium
bromide): 3443 (broad, N-H and O-H str of porphyrin), 1600,
1472 (C═C, C═N in plane bend), 978, 922 (porphyrin
macrocyclic bend), 822.6, 795 (s, aromatic C═C-H out-of-plane
3-(4-Methylazophenyl)-4-hydroxy-5-methoxybenzaldehyde
1
(1b).
Yield 32.6% (2.2 g); mp: 138ꢀC; IR (potassium
bend) cmÀ1; H-NMR (deuteriochloroform): d 13.73 (s, 4H, 4Â
bromide): 3426 (b, O-H stretch), 1686 (s, -CHO stretch), 1598,
OH), 9.06 (s, 8H, pyrrole C-H), 8.46 (s, 4H, C2-H), 7.90 (s, 4H,
C6-H), 7.86–7.84 (d, 8H, C2 and C6 azophenyl-H), 7.33–7.31
(d, 8H, C3 and C5 azophenyl-H), 4.08 (s, 12H, 4Â O-CH3), 2.43
(s, 12H, 4Â CH3), 2.61 (s, 2H, porphyrin N-H). FABMS: m/z
1272 (M++2) requires 1270. Anal. Calcd for C76H62O8N12: C,
71.81; H, 4.88; N, 13.23. Found: C, 71.69; H, 4.81; N, 13.28%.
meso-Tetrakis{3-(4-methoxyazophenyl)-4-hydroxy-5-methoxy-
1482, 1436, 1408 (m, aromatic C═C, N═N stretch) cmÀ1; H-
1
NMR (deuteriochloroform): d 14.25 (s, 1H, OH), 9.94 (s, 1H,
CHO), 8.06 (s, 1H, ArC2-H), 7.8 (d, 2H, C2 and C6-H of
azophenyl), 7.47 (s, 1H, ArC6-H), 7.35 (d, 2H, C3 and C5-H of
azophenyl), 4.00 (s, 3H, O-CH3), 2.45 (s, 3H, CH3); MS: m/z
271 (M + H)+ requires 270. Anal. Calcd for C15H14N2O3: C,
66.66; H, 5.18; N, 10.37. Found: C, 65.58; H, 5. 07; N, 10. 51%.
phenyl}porphyrin (2c).
Yield 22.5%; mp> 300ꢀC; UV–vis:
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

March 2013
Synthesis and Characterization of Azobenzene–Porphyrins
271
l
max nm (deuteriochloroform) (log x): 370 (5.47), 440 (5.84), 520
Technology, New Delhi, India, for sanctioning Women
Scientist Scheme A, and we are thankful to the Indian Institute
of Chemical Technology, Hyderabad, India for providing
spectroscopic facilities.
(4.77), 555 (4.68), 600 (4.31), 655 (4.73); IR (potassium
bromide): 3437 (broad, N-H and O-H str of porphyrin), 1579,
1452, 1413 (C═C, C═N in plane bend), 979, 922 (porphyrin
macrocyclic bend), 822, 796 (s, aromatic C═C-H out-of-plane
bend) cmÀ1 1H-NMR (deuteriochloroform): d 13.76 (s, 4H,
;
4Â OH), 9.04 (s, 8H, pyrrole C-H), 8.42 (s, 4H, C2-H), 7.92
(s, 4H, C6-H), 7.87–7.85 (d, 8H, C2 and C6 azophenyl-H),
7.53–7.51 (d, 8H, C3 and C5 azophenyl-H), 4.08, (s, 12H, 4Â
O-CH3), 3.91(s, 12H, 4Â O-CH3), 2.65 (s, 2H, porphyrin
N-H); FABMS: m/z 1336 (M++2) requires 1334. Anal. Calcd
for C76H62O12N12: C, 68.36; H, 4.64; N, 12.59. Found: C,
68.12; H, 4.51; N, 12.48%.
REFERENCES AND NOTES
[1] Lee, S. J.; Hupp, J. T. Coord Chem Rev 2006, 250, 1710.
[2] Giribabu, L.; Kumar, V. C..; Reddy, P. Y. Chem Asian J 2007, 2,
1574. (b) Giribabu, L.; Rao, T. A.; Maiya, B. G. Inorg Chem 1999, 38, 4971.
[3] Giribabu, L.; Kumar, V. C.; Reddy, P. Y. J Porphyrins &
Phthalocyanines 2006, 10, 1007.
[4] Rao, S. V.; Srinivas, N. K. M. N.; Rao, D. N.; Giribabu, L.;
Maiya, B. G.; Philip, R.; Kumar, G. R. Optics Commun 2000, 182, 255.
[5] Belser, P.; Bernhard, S.; Blum, C.; Beyeler, L.; DeCola, D.;
Balzani, V. Coord Chem Rev 1999, 190, 155.
[6] Tong, X.; Pelletier, M.; Laisa, A.; Zhao, Y. Angew Chem Int
Ed 2008, 47, 3596. (and references therein).
[7] Yukata, T.; Mori, I.; Kurihara, M.; Mizutani, J.; Kubo, K.;
Furusho, S.; Matsumura, K.; Tamai, N.; Nishihara, H. Inorg Chem
2001, 40, 4986.
meso-Tetrakis{3-(4-nitroazophenyl)-4-hydroxy-5-methoxy-
phenyl}porphyrin (2d). Yield 10.6%; mp > 300ꢀC; UV–vis:
lmax nm (deuteriochloroform) (log x): 360 (5.52), 430 (5.80),
525 (4.97), 560 (4.69), 595 (4.61), 660 (4.50); IR (potassium
bromide): 3422 (broad, N-H and O-H str of porphyrin), 1592,
1423, 1493, 1402 (C═C, C═N in plane bend), 970, 924
(porphyrin macrocyclic bend), 855, 798 (s, aromatic C═C-H
1
out-of-plane bend)cmÀ1; H-NMR (deuteriochloroform): d 13.72
[8] Tsuchiya, S. J Am Chem Soc 1999, 121, 48.
(s, 4H, 4Â OH), 9.06 (s, 8H, pyrrole C-H), 8.54–8.52 (d, 8H, C3
and C5-H of azophenyl), 8.44 (s, 4H, C2-H), 7.90 (s, 4H, C6-H),
7.86–7.84 (d, 8H, C2 and C6 azophenyl-H), 4.06 (s, 12H, 4Â
O-CH3), 2.62 (s, 2H, porphyrin N-H). FABMS: m/z 1396
(M+ + 2) requires 1394. Anal. Calcd for C72H50O16N16: C, 61.98;
H, 3.58; N, 16.06. Found: C, 61.82; H, 3.51; N, 15.92%.
[9] Liu, X. G.; Feng, Y. Q.; Zhao, Y.; Chen, H. L.; Li, X. G. Dyes
& Pigments 2007, 75, 413.
[10] Hunter, C. A.; Sarson, L. D. Tetrahedron Letts 1996, 37, 699.
[11] Yamamura, T.; Momotake, A.; Arai, T. Tetrahedron Letts
2004, 45, 9219.
[12] Reddy, D. R.; Maiya, B. G. J Phys Chem A 2003, 107, 6326.
(b) Reddy, D. R.; Maiya, B. G. Chem Commun 2001, 37, 117.
[13] Peters, M. V.; Goddard, R.; Hecht, S. J Org Chem 2006, 71,
7846.
Acknowledgments. One of the authors (M. Amaravathi)
gratefully acknowledges the Department of Science and
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Featured Compounds
1
1a
1b
Compound Details
Structure Search
Structure Search
Structure Search
Compound Details
Structure Search
Structure Search
Structure Search
Compound Details
Structure Search
Structure Search
Structure Search
1c
1d
2
Compound Details
Compound Details
Compound Details
2a
2b
2c
Compound Details
Compound Details
Compound Details
FC-1

Featured Compounds
2d
Compound Details
Structure Search
FC-2