One-pot synthesis of β-carboxy tetra aryl porphyrins: potential applications to dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tetlet.2010.03.088

A facile one-pot conversion of 2-formyl-5,10,15,20-tetra aryl-substituted porphyrins and their zinc derivatives to the corresponding 2-carboxy-5,10,15,20-tetra aryl-substituted porphyrins was achieved for the first time by using hydroxylamine hydrochloride and phthalic anhydride. All these substituted carboxy porphyrins were completely characterized by using mass, CHN analysis, ¹H NMR, UV-vis, Fluorescence spectroscopies, and cyclic voltammetry. Both the absorption maxima and emission maxima were red shifted by 5-7 nm. The LUMO of these porphyrins are above the TiO₂ conduction band and HOMO was below the redox electrolytes. These carboxy porphyrins are potential applications as sensitizers to dye-sensitized solar cell.

Full text of DOI:10.1016/j.tetlet.2010.03.088

Tetrahedron Letters 51 (2010) 2865–2867
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
One-pot synthesis of b-carboxy tetra aryl porphyrins: potential applications to
dye-sensitized solar cells
*
P. Silviya Reeta, Jaipal Kandhadi, Giribabu Lingamallu
Nanomaterials Laboratory, Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 607, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile one-pot conversion of 2-formyl-5,10,15,20-tetra aryl-substituted porphyrins and their zinc deriv-
atives to the corresponding 2-carboxy-5,10,15,20-tetra aryl-substituted porphyrins was achieved for the
first time by using hydroxylamine hydrochloride and phthalic anhydride. All these substituted carboxy
porphyrins were completely characterized by using mass, CHN analysis, 1H NMR, UV–vis, Fluorescence
spectroscopies, and cyclic voltammetry. Both the absorption maxima and emission maxima were red
shifted by 5–7 nm. The LUMO of these porphyrins are above the TiO2 conduction band and HOMO was
below the redox electrolytes. These carboxy porphyrins are potential applications as sensitizers to dye-
sensitized solar cell.
Received 16 February 2010
Revised 17 March 2010
Accepted 19 March 2010
Available online 27 March 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Porphyrins and their metal complexes are involved in wide-
spread applications that include self-assembly, sensors, liquid
crystalline materials, non-linear optical studies, light emitting
diodes, and as sensitizers in dye-sensitized solar cells (DSCs) for
the conversion of sunlight into electricity.1 Of these, conversion
of sunlight into electricity using photovoltaic devices is particu-
larly interesting. In this regard, DSC is currently attracting para-
mount importance because of their low cost, easy processing,
and high conversion efficiency than the conventional solid-state
photovoltaic devices.2 In these cells, dye sensitizer is one of the
key components for high power conversion efficiencies. The most
successful charge-transfer sensitizers employed so far in such
cells are bis(tetrabutylammonium)-cis-di(thiocyanato)-N,N0-bis(4-
carboxylato-40-carboxylic acid-2,20-bipyridine)ruthenium(II) (the
N719 dye) and trithiocyanato 4,40400-tricaboxy-2,20:60,200-terpyri-
dine ruthenium(II) (the black dye) with an efficiency of >12%.3 In
spite of this, the main drawbacks of these sensitizers are the lack
of absorption in the red region of the visible spectrum as well as
rarity of the metal in the earth crust. As Ru dyes are exclusive, me-
tal-free organic molecules, tetrapyrrolic compounds such as por-
phyrins and phthalocyanines are potential alternative sensitizers
for DSC applications based on their thermal as well as electronic
properties.4
rins remained around 3%. The ability to modify and tune the photo-
physical properties of synthetic porphyrins via the introduction of
specific substituents either at meso- or at pyrrole-b position/s in
order to further improve the efficiency of porphyrin-based DSC
devices is being explored. Imahori and co-workers have used quin-
oxaline-fused porphyrins at pyrrole-b position containing either
one or two carboxylic-anchoring groups and achieved the effi-
ciency of 5.2%.6 Officer and co-workers have reported a combina-
tion of conjugated ethenyl or diethenyl linker in the pyrrole-b
position and a carboxylate-binding group gives the device efficien-
cies up to 7.1%.7 All these porphyrins have carboxylic anchoring
groups present on the linker (either on extended-p conjugation
or extended aromatic ring) and are not directly connected to pyr-
role-b position. If the anchoring carboxylic group is directly con-
nected to pyrrole-b position, then the porphyrin in excited state
can efficiently inject electron into TiO2 conduction band and it will
improve the efficiency as well as durability of device. To the best of
our knowledge such type of molecules have not been reported in
the literature so far. Here in, we report the synthesis and character-
ization of free-base and zinc tetraaryl substituted porphyrins hav-
ing carboxylic acid group directly connected to pyrrole-b position.
The 2-formyl-5,10,15,20-tetra aryl-substituted porphyrins and
their zinc derivatives were synthesized as per the literature proce-
dure.8 The oxidation of 2-formyl tetra unsubstituted porphyrins to
the corresponding carboxylic acid has been achieved by the use of
a mixture of chromium trioxide and sulfuric acid.9 Officer and
co-workers have applied the same reaction conditions to the
2-formyl-5,10,15,20-tetra aryl-substituted porphyrins and failed
to convert into corresponding carboxylic group even after pro-
longed heating.8 Other strong oxidizing agents KMnO4, m-chloro-
perbenzoic acid, and Corey’s cyanohydrins oxidation conditions
have also failed and led to a very complex mixture of products.10
Plants and bacteria capture solar energy using porphyrin-based
chromophores for converting it into chemical energy. Some metal-
loporphyrins have been tested for the photosensitization of
wideband-gap semiconductors, the most common being either
free-base or its zinc derivative of the meso-benzoic acid substituted
porphyrins.5 However, the efficiency of meso-substituted porphy-
* Corresponding author. Tel.: +91 40 27193186; fax: +91 40 27160921.
E-mail address: giribabu@iict.res.in (G. Lingamallu).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.088

2866
P. Silviya Reeta et al. / Tetrahedron Letters 51 (2010) 2865–2867
R
R
CHO
COOH
N
N
N
N
N
NH2OH.HCl/N(C2H5)3
Phthalic anhydride
N
N
R
M
R
M
R
R
N
R
R
R = H and M = 2H, 1; R = H and M = Zn(II), Zn1
R = -CH3 and M = 2H, 2; R = -CH3 and M = Zn(II), Zn2
R = -OCH3 and M = 2H, 3; R = -OCH3 and M = Zn(II), Zn3
R = -tert-butyl and M = 2H, 4; R = -tert-butyl and M = Zn(II), Zn4
Scheme 1. Synthesis of b-carboxy tetra aryl porphyrins.
into their corresponding carboxylic acids by using phthalic acid un-
der solvent-free condition.12 We have not used aq ammonia in this
reaction and by refluxing the reaction mixture for a prolonged
time, the formyl porphyrin converted to corresponding carboxylic
porphyrin. Initially, we have used this one-pot method for the con-
version of 2-formyl-5,10,15,20-tetra phenyl porphyrin (1) to 2-car-
boxy-5,10,15,20-tertra phenyl porphyrin (80%).13 We have
extended this methodology to other tetra aryl-substituted porphy-
rins having electron-releasing groups on phenyl ring, viz. –CH3,
–OCH3, –tert-butyl groups, and their zinc derivatives. Both free-
base and their zinc tetra aryl-substituted carboxy porphyrins were
obtained in >80% yield. All these 2-carboxy tetra aryl-substituted
porphyrins were completely characterized by mass, CHN analysis,
IR, 1H NMR, UV–vis, Fluorescence spectroscopy, and cyclic voltam-
metry (see Fig. 1).
The absorption spectra of free-base and zinc carboxy porphyrin
were measured in CH2Cl2 solvent. The absorption spectra of all
free-base carboxy porphyrins have an intense Soret band and four
less intense Q-bands whereas its zinc derivatives have only two
less intense Q-bands and an intense Soret band like its parent tet-
raaryl porphyrin. However, the absorption maxima of both Soret
and Q-bands red shifted by 5–7 nm, probably due to the presence
of electron-withdrawing carboxyl group. The emission spectra of
both free-base and its zinc derivatives of carboxy porphyrins were
measured in CH2Cl2 solvent. The emission maxima of all 2-carboxy
porphyrins are red shifted by 5–7 nm when compared to those of
its corresponding parent tetraaryl porphyrins. The singlet excited
state (E0-0) of free-base carboxy porphyrins is ranging from 1.85
to 1.90 0.5 eV and that of its zinc derivatives is from 2.03 to
2.09 0.05 eV. The redox potentials of carboxy porphyrins were
Figure 1. Absorption (___) and emission (——) spectra of Zn1 in CH2Cl2.
The exact reason is not clear and this might be due to the steric
hindrance of aryl groups.8
We have adopted an one-pot, mild reaction condition using
hydroxylamine hydrochloride and phthalic anhydride for the con-
version of 2-formyl-5,10,15,20-tetra aryl-substituted porphyrins to
the corresponding carboxylic acid porphyrins (Scheme 1). By using
this method, Wang and Lin have converted simple alky or aryl
aldehydes to the corresponding nitriles in good yields.11 Aq Ammo-
nia has been used to quench the reaction at nitrile stage. Whereas
in Mathew’s dry hydrolysis reaction, the nitriles were converted
Table 1
UV–vis, emission and electronchemical dataa
, Mꢀ1 cmꢀ1
)
kem
,
c, nm (u)
E1/2
E0-0 (eV)
d
e
Compound
kmaxb, nm (log
e
max
Ox
Red
1
2
3
4
Zn1
Zn2
Zn3
Zn4
424 (5.77), 520 (4.28), 557 (3.90), 598 (3.78), 654 (3.65)
426 (5.64), 523 (4.51), 559 (4.18), 600 (4.04), 661 (3.90)
427 (5.53), 524 (4.51), 561 (4.17), 600 (4.00), 659 (3.91)
431 (5.10), 526 (4.32), 563 (4.11), 602 (3.97), 662 (.3.89)
420 (5.88), 554 (4.38), 593 (3.95)
428 (5.79), 556 (4.45), 594 (4.20)
431 (5.42), 559 (4.38), 600 (3.97)
429 (5.67), 557 (4.42), 594 (4.20)
663, 725 (0.09)
669, 728 (0.10)
675 (0.07)
668, 727 (0.09)
595, 644 (0.041)
606, 658 (0.043)
610, 660 (0.038)
619, 662 (0.047)
0.92
0.94
0.90
0.88
0.79
0.77
0.75
0.74
ꢀ1.24
ꢀ1.27
ꢀ1.28
ꢀ1.30
ꢀ1.36
ꢀ1.40
ꢀ1.39
ꢀ1.41
1.90
1.86
1.87
1.85
2.09
2.06
2.06
2.03
a
b
c
Solvent: CH2Cl2.
Error limits, kmax 1 nm, log
u, 10%.
e, 10%.
d
e
0.1 M TBAP, reference electrode: standard calomel electrode, working: glassy carbon, auxiliary electrode: Pt wire, error limits: E1/2
Error limits, 0.05 eV.
, 0.03 V.

P. Silviya Reeta et al. / Tetrahedron Letters 51 (2010) 2865–2867
2867
5. (a) Campbell, W. M.; Burrell, A. K.; Officer, D. L.; Jolley, K. W. Coord. Chem. Rev.
2004, 248, 1363; (b) Jasieniak, J.; Johnston, M.; Waclawick, E. R. J. Phys. Chem. B
2004, 108, 12962.
6. Eu, S.; Hayashi, S.; Umeyama, T.; Matano, Y.; Araki, Y.; Imahori, H. J. Phys. Chem.
C 2008, 112, 4396.
7. Campbell, W. M.; Jolley, K. W.; Wagner, P.; Wagner, K.; Walsh, P. J.; Gordon, K.
C.; Schmidt-Mende, L.; Nazeeruddin, M. K.; Wang, Q.; Gratzel, M.; Officer, D. L.
J. Phys. Chem. C 2007, 111, 11760.
8. Bonfantini, E. E.; Burrell, A. K.; Campbell, W. M.; Crossley, M. J.; Gosper, I. J.;
Hardin, M. M.; Officer, D. L.; Reid, D. C. W. J. Porphyrins Phthalocyanines 2002, 6,
748.
measured using cyclic voltammetry technique. It has two revers-
ible or quasireversible reductions and oxidations. From Table 1, it
is clear that the reduction potential of all new carboxy porphyrins
are below the reduction of potentials of redox electrolyte (Iꢀ/I3ꢀ).14
The dye easily regenerates by taking electroꢁn from redox electro-
lyte. The excited state oxidation potentials ðEoxÞ of these sensitizers
which corresponds to the LUMO were found to be above the TiO2
conduction band and the electron in the excited state can effi-
ciently inject into the conduction band (ꢀ0.8 V vs SCE).15 The pho-
tovoltaic device studies by using these sensitizers are currently
under progress.
9. Clezy, P. S.; Barrett, J. Biochem. J. 1961, 78, 798.
10. (a) Corey, E. J.; Gilman, N. W.; Canem, B. E. J. Am. Chem. Soc. 1968, 90, 5616; (b)
Godfrey, I. M.; Sargent, M. V.; Elix, J. A. J. Chem. Soc., Perkin Trans. 1 1974, 1353.
11. Wang, E.-C.; Lin, G.-J. Tetrahedron Lett. 1998, 39, 4047.
In conclusion, we have used one-pot mild reaction conditions
for the oxidation of 2-formyl-5,10,15,20-tetra aryl-substituted por-
phyrin to the corresponding 2-carboxy-5,10,15,20-tetra aryl-
substituted porphyrin. From the singlet excited state and reduction
potential, it is clear that the excited electron can be easily injected
into the TiO2 conduction and the oxidized state of dye can easily
get reduced by taking electron from redox electrolyte. These sensi-
tizers are potential applications to dye-sensitized solar cells.
12. (a) Mathews, J. A. J. Am. Chem. Soc. 1898, 20, 648; (b) Mathews, J. A. J. Am. Chem.
Soc. 1896, 18, 679.
13. Hydroxylamine hydrochloride (0.38 g, 5.5 mmol), triethyl amine (0.55 g,
5.5 mmol), and the corresponding 2-formyl-5,10,15,20-tetra aryl substituted
porphyrin (5 mmol) were added in 50 ml of dry acetonitrile under nitrogen
atmosphere. The resulting reaction mixture was stirred for 30 min. To this
phthalic anhydride (0.063 g, 0.5 mmol) was added and the reaction mixture
was refluxed for 7 h. The solvent was removed under reduced pressure and the
solid material was obtained. The obtained solid material was subjected to silica
gel column chromatography and eluted with CHCl3/CH3OH (95:5) mixture and
the purple color band was collected and recrystallized from CH2Cl2/hexane
mixture. Analytical data: Compound 1: Yield: 80% (2.63 g). Anal. Calcd for
C45H30N4O2: C, 82.05; H, 4.59; N, 8.51%. Found: C, 82.00; H, 4.55; N, 8.53%. MS:
m/z 658, requires 658.75. 1H NMR (CDCl3) (300 MHz) d ppm: 9.00 (s, 1H), 8.85
(m, 6H), 8.15 (m, 8H), 7.70 (m, 12H) ꢀ2.71 (b, 2H). Compound 2: Yield: 84%
(2.99 g). Anal. Calcd for C49H38N4O2: C, 82.33; H, 5.36; N, 7.84%. Found: C,
82.60; H, 5.05; N, 7.90%. MS: m/z 714, requires 712.85. 1H NMR (CDCl3)
(300 MHz) d ppm: 9.02 (s, 1H), 8.85 (m, 6H), 8.15 (m, 8H), 7.75 (m, 8H), 2.60 (s,
12H), ꢀ2.71 (b, 2H). Compound 3: Yield: 85% (3.31 g). Anal. Calcd for
C49H38N4O6: C, 75.56; H, 4.92; N, 7.19%. Found: C, 75.60; H, 4.95; N, 7.15%.
MS: m/z 778, requires 778.85. 1H NMR (CDCl3) (300 MHz) d ppm: 9.00 (s, 1H),
8.85 (m, 6H), 8.15 (m, 8H), 7.70 (m, 12H), 3.73 (s, 12H) ꢀ2.71 (b, 2H).
Compound 4: Yield: 87% (3.75 g). Anal. Calcd for C61H62N4O2: C, 82.96; H, 7.08;
N, 6.34%. Found: C, 83.00; H, 7.10; N, 6.30%. MS: m/z 882, requires 883.12. 1H
NMR (CDCl3) (300 MHz) d ppm: 9.05 (s, 1H), 8.80 (m, 6H), 8.18 (m, 8H), 7.75
(m, 8H), 1.62 (s, 36H), ꢀ2.65 (b, 2H). Compound Zn1: Yield: 91% (3.28 g). Anal.
Calcd for C45H28N4O2Zn: C, 74.85; H, 3.91; N, 7.76%. Found: C, 74.81; H, 3.89; N,
7.75%. MS: m/z 720, requires 720.15. 1H NMR (CDCl3) (300 MHz) d ppm: 8.95
(m, 7H), 8.18 (m, 6H), 7.95 (d, 2H), 7.75 (m, 12H). Compound Zn2: Yield: 89%
(3.45 g). Anal. Calcd for C49H36N4O2Zn: C, 75.62; H, 4.66; N, 7.20%. Found: C,
75.60; H, 4.70; N, 7.15%. MS: m/z 776, requires 776.21. 1H NMR (CDCl3)
(300 MHz) d ppm: 8.95 (m, 7H), 8.15 (m, 6H), 7.95 (d, 2H), 7.60 (m, 12H), 2.72
(s, 12H). Compound Zn3: Yield: 91% (3.82 g). Anal. Calcd for C49H36N4O6Zn: C,
69.88; H, 4.31; N, 6.65%. Found: C, 69.90; H, 4.30; N, 6.65%. MS: m/z 840,
requires 840.19. 1H NMR (CDCl3) (300 MHz) d ppm: 8.95 (m, 7H), 8.18 (m, 6H),
7.95 (d, 2H), 7.70 (m, 12H), 3.76 (s, 12H). Compound Zn4: Yield: 90% (4.25 g).
Anal. Calcd for C61H60N4O2Zn: C, 77.40; H, 6.39; N, 5.92%. Found: C, 77.35; H,
6.40; N, 5.90%. MS: m/z 945, requires 945.19. 1H NMR (CDCl3) (300 MHz) d
ppm: 8.95 (m, 7H), 8.18 (m, 6H), 7.95 (d, 2H), 7.75 (m, 12H), 1.80 (s, 36H).
14. Giribabu, L.; Chandrasekharam, M.; Kantam, M. L.; Reddy, V. G.; Satyanarayana,
D.; Rao, O. S.; Reddy, P. Y. Ind. J. Chem. 2006, 45A, 629.
Acknowledgments
We are thankful to the projects GAP-0220 and SR/S1/IC21/2008
for financial support of this work.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.03.088.
References and notes
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15. aꢁThe excited state oxidation potentials was calculated by using the equation,
ox
1=2
Eox ¼ E ꢀ E0-0. It was found that ꢀ0.98, ꢀ0.88, ꢀ0.97 and ꢀ0.97 V for 1, 2, 3
and 4, respectively in case of free-base porphyrins whereas in case of zinc
derivatives ꢀ1.30, ꢀ1.31, ꢀ1.31 and 1.29 V for Zn1, Zn2, Zn3 and Zn4,
respectively.; (b) Li, C.; Zhou, J.-H.; Chen, J.-R.; Chen, Y.-S.; Zhang, X.-H.; Ding,
H.-Y.; Wang, W.-B.; Wang, X.-S.; Zhang, B.-W. Chin. J. Chem. 2006, 24, 537.