Facile synthesis, characterization, and fluorescence studies of novel porphyrin appended thiazoles

Article abstract of DOI:10.1002/jhet.1009

A facile and high yielding synthesis of porphyrin appended thiazoles from the reaction of 5-(4-thiocarboxamidophenyl)-10,15,20-triphenylporphyrin with α-bromo ketones has been described. The fluorescence studies of synthesized porphyrin appended thiazoles in chloroform indicate that porphyrin π system is not greatly perturbed by substitution of a thiazole moiety at meso-phenyl ring even in the excited state.

Full text of DOI:10.1002/jhet.1009

January 2013
Facile Synthesis, Characterization, and Fluorescence Studies of Novel
Porphyrin Appended Thiazoles
125
a
a
a
b
b
*
Bhupendra Mishra, K. P. Chandra Shekar, Anil Kumar, S. Phukan, S. Mitra,
and Dalip Kumara
aChemistry Group, Birla Institute of Technology and Science, Pilani 333 031, India
bDepartment of Chemistry, North-Eastern Hill University, Permanent Campus, Umshing, Shillong 793 022, India
*
E-mail: anilkadian@gmail.com
Received August 3, 2010
DOI 10.1002/jhet.1009
View this article online at wileyonlinelibrary.com.
A facile and high yielding synthesis of porphyrin appended thiazoles 5 from the reaction of 5-(4-thiocarbox-
amidophenyl)-10,15,20-triphenylporphyrin with a-bromo ketones has been described. The fluorescence studies
of synthesized porphyrin appended thiazoles 5 in chloroform indicate that porphyrin p system is not greatly
perturbed by substitution of a thiazole moiety at meso-phenyl ring even in the excited state.
J. Heterocyclic Chem., 50, 125 (2013).
INTRODUCTION
preliminary results on fluorescence properties of novel por-
phyrin appended thiazoles (Scheme 1).
Porphyrins are tetrapyrrolic heterocyclic molecules that
play important role in many biological processes such as
light harvesting [1], oxygen transport [2], metabolism,
and catalysis [3]. In recent years, there has been a growing
interest in the use of porphyrins and related compounds as
therapeutic drugs, especially as photosensitizers in photo-
dynamic therapy of cancer [4]. The presence of a conju-
gated double-bond system in the tetrapyrrole nucleus is
the structural feature responsible for the strong and charac-
teristic absorption and fluorescence [5]. Biological effects
of porphyrins largely depend on their physicochemical
properties, which in turn lead to important changes in their
photophysical behavior [6]. The photochemical properties
of porphyrins are largely dependent on the nature of substi-
tuents and the type of metal in the porphyrin cavity and can
be tuned appropriately by modification at b-pyrrolic and
peripheral positions [7]. As a result, functional group
interconversion and synthetic transformations related to
porphyrin chemistry are continuously being explored and
improved. In literature, there are various methods available
for the synthesis of porphyrins molecules [8]; however,
methods for structural modification of porphyrin molecules
are rare. Thus, development of facile, efficient, and versatile
synthetic strategy to functionalize porphyrin molecules with
appended heterocycles is of high interest to broaden their
scope of utilizations. In continuation of our efforts toward
the development of novel porphyrin molecules for anticancer
agents, herein we report the synthesis, characterization, and
RESULTS AND DISCUSSION
5-(4-Carbomethoxyphenyl)-10,15,20-triphenyporphyrin 1
was prepared by condensation of benzaldehyde, 4-carboxy-
methylbenzaldehyde, and pyrrole under Adler–Longo [9]
conditions followed by the hydrolysis and reaction with
thionyl chloride and ammonia as shown in Scheme 1.
Porphyrin thiamide 3 was obtained from the reaction of
5-(4-carboxamido-phenyl)-10,15,ꢀ20-triphenylporphyrin
2
with Lawesson’s reagent at 100 C in toluene. Presence of
a characteristic peak at 1618 cmꢁ1 in IR and a peak at
m/z 674 in ESI–MS confirmed the structure of 3. Porphyrin
thiazoles (5a–g) were prepared by the reaction of 3 with
appropriate a-bromoketones (4a–g) in ethanol under
refluxing conditions in high yields (75–85%) (Table 1,
Scheme 2).
The porphyrin thiazoles (5a–g) showed [M+ H]+ ion peak
in the mass spectra. Along with other peripheral aryl and
b-pyrrolic protons, a singlet peak appeared in 1H NMR spec-
tra at around dH 7.50–7.58 corresponding to thiazole H-500
for 5a–d and 5g, whereas this singlet was shifted downfield
at dH 8.24 and 8.64, respectively for 5e and 5f.
All the synthesized prophyrins are readily soluble in
CHCl3 displaying a very sharp and intense B (Soret) band
centered at 420 nm, accompanied by four weaker Q bands
(515, 550, 590, and 645 nm) and a shoulder at around
© 2013 HeteroCorporation

126
B. Mishra, K. P. C. Shekar, A. Kumar, S. Phukan, S. Mitra, and D. Kumar
Vol 50
Scheme 1. Reagents and conditions: (i) KOH, MeOH: H2O, 90 ꢀC, 30 h, 82%; (ii) SOCl2, toluene, 110 ꢀC, 1 h; (iii) NH3 (g), CH2Cl2, 0–25 ꢀC, 45 min,
85%; and (iv) Lawesson’s reagent, toluene, 100 ꢀC, 1 h, 85%.
400nm in the absorption spectra (Fig. 1). Excitation of the
Table 1
samples at 515 nm gives two fluorescence peaks in all cases,
namely at 648 and 710 nm (Fig. 2). All bands are of p!p*
Synthesis of porphyrin thiazoles 5a–f.
0
1
type. The Soret band is due to an allowed 1A1g! Eu transi-
Sr. No.
R
Time (h)
Yield (%)
tion; and consequently, the intensity of this band is very high
[10]. The characteristic peak positions of absorption spec-
trum for individual porphyrin derivatives are given in Table 2
along with the molar absorption coefficient of each of them.
Very small shift in both the Soret band and Q band position
for the synthesized derivatives from the model compound
5,10,15,20-tetraphenylporphyrin (H2TPP) indicates little
ground state interaction between the porphyrin moiety and
the thiazole ring. Although the main fluorescence peak
position (ca. 648nm) is also insensitive to the substitution,
the other fluorescence band shows a blue-shift of ca.
2–6 nm from the model compound H2TPP (712 nm). This
shift is more prominent in 5e, where a strong electron
deficient pyridyl ring is involved. Table 2 also includes
the relative fluorescence yield of all the synthesized pro-
phyrin derivatives along with that of the model compound
H2TPP. It is seen that the fluorescence quantum yield (ff)
is reduced to almost half by the substitution.
5a
5b
5c
5d
5e
5f
C6H5
4-CH3OC6H4
4-BrC6H4
2-Thienyl
2-Pyridyl
2
3
2
4
4
8
8
82
80
85
80
79
77
75
3-Coumarinyl
3-Indolyl
5g
Scheme 2. Reagents and conditions: (i) RCOCH2Br (4a–g), EtOH, 80 ꢀC.
The most pronounced reduction in ff is again observed
for 5e, which may be because of facile electron transfer
from the excited state of porphyrin moiety to the strong
acceptor pyridyl ring. In porphyrin 5e, pyridyl ring is
5b
H2TPP
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.03
0.02
0.01
0.00
5a
5e
5d
5c
1000
800
600
400
200
0
500
550
600
650
Wavelength /nm
350
400
450
500
550
600
650
700
600
650
700
750
Wavelength /nm
Wavelength /nm
Figure 1. Absorption spectrum of 5b (line and symbol) along with the model
compound 5,10,15,20-tetraphenylporphyrin (H2TPP) (solid line) in chloro-
form. Inset shows the zoomed spectrum in the region of Q-bands for 5b.
Figure 2. Representative fluorescence emission spectra of some of the
synthesized porphyrin thiazoles 5 in chloroform (lexc. = 515 nm).
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

January 2013
Facile Synthesis, Characterization, and Fluorescence Studies of Novel
Porphyrin Appended Thiazoles
127
Table 2
Absorption and emission data of porphyrins 5a–g in chloroform.
(absorption) lmax nm (e/104 Mꢁ1 cmꢁ1
[QY(0,1) [QY(0,0) [QX(0,1)
)
(Emission) lmax nm
Quantum yield (Φf)
Compound
(Soret)
]
]
]
[QX(0,0)]
5a
5b
5c
5d
5e
5f
5g
H2TPP
420 (66.00)
420 (72.00)
420 (65.00)
420 (88.00)
420 (55.00)
420 (32.00)
420 (220.0)
418 (250.0)
516 (5.80)
516 (6.30)
516 (5.90)
516 (10.0)
516 (7.50)
516 (4.50)
516 (19.0)
515 (19.0)
552 (4.40)
552 (4.80)
552 (4.50)
552 (8.40)
551 (6.30)
551 (3.90)
552 (14.0)
550 (15.0)
590 (3.90)
592 (4.20)
591 (4.00)
591 (7.70)
590 (5.90)
594 (3.60)
591 (12.0)
592 (13.0)
646 (3.70)
646 (4.00)
647 (3.80)
643 (7.40)
647 (5.70)
644 (3.60)
647 (12.0)
647 (13.0)
648
648
648
648
648
649
648
648
706
710
710
710
712
706
707
712
0.048
0.053
0.040
0.039
0.040
0.025
0.047
0.110
H2TPP: 5,10,15,20-tetraphenylporphyrin.
(3ꢃ 50mL). The organic layers were collected, dried over
anhydrous Na2SO4 and then evaporated. The residue was purified
by column chromatography on silica gel using CHCl3-hexane
(7:3v/v) as eluent to afford 1 (second fraction in column) in 10%
yield (4.09 g). H NMR (CDCl3, 400MHz) d 9.19–8.49 (m, 8H),
8.37–8.34 (m, 2H), 8.24–8.09 (m, 6H), 7.93–7.60 (m, 9H),
connected with porphyrin through a linking bridge; it
would make up a “donor-space-acceptor” intramolecular
photoinduced electron transfer system. The fluorescence
of porphyrin is quenched by way of transfer of the excited
state electron from porphyrin to pyridyl ring through the
thiazole spacer. The reduced fluorescence quantum yields
compared with H2TPP indicate that there is distortion in
the electronic structure of porphyrin ring because of
appended heterocyclic thiazole ring on aryl group.
1
7.20–7.14 (m, 2H), 4.02 (s, 3H), ꢁ2.88 (s, 2H).
Preparation of 5-(4-carboxamidophenyl)-10,15,20-triphenyl-
porphyrin 2. To a stirred suspension of 5-(4-carbomethoxyphenyl)-
10,15,20-triphenyl-porphyrin (1) (200 mg, 0.297mmol) in methanol
(15 mL) was added KOH (30mL, 1N aqueous solution) and heated
under reflux for 30h. After completion of the reaction as indicated
by TLC, the mixture was cooled to room temperature; solid was
filtered, washed with water and 1N HCl solution. The residue so
obtained was dissolved in CHCl3: MeOH, dried over anhydrous
Na2SO4 and evaporated to dryness to afford 5-(4-carboxyphenyl)-
10,15,20-triphenylporphyrin as a purple solid in 82% yield.
EXPERIMENTAL
The aldehydes, pyrrole, and Yb(OTf)3 were purchased from
Sigma-Aldrich (India). All other reagents and solvents were pur-
chased from Merck (India) and Spectrochem, India Ltd. The synthe-
sized porphyrin derivatives (5a–g), H2TPP were characterized by
UV–vis and fluorescence spectroscopy. 1H NMR spectra were
recorded on Brucker Heaven 11400 (400 MHz), and mass spectra
were recorded on QSTAR Elite LX/MS/MS from applied biosys-
tems. Steady-state absorption spectra were recorded on a Perkin
Elmer Lambda25 absorption spectrophotometer. Fluorescence
spectra were taken in a Hitachi FL4500 spectrofluorimeter, and all
the spectra were corrected for the instrument response function.
Quartz cuvettes of 10 mm optical path length received from Perki-
nElmer, USA (part no. B0831009) and Hellma, Germany (type
111-QS) were used for measuring absorption and fluorescence spec-
tra, respectively. For fluorescence emission, the sample was excited at
515 nm, whereas excitation spectra were obtained by monitoring at
the respective emission maximum. In both the cases, 5 nm bandpass
was used in the excitation and emission side. Fluorescence quantum
yields (ff) were calculated by comparing the total fluorescence inten-
sity under the whole fluorescence spectral range by taking H2TPP as
standard. The relative experimental error of the measured quantum
yield was estimated within ꢂ5%.
5-(4-Carboxyphenyl)-10,15,20-triphenylporphyrin
(150 mg,
0.228mmol) was dissolved in dry toluene (15 mL) and added
thionyl chloride (200mL, 0.273mmol) and heated under reflux
for 1 h. After removal of solvent under vacuum, the greenish
residue obtained was dissolved in dry CH2Cl2 (10mL) and cooled
in an ice bath. In the cooled reaction mixture, ammonia gas was
purged until all acid chloride was consumed (10 min). The
mixture was stirred at room temperature for 30 min and diluted
with water (20 mL). The organic phase was separated, dried over
anhydrous Na2SO4, and distilled off to afford 2 as a purple solid
(125mg, 85%).
5-(4-Thiocarboxamidophenyl)-10,15,20-triphenylporphyrin
3. Lawesson’s reagent (70 mg, 0.175 mmol) was added to stirred
suspension of 2 (110mg, 0.167mmol) in dry toluene (25mL) and
heated the reaction mixture at 100 ꢀC for 1 h. After completion of
reaction, the solvent was evaporated under vacuum and the
residue so obtained was purified by column chromatography on
silica gel using CHCl3: MeOH (99:1v/v) as eluent to obtain 3 as
purple solid (95mg, 85%).
5-{40-(400-Phenyl)-200-thiazolyl}-10,15,20-triphenylporphyrin
5a. To a solution of 3 (50mg, 0.074mmol) in ethanol (5mL) was
added phenacyl bromide 4a (14 mg, 0.074 mmol) and the reaction
mixture was heated under reflux for 4 h. After completion of the
reaction, solvent was evaporated and the resulting mixture was
extracted with CHCl3, washed with water (2ꢃ 20 mL) and dried
over anhydrous Na2SO4. The solvent was removed under reduced
pressure on a rotary evaporator to give crude 5a, which was
5-(4-Carbomethoxyphenyl)-10,15,20-triphenylporphyrin 1.
To a refluxing solution of methyl 4-formylbenzoate (10 g,
61mmol) in propionic acid (280 mL) were added freshly distilled
benzaldehyde (17.8 g, 167 mmol) and pyrrole (22.25 g, 333mmol)
simultaneously at the same rates. The reaction mixture was heated
under reflux for 1 h. After completion of the reaction, the
propionic acid was distilled off under vacuum, the residue was
cooled to room temperature and basified to pH (~8) with saturated
sodium carbonate solution and extracted with chloroform
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

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B. Mishra, K. P. C. Shekar, A. Kumar, S. Phukan, S. Mitra, and D. Kumar
Vol 50
purified by column chromatography on silica gel using chloroform
as eluent. 5a: yield 82% (47mg); UV–vis [lmax, nm] in CHCl3:
NMR (CDCl3, 400 MHz) d = ꢁ2.76 (s, 2H, NH), 7.30–7.32
(m, 2H), 7.45–7.47 (m, 1H), 7.55 (s, 1H), 7.73–7.81 (m, 9H),
7.95 (d, J = 2.6 Hz, 1H), 8.22–8.25 (m, 7H), 8.32–8.34 (m, 3H),
8.48 (dd, J = 1.8 Hz, J = 6.4 Hz, 2H), 8.86–8.93 (m, 8H,
b-pyrrole); MS m/z calcd for C55H36N6S 812.2722, found
813.3018 (M + H)+.
1
420, 516, 552, 590, 646: H NMR (CDCl3, 400MHz) d = ꢁ2.77
(s, 2H, NH), 7.41–7.45 (m, 1H), 7.54 (t, J = 7.6Hz, 2H), 7.64
(s, 1H, 500-H thiazole), 7.73–7.79 (m, 9H), 8.12–8.14 (m, 2H),
8.21–8.24 (m, 6H), 8.34 (d, J = 8.2 Hz, 2H), 8.45 (d, J = 7.3Hz,
2H), 8.87–8.92 (m, 8H, b-pyrrole); MS m/z calcd for C53H35N5S
773.2613, found 774.2920 (M+ H)+.
Acknowledgments. The authors thank the Department of Science
and Technology, New Delhi for the financial support and SAIF,
Panjab University for providing analytical support.
5b was obtained in 80% yield (48 mg) starting from 3 and
2-bromo-1-(4-methoxyphenyl)ethanone 4b as described for 5a:.
1
UV–vis [lmax, nm] in CHCl3: 420, 516, 552, 592, 646; H NMR
(CDCl3, 400MHz) d = ꢁ2.77 (s, 2H, NH), 3.90 (s, 3H,–OCH3),
7.05 (d, J = 8.8Hz, 2H), 7.50 (s, 1H, 500-H thiazole), 7.74–7.79
(m, 9H), 8.07 (d, J = 8.9Hz, 2H), 8.21–8.24 (m, 6H), 8.33
(d, J =8.2 Hz, 2H), 8.44 (d, J = 7.4 Hz, 2H), 8.87–8.92, (m, 8H,
b-pyrrole); MS m/z calcd for C54H37N5OS 803.2719, found
804.3091 (M + H)+.
REFERENCES AND NOTES
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5c was obtained in 85% yield (48 mg) starting from 3 and
2-bromo-1-(4-bromophenyl)ethanone 4c as described for 5a;.
1
UV–vis [lmax, nm] in CHCl3: 420, 516, 552, 591, 647; H NMR
[5] (a) Wenga, Y. Q.; Tenga, Y. L.; Yuea, F.; Zhonga, Y. R.; Ye, B.
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(CDCl3, 400MHz) d = ꢁ2.77 (s, 2H, NH), 7.63–7.67 (m, 2H),
7.72–7.81 (m, 9H), 8.21–8.24 (m, 6H), 8.32–8.36 (m, 3H), 8.44
(d, J = 8.2 Hz, 2H), 8.72–8.74 (m, 2H), 8.86–8.91 (m, 8H,
b-pyrrole); MS m/z calcd for C53H34BrN5S 851.1718, found
852.4110 (M + H)+.
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5d was obtained in 80% yield (46mg) starting from 3 and
2-bromo-1-(thiophen-2-yl)ethanone 4d as described for 5a;.
1
UV–vis [lmax, nm] in CHCl3: 420, 516, 552, 591, 643; H NMR
(CDCl3, 400MHz) d =ꢁ2.76 (s, 2H, NH), 7.16 (dd, J = 3.9Hz,
J = 5.0Hz, 1H), 7.38 (dd, J = 5.2 Hz, J = 1.0Hz, 1H), 7.49 (s, 1H,
500-H thiazole), 7.64 (dd, J = 3.5Hz, J = 1.0Hz, 1H), 7.73–7.81
(m, 9H), 8.21–8.23 (m, 6H), 8.33 (d, J = 8.0Hz, 2H), 8.42
(d, J = 7.0Hz, 2H), 8.85–8.90 (m, 8H, b-pyrrole); MS m/z calcd
for C51H33N5S2 779.2177, found 780.2522 (M+ H)+.
5e was obtained in 79% yield (45 mg) starting from 3 and
2-bromo-1-(pyridin-2-yl)ethanone hydrobromide 4e as described
for 5a;. UV–vis [lmax, nm] in CHCl3: 420, 516, 551, 590, 647;
1H NMR (CDCl3, 400 MHz) d = ꢁ2.76 (s, 2H, NH), 7.29–7.32
(m, 1H), 7.73–7.81 (m, 9H), 7.89 (t, J = 7.7Hz, 1H), 8.21–8.25
(m, 7H), 8.34 (dd, J =1.7 Hz, J = 6.4 Hz, 2H), 8.45–8.41 (m, 3H),
8.69–8.71 (m, 1H), 8.86–8.91 (m, 8H, b-pyrrole): MS m/z calcd
for C52H34N6S, 774.2566, found 775.2920 (M+ H)+.
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5f was obtained in 77% yield (48 mg) starting from 3 and
3-(2-bromoacetyl)-2H-chromen-2-one 4f as described for 5a;.
1
UV–vis [lmax, nm] in CHCl3: 420, 516, 551, 594, 644; H NMR
(CDCl3, 400MHz) d = ꢁ2.76 (s, 2H, NH), 7.35–7.38 (m, 1H),
7.45 (d, J = 8.4 Hz, 1H), 7.57–7.58 (m, 1H), 7.74–7.81 (m, 10H),
8.22–8.24 (m, 6H), 8.37 (dd, J = 1.7Hz, J = 6.4 Hz, 2H), 8.47
(dd, J = 1.8Hz, J = 6.4Hz, 2H), 8.64 (s, 1H), 8.86–8.92 (m, 8H,
b-pyrrole), 9.05 (s, 1H); MS m/z calcd for C56H35N5O2S
841.2511, found 842.2730 (M+ H)+.
5g was obtained in 75% yield (45 mg) starting from 3 and
2-bromo-1-(1H-indol-3-yl)ethanone 4g as described for 5a;.
UV–vis [lmax, nm] in CHCl3: 420, 516, 552, 591, 647; 1H
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet