1. Indian Journal of Chemistry
Vol. 41A, September 2002, pp. 1860-1 863
Synthesis of single strapped 21-thia
tetraphenylporphyrin systems
P Boobalan, Iti Gupta & M Ravikanth'
Department of Chemistry, Indian Institute of Technology,
Powai, Mumbai 400076, India
I?eceived 8 October 2001; revised 14 March 2002
Synthesis and characterization of two single strapped 2 1-thia
tetraphenylporphyrins have been reported. The two adjacent
phenyls that are present at the meso position of 2 1-thia
tetraphenylporphyrin are linked by either rigid aromatic group or
flexible alkyl chain. Absorption and fluorescence studies indicated
that the rigid aromatic group induced more nonplanarity in the
porphyrin ring than flexible alkyl chain.
The OCCUlTence of porphyrin rings in nature as ligands
in such crucial and multiple roles as those spanned by
the heme proteins, chlorophyll, vitamin B12 etc
suggest that some advantages might be associated
with this macrocyclic structurel. One of the main
reasons for nature to choose porphyrin ligands for
doing such diverse functions in biology is that the
porphyrin macrocycle is conformationally tlexible
and can adopt a range of nonplanar conformations
needed for a variety of biological functions. The
X-ray structures solved for several biomolecules
support this observation I. For example, the domed
porphyrins present in several heme proteins involved
In oxygen transport, peroxide reduction and
disproportionating the mitochondrial electron
transport chain and drug metabolism. Nonplanar
conformational distortion of tetrapyrrole pigments of
photosynthetic reaction centres have been suggested
to control the photophysical properties. For a better
understanding of the biological role of nonplanar
porphyrins, many laboratories have embarked on
studies of synthetic nonplanar tetrapyrrole
I . 12 N I . . hmacrocyc es til recent years'. onp ananty In t e
porphyrin macrocycle has been induced by steric
constraints including the strapping, capping etc,
changing the central metal ion, ring reduction and
oxidation, core modification and crystal packing
effects I. Core modification of porphyrin rings by
introducing thiophene, furan, selenophene,
tellurophene in place of pyrrole leads to novel core
modified porphyrins which exhibit interesting
R M
8r 8r
- C*C- 2H 1H2 - H2
8r 8r
--(CH2m- 2H ~
properties in terms of both aromatic character and
their ability to stabilize metals in unusual oxidation
states1.3. The X-ray structures solved for thiaporphyrin
and its metal derivatives indicates that the three
pyrroles and thiophene are not in one plane and the
porphyrin is nonplanar:1a.b. Recently, we are exploring
core modified porphyrin chemistry and we have
prepared Iight-harvesti ng systems,la.d, B-substituted
porphyrins4" and water-soluble porphyrin s~r. This
paper reports the synthesis and characterisation of
single strapped 21-thia tetraphenylporphyrin systems
(Structure I). The two adjacent lIleso-phenyls of the
porphyrin ring are linked with either rigid aromatic
group or long flexible alkyl chain. IH NMR,
absorption and fluorescence studies indicates that the
rigid aromatic group induces more deformation in the
porphyrin ring than the flexible alkyl chain.
Experimental
Synthesis of 2J-tliiatefraphelzylporphyrin (1)
2,5-Bis(phenylhydroxymethyl)thiophene (500 mg,
1.51 mmol) and 2,3,5,6-tetrabromo-p-phenylene
dimethylenedioxybenzaldehyde (I g. 1.51 mmol)
were dissolved in 250 ml propionic acid. Pyrrole
(0.350 ml, 5.04 mmol) was added and the reaction
mixture was then refluxed for 2 h. The reaction

2. ----
/
,
NOTES 186 1
mixture was cooled to room temperature and left
aside for overnight. The bl ack crude compound was
washed several times with hot water and dried. The
crude solid was dissolved in CH2C12 and slurry was
prepared with silica gel. Thi s was chromatographed
over silica gel (60-120 mesh) to remove
nonporphyrinic materials from mi xture of porphyri ns.
The porphyrin mixture which contains atleast four
other porphyrins along with the desired porphyrin was
subjected to silica gel chromatography using
petroleum etheriCl-I2C12 (7 :3) and eluted slowly. The
first band was tetrapheny l-2 1,23-dithiaporphyrin
(S2TPP) which was collected. The required strapped
2 1-monothiatetraphenylporphyrin, 1 was moved as
second band which was collected and evaporated.
This was rechromatographed on siIica gel using the
same solvent mixture to afford the pure compound.
(60 mg, 3.6%). 11-1 NMR (COCll , 8 in ppm) -2.90 (s,
11-1, NI-I), 4.95 (d, 21-1, OCI-I2), 5.95 (d, 21-1, OCI-I2),
7048 (m, 21-1, Ar), 7.65 (d, 2H, Ar), 7.79 (m, 61-1, Ar),
8.05 (m, 21-1, Ar), 8.29 (m, 41-1, Ar), 8.54 (s, 21-1 , ~­
py), 8.59 (d, 2H, ~-py), 8.65 (m, 21-1 , ~-py), 9.72 (s,
21-1, ~-thi ophene) . FAB-MS CS2 1-1 3 IN lS02 Br~ calc. av
mass 1081.52, obsd. mlz 108 1 (M+). Anal. caled: C,
57.7; 1-1, 2.89; N, 3.89. Found: C, 58.6; 1-1, 2.92; N,
3.96. UV-vis O"maxinm, EI I03
dm.1mor 1
cm-
l
) 431
( 111.0), 513 ( 11.0), 545 (2.9), 6 16 ( 104), 677 ( 1.8).
Fluorescence O.cx = 5 15 nm, Am"x) 681, 753 (q) =
0.0089). UV-vi s (Am"xinm, EII0
3
dm
3
mol-
1
cm-
l
) for
11-13
2
+ 457(74.0), 439(sh) (50.0), 551 (3.80), 603
(3.70), 669 (2.80).
Syllthesis of 21-thiatetraphellylporphyrill (2)
2,5-Bis(phenylhydroxymethyl)thiophene ( 100 mg,
0.337 mmol), 2,2' ( I ,6-hexyIdioxy-benzaldehyde)
( 135 mg, 00404 mmol), and pyrrole (0.062 ml, 0.895
mmol ) were condensed in 25 ml propionic acid
followed by puri fication under simi lar col umn
chromatographic conditions mentioned for the
synthesis of 1 yiclded the required 2 (10 mg, 4%). 11-1
NMR (COCI3, 8 in ppm) - 2.63 (s, 11-1, NI-I), 1.25 (s,
81-1, CI-I2), 3.78 (m, 4H, OCI-I2), 7042 (m, 21-1, Ar),
7.78 (m, 81-1, Ar), 8. 13 (m, 21-1, -Ar), 8.25 (m, 41-1, Ar),
8.65 (m, 41-1, ~-py), 8.83 (d, 21-1, ~-py), 9.71 (s, 21-1, ~­
th iophene). FAB-MS Cso1-l3<)N3S02 calc. av mass
745.73, obsd. mlz 746 (M+). Anal. caled: C, 80.5; 1-1,
5.27; N, 5.63. Found: C, 8 1.6; 1-1, 5.92; N, 5.71 . UV-
vis (Alllaxinm, Ell 03
dm3
mol-1em- I) 430 ( 124.0), 516
( 14.2), 550 (3.9), 615 (2.3), 676 (3.7). Fluorescence
(Aex =515 nm, Amax) 680, 752 (</> =0.0124).
H&O 8r 8r Hyo
r o- c t}-c-o~ + 3 o~+ 1 '" I H;'}---( H, V N
Sr Br H
Reflux 2 h
~-!J + Three N4Porphynns
o 1
STPPH
I: 'hca Gel Column Chromatography
l
Schcme I- Synlhelic scheme for the prcparalion of I.
Results and discussion
The single strapped 2 1-monothiaporphyrins, 1 and
2 were prepared as outlined in Scheme I. The required
precursors thiophene diol
s
and dialdehydes
6
were
prepared by the literature procedure. Condensation of
one equivalent of thiophene diol, one equi valent of
dialdehyde and three equi valents of pyrrole in
propionic acid resulted in the formation of mi xture of
porphyrins. The porphyrin mi xture was expected to
contain four other porphyrins along with the desired
porphyrin 1 or 2. The TLC analysis showed an
indication of formation of only two major compounds
and two minor compounds. S2TPP was the maj or
component along with the desired porphyrin and two
N~ porphyrins were formed in tracc amounts. Aftcr
doing the filtration column on silica gel with C1-I2C12
to remove the nonporphyrinic impurities, the mi xture
of porphyrins were loaded on silica gel column and
eluted slowly with pet etherldichloromethane mi xture
(7:3). The desired porphyrins 1 and 2 were moved as
a second band. Both 1 and 2 were characterized by 11-1
NMR, FAB mass, absorption and emission
spectroscopies. In 11-1 NMR, both 1 and 2 showed a
singlet for thiophene protons as in parent STPPI-I .
However, the splitting of pyrrole signals was varied in

3. 1862 INDIAN J CHEM, SEC A, SEPTEMBER 2002
both 1 and 2 compared to STPPH. All three pyrrole
signals in STPPH appeared as doublets and the
pyrrole which was opposite to thiophene experienced
more down field shift compared to the other two
pyrroles7
. In 1, the pyrrole which was opposite to
thiophene experienced more upfield shift indicating
that the 2,3,5,6-tetrabromo xylene strap inducing the
nonplanarity in the porphyrin ring which results in the
shift of pyrrole protons due to change in the ring
current. The other two pyrroles also showed little
upfield shift and complex multiplet unlike the parent
STPPH. However, the hexyl strap in 2 didn't induce
much structural change in the porphyrin as evidenced
in no shift of pyrrole protons compared to STPPH.
However, unlike doublets for pyrroles in STPPH, the
pyrroles in 2 appeared as complex multiplet. The NH
proton of 1 also experienced upfield shift compared to
STPPH and 2 indicating that the 2,3,5,6-tetrabromo
p-xylene strap altering the structure of the porphyrin.
The bridging -OCH2 protons in 1 appeared as two
doublets unlike the singlet observed in corresponding
dialdehyde suggesting that they are inequivalent in 1.
The - OCH2 protons also experienced shi fts due to
ring current effect of the porphyrin ri ng. Thus, the' H
NMR studies indicates that the structure of 1 is more
altered due to rigid aromatic strap than 2. The FAS
mass showed molecular ion peak at 1081 for 1 and
746 for 2 confirming the products.
The absorption spectra of 1 and its dication, 1H,2+
was recorded in toluene. It is seen that both 1 and 2
show four Q-bands and one Soret band with no shifts
in the peak positions W.r.t. STPPH. However, the
extinction coefficients of all bands were drastically
reduced compared to that of STPPH supporti ng the
change of structure in the porphyrin ring caused by
the bridging groups. The change was more
pronounced with 2,3,5,6-tetrabromo p-xylyl strap
indicating more nonplanarity in the porphyrin ring of
1 than in 2. The addition of trifluoroacetic acid to the
porphyrin in toluene results in the formation of
dication, 1H/+ in which all three nitrogens have
protons. The addition of protons to the inner nitrogens
results in the steric crowding in the core. Due to
crowding in the porphyrin core, the pyrroles prefers to
tilt in such way that the pyrroles are almost in plane
with phenyl groups. Due to this, the resonance
interaction between meso-phenyl and porphyrin group
increases which reflects in red shifts in the absorption
bands. This is the case of dication of STPPH which
show only one Q-band which was red shifted
compared to STPPH. However, the dication, 1H32+
showed three Q-bands which were blue shifted W.r.t 1
indicating that the meso-phenyl groups were not in
plane with porphyrin ring. This is due to the strap
which prevents the rotation of lIIeso-phenyl groups
into the plane of the porphyrin rin"". The emission
spectra of 1 and 2 recorded in toluene (68 1 and 750
nm) didn't show any shift compared to STPPH.
However, the quantum yields measured for 1 (<I> =
0.0089) and 2 (<I> = 0.0124) are lower than STPPH (<I>
= 0.0 168) indicating that the straps causes some
deformation in the porphyrin ring. The reduction in
quantum yield was much lower for 1 than 2
suggesting that the 2,3,5,6-tetrabrorno p-xy lyl group
causing more deformation. An add itional heavy
halogen effect in 1 due to the presence of bromines on
bridging phenyl also contributes to the low quantum
yield of 1 than 2 and STPPH.
It can be concluded that two eli fferent single
strapped 21-monothiaporphyrins were synthesized for
the first time. The straps were varied from flexible
hexyl chain to rigid 2,3,5,6-tetrabromo p-xylyl
groups. The rigid aromatic bridging group caused
more deformation In the porphyrin ring as evidenced
in 'H NMR, absorption and fluorescence data. Efforts
are being made synthesize of doubly strapped core
modified porphyrins, to study the effect of
deformation in the porphyrin ring on ground and and
excited properties of core modified porphyrins.
Acknowledgement
Financial assistance from the Council of Scientific
ancllndustrial Research is gratefully acknowledged.
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4. NOTES 1863
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