Synthesis and fluorescence properties of covalently linked homo- and hetero-porphyrin dyads containing meso-tolyl porphyrin and meso-furyl porphyrin sub-units

Article abstract of DOI:10.1016/j.tet.2007.01.005

Three porphyrin building blocks with N₄, N₃S and N₂S₂ cores having three meso-furyl groups and one meso-iodophenyl group were synthesized and characterized. The porphyrin building blocks were used to synthesize

Full text of DOI:10.1016/j.tet.2007.01.005

Tetrahedron 63 (2007) 2455–2465
Synthesis and fluorescence properties of covalently linked
homo- and hetero-porphyrin dyads containing meso-tolyl
porphyrin and meso-furyl porphyrin sub-units
*
Smita Rai and M. Ravikanth
Department of Chemistry, Indian Institute of Technology, Powai, Mumbai 400076, India
Received 21 September 2006; revised 29 November 2006; accepted 4 January 2007
Available online 7 January 2007
Abstract—Three porphyrin building blocks with N4, N3S and N2S2 cores having three meso-furyl groups and one meso-iodophenyl group
were synthesized and characterized. The porphyrin building blocks were used to synthesize six porphyrin dyads such as N4–N4, N3S–
N3S, N2S2–N2S2, N4–N3S, N4–N2S2 and N3S–N2S2 containing meso-tolyl and meso-furyl porphyrin sub-units under mild Pd(0) mediated
coupling conditions. Steady state fluorescence studies indicated an efficient energy transfer from the meso-tolyl porphyrin sub-unit to the
meso-furyl porphyrin sub-unit in all six dyads. This study supported the argument that the meso-furyl porphyrins can be used as good energy
acceptors when meso-aryl porphyrins act as energy donors in their metal free form.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
porphyrin unit upon irradiation of one porphyrin sub-unit to
study energy transfer properties. Selective excitation of the
porphyrin unit can be easily achieved if the porphyrin sub-
units in the dyad are not identical (unsymmetrical). In these
unsymmetrical porphyrin dyads, the two different types of
macrocycles in the metal free form can be connected either
covalently or non-covalently to study the energy transfer
properties. We3 and others4 have synthesized a series of
unsymmetrical porphyrin dyads containing two different
types of porphyrin macrocycles, such as porphyrin–phthalo-
cyanine, porphyrin–corrole and porphyrin–hetero-atom
substituted porphyrin and studied their energy transfer prop-
erties. Recently, we synthesized5 a series of porphyrins with
different porphyrin cores such as N4, N3S, N3O, N2S2, etc.
having five-membered furyl groups in place of six-mem-
bered aryl groups, and the studies showed that the electronic
properties are greatly altered on the introduction of furyl
groups in place of aryl groups at meso positions. Our studies
indicated that the meso-furyl porphyrins have favourable
energetics for electronic communication with meso-aryl
porphyrins. Thus assembling a meso-furyl porphyrin and
a meso-aryl porphyrin in a single unit may be expected to
lead to interesting excited state properties. In this paper,
we synthesized mono-functionalized meso-furyl porphyrin
building blocks with N4, N3S and N2S2 porphyrin cores
(Chart 1) and used them for the synthesis of covalently
linked diphenyl ethyne bridged three homo- and three het-
ero-porphyrin dyads (Chart 2). The preliminary fluorescence
study indicated the possibility of energy transfer from the
meso-aryl porphyrin to the meso-furyl porphyrin in metal
free form.
Porphyrin dyads containing an energy donor and acceptor
have been the topic of interesting research because they
are very useful to understand the process of photosynthesis.
A variety of covalently and non-covalently linked porphyrin
dyads with various constraints, such as changing the nature
of the spacers, the distance and orientation of donor with re-
spect to acceptor, have been synthesized and their excited
state properties have been well documented in the litera-
ture.1 Investigation of the energy transfer mechanism in
these dyads would not only yield valuable information on
the intrinsic properties of reaction centre proteins but would
also be helpful in the construction of solar energy cells, mo-
lecular photonic wires and molecular optoelectronic gates.2
A common feature among the various porphyrin dyads re-
ported in the literature is that they invariably contain similar
porphyrin cores (N4). However, the energy transfer proper-
ties of such symmetrical dyads have been studied by creating
an energy gradient between the two porphyrin sub-units on
insertion of metals such as Zn(II), Mg(II) and Cd(II) in
one porphyrin sub-unit and leaving the other porphyrin
sub-unit in the free base form. On excitation, the energy
from the metallated porphyrin, which is at higher energy is
transferred to the free base porphyrin, which is at low energy.
However, in these symmetrical porphyrin dyads it is some-
times difficult to achieve the selective excitation of the donor
* Corresponding author. Tel.: +91 22 25767176; fax: +91 22 25723480;
e-mail: ravikanth@chem.iitb.ac.in
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.01.005

2456
S. Rai, M. Ravikanth / Tetrahedron 63 (2007) 2455–2465
O
O
O
S
N
N
HN
S
N
N
N
I
I
I
O
O
O
NH
S
N
NH
O
O
O
1
2
3
Chart 1. Structures of meso-furyl porphyrin building blocks.
2. Results and discussion
of 1 equiv of 4-iodobenzaldehyde, 3 equiv of furan-2-alde-
hyde with 4 equiv of pyrrole under standard porphyrin form-
ing conditions.6 This condensation resulted in the formation
of a mixture of six porphyrins as expected, and the desired
compound 1 was separated from the mixture by column
chromatography. The compound was characterized by NMR,
mass, elemental analysis and absorption spectroscopic
2.1. Synthesis of mono-functionalized meso-furyl
porphyrin building blocks 1–3
The mono-functionalized meso-furyl porphyrin building
block 1 was synthesized by mixed aldehyde condensation
CH3
CH3
O
O
N
HN
N
NH
N
N
O
N
HN
N
S
N
N
O
H3C
H3C
NH
HN
NH
HN
O
O
7
O
4
CH3
CH3
CH3
CH3
O
N
S
S
N
N
O
N
HN
N
S
N
N
S
O
H3C
H3C
NH
N
HN
NH
O
O
5
8
CH3
CH3
CH3
CH3
O
O
N
S
S
S
N
N
S
N
S
N
S
N
N
O
O
H3C
H3C
N
NH
S
O
O
6
9
CH3
CH3
Homo dyads
Chart 2. Structures of homo- and hetero-porphyrin dyads.
Hetero dyads

S. Rai, M. Ravikanth / Tetrahedron 63 (2007) 2455–2465
2457
techniques. The M+ ion peak in the mass spectrum and
matching elemental analysis confirmed compound 1. In the
1H NMR spectrum, the eight pyrrole protons, which gener-
ally appeared as singlet in symmetrically substituted porphy-
rin such as 5,10,15,20-meso-tetrafuryl porphyrin,5a appeared
in 1 as two sets of signals due to unsymmetric substitution.
The absorption spectrum showed three Q-bands and one
Soret band with the peak positions closely matched with
those of 5,10,15,20-meso-tetrafuryl porphyrin.5
The mono-functionalized meso-furyl-21-thiaporphyrin 2
was synthesized by condensing 1 equiv of thiophene diol
11 with 2 equiv of furan-2-aldehyde and 3 equiv of pyrrole
under mild acid catalyzed conditions. The crude porphyrin
mixture containing four different porphyrins was separated
by column chromatography on silica and the desired
mono-functionalized meso-furyl-21-thiaporphyrin 2 was af-
forded as a purple solid in 8% yield. The M+ ion peak in ES
MS spectrum and matching elemental analysis confirmed
1
the compound 2. In H NMR, the two thiophene protons
To synthesize the mono-functionalized meso-furyl porphy-
rins with N3S (2) and N2S2 (3) cores, we required the
symmetrical and unsymmetrical thiophene diols. The sym-
metrical thiophene diol, 2,5-bis(2-furylhydroxymethyl)thio-
phene 10 was prepared by following our earlier procedure.5a
The unknown unsymmetrical thiophene diol, 2-(4-iodophe-
nylhydroxymethyl)-5-(2-furylhydroxymethyl)thiophene 11
was prepared in two steps starting from thiophene (Scheme
1). The required thiophene mono-ol, 2-(2-furylhydroxyme-
thyl)thiophene 12 was prepared in 40% yield by treating
thiophene with 1.2 equiv of n-BuLi followed by 1.2 equiv
of furan-2-aldehyde in THF at 0 ꢀC and purified by column
chromatography. The unsymmetrical thiophene diol 11 was
prepared by treating the mono-ol 12 with 2 equiv of n-BuLi
followed by 1.2 equiv of 4-iodobenzaldehyde in THF at
0 ꢀC. The crude diol was purified by silica gel column chro-
matography to afford pure diol 11 as a yellow oily liquid in
22% yield. The ES MS mass spectrum of 11 showed
M+ꢁOH peak and elemental analysis was in agreement
with the composition of diol 11. In 1H NMR, the thiophene
protons appeared as a multiplet due to unsymmetric substi-
tution. Furthermore, the presence of a broad singlet at
2.20 ppm in the 1H NMR spectrum and a very strong broad
peak at 3400 cmꢁ1 in IR spectrum confirmed the presence of
hydroxyl groups.
appeared as two sets of well defined doublets at 9.67 and
10.12 ppm due to the unsymmetrical substitution. The six
pyrrole protons appeared as four sets of signals. The three
sets of chemically non-equivalent furyl protons appeared
as three sets of signals, and the inner NH proton appeared
as a broad singlet at ꢁ2.62 ppm.
The mono-functionalized meso-furyl 21,23-dithiaporphyrin
building block 3 was prepared by condensing the same
thiophene diol 11 with unknown 5,10-bis(2-furyl)-16-thia-
15,17-dihydrotripyrrane 13 under mild acid catalyzed
conditions. The unknown 13 was prepared by treating diol
10 with excess pyrrole in the presence of an acid, followed
by column chromatographic purification. Condensation of
1 equivof diol 11 and tripyrrane 13 under mild acid catalyzed
porphyrin forming conditions followed by column chromato-
graphic purification yielded the mono-functionalized meso-
furyl-21,23-dithiaporphyrin 3 as single product in 11% yield.
The ES MS mass spectrum showed M+ ion peak and the ele-
mental analysis matched the expected composition of the
compound. In 1H NMR spectrum, the four thiophene protons
appeared as three sets of signals and the four pyrrole protons
appeared as two sets of signals. The chemically non-equiva-
lent furyl protons appeared as complex multiplet in the 7.50–
8.05 ppm region.
n-BuLi, TMEDA
O
S
12
H
S
OH
CHO
O
I
n-BuLi
TMEDA
I
O
O
S
NH HN
13
I
CHO
+
N
H
CHO
O
H
H
S
OH
BF3·OEt2
DDQ
OH
BF3·OEt2
DDQ
11
(22%)
O
O
S
N
N
S
N
N
I
I
O
NH
O
S
O
O
2 (8%)
(11%)
3
Scheme 1. Synthesis of N3S 2 and N2S2 3 porphyrin building blocks.

2458
S. Rai, M. Ravikanth / Tetrahedron 63 (2007) 2455–2465
2.2. Synthesis of diphenyl ethyne bridged porphyrin
dyads
confirming the identities of the dyads. The 1H NMR spectra
of dyads 4–6 were clean and the resonances of the dyads
were assigned on the basis of the spectra observed for the
corresponding two monomers taken independently. In N4–
N4 dyad 4, the 16 pyrrole protons appeared as 2 sets of
signals: 6 pyrrole protons appeared as a multiplet in 9.10–
9.30 ppm region and the 10 pyrrole protons appeared as
complex multiplet in 8.89–8.93 ppm region. The meso-tolyl,
meso-furyl and bridging phenyl group protons appeared
as overlapping signals in 7.05–8.22 ppm region. The NH
protons appeared as two broad singlets at ꢁ2.60 and
2.2.1. Synthesis of homo-porphyrin dyads. The porphyrin
dyads containing two porphyrin sub-units with the same por-
phyrin core such as N4–N4 (4), N3S–N3S (5) and N2S2–N2S2
(6) were synthesized by adopting Lindsey’s mild, copper
free, palladium(0) mediated Sonogashira coupling condi-
tions.7 The three required mono-functionalized meso-aryl
porphyrins such as 5,10,15-tris(4-tolyl)-20-(4-ethynylphe-
nyl)porphyrin8 (N4 core) 14, 5,10,15-tris(4-tolyl)-20-
(4-ethynylphenyl)-21-thiaporphyrin3d (N3S core) 15 and
5,10,15-tris(4-tolyl)-20-(4-ethynylphenyl)-21,23-dithiapor-
phyrin3e (N2S2 core) 16 which were synthesized by follow-
ing reported methods. The porphyrin dyad 4 was synthesized
by coupling 1 with 14 in toluene/triethylamine at 35 ꢀC in
the presence of a catalytic amount of Pd2(dba)3 and AsPh3.
The progress of the reaction was followed by TLC analysis,
which showed the starting materials as minor spots and the
required dyad as a major spot, after 12 h.
1
ꢁ2.74 ppm. The H NMR spectrum of dyad 5 showed one
multiplet at 10.19 ppm and two doublets at 9.74 and
9.82 ppm for the four thiophene protons. The 12 pyrrole pro-
tons in 5 appeared as 5 sets of signals in 8.56–9.55 ppm
region and the 2 NH protons appeared as broad singlet at
ꢁ2.62 ppm. Similarly, dyad 6 showed two sets of multiplets
for the eight thiophene protons and two sets of multiplets for
the eight pyrrole protons. The other aryl and furyl protons
appeared in 6.90–8.10 ppm region. A comparison of chem-
ical shifts of the various protons of the dyads 4–6 with those
of the corresponding monomeric porphyrin sub-units indi-
cates only minor differences suggesting that the two por-
phyrinic sub-units in the dyads interact very weakly. This
is consistent with the earlier reports on the covalently linked
porphyrin dyads.2
The crude compound was purified twice by column chroma-
tography on silica and the desired dyad 4 was obtained as
a purple solid in 56% yield. Similarly, the N3S–N3S dyad
5 was prepared by coupling of 2 and 15 and N2S2–N2S2
dyad 6 by coupling of 3 and 16 under identical palladium(0)
coupling conditions as mentioned for dyad 4 (Scheme 2).
The coupling reactions worked smoothly and required two
straightforward column chromatographic purifications to af-
ford pure dyads in decent yields. Dyads 4–6 are highly sol-
uble in all common organic solvents and were characterized
by all spectroscopic techniques. Dyads 4–6 showed M+ꢁ2,
M+ and M++1 ion peaks, respectively, in mass spectra
2.2.2. Synthesis of hetero-porphyrin dyads. The hetero-
porphyrin dyads such as N4–N3S (7), N4–N2S2 (8) and
N3S–N2S2 (9) were synthesized by coupling appropriate
porphyrin building blocks under similar palladium(0) cou-
pling conditions. The coupling of 2 and 14 in the presence
of Pd2(dba)3/AsPh3 at 35 ꢀC overnight followed by column
CH3
O
S
N
S
N
N
S
H
I
+
H3C
O
N
S
O
Pd2(dba)3/AsPh3
CH3
16
Toluene/TEA, 35 °C
3
CH3
O
S
N
N
S
N
S
H3C
O
S
N
O
6 (48%)
CH3
Scheme 2. Synthesis of porphyrin dyad 6.

S. Rai, M. Ravikanth / Tetrahedron 63 (2007) 2455–2465
2459
chromatographic purification yielded dyad 7 in 45% yield.
Dyad 8 was obtained in 48% yield by coupling of 3 and 14
under similar Pd(0) coupling conditions followed by column
chromatographic purification. Similarly dyad 9 was obtained
in 42% yield by coupling 3 and 15 under same reaction con-
ditions and purification by column chromatography. The
three dyads 7–9 have good solubility in most of the common
organic solvents. Dyads 7–9 were confirmed by ES MS mass
spectra, which showed a strong M+ ion peak for all three
dyads. 1H NMR spectroscopy has been used to characterize
the dyads 7–9. In dyad 7, which contains N4 and N3S por-
phyrin sub-units, the two thiophene protons of N3S porphy-
rin sub-unit appeared as two sets of multiplets at 10.12 and
9.75 ppm regions. The 14 pyrrole protons, 8 belonging to
the N4 porphyrin sub-unit and 6 belonging to N3S porphyrin
sub-unit appeared as 2 sets of multiplets in 8.84–8.95 ppm
region. The meso-furyl and meso-aryl protons also appeared
in 7.11–8.18 ppm region. The two inner NH protons of N4
porphyrin sub-unit were observed as broad singlet at
ꢁ2.78 ppm and the inner NH proton of N3S porphyrin
sub-unit was observed as broad singlet at ꢁ2.38 ppm. Sim-
ilarly in dyad 8, which contains N4 and N2S2 porphyrin
sub-units, the 4 thiophene protons appeared as 3 sets of sig-
nals in the region 9.72–10.13 ppm and 12 pyrrole protons as
a complex multiplet in 8.71–9.04 ppm region. The N2S2
porphyrin sub-unit did not have any inner NH protons
and the two inner NH protons of N4 porphyrin sub-unit
appeared as broad singlet at ꢁ2.70 ppm. In dyad 9 contain-
ing N3S and N2S2 porphyrin sub-units, the six thiophene pro-
tons appeared as two sets of signals in 9.60–10.20 ppm
region. The 10 pyrrole protons appeared as 2 sets of multi-
plets and the inner NH proton of N3S porphyrin sub-unit
appeared as broad singlet at ꢁ2.69 ppm. Thus, the proton
NMR spectra clearly support the proposed structures of all
the dyads 4–9.
The monomeric furyl porphyrin building blocks showed
three Q-bands and one strong Soret band and the peak max-
ima matched closely with their corresponding tetrafuryl por-
phyrins reported previously.5 The absorption spectra of dyad
6 along with the corresponding monomers, in both the
Q-band and the Soret region are shown in Figure 1. As is clear
2.3. Absorption properties
The absorption spectra of meso-furyl porphyrin building
blocks 1–3, dyads 4–9 and their corresponding 1:1 mixtures
of monomers were recorded in toluene and the data are pre-
sented in Table 1. The literature data3d,3e,8 for meso-aryl por-
phyrin building blocks 14–16 are also included in Table 1.
Figure 1. (a) Comparison of absorption spectra of dyad 6 (———) and its
corresponding monomers 3 (- - - - -) and 16 (///) recorded in toluene.
(b) Comparison of absorption spectra of dyad 8 ( ) and its corre-
sponding monomers 3 (- - - - -) and 14 ( ) recorded in toluene.
Table 1. Absorption data of porphyrin monomers, porphyrin dyads and their corresponding 1:1 mixture of monomers recorded in toluene
Porphyrin
Soret band l (nm) (log 3)
Absorption Q-bands l (nm) (log 3)
1
2
3
14
15
16
430 (8.83)
440 (4.7)
454 (2.1)
422 (28.9)
430 (5.4)
437 (5.3)
423 (18.9)
422 (38.6)
433 (4.97)
432 (5.01)
440 (5.08)
438 (4.76)
423 (12.1)
422 (18.8)
422 (13.0), 454 (5.3)
422 (18.3)
433 (4.96), 459 (4.47)
432 (5.02)
523 (6.9)
525 (5.1)
522 (br)
563 (4.8)
599 (br)
622 (br)
—
660 (2.5)
691 (2.0)
717 (2.1)
652 (5.9)
676 (3.5)
697 (3.6)
653 (4.2), 659 (sh)
652 (10.5)
681 (3.2), 706 (2.0)
679 (3.2)
699 (3.3), 730 (2.0)
698 (3.0)
653 (2.6), 690 (sh)
652 (4.1)
653 (4.2), 728 (3.2)
652 (4.6), 714 (sh)
681 (3.31), 727 (2.90)
679 (3.50), 731 (sh)
566 (18.1)
573 (4.5)
551 (7.5)
547 (3.8)
548 (3.9)
553 (6.2)
551 (17.9)
552 (3.5), 574 (sh)
551 (3.4)
552 (3.6)
550 (3.2)
553 (4.2)
551 (6.0)
553 (7.8)
551 (6.4)
552 (3.61)
551 (3.69), 578 (sh)
517 (13.4)
514 (4.3)
514 (4.3)
518 (10.2)
517 (27.1)
516 (3.9)
515 (3.8)
517 (4.0)
516 (3.6)
517 (6.3)
517 (9.9)
518 (10.2)
516 (10.1)
517 (3.88)
516 (3.98)
594 (3.9)
617 (3.4)
632 (3.2)
594 (3.9)
594 (11.1)
620 (3.0)
620 (3.0)
636 (3.0)
634 (2.7)
594 (2.2)
593 (3.5)
589 (br)
593 (4.1)
619 (3.27)
619 (3.41)
4
1:1 mix of 1 and 14
5
1:1 mix of 2 and 15
6
1:1 mix of 3 and 16
7
1:1 mix of 2 and 14
8
1:1 mix of 3 and 14
9
1:1 mix of 3 and 15

2460
S. Rai, M. Ravikanth / Tetrahedron 63 (2007) 2455–2465
Table 2. Fluorescence data of homo- and hetero-porphyrin dyads and appro-
priate porphyrin monomers
from Figure 1 and the data presented in Table 1, the absorp-
tion spectra of the dyads resembles, but does not equal, the
sum of the spectra of the corresponding monomeric units.
The absorption spectrum of dyad 4 was not significantly
changed in shape, position and bandwidth compared to its
1:1 mixture of corresponding monomers. However, the
extinction coefficients of dyad 4 were significantly reduced
as compared to the 1:1 mixture suggesting a weak interac-
tion between the two porphyrin sub-units. Dyad 5 showed
six Q-bands at 516, 552, 574, 620, 681 and 706 nm and
one strong broad Soret band at 433 nm. The peak at
706 nm is mainly because of meso-furyl N3S porphyrin
sub-unit and the peak at 681 nm is due to meso-aryl N3S por-
phyrin sub-unit. The peak at 706 nm was not observed in the
1:1 mixture of monomers because the extinction coefficients
of meso-aryl porphyrins are much higher as compared
to meso-furyl porphyrins. Hence the absorption spectra of
the 1:1 mixture show absorption bands, which correspond
mainly to meso-aryl porphyrin. But in covalently linked
dyad 5, due to weak interaction between the porphyrin
sub-units, the extinction coefficients of both the porphyrin
sub-units were significantly reduced and the dyad exhibits
the spectral features of both the porphyrin sub-units. Simi-
larly, the dyad 6 containing two N2S2 porphyrin sub-units
showed five Q-bands and one broad Soret band with a reduc-
tion in extinction coefficients as compared to the 1:1 mix-
ture of monomers. The hetero-dyads 7–9 containing two
different porphyrin cores also showed similar absorption
spectral features as observed for homo-dyads 4–6. In dyad
7 containing N4 and N3S porphyrin sub-units, five Q-bands
and one broad Soret band with a reduction in extinction
coefficients as compared to the 1:1 mixture was observed.
Dyads 8 and 9 containing N4–N2S2 and N3S–N2S2 porphy-
rin sub-units, respectively, showed a split Soret band in
addition to five Q-bands unlike their corresponding 1:1 mix-
ture of monomers, which showed one broad Soret band and
five Q-bands. Thus, the absorption spectral studies of dyads
4–9 and their corresponding 1:1 porphyrin mixture showed
that the two porphyrin sub-units in dyads interact very
weakly.
Compound
lex
Ff (Donor)
Ff (Acceptor)
H2TPPa
STPPHa
S2TPPa
H2TFPa
STFPHa
S2TFPa
1
415
420
430
430
440
450
430
440
450
415
430
420
440
430
450
415
440
415
450
420
450
0.11
0.0168
0.0076
—
—
—
—
—
—
0.00812
—
0.00116
—
0.00046
—
0.00240
—
0.00385
—
—
—
—
0.0109
0.0021
0.0019
0.0102
0.0020
0.0018
—
0.00990
—
0.00201
—
0.00182
—
0.00193
—
2
3
4
5
6
7
8
9
0.00189
—
0.00189
0.00118
—
a
Data taken from Ref. 5b.
excited at 430 nm, strong emission was mainly observed
from the meso-phenyl N2S2 porphyrin sub-unit (Fig. 2a).
These results indicate that the meso-furyl N2S2 porphyrin
can act as an energy acceptor and there is an efficient
energy transfer from meso-phenyl N2S2 porphyrin sub-
unit to meso-furyl N2S2 porphyrin sub-unit in dyad 6.
The excitation spectrum recorded for dyad 6 at 790 nm
matches exactly with its absorption spectrum, further con-
firming the efficient energy transfer between the porphyrin
sub-units.
Similarly, the other two homo-dyads 4 and 5 showed effi-
cient energy transfer from meso-phenyl porphyrin to meso-
furyl porphyrin. These observations are in line with diphenyl
ethyne bridged dyads containing meso-aryl metallated por-
phyrin and meso-aryl free base porphyrin sub-units.9 The
hetero-dyads containing two different types of porphyrin
cores also showed similar energy transfer between the por-
phyrin sub-units. As shown in Figure 2b, in dyad 9 contain-
ing meso-phenyl N3S porphyrin sub-unit and meso-furyl
N2S2 porphyrin sub-unit, on excitation at 420 nm where
meso-phenyl N3S porphyrin sub-unit absorbs strongly, the
emission of meso-phenyl N3S porphyrin was quenched by
93% and the strong emission from meso-furyl N2S2 porphy-
rin sub-unit was observed. It is also clear from Figure 2b that
when the 1:1 mixture of meso-phenyl N3S porphyrin 15 and
meso-furyl N2S2 porphyrin 3 was excited at 420 nm, emis-
sion was observed mainly from the meso-phenyl N3S por-
phyrin sub-unit. The excitation spectrum of dyad 9
recorded at 790 nm closely matched with its absorption
spectrum. These observations support an efficient energy
transfer between the porphyrin sub-units in hetero-dyad 9.
Similar observations were made in dyads 7 and 8 supporting
an energy transfer from meso-phenyl porphyrin sub-unit to
meso-furyl porphyrin sub-unit in hetero-dyads as well.
Thus, the steady state fluorescence studies clearly indicate
that in both homo- and hetero-dyads, an efficient energy
transfer occurs from meso-phenyl porphyrin sub-unit to
meso-furyl porphyrin sub-unit. Detailed time-resolved
2.4. Fluorescence properties
The fluorescence properties of homo- and hetero-porphy-
rin dyads 4–9 and their corresponding 1:1 mixture of
monomers and the appropriate reference compounds
were recorded in toluene at room temperature and the
data for fluorescence yields are presented in Table 2.
The emission spectra of homo-dyad 6 and hetero-dyad 9
and their 1:1 mixture of corresponding monomers are
shown in Figure 2. As is evident from Figure 2a, in
homo-dyad 6, which is composed of meso-phenyl N2S2
and meso-furyl N2S2 porphyrin sub-units, excitation at
450 nm where the meso-furyl N2S2 porphyrin sub-unit
absorbs strongly, results in typical meso-furyl N2S2 por-
phyrin emission with a quantum yield resembling that of
S2TFP (Table 2). Illumination of dyad 6 at 430 nm where
meso-phenyl N2S2 porphyrin sub-unit absorbs strongly,
causes the emission of meso-phenyl N2S2 porphyrin sub-
unit to be quenched by 94% and the strong emission
from meso-furyl N2S2 porphyrin sub-unit was observed
(Fig. 2a). However, when the 1:1 mixture of meso-phenyl
N2S2 porphyrin 16 and meso-furyl N2S2 porphyrin 3 was

S. Rai, M. Ravikanth / Tetrahedron 63 (2007) 2455–2465
2461
4. Experimental
4.1. Data for compounds
1
4.1.1. General. H and 13C NMR spectra were recorded
using a Varian 400 MHz spectrometer using tetramethyl-
silane as an internal standard and chemical shifts are reported
in d (ppm), referred to 1H (of residual proton; d 7.26) and 13
C
(d 77.0) signals of CDCl3. Absorption and fluorescence spec-
tra were obtained with Perkin–Elmer Lambda 35 and Perkin–
Elmer Lambda 55 models, respectively. Infrared spectra
were recorded on a Nicolet Impact-400 FT-IR spectrometer
and the ES MS mass spectra were recorded with a Q-Tof
micro (YA-105) mass spectrometer. Elemental analyses were
conducted using Thermo Finnigan Flash EA 1112. Diethyl
ether, n-hexane, THF and toluene were obtained from S.D.
Fine chemicals, India, and dried over sodium benzophenone
ketyl and distilled prior to use. Triethylamine was dried over
CaH2 and distilled prior to use. BF3$OEt2, 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ), N,N,N0,N0-tetramethyl-
ethylenediamine (TMEDA), AsPh3 and Pd2(dba)3 were used
as obtained from Sigma–Aldrich Chemical Co. All other
chemicals used for the synthesis were of reagent grade unless
otherwise specified. Column chromatography was per-
formed on silica (Merck, 60–120 mesh) obtained from Sisco
Research Laboratories, India. 2,5-Bis[2-furylhydroxy-
methyl]thiophene3a (symmetrical diol) 10, 5,10,15-tris(4-
tolyl)-20-(4-ethynylphenyl)porphyrin8 (N4 core) 14, 5,10,15-
tris(4-tolyl)-20-(4-ethynylphenyl)-21-thiaporphyrin3d (N3S
core) 15 and 5,10,15-tris(4-tolyl)-20-(4-ethynylphenyl)-
21,23-dithiaporphyrin3e (N2S2 core) 16 were synthesized
by the reported methods.
4.1.2. 2-(2-Furylhydroxymethyl)thiophene (12). Dry, dis-
tilled ether (40 mL) was added to a 250 mL three-necked,
round-bottomed flask fitted with a rubber septum and gas
inlet tube; the flask was flushed with argon for 10 min. Tet-
ramethylethylenediamine (TMEDA) (4.6 mL, 30.42 mmol)
and n-BuLi (20 mL of ca. 15% solution in hexane) were
added to the stirred solution and the reaction temperature
maintained at 0 ꢀC in an ice bath. Thiophene (2.0 mL,
25.35 mmol) was added and the solution was stirred for
1 h. As the reaction progressed, a white turbid solution
formed indicating the formation of 2-lithiated salt of thio-
phene. An ice-cold solution of furan-2-aldehyde (2.5 mL,
30.42 mmol) in dry THF (40 mL) was then added and
stirred for an additional 15 min at 0 ꢀC. The reaction mix-
ture was brought to room temperature. The reaction was
quenched by adding an ice-cold NH4Cl solution (50 mL,
ca. 1 M). The organic layer was diluted with ether and
washed several times with water and brine and dried over
anhydrous Na2SO4. The solvent was removed in a rotary
evaporator under reduced pressure to afford the crude com-
pound. TLC analysis showed two spots mainly correspond-
ing to unchanged aldehyde and the desired mono-ol 12.
The crude compound was loaded on silica and eluted
with petroleum ether. The unchanged aldehyde was re-
moved with petroleum ether/ethylacetate (98:2) solvent
mixture and the desired mono-ol 12 was collected with pe-
troleum ether/ethylacetate (96:4) solvent mixture (Rf 0.60).
The solvent was removed in a rotary evaporator to afford
12 as a light yellow solid (2.1 g, 40% yield). Mp 38–
40 ꢀC. IR (KBr, cmꢁ1): n¼3280 (OH). 1H NMR
Figure 2. (a) Comparison of emission spectra of dyad 6 (- - - - -) and 1:1
mixture of 3 and 16 (///) recorded at excitation wavelength 430 nm
in toluene. The excitation spectrum of dyad 6 (———) recorded at emission
wavelength 790 nm is also shown. (b) Comparison of emission spectra of
dyad 9 (
excitation wavelength 420 nm and the excitation spectrum of dyad 9
) recorded at emission wavelength 790 nm in toluene.
) and 1:1 mixture of 3 and 15 (
) recorded at
(
studies are now required to quantify the energy transfer
dynamics.
3. Conclusions
Mono-functionalized meso-furyl porphyrin building blocks
with N4, N3S and N2S2 porphyrin cores were synthesized
and characterized. The meso-furyl porphyrin building blocks
were used further to synthesize three homo-dyads containing
two similar porphyrin cores and three hetero-dyads contain-
ing two dissimilar porphyrin cores. The absorption studies
indicated a weak interaction between the two porphyrin
sub-units. The steady state fluorescence studies supported
an energy transfer from meso-aryl porphyrin to meso-furyl
porphyrin in dyads on selective excitation of meso-aryl por-
phyrin sub-unit. Thus, we have demonstrated that by intro-
ducing meso-furyl groups in place of meso-aryl groups, the
energy levels are altered and the meso-furyl porphyrins
can act as good energy acceptors when these porphyrins
are connected to meso-aryl porphyrins.

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S. Rai, M. Ravikanth / Tetrahedron 63 (2007) 2455–2465
(400 MHz, CDCl3, d in ppm): 2.86 (1H, br s, OH), 6.03
(1H, s, CHOH), 6.27–6.34 (2H, m, furyl), 6.97–7.00 (2H,
m, thiophene), 7.27–7.29 (1H, m, furyl), 7.35–7.40 (1H,
m, thiophene). 13C NMR (100 MHz, CDCl3, d in ppm):
67.60 (CHOH), 104.00, 116.02, 146.93, 148.04 (furyl),
126.24, 126.37, 126.83, 144.26 (thiophene). ES MS:
C9H8O2S, calcd av mass 180.2, obsd m/z 163.2 (M+ꢁ17,
100%). Anal. Calcd: C, 59.98; H, 4.47; S, 17.79. Found:
C, 59.70; H, 4.21; S, 17.62.
was removed under reduced pressure and the unreacted
pyrrole was removed by vacuum distillation at room tem-
perature. The resulting viscous dark yellow liquid was
purified by column chromatography (silica gel 60–120
mesh, ethyl acetate/petroleum ether (10:90)). After the
initial tailing material, a pale orange band eluted (Rf 0.21),
which gave a brown solid identified as 13 in 55%
yield (390 mg). Mp 80–82 ꢀC. IR (KBr, cmꢁ1): n¼3402
1
(NH). H NMR (400 MHz, CDCl3, d in ppm): 5.53 (2H,
s, meso), 5.99–6.09 (6H, m, pyrrole [4H]+furyl [2H]),
6.25–6.28 (2H, m, furyl), 6.57–6.60 (4H, m, pyrrole
[2H]+thiophene [2H]), 7.29 (2H, m, furyl), 8.05 (2H, br s,
NH). 13C NMR (100 MHz, CDCl3, d in ppm): 55.22
(meso), 107.01, 110.35, 143.69, 154.68 (furyl), 107.15,
108.02, 117.70, 130.31 (pyrrole), 125.16, 142.06 (thio-
phene). ES MS: C22H18N2O2S, calcd av mass 374.4,
obsd m/z 375.4 (M++1, 100%). Anal. Calcd: C, 70.57; H,
4.84; N, 7.48; S, 8.56. Found: C, 70.69; H, 4.72; N, 7.43;
S, 8.42.
4.1.3. 2-(4-Iodophenylhydroxymethyl)-5-(2-furylhydroxy-
methyl)thiophene (11). In a three-necked round-bottomed
flask equipped with rubber septum, gas inlet and gas outlet
tube, freshly distilled dry diethyl ether (40 mL) was taken
and a positive pressure of nitrogen was maintained. Mono-
ol 12 (2.00 g, 11.10 mmol) was added to it and the temper-
ature of the reaction flask was maintained at 0 ꢀC. TMEDA
(4.17 mL, 27.75 mmol) followed by n-BuLi (17.34 mL of
ca. 15% solution in hexane) was added dropwise over
10 min into the stirred solution and the reaction mixture
was allowed to stir for 30 min at 0 ꢀC. An ice-cold solution
of 4-iodobenzaldehyde (3.86 g, 16.6 mmol) in dry THF
(40 mL) was added using siphon apparatus from another
round-bottomed flask. The mixture was stirred for 15 min
and an ice-cold solution of NH4Cl (50 mL, ca. 1 M) was
added to quench the excess n-BuLi. The organic layer was
separated from the aqueous layer, and the aqueous layer
was extracted several times with ether. All the organic layers
were combined and washed with water and brine and dried
over anhydrous Na2SO4. The solvent was removed on
a rotary evaporator under reduced pressure to afford the
crude compound. TLC analysis showed three major spots
corresponding to the unreacted 4-iodobenzaldehyde, un-
reacted mono-ol and the desired diol 11. The aldehyde and
the mono-ol were removed by silica gel column chromato-
graphy using petroleum ether/ethyl acetate (90:10) as an el-
uent and the major diol fraction 11 was collected using
petroleum ether/ethyl acetate (80:20, Rf 0.22). The solvent
was removed on a rotary evaporator under reduced pressure
to afford the pure diol 11 as a yellow oily compound
(500 mg, 22%). IR (neat, cmꢁ1): n¼3400, 2955, 2929,
1965, 1652, 1515, 1483, 1413, 1263, 1006, 884, 814, 735,
4.1.5. 5-(4-Iodophenyl)-10,15,20-tris(2-furyl)porphyrin
(1). In a 500 mL one-necked round-bottomed flask
fitted with argon bubbler, furan-2-aldehyde (0.56 mL,
6.81 mmol), pyrrole (0.63 mL, 9.08 mmol) and 4-iodobenz-
aldehyde (500 mg, 2.27 mmol) were dissolved in 230 mL
of CH2Cl2. After purging with argon for 10 min, the conden-
sation of furan-2-aldehyde, pyrrole and iodobenzaldehyde
was initiated by adding a catalytic amount of BF3$OEt2
(91 mL of 2.5 M stock solution). The reaction mixture was
stirred at room temperature for 1 h. The progress of the
reaction was checked by taking aliquots of the reaction
mixture at regular intervals and oxidizing with DDQ. Re-
cording the absorption spectra clearly confirmed the forma-
tion of porphyrin. After 1 h, DDQ (385 mg, 1.70 mmol) was
added and the reaction mixture was stirred in air for an
additional 1 h. The solvent was removed under reduced
pressure and the crude compound was purified by silica
gel column chromatography. TLC analysis indicated the
formation of a mixture of six porphyrins: meso-tetrafuryl
porphyrin H2TFP, the desired mono-functionalized N4
porphyrin 1, cis and trans mixture of N4 porphyrin, tri-func-
tionalized N4 porphyrin and tetra-functionalized N4 por-
phyrin. The crude compound containing a mixture of six
porphyrins was subjected to silica gel column chromato-
graphy and the desired mono-functionalized N4 porphyrin
1 with small impurities was obtained as the fifth band using
petroleum ether/dichloromethane (75:25). The compound
with small impurities was further subjected to second silica
gel column chromatography using same solvent mixture and
afforded the pure 1 as a purple solid (0.09 g, 8%). Mp
1
597. H NMR (400 MHz, CDCl3, d in ppm): 2.20 (2H, br
s, OH), 5.80–5.95 (2H, m, CHOH), 6.25–6.31 (1H, m, furyl),
6.68–6.69 (1H, m, furyl), 6.78–6.80 (1H, m, thiophene),
6.95–6.98 (1H, m, thiophene), 7.12 (2H, d, J¼8.0 Hz,
aryl), 7.32–7.36 (1H, m, furyl), 7.64 (2H, d, J¼8.0 Hz,
aryl). 13C NMR (100 MHz, CDCl3, d in ppm): 67.72,
73.45 (CHOH), 101.00, 129.33, 134.56, 136.04 (aryl),
105.32, 114.44, 146.00, 146.91 (furyl), 124.57, 128.13,
140.29, 143.71 (thiophene). ES MS: C16H13O3SI, calcd
av mass 412.2, obsd m/z 394.9 (M+ꢁ17, 100%). Anal.
Calcd: C, 46.61; H, 3.17; S, 7.77. Found: C, 46.79; H,
3.09; S, 7.72.
1
>300 ꢀC. H NMR (400 MHz, CDCl3, d in ppm): ꢁ2.78
(2H, s, NH), 6.90 (3H, s, furyl), 7.40–7.46 (3H, br m, furyl
[1H]+aryl [2H]), 7.60–7.68 (3H, br m, furyl [1H]+aryl
[2H]), 7.75–7.77 (1H, br m, furyl), 7.99 (3H, s, furyl),
8.71 (2H, s, b-pyrrole), 9.01–9.06 (6H, br m, b-pyrrole).
13C NMR (100 MHz, CDCl3, d in ppm): 100.12, 128.15,
134.50, 142.28 (aryl), 110.78, 116.03, 147.22, 157.16,
157.62 (furyl), 129.75, 130.88, 131.55, 131.78, 132.90,
132.99, 133.37, 134.03, 140.07, 140.33, 141.05, 141.50
(pyrrole), 139.46, 154.70 (meso). ES MS: C38H23N4O3I,
calcd av mass 710.5, obsd m/z 710.8 (M+, 100%). Anal.
Calcd: C, 64.23; H, 3.26; N, 7.89. Found: C, 64.31; H,
3.32; N, 7.95.
4.1.4. 5,10-Bis(2-furyl)-16-thia-15,17-dihydrotripyrrane
(13). A mixture of 10 (500 mg, 1.81 mmol) and pyrrole
(5.0 mL, 72.44 mmol) was degassed by bubbling with N2
for 10 min. BF3$OEt2 (274 mL, 1.81 mmol) was added and
the reaction mixture was stirred for 30 min at room temper-
ature. The mixture was then diluted with CH2Cl2 (100 mL),
washed with 0.1 M NaOH, followed by water washing. The
organic layer was dried over anhydrous Na2SO4. The solvent

S. Rai, M. Ravikanth / Tetrahedron 63 (2007) 2455–2465
2463
4.1.6. 5-(4-Iodophenyl)-10,15,20-tris(2-furyl)-21-mono-
thiaporphyrin (2). Samples of diol 11 (500 mg,
1.21 mmol), furan-2-aldehyde (0.20 mL, 2.42 mmol) and
pyrrole (0.25 mL, 3.63 mmol) were dissolved in dichloro-
methane (121 mL) taken in 250 mL round-bottomed flask
and nitrogen was purged for 10 min. BF3$OEt2 (50 mL of
2.5 M stock solution) was added to catalyze the reaction
and the stirring was continued for 1 h. DDQ (410 mg,
1.81 mmol) was added to oxidize the porphyrinogen to
porphyrin and stirring was continued for an additional 1 h
in air. TLC analysis indicated the formation of a mixture
of four porphyrins: tetrafuryl porphyrin (H2TFP), the de-
sired 21-thiaporphyrin 2 and cis and trans mixture of
21,23-dithiaporphyrins. The crude compound containing
a mixture of four porphyrins was subjected to silica gel
column chromatography and the desired N3S porphyrin
2 with small impurities was obtained as the first band using
petroleum ether/dichloromethane (80:20). The compound
was further subjected to second silica gel column chromato-
graphy using same solvent mixture and afforded pure 2 as a
purple solid (0.07 g, 8%). Mp >300 ꢀC. 1H NMR (400 MHz,
CDCl3, d in ppm): ꢁ2.62 (1H, s, NH), 7.39 (3H, d,
J¼2.0 Hz, furyl), 7.91 (2H, d, J¼8.0 Hz, aryl), 8.01–8.05
(3H, m, furyl), 8.11 (2H, d, J¼8.0 Hz, aryl), 8.79–8.81
(3H, m, furyl), 8.60 (1H, d, J¼4.4 Hz, b-pyrrole), 8.97
(1H, d, J¼4.0 Hz, b-pyrrole), 9.09 (2H, s, b-pyrrole), 9.20
(2H, s, b-pyrrole), 9.67 (1H, d, J¼4.8 Hz, b-thiophene),
10.12 (1H, d, J¼5.2 Hz, b-thiophene). 13C NMR
(100 MHz, CDCl3, d in ppm): 100.10, 128.05, 134.53,
142.35 (aryl), 110.80, 116.14, 147.21, 157.11, 157.65
(furyl), 126.77, 127.70, 137.74, 138.65 (thiophene),
129.75, 130.88, 132.90, 133.37, 134.03, 140.07, 141.05
(pyrrole), 139.21, 154.66 (meso). ES MS: C38H22N3O3SI,
calcd av mass 727.5, obsd m/z 727.9 (M+, 100%). Anal.
Calcd: C, 62.73; H, 3.04; N, 5.77; S, 4.40. Found: C,
62.99; H, 3.12; N, 5.63; S, 4.32.
4.1.8. 4-[5,10,15-Tris(4-tolyl)-20-porphinyl]-40-[5,10,15-
trifuryl-20-porphinyl]diphenyl-ethyne (4). A solution of
14 (15.0 mg, 0.02 mmol) and 1 (16.0 mg, 0.02 mmol) in
dry toluene/triethylamine (3:1, 30 mL) was taken in
a 100 mL two-necked round-bottomed flask. The flask was
fitted with a reflux condenser, gas inlet and gas outlet tubes
for nitrogen purging. The reaction vessel was placed in an oil
bath pre-heated to 35 ꢀC. After purging nitrogen for 15 min,
AsPh3 (8.0 mg, 0.003 mmol) followed by Pd2(dba)3
(3.0 mg, 0.003 mmol) was added and the reaction was
stirred at 35 ꢀC for 12 h. TLC analysis of the reaction mix-
ture indicated the virtual disappearance of spots correspond-
ing to starting materials and the appearance of a new spot
corresponding to dyad 4. The solvent was removed under
vacuum and the crude compound was purified by silica gel
chromatography using petroleum ether/dichloromethane
(80:20) to remove the excess AsPh3 and the small amounts
of monomeric porphyrins. The desired dyad 4 was then col-
lected with petroleum ether/dichloromethane (65:35). The
solvent was removed on rotary evaporator under vacuo and
afforded dyad 4 as a violet solid (11.0 mg, 56%). Mp
>300 ꢀC. IR (KBr film, cmꢁ1): n¼3069, 2925, 2853,
1965, 2110, 1648, 1465, 967, 800, 656, 618. 1H NMR
(400 MHz, CDCl3, d in ppm): ꢁ2.74 (2H, br s, NH),
ꢁ2.60 (2H, br s, NH), 2.80 (9H, s, CH3), 7.05–7.09 (3H,
br m, furyl), 7.42–7.43 (3H, m, furyl), 7.57–7.59 (6H, m,
aryl), 8.12–8.14 (13H, m, furyl [3H]+aryl [10H]), 8.18–
8.22 (4H, m, aryl), 8.89–8.93 (10H, m, b-pyrrole), 9.10–
9.30 (6H, br m, b-pyrrole). 13C NMR (100 MHz, CDCl3,
d in ppm): 21.14, 21.64 (aryl CH3), 100.10 (ethynyl),
128.14, 129.01, 131.26, 134.72 (aryl), 130.30, 130.39,
130.45, 131.10 (pyrrole), 108.43, 113.70, 144.56, 154.42
(furyl), 135.92, 137.50, 137.58, 139.40, 146.11, 146.24,
145.20, 146.10 (meso). ES MS: C87H58N8O3, calcd av
mass 1263.4, obsd m/z 1261.2 (M+ꢁ2, 50%). Anal. Calcd:
C, 82.71; H, 4.63; N, 8.87. Found: C, 82.86; H, 4.78; N, 8.92.
4.1.7. 5-(4-Iodophenyl)-10,15,20-tris(2-furyl)-21,23-di-
thiaporphyrin (3). A solution of diol 11 (500 mg,
1.21 mmol) and 16-thiatripyrrane 13 (470 mg, 1.21 mmol)
in dichloromethane (121 mL) was condensed by the addition
of a catalytic amount of BF3$OEt2 (60 mL of 2.5 M stock so-
lution) for 1 h. DDQ (275 mg, 1.21 mmol) was then added
and the reaction mixture was stirred in air for an additional
1 h. TLC analysis showed the formation of the required com-
pound as the sole product. The solvent was removed under re-
duced pressure and the crude compound was purified by
silica gel column chromatography using petroleum ether/di-
chloromethane (65:35) as eluent to afford the desired porphy-
rin 3 as a purple solid (0.09 g, 11%). Mp >300 ꢀC. 1H NMR
(400 MHz, CDCl3, d in ppm): 7.50–7.55 (3H, m, furyl), 7.61–
7.68 (5H, m, furyl [3H]+aryl [2H]), 7.70–7.75 (2H, m, aryl),
8.02–8.05 (3H, m, furyl), 8.62 (1H, d, J¼4.8 Hz, b-pyrrole),
8.99–9.00 (3H, m, b-pyrrole), 9.61 (1H, d, J¼4.8 Hz, b-thio-
phene), 10.04 (1H, d, J¼5.2 Hz, b-thiophene), 10.10 (2H, s,
b-thiophene). 13C NMR (100 MHz, CDCl3, d in ppm):
100.00, 124.42, 133.16, 143.30 (aryl), 112.54, 113.05,
144.65, 148.26 (furyl), 124.09, 127.67, 135.09, 136.53 (thio-
phene), 128.81, 129.04, 138.44, 141.29 (pyrrole), 139.06,
147.65 (meso). ES MS: C38H21N2O3S2I, calcd av mass
744.6, obsd m/z 746.1 (M+, 100%). Anal. Calcd: C, 61.29;
H, 2.84; N, 3.76; S, 8.61. Found: C, 61.20; H, 2.90; N,
3.80; S, 8.52.
4.1.9. 4-[5,10,15-Tris(4-tolyl)-21-monothia-20-por-
phinyl]-40-[5,10,15-trifuryl-21-monothia-20-porphinyl]-
diphenylethyne (5). A solution of 15 (15.0 mg, 0.02 mmol)
and 2 (15.6 mg, 0.02 mmol) in dry toluene/triethylamine
(3:1, 30 mL) was coupled by the addition of AsPh3
(8.0 mg, 0.003 mmol) followed by Pd2(dba)3 (3.0 mg,
0.003 mmol) under the same reaction conditions described
for dyad 4. Purification by silica gel column chromatography
using petroleum ether/dichloromethane (65:35) afforded
dyad 5 as a violet solid (11.2 mg, 40%). Mp >300 ꢀC. IR
(KBr film, cmꢁ1): n¼3070, 2925, 2853, 1965, 2110, 1648,
1460, 970, 800, 656, 615. 1H NMR (400 MHz, CDCl3, d in
ppm): ꢁ2.62 (2H, br s, NH), 2.70 (9H, s, CH3), 7.03–7.05
(3H, m, furyl), 7.52–7.64 (9H, m, furyl [3H]+aryl [6H]),
7.97–8.19 (17H, m, furyl [3H]+aryl [14H]), 8.56–8.58 (1H,
m, b-pyrrole), 8.62–8.67 (3H, m, b-pyrrole), 8.87–8.89
(4H, m, b-pyrrole), 9.04 (1H, d, J¼4.8 Hz, b-pyrrole),
9.55–9.62 (3H, m, b-pyrrole), 9.74 (1H, d, J¼5.2 Hz, b-thio-
phene), 9.82 (1H, d, J¼5.2 Hz, b-thiophene), 10.18–10.20
(2H, m, b-thiophene). 13C NMR (100 MHz, CDCl3, d in
ppm): 21.65, 22.86 (aryl CH3), 128.40, 128.97, 134.31,
135.29 (aryl), 130.31, 130.58, 137.55, 139.28 (thiophene),
131.15, 131.64, 132.73 (pyrrole), 110.42, 113.70, 143.38,
154.54 (furyl), 140.22, 141.63, 144.20, 147.22, 146.01
(meso). ES MS: C87H56N6S2O3, calcd av mass 1298.2,
obsd m/z 1298.4 (M+, 60%). Anal. Calcd: C, 80.49; H,

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S. Rai, M. Ravikanth / Tetrahedron 63 (2007) 2455–2465
4.35; N, 6.47; S, 4.94. Found: C, 80.53; H, 4.48; N, 6.52; S,
4.99.
0.03 mmol) in dry toluene/triethylamine (3:1, 30 mL) were
coupled in the presence of AsPh3 (9.79 mg, 0.004 mmol)
and Pd2(dba)3 (4.11 mg, 0.004 mmol) at 35 ꢀC for 15 h. Col-
umn chromatography on silica using petroleum ether/
dichloromethane (60:40) gave dyad 8 as purple solid
(18.6 mg, 48%). Mp >300 ꢀC. IR (KBr film, cmꢁ1):
n¼3055, 2922, 2850, 2112, 1454, 990, 750. 1H NMR
(400 MHz, CDCl3, d in ppm): ꢁ2.70 (2H, br s, NH), 2.70
(9H, s, Me), 7.05–7.09 (3H, m, furyl), 7.42–7.46 (3H, m,
furyl), 7.50–7.59 (10H, m, aryl), 8.04–8.14 (10H, m, aryl),
8.18–8.22 (3H, m, furyl), 8.71–9.04 (12H, m, b-pyrrole),
9.72–9.74 (1H, m, b-thiophene), 10.04 (1H, s, b-thiophene),
10.09–10.13 (2H, m, b-thiophene). 13C NMR (100 MHz,
CDCl3, d in ppm): 21.11, 21.58 (aryl CH3), 102.05 (ethynyl),
124.62, 124.79, 128.52, 130.06, 131.24, 134.11 (aryl),
130.32, 130.41, 137.81, 138.00 (thiophene), 131.15,
131.39, 132.60, 132.71, 133.28, 133.88 (pyrrole), 110.20,
114.82, 144.26, 156.25 (furyl), 135.41, 136.16, 137.70,
139.81, 146.20, 147.89 (meso). ES MS: C87H56N6O3S2,
calcd av mass 1297.4, obsd m/z 1296.7 (M+, 50%). Anal.
Calcd: C, 80.54; H, 4.35; N, 6.48; S, 4.94. Found: C, 80.61;
H, 4.38; N, 6.42; S, 4.99.
4.1.10. 4-[5,10,15-Tris(4-tolyl)-21,23-dithia-20-por-
phinyl]-40-[5,10,15-trifuryl-21,23-dithia-20-porphinyl]-
diphenylethyne (6). Coupling of N2S2 porphyrin 3 (15.0 mg,
0.02 mmol) and 5-(4-iodophenyl)-10,15,20-tris(2-furyl)-
21,23-dithiaporphyrin 16 (16.0 mg, 0.02 mmol) in dry tolu-
ene/triethylamine (3:1, 30 mL) in the presence of AsPh3
(8.0 mg, 0.003 mmol) and Pd2(dba)3 (3.0 mg, 0.003 mmol)
at 35 ꢀC overnight followed by silica gel column chromato-
graphic purification using petroleum ether/dichloromethane
(60:40) gave dyad 6 as a violet solid (13.4 mg, 48%). Mp
>300 ꢀC. IR (KBr film, cmꢁ1): n¼3068, 2923, 2854, 1964,
2110, 1648, 1465, 967, 802, 656, 616. 1H NMR (400 MHz,
CDCl3, d in ppm): 2.62 (9H, s, CH3), 6.90 (3H, s, furyl),
7.29–7.31 (6H, m, aryl), 7.52–7.54 (4H, d, J¼7.2 Hz, aryl),
7.64–7.80 (6H, m, furyl), 8.04–8.10 (10H, m, aryl), 8.53–
8.62 (4H, m, b-pyrrole), 8.88–8.91 (4H, m, b-pyrrole),
9.55–9.62 (4H, m, b-thiophene), 9.95–10.01 (4H, m, b-thio-
phene). 13C NMR (100 MHz, CDCl3, d in ppm): 21.50, 22.69
(aryl CH3), 101.41 (ethynyl), 128.31, 128.98, 134.16, 134.36
(aryl), 130.64, 130.70, 131.73, 131.80, 137.56, 138.55 (thio-
phene), 131.92, 132.55, 132.90, 133.08, 139.10, 140.09 (pyr-
role), 110.21, 114.30, 143.30, 157.89 (furyl), 145.29, 146.01,
146.20, 146.33, 154.33 (meso). ES MS: C87H54N4S4O3, calcd
av mass 1332.1, obsd m/z 1333.1 (M++1, 30%). Anal. Calcd:
C, 78.39; H, 4.08; N, 4.20; S, 9.62. Found: C, 78.48; H, 4.18;
N, 4.32; S, 9.69.
4.1.13. 4-[5,10,15-Tris(4-tolyl)-21-monothia-20-por-
phinyl]-40-[5,10,15-trifuryl-21,23-dithia-20-porphinyl]-
diphenylethyne (9). A solution of 15 (15.0 mg, 0.02 mmol)
and 3 (14.0 mg, 0.02 mmol) in dry toluene/triethylamine
(3:1, 30 mL) was coupled in the presence of AsPh3
(8.0 mg, 0.003 mmol) and Pd2(dba)3 (3.0 mg, 0.003 mmol)
at 35 ꢀC for 12 h. Column chromatography purification on
silica gel using petroleum ether/dichloromethane (50:50)
gave dyad 9 as a violet solid in 42% yield (12.0 mg). Mp
>300 ꢀC. IR (KBr film, cmꢁ1): n¼3065, 2925, 2853, 1965,
2110, 1648, 1465, 967, 800, 656, 620. 1H NMR (400 MHz,
CDCl3, d in ppm): ꢁ2.69 (1H, br s, NH), 2.68 (9H, s, CH3),
7.04–7.06 (3H, m, furyl), 7.34–7.41 (8H, m, aryl), 7.50–
7.54 (3H, m, furyl), 7.58–7.63 (4H, m, aryl), 7.90–7.98
(4H, m, aryl), 8.04–8.12 (4H, m, aryl), 8.16–8.22 (3H, m,
furyl), 8.51–8.52 (6H, m, b-pyrrole), 8.92–9.01 (4H, m, b-
pyrrole), 9.60–9.70 (4H, m, b-thiophene), 10.13–10.20 (2H,
m, b-thiophene). 13C NMR (100 MHz, CDCl3, d in ppm):
21.69, 22.09 (aryl CH3), 100.24 (ethynyl), 128.49, 129.06,
133.25, 134.29 (aryl), 130.01, 131.23, 131.47, 137.72,
138.55 (thiophene), 132.80, 132.55, 132.90, 134.54,
136.87, 139.26, 140.09 (pyrrole), 110.21, 114.30, 142.01,
157.72 (furyl), 145.29, 146.01, 146.20, 150.62, 154.67
(meso). ES MS: C87H55N5S3O3, calcd av mass 1315.3, obsd
m/z 1315.2 (M+, 60%). Anal. Calcd: C, 79.45; H, 4.21; N,
5.32; S, 7.31. Found: C, 79.53; H, 4.28; N, 5.39; S, 7.39.
4.1.11. 4-[5,10,15-Tris(4-tolyl)-20-porphinyl]-40-[5,10,15-
trifuryl-21-monothia-20-porphinyl]diphenylethyne (7).
Samples of 14 (10.0 mg, 0.014 mmol) and 5-(4-
iodophenyl)-10,15,20-tris(2-furyl)-21-monothiaporphyrin 2
(10.0 mg, 0.014 mmol) in dry toluene/triethylamine (3:1,
30 mL) were coupled in the presence of AsPh3 (5.35 mg,
0.002 mmol) and Pd2(dba)3 (1.92 g, 0.002 mmol) at 35 ꢀC
for 12 h under nitrogen atmosphere. The crude reaction mix-
ture was purified by column chromatography and the desired
dyad 7 was collected using petroleum ether/dichlorometh-
ane (50:50). The solvent was removed on rotary evaporator
under vacuum and dyad 7 was obtained as a purple solid
(8.1 mg, 45%). Mp >300 ꢀC. IR (KBr film, cmꢁ1):
n¼3073, 2930, 2862, 2112, 1450, 990, 752. 1H NMR
(400 MHz, CDCl3, d in ppm): ꢁ2.38 (1H, s, NH), ꢁ2.78
(2H, s, NH), 2.73 (9H, s, CH3), 7.11–7.13 (3H, m, furyl),
7.38–7.40 (3H, m, furyl), 7.54–7.56 (8H, m, aryl), 7.74–
7.76 (4H, m, aryl), 8.09–8.11 (8H, m, aryl), 8.16–8.18
(3H, m, furyl), 8.84–8.86 (9H, m, b-pyrrole), 8.93–8.95
(5H, m, b-pyrrole), 9.75 (1H, s, b-thiophene), 10.12 (1H,
s, b-thiophene). 13C NMR (100 MHz, CDCl3, d in ppm):
21.13, 21.61 (aryl CH3), 102.11 (ethynyl), 124.74, 124.99,
128.14, 129.16, 131.33, 134.26 (aryl), 130.32, 130.44,
137.81, 138.44 (thiophene), 131.19, 131.49, 132.45,
132.65 (pyrrole), 109.23, 114.70, 144.53, 156.50 (furyl),
135.39, 136.45, 137.81, 139.80, 146.24, 147.86 (meso).
HRMS: C87H57N7O3S, calcd av mass 1280.5210, obsd m/z
1280.5215 (M+, 45%). Anal. Calcd: C, 81.60; H, 4.49; N,
7.66; S, 2.50. Found: C, 81.65; H, 4.54; N, 7.72; S, 2.59.
Acknowledgements
This research was supported by Department of Science and
Technology and Department of Atomic Energy, Govt. of
India, New Delhi. NMR and mass spectra were obtained at
Department of Chemistry, IIT-Bombay.
References and notes
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trifuryl-21,23-dithia-20-porphinyl]diphenylethyne (8).
Samples of 14 (20.0 mg, 0.03 mmol) and 3 (22.0 mg,
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