Synthesis of covalently linked unsymmetrical porphyrin pentads containing three different porphyrin subunits

Article abstract of DOI:10.1021/jo801455w

(Chemical Equation Presented) The tetrafunctionalized AB₃-type porhyrin building blocks containing two different types of functional groups with N₄, N₃O, N₃S, and N₂S ₂ porphyrin cores were

Full text of DOI:10.1021/jo801455w

Synthesis of Covalently Linked Unsymmetrical Porphyrin Pentads
Containing Three Different Porphyrin Subunits
Smita Rai and Mangalampalli Ravikanth*
Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India
raVikanth@chem.iitb.ac.in
ReceiVed July 4, 2008
The tetrafunctionalized AB3-type porphyrin building blocks containing two different types of functional
groups with N4, N3O, N3S, and N2S2 porphyrin cores were synthesized by following various synthetic
routes. The AB3-type tetrafunctionalized N4 porphyrin building block was synthesized by a mixed
condensation approach, the N3S and N3O porphyrin building blocks by a mono-ol method, and N2S2
porphyrin building block by an unsymmetrical diol method. The tetrafunctionalized porphyrin building
blocks were used to synthesize monofunctionalized porphyrin tetrads containing two different types of
porphyrin subunits by coupling of 1 equiv of tetrafunctionalized N4, N3O, N3S, and N2S2 porphyrin building
block with 3 equiv of monofunctionalized ZnN4 porphyrin building block under mild copper-free Pd(0)
coupling conditions. The monofunctionalized porphyrin tetrads were used further to synthesize
unsymmetrical porphyrin pentads containing three different types of porphyrin subunits by coupling 1
equiv of monofunctionalized porphyrin tetrad with 1 equiv of monofunctionalized N2S2 porphyrin building
blocks under the same mild Pd(0) coupling conditions. The NMR, absorption, and electrochemical studies
on porphyrin tetrads and porphyrin pentads indicated that the monomeric porphyrin subunits in tetrads
and pentads retain their individual characteristic features and exhibit weak interaction among the porphyrin
subunits. The steady state and time-resolved fluorescence studies support an efficient energy transfer
from donor porphyrin subunit to acceptor porphyrin subunit in unsymmetrical porphyrin tetrads and
porphyrin pentads.
Introduction
properties of the photosynthesis reaction center but also their
use for various other applications.2 A common feature among
the various multiporphyrin arrays reported in the literature is
Natural photosynthetic systems employ elaborate light-
harvesting complexes to capture dilute sunlight and funnel the
captured energy to the reaction center through rapid and efficient
transfer processes.1 A major objective in the field of artificial
photosynthesis is to create synthetic light-harvesting complexes.
In recent times, several covalently linked multiporphyrin arrays
have been synthesized and their excited-state properties have
been explored to understand not only the light-harvesting
(2) (a) Wagner, R. W.; Seth, J.; Yang, S.-I.; Kim, D.; Bocian, D. F.; Holten,
D.; Lindsey, J. S. J. Org. Chem. 1998, 63, 5042. (b) Li, J.; Ambroise, A.; Yang,
S.-I.; Diers, J. R.; Seth, J.; Wack, C. R.; Bocian, D. F.; Holten, D.; Lindsey,
J. S. J. Am. Chem. Soc. 1999, 121, 8927. (c) Song, H.-E.; Kirmaier, C.; Schwartz,
J. K.; Hindin, E.; Yu, L.; Bocian, D. F.; Lindsey, J. S.; Holten, D. J. Phys.
Chem. B 2006, 110, 19131. (d) Biemans, H. A. M.; Rowa, A. E.; Verhoeven,
A.; Vanoppen, P.; Latterini, L.; Foekema, J.; Schenning, A. P. H. J.; Meijer,
E. W.; de Schryver, F. C.; Nolte, R. J. M. J. Am. Chem. Soc. 1998, 120, 11054.
(e) Sugiura, K.; Fujimoto, Y.; Sakata, Y. Chem. Commun. 2000, 1105. (f) Cho,
H. S.; Rhee, H.; Song, J. K.; Min, C.-K.; Takase, M.; Aratani, N.; Cho, S.;
Osuka, A.; Joo, T.; Kim, D. J. Am. Chem. Soc. 2003, 125, 5849. (g) Shoji, O.;
Okada, S.; Satake, A.; Kobuke, Y. J. Am. Chem. Soc. 2005, 127, 2201.
(1) (a) McDermott, G.; Prince, S. M.; Freer, A. A.; Hawthornthwaite-Lawless,
A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995, 374, 517. (b)
Koepke, J.; Hu, X.; Muenke, C.; Schulten, K.; Michel, H. Structure 1996, 4,
581. (c) Pullerits, T.; Sundstrom, V. Acc. Chem. Res. 1996, 29, 381.
8364 J. Org. Chem. 2008, 73, 8364–8375
10.1021/jo801455w CCC: $40.75 2008 American Chemical Society
Published on Web 10/02/2008

Synthesis of Unsymmetrical Porphyrin Pentads
that they invariably contain similar porphyrin cores (N4). The
energy transfer properties of such symmetrical porphyrin arrays
containing N4 porphyrin cores have been studied by creating
an energy gradient between the two porphyrin subunits by
insertion of a metal such as Zn(II), Mg(II), and Sn(II) in the
one of the porphyrin subunits and leaving the other porphyrin
subunit in the free base form.3 Thus, porphyrin arrays like star-
shaped porphyrin pentads4 containing four peripheral metalated
porphyrin subunits and one central free base porphyrin subunit
were synthesized. These systems act as a light-harvesting array
containing multiple photoactive energy donors funnelling energy
to one low lying energy acceptor. However, in these symmetrical
porphyrin arrays, the absorption bands of metalated porphyrin
may overlap with the free base porphyrin subunit of porphyrin
arrays; hence, the selective excitation of donor porphyrin subunit
is difficult. Furthermore, the emission bands of donor and
acceptor porphyrin subunits also considerably overlap with each
other which in some cases causes problem for accurate estima-
tion of singlet-singlet energy-transfer parameters. To circum-
vent these problems, recently the efforts have been directed
in design and synthesis of unsymmetrical arrays containing
two different macrocycles such as porphyrin-chlorin,5 por-
phyrin-corrole,6 porphyrin-pheophorbide,7 and porphyrin-
phthalocyanine8 macrocycles. These unsymmetrical arrays are
useful to study singlet-singlet energy transfer and also to obtain
fast initial charge transfer and a slow back reaction, thus giving
a long-lived charge-transfer state.
We recently investigated the synthesis of a variety of ꢀ- and
meso-substituted core-modified porphyrins to study their elec-
tronic properties.9 The modification of porphyrin core by
replacing one or two inner nitrogens with other heteroatoms
such as sulfur, oxygen, selenium, and tellurium forms a group
of core-modified porphyrins10 containing different kinds of
porphyrin cores such as N3S, N2S2, N3O, N2SO, N2OS, N3Se,
N3Te, N2Se2, etc. The core-modified porphyrins exhibit interest-
ing properties in terms of both aromatic character and their
ability to stabilize metals in unusual oxidation states.10 The
electronic properties of core-modified porphyrins are quite
different from normal porphyrins (N4 core). An assembly of
such core-modified porphyrin and normal porphyrin (N4 core)
or with any other macrocycle such as corrole, phthalocyanine,
etc. would offer unique dyads or higher oligomers which are
expected to have unusual electronic structure and interesting
photophysical properties. Van Patten and co-workers11 on the
basis of computational studies predicted that a set of porphyrins
such as N4, N3O, N3S, N2OS, and N2S2 porphyrins arranged in
a linear series with a progressive decrease in energy levels could
provide the basis for an energy cascade. We synthesized a series
of unsymmetrical porphyrin dyads12 containing two different
macrocycles such as N4-N3O, N4-N3S, N4-N2S2, N3O-N3S,
N3S-N2S2, etc., and preliminary photophysical studies sup-
ported an efficient energy transfer from one porphyrin subunit
to another in these systems. However, except our own few
examples of unsymmetrical porphyrin oligomers containing
core-modified porphyrins, the reports on unsymmetrical por-
phyrin arrays containing core-modified porphyrin as one of the
porphyrin subunit are almost scarce due to lack of proper
synthetic methods to synthesize the functionalized core-modified
porphyrin building blocks. Furthermore, the examples of co-
valently linked unsymmetrical arrays comprised of five or more
macrocycles with two different macrocycles are very few in
literature. Lindsey and co-workers synthesized multiporphyrin-
phthalocyanine arrays such as pentads8a and nonads8b compris-
ing four porphyrins and one phthalocyanine and eight porphyrins
and one phthalocyanine, respectively, and demonstrated an
efficient energy transfer from porphyrins to phthalocyanine in
these novel systems (Chart 1). We synthesized unsymmetrical
porphyrin pentad containing four peripheral N4 porphyrin
subunits and one central N2S2 porphyrin subunit and showed
an efficient singlet-singlet energy transfer from peripheral N4
porphyrin subunits to central N2S2 porphyrin subunit (Chart 1).13
Recently, we also assembled three different types of porphyrin
subunits using both covalent and noncovalent approaches (Chart
1).14 Except for our one above-mentioned unsymmetrical
porphyrin triad, to the best of our knowledge, there are no reports
on unsymmetrical arrays containing more than two types of
macrocycles. In this paper, we synthesized the new AB3 type
tetrafunctionalized porphyrins with N4, N3O, N3S, and N2S2
cores by modifying the available methods. The tetrafunction-
alized porphyrin building blocks were then used for the synthesis
of four monofunctionalized porphyrin tetrads containing two
different types of porphyrin subunits. In the last step, the
monofunctionalized porphyrin tetrads were used to synthesize
four unsymmetrical pentads containing three different types of
porphyrins 1-4 (Chart 2). The preliminary photophysical studies
on pentads 1-4 clearly demonstrated an energy transfer from
three peripheral porphyrin subunits to central porphyrin subunit
and then from the central porphyrin subunit to the other
peripheral porphyrin subunit.
(3) (a) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35,
57. (b) Burrell, A. K.; Officer, D. L.; Plieger, P. G.; Reid, D. C. W. Chem. ReV.
2001, 101, 2751.
(4) (a) Prathapan, S.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc. 1993,
115, 7519. (b) Seth, J.; Palaniappan, V.; Johnson, T. E.; Prathapan, S.; Lindsey,
J. S. J. Am. Chem. Soc. 1994, 116, 10578.
Results and Discussion
(5) Arnold, D. P.; Hartnell, R. D. Tetrahedron 2001, 57, 1335.
Synthesis of AB3-Type Tetrafunctionalized Porphyrin
Building Blocks. To synthesize covalently linked diphenyl
ethyne-bridged unsymmetrical porphyrin pentads 1-4 contain-
ing three different types of porphyrin subunits (Chart 2), the
AB3 type of tetrafunctionalized porphyrin building blocks with
(6) (a) Kadish, K. M.; Fremond, L.; Ou, Z.; Shao, J.; Shi, C.; Anson, F. C.;
Burdet, F.; Gros, C. P.; Barbe, J. M.; Guilard, R. J. Am. Chem. Soc. 2005, 127,
5625. (b) Guilard, R.; Gros, C. P.; Barbe, J. M.; Espinosa, E.; Jerome, F.; Tabard,
A. Inorg. Chem. 2004, 43, 7441. (c) Guilard, R.; Burdet, F.; Barbe, J. M.; Gros,
C. P.; Espinosa, E.; Shao, J.; Ou, Z.; Zhan, R.; Kadish, K. M. Inorg. Chem.
2005, 44, 3972. (d) Kadish, K. M.; Shao, J.; Ou, Z.; Zhan, R.; Burdet, F.; Barbe,
J. M.; Gros, C. P.; Guilard, R. Inorg. Chem. 2005, 44, 9023. (e) Kadish, K. M.;
Shao, J.; Ou, Z.; Fremond, L.; Zhan, R.; Burdet, F.; Barbe, J. M.; Gros, C. P.;
Guilard, R. Inorg. Chem. 2005, 44, 6744. (f) Gros, C. P.; Brisach, F.; Meristoudi,
A.; Espinosa, E.; Guilard, R.; Harvey, P. D. Inorg. Chem. 2007, 46, 125.
(7) Shinoda, S.; Tsukube, H.; Nishimura, Y.; Yamazaki, I.; Osuka, A.
Tetrahedron. 1997, 53, 13657.
(8) (a) Li, J.; Diers, J. R.; Seth, J.; Yang, S. I.; Bocian, D. F.; Holten, D.;
Lindsey, J. S. J. Am. Chem. Soc. 1999, 64, 9090. (b) Li, J.; Lindsey, J. S. J.
Org. Chem. 1999, 64, 9101. (c) Yang, S. I.; Li, J.; Cho, H. S.; Kim, D.; Bocian,
D. F.; Holten, D.; Lindsey, J. S. J. Mater. Chem. 2000, 10, 283.
(9) Gupta, I.; Ravikanth, M. Coord. Chem. ReV. 2006, 250, 468.
(10) Latos-Grazynski, L. In The Porphyrin Handbook; Kadish, K. M.; Smith,
K. M.; Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 2, p 361.
(11) Van patten, P. G.; Shreve, A. P.; Lindsey, J. S.; Donhoe, R. J. J. Phys.
Chem. B 1998, 102, 4209.
(12) (a) Punidha, S.; Ravikanth, M. Tetrahedron 2004, 60, 8437. (b) Gupta,
I.; Ravikanth, M. J. Org. Chem. 2004, 69, 6796. (c) Punidha, S.; Agarwal, N.;
Ravikanth, M. Eur. J. Org. Chem. 2005, 2500. (d) Gupta, I.; Frohlich, R.;
Ravikanth, M. Chem. Commun. 2006, 3726. (e) Punidha, S.; Agarwal, N.; Gupta,
I.; Ravikanth, M. Eur. J. Org. Chem. 2007, 1168. (f) Rai, S.; Ravikanth, M.
Tetrahedron 2007, 11, 2455.
(13) Kumaresan, D.; Agarwal, N.; Ravikanth, M. J. Chem. Soc., Perkin Trans.
1 2001, 1644.
(14) Punidha, S.; Ravikanth, M. Synlett 2005, 14, 2199.
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Rai and Ravikanth
CHART 1. Structures of Unsymmetrical Porphyrin Arrays Containing More than One Type of Macrocycle
different cores such as N4, N3O, N3S, and N2S2 are required.
Since the diphenyl ethyne bridges were used to connect the
porphyrin subunits, the porphyrin building blocks bearing
functional groups such as iodophenyl and ethynylphenyl groups
at meso-positions are required for Pd-mediated coupling reac-
tions. The AB3 type of meso-functionalized N4 porphyrin
building block 5 containing three iodophenyl groups and one
protected ethynylphenyl group at the meso positions was
synthesized by condensing 3 equiv of 4-iodobenzaldehyde with
1 equiv of 4-(3-hydroxy-3-methylbut-1-ynyl)benzaldehyde and
4 equiv of pyrrole in propionic acid at refluxing temperature
for 3 h.15 After standard workup and one filtration column on
silica gel using dichloromethane as solvent, the TLC analysis
showed the formation of statistical mixture of six porphyrins.
The desired N4 porphyrin building block 5 was separated from
rest of the porphyrin mixture by silica gel column chromatog-
raphy and isolated pure 5 as purple solid in 10% yield. However,
to synthesize AB3 type N3O 6, N3S 7, and N2S2 8 porphyrin
building blocks, there are no proper synthetic methods available
in literature. Recently we developed mono-ol12b and unsym-
(15) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour, J.;
Korsakoff, L. J. Org. Chem. 1967, 32, 476.
8366 J. Org. Chem. Vol. 73, No. 21, 2008

Synthesis of Unsymmetrical Porphyrin Pentads
CHART 2. Unsymmetrical Porphyrin Pentads Containing Three Different Types of Porphyrin Subunits
metrical diol12c methods to synthesize the monofunctionalized
core-modified porphyrins and functionalized symmetrical diol
method14 to synthesize tetrafunctionalized A4 type N2S2 por-
phyrin building blocks. The AB3 type core-modified porphyrin
building blocks were prepared by modifying the mono-ol and
unsymmetrical diol methods.
condensing 1 equiv of mono-ol 9 and 10, respectively, with 3
equiv of 4-iodobenzaldehyde and 3 equiv of pyrrole in propionic
acid at refluxing temperature for 3 h (Scheme 1). The condensa-
tion resulted in the formation of mixture of two porphyrins:
the desired AB3-type tetrafunctionalized N3O porphyrin 6 or
N3S porphyrin 7 and 5,10,15,20-tetrakis(4-iodophenyl)porphyrin
with N4 core. The mixture of two porphyrins was separated by
column chromatography and afforded 6 and 7 in 6-7% yields.
The AB3-type tetrafunctionalized N2S2 porphyrin building block
8 was synthesized by condensing 1 equiv of diol 12 and 16-
thiatripyrrane 13 in propionic acid at refluxing temperature for
2 h (Scheme 1). The condensation resulted in the formation of
N2S2 porphyrin 8 as sole product. The crude compound was
purified by column chromatography and afforded 8 as purple
solid in 9% yield.
The required precursors, the furan mono-ol, 2-{[4-(3-hydroxy-
3-methylbut-1-ynyl)phenyl]hydroxymethyl}furan 9, thiophene
mono-ol, 2-{[4-(3-hydroxy-3-methylbut-1-ynyl)phenyl]hydroxy-
methyl}thiophene 10, unsymmetrical thiophene diol, 2-[(4-
iodophenyl) hydroxyl methyl]-5-{[4-(3-hydroxy-3-methyl but-
1-ynyl) phenyl]-hydroxylmethyl} thiophene 11, symmetrical
thiophene diol, 2,5-bis[(4-iodo phenyl)hydroxymethyl]thiophene
12, and symmetrical 16-thiatripyrrane, 5,10-di(4-iodophenyl)-
16-thia-15,17-dihydrotripyrrane 13 were synthesized as shown
in Scheme 1. The mono-ols 9 and 10 were synthesized16 by
treating furan and thiophene, respectively, with 1.2 equiv of
n-BuLi followed by 1.2 equiv of 4-(3-hydroxy-3-methylbut-1-
ynyl)benzaldehyde in THF at 0 °C. The crude compounds were
subjected to column chromatographic purification and the pure
mono-ols 9 and 10 were collected as white solids in 24% and
40% yields, respectively. The unsymmetrical diol 11 was
synthesized similarly in 32% yield by treating mono-ol 10 with
n-BuLi followed by 4-iodobenzaldehyde in THF at 0 °C and
the resultant crude compound was purified by column chroma-
tography on silica. The symmetrical thiophene diol 12 was
synthesized in 65% yield by treating thiophene with 2.5 equiv
of n-BuLi followed by treatment with 2.2 equiv of p-iodoben-
zaldehyde. The 16-thiatripyrrane 13 was synthesized17 by
reacting thiophene diol 12 with excess of pyrrole under mild
acid-catalyzed conditions, and the crude compound was purified
by silica gel column chromatography. The precursor compounds
9-13 were characterized by mp, IR, mass, NMR, and elemental
analysis techniques.
The tetrafunctionalized N4, N3O, N3S, and N2S2 porphyrins
5-8 were characterized with ES-MS mass, NMR, absorption,
fluorescence and elemental analysis techniques. All four tet-
rafunctionalized porphyrins 5-8 showed molecular ion peak
in ES-MS mass spectra confirming the identity of the com-
pounds (Supporting Information). In the 1H NMR spectrum of
5, the eight ꢀ-pyrrole protons appeared as singlet although
multiplet is expected due to the unsymmetric substitution of
N4 porphyrin building block. Similarly, the N3O porphyrin
building block 6 showed a singlet for two ꢀ-furan protons and
three sets of signals for six ꢀ-pyrrole protons. The N3S porphyrin
7 showed a multiplet for two ꢀ-thiophene protons and three
sets of signals for six ꢀ-pyrrole protons (Supporting Informa-
tion). The four ꢀ-thiophene and four ꢀ-pyrrole protons in N2S2
porphyrin building block 8 appeared as two singlets. Thus,
except N3S porphyrin 7, the other three porphyrins N4 5, N3O
6, and N2S2 8 porphyrins did not show the splitting of ꢀ-pyrrole,
ꢀ-furan, and ꢀ-thiophene protons, although these porphyrins are
unsymmetrically substituted. The absorption and emission
spectra of N4 5, N3O 6, N3S 7, and N2S2 7 porphyrins showed
characteristic porphyrin bands with peak maxima matching
closely with corresponding symmetrically substituted meso-
tetratolylporphyrins11 such as H2TTP, OTTPH, STTPH, and
The AB3-type tetrafunctionalized N3O and N3S porphyrin
building blocks 6 and 7, respectively, were synthesized by
(16) Ulman, A.; Manassen, J. J. Chem. Soc., Perkin Trans. 1979, 1066.
(17) Heo, P.-Y.; Shin, K.; Lee, C.-H. Bull. Korean Chem. Soc. 1996, 17,
515.
J. Org. Chem. Vol. 73, No. 21, 2008 8367

Rai and Ravikanth
SCHEME 1. Synthesis of AB3-Type Tetrafunctionalized Porphyrins 6-8
S2TTP, respectively. The elemental analyses were also in
agreement with the composition of the porphyrin building blocks
5-8.
chromatography and isolated pure tetrad 14 in 52% yield.
Similarly, the tetrad (ZnN4P)3-N3OP 15 was prepared in 30%
yield by coupling of 18 and 6 under similar Pd(0) coupling
conditions. The tetrad (ZnN4P)3-N3SP 16 was obtained in 48%
yield by coupling of 18 and 7 under similar reaction conditions.
Coupling of 17 and 8 under the same mild Pd(0) conditions
followed by silica gel column chromatographic purification
afforded tetrad (ZnN4P)3-N2S2P 17 in 50% yield. To afford
the tetrads containing monoethynyl functional group 14a-17a,
the deprotection of ethynyl group of tetrads 14-17 was carried
out by treating 14-17 with KOH in benzene/methanol at 80
°C (Scheme 2).
Synthesis of Covalently Linked Unsymmetrical Porphy-
rin Tetrads 14-17 and Pentads 1-4. The tetrafunctionalized
porphyrin building blocks containing three meso-iodophenyl
groups and one 3-hydroxy-3-methylbut-1-ynylphenyl group with
different cores 5-8 were used to synthesize the monofunction-
alized porphyrin tetrads containing two different macrocycles
such as (ZnN4P)3-N4P 14, (ZnN4P)--N3OP 15, (ZnN4P)3-
N3SP 16, and (ZnN4P)-N2S2P 17 as shown in Scheme 2. The
other desired monofunctionalized porphyrin building block, 5-[4-
(3-hydroxy-3-methylbut-1-ynyl)-10,15,20-tri(p-tolyl)zinc(II)por-
phyrin 18 was synthesized by following literature procedure.18
The diphenylethyne bridged tetrads were synthesized under mild
copper-free Pd(0) coupling conditions.19 The tetrad 14 contain-
ing three ZnN4 porphyrin subunits and one N4 porphyrin subunit
was synthesized by coupling of 3.2 equiv of 18 and 1 equiv of
5 in toluene/triethylamine at 40 °C in the presence of a catalytic
amount of AsPh3 and Pd2(dba)3 overnight. The TLC analysis
after 12 h showed the appearance of new major spot along with
two other minor spots corresponding to the starting materials.
The crude compound was subjected twice to silica gel column
In the final step, the monofunctionalized tetrads 14a-17a
have been used to synthesize covalently linked unsymmetrical
porphyrin pentads 1-4 containing three different porphyrin
subunits (Scheme 3). The other required monofunctionalized
N2S2 porphyrin building blocks such as 5-(4-iodophenyl)-
10,15,20-tri(p-tolyl)-21,23-dithiaporphyrin12c 19 and 5-(4-io-
dophenyl)-10,15,20-tri(2-furyl)-21,23-dithiaporphyrin12f 20 were
prepared as reported earlier. The (ZnN4)3-N4-N2S2 porphyrin
pentad 1 was synthesized by coupling of tetrad 14 and N2S2
porphyrin building block 19 in toluene/triethylamine at 35 °C
in the presence of a catalytic amount of AsPh3/Pd2(dba)3.19 The
progress of the reaction was monitored with TLC and the
reaction was stopped after the disappearance of spots corre-
sponding to starting materials and the appearance of new spot
corresponding to unsymmetrical porphyrin pentad 1. The crude
(18) Cho, W.-S.; Kim, H.-J.; Littler, B. J.; Miller, M. A.; Lee, C.-H.; Lindsey,
J. S. J. Org. Chem. 1999, 64, 7890.
(19) (a) Wagner, R. W.; Johnson, T. E.; Li, F.; Lindsey, J. S. J. Org. Chem.
1995, 60, 5266–5273. (b) Farina, A.; Krishnan, V. B. J. Am. Chem. Soc. 1991,
113, 9585.
8368 J. Org. Chem. Vol. 73, No. 21, 2008

Synthesis of Unsymmetrical Porphyrin Pentads
SCHEME 2. Synthesis of Covalently Linked Unsymmetrical (ZnN4)3-N3X Porphyrin Tetrads
compound was subjected twice to silica gel column chroma-
tography and afforded (ZnN4)3-N4-N2S2 porphyrin pentad 1
in 52% yield. The pentads (ZnN4)3-N3O-N2S2 2 and
(ZnN4)3-N3S-N2S2 3 were prepared by coupling of tetrads 15
and 16, respectively, with N2S2 porphyrin building block 19
under similar mild Pd(0)-mediated coupling conditions.19 The
crude pentads 2 and 3 were purified by column chromatography
and afforded pure compounds 2 and 3 in 30-40% yields. The
pentad (ZnN4)-N2S2-N2S2 4 was prepared by coupling of
tetrad 17 and N2S2 porphyrin building block 20 under same
Pd(0) mediated coupling conditions19 and afforded in 45% yield.
All coupling reactions worked smoothly and the progress of
the reaction was followed easily by tlc analysis at regular
intervals. Two silica gel column chromatographic purifications
are required to afford pure tetrads 14-17 and pentads 1-4 in
decent yields and all compounds are readily soluble in most of
the common organic solvents. The tetrads 14-17, their depro-
tected forms 14a-17a, and pentads 1-4 were characterized by
ES-MS mass, NMR, elemental analysis, absorption, cyclic
voltammetry, steady-state, and time-resolved fluorescence tech-
niques. The tetrads 14-17, deprotected tetrads 14a-17a, and
pentads 1-4 showed molecular ion peak in ES-MS mass spectra
confirming the identity of the compounds (Supporting Informa-
tion).
NMR Studies of Tetrads 14-17, and Pentads 1-4. NMR
spectroscopy was used to characterize the tetrads 14-17 depro-
tected tetrads 14a-17a and pentads 1-4 in detail. Since tetrads
14-17 and pentads 1-4 have two and three different types of
porphyrin subunits respectively, the 1H NMR resonances of tetrads
and pentads were assigned on the basis of spectra observed for
the two and three constituted porphyrin monomers of corresponding
tetrad and pentad taken independently.
In the 1H NMR spectrum of tetrad 14 containing three ZnN4
porphyrin subunits and one N4 porphyrin subunits, the 32
ꢀ-pyrrole protons; 24 protons corresponding to three ZnN4
porphyrin subunits and 8 protons corresponding to N4 porphyrin
subunit were appeared as overlapping signals in 8.60-9.10 ppm
region. The two inner NH protons of N4 porphyrin subunit
appeared at -2.80 ppm. In tetrad 15 containing three ZnN4
porphyrin subunits and one N3O porphyrin subunit, although
the 30 pyrrole signals were appeared as series of overlapping
signals in 8.80-9.00 ppm region, the two ꢀ-furan protons were
appeared separately as multiplet in 9.16-9.18 ppm region
1
confirming the composition of the tetrad. Similarly in the H
NMR spectrum of tetrad 16 containing one N3S porphyrin and
three ZnN4 porphyrin subunits, the two ꢀ-thiophene protons of
N3S porphyrin subunit were observed as two sets of doublets
in 9.60-9.80 ppm region and the ꢀ-pyrrole protons of all four
J. Org. Chem. Vol. 73, No. 21, 2008 8369

Rai and Ravikanth
SCHEME 3. Synthesis of Covalently Linked Unsymmetrical (ZnN4)3-N2XY-N2S2 Porphyrin Pentads 1-4
porphyrin subunits were appeared as overlapping signals in
8.60-9.00 ppm region. The tetrad 17 containing one N2S2
porphyrin and three ZnN4 porphyrin subunits also showed two
sets of doublets for four ꢀ-thiophene protons of N2S2 porphyrin
The pentads 1-4 containing three different porphyrin subunits
showed resonances corresponding to all three porphyrin subunits
in 1H NMR spectra (Supporting Information). For, e.g., pentad
2 containing ZnN4, N3O, and N2S2 porphyrin subunits, the
specific resonances corresponding to each porphyrin subunit
such as signals at 9.2 ppm of ꢀ-furan protons of N3O porphyrin
subunit and signals at 9.8 ppm of ꢀ-thiophene protons of N2S2
porphyrin subunit were present (Supporting Information).
Similarly, the other three pentads 1, 3, and 4 also showed
features corresponding to three different porphyrin subunits. A
1
subunit in 9.60-9.80 ppm as expected. The H NMR spectra
of deprotected tetrads 14a-17a also exhibited same features
as described for tetrads 14-17. The signal at δ ) 3.3 ppm
corresponding to ethyne CH proton in 1H NMR spectra
confirmed the identity of the porphyrin tetrads 14a-17a
containing one phenylethynyl functional group.
8370 J. Org. Chem. Vol. 73, No. 21, 2008

Synthesis of Unsymmetrical Porphyrin Pentads
TABLE 1. Absorption Data of Porphyrin Tetrads 14-17 and a
TABLE 2. Absorption Data of Porphyrin Pentads 1-4 and 1:1
Mixture of Their Corresponding Tetrads and Monomers Recorded
in Toluene
3:1 Mixture of Corresponding Monomers Recorded in Toluene
Soret
band λ (nm)
absorption Q-bands λ (nm)
Soret
porphyrin
(ꢀ × 10-4
)
(ꢀ × 10-3
)
band λ (nm)
absorption Q-bands λ (nm)
compd
(ꢀ × 10-4
)
(ꢀ × 10-3
)
14
420 (45.2)
516 (22.2), 551 (9.5), 593 (6.4), 649 (5.5)
1
420 (40.2)
436 (36.4)
516 (22.2), 551 (9.5), 593 (6.2), 649 (5.1),
699 (3.2)
3:1 mixture of 419 (101.1) 515 (29.2), 550 (48.9), 590 (14.4), 650 (15.6)
18 and 5
1:1 mixture of
14 and 19
420 (99.2)
436 (89.5)
515 (42.6), 550 (16.8), 592 (13.9), 650 (20.2)
698 (10.1)
15
423 (45.2)
422 (90.2)
515 (21.2), 552 (9.2), 592 (8.4), 619 (6.4),
673 (5.5)
2
424 (31.8)
436 (20.8)
516 (20.2), 551 (8.9), 590 (6.2), 616 (5.8)
672 (4.2), 699 (sh)
3:1 mixture of
515 (27.3), 550 (42.9), 593 (16.1), 618 (15.2),
672 (12.2)
18 and 6
1:1 mixture of
15 and 19
421 (97.2)
436 (sh)
515 (46.5), 550 (16.6), 590 (13.9), 616 (19.2)
672 (10.6), 699 (7.4)
16
423 (sh)
430 (18.6)
516 (15.5), 550 (5.4), 592 (4.2), 624 (2.4)
679 (3.8)
3
423 (23.9)
436 (20.2)
515 (17.5), 552 (11.2), 590 (5.4), 635 (4.4),
680 (sh), 699 (3.2)
3:1 mixture of
18 and 7
420 (89.2)
515 (26.2), 550 (38.7), 592 (15.9), 625 (14.8),
678 (9.2)
1:1 mixture of
16 and 19
432 (92.5)
436 (sh)
516 (45.8), 551 (22.3), 590 (15.6), 636 (18.6)
679 (4.5), 699 (7.2)
17
422 (21.3)
435 (sh)
514 (19.3), 552 (5.8), 592 (1.4), 631 (3.5)
699 (3.6)
4
422 (25.4)
454 (sh)
517 (17.8), 551 (12.4), 573 (5.6), 590 (5.3),
702 (3.8), 717 (sh)
3:1 mixture of
18 and 8
420 (88.7)
515 (27.2), 552 (38.9), 592 (16.2), 631 (14.8),
699 (9.9)
1:1 mixture of
17 and 20
422 (93.9)
452 (sh)
515 (46.9), 551 (25.6), 573 (16.2), 590 (19.2)
700 (10.1), 717 (6.8)
comparison of chemical shifts of the various protons of tetrads
14-17 and pentads 1-4 with those of corresponding monomeric
porphyrin units indicate only minor differences, suggesting that
the porphyrin subunits in tetrads 14-17 and pentads 1-4
interact very weakly.
in the Soret region with peak maxima at 423 and 436 nm (Table
2). In this pentad 3, the absorption bands at 699, 635, and 436
nm mainly correspond to N2S2 porphyrin subunit; 680 nm is
exclusively from the N3S porphyrin subunit, and bands at 590,
550, and 423 nm are mainly because of the ZnN4 porphyrin
subunit. Unlike tetrads 14-17 which showed a broad Soret band,
the pentads 1-4 showed a split Soret band. This is due to the
relatively large difference between the Soret absorption peak
maxima of the three different macrocycles of the pentad.
Similarly, the absorption spectra of the other three pentads 1,
2, and 4 are nearly the sum of the spectra of the corresponding
three different macrocycles of those pentads (Table 2). Although
the peak maxima of absorption bands of pentads is nearly same
as those of their corresponding monomers, the extinction
coefficients of pentads are much lower than 1:1 mixture of their
corresponding tetrad and N2S2 porphyrin monomer (Table 2)
indicating that the porphyrin subunits in the pentad interact very
weakly.
Absorption Properties of Tetrads 14-17 and Pentads
1-4. The absorption spectra of tetrads 14-17 and their
corresponding porphyrin monomers in 3:1 ratio and pentads 1-4
and their corresponding 1:1 mixture of tetrad and N2S2 porphyrin
monomers were recorded in toluene and data are presented in
Tables 1 and 2, respectively. The comparison of absorption
spectra of tetrad 17 and 3:1 mixture of monomers of ZnN4
porphyrin and N2S2 porphyrin recorded in toluene is shown in
Figure 1. Inspection of Figure 1 and Table 1 indicates that the
absorption spectra of tetrads showed features of both monomeric
porphyrin subunits11 with almost no shifts in peak maxima
indicating that the constituted monomeric porphyrins retained
their properties in tetrads. For example, in tetrad 17, the
absorption bands at 699, 631, and 514 nm in the Q-band region
mainly correspond to N2S2 porphyrin subunit and the bands at
552 and 592 nm belong to the ZnN4 porphyrin subunit.
Similarly, the Soret band at 435 nm corresponds to the N2S2
porphyrin subunit and the band at 422 nm appeared as shoulder
corresponds to ZnN4 porphyrin subunits. The other three tetrads
14-16 also showed similar absorption features. The Soret band
in tetrads 14-16 did not show any splitting but broadened
because the peak maxima of the corresponding monomers of
the tetrads 14-16 were close to each other. However, the
extinction coefficients of all absorption bands of tetrads 14-17
compared to their 3:1 mixture of corresponding monomers
(Table 1) were quite different, indicating that the porphyrin
subunits in tetrads 14-17 interact very weakly.
The comparison of absorption spectra of (ZnN4)3-N3S tetrad
16 and (ZnN4)--N3S-N2S2 pentad 3 is shown in Figure 2. The
absorption spectra of pentad 1-4 are just linear combination
of the spectra of all three different macrocycles with only minor
differences in wavelength maxima and band shapes. For
example, the (ZnN4)--N3S-N2S2 pentad 3 (Figure 2) showed
six absorption bands in the Q-band region with absorption
maxima at 515, 552, 590, 635, 680, and 699 nm and two bands
FIGURE 1. Comparison of Q-band and Soret band (inset) absorption
spectra of (ZnN4P)3-N2S2P tetrad 17 (s) and 3:1 mixture of corre-
sponding monomers (---) recorded in toluene.
J. Org. Chem. Vol. 73, No. 21, 2008 8371

Rai and Ravikanth
TABLE 3. Electrochemical Redox Data (V) of Porphyrin
Monomers, Tetrads 14-17, and Pentads 1-4 in Dichloromethane
Containing 0.1 M TBAP as Supporting Electrolyte
ECT
compd
oxidation
reduction
-1.35
(M+, M-) E0-0
18 0.77 1.08
5
6
7
8
20
14 0.73 0.98 1.28
15 0.72 0.92 1.23
16 0.72 1.02 1.39
17 0.71 1.01 1.38 -0.98
1
2
3
4
-1.71
2.06
1.90
1.85
1.82
1.77
1.03 1.30
1.10
1.11 1.28
1.18
0.92 1.24 -0.82
-1.11
-1.52
-1.20
-1.03 -1.35
-1.23
-0.94
-1.18
1.63
-1.18 -1.55 -1.75
-1.16 -1.54 -1.75
-1.46 -1.69
1.91
1.88
1.74
1.61
1.62
1.62
1.62
1.52
-1.02
-1.22
-1.18 -1.53 -1.75
-1.29 -1.51 -1.74
-1.72
0.72 1.03 1.31 -0.93
0.72 0.91 1.24 -0.90
0.70 1.02 1.29 -0.92 -1.10 -1.22 -1.40 -1.64
0.72 0.98 1.25 -0.80 -1.03 -1.27 -1.56 -1.73
FIGURE 2. Q-band absorption spectra of 3 (s) and 16 (---) recorded
in toluene. The inset shows their corresponding Soret band absorption
spectra. The concentrations used for Q- and Soret band spectra were 5
× 10-5 M and 5 × 10-6 M, respectively.
at 0.73 V was due to oxidation of the ZnN4P subunit because
it is easier to oxidize as compared to free base N4P. The
oxidation at 1.28 V was assigned to the oxidation of N4P,
and oxidation at 0.98 V was due to the oxidation of both
ZnN4P and N4P subunits. Similarly, the three reductions of
14 at -1.18, -1.55, and -1.75 V were assigned as follows:
the first reduction at -1.18 V was due to N4P porphyrin, the
last reduction at -1.75 V was assigned to ZnN4P porphyrin,
and reduction at -1.55 V was due to to both N4P and ZnN4P
porphyrin subunits. Similarly, the tetrads 15-17 also exhib-
ited oxidation and reduction waves, and the peak potentials
were in the same range of their corresponding porphyrin
monomers (Table 3). Pentads 1-4 also exhibited similar
redox features. Although all three different porphyrin subunits
of pentads 1-4 have one or two overlapping redox potentials,
it is noted that atleast one oxidation or reduction wave which
is an exclusive characteristic feature of each porphyrin
subunit of the pentad is present. For example, in ZnN4 por-
phyrin, (ZnN4)3-N3S porphyrin tetrad 16 and (ZnN4)3-
N3S-N2S2 porphyrin pentad 3 series, the ZnN4 porphyrin
shows a specific oxidation at 0.71 V, the (ZnN4)3-N3S
porphyrin tetrad 16 showed a specific reduction at -1.02 V
corresponding to N3S porphyrin subunit and (ZnN4)3-
N3S-N2S2 porphyrin pentad 3 showed a specific reduction
at -0.92 V characteristic of the N2S2 porphyrin subunit of
pentad 3. These specific oxidation and reduction potentials
help in the characterization of these compounds. Thus, the
electrochemical studies were in agreement with the absorption
studies of tetrads 14-17 and pentads 1-4 and studies support
that the porphyrin subunits in tetrads and pentads interact
very weakly.
Steady-State and Time-Resolved Fluorescence Studies
of Tetrads 14-17 and Pentads 1-4. The steady-state fluo-
rescence properties of tetrads 14-17, pentads 1-4, and ap-
propriate reference compounds were recorded in toluene at room
temperature (Table 4). In tetrads 14-17, the ZnN4 porphyrin
subunit acts as an energy donor and absorbs strongly at 550
nm, and N4, N3O, N3S, and N2S2 porphyrin subunits act as
energy acceptors and do not absorb strongly at 550 nm. The
emission spectra of tetrad 17 along with its 3:1 mixture of ZnN4
and N2S2 porphyrins using excitation wavelength of 550 nm
and an excitation spectrum of tetrad 17 using emission
wavelength of 750 nm recorded in toluene are shown in Figure
4. Inspection of Figure 4 indicates that on illumination of tetrad
17 at 550 nm where the ZnN4 porphyrin subunit absorbs
strongly, the emission of ZnN4 causes the porphyrin subunit to
FIGURE 3. Comparison of cyclic voltammograms (scan rate, 50
mVs-1) of (a) tetrad 16 and (b) pentad 3 in CH2Cl2 containing 0.1 M
TBAP as supporting electrolyte.
Electrochemical Properties of Tetrads 14-17. The redox
chemistry of covalently linked unsymmetrical porphyrin
tetrads 14-17 containing two different macrocycles and
porphyrin pentads 1-4 containing three different macrocycles
was followed by cyclic voltammetry at a scan rate of 50 mV/s
and differential pulse voltammetry at a scan rate of 20 mV/s
using tetrabutylammonium perchlorate as supporting elec-
trolyte (0.1 M) in dichloromethane. In general, the oxidation
and reduction waves of two porphyrin subunits in tetrads
14-17 and three porphyrin subunits in pentads 1-4 were
present and the peak potentials were matching closely with
the corresponding monomeric porphyrins. The tetrads 14-17
showed three oxidations and three reductions (Figure 3). Both
oxidations and reductions are reversible or quasi-reversible
(Ep ) 60-120 mV), and all potentials were assigned easily
on the basis of electrochemical data of their corresponding
monomers11 (Table 3). For example, the tetrad 14 showed
three oxidations at 0.73, 0.98, and 1.28 V. The first oxidation
8372 J. Org. Chem. Vol. 73, No. 21, 2008

Synthesis of Unsymmetrical Porphyrin Pentads
TABLE 4. Emission Data of Tetrads 14-17 and Pentads 1-4 in
Toluene
-1
compd
Φf (donor)
% donor
τDA (ps)
ΦENT
KENT (ps)
ZnTPP
0.033
2070
66
89
99
136
80
112
128
142
14
15
16
17
1
2
3
0.0006
0.0008
0.0010
0.0006
0.0007
0.0008
0.0012
0.0010
98
98
97
98
98
98
96
97
0.97
0.95
0.95
0.94
0.96
0.94
0.94
0.93
68
93
104
145
83
118
136
152
4
be quenched by 98%, and the strong emission was observed
from N2S2 porphyrin subunit. However, when a 3:1 mixture of
ZnN4 porphyrin and N2S2 porphyrin was excited at 550 nm,
the strong emission was exclusively observed from ZnN4
porphyrin. The excitation spectrum of tetrad 17 recorded at 750
nm matched with the absorption spectrum. These results
indicated that there is an efficient singlet-singlet energy transfer
from ZnN4 porphyrin subunit to N2S2 porphyrin subunit in tetrad
17. Similarly, for the tetrads 14-16 on excitation at 550 nm,
the emission from ZnN4 porphyrin was quenched by 97-98%
and the strong emission was observed from N4, N3O, and N3S
porphyrin subunits, respectively, supporting an efficient energy
transfer from the ZnN4 porphyrin subunit to the respective
acceptor porphyrin subunit in tetrads 14-16.
FIGURE 4. Comparison of emission spectra of (ZnN4P)3-N2S2P tetrad
17 (· · ·), and 3:1 mixture of 8 and 18 (---) recorded at an excitation
wavelength 550 nm in toluene. The excitation spectrum of tetrad 17
(s) recorded at emission wavelength 750 nm is also shown.
The emission spectra of pentad 3 and a 3:1:1 mixture of the
corresponding monomers recorded at 550 nm is shown in Figure
5. As is evident from Figure 5, in the pentad 3, excitation at
550 nm, where the ZnN4 porphyrin subunit absorbs strongly (φ
) 0.033 for ZnTPP), the emission of the ZnN4 porphyrin subunit
was quenched by 97% (φ ) 0.0008) and the strong emission
from the N2S2 porphyrin subunit was observed. However, when
a 3:1:1 mixture of corresponding monomers was irradiated at
550 nm, a strong emission was observed mainly from the ZnN4
porphyrin subunit (Figure 5). These results indicate that there
is an efficient energy transfer from the ZnN4 porphyrin subunit
to the N2S2 porphyrin subunit in pentad 3. The energy transfer
that occurs in 3 is independent of the excitation wavelength.
The excitation spectrum recorded for 3 at λem ) 730 nm matches
with the absorption spectrum, further confirming the efficient
energy transfer among the subunits.
The tetrads 14-17and pentads 1-4 were studied by time-
resolved fluorescence spectroscopic technique which also sup-
port the efficient energy transfer within tetrads 14-17 and
pentads 1-4. The tetrads 14-17 were excited at 406 nm and
monitored at two different wavelengths corresponding to the
emission peak maxima of donor ZnN4 porphyrin subunit and
acceptor porphyrin such as N4 in 14, N3O in 15, N3S in 16, and
N2S2 in 17. The fluorescence decays of tetrads 14-17 monitored
at emission peak maxima of acceptor porphyrin subunit was
fitted to single exponential and the lifetime was close to the
corresponding porphyrin monomer. The fluorescence decays of
tetrads 14-17 monitored at emission peak maxima of donor
ZnN4 porphyrin subunit were fitted to two or three exponential
with a dominant contribution from one component (Supporting
Information). The other one or two minor components observed
were attributed to the monomeric porphyrin impurities present
in the tetrad. The major component decay is generally the
FIGURE 5. Comparison of emission spectra of pentad 3 (s) and 3:1:1
mixture of corresponding monomers (---) recorded at excitation
wavelength 550 nm in toluene.
quenched lifetime of donor ZnN4 porphyrin subunit. The donor
ZnN4porphyrin subunit was quenched due to singlet-singlet
energy transfer from donor ZnN4 porphyrin subunit to the
acceptor porphyrin subunit in tetrad such as N4 in 14, N3O in
15, N3S in 16, and N2S2 porphyrin subunit in 17. The rate of
energy transfer (kENT) and the efficiency of energy transfer
(φENT) were calculated20 from the measured lifetime (τDA) of
the ZnN4 porphyrin subunit in tetrads and ZnTPP (τD) using
the following equations:
kENT ) 1 ⁄ τDA - 1 ⁄ τD
ꢁENT ) kENTτD
(1)
(2)
The kENT and φENT data indicate that the efficiencies (>90%)
are almost same as that of diphenylethyne bridged porphyrin
dyads reported in literature21 but the energy transfer rates are
found to be slower than the reported dyads.2a,b,20 Furthermore,
we evaluated through-bond (TB) and through-space (TS)
contributions to the overall rate of energy transfer in terms of
Fo¨rster theory.21 The Fo¨rster process is mediated by the
donor-acceptor distance and orientation as well as their
(20) Hsiao, J.-S. B. P; Krueger, B. P.; Wagner, R. W.; Johnson, T. E.;
Delaney, J. K.; Mauzerall, D. C.; Fleming, G. R.; Lindsey, J. S.; Bocian, D. F.;
Donohoe, R. J. J. Am. Chem. Soc. 1996, 118, 11181.
(21) Fo¨rster, Th. Ann. Phys. 1948, 2, 55.
J. Org. Chem. Vol. 73, No. 21, 2008 8373

Rai and Ravikanth
TABLE 5. Through-Bond and through-Space Energy Transfer
Rates and Contributions for Covalently Linked Porphyrin Tetrads
14-17 and Pentads 1-4
-1
-1
compd
J (10-14
)
KTS (ps)
KTB (ps)
ꢂTS
ꢂTB
14
15
16
17
1
2
3
4.22
4.05
1.91
4.31
4.48
4.52
4.97
7.98
473
498
1040
763
617
1612
546
79
116
116
178
99
127
181
193
0.14
0.19
0.10
0.18
0.13
0.07
0.24
0.20
0.86
0.81
0.90
0.82
0.86
0.92
0.75
0.80
4
719
emission and absorption spectral overlap characteristics. In the
Fo¨rster theory of energy transfer, the rate is given by
kTS ) (8.8 × 1023)K2ꢁfJn-4τD-1R-6
where K2 is the orientation factor, φf is the fluorescence quantum
yield of the donor in the absence of acceptor, J (in cm6 mmol-1
(3)
)
is the spectral overlap integral, n is the solvent refractive index
(1.49 for toluene), τD is the donor lifetime in the absence of
acceptor and R is the center-to-center distance of donor-acceptor
in Å. The overlap integral J is calculated from the equation
J ) FD(ν)ε(ν)ν-1dν
(4)
FIGURE 6. Energies of singlet and charge-transfer states of the pentad
2 containing ZnN4, N3O, and N2S2 porphyrin subunits.
where FD (ν) is the fluorescence intensity of the donor in
wavenumber units with total intensity normalized to unity, ꢀ(ν)
is the absorbance coefficient of the acceptor and ν is the
wavenumber in cm-1. The center-to-center distance of
donor-acceptor was taken as 20 Å based on MM+ force field
calculations, and 1.125 was used for K2. The relative contribu-
tions of TS (ꢂTS) and TB (ꢂTB) energy transfer to overall rate
were calculated using the following equations:20
accounts for greater than 90% of the observed rate (Table 5).
The energy level diagram shown in Figure 6 suggest that the
photoinduced electron transfer was also partly responsible in
addition to energy transfer for quenching of ZnN4 porphyrin
emission in pentads 1-4. A detailed photophysical studies are
required to understand the excited-state dynamics of porphyrin
pentads with three different porphyrin subunits reported in this
paper.
kENT ) kTB + kTS
ꢂTS ) kTS ⁄ kENT
ꢂTB ) kTB ⁄ kENT
(5)
(6)
(7)
Conclusions
In conclusion, we synthesized four new AB3-type tetrafunc-
tionalized porphyrin monomeric building blocks with N4, N3O,
N3S, and N2S2 cores by modifying the existing methods. The
tetrafunctionalized porphyrin building blocks were used to
synthesize the monofunctionalized porphyrin tetrads by coupling
of one equivalent of AB3 type tetrafunctionalized porphyrin
building block with 3 equiv of monofunctionalized ZnN4
porphyrin building blocks under Pd(0) coupling conditions. In
the final step, the monofunctionalized porphyrin tetrads were
coupled with monofunctionalized N2S2 porphyrin building
blocks under similar Pd(0) coupling conditions and afforded
porphyrin pentads containing three different porphyrin cores in
decent yields. The unsymmetrical porphyrin tetrads containing
two different porphyrin cores and unsymmetrical porphyrin
pentads containing three different porphyrin cores were char-
acterized by spectral and electrochemical techniques. The NMR,
absorption, and electrochemical studies on unsymmetrical
porphyrin tetrads and pentads indicate that the two and three
types of porphyrin subunits in tetrads and pentads respectively
retain their individual properties supporting weak interaction
among the porphyrin subunits. The steady state and time-
resolved fluorescence studies support an efficient energy transfer
at singlet state from peripheral donor ZnN4 porphyrin subunits
to N4, N3S, N3O, and N2S2 porphyrin subunits in tetrads and
peripheral ZnN4 porphyrin subunits to acceptor N2S2 porphyrin
subunit mediated via central N4, N3O, N3S, and N2S2 porphyrin
The data presented in Table 5 reveals that the TB contribution
accounts for greater than 90% of the observed rate as noted
previously for similar kind of diarylethyne bridged porphyrin
arrays.2a,b,20 In addition to energy transfer, the electron-transfer
quenching interaction may also be possible in these systems.
Hence we evaluated the single state energy levels and charge
transfer states using fluorescence and redox potential data22
(Figure 6). As shown in Figure 6, the quenching of ZnN4
porphyrin fluorescence is also partly due to the photoinduced
electron transfer which is also possible in these tetrads.
The time-resolved emission studies also support the efficient
energy transfer in pentads 1-4. The fluorescence decays of
pentads were fitted to two or three exponential with one major
component (95%) and one or two minor components. The minor
components are due to small impurities of monomeric porphy-
rins present in the pentads. The major component decay was
attributed to the decreased lifetime of ZnN4 porphyrin subunit
in pentads due to the singlet-singlet excitation energy transfer
from the ZnN4 porphyrin subunit to the terminal N2S2 porphyrin
subunit. The ΚENT and φENT calculated for pentads 1-4 indicate
that the efficiencies (90%) are good, but the energy transfer rates
are slower than the reported porphyrin arrays. Furthermore, it
is established that in pentads also through-bond contribution
(22) Giribabu, L.; Rao, T. A.; Maiya, B. G. Inorg. Chem. 1999, 38, 4971.
8374 J. Org. Chem. Vol. 73, No. 21, 2008

Synthesis of Unsymmetrical Porphyrin Pentads
(ZnN4)3-N3S-N2S2 Porphyrin Pentad 3. Coupling of tetrad
16a (10.0 mg, 3.46 µmol) and 5-(4-iodophenyl)-10,15,20-tri(p-
tolyl)-21,23-dithiaporphyrin 20 (2.83 mg, 3.46 µmol) in dry toluene/
triethylamine (3:1, 30 mL) in the presence of AsPh3 (1.28 mg, 4.18
µmol) and Pd2(dba)3 (0.48 mg, 0.52 µmol) at 35 °C for 10 h
followed by slica gel column chromatographic purification using
dichloromethane/methanol (97:3) afforded the desired pentad 3 as
a violet solid in 40% yield (5 mg). 1H NMR (400 MHz, CDCl3, 25
°C): δ ) -2.69 (brs, 1H), 2.74 (s, 36H), 7.30-7.38 (m, 24H),
7.53 (d, J ) 8.0 Hz, 8H), 7.58-7.64 (m, 10H), 7.80-7.84 (m,
4H), 7.89 (d, J ) 8.0 Hz, 4H), 7.92-7.97 (m, 6H), 8.05-8.09 (m,
8H), 8.10-8.17 (m, 12H), 8.22 (d, J ) 8.0 Hz, 4H), 8.60-8.66
(m, 8H), 8.64-8.72 (m, 10H), 8.76 (d, J ) 4.4 Hz, 6 H), 8.82 (d,
J ) 4.4 Hz, 6 H), 8.93-8.96 (m, 4 H), 9.62 (d, J ) 4.6 Hz, 1 H),
9.68-9.72 (m, 4 H), 9.77 (d, J ) 4.6 Hz, 1 H) ppm. 13C NMR
(100 MHz, CDCl3, 25 °C): δ ) 21.4, 31.4, 81.8, 91.0, 123.9, 127.3,
127.4, 128.2, 128.9, 131.3, 132.6, 134.1, 134.3, 135.7, 136.5, 137.5,
137.9, 139.1, 141.1, 144.1, 147.3, 152.7, 154.5 ppm. Anal. Calcd:
C, 80.71; H, 4.43; N, 6.67; S, 2.69. Found: C, 80.62; H, 4.58; N,
6.48; S, 2.84.
(ZnN4)3-N2S2-N2S2 Porphyrin Pentad 4. Samples of 17a
(10.0 mg, 3.44 µmol) and 5-(4-iodophenyl)-10,15,20-tri(2-furyl)-
21,23-dithiaporphyrin 20 (2.81 mg, 3.44 µmol) were coupled in
dry toluene/triethylamine (3:1, 15 mL) in the presence of a catalytic
amount of AsPh3 (1.26 mg, 4.12 µmol) and Pd2(dba)3 (0.47 mg,
0.51 µmol) at 35 °C under nitrogen for 10 h. The crude compound
was purified by silica gel column chromatography using dichlo-
romethane/methanol (96:4) and afforded pure pentad 4 in 45% yield
(5 mg). Mp > 300 °C. IR (KBr film): ν ) 3342, 3056, 3034, 2930,
2859, 2111, 1455, 992, 756 cm-1. 1H NMR (400 MHz, CDCl3, 25
°C): δ ) 2.73 (s, 27 H), 7.02-7.06 (m, 3H), 7.38-7.44 (m, 10H),
7.52-7.60 (m, 12H), 7.60-7.68 (m, 6H), 7.70-7.82 (m, 11H),
7.86-8.00 (m, 11H), 8.08-8.26 (m, 24H), 8.78-8.81 (m, 2H),
8.95-9.00 (m, 22H), 9.58-9.72 (m, 5H), 10.02-10.11 (m, 3H)
ppm. 13C NMR (100 MHz, CDCl3, 25 °C): δ ) 21.8, 31.6, 82.7,
91.8, 124.6, 127.1, 127.7, 128.4, 128.9, 131.4, 132.8, 134.8, 134.6,
135.8, 136.9, 137.2, 137.8, 140.1, 141.2, 144.6, 147.8, 152.9, 154.6
ppm. ES-MS: C231H144N16O3S4Zn3 calcd avg mass 3516.2, obsd
m/z 3515.0 [M+ - H, 25]. Anal. Calcd: C, 78.91; H, 4.13; N, 6.37;
S, 3.65. Found: C, 78.86; H, 4.22; N, 6.28; S, 3.52.
subunits in pentads. More studies are required to understand
the excited-state dynamics of these novel pentads containing
three different cores. The synthetic strategy shown in this paper
would help in designing and synthesizing more elaborate novel
unsymmetrical porphyrin arrays containing different porphyrin
subunits.
Experimental Section
(ZnN4)3-N4-N2S2 Porphyrin Pentad (1). Samples of tetrad
14a (10.0 mg, 3.49 µmol) and 5-(4-iodophenyl)-10,15,20-tri(p-
tolyl)-21,23-dithiaporphyrin 20 (2.85 mg, 3.49 µmol) in dry toluene/
triethylamine (3:1, 20 mL) were coupled in the presence of catalytic
amounts of AsPh3 (1.28 mg, 4.18 µmol) and Pd2(dba)3 (0.48 mg,
0.52 µmol) at 35 °C for 15 h. The formation of pentad was
confirmed by the appearance of new spot on TLC as well as the
characteristic absorption bands observed in UV-vis spectroscopy.
The crude compound was purified by silica gel column chroma-
tography, and the pure porphyrin pentad 1 was collected using
dichloromethane/methanol (93:7) as a purple solid (6 mg, 52%).
Mp > 300 °C. IR (KBr film): ν ) 3356, 3058, 2930, 2875, 2110,
1451, 990, 853, 752 cm-1. 1H NMR (400 MHz, CDCl3, 25 °C): δ
) -2.84 (s, 2H), 2.72 (s, 36H), 7.42-7.49 (m, 30H), 7.80 (d, J )
8.0 Hz, 10H), 8.09-8.12 (m, 40H), 8.78-8.90 (m, 36H), 9.59-9.64
(m, 4H) ppm. 13C NMR (100 MHz, CDCl3, 25 °C): 22.4, 90.9,
93.8, 124.5, 126.2, 126.1, 127.6, 128.9, 130.1, 130.9, 132.6, 133.6,
134.5, 134.9, 135.6, 137.2, 140.4, 142.8, 142.7, 143.4, 145.2, 147.2,
147.9, 149.1, 150.4, 151.6, 153.8 ppm. Anal. Calcd: C, 81.10; H,
4.48; N, 7.09; S, 1.80. Found: C, 81.32; H, 4.62; N, 7.20; S, 1.64.
(ZnN4)3-N3O-N2S2 Porphyrin Pentad 2. Samples of porphy-
rin tetrad 15a (10.0 mg, 3.49 µmol) and 5-(4-iodophenyl)-10,15,20-
tri(p-tolyl)-21,23-dithiaporphyrin 20 (2.85 mg, 3.49 µmol) in dry
toluene/triethylamine (3:1, 30 mL) were coupled in the presence
of AsPh3 (1.28 mg, 4.18 µmol) and Pd2(dba)3 (0.48 mg, 0.52 µmol)
at 35 °C for 12 h. The crude reaction mixture was purified by silica
gel chromatography using dichloromethane /methanol (90:10) and
afforded pentad 2 as a purple solid in 30% yield (4 mg). Mp >
300 °C. IR (KBr film): ν ) 3348, 3055, 2923, 2854, 1452, 995,
750 cm-1. 1H NMR (400 MHz, CDCl3, 25 °C): δ ) 2.75 (s, 36H),
7.30-7.38 (m, 36H), 7.50-7.64 (m, 12H), 7.88-8.00 (m, 10H),
8.05-8.18 (m, 22H), 8.62-8.70 (m, 20H), 8.75 (d, J ) 4.2 Hz,
6H), 8.81 (d, J ) 4.2 Hz, 8H), 9.15 (br s, 2H), 9.61 (d, J ) 4.4 Hz,
1H), 9.68-9.72 (m, 3H) ppm. 13C NMR (100 MHz, CDCl3, 25
°C): 13C NMR (100 MHz, CDCl3, 25 °C): 22.6, 90.7, 93.5, 124.4,
126.7, 126.8, 127.8, 128.2, 129.5, 130.8, 132.8, 133.2, 134.3, 134.8,
135.3, 137.1, 140.5, 142.3, 142.8, 143.6, 145.4, 146.9, 147.3, 149.5,
150.2, 151.2, 153.4 ppm. ES-MS: C240H157N17OS2Zn3 calcd avg
mass 3555.2, obsd m/z 3555.3 [M+, 25]. Anal. Calcd C, 81.08; H,
4.45; N, 6.70; S, 1.80. Found: C, 81.22; H, 4.62; N, 6.88; S, 1.98.
Acknowledgment. We thank BRNS, CSIR, and DST for
financial support.
Supporting Information Available: Experimental for com-
1
pounds 5-17 and copies of ES-MS and H NMR spectra of
selected compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO801455W
J. Org. Chem. Vol. 73, No. 21, 2008 8375