Sn(IV) porphyrin based axial-bonding type porphyrin triads containing heteroporphyrins as axial ligands

Article abstract of DOI:10.1021/ic901920y

The thiaporphyrin building blocks with N₃S and N ₂S₂ cores containing one hydroxyphenyl functional group at the meso position were synthesized by adopting the unsymmetrical thiophene diol method. These monohydroxy thiaporphyrins were used to construct the first examples of axial bonding type Sn(IV) porphyrin triads in which Sn(IV) porphyrin acts as basal unit and the two thiaporphyrin units as axial ligands by treating with SnTTP(OH)₂ in benzene at refluxing temperature. The axial bonding type triads were confirmed by mass, 1D and 2D NMR studies. The absorption and electrochemical studies support weak ground state interaction among the porphyrin subunits within the porphyrin triads. The fluorescence studies indicate there is a possibility of energy transfer at the singlet state from basal Sn(IV) porphyrin unit to axial thiaporphyrin units.

Full text of DOI:10.1021/ic901920y

2692 Inorg. Chem. 2010, 49, 2692–2700
DOI: 10.1021/ic901920y
Sn(IV) Porphyrin Based Axial-Bonding Type Porphyrin Triads Containing
Heteroporphyrins as Axial Ligands
Vijayendra S. Shetti and Mangalampalli Ravikanth*
Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India
Received September 29, 2009
The thiaporphyrin building blocks with N3S and N2S2 cores containing one hydroxyphenyl functional group at the meso
position were synthesized by adopting the unsymmetrical thiophene diol method. These monohydroxy thiaporphyrins
were used to construct the first examples of axial bonding type Sn(IV) porphyrin triads in which Sn(IV) porphyrin acts
as basal unit and the two thiaporphyrin units as axial ligands by treating with SnTTP(OH)2 in benzene at refluxing
temperature. The axial bonding type triads were confirmed by mass, 1D and 2D NMR studies. The absorption and
electrochemical studies support weak ground state interaction among the porphyrin subunits within the porphyrin
triads. The fluorescence studies indicate there is a possibility of energy transfer at the singlet state from basal Sn(IV)
porphyrin unit to axial thiaporphyrin units.
Introduction
self-assembling systems. When porphyrins bearing external
coordination functionalities bind to other metallopor-
phyrins, assemblies of axially connected chromophores com-
monly known as “axial bonding type” arrays are generated.3
Tailor-made metalloporphyrins bearing nitrogen and oxygen
donor functionalities have been designed that spontaneously
self-assemble in discrete arrays by virtue of the independent
coordination properties of Zn(II), Ru(II), Rh(III), and
Sn(IV) centers.
Tin(IV) porphyrins offer the chemist many advantages
because of the particular properties conferred by the highly
charged main group metal center.4 The Sn(IV) porphyrins
are very stable, diamagnetic, and usually six coordinate with
trans-diaxial anionic or occasionally neutral ligands. The
particular oxophilicity of the Sn(IV) center confers a notable
preference for coordination of carboxylates and aryloxides,
and these properties have been exploited by different workers
for the preparation of elegant and elaborate multiporphyrin
arrays.4 The oxophilic nature of Sn(IV) ions has been used by
Maiya and co-workers5 to construct novel axial-bonding
type porphyrin arrays such as porphyrin trimers, hexamers,
Multiporphyrin arrays have attracted attention because of
their use in opto-electronic device applications and light
harvesting.1 Porphyrins and metalloporphyrins can be as-
sembled into arrays either by covalent strategies1 or by
adopting non-covalent strategies.2 Conventional linear syn-
thetic strategies are quite limiting since these strategies
normally involve many sequential steps, separation of statis-
tical mixtures, and extensive chromatographic purification,
always resulting in a low product yield. Non-covalent stra-
tegies have emerged as a viable alternative to covalent
synthesis in the construction of multiporphyrin arrays. Both
coordination and multiple hydrogen bonds or their combina-
tion have been used as the source of the non-covalent
bonding interactions.2 To construct non-covalent porphyrin
arrays, the porphyrins and metalloporphyrins were exploited
essentially in two ways: (1) Metalloporphyrins can be accep-
tor building blocks provided that the metal atom inside the
porphyrin core has at least one axial site available for
coordination. (2) Porphyrins can behave as donor building
blocks, provided that they have ligands appended to the
periphery that can suitably coordinate to metal centers. These
two kinds of porphyrin modules, acceptor and donor, can be
freely and rationally combined to build multiporphyrin
(3) (a) Aoyama, Y.; Kamohara, T.; Yamagishi, A.; Toi, H.; Ogoshi, H.
Tetrahedron Lett. 1987, 28, 2143–2146. (b) Stibrany, R. T.; Vasudevan, J.;
Knapp, S.; Potenza, J. A.; Emge, T.; Schugar, H. J. J. Am. Chem. Soc. 1996, 118,
3980–3981. (c) Gerasimchuk, N. N.; Mokhir, A. A.; Rodgers, K. R. Inorg. Chem.
1998, 37, 5641–5650. (d) Funatsu, K.; Imamura, T.; Ichimura, A.; Sasaki, Y.
Inorg. Chem. 1998, 37, 4986–4995. (e) Vasudevan, J.; Stibrany, R. T.; Bumby, J.;
Knapp, S.; Potenza, J. A.; Emge, T. J.; Schugar, H. J. J. Am. Chem. Soc. 1996,
118, 11676–11677. (f) Kariya, N.; Imamura, T.; Sasaki, Y. Inorg. Chem. 1998, 37,
1658–1668.
(4) Arnold, D. P.; Blok, J. Coord. Chem. Rev. 2004, 248, 299–319.
(5) (a) Giribabu, L.; Rao, T. A.; Maiya, B. G. Inorg. Chem. 1999, 38,
4971–4980. (b) Kumar, A. A.; Giribabu, L.; Reddy, D. R.; Maiya, B. G. Inorg.
Chem. 2001, 40, 6757–6766.
*To whom correspondence should be addressed. E-mail: ravikanth@
chem.iitb.ac.in.
(1) (a) Suslick, K. S.; Rakow, N. A.; Kosal, M. E.; Chou, J.-H.
J. Porphyrins Phthalocyanines 2000, 4, 407–413. (b) Burrell, A. K.; Officer,
D. L.; Plieger, P. G.; Reid, D. C. W. Chem. Rev. 2001, 101, 2751–2796. (c)
Shinokubo, H.; Osuka, A. Chem. Commun. 2009, 1011–1021.
(2) (a) Imamura, T.; Fukushima, K. Coord. Chem. Rev. 2000, 198, 133–
156. (b) Wojaczynski, J.; Latos-Grazynski, L. Coord. Chem. Rev. 2000, 204,
113–171.
r
pubs.acs.org/IC
Published on Web 02/19/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2693
Scheme 1. Synthesis of Monohydroxy Porphyrin Building Blocks 3 and 4
and nonamers, and they have studied their electrochemical and
photophysical properties in detail. Sanders and co-workers6
and others7 have also used the oxophilic nature of Sn(IV)
centers along with complementary binding properties of
Ru(II) or Zn(II) toward nitrogen and synthesized several
supramolecular porphyrin arrays. A common feature among
the axial bonding type Sn(IV) porphyrin arrays reported in
literature4-7 so far is that all porphyrin subunits in these
arrays have similar donor atoms (pyrrole nitrogens) in the
porphyrin core. One area of porphyrin chemistry that has
received less attention but may lead to systems of interesting
structural and electronic properties is the characterization of
porphyrin arrays containing two dissimilar macrocycles such
as porphyrin-chlorin,8a porphyrin corrole,8b porphyrin-
phthalocyanine8c and porphyrin-heteroporphyrin.8d We9a-d
and others9e,f reported synthesis of covalent and non-cova-
lent porphyrin arrays containing two dissimilar cores such as
(8) (a) Paolesse, R.; Pandey, R. K.; Forsyth, T. P.; Jaquinod, L.;
Gerzevske, K. R.; Nurco, D. J.; Senge, M. O.; Licoccia, S.; Boschi, T.;
Smith, K. M. J. Am. Chem. Soc. 1996, 118, 3869–3882. (b) Flamingi, L.;
Ventura, B.; Tasior, M.; Gryko, D. T. Inorg. Chim. Acta 2007, 360, 803–813. (c)
Yang, S. I.; Li, J.; Cho, H. S.; Kim, D.; Bocian, D. F.; Holten, D.; Lindsey, J. S.
J. Mater. Chem. 2000, 10, 283–296. (d) Gupta, I.; Ravikanth, M. Coord. Chem.
Rev. 2006, 250, 468–518.
(9) (a) Gupta, I.; Ravikanth, M. J. Org. Chem. 2004, 69, 6796–6811. (b)
Punidha, S.; Ravikanth, M. Tetrahedron 2004, 60, 8437–8444. (c) Punidha, S.;
Ravikanth, M. Tetrahedron 2008, 64, 8016–8028. (d) Punidha, S.; Sinha, J.;
Kumar, A.; Ravikanth, M. J. Org. Chem. 2008, 73, 323–328. (e) Berlicka, A.;
Pacholska, E.; Latos-Grazynski, L. J. Porphyrins Phthalocyanines 2003, 7, 8–
16. (f) Pandian, R. P.; Chandrashekar, T. K. Inorg. Chem. 1994, 33, 3311–3324.
(6) (a) Hawley, J. C.; Bampos, N. B.; Abraham, R. J.; Sanders, J. K. M.
Chem. Commun. 1998, 661–662. (b) Kim, H.-J.; Bampos, N.; Sanders, J. K. M.
J. Am. Chem. Soc. 1999, 121, 8120–8121. (c) Maiya, B. G.; Bampos, N.; Kumar,
A. A.; Feeder, N.; Sanders, J. K. M. New J. Chem. 2001, 25, 797–800. (d) Webb,
S. J.; Sanders, J. K. M. Inorg. Chem. 2000, 39, 5920–5929.
(7) (a) Fallon, G. D.; Lee, M. A.-P.; Langford, S. J.; Nichols, P. J. Org.
Lett. 2002, 4, 1895–1898. (b) Langford, S. J.; Latter, M. J.; Beckmann, J. Inorg.
Chem. Commun. 2005, 8, 920–923. (c) Langford, S. J.; Woodward, C. P.
CrystEngComm 2007, 9, 218–221. (d) Kim, H.-J.; Jo, H. J.; Kim, J.; Kim,
S.-Y.; Kim, D.; Kim, K. CrystEngComm 2005, 7, 417–420. (e) Jo, H. J.; Jung,
S. H.; Kim, H.-J. Bull. Korean Chem. Soc. 2004, 25, 1869–1873.

2694 Inorganic Chemistry, Vol. 49, No. 6, 2010
Shetti and Ravikanth
Scheme 2. Synthesis of Sn(IV)porphyrin Based Triads 1 and 2
porphyrin with N4 core and thiaporphyrins with N3S and
N2S2 cores. Thiaporphyrins are resulted from the replace-
ment of one or two nitrogens by the thiophene sulfurs and
possess very interesting chemical and physical properties
which are different from N4 porphyrins. A perusal of litera-
ture reveals that there are no examples on Sn(IV) porphyrin
based axial bonding type porphyrin arrays containing thia-
porphyrins as axial ligands. Thus an assembly containing a
thiaporphyrin with N2S2 or N3S core as axial porphyrin and
Sn(IV) derivative of N4 porphyrin as basal porphyrin offers a
unique axial bonding type triads which may possess interest-
ing physicochemical properties. In this paper, we used Sn-
(IV)TTP(OH)2 as a basal scaffold to synthesize first examples
of porphyrin triads 1 and 2 containing heteroporphyrins as
axial ligands by adopting the axial bonding approach. We
also report the photophysical and electrochemical properties
of these two novel axial bonding type porphyrin trimeric
arrays.
4-hydroxyphenyl groups at meso-position whichwere synthe-
sized as shown in Scheme 1. The precursor unsymmetrical
thiophene diol, 2-[(4-hydroxyphenyl)hydroxymethyl]-5-[(p-
tolyl)hydroxymethyl]thiophene 5 was synthesized by treating
thiophene mono-ol, 2-[(p-tolyl)hydroxymethyl]thiophene9a
with 2 equiv of n-BuLi followed by 1.2 equiv of 4-hydroxy-
benzaldehyde in tetrahydrofuran (THF) at 0 °C. Column
chromatographic purification on silica gel afforded the un-
symmetrical thiophene diol 5 as yellow semisolid in 32%
yield. The diol 5 was confirmed by a molecular ion peak in the
1
ES-MS mass spectrum, clean H NMR, and matching ele-
mental analysis. The other precursor 16-thiatripyrrane 6 was
synthesized by following a reported procedure.10 The 21,23-
dithiaporphyrin building block, 5-(4-hydroxyphenyl)-
10,15,20-tri(p-tolyl)-21,23-dithiaporphyrin 3 was synthesized
by condensing 1 equiv of unsymmetrical diol 5 with 1 equiv of
16-thiatripyrrin 6 in propionic acid at refluxing tempera-
ture.11 After workup and one filtration column, the thin-
layer chromatography (tlc) analysis showed the formation of
21,23-dithiaporphyrin building block 3 as sole product.
Results and Discussion
To synthesize the two trimeric arrays 1 and 2, we need an
access to unknown monofunctionalized 21,23-dithiapor-
phyrin (N2S2 core) 3 and 21-monothiaporphyrin 4 having
(10) Heo, P. Y.; Lee, C. H. Bull. Korean Chem. Soc. 1996, 17, 515–520.
(11) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2695
Figure 1. 1H NMR spectra recorded in CDCl3 with various proton assignments for triad 2 and the partial 1H-1H COSY spectrum of 2.
The crude compound was subjected to silica gel chromato-
graphic purification and afforded pure 21,23-dithiaporphyr-
in building block 3 as purple solid in 8% yield. The 21-
thiaporphyrin building block 4 was synthesized similarly by
condensing 1 equiv of unsymmetrical thiophene diol 5 with
2 equiv of p-tolualdehyde and 3 equiv of pyrrole in propionic
acid under refluxing conditions. The tlc analysis of crude
compound showed the formation of two compounds:
5,10,15,20-tetratolylporphyrin (H2TTP) and the desired 21-
thiaporphyrin building block 4. In general, this condensation
should also give a cis and trans mixture of 21,23-dithiapor-
phyrin building blocks,12 but we did not observe the forma-
tion of cis and trans mixture of 21,23-dithiaporphyrins
because of the low reactivity of diol 5. The mixture was
separated by column chromatography on silica and afforded
pure 21-thiaporphyrin building block 4 in 3% yield. Since
the yield of 4 was low, we adopted an alternate method
to synthesize 4. The unsymmetrical diol 2-[(4-meth-
oxyphenyl)hydroxymethyl]-5-[(p-tolyl)hydroxymethyl]thioph-
ene 7 was prepared by treating 2-[(p-tolyl)hydroxymethyl]thio-
phene with p-anisaldehyde under n-BuLi conditions and
purified by column chromatography. The diol 7 was then
condensed in next step with p-tolualdehyde and pyrrole under
Lindsey’s mild porphyrin forming conditions.13 The condensa-
tion resulted in the formation of a mixture of porphyrins, and
the required porphyrin 8 was separated from the mixture by
column chromatography to afford pure 8 in 12% yield. In the
final step, the porphyrin 8 was treated with 49% HBr-H2O
at reflux to convert the methoxy porphyrin 8 to hydroxy
porphyrin 4. The crude compound was subjected to column
chromatographic purification and afforded porphyrin 4 in
60% yield. Although the modified method involves one addi-
tional step, the desired N3S porphyrin building block 4 can be
prepared in good yield. The new porphyrins 3, 4, and 8 were
1
characterized by ES-MS, H and 13C NMR, absorption and
elemental analysis techniques.
The triads 1 and 2 were synthesized as shown in Scheme 2.
The triad 1 containing 21, 23-dithiaporphyrin subunit as
axial ligands was synthesized by refluxing 1 equiv of SnTTP-
(OH)2 with 2 equiv of 21,23-dithiaporphyrin building block 3
in benzene at refluxing temperature for 12 h. The benzene was
removed on a rotary evaporator, and the crude compound
was subjected to basic alumina column chromatography
using dichloromethane as eluent. The fast moving band of
(12) Punidha, S.; Agarwal, N.; Ravikanth, M. Eur. J. Org .Chem. 2005,
2500–2517.
(13) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.;
Marguerettaz, A. M. J. Org. Chem. 1987, 52, 827–836.

2696 Inorganic Chemistry, Vol. 49, No. 6, 2010
Shetti and Ravikanth
current effect of the adjacent porphyrins and appeared at a
different region as compared to their corresponding mono-
meric porphyrins 3 and 4, respectively. Thus, in triad 2, all
eight β-pyrrole protons of Sn(IV)porphyrin of type a reso-
nate at 9.38 ppm which were downfield shifted compared to
SnTTP(OH)2 in which the β-pyrrole protons resonate at
9.13 ppm. On the other hand the β-pyrrole protons of axial
N3S porphyrin experienced upfield shifts compared to corres-
ponding monomeric 21-thiaporphyrin. The two β-pyrrole
protons of type b and two others of type c which were facing
the central Sn(IV) porphyrin appeared at 8.37 ppm and 8.41
ppm, respectively. The remaining eight β-pyrrole protons of
axial porphyrin of type d which were away from the central
Sn(IV) porphyrin appeared as two doublets at 8.57 ppm,
8.65 ppm and as multiplet at 8.88 ppm. Similarly, the two
-thiophene protons of type e appeared as a doublet at
9.14 ppm and the other two β-thiophene protons of type f
appeared as a doublet at 9.50 ppm. All these protons were
upfield shifted compared to corresponding monomeric
21-thiaporphyrin 4. The inner NH proton of axial N3S
porphyrin also experienced slight upfield shift and appeared
at -2.78 ppm. However, the most prominent ring current
effects were observed for phenoxo groups of the N3S por-
phyrin that are bound to the Sn(IV) center. These protons,
being affected by both the inherent deshielding effect of the
axial N3S porphyrin and the shielding effect of the basal
Sn(IV) porphyrin, resonate at 6.60 ppm as a doublet corres-
ponding to four protons of type g and at 2.40 ppm corres-
ponding to four protons of type h as detected by the proton
connectivity pattern in the 1H-1H COSY spectrum
(Figure 1). Similar observations were made for triad 1
(Supporting Information). In triad 1 also, the maximum
upfield shifts were noted for meta (g type) and ortho protons
(h type) of phenoxo groups of the axial porphyrin which
appeared at 6.60 and 2.40 ppm, respectively. 1H-13C hetero-
nuclear single-quantum coherence (HSQC) experiments were
performed on trimers 1 and 2 to understand the connectivity
of various protons and carbon atoms.
The absorption spectra of triads 1 and 2 recorded in
dichloromethane are shown in Figure 2, and the data of
triads along with their corresponding porphyrin monomers
are presented in Table 1. The absorption spectra of triads 1
and 2 showed features of both their corresponding porphyrin
monomers and appear to be nearly a superposition of the
spectra of the corresponding monomers with slight shifts in
their peak maxima. For example, the absorption spectrum of
triad 1 showed bands at 426, 442, 519, 559, 601, 637, and 703
nm. In this, the bands at 426, 559, and 601 are mainly due to
the Sn(IV) porphyrin subunit, and the bands at 442, 637, and
703 nm are mainly due to the axial 21, 23-dithiaporphyrin
subunit. These observations indicate that the porphyrin
subunits in triads 1 and 2 interact weakly. The electrochemi-
cal studies were carried out on triads 1 and 2 along with their
corresponding porphyrin monomers in CH2Cl2 using TBAP
as supporting electrolyte, and redox potentials were analyzed
based on the data of their corresponding monomers. The
reduction waves of triads 1 and 2 are presented in Figure 3,
and the data of triads 1 and 2 along with their appropriate
porphyrin monomers are presented in Table 1. Generally, the
triads exhibited two oxidations and two or three reductions.
The oxidations are irreversible and difficult to assign, but the
reductions are reversible, diffusion controlled, and one
electron-transfer reactions. Nonetheless, on the basis of the
Figure 2. Q-band absorption spectra of triads (a) 1 and (b) 2 recorded in
dichloromethane. The inset shows their Soret band absorption spectra.
the desired triad was collected and recrystallized from di-
chloromethane/n-hexanes to afford pure triad 1 in 60% yield.
Similarly triad 2 containing N3S porphyrin as axial ligands
was synthesized by reacting SnTTP(OH)2 with 21-thiapor-
phyrin 4 under the same reaction conditions used for triad 1
followed by column chromatographic purification on basic
alumina, and recrystallization afforded triad 2 in 60% yield.
The triad formation reactions worked smoothly and required
simple column chromatographic purification to afford pure
triads in good yields. The triads 1 and 2 were freely soluble in
common organic solvents and were characterized by elemen-
tal analysis, mass, NMR, absorption, electrochemistry, and
fluorescence spectroscopic techniques. The matrix-assisted
laser desorption/ionization-time of flight (MALDI-TOF)
mass spectra of triads 1 and 2 showed a strong peak
corresponding to M-(O porphyrin).14 The elemental analyses
were matched closely with the expected composition of triads
confirming the identity of the triads 1 and 2. The 1H NMR
spectra of triad 2 is presented in Figure 1 along with proton
assignments, which are made on the basis of the resonance
position and integrated intensity data as well as the proton-
1
to-proton connectivity information revealed in the H-1H
COSY spectra to arrive at the structures of these new
compounds. It is clear from the NMR data that certain
protons of the basal Sn(IV) porphyrin and axial heteropor-
phyrins in triads 1 and 2 experienced shifts because of the ring
(14) Kim, H. J.; Jeon, W. S.; Lim, J. H.; Hong, C. S.; Kim, H.-J.
Polyhedron 2007, 26, 2517–2522.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2697
Table 1. UV-vis and Redox Potential Data (V) for Triads 1 and 2 along with Their Corresponding Monomers
UV-vis data, λmax nm (log ε)
potential (V) V vs SCE
CT [SnP-(N2XY)2
]
ECT [SnPþ(N2XY)2
]
þ
-
compound
Soret band
429 (6.15)
Q-bands
oxidation
reduction
E
SnTTP(OH)2
563 (4.30)
603 (4.24)
516 (4.42)
550 (4.01)
635(3.19)
699 (3.78)
519 (4.88)
559 (4.84)
601 (4.33)
637 (3.78)
703 (4.36)
515 (4.17)
551 (3.78)
618 (3.34)
679 (3.58)
518 (4.48)
558(4.42)
601 (4.0)
1.39
1.18
-0.96
-0.94
-1.36
3
1
437 (5.43)
-1.23
-1.29
426 (5.89)
442 (5.89)
1.00
1.53
-0.97
-1.38
1.97
2.50
4
2
430 (5.27)
1.11
0.93
1.51
1.28
-1.03
-1.11
-1.35
427 (5.65)
437 (5.54)
-1.40
2.03
2.39
683 (3.95)
Figure 3. Reduction waves for triads (a) 1 and (b) 2 in dichloromethane
containing 0.1 M TBAP as supporting electrolyte recorded at 50 mV/s
scan speed.
redox data of the individual monomers, we attempted to
assign the peaks to the basal Sn(IV) porphyrin and axial
heteroporphyrin separately. Analysis of the redox data in
Table 1 revealed that the redox potentials of triads 1 and 2
were in the same range as those of their corresponding
monomers supporting a weak interaction among porphyrin
subunits in triads 1 and 2.
The steady state fluorescence properties of triads 1 and 2
were studied in dichloromethane by using excitation wave-
lengths 515 and 560 nm. From the absorption data, it is clear
thatthese triadscan beselectively excitedatSn(IV) porphyrin
or the heteroporphyrin absorption maximum. The Sn(IV)
porphyrin absorbs more strongly at 560 nm whereas the
heteroporphyrin subunit absorbs strongly at 515 nm. The
fluorescence spectra of triad 1 at 515 and 560 nm and 2:1
mixture of 3 and SnTTP(OH)2 are shown in Figure 4a
whereas the triad 2 at 515 and 560 nm along with 2:1 mixture
of 4 and SnTTP(OH)2 at 560 nm are shown in Figure 4b. The
photophysical data of triads 1 and 2 along with the corre-
sponding monomeric porphyrins are presented in Table 2.
The triads 1 and 2 showed mainly three emission bands; two
corresponding to Sn(IV) porphyrin subunit (600, 650 nm)
and thethird oneat680 nmin triad 2 and704 nm in triad 1 are
mainly due to axial heteroporphyrin subunits (Figure 4). For
the triads 1 and 2 on excitation at 515 nm where axial
porphyrin subunit was the dominant absorber, the fluores-
Figure 4. Comparison of normalized emission spectra of triads (a) 1
and (b) 2 along with the 1:2 mixture of their monomers at λex = 560 and
515 nm recorded in dichloromethane.
cence was exclusively observed from the axial heteroporphyrin
subunit. The quantum yields measured at 515 nm for axial
porphyrin subunits in triads 1 and 2 (Table 2) were found
to be lower than that of their corresponding porphyrin
monomers 3 and 4, respectively, indicating the quenching
of the fluorescence of axial heteroporphyrin because of its

2698 Inorganic Chemistry, Vol. 49, No. 6, 2010
Shetti and Ravikanth
Table 2. Fluorescence Data for Triads 1 and 2 along with Their Corresponding
Monomers Recorded in Dichloromethane
leads to a charge transfer state of the type (N2XYPþSnP-)
and involves the free energy change ΔG(PET) which is
calculated using the following equation:
compound
λex nm
Φ
N2XYP (%Q)a
Φ
SnP (%Q)a
ΔGðPETÞ ¼ EoxðN2XYPÞ -EredðSnIVPÞ -E0 -0ðSnIVPÞ
ð2Þ
SnTTP(OH)2
560
515
515
560
515
515
560
0.025
3
1
0.0076
0.0025 (67)
0.0012 (84)
0.0168
0.0067 (58)
0.0036 (77)
0.0010 (84)
4
2
ThechangeinfreeenergyΔG(PET) wasfoundtobe-0.07 eV
for 1 and -0.01 eV for 2 (Figure 5). Thus, the quenching of
the Sn(IV) porphyrin in triads 1 and 2 can be rationalized in
terms of the intramolecular energy transfer from the basal
Sn(IV) porphyrin to the axial heteroporphyrin competing
with the photoinduced electron transfer from the ground
state of the axial heteroporphyrin to the basal Sn(IV)
porphyrin. Systematic time-resolved absorption and fluores-
cence studies are required to probe the excited state dynamics
of these novel axial bonding type porphyrin triads.
0.0052 (79)
a (%Q) denotes percentage of quenching of fluorescence quantum yield.
coordination to heavy Sn(IV) porphyrin subunit in the triads.
However, when triads 1 and 2 were excited at 560 nm where
the Sn(IV) porphyrin subunit absorbs 3-4 times more
strongly than the axial heteroporphyrin subunits, the fluore-
scence of the Sn(IV) porphyrin subunit was quenched by
78-84% as compared to the SnTTP(OH)2, and major
emission was noted from the heteroporphyrin subunit.
However, when a 2:1 mixture of the corresponding mono-
mers of triads 1 and 2 were excited at 560 nm, the major
emission was noted from the Sn(IV) porphyrin, and some
amount of emission was observed from heteroporphyrin
subunit. The excitation spectra of triads 1 and 2 recorded
at 720 and 700 nm, respectively, matched closely with the
corresponding absorption spectra of triads 1 and 2. All these
observations suggest that in triads 1 and 2, on excitation at
560 nm, there is a possibility of energy transfer at the singlet
state from the basal Sn(IV) porphyrin subunit to the axial
N2S2 porphyrin subunits in triad 1 and N3S porphyrin
subunits in triad 2. Furthermore, the energy transfer effi-
ciency can be estimated by the two factors, the donor factor
and the acceptor factor.15 Comparing the emission spectra of
triads 1 and 2 with that of the unlinked equimolar mixture
(2:1) of their components, the decreased value (ΔI1) of the
integrated fluorescence intensities from the corrected fluore-
scence spectra of the SnTTP moiety (donor) was obtained,
and the percentage of the transferred energy from the donor
is Pd = ΔI1/I1, where I1 is the integrated fluorescence
intensity of the unconjugated donor [Sn(IV)porphyrin].
For the acceptor, the percentage of accepted energy is
Pa = ΔI2/ΔI1φF, where ΔI2 is the increased value of the
integrated fluorescence intensities from the acceptor part in
the trimer compared with the unconjugated acceptor (i.e.,
N3S and N2S2 porphyrins) and φF is the fluorescence quan-
tum yield of the acceptor. Combining these two factors, the
efficiency (E) of energy transfer from the donor to the
acceptor can be given by eq 1:
Conclusions
In this paper, for the first time, we have demonstrated the
application ofthe dihydroxoSn(IV) porphyrinas a scaffold in
the construction of axial bonding type of arrays involving
heteroporphyrins as axial ligands. The absorption and elec-
trochemical studies support weak ground state interaction
among the porphyrin subunits within the triads. The fluore-
scence studies indicate that the basal Sn(IV)porphyrin
can act as an energy donor and transfer its energy to the
heteroporphyrin subunit. Thus, we conclude that the energy
transfer from the Sn(IV) porphyrin subunit to the axial
heteroporphyrin subunit is the major cause for quenching
the fluorescence of the Sn(IV) porphyrin subunit although
the photoinduced electron transfer within the porphyrin
subunits in the triads cannot be ruled out completely. We
hope the availability of more such axial bonding type arrays
containing different types of macrocycles as axial ligands
would help in building materials for future applications.
Experimental Section
All general chemicals and solvents were procured from SD
Fine Chemicals, India. Column chromatography was per-
formed using silica gel and basic alumina obtained from Sisco
Research Laboratories, India. Tetrabutylammonium per-
chlorate was purchased from Fluka and used without further
purifications.
1H NMR spectra were recorded with Varian 400 MHz
instrument using tetramethylsilane as an internal standard.
13C NMR spectra were recorded on a Varian spectrometer
1
operating at 100.6 MHz. H-13C HSQC experiments were
performed on a Bruker AVIII 400 MHz instrument using a
BBFO probe. All NMR measurements were carried out at
room temperature in deuterochloroform. Absorption and
steady state fluorescence spectra were obtained with Perkin-
Elmer Lambda-35 and Lambda-55 instruments, respectively.
The fluorescence quantum yields (Φf) of Sn(IV) porphyrins
were estimated from the emission and absorption spectra by a
comparative method.16 ES-MS spectra were recorded with a
Q-Tof Micromass spectrometer. MALDI-TOF spectra were
obtained from Axima-CFR manufactured by Kratos Analy-
ticals. Cyclic voltammetric (CV) and differential pulse voltam-
metric (DPV) studies were carried out with a BAS elec-
trochemical system utilizing the three electrode configuration
consisting of a glassy carbon (working electrode), platinum
wire (auxiliary electrode), and saturated calomel (reference
E ¼ PdPa ¼ ΔI2=I1φF
ð1Þ
Thus, the efficiencies of energy transfer in 1 and 2 are 68 and
70%, respectively, as calculated from the data in Figure 4 and
using the above equation. Besides energy transfer which is
majorly responsible for quenching the fluorescence of the
Sn(IV) porphyrin subunit, the photoinduced electron
transfer (PET) from the ground state of the axial hetero-
porphyrin subunit (N2XYP) to the singlet state of the Sn(IV)
porphyrin (SnP) may also be partly responsible for quench-
ing the fluorescence of the donor subunit. This PET reaction
(15) Durmus, M.; Chen, J. Y.; Zhao, Z. X.; Nyokong, T. Spectrochimica
Acta Part A 2008, 70, 42–49.
(16) Gupta, I.; Ravikanth, M. Inorg. Chim. Acta 2007, 360, 1731–1742.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2699
Figure 5. Generalized energy level diagram illustrating singlet and charge transfer states for triads 1 and 2.
electrode) electrodes in dry dichloromethane using 0.1 M
tetrabutylammonium perchlorate as supporting electrolyte.
2-[(4-Hydroxyphenyl)hydroxymethyl]-5-[(p-tolyl)hydroxymethyl]-
thiophene (5). A sample of 2-[(p-tolyl)hydroxymethyl] thiophene
(1 g, 4.90 mmol) was charged in a two necked round bottomed
flask equipped with nitrogen bubbler and rubber septum. Dry
diethyl ether (30 mL) and N,N,N0,N0-tetramethylethylenediamine
(TMEDA) (1.48 mL, 9.79 mmol) were added to it followed by
careful addition of n-BuLi (6.12 mL of ca. 15% solution in hexane,
9.79 mmol) at 0 °C. The resultant mixturewas allowed tostirat the
same temperature for 1 h. Ice cold dry THF solution of 4-hydroxy
benzaldehyde (0.72 g, 5.87 mmol) was added to the above mixture
and allowed to stir for 6 h at room temperature. The reaction was
quenched by adding saturated ammonium chloride solution in
water and extracting with dichloromethane. The organic layer was
collected and dried over sodium sulfate and evaporated in vacuo.
The crude compound was loaded on a silica gel column and eluted
with petroleum ether/ethyl acetate (65:35) as a yellowish semi
solid. (0.51 g, yield 32%) 1H NMR (400 MHz, CDCl3) δ 2.25 (s,
3H, CH3), 3.00 (s, 2H, OH), 5.82-5.85 (m, 2H, CHOH),
6.62-6.65 (m, 2H, thiophene), 7.16 (d, J = 8.2 Hz, 2H, Ar),
7.27 (d, J = 7.8 Hz, 2H, Ar), 7.78 (d, J = 7.8 Hz, 2H, Ar), 8.42 (d,
J = 8.2 Hz, 2H, Ar) ppm; 13C NMR (100 MHz, CDCl3) δ 22.5,
70.5, 71.5, 120.1, 124.3, 124.5, 128.2, 129.3, 134.6, 136.0, 139.4,
140.2, 143.7, 145.4, 146.1, 156.9 ppm; ES-MS: m/z (%) = 309.3
(M - 17)þ (100); Elemental analysis: Calcd (%) for C19H18O3S: C
69.91, H 5.56, S 9.82; Found: C 69.85, H 5.61, S 9.78.
5-(4-Hydroxyphenyl)-10,15,20-tri(p-tolyl)-21,23-dithiaporphyrin
(3). The samples of 5 (0.5 g, 1.53 mmol) and 16-thiatripyrrane 6
(0.65 g, 1.53 mmol) in propionic acid (200 mL) were refluxed for
4 h. The propionic acid was removed under vacuum, and the
resulted crude compound was washed several times with water and
oven-dried. The crude compound was subjected to silica gel
column, and the desired porphyrin 3 was eluted using petroleum
ether/dichloromethane (15:85) after which the solvent was re-
moved under vacuum to get 3 as a purple solid. (90 mg, Yield
1
8%) mp >300 °C; H NMR (400 MHz, CDCl3) δ 2.70 (s, 9H,
CH3), 5.13 (s, 1H, OH), 7.22 (d, J = 8.7 Hz, 2H, Ar), 7.61 (d, J =
7.8 Hz, 6H, Ar), 8.09 (d, J = 8.2 Hz, 2H, Ar), 8.12 (d, J = 7.8 Hz,
6H, Ar), 8.68 (s, 4H, β-pyrrole), 9.68 (s, 4H, β-thiophene) ppm; 13
C
NMR (100 MHz, CDCl3) δ 21.6, 123.6, 124.1, 127.4, 128.1, 130.0,
134.3, 135.2, 136.0, 137.5, 138.3, 142.3, 146.3, 147.5, 149.2, 152.2,
154.3 ppm; ES-MS: m/z (%) = 707.2 (M þ H)þ (100); Elemental
analysis: Calcd (%) for C47H34N2OS2: C 79.85, H 4.85, N 3.96, S
9.07; Found: C 79.90, H 4.80, N 3.92, S 9.11.
2-[(4-Methoxyphenyl)hydroxymethyl]-5-[(p-tolyl)hydroxy-
methyl]thiophene (7). The diol 7 was prepared from 2-[(p-
tolyl)hydroxymethyl]thiophene (1 g, 4.90 mmol) similarly by
following the procedure described for diol 5 using 4-methoxyben-
zaldehyde in place of 4-hydroxybenzaldehyde. (0.8 g, yield 48%)
1H NMR (400 MHz, CDCl3) δ 2.33 (s, 3H, CH3), 2.47 (bs, 2H,
OH), 3.78 (s, 3H, OCH3), 5.89 (d, J = 5.4 Hz, 2H, CHOH),
6.65-6.67 (m, 2H, thiophene), 6.85 (d, J = 8.8 Hz, 2H, Ar), 7.14
(d, J = 7.6 Hz, 2H, Ar), 7.25-7.33 (m, 4H, Ar) ppm; 13C NMR

2700 Inorganic Chemistry, Vol. 49, No. 6, 2010
Shetti and Ravikanth
(100 MHz, CDCl3) δ 21.3, 55.4, 72.3, 114.0, 124.4, 126.4, 127.8,
129.3, 135.3, 137.9, 140.1, 148.5, 159.4 ppm; ES-MS: m/z (%) =
323.1 (M-17)þ (100); Elemental analysis: Calcd (%) for
C20H20O3S: C 70.56, H 5.92, S 9.42; Found: C 70.60, H 5.88, S
9.45.
The compound 4 was also synthesized alternatively by reflux-
ing porphyrin 8 (0.1 g, 14 mmol) in 30 mL of HBr-water (49%)
for 4 h. The reaction mixture was extracted with dichloro-
methane and was washed several times with water and dilute
ammonia solution (25% v/v). The organic layer was evaporated
under reduced pressure and subjected to silica gel column
chromatographic purification using dichloromethane to afford
pure 4. (58 mg, 60%).
5-(4-Methoxyphenyl)-10,15,20-tri(p-tolyl)-21-thiaporphyrin (8).
Samples of unsymmetrical diol 7 (0.5 g, 1.4 mmol), p-tolualdehyde
(0.33 mL, 2.8 mmol), and pyrrole (0.29 mL, 4.2 mmol) were
dissolved in dichloromethane (300 mL), and the reaction flask was
degassed with nitrogen for 10 min with stirring. The condensation
was initiated by adding BF3.OEt2 (0.4 mL of 2.5M solution) while
stirring the reaction mixture at room temperature for 1 h under a
nitrogen atmosphere. 2,3-Dichloro-5,6-dicyano-benzoquinone
(DDQ) (0.32 g, 1.4 mmol) was then 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 using petroleum
ether/dichloromethane (60:40) to afford porphyrin 8. (125 mg,
Yield 12%) 1H NMR (400 MHz, CDCl3) δ -2.68 (s, 1H, NH),
2.69 (s, 3H, CH3), 4.08 (s, 3H, OCH3), 7.34 (d, J = 8.5 Hz, 2H,
Ar), 7.54 (d, J = 7.6 Hz, 4H, Ar), 7.61 (d, J = 7.9 Hz, 2H, Ar),
8.07 (d, J = 7.9 Hz, 4H, Ar), 8.12-8.18 (m, 4H, Ar), 8.60-8.62
(m, 2H, pyrrole), 8.68 (d, J = 4.2 Hz, 2H, pyrrole), 8.93-8.94 (m,
2H, pyrrole), 9.75 (s, 2H, thiophene) ppm; 13C NMR (100 MHz,
CDCl3) δ 21.7, 110.2, 113.3, 127.5, 128.5, 133.2, 133.7, 134.4,
135.6, 138.3, 139.2, 139.7, 147.3, 154.6, 159.7 ppm; ES-MS: m/z
(%) = 704.4 (M þH)þ (100); Elemental analysis: Calcd (%) for
C48H37N3OS: C 81.90, H 5.30, N 5.97, S 4.56; Found: C 81.95, H
5.25, N 5.98, S 4.54.
General Synthesis of Triads (1) and (2). The triads 1 and 2 were
synthesized by refluxing Sn(IV)TTP(OH)2 (12 μmol) and mono-
hydroxyporphyrins 3 and 4 (24 μmol), respectively, in dry
benzene (10 mL) for 12 h under a nitrogen atmosphere. The
solvent was evaporated under reduced pressure, and the resulted
residue was subjected to basic alumina column. The desired
product was eluted with dichloromethane and was recrystallized
using dichloromethane/n-hexane mixture to afford trimers
1 and 2 in 60% yield.
Porphyrin Triad (1). Mp >300 °C, 1H NMR (400 MHz,
CDCl3) δ 2.49 (d, J = 8.5 Hz, 4H, Ar), 2.68-2.70 (m, 30H,
CH3), 6.65 (d, J = 8.5 Hz, 4H, Ar), 7.58-7.73 (m, 23H, phenyl),
8.02-8.11 (m, 13H, phenyl), 8.41-8.45 (m, 8H, [β-pyrrole þ
phenyl]), 8.65 (s, 4H, β-pyrrole), 9.04 (d, J = 4.9 Hz, 2H,
β-thiophene), 9.39 (s, 8H, β-pyrrole Sn), 9.41 (d, J = 4.9 Hz,
2H, β-thiophene), 9.62 (s, 4H, β-thiophene) ppm; MALDI-
TOF: m/z (%) = 1493.1 (M-C47H34N2OS2)þ (90); Elemental
analysis: Calcd (%) for C142H102N8O2S4Sn: C 77.71, H 4.96, N
4.97, S 5.68; Found: C 77.81, H 4.93, N 5.70, S 5.65.
Porphyrin Triad (2). Mp >300 °C, 1H NMR (400 MHz,
CDCl3) δ -2.78 (s, 2H, NH), 2.49 (d, J = 8.5 Hz, 4H, Ar),
2.68-2.70 (m, 30H, CH3), 6.65 (d, J = 8.5 Hz, 4H, Ar),
7.50-7.74 (m, 22H, phenyl), 8.0-8.09 (m, 10H, phenyl), 8.10
(d, J = 7.9 Hz, 4H, phenyl), 8.37 (d, J = 4.6 Hz, 2H, β-pyrrole),
8.41-8.42 (m, 6H, [β-pyrrole þ phenyl]), 8.57 (d, J = 4.6 Hz, 2H,
β-pyrrole), 8.65 (d, J = 4.5 Hz, 2H, β-pyrrole), 8.88-8.89 (m, 4H,
β-pyrrole), 9.14 (d, J = 5.2 Hz, 2H, β-thiophene), 9.37-9.39 (m,
8H, β-pyrrole Sn), 9.50 (d, J = 5.2 Hz, 2H, β-thiophene) ppm;
MALDI-TOF: m/z (%) = 1476.7 (M-C47H35N3OS)þ (25); Ele-
mental analysis: Calcd (%) for C142H104N10O2S2Sn: C 78.91, H
5.13, N 6.30, S 2.89; Found: C 78.93, H 5.15, N 6.27, S 2.88.
5-(4-Hydroxyphenyl)-10,15,20-tri(p-tolyl)-21-thiaporphyrin (4).
Samples of unsymmetrical diol 5 (0.5 g, 1.53 mmol), p-tolualde-
hyde (0.36 mL, 3.06 mmol), and freshly distilled pyrrole (0.32 mL,
4.60 mmol) were refluxed in 200 mL of propionic acid for 4 h. The
propionic acid was removed under reduced pressure, and the
residue was washed thoroughly with water and oven-dried. The
crude compound was subjected to silica gel column chromato-
graphy, and the desired porphyrin 4 was collected using dichlor-
omethane as eluent. (35 mg, Yield 3%) mp >300 °C; 1H NMR
(400 MHz, CDCl3) δ -2.69 (s, 1H, NH), 2.73 (s, 9H, CH3), 7.28
(m, 2H, Ar), 7.55 (d, J = 7.9 Hz, 4H, Ar), 7.62 (d, J = 8.2 Hz, 2H,
Ar), 8.00-8.10 (m, 8H, Ar), 8.60-8.61 (m, 2H, β-pyrrole), 8.68-
8.69 (m, 2H, β-pyrrole), 8.93 (s, 2H, β-pyrrole), 9.75 (s, 2H,
β-thiophene) ppm; 13C NMR (100 MHz, CDCl3) δ 21.7, 114.7,
123.9, 127.5, 128.5, 129.0, 130.0, 131.5, 133.1, 133.8, 134.4, 135.6,
137.7, 138.3, 139.2, 141.9, 147.2, 154.6, 155.7, 157.7 ppm; ES-MS:
m/z (%) = 690.5 (M þ H)þ (100); Elemental analysis: Calcd (%)
for C47H35N3OS: C 81.83, H 5.11, N 6.09, S 4.65; Found: C 81.85,
H 5.08, N 6.11, S 4.63.
Acknowledgment. M.R. thanks Council of Scientific and
Industrial Research (CSIR) and Department of Science and
Technology (DST) for financial support, and V.S. thanks CSIR
for a fellowship.
Supporting Information Available: Mass and NMR spectra of
selected compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.