Porphyrins linked directly to the 5,5′ positions of 2,2′-bipyridine: A new supramolecular building block and switch

Article abstract of DOI:10.1021/ic025985v

The synthesis and coordination chemistry of two porphyrin dimers linked either at the 5,5′ or the 4,4′ positions of 2,2′-bipyridine are described. These compounds, which may serve a molecular tectons for the constructions of a variety of supramolecular arrays of diverse function, reveal that the ground- and excited-state electronic communication between the chromophores is only moderately affected by the complexation state of the bipyridyl moiety. The nature of the metal ion chelated by the bipyridine only slightly perturbs the ground-state spectra, and differences observed in the excited state are largely ascribed to the heavy atom effect. This work also shows that conformational changes in structural subunits, in this case induced by bipyridyl complexation of various metal ions, do not necessarily require reorganization of supramolecular systems.

Full text of DOI:10.1021/ic025985v

Inorg. Chem. 2003, 42, 2075−2083
Porphyrins Linked Directly to the 5,5′ Positions of 2,2′-Bipyridine: A
New Supramolecular Building Block and Switch
Kai Fan Cheng, Charles Michael Drain,* and Klaus Grohmann
Department of Chemistry and Biochemistry, Hunter College and Graduate Center of the City
UniVersity of New York, 695 Park AVenue, New York, New York 10021
Received August 29, 2002
The synthesis and coordination chemistry of two porphyrin dimers linked either at the 5,5′ or the 4,4′ positions of
2,2′-bipyridine are described. These compounds, which may serve a molecular tectons for the constructions of a
variety of supramolecular arrays of diverse function, reveal that the ground- and excited-state electronic communication
between the chromophores is only moderately affected by the complexation state of the bipyridyl moiety. The
nature of the metal ion chelated by the bipyridine only slightly perturbs the ground-state spectra, and differences
observed in the excited state are largely ascribed to the heavy atom effect. This work also shows that conformational
changes in structural subunits, in this case induced by bipyridyl complexation of various metal ions, do not necessarily
require reorganization of supramolecular systems.
Introduction
efficiently harvest light due to their strong absorption bands,
they are remarkably robust considering their rich photo-
chemical and redox properties, and there are a variety of
means to fine-tune these properties via peripheral substituent
modifications or metal complexation. Since their discovery
at the end of the 19th century, bipyridines such as 2,2′-
bipyridine (BiPy) and their coordination chemistry have been
well studied for many of the same reasons.2,16 The combina-
tion of porphyrins and BiPy techtons17-20 continues to
develop in the arena of molecular-scale information process-
ing systems, such as in highly conjugated porphyrin-based
assemblies.21-23 Terpyridyl and similar ligands appended to
porphyrins are of continuing interest in the formation of
With the rapidly developing field of molecular-based
materials, the preparation of supramolecular organic photonic
devices is an important research area in view of both
fundamental science and applications development.1 The
practical applications of supramolecular photonics2 include
solar energy conversion into electrical or chemical energy3-7
and as components of nanoscaled devices.1,8-10 Porphyrins
are appealing subunits for materials11-15 because they can
* To whom correspondence should be addressed. E-mail: cdrain@
hunter.cuny.edu.
(1) Alivisatos, A. P.; Barbara, P. F.; Castleman, A. W.; Chang, J.; Dixon,
D. A.; Klein, M. L.; McLendon, G. L.; Miller, J. S.; Ratner, M. A.;
Rossky, P. J.; Stupp, S. I.; Thompson, M. E. AdV. Mater. 1998, 10,
1297-1336.
(2) Kalyanasundaram, K.; Gra¨tzel, M. Coord.Chem. ReV. 1998, 177, 347-
414.
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D. G.; Sommers, H. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6498-
6502.
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Mater. Chem. 2000, 10, 645-650.
(15) (a) Drain, C. M.; Mauzerall, D. Bioelectrochem. Bioenerg. 1990, 24,
263-268. (b) Drain, C. M.; Christensen, B.; Mauzerall, D. Proc. Natl.
Acad. Sci. U.S.A. 1989, 89, 6959-6962. (c) Drain, C. M.; Mauzerall,
D. C. Biophys. J. 1992, 63, 1556-1563.
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3590.
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48.
(6) Milgrom, L. R. J. Chem. Soc., Perkin Trans. 1 1984, 1483-1487.
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P. Sol. Energy Mater. Sol. Cells 1995, 38, 127-134.
(8) Dagani, R. Chem. Eng. News 1998, 76, 35-46.
(9) Dagani, R. Chem. Eng. News 1998, 76, 70-78.
(10) Dagani, R. Chem. Eng. News 2000, 78, 1-19.
(11) Burrell, A.; Officer, D.; Plieger, P.; Reid, D. Chem. ReV. 2001, 101,
2751-2796.
(17) Goldberg, I. Chem. Eur. J. 2000, 6, 3863-3870.
(18) Diskin-Posner, Y.; Dahal, S.; Goldberg, I. Angew. Chem., Int. Ed. 2000,
39, 1288-1292.
(19) Kumar, R. K.; Balasubramanian, S.; Goldberg, I. Inorg. Chem. 1998,
37, 541-552.
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Macrocyclic Chem. 2000, 37, 219-230.
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Chemistry; Sauvage, J. P., Ed.; John Wiley & Sons Ltd.: New York,
1999; pp 285-368.
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10.1021/ic025985v CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/22/2003
Inorganic Chemistry, Vol. 42, No. 6, 2003 2075

Cheng et al.
a
Scheme 1
a Upon chelation by many transition metal ions, the 2,2′-bipyridine moiety becomes rigid and planar, thus making the two porphyrins at the 5,5′ positions
in 1 or Zn21 coplanar on average, and somewhat increases the electronic communication between the chormophores (left). The 2,2′-bipyridyl moieties of
two 1 or Zn21 species can bind to a tetrahedral metal ion (right) or metals with different coordination geometries. The axial coordination of the zinc ions
of two Zn21 species by two DABCO moieties creates a supramolecular square (center), which maintains its structural integrity upon metal ion binding by
the two 2,2′-bipyridines (bottom), thus demonstrating that conformational changes in a component need not alter supramolecular structure. For clarity, the
t-butylphenyl groups are not shown.
catenanes and other topologically complex structures, and
there are a variety of other approaches to self-assembled
porphyrin systems.11,21-23 Certainly, one of the most exploited
methods to assemble porphyrin arrays, layers, and crystals
is to use pyridyl porphyrins and transition metals. The 60°,
120°, or 180° direction of the respective 2-, 3-, or 4-pyridyl
nitrogens, combined with the various coordination geometries
of transition metals, and the axial positions of metal
derivatives, can result in a very large number of porphyrinic
materials, both discrete and polymeric.11,23,24 Therefore, one
of the central issues in supramolecular porphyrin chemistry
is the role of the pyridyl-metal-pyridyl linker in mediating
electron and energy transfer among the chromophores.25
arrays, herein we report the synthesis, characterization, and
functionality of 5,5′-bis[5′-(10,15,20-(4-tertbutylphenyl)por-
phrinyl)]-2,2′-bipyridine, 1, wherein the porphyrins are
appended 180° from each other in any conformation of the
BiPy (Scheme 1). In this system, the covalent bond between
the 2 and 2′ positions is conjugated to the 5,5′ positions
where the porphyrins are attached and the pyridyl nitrogens
are not conjugated to the 5 positions. The spectral properties
of this supramolecular techton are compared to 4,4′-bis[5′-
(10,15,20-(4-tertbutylphenyl)porphyrinyl]-2,2′-bipyridine, 2,
wherein the relative angle between the porphyrin planes
changes with the conformation of the BiPy moiety (Scheme
2). A meso alkyl derivative of 2 was reported earlier.26 In
contrast, the covalent bond in this latter system is not
conjugated to the 4,4′ positions bearing the porphyrins, but
the pyridyl nitrogens are. As in all meso aromatic porphyrin
compounds, the dihedral angle between the porphyrin
macrocycle and the meso pyridyl substituent is on average
90° and can vary (∼60° in solution. The orthogonality of
the porphyrins to the pyridines minimizes conjugation;
nevertheless, it is well-known that the wavelengths of the Q
and B bands in the ground-state electronic spectra of meso
To develop new building blocks that also address the role
of the linker in the photophysics of supramolecular porphyrin
(23) (a) Chambron, J.-C.; Heitz, V.; Sauvage, J.-P. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: New York, 2000; Vol. 6, pp 1-42. (b) Bruce, J. I.; Chambron,
J.-C.; Ko¨lle, P.; Sauvage, J.-P. J. Chem. Soc., Perkin Trans. 1 2002,
1226-1231.
(24) (a) Chou, J.-H.; Kosal, M. E.; Nalwa, H. S.; Rakow, N. A.; Suslick,
K. S. The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: New York, 2000; Vol. 6, pp 43-131. (b)
Drain, C. M.; Hupp, J. T.; Suslick, K. S.; Wasielewski, M. R.; Chen,
X. J. Porphryins Phthalocyanines 2002, 6, 243-258.
(25) Prodi, A.; Kleverlaan, C. J.; Indelli, M. T.; Scandola, F.; Alessio, E.;
Iengo, E. Inorg. Chem. 2001, 40, 3498-3504.
(26) Tomohiro, Y.; Satake, A.; Kobuke, Y. J. Org. Chem. 2001, 66, 8442-
8446.
2076 Inorganic Chemistry, Vol. 42, No. 6, 2003

Porphyrins as Supramolecular Building Blocks
a
Scheme 2
a Upon chelation by many transition metal ions, the 2,2′-bipyridine moiety becomes rigid and planar, thus locking the two porphyrins at the 4,4′ positions
in 2 or Zn22 in a syn conformation (left), and also somewhat increases the electronic communication between the chormophores. Similar to 1, the BiPy unit
can bind a variety of metal ions, but the resultant geometry of the macrocycles is quite different (right). For clarity, the t-butylphenyl groups are not shown.
4-pyridyl porphyrins are shifted by several nanometers upon
pyridyl coordination to a variety of metal ions.25,27,28 Also,
metalation of the porphyrin significantly alters the basicity
of the para pyridyl nitrogen. These are clear indications that
there is significant electronic communication between the
macrocycle and the pyridyl subunits.
disassembly, as well demonstrated by techton 2 and its
derivatives, whereas in compound 1 the conformational
changes resulting upon binding of a metal ion by the BiPy
moiety leave the structure of a supramolecular assembly
intact. This latter feature is demonstrated by the observation
that the structural integrity of a supramolecular square
composed of two Zn21 moieties and two 1,2-diazabicyclo-
[2,2,2]octane (DABCO) units (Scheme 1) remains intact even
though the 2,2′-bipyridine moieties change conformation
upon binding transition metals.
The BiPy moiety is normally not planar, but it adopts a
planar conformation upon metal ion binding. The planar
structure electronically couples, via conjugation, the two
aromatic pyridine systems with the atomic orbitals of the
metal contributing to varying degrees toward the overall
molecular orbitals of the complex. As such, the complexation
of a metal ion by the BiPy moiety results in increased
electronic coupling between the two 5,5′ attached chro-
mophores because the BiPy becomes planar. Thus, the BiPy
moiety serves as a metal-gated switch to electronically couple
the porphyrins (Scheme 1). When the BiPy unit of compound
2 binds a metal, the two porphyrins rotate from an average
anti-like conformation to lock into a syn-like conformation
so that they are on the same side of the BiPy and can serve
as receptor to ditopic ligands of appropriate size and
orientation (Scheme 2). For 2, electronic communication is
mediated also by the adoption of a planar structure by the
BiPy, but the nitrogens of the bis chelate are conjugated to
the point of attachment of the macrocycles.
The primary goals of the present work are (1) to compare
the ground- and excited-state spectra of these two structural
isomers when the bipyridyl linker is coordinated to various
metal ions in order to gain insights into the role of this motif
in the photophysical properties of supramolecular porphyrin
arrays assembled by exocyclic coordination to metal ions,
and (2) to design and characterize a new techton for the self-
assembly of porphyrinic materials. To this end, we have
examined the electronic absorption and emission spectra of
1 and its zinc metalloporphyrin derivative, Zn21, and
compared these results to those of 2 and Zn22. These
reactions are summarized in Schemes 1 and 2.
Experimental Section:
Reagents and Physical Measurements. All solvents were
distilled using standard methods. Silica gel (Baker, 60 µm average
particle size) and alumina (Fisher, 80-200 mesh) were used for
column chromatography. The CHCl3 was reagent grade and used
directly. The methyl-THF was of reagent grade and used as
obtained. The THF was freshly distilled from Na, and methanol
was distilled from CaH2. The other solvents were distilled either
from CaH2 or P2O5. 2,2′-Bipyridine-4,4′-dicarboxaldehyde was
purchased from Aldrich Chemical Co., and 2,2′-bipyridine-5,5′-
dicarboxaldehyde was prepared according to the literature.32,33 1
H
NMR spectroscopy was performed at 300 MHz on a Varian U-300
spectrometer using chloroform-d solvent as internal reference. UV-
vis spectra were recorded on a Varian Bio-100 spectrometer.
Electrospray mass spectroscopy spectra were obtained from a
Hewlett-Packard HP-1100 LC/MS in positive-ion mode.
Schemes, procedures, and characterization of the intermediates
for the unsuccessful directed synthesis of 2 are presented in the
Supporting Information. In general for titration experiments, an
aliquot of the solution containing the given metal ion was added
to the porphyrin solution, the mixture was stirred at room
temperature for ∼15 min (for the Pd(II) compounds, the mixture
was heated to ∼50 °C), and spectra were recorded at ∼20 °C. Under
the conditions described for the titrations, and consistent with known
BiPy chemistry, plots of either absorption intensity or chemical
shifts versus mole ratio indicate a 1:1 stoichiometry of the porphyrin
derivatives to the metal ion coordinated to the BiPy unit.
In most supramolecular systems, a conformational change
in one of the structural elements results in a substantial
change in the structure of the supermolecule29-31 or even
(29) Lehn, J.-M. Chem. Eur. J. 2000, 6, 2097-2102.
(30) Funeriu, D. P.; Lehn, J.-M.; Fromm, K. M.; Fenske, D. Chem. Eur. J.
2000, 6, 2103-2111.
(31) Linke, M.; Chambron, J.-C.; Heitz, V.; Sauvage, J.-P.; Semetey, V.
Chem. Commun. 1998, 2469-2470.
(32) Ebmeyer, F.; Vogtle, F. Chem. Ber. 1989, 122, 1725-1727.
(33) Al-Sayah, M. H.; Salameh, A. S. Arabian J. Sci. Eng. 2000, 25, 67-
70.
(27) Drain, C. M.; Lehn, J.-M. Chem. Commun. 1994, 2313-2315.
(28) Drain, C. M.; Nifiatis, F.; Vasenko, A.; Batteas, J. Angew. Chem.,
Int. Ed. 1998, 37, 2344-2347.
Inorganic Chemistry, Vol. 42, No. 6, 2003 2077

Cheng et al.
5,5′-Bis[5′-(10,15,20-tri-4-tert-butylphenyl)porphyrinyl]-2,2′-
bipyridine (1). A mixture of 2,2′-bipyridine-5,5′-dicarboxaldehyde
(25 mg, 0.12 mmol) and pyrrole (0.07 mL, 0.96 mmol) in a 15 mL
solvent mixture of acetic acid and nitrobenzene (3:2 v/v) was
refluxed for 15 min, and then, 4-tert-butylbenzaldehyde (0.12 mL,
0.72 mmol) was added to the reaction mixture. The final mixture
was refluxed in air for 18 h at 140 °C. Then, the excess solvent
was removed by vacuum distillation. The crude dark solid was
dissolved in chloroform and filtered through a pad of silica (5%
EtOH in chloroform). The filtrate was concentrated, redissolved in
CHCl3, and chromatographed (silica 37 g, chloroform). The second
purple band was collected. Four-fifths of the solvent was removed,
and the crude product was crystallized with acetonitrile at 0 °C,
affording a purple precipitate that was centrifuged and the pellet
dried in vacuo (3.7 mg, 2%). 1H NMR (chloroform-d): δ 9.71 (d,
J ) 2.5 Hz, 2H, H6 in 2,2′-bipyridyl), 9.22 (d, J ) 8 Hz, 2H, H3
in 2,2′-bipyridyl), 9.03 (d, J ) 5 Hz, 4H, pyrrole-Hâ), 8.99 (d, J
) 5 Hz, 4H, pyrrole-Hâ), 8.93 (s, 8H, pyrrole-Hâ), 8.87 (dd, J1 )
2.5 Hz, J2 ) 8 Hz, 2H, H4 in 2,2′-bipyridyl), 8.19 (m, 12H, H2 in
4-tert-butylphenyl), 7.80 (m, 12H, H3 in 4-tert-butylphenyl), 1.65
(s, 54H, CH3 in 4-tert-butylphenyl), -2.66 (br s, 4H, inner-NH).
UV-vis (methyl-THF): λmax ) 422, 516, 552, 594, 650 nm. ESMS
obsd 1566.8, calcd 1566.0 (C110H104N10).
vis (methyl-THF): λmax ) 430, 557, 597 nm. ESMS obsd 1694.3,
calcd 1693.0 (C110H100N10Zn2).
5-Mesityldipyrromethane (3). A mixture of pyrrole (18.8 mL,
271 mmol) and mesitaldehyde (2 mL, 13.5 mmol) was flushed with
nitrogen for 15 min and then treated with TFA (0.11 mL, 2.5 mmol),
and the mixture was stirred under nitrogen at room temperature
for 1 h and then quenched with triethylamine (0.5 mL). Toluene
(100 mL) was added, and the organic phase was washed with brine
(2 × 50 mL) and dried (MgSO4). The solvent was removed under
vacuum to give a black oil. Vacuum distillation removed the excess
pyrrole to yield a highly colored solid that was washed with ethyl
acetate. The solvent was evaporated until a solid began to form.
Then, 50 mL of cyclohexanes was added to precipitate a yellow
solid that was filtered and recrystallized from ethanol/water (10:1)
to give yellow crystals (0.88 g, 25%). 1H NMR (chloroform-d): δ
7.95 (br, 2H), 6.87 (s, 2H), 6.67 (m, 2H), 6.19 (m, 2H), 6.02 (m,
2H), 5.93 (s, 1H), 2.29 (s, 3H), 2.07 (s, 6H).
5,5′-Bis(2,2′-dipyrromethyl)-2,2′-bipyridine (4). A mixture of
pyrrole (6.5 mL, 94 mmol) and 2,2′-bipyridine-5,5′-dicarboxalde-
hyde (200 mg, 0.94 mmol) was flushed with nitrogen for 15 min
and treated with TFA (0.06 mL, 0.75 mmol). The solution darkened,
and the mixture was stirred for 15 min. Aqueous NaOH (0.1 N, 20
mL) and 20 mL of ethyl acetate were added, and the layers were
separated. The organic layer was washed with water (2 × 20 mL).
The combined organic layers were washed with brine and dried
(MgSO4), and vacuum distillation removed the excess pyrrole.
Precipitation from hexane/CH2Cl2 gave a yellow solid, which was
5,5′-Bis[5′-(10,15,20-tri-4-tert-butylphenyl)porphyrinatozinc-
(II)]-2,2′-bipyridine (Zn2 1). Compound 1 (3.0 mg, 1.9 µmol) was
dissolved in 10 mL of CHCl3, and then, 6 drops of saturated
Zn(CH3CO2)2 in methanol was added to the solution. The resulting
porphyrin solution was allowed to reflux for 15 min (the process
was monitored by UV-vis spectroscopy). The reaction was
quenched by adding 10 mL of water, and then, the layers were
1
filtrated and dried in vacuo (0.25 g, 60%). H NMR (chloroform-
d/DMSO-d6): δ 9.20 (br, 4H), 8.40 (s, 2H), 8.16 (d, J ) 8 Hz,
2H), 7.52 (dd, J1 ) 2 Hz, J2 ) 8 Hz, 2H), 6.61 (m, 4H), 5.99 (m,
4H), 5.77 (s, 4H), 5.45 (s, 2H).
separated. The organic layer was dried (MgSO4), and concentrated
1
to
/ of the volume. The resulting porphyrin solution was
5
crystallized from acetonitrile at 0 °C to afford a reddish precipitate.
Results and Discussion
The solid was centrifuged and the pellet dried in vacuo (3.1 mg,
1
Synthesis. Compounds 1 and 2 were synthesized by mixed
aldehyde condensations based on a modified Adler synthe-
sis34 in ∼3% yield (Scheme 3) using the respective 2,2′-
bipyridine-5,5′-dicarboxaldehyde and 2,2′-bipyridine-4,4′-
dicarboxaldehyde. The directed synthesis of each using a
MacDonald-type 2 + 2 coupling strategy (compounds 3 and
4 in Scheme 4) was unsuccessful using a variety of reaction
conditions with either the 4-tert-butlybenzaldehyde or the
mesityl aldehyde.35-39 While the precursors and their inter-
mediates for both 1 and 2 were readily synthesized and char-
acterized according to literature methods, these reaction
sequences failed in the porphyrinogen/porphyrin forming step
as was observed earlier for a compound similar to 2.26 The
mesityl aldehydes are reported to minimize scrambling and
increase the solubility (i.e., reactivity) of the intermediates,
yet the synthesis of these derivatives also fails at the crucial
porphyrin-forming step. The previous synthesis of a meso
alkyl version of 2 involved the directed synthesis of a
95%). H NMR (chloroform-d/THF-d8 6:1): δ 9.63 (d, J ) 2.5
Hz, 2H, H6 in 2,2′-bipyridyl), 9.14 (d, J ) 8 Hz, 2H, H3 in 2,2′-
bipyridyl), 8.99 (m, 8H, pyrrole-Hâ), 8.89 (s, 8H, pyrrole-Hâ), 8.78
(dd, J1)2.5 Hz, J2)8 Hz, 2H, H4 in 2,2′-bipyridyl), 8.13 (m, 12H,
H2 in 4-tert-butylphenyl), 7.72 (m, 12H, H3 in 4-tert-butylphenyl),
1.59 (s, 36H, CH3 in 4-tert-butylphenyl), 1.58 (s, 18H, CH3 in
4-tert-butylphenyl). UV-vis (methyl-THF): λmax ) 430, 558, 599
nm. ESMS obsd 1694.3, calcd 1693.0 (C110H100N10Zn2).
4,4′-Bis[5′-(10,15,20-tri-(4-tert-butylphenyl)porphyrinyl)]-2,2′-
bipyridine (2). A sample of 2,2′-bipyridine-4,4′-dicarboxaldehyde
(25 mg, 0.12 mmol) was treated identically as for 1, affording 5.6
mg (3%) of reddish purple solid. 1H NMR (chloroform-d): δ 9.64
(s, 2H, H3 in 2,2′-bipyridyl), 9.07 (d, J ) 4.8 Hz, 2H, H6 in 2,2′-
bipyridyl), 8.98 (s, 8H, pyrrole-Hâ), 8.92 (s, 8H, pyrrole-Hâ), 8.24
(dd, J1 ) 1.4 Hz, J2 ) 4.0 Hz, 2H, H5 in 2,2′-bipyridyl), 8.18 (d,
J ) 8.1 Hz, 12H, H2 in 4-tert-butylphenyl), 7.78 (d, J ) 8.1 Hz,
12H, H3 in 4-tert-butylphenyl), 1.63 (s, 54H, CH3 in 4-tert-
butylphenyl), -2.68 (br s, 4H, inner-NH). UV-vis (methyl-THF):
λmax ) 422, 515, 550, 593, 648 nm. ESMS obsd 1566.8, calcd
1566.0 (C110H104N10).
4,4′-Bis[5′-(10,15,20-tri-(4-tert-butylphenyl)porphyrinatozinc)]-
2,2′-bipyridine (Zn22). A sample of compound 2 (5.0 mg, 3.2
µmol) was treated identically as for Zn21, affording 5.1 mg (95%)
(34) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.
(35) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J.; Lindsey, J. S. J. Org.
Chem. 2000, 65, 7323-7344.
1
of a reddish solid. H NMR (chloroform-d): δ 9.63 (s, 2H, H3 in
(36) Gryko, D.; Lindsey, J. S. J. Org. Chem. 2000, 65, 2249-2252.
(37) Geier, G. R.; Littler, B. J.; Lindsey, J. S. J. Chem. Soc., Perkin Trans.
2 2001, 2, 701-711.
2,2′-bipyridyl), 9.06 (d, J ) 4.8 Hz, 2H, H6 in 2,2′-bipyridyl), 9.03
(s, 8H, pyrrole-Hâ), 8.97 (s, 8H, pyrrole-Hâ), 8.23 (dd, J1 ) 1.4
Hz, J2 ) 4.0 Hz, 2H, H5 in 2,2′-bipyridyl), 8.17 (d, J ) 8.1 Hz,
12H, H2 in 4-tert-butylphenyl), 7.76 (d, J ) 8.1 Hz, 12H, H3 in
4-tert-butylphenyl), 1.63 (s, 54H, CH3 in 4-tert-butylphenyl). UV-
(38) Lindsey, J. S.; Lee, C.; Miller, M. A.; Littler, B. J.; Kim, H.; Cho, W.
J. Org. Chem. 1999, 64, 7890-7901.
(39) Littler, B. J.; Lindsey, J. S.; Miller, M. A.; Hung, C. H.; Wagner, R.
W.; O’Shea, D. F.; Boyle, P. D. J. Org. Chem. 1999, 64, 1391-1396.
2078 Inorganic Chemistry, Vol. 42, No. 6, 2003

Porphyrins as Supramolecular Building Blocks
Scheme 3
Scheme 4
synthetic point of view, this method for 2 both is more time-
consuming and provides a comparable overall yield from
the starting aldehydes. As noted with similar synthetic
procedures that use the Adler method to make porphyrins
bearing both heterocyclic and phenyl moieties, allowing the
less reactive heterocyclic aldehyde to react with the pyrroles
for ∼15 min before addition of the arylaldehyde significantly
increases the yield of the porphyrins bearing the heterocyclic
groups.40,41 The yield from the Adler-based synthesis is
somewhat less than the expected ∼4% estimated from the
statistical reaction of the aldehydes based on stoichiometry,
and a 25% yield for each porphyrin. However, this is greatly
ameliorated by the ease of scale-up of the one-step synthesis.
We obtain only traces of porphyrin using the five-step
reaction sequence outlined in Scheme 4 which would have
an overall yield of ∼2% had the two porphyrin-forming
reactions occurred with 25% yields each.
Electronic Spectra. The two halves of free 2,2′-bipyridine
are not coplanar, and the nitrogens generally point away from
each other; therefore, there is little conjugation between the
two pyridyl rings. Meso aryl groups on porphyrins are also
noncoplanar with the macrocycle, but the effects of substitu-
tions on the aryl ring are readily observed in the electronic
spectra, indicating some degree of electronic coupling
between the two. Therefore, we expected and observed that
(40) Shi, X.; Barkigia, K. M.; Fajer, J.; Drain, C. M. J. Org. Chem. 2001,
66, 6513-6522.
monopyridylporphyrin followed by a coupling reaction to
form the pyridyl moiety.26 Though more elegant from a
(41) Drain, C. M.; Shi, X.; Milic, T.; Nifiatis, F. Chem. Commun. 2001,
287-288.
Inorganic Chemistry, Vol. 42, No. 6, 2003 2079

Cheng et al.
Figure 1. UV-vis spectra of 5-(4-pyridyl)-10,15,20-tri(4-tert-butylphenyl)-
porphyrin (A), 1 (B), and Zn21 (C) in 2-methyl-THF (4 µM).
the two free-base porphyrins of both compounds 1 and 2
linked by the free-base 2,2′-bipyridine are reasonably
electronically isolated. There is a ∼5 nm red shift and ∼50%
increase in fwhm of the Soret band of 1, and for 2, the
broadening is only ∼25%, both versus a monopyridyl-
porphyrin analogue. These are somewhat greater than most
simple substituent effects for porphyrinic systems (Figure
1). The UV-vis spectrum indicates compound 1 has an
increased proclivity to aggregate compared to 2.
Upon metal ion coordination, the two pyridines of 2,2′-
bipyridine become coplanar, and the degree of conjugation
between the halves increases. The combination of the BiPy
and the metal orbitals results in a system that allows greater
electronic communication between the porphyrin chro-
mophores. In 1, the 5 and 5′ positions are conjugated to the
covalent bond and not with the pyridyl nitrogens. Conversely
for 2, the nitrogens are conjugated to the point of attachment,
and the covalent bond is not. As expected, when the BiPy
linker is coordinated by a variety of metal ions such as
Cu(I), Ni(II), or Pd(II), the UV-vis spectra indicate an
increase in the electronic communication between the two
porphyrins in 1 (Figures 2 and 3) and 2 (Figure 4). For the
Pd(II) adducts of 1 and 2, this is indicated by a ∼2 nm blue
shift and a further ∼20% broadening of the Soret band, as
well as red shifts in the Q-bands (Figure 2). Several isosbestic
points are found in the titration experiments for the free-
base compounds, indicating the formation of only one
product. For comparison, a linear dimer formed from the
coordination of two para pyridylporphyrins to Pd(II)Cl2
results in a 2-3 nm red shift and a ∼10% broadening of the
Soret band.27 Thus, in addition to the substituent effects, the
ground-state electronic communication between chromo-
phores is mediated by both the covalent bond and by the
BiPy chelated metal ion, though the effects are small.
Unexpectedly, when both of the porphyrins are metalated
with zinc, the equilibrium between the free-base 2,2′-
bipyridine conformations shifts such that a significant
population of the coplanar form exists, as observed by the
dual Soret bands in the optical spectra for Zn21 (Figure 1),
and to a different extent for Zn22 (Figure 4, initial spectra).
The peak at 430 nm is consistent with the known red shift
of porphyrin Soret bands upon Zn(II) coordination, and the
Figure 2. UV-visible spectra as 1 equiv (in 6 e5 µL aliquots) of
Pd(II)Ac2 is titrated into 3 mL of a 5 µM solution of 1 (top) and Zn21
(bottom)in 2-methyl-THF.
Figure 3. Soret region of the UV-vis spectra as 1 equiv (in 6 e5 µL
aliquots) of Ni(II)Ac2 or Cu(I)ClO4 is titrated into of 3 mL of a 5 µM
solution of 1 (left), and Zn21 (right) in 2-methyl-THF; i ) initial, f ) final.
426 nm band represents the fraction of molecules with the
BiPy in a planar conformation. This latter observation also
is consistent with the known increase in electron density on
the nitrogen(s) of 4 pyridyl porphyrins, as measured by
coordination chemistry and basicity, upon zinc metala-
tion.27,28,40,41 This conclusion is also supported by the
observation that when the Pd(II) species is titrated into a
solution of Zn21, the 430 nm band disappears, to yield a
broad Soret band similar to the one found for the free base
2080 Inorganic Chemistry, Vol. 42, No. 6, 2003

Porphyrins as Supramolecular Building Blocks
These observations in the electronic spectra are consistent
with the Gouterman four-orbital model.42 For both Zn21 and
Zn22, the largest changes in terms of energetics are observed
in the Q(0,0) bands which correspond to the lowest energy
1(π,π*) state. The highest occupied molecular orbitals of zinc
porphyrins are a2u and a1u, and the former has significant
electron density at the meso positions (bearing the BiPy
moiety) and the pyrrole nitrogens whereas the latter has
electron density on the pyrrole R and â positions.
NMR Spectra. Chelation of Pd(II)Ac2 by the 2,2′-BiPy
1
subunit for both 1 and 2 was also examined by H NMR
titrations whereby 1.5 mg of 1 or 2 was dissolved in 1 mL
of CDCl3 and the spectrum was taken of the initial solution
and after each addition of 0.2 equiv of Pd(II)Ac2 dissolved
in CDCl3. The highly characteristic shift in the pyridyl
protons upon coordination of a palladium ion indicates clean
formation of the Pd adducts of 1 and 2. For Zn21 in 6:1
CDCl3/THF-d8, the singlet for the pyridyl 6H moves from
9.63 to 9.25 ppm, the doublet for the pyridyl 3H moves from
9.13 to 9.05 ppm, the doublet for the pyridyl 4H moves from
8.78 to 8.65 ppm, and the pyrrole doublet of doublets at 8.98
ppm (which arise from the pyrrole nearest the BiPy unit)
changes to complex multiplets at 9.00 and 8.75 ppm. The
single pyrrole resonance from the outer pyrroles significantly
broadens into three closely spaced peaks, and the t-butyl
methyls move from 1.59 to 1.61 ppm. Similar 1H NMR shifts
are observed for 2. The changes in chemical shifts are
consistent with those observed for other pyridylporphyrin
complexes,25,27,28 and the increased peak width is likely due
to the increased rigidity of the system, and slight increase
in molar mass. Significantly, the changes in chemical shift
of the pyrrole protons for both 1 and 2 when the BiPy unit
binds Pd(II) further indicates the substituent effects, but note
that these changes in pyrrole chemical shifts are about the
same as those observed when two 5-(4-pyridyl)-10,15,20-
tri-(4-tertbutylphenyl)porphyrins bind the same metal ion.
Therefore, the covalent bond between the pyridines in BiPy
in 1 and 2 has only minor effects on the ring current of the
macrocycles. The predominant causes of the spectral changes
are the metal ion and the conformational change of the BiPy
induced by metal ion binding.
Coordination Chemistry. The coordination chemistry of
these two compounds has yet to be fully explored; however,
the more than ample precedents in BiPy chemistry indicate
that these will be good tectons for a variety of supramolecular
arrays and polymers. Control of temperature, relative stoi-
chiometry, and choice of metal ion allow one to design
systems wherein these two compounds merely complex the
metal ion, such as the work herein, or form higher ordered
adducts (such as 2:1 or 3:1).16 The structure of these
complexes will be governed by the metal ion binding
properties as well as the steric interactions of the porphyrins.
For example, the tetrahedral coordination geometries of some
metal ions such as Cu(I) and Zn(II) allow the formation of
dimers of 1 or 2 bound together by the coordination of the
BiPy moieties to the metal. In these cases, the four chro-
Figure 4. Soret region of the UV-vis spectra as 1 equiv (in 6 e5 µL
aliquots) of Ni(II)Ac2 or Cu(I)ClO4 is titrated into of 3 mL of 5 µM solutions
of 2 (left) and Zn22 (right) in 2-methyl-THF; i ) initial, f ) final. The
Pd(II) titration can be found in the Supporting Information.
compound upon coordination of this metal by the BiPy
subunit. Again, the small red shift of the Zn21-Pd(II)
complex is due to the zinc metalloporphyrin (Figures 2 and
3). Note that, even at low concentrations (<10 µM) of Zn21,
the split Soret band remains, so this observation is not due
to changes in the aggregation state. Note also that for 2 there
is a ∼3 nm blue shift of the Soret upon complexation of the
same set of metal ions (Figure 4). When metal ions bind the
BiPy of Zn22, the split Soret band also evolves into a broader
peak centered to the blue region of that of Zn22. Thus, the
largest contributor to the observed spectral shifts is the
locking-in of the planar conformation of the BiPy by metal
ion binding, vide infra.
The nature of the metal ion coordinated by the BiPy moiety
for both 1 and 2 makes little difference in the ground-state
spectra as first row transition metals, second row metals, and
divalent and univalent ions result in similar characteristic
changes in the optical spectra with only subtle differences
between them. The excited-state spectra reveal several
features that are worth noting. As expected for virtually any
fluorophore, upon coordination of a metal ion by the BiPy
moiety, the fluorescence emission spectra decrease in
intensity relative to those of the parent compounds due to
the heavy atom effect. Second, upon BiPy coordination to
some metal ions, there are pronounced differences in both
the relative intensities and the energies of the emission bands
relative to the unbound species. These differences likely arise
from the different redox potentials of the different metal ions
rather than enhanced electronic coupling of the chromophores
and may represent a means to fine-tune the excited-state
properties. The fluorescence emission and excitation spectra
for 1, Zn21, 2, and Zn22 where the BiPy is complexed to
Zn(II) or Pd(II) are essentially similar to those of the
uncomplexed species, albeit with reduced intensity due to
the heavy atom effect. This latter observation confirms the
general conclusion that the conformational changes of the
bis-porphyrin BiPy compounds upon coordination to metal
ions result in modest electronic coupling of the chromo-
phores.
(42) Gouterman, M. The Porphyrins; Dolphin, D., Ed.; Academic Press:
New York, 1978; Vol. 3, pp 1-165.
Inorganic Chemistry, Vol. 42, No. 6, 2003 2081

Cheng et al.
Figure 5. (A) The broad Soret band in the UV-vis spectra of Zn21 (70 µM in chloroform, 1 mm path length cuvette) indicates some aggregation, but
disaggregation occurs as 1 equiv of DABCO is titrated into the solution to form a supramolecular square (Scheme 1). (B) The addition of 1 equiv of
Pd(II)Ac2 per Zn21 changes the conformation of the BiPy without changing the structure of the supermolecular square. (C) Changes in the optical spectra
are nearly complete after only ∼0.5 equiv of DABCO is added to Zn22. (D) The addition of 1 equiv of Pd(II)Ac2 per Zn22.
mophores are well separated (Schemes 1 and 2). The optical
spectra of both of these simple coordination dimers are
unremarkable such that neither dimer displays properties
much different from those of the monomers coordinated by
the same metal ion. The relative orientation of the porphyrins
in coordination oligomers or arrays based on compounds 1
and 2 will be quite different. Coordination of the BiPy moiety
of the 5,5′ derivative to square planar metal ions will force
the macrocycles to be cofacial thereby altering the photo-
physical properties, as exhibited by further blue shifts in the
Soret band. The coordination of the 4,4′ derivative to square
planar metals leaves the chromophores separated, and the
photophysical properties are nearly indistinguishable from
the simple monomer coordinated to the same metal ion. The
full characterization of self-assembled arrays of these
molecules will be reported later.
Supramolecular Squares. Since the first publications on
designed coordination squares of porphyrins,27 there have
been numerous reports on the application of these concepts
to the design of functional materials.24,27,43 In all of the
porphyrinic squares, and most supramolecular systems in
general, a conformational change in the molecules that
constitute the sides either is not possible because they are
locked into place or results in the disassembly of the
supramolecular system, which may or may not reassemble
into a different structure.30 Compound 1 and its metallopor-
phyrin derivatives, however, show that this is not a priori
true. As a demonstration, it is possible to form a supramo-
lecular square using two Zn21 moieties and two DABCO
molecules (Scheme 1). At 70 µM in chloroform, the
supramolecular square is the predominant species according
to well established equilibria and exhibits the characteristic
red shift in the electronic spectra upon axial coordination of
the zinc porphyrins by nitrogenous ligands44 (Figure 5). In
the process of forming the square, the DABCO disrupts the
aggregates of Zn21 found at this concentration, and note that
the Soret band of the final spectrum, “f” (peak maxima at
426, 430, 562, 606 nm) is essentially identical to that of this
compound at 5 µM, with one equivalent of piperidine. Note
that the nominal Soret bands are essentially the same as that
of unligated Zn21, but the Q-bands are red shifted by ∼ 5
nm. A plot of ∆A versus DABCO equivalents confirms the
1:1 stoichiometry. The porphyrins in each Zn21 unit are now
coplanar. This is in contrast to the “average” coplanarity
arising from the small oscillations about the pyridyl-
porphyrin bonds when the BiPy unit in Zn21 is bound by
metal ions (peak maxima at 426, 560, 607 nm). Then, as
the Pd(II) species is titrated into the solution containing the
square, there is a substantial red shift in the lowest energy
Q-band (12 nm), a smaller shift in the intermediate Q-band
(3 nm), and the red Soret band at 430 nm disappears. The
two BiPy units are now also locked into a planar conforma-
tion, and the porphyrins in each subunit in the final structure
are not only coplanar but also in register with the opposing
subunit. This is further indication that the planar, metal-bound
(43) (a) Melinda, H. K.; Kurt, D. B.; Joseph, T. H. Coord. Chem. ReV.
2000, 205, 201-228. (b) Fujita, M.; Yazaki, J.; Ogura, K. J. Am. Chem.
Soc. 1990, 112, 5645-5647.
(44) Anderson, H. L. Inorg. Chem. 1994, 33, 972-981.
2082 Inorganic Chemistry, Vol. 42, No. 6, 2003

Porphyrins as Supramolecular Building Blocks
state of the BiPy is the main reason for the observed changes
in the electronic spectra. On the other hand, with Zn22, a
plot of ∆A versus DABCO equivalents indicates about half
an equivalent of DABCO binds per Zn22. In this case, the
addition of the Pd(II) species shifts the Q-bands to the red,
and there is a broadening of the Soret band (Figure 5).
The fluorescence of both the supramolecular square and
the Zn22-DABCO complex is typical of axial binding of
nitrogenous ligands to the Zn(II) centers.45 As observed in
other porphyrin assemblies mediated by Pd(II) coordina-
tion,12,27,28 there are small red shifts in the emission spectra
of Zn21‚Pd(II) and Zn22‚Pd(II) that reflect the observed shifts
in the ground-state spectra. However, note that when two
Pd(II) ions bind to the square the fluorescence is quenched
by ∼50%, but when one Pd(II) binds to the Zn22-DABCO
complex the emission is quenched by ∼80%. The smaller
magnitude of the quenching by Pd(II) is consistent with other
rigidly assembled porphyrin systems14 and aggregates.46
Taken together the absorption data indicate that the average
coplanarity of the chromophores is not the only cause of the
spectral shifts, but it is a combination of the planar
conformation of the BiPy and the substituent effects caused
by metal ion binding to the BiPy. The further red shift of
the electronic spectra of the supramolecular square is largely
due to the highly rigid structure with the BiPy units locked
into a planar conformation.
pounds indicate that the spatial arrangement of the porphyrins
further dictates the photophysical properties, as expected.
Thus, larger and more complex porphyrin arrays may be
synthesized by coordinating the appropriate transition metal
ions bearing multitopic ligands to the BiPy moiety.23 These
molecules will serve as part of a library of building blocks
for the self-assembly of nanoscaled photonic materials.47-51
Since the porphyrins only subtly alter the 2,2′-bipyridine
metal ion binding properties, the well established coordina-
tion chemistry of this subunit can be exploited. However,
steric considerations may well prevent or complicate the
formation of certain bis- and tris(BiPy) complexes that are
well-known in the literature, especially for 1 and its
derivatives. Importantly, the results on the ladder complex
of Zn21 demonstrate that there is no a priori requirement
that structural rearrangements or reorganization of supramo-
lecular systems will result from structural changes in
component molecules, even though these structural perturba-
tions result in significantly different photophysical properties.
Acknowledgment. This article is dedicated to Richard
W. Franck on the occasion of his 67th birthday. Funding
from the National Science Foundation (Grants CHE-0135509
and IGERT DGE-9972892) and the PSC-CUNY fund is
gratefully acknowledged. Hunter College Chemistry infra-
structure is partially supported by the National Institutes of
Health RCMI program, GM3037. Ngee Ai Thai is thanked
for her valuable assistance in the preparation of compound
1.
Conclusions
The new bis-porphyrin-2,2′-bipyridine system 1 and its
Zn complex were synthesized and characterized by UV-
vis, NMR and ESI-MS. The rigid architectures of the
molecules are in contrast to other porphyrin-bipyridine
systems such as 2, or similar derivatives.26 When the BiPy
subunit binds metal ions, it becomes planar, and facilitates
electronic coupling between the chromophores of 1, 2, and
their metalloporphyrin derivatives. These compounds (1,
Zn21, 2, and Zn22) can serve as sensors for various metal
ions with the sensitivity dictated by the binding constant of
the BiPy for the given metal ion. In a second functional
mode, these compounds can serve as an electronic switch
gated by complexation and decomplexation of these ions.
Initial studies comparing the ground- and excited-state spectra
of supramolecular systems constructed from these com-
Supporting Information Available: Procedures for the syn-
thesis of the intermediates for the directed synthesis of 2; 1H NMR
data for the Pd(II) adducts of Zn21, Zn22, and 2; UV-vis spectra
for the Pd(II) titration into 2 and Zn22; fluorescence data for the
supramolecular square titrated with Pd(II); and a table of λmax values
for all compounds. This material is available free of charge on the
Internet at http://pubs.acs.org.
IC025985V
(47) Abrahams, B. F.; Hoskins, B. F.; Michall, D. M.; Robson, R. Nature
1994, 369, 727-729.
(48) Burrell, A. K.; Officer, D. L.; Reid, D. C. W. Angew. Chem., Int. Ed.
1995, 34, 900-902.
(49) Iengo, E.; Zangrando, E.; Minatel, R.; Alessio, E. J. Am. Chem. Soc.
2002, 124, 1003-1013.
(45) Taylor, P. N.; Anderson, H. L. J. Am. Chem. Soc. 1999, 121, 11538-
(50) Lindsey, J. S.; Prathapan, S.; Johnson, T. E.; Wagner, R. W.
Tetrahedron 1994, 50, 8941-8968.
11545.
(46) Xu, W.; Guo, H.; Akins, D. L. J. Phys. Chem. B 2001, 105, 1543-
(51) Prathapan, S.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc. 1993,
115, 7519-7520.
1546.
Inorganic Chemistry, Vol. 42, No. 6, 2003 2083