Self-assembly of oligomeric porphyrin rings

Article abstract of DOI:10.1021/ol000129d

(equation presented) A cobalt porphyrin equipped with two different but geometrically complementary pyridine ligands self-assembles to form an unusually stable complex with approximately 12 porphyrin monomers arranged in a macrocyclic array.

Full text of DOI:10.1021/ol000129d

ORGANIC
LETTERS
2000
Vol. 2, No. 16
2435-2438
Self-Assembly of Oligomeric Porphyrin
Rings
Richard A. Haycock, Christopher A. Hunter,* David A. James,
Ulrike Michelsen, and Liam R. Sutton
Krebs Institute for Biomolecular Science, Department of Chemistry,
UniVersity of Sheffield, Sheffield S3 7HF, U.K.
c.hunter@shef.ac.uk
Received May 18, 2000
ABSTRACT
A cobalt porphyrin equipped with two different but geometrically complementary pyridine ligands self-assembles to form an unusually stable
complex with approximately 12 porphyrin monomers arranged in a macrocyclic array.
Porphyrin arrays have attracted considerable interest as
synthetic targets due to the potential for exploitation as
photochemical devices.1 Organized oligomeric assemblies of
the related chlorophyll chromophores are used in biological
light-harvesting complexes and reaction centers for the
trapping and conversion of solar energy in photosynthesis.2
A number of synthetic strategies have been developed for
the construction of multiporphyrin assemblies, and self-
assembly has emerged as one of the most promising
approaches.3 We recently reported the self-assembly of long-
chain polymers from a cobalt porphyrin symmetrically
functionalized with two pyridine ligands, Co1 (Figure 1).4
We now describe the synthesis of an unsymmetrically
functionalized analogue and show that it exhibits completely
different self-assembly properties.
(1) Sessler, J. L.; Wang, B.; Harriman, A. J. Am. Chem. Soc. 1993, 115,
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2316. Chi, X.; Guerin, A. J.; Haycock, R. A.; Hunter, C. A.; Sarson, L. D.
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10.1021/ol000129d CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/21/2000

Scheme 1a
a Conditions: 4-pyridinecarbonyl chloride; (b) i. Mg, ii. 4-bromo-
pyridine; (c) i. KMnO4, ii. SOCl2.
concentration profile for the symmetrical derivative Co1. The
retention times for Co1 are significantly shorter than those
for the corresponding porphyrin monomer Co3 and porphyrin
dimer (Co4)2 and are indicative of polymerization. As the
concentration of cobalt porphyrin increases, the length of
the polymeric assembly increases as expected.4 In contrast,
the retention time for Co2 is relatively insensitive to
concentration (Figure 2b). A single species is formed over
the concentration range 5-500 µM, and only when the
concentration increases further do higher molecular weight
polymers begin to emerge. The behavior of Co2 is charac-
teristic of a cooperative self-assembly process to form a
special structure with unusual stability. The most likely
candidate is a macrocyclic architecture, where the intramo-
lecular interactions formed in the cyclization process confer
additional stability relative to the intermolecular interactions
required to make open-chain polymers.6 We have endeavored
to obtain some supporting evidence for this hypothesis by
studying the dimerization of model compounds.
Figure 1. Self-assembly of Co1 to form long-chain polymeric
complexes, showing the key monomer-monomer interaction
interaction in detail and a cartoon of the corresponding polymer.
The structural parameter d1 used for calibration of the GPC data is
indicated.
H22 was prepared from the corresponding diaminopor-
phyrin using sequential acid chloride couplings as illustrated
in Scheme 1. Metalation with cobalt acetate in chloroform/
methanol afforded Co2 in quantitative yield. We have
previously shown that the best method for characterizing the
assembly properties of cobalt porphyrin oligomers is gel
permeation chromatography (gpc),5 and Figure 2 shows the
results obtained for Co2.
H24 and H25 were prepared from the corresponding
aminoporphyrins according to Scheme 2 and were quanti-
tatively metalated with zinc acetate. UV/visible dilution
studies were used to determine the association constants for
the zinc porphyrin dimers. The Zn4‚Zn5 association con-
stant (1.0 ( 0.2 × 106 M-1) is significantly larger than the
self-dimerization constants for the formation of Zn5‚Zn5 (4
( 1 × 104 M-1) and Zn4‚Zn4 (4 ( 1 × 105 M-1). Thus, the
two different ligand side arms constitute a geometrically
The behavior of this system is remarkably different from
the other symmetrically substituted porphyrin deriva-
tives which we have studied. Figure 2a shows a typical
(5) Other techniques have proved less suitable for characterizing these
systems: the major species detected by mass spectrometry (FAB, ESMS,
or Maldi) is always the monomer; the molecular weights of the oligomers
are too high for reliable vapor pressure osmometry; the paramagnetic
cobalt(II) center makes the 1H NMR spectra very broad; ligand coordination
can be detected by UV/visible absorption spectroscopy, but this only
confirms that the cobalt centers are fully bound to pyridine in these
compounds.
(6) Chi, X.; Guerin, A. J.; Haycock, R. A.; Hunter, C. A.; Sarson, L. D.
J. Chem. Soc., Chem. Commun. 1995, 2563.
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Org. Lett., Vol. 2, No. 16, 2000

Scheme 2a
a (a) 4-Pyridinecarbonyl chloride; (b) 3-(4-pyridyl)benzoyl chlo-
ride.
porphyrins in the heterodimer (Figure 3) which is clearly
consistent with proposed macrocyclization of Co2. The two
different ligands in Co2 are attached trans across the
porphyrin, and so the 30° bend at the level of the monomer-
monomer interaction must propagate in the same direction
Figure 2. Normalized GPC traces (5 µm mixed bed column) for
(a) Co1 at concentrations between 55 and 7000 µM and (b) Co2 at
concentrations between 5 and 2000 µM. tR is the retention time
and A is the absorbance at 440 nm. Both figures show the
corresponding GPC traces for reference compounds Co3 and Co4.
Co3 cannot oligomerize because it is functionalized with benzoyl
rather than pyridyl side arms as shown.4 Co4 is functionalized with
only one pyridyl side arm and therefore forms a dimer (see Scheme
2 for structure).4
complementary pair, and this leads to preferential formation
of the heterodimer shown in Figure 3. Models show that the
heterodimer can adopt a strain-free conformation which is
not accessible to the other two homodimers (compare the
ideal metal-pyridine bond angles in Figure 3 with those in
Figure 1 which are highly distorted). This structure leads to
an inclination of about 30° between the planes of the two
Figure 3. Model compounds Zn4 and Zn5 form a 1:1 complex
rather than homodimers due to the complementarity of the two
different ligand side arms. This leads to an inclination of about
30° between the planes of the two porphyrins. The structural
parameter d2 used for calibration of the GPC data is indicated.
Org. Lett., Vol. 2, No. 16, 2000
2437

along the oligomer, leading to either a macrocyclic or helical
structure. This suggests that the stable structure observed by
GPC is a cyclic dodecamer (Figure 4).
a value of N ) 11.5 for the Co2 assembly which is close to
the expected value for a monomer-monomer wedge angle
of 30°.
These experiments show that changing the geometry of
the monomer unit can have dramatic effects on the structure
and self-assembly properties of cobalt porphyrin arrays. Co1
which is symmetrically substituted with two identical ligand
side arms forms linear polymers (Figure 1). Co2 which is
unsymmetrically functionalized with complementary but
different ligand side arms self-assembles at much lower
concentrations to form a more stable structure which resists
polymerization. The best explanation based on the results
obtained from GPC, modeling, and the behavior of zinc
porphyrin model compounds is the formation of a macro-
cyclic dodecamer (Figure 4) which only opens up to form
higher molecular weight polymers once the concentration
increases above the effective molarity for macrocylisation
(about 0.5 mM).7 The macrocyclic architecture resembles
the bacterial light-harvesting complexes LH1 and LH2 where
close coupling of the chromophores around the ring facilitates
rapid and efficient energy transfer over long distances.2,8 The
chromophores are too far apart to show strong excitonic
coupling in the UV/visible absorption spectrum, and unfor-
tunately, cobalt quenches the porphyrin fluorescence, which
makes any analysis of the energy transfer properties of these
synthetic analogues problematic. However, the approach can
clearly be applied to other metalloporphyrin-ligand combina-
tions, and work on such systems is in progress.
Figure 4. Co2 self-assembles to form a macrocyclic oligomer. A
bend of 30° at the level of the Co2 monomer-monomer interaction
leads to the dodecamer shown, and this structure is consistent with
the GPC retention time (the structural parameter d2 used for
calibration of the GPC data is indicated).
The GPC results can be calibrated using the Co1-Co4
chain stopper system as described previously,4 and this
calibration was used to estimate the size of the self-assembled
structure formed by Co2. Since the dimensions of a macro-
cyclic oligomer scale with the diameter of the ring, whereas
the dimensions of a linear polymer simply increase with
length, a geometric correction must be applied to the retention
time calibration in order to relate these two systems:
Acknowledgment. This research was supported by the
EPRSC (R.A.H., U.M.), the BBSRC (D.A.J., L.R.S.), and
the Lister Institute (C.A.H.).
Supporting Information Available: Spectroscopic data
for H22, Co2, Zn4, and Zn5. This material is available free
of charge via the Internet at http://pubs.acs.org.
tR for Co1 ) c log(Nd1)
tR for Co2 ) c log(Nd2/π)
(1)
(2)
OL000129D
(7) Given the accuracy of the gpc experiment and the width of the peaks,
it is not possible to pin down the structure to a dodecamer as opposed to
another cyclic oligomer close in size or a mixture of two or three such
species. However, the structure is clearly a macrocycle of approximately
these dimensions.
(8) van Grondelle, R.; Dekker, J. P.; Gillbro, T.; Sundstrom, V. Biochim.
Biophys. Acta Bioenergetics 1994, 1187, 1. Karrasch, S.; Bullough, P. A.;
Ghosh, R. EMBO J. 1995, 14, 631. Oling, F.; Boekema, E. J.; Dezarate, I.
O.; Visschers, R.; van Grondelle, R.; Keegstra, W.; Brisson, A.; Picorel,
R. Biochim. Biophys. Acta Bioenergetics 1996, 1273, 44.
where c is a constant, N is number of monomer units in the
assembly, and d1 and d2 are the monomer repeat lengths
illustrated in Figures 1, 3, and 4. The repeat lengths were
determined by using models which assume that the porphyrin
side arms all lie flat in one plane (d1 ≈ 14 Å and d2 ≈ 19
Å). The GPC data in Figure 2 together with eqs 1and 2 give
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Org. Lett., Vol. 2, No. 16, 2000