Light-harvesting macroring accommodating a tetrapodal ligand based on complementary and cooperative coordinations

Article abstract of DOI:10.1021/ja048118t

A porphyrin macroring, mimicking the light-harvesting complex of photosynthetic purple bacteria, was synthesized by self-assembling trisporphyrinatoZn(II) complexes with imidazolyl substituents at both molecular terminals. Very strong complementary coordination of imidazolyl to Zn(II) afforded exclusively the cyclic trimer of trisporphyrin under dilute conditions at 27 °C in CHCl₃/MeOH = 9/1 (v/v). ¹H NMR spectra of the macroring indicate the existence of two topological isomers, one symmetric and one asymmetric. Use of three noncoordinated porphyrinatoZn(II) sites allows a tetrapodal ligand to be incorporated into the cavity of the macroring. A Job plot and the clear bending behavior on UV-vis titration indicated the formation a 1:1 complex; the heterogeneity of two topological isomers was not observed, and both isomers similarly accommodated the tetrapodal ligand. The association constant obtained by curve fitting analysis was 8 × 10⁸ M^-1 in toluene. This large association constant reflects the cooperative nature of three coordination sites. Copyright

Full text of DOI:10.1021/ja048118t

Published on Web 06/25/2004
Light-Harvesting Macroring Accommodating a Tetrapodal Ligand Based on
Complementary and Cooperative Coordinations
Yusuke Kuramochi, Akiharu Satake, and Yoshiaki Kobuke*
Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama,
Ikoma, Nara 630-0192, Japan
Received April 1, 2004; E-mail: kobuke@ms.naist.jp
Light-harvesting complexes (LHs) contained in photosynthetic
purple bacteria are some of the most beautiful and efficient
molecular structures found in nature. These complexes exist in two
types of cyclic arrangements of Bchl a: the LH1 arrangement,
which includes the reaction center,1 and the LH2 arrangement
around the periphery of LH1.2 The most fascinating basic structure
is the higher-ordered barrel structure of LH1 and B850 in LH2.3
These chromophore arrays exist as strongly coupled excitons and
play important roles in the rapid and efficient transfer of absorbed
solar energy to the reaction center. Various types of multiporphyrin
systems have been described by investigations into the high
efficiency of the energy transfer process and for application to
artificial photosynthetic systems.4 However, light-harvesting cyclic
arrays of porphyrins showing strong excitonic coupling such as LH
systems have hardly been described so far. Under these circum-
stances, we have recently reported that hexameric and pentameric
macrorings, which exhibited large split Soret bands (∆E ∼ 2000
cm-1), mimicked the structure of B850 in LH2 by assembling
gable-porphyrinatoZn(II) complexes with imidazolyl groups in a
supramolecular fashion based on the complementary coordination
of imidazolyl to Zn(II).5,6
dinated porphyrinatoZn(II) sites in the cyclic trimer afford scaffolds
that can accommodate a functional molecule corresponding to the
reaction center. Furthermore, this molecule is composed of three
porphyrin units connected at an angle of 120° by m-phenylene
moieties. Therefore, its cyclic trimer, (1Zn)3, should be strain-free,
whereas dimer or tetramer formation will be improbable because
of high angle strain energies, and expected as a specific product
on supramolecular cyclization.
Free-base trisporphyrin 1Fb was synthesized in 20% yield by
condensation of 5-(1-methylimidazol-2-yl)-10,15-bis(3-allyloxypro-
pyl)-20-(3-formylphenyl)porphyrin 3Fb with meso-methoxycarbon-
ylethyldipyrromethane 4. Zn(II) metal ion was introduced into 1Fb
by treatment with a methanol solution of Zn(OAc)2 in CHCl3. GPC
analysis of the as-prepared (before reorganized) mixture showed a
sharp peak accompanied by a broad shoulder at a shorter retention
time (Figure S1, Supporting Information). To turn the mixture into
a single product, it was diluted to 0.02 mM in CHCl3/MeOH )
9/1 (v/v) and allowed to stand at 27 °C for 1 day in the dark. The
equilibrium of the complementary coordination of imidazolyl to
Zn(II) is expected to drive linear oligomers and any larger ring
compounds to form the more stable cyclic trimers.6 After this
reorganization, the broad shoulder disappeared completely, leaving
the sharp peak as the sole product in the GPC chromatogram. This
cyclic trimer was stable for at least several days7 even under
extremely dilute conditions (∼10-7 M) in nonpolar solvents such
as CHCl3 (including 0.5% EtOH as a stabilizer) and toluene. To
determine the structure of the complex by mass spectrometry, the
coordination pairs were covalently linked by a multisite ring-closing
metathesis reaction.8 After covalent linking, the mass spectrum
showed only a single peak corresponding to the 1Zn trimer,
C-(1Zn)3 ( m/z found 5816.4, calcd for [M + H]+ 5816.5) (Figure
S2, Supporting Information). The covalently linked trimer C-(1Zn)3
maintains its cyclic structure even in highly coordinating solvents
such as pyridine. This feature enables us to evaluate the photo-
physical properties of the complex in polar media, facilitating the
electron-transfer reaction.
Scheme 1. Formation of Self-assembled Cyclic Trisporphyrin
Trimers, (N-(1Zn)3)a
The proton NMR spectra of the noncovalently linked cyclic
trimer, N-(1Zn)3, were complex. However, use of HH-COSY,
TOCSY, and HMQC measurements allowed the assignment of all
the signals as a 2:3 mixture of two topological isomers, D3h-
symmetric and asymmetric. The key signals were protons inside
and outside the ring. In the reference dimer, 5Zn (whose structure
was illustrated in Supporting Information Scheme 1), the corre-
sponding Im-4 protons were observed around 2.1 ppm, with an
upper field shift of 5.7 ppm from 7.8 ppm in the free base 5Fb. In
the case of N-(1Zn)3, the outside Im-4 protons appeared at 2.26
ppm in the D3h-symmetric isomer and at 2.30, 2.23, and 2.20 ppm
in the asymmetric isomer. The inside Im-4 protons appeared at 1.72
ppm in the D3h-symmetric isomer and 1.89, 1.86, and 1.69 ppm in
the asymmetric. The inside protons are discriminated from the
a Two topological isomers, one D3h-symmetric (left bottom) and one
asymmetric (right bottom), exist.
We have now applied this supramolecular methodology to
trisporphyrin 1 and have constructed a novel light-harvesting cyclic
trimer of 1Zn. Only the terminal two porphyrins in 1Zn are used
in the ring-forming complementary coordination. Three noncoor-
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10.1021/ja048118t CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
The fluorescence quantum yields (Φf) of Znporphyrin mono-
mer,11 dimer (5Zn)2, and nonameric ring N-(1Zn)3 in toluene were
3.4, 4.9, and 5.1%, respectively. It is noteworthy that the enhanced
fluorescence of the dimer (5Zn)2 compared to the monomer was
not quenched at all and was maintained even for N-(1Zn)3 by the
accumulation of nine chromophores in close vicinity. These facts
suggest that macroring formation is an excellent way to harvest
light. Extensive investigations of other photophysical properties are
now underway.
In conclusion, we have successfully synthesized a new model
of light-harvesting complexes by specific trimerization of the
trisporphyrin, 1Zn, in a supramolecular fashion. A tetrapodal ligand
was incorporated into the cavity of the macroring with excellent
cooperativity. The residual arm of 2 in the composite 2/(1Zn)3 can
be further modified with energy and electron acceptors. This simple
method, based on complementary and cooperative coordination,
seems to be an efficient way to construct LH1 and its composites
with reaction centers.
Figure 1. UV-vis titration of N-(1Zn)3 (4 × 10-8 M) in toluene with
tetrapodal ligand 2. Inset: Plot of ∆A at 443 nm as a function of ratio
[2]/[N-(1Zn)3].
outside protons by further shifts of 0.4-0.5 ppm caused by
anisotropic effects from other porphyrin members. Similar behavior
was observed for Im-5 and N-Me protons (see Supporting Informa-
tion, Figures S3 and S4). The clear differentiation of these protons
into a single species or three different species assigns the ring
structures as D3h-symmetric and asymmetric topological isomers,
respectively.
Acknowledgment. This work was supported by CREST of the
Japan Science and Technology Corporation and a Grant-in-Aid for
Scientific Research (No. 15205020) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
Supporting Information Available: Synthetic details, GPC, MS,
and NMR spectra, and UV-vis titration in CHCl3 and Job plots of
N-(1Zn)3 with 2 (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
The incorporation of a guest-molecule into the inner sphere of
N-(1Zn)3 was examined by UV-vis titration. For this purpose,
the tetrapodal guest 2 was synthesized from tetra(4-iodophenyl)-
methane9 and 4-ethynylpyridine by Sonogashira coupling.10 When
a N-(1Zn)3 solution (4 × 10-8 M) in toluene was titrated by guest
2, the Soret band at 443 nm and the Q-band at 564 nm were shifted
to longer wavelengths. Figure 1 shows the spectral change during
the titration and the plot of ∆A at 443 nm as a function of the ratio
[2]/[N-(1Zn)3]. A Job plot (Supporting Information, Figure S6)
and the clear bending behavior observed at [2]/[N-(1Zn)3] ) 1
(Figure 1 inset) indicate the formation of a strong 1:1 complex.
Heterogeneity of the two topological isomers was not observed in
the titration, and both isomers similarly accommodated guest 2.
The association constants obtained by curve fitting analysis were
8 × 108 M-1 in toluene and 1.2 × 107 M-1 in CHCl3 (including
0.5% EtOH as a stabilizer). Such large association constants for
the composite reflect the cooperative nature of the three coordination
sites. Furthermore, these high-stability constants are of great advan-
tage in such a supramolecular architecture in homogeneous solution
because simple mixing is sufficient to generate the desired product.
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