2.35 by gel permeation chromatography (GPC) analysis refer-
enced to a polystyrene standard. MALDI-TOF-MS spectros-
copy of the assemblies gave only the fragment (R)- and (S)-3
(m/z 2050), in which no peaks corresponding to oligonuclear
complexes were detected. Therefore, the chiral-twisted iron(III)
porphyrin dimers formed high-molecular-weight assemblies
through the formation of the l-oxo dimer between two iron
porphyrin moieties.
In conclusion, we have demonstrated the spontaneous for-
mation of optically active porphyrin assemblies through the
formation of l-oxo dimers among chiral-twisted porphyrin
dimers. Predetermined design of the building blocks can lead
to large porphyrin assemblies through self-assembling pro-
cesses. Optically active porphyrin assemblies should be of
great interest in chiral recognition and enantioselective cata-
lytic processes by the use of their chiral grooves.
Fig. 2 (A) Absorption spectral changes of (R)-3 by mixing with
aqueous solutions at various pH: pH \ 7.0, 9.5, 9.8, 10.5, 12.4. (B)
Circular dichroism spectra of (R)-3 and assembled (R)-3 in CH Cl .
Acknowledgements
This research was supported by a Grant-in-Aid for COE
Research ““Advanced Fiber/Textile Science and TechnologyÏÏ
(No. 10CE2003) and ScientiÐc Research (No. 11450366) from
the Ministry of Education, Science, Sports, Culture of Japan.
2
2
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Scheme 2
absorbance of the Soret band increased with increasing pH
and saturated at pH 12.4 (Fig. 2A). The isosbestic points
showed an equilibrium between the two species at di†erent
pH. IR spectroscopy of (R)-3 after treatment with a basic
aqueous solution displayed a new band at 850 cm~1, indicat-
ing the formation of a l-oxo dimer between the two Fe(III)
porphyrin moieties.8 Furthermore, a mononuclear tetrakis(3,4,
5-trimethoxyphenyl)porphyriniron(III) complex has also been
converted to the l-oxo dimer under the same conditions and
was characterized by MALDI-TOF-MS and IR.9 The exami-
nation of CPK molecular models revealed that the intramole-
cular formation of l-oxo dimers in (R)- and (S)-3 is sterically
impossible.10 The absorption spectra above pH 12.4 provided
strong evidence of intermolecular formation of l-oxo dimers
between two iron porphyrin moieties in di†erently twisted
porphyrin dimers. The twisted porphyrin dimer could have
assembled into the porphyrin assembly as shown in Scheme 2.
Fig. 2B shows the CD spectral change of (R)-3 when mixed
with a basic aqueous solution at pH 12.4. Iron complexes (R)-
and (S)-3 exhibited positive and negative CD signs for the
Soret band, respectively. The magnitude of the CD signals for
the Soret band of (R)-3 increased as a result of the formation
of the l-oxo dimer, and the CD spectral change corresponds
to the absorption spectral change. An opposite response of the
CD spectra was observed in the formation of the l-oxo dimer
3
4
5
6
L. Pu, Chem. Rev., 1998, 98, 2405 and refs. therein.
N. Harada and K. Nakanishi, Circular Dichroic Spectroscopy-
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7
8
9
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M. Takeuchi, T. Imada and S. Shinkai, J. Am. Chem. Soc., 1996,
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J. W. Buchler, Porphyrins and Metalloporphyrins, ed. K. M.
Smith, Elsevier, Amsterdam, 1975.
10 FeÈFe distance in 3 was estimated to be ca. 1.1 nm by CPK
models.
of (S)-3. The molecular weight (M ) and polydispersity
w
(M /M ) were roughly estimated to be 6.0 ] 104 g mol~1 and
L etter b000411l
w
n
114
New J. Chem., 2000, 24, 113È114