A porphyrin square: Synthesis of a square-shaped π-conjugated porphyrin tetramer connected by diacetylene linkages

Article abstract of DOI:10.1039/b002699i

A square-shaped π-conjugated porphyrin tetramer has prepared: the excitation energies of both the Soret and Q bands, 19 880 (503) and 15 180 cm^-1 (659 nm), respectively, are quite low compared with those for the monomer and reported cyclic porphyrin oligomers.

Full text of DOI:10.1039/b002699i

A porphyrin square: synthesis of a square-shaped p-conjugated porphyrin
tetramer connected by diacetylene linkages
Ken-ichi Sugiura, Yoshinobu Fujimoto and Yoshiteru Sakata*
The Institute of Scientific and Industrial Research (ISIR), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka
567-0047, Japan. E-mail: sakata@sanken.osaka-u.ac.jp
Received (in Cambridge, UK) 5th April 2000, Accepted 9th May 2000
Published on the Web 8th June 2000
A square-shaped p-conjugated porphyrin tetramer has
prepared: the excitation energies of both the Soret and Q
bands, 19 880 (503) and 15 180 cm21 (659 nm), respectively,
are quite low compared with those for the monomer and
reported cyclic porphyrin oligomers.
properties of a new cyclic p-conjugated porphyrin tetramer 1
(Fig. 1).
The synthetic route to 1 is shown in Scheme 1. An important
key intermediate, 5,10-bis(3,5-di-tert-butylphenyl)porphyrin 3,
which acts as the corners of the square 1 was prepared in 3%
yield by the coupling reaction of 3,5-di-tert-butylphenylbenzal-
dehyde and pyrrole with tripyrrane 2, as reported by Taniguchi
and coworkers,16 in the presence of powdered NaCl.17 After
metallation of 3, functionalization reactions were performed at
the meso-positions, i.e. bromination (affording 5), introduction
of acetylene groups (to give 6), and the removal of the TMS
groups (providing 7).† Glaser–Hay coupling4 of 7 at a 2.6 mM
concentration gave moderately soluble, brown coloured crude
products. The resulting reaction mixture was analyzed by
matrix-assisted laser desorption ionization time-of-flight mass
spectrometry to give a main peak for M+ at m/z 3160, along with
a weak one at m/z 2371. This clearly indicates the tetramer was
formed exclusively (m/z calc. for M+ of the cyclic tetramer,
Interdisciplinary research on porphyrin oligomers (POs) has
been attracting much attention over the last two decades.1
Studies are focused on the elucidation of the function of
naturally occurring POs, such as the cyclophane-like dimeric
special pair in the photosynthetic reaction center,1 as well as the
creation of component molecules for novel advanced materi-
als.2–8 Recently, many efforts have been made to prepare cyclic
porphyrin oligomers (CPOs) motivated by the wheel-shaped
oligomeric porphyrinoid architecture found in the light harvest-
ing proteins.9 After the pioneering work of Sanders and
Anderson in 1989,10 several CPOs have been reported, in which
each porphyrin is connected by covalent11–13 or coordination
bonds.14 However, the linkages are only limited to aryl-based
meso-groups in which the porphyrin and linker p-systems are
orthogonal. Therefore, the electronic interaction between the
neighbouring chromophores in the ground state remains small.7
With the aim of creating a highly conjugated CPO, we chose a
diacetylene unit to connect porphyrins directly at the meso-
position.4–8, 15 Here we report the synthesis and some basic
C
C
208H200N16Ni4: 3159; m/z calc. for M+ of the acyclic trimer,
156H152N12Ni3: 2371). Separation of the tetramer was success-
fully achieved using preparative gel permeation chromatog-
raphy. The absence of C·C–H and asymmetric C·C stretching
bands in the IR spectrum of 1, characteristically observed for 7
at 3310s and 2106m cm21, respectively, suggest the tetramer to
be cyclic. The simple pattern of 1H-NMR peaks† also supports
the cyclic structure having D4h symmetry (vide infra). In a
Scheme 1 Reagents and conditions: (i) 2.0 eq. 3,5-di-tert-butylphe-
nylbenzaldehyde, TFA, NaCl, CH2Cl2; (ii) excess Ni(CH3CO2)2·4H2O,
(CH2Cl)2, reflux; (iii) 2.05 eq. NBS, CH2Cl2, 220 °C, (iv) 3.0 eq. TMS–
C·CH, Cu2Cl2, Pd(PPh3)4, NEt2H, (v) nBu4NF, CH2Cl2 (vi) Cu2Cl2,
TMEDA, CH2Cl2. The values in parentheses for 7 are 1H-NMR assign-
ments.
Fig. 1 Compound 1 along with 1H-NMR assignments.
DOI: 10.1039/b002699i
Chem. Commun., 2000, 1105–1106
This journal is © The Royal Society of Chemistry 2000
1105

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This work was supported in part by a Grant-in-Aid for
Scientific Research on Priority Areas (no. 11136222 ‘Metal-
assembled Complexes’ to K-i. S., no. 11166238 ‘Molecular
Physical Chemistry’ to K-i. S., and no. 10146103 ‘Creation of
Characteristic Delocalized Electronic Systems’ to Y. S.) from
the Ministry of Education, Science, Sports and Culture, Japan.
We appreciate the technical assistance provided by the
Materials Analysis Centre of ISIR, Osaka University as well as
helpful discussions with Professor S. Taniguchi, Ibaraki
National College of Technology, Japan.
Notes and references
† Data: 3: 1H-NMR (CDCl3) d 10.21 (s, 2H), 9.44 (s, 2H), 9.33 (d, J = 5,
2H), 9.06 (d, J = 5, 2H), 8.97 (s, 2H), 8.09 (d, J = 2, 4H), 7.81 (t, J = 2
Hz, 2H), 1.54 (s, 36H), 23.27 (bs, 2H). 4: 1H-NMR (CDCl3) d 9.88 (s, 2H),
9.23 (s, 2H), 9.17 (d, J = 5, 2H), 8.98 (d, J = 5, 2H), 8.90 (s, 2H), 7.92 (d,
J = 2, 4H), 7.76 (t, J = 2 Hz, 2H), 1.50 (s, 36H). 5: 1H-NMR (CDCl3) d
9.39 (s, 2H), 9.34 (d, J = 5, 2H), 8.69 (d, J = 5, 2H), 8.62 (s, 2H), 7.72 (d,
J = 2, 4H), 7.64 (t, J = 2 Hz, 2H), 1.39 (s, 36H). 6: 1H-NMR (CDCl3) d
9.50 (s, 2H), 9.41 (d, J = 5, 2H), 8.75 (d, J = 5, 2H), 8.64 (s, 2H), 7.79 (d,
J = 2, 4H), 7.70 (t, J = 2, 2H), 1.45 (s, 36H), 0.54 (s, 18H). UV–vis lmax
(CHCl3): 310 (log e = 3.77), 438 (4.89), 554 (3.76) nm. 7: 1H-NMR
(CDCl3) d 9.55 (s, 2H), 9.46 (d, J = 5, 2H), 8.79 (d, J = 5, 2H), 8.68 (s,
2H), 7.81 (d, J = 2, 4H), 7.72 (t, J = 2, 2H), 4.05 (s, 2H), 1.46 (s, 36H) ppm.
1: 1H-NMR (CDCl3) d 10.09 (s, 8H), 9.66 (d, J = 5, 8H), 8.88 (d, J = 5,
8H), 8.68 (s, 8H), 7.87 (d, J = 2, 16H), 7.75 (t, J = 2, 8H), 1.55 (s, 144H).
UV–vis lmax (CHCl3): 324 (log e = 3.94), 423 (sh, 4.37), 503 (4.85), 562
(4.12), 659 (4.44) nm.
Fig. 2 Absorption spectra of 1 (solid line) and 6 (dotted line) in CHCl3.
typical reaction, 41.3 mg of 7, 400.0 mg of Cu2Cl2, 0.55 mL of
TMEDA, and 50 mL of dry CH2Cl2 gave 8.9 mg of the tetramer
1 as a lustrous dark-green solid (22% yield). It should be
emphasised that this relatively high yielding formation of cyclic
1 occurred in the absence of template;12 the isolated yield of the
structurally similar square was 7% at most.11
1
The H-NMR chemical shift of the internal protons (Ha:
10.09 ppm) is informative about the structure of 1 from the
following two view points. First, compared to the corresponding
protons of 7 (HaA: 9.55 ppm), Ha is shifted to low field by +0.54
ppm. Although a similar downfield shift was observed for the
linear porphyrin oligomers connected by the diacetylene
linkages, e.g. +0.22 ppm for the dimer,8 the shift for 1 is more
than double this value. Second, the difference in the chemical
shifts of the internal (Ha) and the external protons (Hb: 9.66
ppm) adjacent to the diacetylene units of 1 is +0.43 ppm. This
value is also larger than that observed for 7 (+0.09 ppm: 9.55
ppm for HaA and 9.46 ppm for HbA). These results suggest that the
Ha-protons of 1 are affected by the ring current, not only of the
two adjacent porphyrin rings, but also of the diagonal porphyrin
in the square and also clearly suggests 1 to be cyclic.
1 Reviews: M. R. Wasielewski, Chem. Rev., 1992, 92, 435; H. Kurreck
and M. Huber, Angew. Chem., Int. Ed. Engl., 1995, 34, 849.
2 J. D. Spence and T. D. Lash, J. Org. Chem., 2000, 65, 1530.
3 K.-i. Sugiura, H. Tanaka, T. Matsumoto, T. Kawai and Y. Sakata, Chem.
Lett., 1999, 1193.
4 H. L. Anderson, S. J. Martin and D. D. C. Bradley, Angew. Chem., Int.
Ed. Engl., 1994, 33, 655.
5 H. L. Anderson, Inorg. Chem., 1994, 33, 972.
6 P. N. Taylor, J. Huuskonen, G. Rumbles, R. T. Aplin, E. Williams and
H. L. Anderson, Chem. Commun., 1998, 909.
7 H. L. Anderson, Chem. Commun., 1999, 2323.
8 V. S.-Y. Lin, S. G. DiMagno and M. J. Therien, Science, 1994, 264,
1105; V. S.-Y. Lin and M. J. Therien, Chem. Eur. J., 1995, 1, 645.
9 T. Pullerits and V. Sundström, Acc. Chem. Res., 1996, 29, 381.
10 H. L. Anderson and J. K. M. Sanders, J. Chem. Soc., Chem. Commun.,
1989, 1714.
11 R. W. Wagner, J. Seth, S. I. Yang, D. Kim, D. F. Bocian, D. Holten and
J. S. Lindsey, J. Org. Chem., 1998, 63, 5042.
12 S. Anderson, H. L. Anderson and J. K. M. Sanders, Acc. Chem. Res.,
1993, 26, 469.
13 J. Li, A. Ambroise, S. I. Yang, J. R. Diers, J. Seth, C. R. Wack, D. F.
Bocian, D. Holten and J. S. Lindsey, J. Am. Chem. Soc., 1999, 121,
8927; O. Mongin, A. Schuwey, M.-A. Vallot and A. Gossauer,
Tetrahedron Lett., 1999, 40, 8347.
14 C. M. Drain and J.-M. Lehn, J. Chem. Soc., Chem. Commun., 1994,
2313 (corrigendum: J. Chem. Soc., Chem. Commun., 1995, 503); K.
Funatsu, A. Kimura, T. Imamura and Y. Sasaki, Chem. Lett., 1995,
765.
15 To the best of our knowledge, the introduction of the acetylene group on
the meso-position was first proposed by Arnold and coworkers in 1978.
D. P. Arnold, A. W. Johnson and M. Mahendran, J. Chem. Soc., Perkin
Trans. 1, 1978, 366; D. P. Arnold and L. J. Nitschinsk, Tetrahedron,
1992, 48, 8781; D. P. Arnold and D. A. James, J. Org. Chem., 1997, 62,
3460; D. P. Arnold, G. A. Heath and D. A. James, J. Porphyrins
Phthalocyanines, 1999, 3, 5.
The electronic spectra of 1 and 6 are shown in Fig. 2. The
spectrum of 1 is composed of two main bands: Q-band-like
band I and Soret-band-like band II.† Reflecting the highly
conjugated structure, band I and band II appear at 15 180 (659)
and 19 880 cm21 (503 nm), respectively. Both band I and band
II are shifted to lower energy compared with those of the
monomer 6 by about 2900 cm21. Although these bands appear
at much longer wavelengths than those of the porphyrin square
connected by meso-aryl-based linkages reported by Lindsey and
coworkers (lmax = 23 260 cm21; 430 nm),11 these wavelengths
are similar to the corresponding linear tetramer.6 The intensity
of band I normalized per chromophore was increased compared
with that of 6. Conversely, the intensity of band II was
decreased. This phenomenon suggests that the degeneracy of
the a1u and a2u orbitals of the porphyrins is lifted7 and that a new
p-electronic system in the square is formed.
In conclusion, a novel and highly conjugated porphyrin
square was prepared. In addition to the expanded p-system of
the molecule, our square has a central cavity corresponding to
the size of molecules such as hexamethylbenzene.18 These
characteristics will allow us to use the square as an advanced
host molecule. We are currently investigating the photo-
chemistry of the corresponding free base and zinc complex, as
well as contraction and expansion of the square by replacing the
diacetylene units with –C·C– and –(C·C–)n (n > 3) linkers,
respectively, keeping the p-conjugation between the four
porphyrins.
16 S. Taniguchi, H. Hasegawa, M. Nishimura and M. Takahashi, Synlett,
1999, 73.
17 F. Li, K. Yang, J. S. Tyhonas, K. A. MacCrum and J. S. Lindsey,
Tetrahedron, 1997, 53, 12 339.
18 J. D. Ferrara, C. Tessier-Youngs and W. J. Youngs, J. Am. Chem. Soc.,
1985, 107, 6719.
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Chem. Commun., 2000, 1105–1106