Table 1 Electronic absorption titration data for 1a, 1b and 6 with DABCO
in CDCl3–DMSO (20/1, v/v) at 293 K
Compound
lfreea/nm
lcomplexa/nm Dlb/nm Kc/M–1 (s.d., %)
1a
1b
6
425 (12)
428 (12)
429 (10)
424 (8)
424 (8)
431 (9)
21
24
2
1.48 3 105 (11)
3.59 3 103 (7)
1.02 3 103 (5)
a Absorption maximum in the Soret region. lfree; in the absence of DABCO,
lcomplex; upon complexation with DABCO. Half band width in parenthesis.
b lcomplex 2 lfree c Determined by computer-assisted least-squares analysis
.
of the absorbance changes.
afford an identical narrow Soret absorption (lmax = 424 nm;
half band width, 8 nm), although monomer 6 exhibited a red
shift upon complexation, induced by the coordination of
DABCO’s nitrogen to the central zinc of the porphyrin
moiety.10 This suggests that the blue shifts observed in 1a and
1b are due to the exitonic interaction between the porphyrin
chromophores tightly fixed in a cofacial manner. It is empha-
sized that the binding constant K for 1a was 41 times larger than
that for 1b, indicating that the introduction of methyl groups at
the 2 and 2A positions in the diarylurea skeleton of 1a effectively
forces the porphyrin moieties to adopt a cofacial orientation.
In summary, we have demonstrated here a facile method for
construction of cofacial porphyrin oligomers by linking por-
phyrin units by a diarylurea linkage as well as the introduction
of appropriate intramolecular steric interactions. As can be seen,
alternative copolymerization of 5 with the corresponding
porphyrin diisocyanate should afford a linear cofacial porphyrin
array. This research is on going.
Fig. 2 Electronic absorption spectra of 1a, 1b, 2 and 6 in the Soret region in
CHCl3–DMSO (20/1, v/v) at 293 K.
Firstly, electronic absorption spectra gave us some informa-
tion about the orientation between the chromophores in the zinc
porphyrin oligomers 1 and 2. The absorption spectra in the Soret
region for 1, 2 and 6 are shown in Fig. 2. The dimer 1b, which
possesses no substituents in the diarylurea skeleton except for
the porphyrins, exhibited a similar absorption spectrum to 6,
whereas the absorption maximum of 1a exhibited a slight blue
shift of 4 nm compared to that of 6, indicating an exitonic
coupling between the transitions in the two porphyrin moieties
adopting the cofacial arrangement.6,7 The trimer 2 also
exhibited the similar blue shift (lmax = 426 nm), indicating a
well-defined cofacial array of three porphyrin units, although
the half band width is a little bit larger than that of 1a (half band
widths; 12 and 14 nm for 1a and 2, respectively).
The cofacial orientation between the porphyrin units in 1a
was also confirmed by formation of a complex with 1,4-diazabi-
cyclo[2.2.2]octane (DABCO). The porphyrin face-to-face dis-
tance in 1a estimated by a molecular modeling study was
7.0 Å,8 suitable distance for binding of DABCO through two
Zn–N coordination interactions. Addition of an equimolar
amount of 1a to a solution of DABCO in CDCl3–DMSO-d6
(20+1, v/v, 0.34 mM) induced a significantly large upfield shift
of the –CH2CH2– signal of DABCO from 2.80 to 24.75 ppm,
which apparently originated from ring current anisotropy of the
two porphyrin rings.9 In Fig. 3 are shown absorption spectra of
1a and 1b in CHCl3–DMSO (20+1, v/v) upon addition of
varying concentrations of DABCO, and the titration data are
summarized in Table 1. An isosbestic point observed in each
spectral change (426 and 427 nm for 1a and 1b, respectively)
indicates 1+1 complex formation in the present condition,
which was supported by the Job plot. The spectra of 1a and 1b
both exhibited blue shifts upon complexation with DABCO to
Notes and references
1 R. W. Wagner and J. S. Lindsey, J. Am. Chem. Soc., 1994, 116, 9759;
J. Seth, V. Palaniappan, T. E. Johnson, S. Prathapan, J. S. Lindsey and
D. F. Bocian, J. Am. Chem. Soc., 1994, 116, 10 578 and references
therein; H. L. Anderson, Chem. Commun., 1999, 2323.
2 Covalently linked porphyrin arrays: K. Sugiura, H. Tanaka, T.
Matsumoto, T. Kawai and Y. Sakata, Chem. Lett., 1999, 1193; E. K. L.
Yeow, K. P. Ghiggino, J. N. H. Reek, M. J. Crossley, A. W. Bosman,
A. P. H. J. Schenning and E. W. Meijer, J. Phys. Chem. B, 2000, 104,
2596; F. Takei, K. Onitsuka, N. Kobayashi and S. Takahashi, Chem.
Lett., 2000, 914; A. Nakano, T. Yamazaki, Y. Nishimura, I. Yamazaki
and A. Osuka, Chem. Eur. J., 2000, 6, 3254.
3 Supramolecular systems of porphyrin assemblies: C. A. Hunter and
R. K. Hyde, Angew. Chem., Int. Ed. Engl., 1996, 35, 1936; A. P. H. J.
Schenning, F. B. G. Benneker, H. P. M. Geurts, X. Y. Liu and R. J. M.
Nolte, J. Am. Chem. Soc., 1996, 118, 8549; S. Knapp, B. Huang, T. J.
Emge, S. Sheng, K. Krogh-Jespersen, J. A. Potenza and H. J. Schugar,
J. Am. Chem. Soc., 1999, 121, 7977; N. Nagata, S. Kugimiya and Y.
Kobuke, Chem. Commun., 2000, 1389; Y. Kuroda, K. Sugou and K.
Sasaki, J. Am. Chem. Soc., 2000, 122, 7833.
4 M. C. Etter, Z. Urbanczyk-Lipkowska, M. Zia-Ebrahimi and T. W.
Panunto, J. Am. Chem. Soc., 1990, 112, 8415; A. Tanatani, I. Azumaya
and H. Kagechika, J. Syn. Org. Chem., Jpn., 2000, 58, 556.
5 J. P. Collman, Z. Wang and A. Straumanis, J. Org. Chem., 1998, 63,
2424.
6 M. Kasha, H. R. Rawls and M. A. El-Bayoumi, in Molecular
Spectroscopy VIII, Butterworths, London, 1965, p. 371.
7 The electronic absorption spectrum of 1a did not exhibit any significant
changes between temperatures 283–323 K. This suggests that 1a
predominantly adopts a cofacial conformation, and does not exist in
equilibrium with other conformers.
8 The molecular modeling was performed at MOPAC PM3 level by using
the PC Spartan Pro program package (Wavefunction Inc., Irvine,
California, 1999).
9 In the presence of an excess of DABCO, no porphyrin ligands other than
1
1a and 1a·DABCO complex were observed in the H NMR spectrum,
indicating following complexation of DABCO to 1a·DABCO scarcely
occurs.
10 Although the possibility exists of the monomer 6 forming a 1+2 complex
with two equivalents of DABCO, 1+1 complex formation is pre-
dominant under dilute conditions: P. N. Taylor and H. L. Anderson,
J. Am. Chem. Soc., 1999, 121, 11 538.
Fig. 3 Electronic absorption spectra of 1a and 1b in the presence of varying
concentrations of DABCO in CHCl3–DMSO (20/1, v/v) at 293 K. (a) [1a],
1.50 mM; [DABCO], 0, 1.22, 2.44, 4.83, 7.75, 11.7, 23.3, 34.8 mM. (b) [1b],
1.64 mM; [DABCO], 0, 0.0260, 0.0515, 0.0827, 0.125, 0.249, 0.372, 0.614,
2.50, 3.67 mM.
558
Chem. Commun., 2001, 557–558