Synthesis and characterization of facially encumbered and soluble porphyrin tapes

Article abstract of DOI:10.1246/cl.2006.946

We report the synthesis and characterization of facially encumbered meso-meso-linked and triply fused porphyrin arrays. Copyright

Full text of DOI:10.1246/cl.2006.946

946
Chemistry Letters Vol.35, No.8 (2006)
Synthesis and Characterization of Facially Encumbered and Soluble Porphyrin Tapes
Toshiaki Ikeda, Akihiko Tsuda, Naoki Aratani, and Atsuhiro Osukaꢀ
Department of Chemistry, Graduate School of Science, Kyoto University,
and Core Research for Evolutional Science and Technology,
Japan Science and Technology Agency, Sakyo-ku, Kyoto 606-8502
(Received April 21, 2006; CL-060478; E-mail: osuka@kuchem.kyoto-u.ac.jp)
We report the synthesis and characterization of facially en-
cumbered meso–meso-linked and triply fused porphyrin arrays.
Cu(I)-catalyzed coupling reaction of 2,4,6-tribromobenzalde-
hyde (1) and 3,5-di-tert-butylphenol (2) afforded 2,4,6-tris(3,5-
di-tert-butylphenoxy)benzaldehyde (3) in 79% yield.6 Acid-
catalyzed condensation of 3 and dipyrrylmethane (4) followed
by oxidation with DDQ provided 5,15-bis[2,4,6-tris(3,5-di-
tert-butylphenoxy)phenyl]porphyrin in 43% yield, which was
converted to Zn(II)–porphyrin B1 quantitatively by insertion
of Zn(II) ion.
Single crystals of B1 suitable for X-ray analysis were grown
by slow diffusion of acetonitrile into a toluene solution
(Figure 1).7 In the molecular structure of B1, the ortho-substitut-
ed 3,5-di-tert-butylphenoxy groups take almost perpendicular
conformations with regard to the porphyrin plane, hence provid-
ing steric hindrance toward ꢁ–ꢁ stacking. Actually, there is no
sign of ꢁ–ꢁ stacking in the solid state. In addition, the Zn(II) ion
in B1 is not coordinated with any solvent molecules despite the
tendency of Zn(II) porphyrin to bind an axial ligand.
Facially encumbered meso–meso-linked porphyrins Bn
were synthesized by Ag(I)-promoted oxidative coupling.1
Monomer B1 was stirred in the presence of 3 equiv. of AgPF6
at 40 ꢁC for 72 h to afford B2 (21%), B3 (8%), and B4 (2%)
along with the recovery of B1 (48%). In contrast to the previous-
ly reported cases, the coupling reaction of B2 was found to be
very sluggish even under rather forcing conditions, probably
because of the serious buttressing steric hindrance arising from
the bulky meso-aryl substituents.
In general, the oxidation of meso-free meso–meso-linked
porphyrins with DDQ-Sc(OTf)3 tends to give polymeric prod-
ucts as a result of intermolecular coupling. Therefore, the syn-
thesis of the porphyrin tapes required the protection of the free
meso-positions before the double ring closure reaction. In con-
Recently, our group has reported singly meso–meso-linked
porphyrin arrays1 and meso–meso, ꢀ–ꢀ, ꢀ–ꢀ, triply linked por-
phyrin arrays (porphyrin tapes),2 both of which are unprecedent-
ed in respects of the molecular length and the extent of ꢁ-conju-
gation. The former arrays have been synthesized up to a discrete
1024-mer, in which the electronic interactions between porphy-
rin units are small owing to the orthogonal conformations. In
contrast, the fully conjugated features of the latter arrays are
apparent from the progressive red-shifts of the absorption
bands, reaching an exceptionally red-shifted absorption band
at 2800 nm for the dodecameric porphyrin tape.2a This feature
is intriguing, since they do not exhibit effective conjugation
length (ECL) behavior3 up to the dodecamer, hence suggesting
ECL of these arrays larger than 12. Despite this promise, the por-
phyrin tapes have serious problems such as poor solubility and
chemical instability, which become more evident with increas-
ing number of porphyrin subunits. Especially, the poor solubility
of porphyrin tapes makes it difficult to characterize them by
spectroscopic methods. These problems are common for exten-
sively ꢁ-conjugated molecules. One of the useful strategies to
solve them is to encapsulate the ꢁ-conjugated system.4,5 In this
paper, synthesis and properties of a new series of porphyrin
arrays that bear bulky aryl groups at 5- and 15-positions are
reported. 2,4,6-Tris(3,5-di-tert-butylphenoxy)phenyl group was
employed for facial encumbrance of porphyrin tapes, which
was hoped to suppress aggregation and to improve the chemical
stability.
5,15-Bis[2,4,6-tris(3,5-di-tert-butylphenoxy)phenyl]porphy-
rinzinc(II) complex B1 was synthesized as shown in Scheme 1.
tBu
tBu
tBu
tBu
Br
OHC
Br
tBu
tBu
O
a)
Br
+
OHC
O
3
OH
2
O
1
tBu
tBu
3
tBu
tBu
O
O
tBu
N
N
N
b)
OHC
tBu
O
+
Ar
Zn
Ar
N
H
N
H
tBu
O
N
O
tBu
tBu
tBu
tBu
4
Ar =
B1
tBu
O
3
tBu
tBu
Scheme 1. Synthetic scheme of B1. a) Cs2CO3, CuCl, AcOEt,
.
toluene, 79%. b) 1) TFA, CH2Cl2, 2) DDQ, 3) Zn(OAc)2 2H2O,
Figure 1. X-ray crystal structure of B1. (a) Top view. (b) Side
view.
MeOH, CHCl3, 43%.
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.8 (2006)
947
Ar
Ar
Ar
tBu
O
N
N
Zn
N
N
N
Zn
N
tBu
O
N
N
N
Ar
N
N
tBu
tBu
Ar
Ar
Ar
N
Zn
N
Zn
N
N
Ar =
Ar
N
n-2
Bn
n-2
N
Zn
N
O
Ar
N
N
Zn
N
N
N
N
tBu
Ar
tBu
Ar
TBn
Ar
Chart 1.
H2 H3
H1
Figure 3. UV–vis absorption spectra of TBn and Tn in CHCl3.
Structures of Tn are shown in Supporting Information.
structures of Q-bands are well preserved up to TB4 compared
to sterically nonencumbered porphyrin tape Tn,2c probably re-
flecting facial encumbrance effects of the bulky aryl groups that
suppress the aggregation.
H2 H3H4
H1
In summary, the synthesis and characterization of the facial-
ly encumbered porphyrin arrays were reported. The bulky
groups are suitable for suppression of aggregation. In addition,
such bulky groups allowed for the synthesis of meso-free por-
phyrin tapes, which may be fabricated towards further elaborat-
H2 H3H4 H5
H1
1
ed molecules. The H NMR spectra of the meso-free porphyrin
tapes revealed the attenuated ring current of porphyrins. Detailed
examinations of these soluble and chemically stable porphyrin
tapes TBn with respect to their promising attributes8 will be wor-
thy of further investigation.
Figurꢀe 2. 1H NMR spectra in CDCl3. a) TB2, b) TB3, and c)
TB4. shows solvents and impurities.
trast, the oxidation of Bn with DDQ-Sc(OTf)3 in toluene at
80 ꢁC for 2 h provided meso-free porphyrin tapes TBn bearing
free meso-postions in moderate yield (23–56%) without any
noticeable amount of oligomeic porphyrin products (Chart 1).
Porphyrin tapes TB2, TB3, and TB4 thus synthesized exhibited
parent molecular ion peaks at m/z 3492.51 (calcd for 3497.49,
This work was partly supported by a Grant-in-Aid for Scien-
tific Research and the 21st Century COE Program.
References and Notes
1
a) A. Osuka, H. Shimidzu, Angew. Chem., Int. Ed. Engl.
1997, 36, 135. b) N. Aratani, A. Osuka, Y. H. Kim, D. H.
Jeong, D. Kim, Angew. Chem., Int. Ed. 2000, 39, 1458.
c) N. Aratani, A. Takagi, Y. Yanagawa, T. Matsumoto, T.
Kawai, Z. S. Yoon, D. Kim, A. Osuka, Chem. Eur. J.
2005, 11, 3389.
C
232H274N8O12Zn2), m/z 5241.40 (calcd for 5243.22,
C348H416N12O18Zn3), and m/z 6986.33 (calcd for 6988.93,
464H542N16O24Zn4), respectively, as revealed by MALDI-
C
TOF mass spectroscopy.
The 1H NMR spectra of TB2, TB3, and TB4 taken in CDCl3
are shown in Figure 2. In contrast to the previously reported por-
phyrin tapes,2b TBn displayed relatively sharp peaks even with-
out axial ligand such as butylamine, suggesting suppressed ag-
gregation. Compared to B1 and B2, meso and ꢀ protons of
TB2, TB3, and TB4 exhibit up-field shifts, which has been inter-
preted in terms of weakened ring current of porphyrin tapes com-
pared with that of porphyrin or meso–meso-linked porphy-
rin.2b,2c The clear spectra of TB3 and TB4 encouraged us to
compare the ring current of inner and outer units of porphyrin
tapes. Signals due to H 1 and H 2 of TB2, TB3, and TB4 appear
2
a) A. Tsuda, A. Osuka, Science 2001, 293, 79. b) A. Tsuda,
H. Furuta, A. Osuka, J. Am. Chem. Soc. 2001, 123, 10304. c)
T. Ikeue, N. Aratani, A. Osuka, Isr. J. Chem. 2005, 45, 293.
a) H. Meier, U. Stalmach, H. Kolshorn, Acta Polym. 1997,
48, 379. b) H. Meier, Angew. Chem., Int. Ed. 2005, 44, 2482.
a) J. J. Michels, M. J. O’Connell, P. N. Taylor, J. S. Wilson,
F. Cacialli, H. L. Anderson, Chem. Eur. J. 2003, 9, 6167. b)
P. H. Kwan, M. J. MacLachlan, T. M. Swager, J. Am. Chem.
Soc. 2004, 126, 8638.
3
4
5
6
7
S. Masuo, H. Yoshikawa, T. Asahi, H. Masuhara, T. Sato,
D.-H. Jiang, T. Aida, J. Phys. Chem. B 2003, 107, 2471.
J.-F. Marcoux, S. Doye, S. L. Buchwald, J. Am. Chem. Soc.
1997, 119, 10539.
ꢀ
ꢀ
3
at nearly the same region, while signals due to H are slightly
ꢀ
shifted to up-field upon elongation of the array. Interestingly
signals due to H and H of TB3 and TB4 are more upfield
ꢀ
4
5
ꢀ
Crystallographic data for B1: C116H140N4O6Zn1, Mr
ꢀ
shifted at ca. 6.8 ppm, suggesting that the ring current of inner
porphyrins is weaker than that of outer porphyrins.
1751.79, triclinic, space group P1 (No. 2), a ¼ 9:692ð3Þ,
˚
b ¼ 12:860ð5Þ, c ¼ 20:454ð6Þ A, ꢂ ¼ 102:36ð1Þ, ꢀ ¼
ꢁ
3
˚
The UV–vis absorption spectra of TBn taken in CHCl3 are
shown in Figure 3. The absorption spectra of TBn exhibit split
Soret bands and significantly red-shifted Q-bands, which are
similar to those of the previously reported porphyrin tapes.2
However, it is worthy to note that the broadening of absorption
bands caused by aggregation was less serious and the vibronic
92:912ð9Þ, ꢃ ¼ 95:12ð1Þ , V ¼ 2473ð1Þ A , Dcalcd ¼ 1:176
g/cm3, Z ¼ 1, R1 ¼ 0:0970 (I > 2ꢄðIÞ), Rw (I > 2ꢄðIÞ) =
0.2500, GOF = 1.036. CCDC 604851.
8
T. K. Ahn, K. S. Kim, D. Y. Kim, S. B. Noh, N. Aratani, C.
Ikeda, A. Osuka, D. Kim, J. Am. Chem. Soc. 2006, 128,
1700.
Published on the web (Advance View) July 22, 2006; doi:10.1246/cl.2006.946