Meso-β doubly linked Zn(II) porphyrin trimers: Distinct anti-versus-syn effects on their photophysical properties

Article abstract of DOI:10.1021/ol9011585

Meso-β doubly linked Zn(II) porphyrin dimer 1 and anti- and syn-trimers 2 and 3 were synthesized by DDQ-Sc(OTf)₃ oxidation of the corresponding meso-β singly linked porphyrin precursors. The absorption and fluorescence spectra and TPA cross-sec

Full text of DOI:10.1021/ol9011585

ORGANIC
LETTERS
2009
Vol. 11, No. 14
3080-3083
Meso-ꢀ Doubly Linked Zn(II) Porphyrin
Trimers: Distinct anti-versus-syn Effects
on Their Photophysical Properties
Toshiaki Ikeda,† Naoki Aratani,† Shanmugam Easwaramoorthi,‡ Dongho Kim,*,‡
and Atsuhiro Osuka*,†
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku,
Kyoto 606-8502, Japan, and Spectroscopy Laboratory for Functional π-electronic
Systems and Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea
osuka@kuchem.kyoto-u.ac.jp; dongho@yonsei.ac.kr
Received May 25, 2009
ABSTRACT
Meso-ꢀ doubly linked Zn(II) porphyrin dimer 1 and anti- and syn-trimers 2 and 3 were synthesized by DDQ-Sc(OTf)3 oxidation of the corresponding
meso-ꢀ singly linked porphyrin precursors. The absorption and fluorescence spectra and TPA cross-section values of 2 and 3 are distinctly
different, reflecting their molecular shapes.
Extensively π-conjugated porphyrin arrays have attracted
considerable attension in light of their possible applications
to organic conducting materials, nonlinear optical (NLO)
materials, and near-infrared dyes.1-6 One of the promising
candidates are porphyrin tapes that have planar structures
and highly π-conjugated networks as a consequence of two
or three covalent linkages.7-10 Recently, our group has
explored two types of porphyrin tapes: meso-meso, ꢀ-ꢀ,
ꢀ-ꢀ triply linked porphyrin tapes9 and meso-ꢀ, meso-ꢀ
doubly linked porphyrin tapes.10 The former tapes are
exceptional molecules which exhibit enhanced Q-band-like
transitions that are extremely red-shifted deeply into the IR
region and large two-photon absorption (TPA) cross-section
values.9 While the latter arrays are also interesting in view
of the progressively enhanced and red-shifted absorption
bands as well as certainly large TPA cross-section values,10
only scattered studies have been reported so far, due mainly
to synthetic difficulty. Meso-ꢀ doubly linked porphyrin arrays
have been limited to the Ni(II) porphyrin case, since they
were synthesized from oxidative oligomerization of a 5,15-
diaryl Ni(II) porphyrin, in which a1u HOMO of Ni(II) plays
a central role for coupling regiochemistry. This synthetic
strategy allowed the isolation of only anti isomers in low
yields (Scheme 1). Although the corresponding meso-ꢀ
doubly linked Zn(II) porphyrin analogues are interesting
molecules for their ground-state and excited-state electronic
properties, such molecules have not been synthesized. Our
attempts to convert the meso-ꢀ doubly linked Ni(II) por-
† Kyoto University.
‡ Yonsei University.
(1) (a) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38,
1350–1377. (b) Chou, J.-H.; Nalwa, H. S.; Kosal, M. E.; Rakow, N. A.;
Suslick, K. S. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M.,
Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 6, Chapter
41
.
(2) (a) Arnold, D. P.; Johnson, A. W.; Mahendran, M. J. Chem. Soc.,
Perkin Trans. 1 1978, 366–370. (b) Arnold, D. P.; Heath, G. A. J. Am.
Chem. Soc. 1993, 115, 12197–12198
(3) (a) Lin, V. S. Y.; DiMagno, S. G.; Therien, M. J. Science 1994,
264, 1105–1111. (b) LeCours, S. M.; Guan, H.-W.; DiMagno, S. G.; Wang,
.
C. H.; Therien, M. J. J. Am. Chem. Soc. 1996, 118, 1497–1503
.
(4) Wytko, J.; Berl, V.; McLaughlim, M.; Tykwinski, R. R.; Schreiber,
M.; Diederich, F.; Boudon, C.; Gisselbrecht, J. P.; Jones, W. E. HelV. Chim.
Acta 1998, 81, 1964–1977
.
(5) (a) Anderson, H. L. Chem. Commun. 1999, 2323–2330. (b) Taylor,
P. N.; Wylie, A. P.; Huuskonen, J.; Anderson, H. L. Angew. Chem., Int.
Ed. 1998, 37, 986–989. (c) Blake, I. M.; Anderson, H. L.; Beljonne, D.;
Bre´das, J.-L.; Clegg, W. J. Am. Chem. Soc. 1998, 120, 10764–10765
.
10.1021/ol9011585 CCC: $40.75
Published on Web 06/12/2009
 2009 American Chemical Society

lation selectivity is ascribed to severe steric hindrance of the
bulky meso-aryl group.12 However, we also found that
the same borylations proceeded more extensively from the
corresponding free base and Ni(II) porphyrin substrates and
that the reaction in THF gave a tetraborylated product
exclusively under the forcing conditions. These results
indicate that the selective diborylation is arising from the
combined steric effects of the bulky aryl groups of 4 and
1,4-dioxane molecule coordinated to the central Zn.
Scheme 1. Oxidative Oligomerization of Ni(II) Porphyrin
With 5 and 6 in hand, we envisoned synthetic routes to 2
and 3 via Suzuki-Miyaura coupling with brominated por-
phyrin 8 and subsequent oxidative ring closure with
DDQ-Sc(OTf)3. For this purpose, ꢀ-borylated porphyrin 7
was prepared from 5,15-bis(3,5-dioctyloxyphenyl)-10-(3,5-
dimethylphenyl)-substituted zinc(II) porphyrin, and mono-
meso-brominated porphyrin 8 was prepared from 5,15-
bis(3,5-dioctyloxyphenyl)-10-phenyl-substituted porphyrin
(for synthetic details, see the Supporting Information).
Meso-ꢀ singly linked porphyrin dimer 9 and anti- and syn-
trimers 10 and 11 were synthesized by Suzuki-Miyaura
coupling as shown in Scheme 3. The diporphyrin 9 was
phyrin arrays to the corresponding Zn(II) porphyrin arrays
failed due to the stability of the former under acidic
conditions. In this paper, we wish to report the rational
synthesis of meso-ꢀ doubly linked Zn(II) porphyrin dimer
and syn- and anti-trimers.
We prepared 5,15-bis(2,4,6-tris(3,5-di-tert-butylphenox-
y)phenyl)-substituted Zn(II) porphyrin 4 as a building block
for the synthesis of extremely long porphyrin tapes that can
escape from serious π-π stacking.11 In the course of this
study, we examined its borylation under Ir-catalyzed condi-
tions,12 since this reaction has been shown to be quite
sensitive to steric effect. In contrast to the effective tetra-
borylation of similar substrates with much less steric
hindrance, the borylation of 4 under our standard conditions
in 1,4-dioxane gave only diborylated products 5 and 6 in a
1:1 ratio (Scheme 2). Curiously, only the diborylation
Scheme 3.
Synthesis of Meso-ꢀ-Linked Porphyrin Arraysa
Scheme 2.
Ir(I)-Catalyzed ꢀ-Borylation of 4a
a Ar1 ) 2,4,6-tris(3,5-di-tert-butylphenoxy)phenyl. B2Pin2 ) bis(pina-
colato)diboron. dtbpy ) 4,4′-di-tert-butyl-2,2′-bipyridyl.
products were obtained under more forcing borylation
conditions such as longer reaction time and/or more catalyst
amount. These two products were separated by repeated
recrystallizations from chloroform/acetonitrile. This dibory-
a Ar2 ) 3,5-dioctyloxyphenyl. Ar3 ) 3,5-dimethylphenyl.
oxidized with 4 equiv of DDQ and Sc(OTf)3 in toluene at
80 °C for 2 h to afford meso-ꢀ doubly linked diporphyrin 1
as a reddish-purple solid (Scheme 4). The 1H NMR spectrum
of 1 shows two singlets for the ꢀ-protons at 9.26 and 9.58
ppm, 12 doublets for the ꢀ-protons in a range of 8.5-10.0
ppm, and two sets of signals for Ar1 and Ar2 protons,
reflecting its low symmetric structure differing in the edge
meso-aryl substituents, phenyl, and Ar3 (Figure 1a). Oxida-
(6) (a) Aratani, N.; Osuka, A. Bull. Chem. Soc. Jpn. 2001, 74, 1361–
1379. (b) Osuka, A.; Shimidzu, H. Angew. Chem., Int. Ed. Engl. 1997, 36,
135–137. (c) Aratani, N.; Osuka, A.; Kim, Y. H.; Jeong, D. H.; Kim, D.
Angew. Chem., Int. Ed. 2000, 39, 1458–1462. (d) Aratani, N.; Takagi, A.;
Yanagawa, Y.; Matsumoto, T.; Kawai, T.; Yoon, Z. S.; Kim, D.; Osuka,
A. Chem.sEur. J. 2005, 11, 3389–3404.
(7) (a) Crossley, M. J.; Burn, P. L. J. Chem. Soc., Chem. Commun. 1991,
1569–1571. (b) Sendt, K.; Johnston, L. A.; Hough, W. A.; Crossley, M. J.;
Hush, N. S.; Reimers, J. R. J. Am. Chem. Soc. 2002, 124, 9299–9309.
(8) (a) Vicente, M. G. H.; Jaquinod, L.; Smith, K. M. Chem. Commun.
1999, 1771–1782. (b) Jaquinod, L.; Siri, O.; Khoury, R. G.; Smith, K. M.
Chem. Commun. 1998, 1261–1262. (c) Vicente, M. G. H.; Cancilla, M. T.;
Lebrilla, C. B.; Smith, K. M. Chem. Commun. 1998, 2355–2356. (d)
Paolesse, R.; Jaquinod, L.; Sala, F. D.; Nurco, D. J.; Prodi, L.; Montalti,
M.; Natale, C. D.; D’Amico, A.; Carlo, A. D.; Lugli, P.; Smith, K. M.
J. Am. Chem. Soc. 2000, 122, 11295–11302. (e) Aihara, H.; Jaquinod, L.;
Nurco, D. J.; Smith, K. M. Angew. Chem., Int. Ed. 2001, 40, 3439–3441.
(f) Nath, M.; Huffman, J. C.; Zaleski, J. M. J. Am. Chem. Soc. 2003, 125,
11484–11485.
(9) (a) Tsuda, A.; Nakano, A.; Furuta, H.; Yamochi, H.; Osuka, A.
Angew. Chem., Int. Ed. 2000, 39, 558–561. (b) Tsuda, A.; Furuta, H.; Osuka,
A. Angew. Chem., Int. Ed. 2000, 39, 2549–2552. (c) Tsuda, A.; Furuta, H.;
Osuka, A. J. Am. Chem. Soc. 2001, 123, 10304–10321. (d) Tsuda, A.; Osuka,
A. Science 2001, 293, 79–82. (e) Ikeue, T.; Aratani, N.; Osuka, A. Isr.
J. Chem. 2005, 45, 293–302. (f) Ikeda, T.; Lintuluoto, J. M.; Aratani, N.;
Yoon, Z. S.; Kim, D.; Osuka, A. Eur. J. Org. Chem. 2006, 3193–3204.
Org. Lett., Vol. 11, No. 14, 2009
3081

The UV/vis absorption spectra of 9-11 display split Soret
bands and red-shifted Q-bands (Figure 2 and Supporting
Scheme 4. Oxidation of Meso-ꢀ Linked Porphyrin Arrays
Figure 2. UV/vis absorption spectra of 10, 11, and meso-meso
linked porphyrin trimer 12 in CHCl3.
Information). Such a split Soret-band is a typical feature of
directly linked porphyrin oligomers and can be explained in
terms of exciton coupling as a result of Coulomic interactions
of transition dipole moments between neighboring two
porphyrins.13 Importantly, the width between two split Soret
bands is larger in meso-meso linked porphyrin trimer 12
(2945 cm-1) than in meso-ꢀ linked porphyrin trimers 10
(1573 cm-1) and 11 (1021 cm-1). Furthermore, it is larger
in anti-trimer 10 than in syn-trimer 11. These observations
suggest that the exciton interactions between porphyrins are
stronger in this order: meso-meso linked porphyrin > anti
meso-ꢀ linked porphyrin > syn meso-ꢀ linked porphyrin.
tion of 10 and 11 with 2 equiv of DDQ and 0.5 equiv of
Sc(OTf)3 in toluene at 30 °C for 72 h afforded doubly linked
porphyrin tapes 2 (49%) and 3 (53%), respectively. The 1H
NMR spectra of 2 and 3 show two singlets and eight doublets
for the ꢀ-protons and two sets of Ar2 protons similarly except
for the signal due to the protons of Ar1, a single set for 2
and two sets for 3, reflecting C2h and C2V symmetries,
respectively (Figure 1b,c). It is interesting to note that the
1H NMR spectra of 2 and 3 are clearly sharper than that of
the previously reported meso-ꢀ doubly linked Ni(II) por-
phyrin trimer bearing 3,5-di-tert-butylphenyl groups.10a This
difference may be ascribed to the large meso-aryl substituents
in the former.11
Figure 3. UV/vis absorption spectra of 1, 2, and 3 in CHCl3.
Figure 3 shows the UV/vis absorption spectra of meso-ꢀ
doubly linked porphyrin tapes. The absorption spectrum of
1 shows three major bands at 429, 568, and 792 nm, which
may correspond to bands I, II, and III of the triply linked
(10) (a) Tsuda, A.; Nakamura, Y.; Osuka, A. Chem. Commun. 2003,
1096–1097. (b) Yoon, M.-C.; Noh, S. B.; Tsuda, A.; Nakamura, Y.; Osuka,
A.; Kim, D. J. Am. Chem. Soc. 2007, 129, 10080–10081.
(11) (a) Ikeda, T.; Tsuda, A.; Aratani, N.; Osuka, A. Chem. Lett. 2006,
35, 946–947. (b) Ikeda, T.; Aratani, N.; Osuka, A. Chem. Asian J. 2009, 4,
in press.
(12) (a) Hata, H.; Shinokubo, H.; Osuka, A. J. Am. Chem. Soc. 2005,
127, 8264–8265. (b) Hata, H.; Yamaguchi, S.; Mori, G.; Nakazono, S.;
Katoh, T.; Takatsu, K.; Hiroto, S.; Shinokubo, H.; Osuka, A. Chem. Asian
J. 2007, 2, 849–859. (c) Hiroto, S.; Hisaki, I.; Shinokubo, H.; Osuka, A.
Angew. Chem., Int. Ed. 2005, 44, 6763–6766.
Figure 1.
1H NMR spectra of 1, 2, and 3 in CDCl3. Letters show
mutual coupling of protons. Asterisks indicate solvent and solvent
impurity.
(13) Kasha, M. Radiat. Res. 1963, 20, 55–70.
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Org. Lett., Vol. 11, No. 14, 2009

porphyrin tapes,9c,d respectively. Similar features are ob-
served for 2, which, however, exhibits band peaks at 427,
852, and 963 nm, indicating large red-shifts only for the latter
two bands. In contrast, the absorption spectrum of 3 is
considerably different, showing band peaks at 422, 522, 604,
734, 927, and 982 nm with molar extinction coefficients that
are apparently smaller than those of 1 and 2. While the
fluorescence is hardly detected from 1 and 2, the syn-isomer
3 emits the fluorescence to a clearly detectable level at 1048
nm with a vibronic structure that is commonly observed for
porphyrins (Figure 4). The fluorescence of porphyrinoids in
polarizability due to the effective π-electron delocalization
along the planar and rigid molecular framework. Further-
more, σ(2) values of 1 and 2 are similar to those of the
corresponding Ni(II) porphyrin tapes (8000 and 16900 GM
for the dimer and trimer).10b This suggests that central metal
has no serious effect on TPA property and the observed σ(2)
values are solely due to π-conjugation pathway of porphy-
rinoids. In contrast, the σ(2) value of 3 is distinctly smaller
than that of 2, indicating that not only π-conjugation length
or size but also its shape is important. The substantial
symmetric charge redistribution after photoexcitation exhib-
ited by the linear π-conjugation pathway of 2 as compared
to the bent one of 3 increases σ(2) values.16 Similar conjuga-
tion-shape effect on TPA was also reported for triply linked
porphyrin tapes; namely, σ(2) values of L-shaped trimer (8700
vs 18500 GM) and T-shaped tetramer (35700 vs 41200 GM)
are smaller than the respective linear systems.16
In summary, meso-ꢀ doubly linked Zn(II) porphyrin
dimers and trimers were synthesized. Although doubly linked
Zn(II) porphyrin tapes dimer 1 and trimer 2 showed similar
absorption to doubly linked Ni(II) porphyrin tape, 3 showed
different absorption from them, indicating it has a different
electronic structure. Our studies have shown that the TPA
cross-section value depends on length and shape of the
π-conjugated system and that the value is larger for linear
than bended π-conjugation.
Figure 4. Fluorescence spectra of 1, 2, and 3 in CHCl3 taken for
excitation at 420 nm (OD ) 1.0).
an infrared region is quite rare14 and interesting in view of
sensing or optical communications.
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research (Nos. 19205006, and 20108001
“pi-Space”) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. N.A. acknowleges a Grant-
in-Aid for Young Scientists (B) and GCOE program
“Integrated Materials Science” for financial support. T.I.
thanks the JSPS Research fellowships for Young Scientists
for support. The work at Yonsei University was supported
by Star Faculty, and World Class University (no.2008-8-
1955), of the Ministry of Education, Science, and Technology
of Korea and the AFSOR/AOARD grant (FA4869-08-1-
4097) (D.K.).
TPA cross-section values (σ(2)) of 1, 2, and 3 in CHCl3
were measured by an open-aperture Z-scan method using
NIR femtosecond optical pulse.15 Because the absorption
spectra of these porphyrinoids spread across a wide spectral
range down to the IR region, excitation wavelengths were
chosen, 1600-1450 nm for 1 and 2000-1800 nm for 2 and
3, to be free from any contribution of one-photon excitation.
The σ(2) values for 1, 2, and 3 were measured to be 7000
(1500 nm), 16800 (1850 nm), and 7900 GM (1850 nm),
respectively. Strong enhancement in σ(2) value as a function
of number of porphyrin units originates from the increased
Supporting Information Available: Experimental details
and spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
(14) (a) Bonifazi, D.; Scholl, M.; Song, F.; Echegoyen, L.; Accorsi, G.;
Armaroli, N.; Diederich, F. Angew. Chem. Int. Ed 2003, 42, 4966–4970.
(b) Ahn, T. K.; Kwon, J. H.; Kim, D. Y.; Cho, D. W.; Jeong, D. H.; Kim,
S. K.; Suzuki, M.; Shimizu, S.; Osuka, A.; Kim, D. J. Am. Chem. Soc.
2005, 127, 12856–12861.
OL9011585
(15) (a) Sheik-Bahae, M.; Said, A. A.; Wei, T.-H.; Hagan, D. J.; van
Stryland, E. W. IEEE J. Quantum Electron. 1990, 26, 760–769. (b) Ahn,
T. K.; Kim, K. S.; Kim, D. Y.; Noh, S. B.; Aratani, N.; Ikeda, C.; Osuka,
A.; Kim, D. J. Am. Chem. Soc. 2006, 128, 1700–1704.
(16) Nakamura, Y.; Jang, S. Y.; Tanaka, T.; Aratani, N.; Lim, J. M.;
Kim, K. S.; Kim, D.; Osuka, A. Chem.sEur. J. 2008, 14, 8279–8289.
Org. Lett., Vol. 11, No. 14, 2009
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