2,5-Thienylene-bridged triangular and linear porphyrin trimers

Article abstract of DOI:10.1002/anie.200802026

(Figure Presented) A question of shape: Palladium-catalyzed cross-coupling of borylporphyrins has led to the synthesis of a β-to-β thienylene-bridged porphyrin dimer and two types of trimer (see picture; gray C; white H, pink Ni, yellow S, blue N). The ladderlike trimer exhibits a very large two-photon absorption cross-section (17300 GM) at 800 nm, while that of the triangular trimer is moderate in comparison (8600 GM).

Full text of DOI:10.1002/anie.200802026

Communications
DOI: 10.1002/anie.200802026
Macrocyclic Porphyrins
2,5-Thienylene-Bridged Triangular and Linear Porphyrin Trimers**
Jianxin Song, So Young Jang, Shigeru Yamaguchi, Jeyaraman Sankar, Satoru Hiroto,
Naoki Aratani, Jae-Yoon Shin, Shanmugam Easwaramoorthi, Kil Suk Kim, Dongho Kim,*
Hiroshi Shinokubo,* andAtsuhiro Osuka*
Highly conjugated porphyrin arrays have received much
attention in light of their potential applications in optoelec-
tronic devices, sensors, photovoltaic devices, nonlinear optical
(NLO) materials, and pigments for photodynamic therapy.[1]
The manipulation of interporphyrinic p conjugation is often
essential to achieve desirable electronic and photophysical
properties. With this in mind, we have reported doubly b-to-b
butadiyne-bridged diporphyrins as the first examples of
multiply b-to-b-bridged diporphyrins.[2] This double-bridging
strategy secures a rigid and coplanar porphyrin conformation,
which induces significant enhancements of two photon
absorption (TPA) properties. In general, large TPA cross-
section values are considered to arise from effective elec-
tronic delocalization, but the general understanding of the
structural factors that influence TPA properties still remains
incomplete.
bridged diporphyrins, we embarked on the synthesis of
doubly b-to-b 2,5-thienylene-bridged diporphyrins with the
expectation that they would display enhanced TPA proper-
ties. Recently, expanded porphyrins with a thiophene-modi-
fied core have been reported to have exceptionally large TPA
cross-section values.[5]
b,b’-Diborylporphyrin 1H[6] was cross-coupled with 2,5-
dibromothiophene under Suzuki–Miyuara coupling condi-
tions. The desired product, doubly 2,5-thienylene-bridged
diporphyrin 2H, was obtained in 29% yield (Scheme 1). The
p Conjugation through thienylene linkages is often quite
effective because of their relatively weak aromaticity as well
as their small rotational barriers.[3] Although several thieny-
lene-bridged porphyrin systems have been reported,[4] no
examples of porphyrins that are directly bridged by thieny-
lene groups at the b positions or contain multiple thienylene
bridges have been reported to date. Encouraged by the large
TPA cross-section values of doubly b-to-b 1,3-butadiyne-
[*] S. Y. Jang, J. Y. Shin, Dr. S. Easwaramoorthi, K. S. Kim,
Prof. Dr. D. Kim
Centre for Ultrafast Optical Characteristics Control andDepartment
of Chemistry
Scheme 1. Suzuki–Miyaura coupling of borylatedporphyrins with 2,5-
dibromothiophene. Reaction conditions: 2,5-dibromothiophene, [Pd2-
(dba)3], PPh3, Cs2CO3, CsF, toluene/DMF/H2O, reflux, 48 h. Ar=3,5-di-
tert-butylphenyl, Bpin=3,3,4,4,-tetramethyl-2,5-dioxaborolanyl, dba=
trans, trans-dibenzylideneacetone.
Yonsei University
Seoul 120-749 (Korea)
Fax: (+82)2-2123-2434
E-mail: dongho@yonsei.ac.kr
Dr. J. Song, S. Yamaguchi, Dr. J. Sankar, S. Hiroto, Dr. N. Aratani,
Prof. Dr. H. Shinokubo, Prof. Dr. A. Osuka
Department of Chemistry
1H NMR and high-resolution ESI-TOF spectra of 2H confirm
its expected structure. To our surprise, the thienylene-bridged
porphyrin trimer 3H was also isolated in 10% yield. The
1H NMR spectrum of 3H is quite simple, exhibiting a singlet
peak for the meso proton, one singlet and two doublets for the
b protons, and a singlet for the thiophene protons, thus
confirming the high symmetry of the trimer. The parent ion
peak of 3H was observed at m/z 2866.6962 (calculated for
[C198H223N12S3]+ 2866.7034 ([M+H]+)) in the high-resolution
ESI-TOF mass spectrum. Complexation of nickel(II) and
zinc(II) ions was carried out by treatment of 2H and 3H with
either Zn(OAc)2 or [Ni(acac)2](acac = acetylacetonato) to
provide dimers 2Ni and 2Zn and trimers 3Ni and 3Ni,
respectively, in quantitative yields.
Graduate School of Science
Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
Fax: (+81)75-753-3970
E-mail: hshino@kuchem.kyoto-u.ac.jp
osuka@kuchem.kyoto-u.ac.jp
[**] The work at Kyoto University was supportedby a Grant-in-Aidfor
Scientific Research (Nos 18685013 and20037034) from MEXT
(Japan). S.Y., J.S., andS.H. thank the JSPS for Research Fellowships.
The work at Yonsei University was supportedby the Star Faculty
program from the Ministry of Education and Human Resources
Development, Korea. S.Y.J., J.Y.S., S.E., andK.S.K. were supported
by BK21 fellowships.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802026.
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Definitive structural assignments of the dimer and trimer
were accomplished by X-ray diffraction analyses of 2Ni and
3Ni, which revealed highly symmetrical structures of the
porphyrin dimer and triangular trimer bridged by 2,5-
thienylene linkages (Figure 1).[7] The porphyrin macrocycles,
the thiophene protons Hth of 3Ni (d = 8.51 ppm), which point
inwards, show a downfield shift relative to those of 2Ni (d =
7.90 ppm). Similar trends are also observed for the corre-
sponding zinc complexes and free-base porphyrins, thereby
suggesting that they adopt similar structures to 2Ni and 3Ni.
Interestingly, the thienylene bridge is able to adapt to the size
of the macrocycle by adopting conformations with the sulfur
atom pointing either into or out of the core, thus maintaining
the overall conjugation of the system.
The UV/Vis absorption and fluorescence spectra of the
ZnII porphyrins measured in CH2Cl2 is shown in Figure 2. The
broadened Soret band of 2Zn is observed (with a shoulder) at
Figure 2. a) UV/Vis absorption andb) emission spectra of 2Zn (c),
3Zn (c), and 8Zn (a) in CH2Cl2. Samples were excitedat the
absorption maxima.
Figure 1. X-ray structures. a) Top view andb) side view of 2Ni and
c) top view andb) side view of 3Ni. The thermal ellipsoids are 50%
probability level. Meso aryl groups are omittedfor clarity.
429 nm, along with an ill-defined Q band at 557 nm. The
absorption spectrum of the triporphyrin 3Zn is significantly
perturbed, and exhibits broader Soret bands with three peaks
at 427, 462, and 494 nm and Q bands at 570 and 601 nm. The
fluorescence spectrum of 3Zn exhibits a typical vibronic
structure with a relatively high fluorescence quantum yield
(FF = 0.056), while the quantum yield of 2Zn is much lower
(FF = 0.008). The reduced fluorescence quantum yield of 2Zn
may arise from the structural distortion of the molecule
caused by the double bridging.
in both 2Ni and 3Ni, are nearly coplanar to each other,
although the porphyrin cores are slightly ruffled, as is often
seen in the case of nickel porphyrins. The maximum displace-
ments of the b-carbon atoms from the mean plane composed
of carbon and nitrogen atoms (except the thiophene bridges)
are 0.546 Š for 2Ni and 0.915 Š for 3Zn. The thiophene
moieties in 2Ni are tilted by 40.58 and 49.38 relative to the
adjacent pyrrole rings, while the tilt angles of the thiophene
moieties range from 26.68 to 36.18 in 3Ni. The meso protons
1
Hmeso appear at d = 12.16 ppm in the H NMR spectrum of
The TPA cross-section values were measured at 800 nm by
using an open-aperture Z-scan method with wavelength-
tunable 120 fs pulses.[8,9] The TPA cross-section values of 2Zn
(7020 GM) and 3Zn (8600 GM) are certainly larger than
2Ni. This signal is shifted substantially downfield from that of
3Ni (d = 10.99 ppm), probably because of the ring current
effect of the adjacent porphyrin moiety. On the other hand,
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those of b,b’-dithienylporphyrin 4Zn (970 GM) and singly
bridged diporphyrin 5Zn (3300 GM, Table 1). The TPA cross-
section value of 2Zn, which is double that of 5Zn, seemingly
originates from the effective p delocalization through the
Table 1: Optical properties of ZnII porphyrins.
(2) [GM][a]
tF [ns][b]
FF
[c]
Compound
s
2Zn
3Zn
4Zn
5Zn
8Zn
7020
8600
970
3300
17300
1.75
1.38
2.44
1.83
1.25
0.008
0.056
0.032
0.099
0.005
[a] TPA cross-section value determined for excitation at 800 nm.
[b] Singlet excited-state lifetime. [c] Fluorescence quantum yield.
Scheme 2. Synthesis of linear trimer 8Zn. Reaction conditions:
[Pd2(dba)3], PPh3, Cs2CO3, CsF, toluene/DMF, reflux, 24 h. Ar=3,5-di-
tert-butylphenyl.
ular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO). The fluorescence lifetime of 3Zn is shorter
than that of 2Zn and nearly identical to that of 8Zn, which
indicates that the slightly more flexible motions of 3Zn
through one thiophene bridge between the porphyrin units
contributes to the acceleration of nonradiative decay chan-
nels. On the other hand, the TPA cross-section value of the
linear trimer 8Zn reached 17300 GM, more than double the
value of its triangular counterpart 3Zn. In this case, p deloc-
alization between the porphyrin units arises from the three
and four thiophene bridges in 3Zn and 8Zn, respectively.
Hence, the increased TPA cross-section value could not be
simply accounted for by the enhanced p delocalization
through an additional thiophene bridge. Other contributing
factors can be one- versus multidimensional p delocalization
and its associated molecular hyperpolarizability, which is in
fact proportional to the TPA cross-section value. In general,
the hyperpolarizability of linear annulenes are much larger
than their corresponding two-dimensional isomers.[11] More-
over, for linear systems, centrosymmetric charge redistribu-
tion has been shown to stimulate a significant enhancement in
TPA cross-section values.[12] Thus, our investigation suggests
that linear one-dimensional p delocalization yields better
TPA properties than delocalization over a triangular shape.
In summary, we have achieved the synthesis of a b-to-b
2,5-thienylene-bridged porphyrin dimer, a triangular trimer,
and a ladderlike trimer. The doubly linked diporphyrin
exhibits a large TPA cross-section value, but the enhancement
of that in the cyclic trimer is not significant. In sharp contrast,
the ladderlike trimer shows a very large TPA cross-section
value by virtue of enhanced charge redistribution and larger
hyperpolarizability along the long molecular axis. Investiga-
tions on higher 1,4-thienylene-bridged porphyrin oligomers
are currently underway.
rigidified planar conformation enforced by the two thiophene
bridges. However, the observed enhancement in the TPA
cross-section values from 2Zn to 3Zn is unexpectedly small
considering that the highly planar structure of 3Zn, which
also contains one additional porphyrin segment, should be
advantageous for global electronic delocalization. This result
may be interpreted as follows: the two porphyrin chromo-
phores in 2Zn can communicate with each other through the
double thiophene bridges, while only a single thiophene
bridge is available for delocalization in 3Zn, thus lowering the
efficacy of the TPA cross-section value per porphyrin unit.
Furthermore, it is known that the dimensionality of the
conjugated backbones has a decisive role in predicting the
TPA properties.[10]
We then decided to synthesize linear porphyrin trimer
8Zn and compare the TPA value with that of the triangular
trimer 3Zn. Suzuki–Miyaura coupling of 6Zn with 7Zn[6]
successfully provided the ladderlike porphyrin trimer 8Zn in
11% yield (Scheme 2). The parent ion peak was observed at
m/z 2948.2549 (calculated for [C188H197N12S4Zn3]+ 2948.2544
([M+H]+)) in the high-resolution ESI-TOF mass spectrum.
1
The H NMR spectrum of 8Zn displays two signals for the
meso proton (d = 12.73 and 12.71 ppm), in agreement with its
ladderlike structure. The absorption spectrum of 8Zn is
similar to that of 2Zn except for the Soret band, which is red-
shifted and intensified. The red-shift of the emission peak of
8Zn from that of 2Zn is not so large (16 nm). To explore the
relationship between the photoexcited dynamics and molec-
ular structure of the porphyrin systems, their fluorescence
lifetimes were determined by a time-correlated single-
photon-counting technique. The fluorescence lifetime
decreased as the number of porphyrin units increased because
of the accelerated nonradiative decay caused by the
decreased energy gap between the highest occupied molec-
Received: April 30, 2008
Keywords: cross-coupling · macrocycles · porphyrinoids ·
.
thiophenes · two-photon absorption
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Chemie
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