Synthesis of doubly β-to-β 1,3-butadiyne-bridged diporphyrins: Enforced planar structures and large two-photon absorption cross sections

Article abstract of DOI:10.1002/anie.200700550

(Figure Presented) Plane and simple! Doubly β-to-β 1,3-butadiyne-bridged diporphyrins have been synthesized efficiently through metal-catalyzed reactions. The double bridges enforce the two porphyrins in a perfectly planar conformation (see picture), whic

Full text of DOI:10.1002/anie.200700550

Angewandte
Chemie
DOI: 10.1002/anie.200700550
Diporphyrins
Synthesis of Doubly b-to-b 1,3-Butadiyne-Bridged Diporphyrins:
Enforced Planar Structures and Large Two-Photon Absorption Cross
Sections**
Ichiro Hisaki, Satoru Hiroto, Kil Suk Kim, Su Bum Noh, Dongho Kim,* Hiroshi Shinokubo,*
and Atsuhiro Osuka*
Covalently linked p-conjugated porphyrin oligomers are
materials of great importance that display efficient energy
and electron transfer, single-molecule conductivity, and non-
linear optical (NLO) properties.[1] Among them, peripherally
ethyne- and 1,3-butadiyne-fabricated porphyrins hold a
central position owing to their attractive electronic proper-
ties.[2–5] As representative examples, Therien and co-workers
reported large hyperpolarizabilities for push–pull meso,-
meso’-bis-arylethynylated porphyrins[3] and Anderson et al.
reported large two-photon-absorption (TPA) properties of
meso-ethyne- and meso-butadiyne-bridged porphyrin oligom-
ers.[4] Most of the electronic properties of these conjugated
porphyrins are considered to depend on the extent of overall
p conjugation, which is a function of molecular coplanarity.
However, there is a continuous distribution of rotational
isomers because of small rotational barriers of triple-bond
linkages[6] which duplicates their photophysics.[7] As an
interesting approach, conformational control has been ach-
ieved with the aid of noncovalent interactions between a zinc
porphyrin and a pyridine coordination site.[4b,8]
Herein, we report the synthesis of doubly b-to-b buta-
diyne-bridged diporphyrins as the first example of multiply
1,3-butadiyne-bridged diporphyrins. An apparent advantage
of this double bridging strategy is a robust, enforced overall
planar conformation, which causes significant enhancements
of TPA properties. Large TPA compounds attract consider-
able interest for several innovative applications such as three-
dimensional (3D) optical memory, deeper-penetrating photo-
dynamic therapy (PDT) of cancer, 3D microfabrication, and
others.[9]
Diporphyrins 1 and 2 were prepared from b-borylated
porphyrins 3a and 3b[10] (Scheme 1). Treatment of mono- and
diborylporphyrins 3a and 3b with Oxone in a THF/acetone/
water suspension[11] successfully gave the corresponding
mono- and dihydroxyporphyrins 4a and 4b in 80% and
88% yields, respectively. The trifluoromethanesulfonyl deriv-
atives were prepared in DMF by reaction with PhNTf2 and
Cs2CO3,[12] and following metallation with Zn(OAc)2 6aZn
and 6bZn were obtained in excellent yields. Installation of
ethynyl groups was conducted through the standard Sonoga-
shira coupling with trimethylsilylacetylene followed by desi-
lylation to provide 8aZn and 8bZn in good yields. Then,
8aZn and 8bZn were subjected to a CuII-mediated oxidative
coupling reaction to furnish the corresponding butadiyne-
bridged porphyrin dimers 1Zn and 2Zn in 82% and 89%
yields, respectively. Demetallation with trifluoroacetic acid
provided 1H and 2H in good yields. Synthesis of 1Ni and 2Ni
was achieved in a similar fashion to 1Zn and 2Zn from NiII
triarylporphyrin as the starting material.
[*] K. S. Kim, S. B. Noh, Prof. Dr. D. Kim
Center for Ultrafast Optical Characteristics Control and
Department of Chemistry
Yonsei University
Seoul 120–749 (Korea)
Fax: (+82)2-2123-2434
E-mail: dongho@yonsei.ac.kr
b-to-b-Linked diporphyrins have been explored to exam-
ine the influences of connectivity difference on the electronic
interactions between the porphyrins,[1e,2a,6a,13,14] but 2H and its
metal complexes (2Zn and 2Ni) are the first examples of
doubly linked diporphyrins. These newly synthesized dipor-
phyrins were fully characterized by high-resolution mass
Dr. I. Hisaki, S. Hiroto, Prof. H. Shinokubo, Prof. Dr. A. Osuka
Department of Chemistry
Graduate School of Science
Kyoto University
and
Core Research for Evolutional Science and Technology (CREST)
Japan Science and Technology Agency
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
1
spectrometry, and H NMR, UV/Vis, and fluorescence spec-
troscopy (Supporting Information). The meso protons (Hm)
are observed at d = 10.42 ppm for 7aZn, d = 10.66 ppm for
7bZn, d = 10.67 for 1Zn, and d = 11.67 ppm for 2Zn. The
observed large downfield shift of Hm in 2Zn probably reflects
the enhanced anisotropic effect of the nearby butadiyne
segments that become distorted upon ring formation (Fig-
ure S8 in the Supporting Information).[15] Similar trends are
also seen for the corresponding NiII complex and free-base
porphyrins, 2Ni and 2H.
[**] This work was partly supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science,
and Technology, Japan. I.H. and S.H. acknowledge the Research
Fellowships of the JSPS for Young Scientists. The work at Yonsei
University was supported by BK21 and the Star Faculty Programs
from the Ministry of Education and Human Resources Develop-
ment.
Figure 1shows the X-ray crystal structure of 2Zn.[16] The
deviation from the mean plane composed of carbon and
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Scheme 1. Synthesis of diporphyrins 1 and 2. Reaction conditions: a) 1) Oxone, THF/acetone/H2O, RT, 1.5 h; 2) PhNTf2, Cs2CO3, DMF, RT, 12 h;
3) Zn(OAc)2, CHCl3, MeOH, RT, 2 h; b) Me3SiCCH, [Pd(PPh3)4], CuI, Et3N, DMF, toluene, 708C, 3 h; c) K2CO3, THF, MeOH, RT, 2 h; d) Cu(OAc)2,
toluene, pyridine, RT, 24 h; e) Cu(OAc)2, THF, pyridine, RT, 24 h. Tf: trifluoromethanesulfonyl; DMF: N,N-dimethylformamide; TMS: trimethylsilyl.
Figure 2 displays the UV/Vis absorption and
fluorescence spectra of ZnII porphyrins measured
in chloroform. Soret and Q-bands are observed,
respectively, at 429 and 555 nm for 7aZn, and at
440 and 564 nm for 7bZn, indicating an increasing
red shift upon the increment of b-ethynyl sub-
stituents. In line with this trend, 7aZn and 7bZn
have fluorescence maxima at 605 and 654 nm and
613 and 663 nm, respectively. On the other hand,
the absorption spectrum of the diporphyrin 1Zn is
significantly perturbed with broader Soret bands
with three distinct peaks at 431, 452, and 479 nm
and Q-bands at 563 and 606 nm. Also, its fluores-
cence spectrum shows a typical porphyrin-like
vibronic structure with a relatively high quantum
yield (FF = 0.12), which is quite similar to those of
the previously reported b-to-b butadiyne-bridged
ZnII diporphyrin.[3a,6a] The diporphyrin 2Zn dis-
plays a more perturbed absorption spectrum, in
which the Soret band is split into peaks at 441and
501nm and Q-bands are observed at 580, 605, and
645 nm. The fluorescence spectrum of 2Zn is
observed in a lower energy region with maxima
at 615, 657, 732, and 779 nm and a considerably
À3
attenuated quantum yield (FF = 9 ” 1 0 ). The
significant red shift may be ascribed to the
expansion of effective p conjugation, and the
reduced fluorescence quantum yield may arise
Figure 1. X-ray crystal structure of 2Zn. Thermal ellipsoids are shown at the 50%
probability level. a) Top view (iPrOH ligands not shown) and b) side view (aryl
from the structural distortion upon the double
bridging.
substituents not shown).
nitrogen atoms of the porphyrin cores and butadiyne bridges
is only 0.053 Š, which indicates its overall remarkably planar
structure. The angles around C63 and C66’ are 174.628 for C2-
C63-C66’ and 177.908 for C63-C66’-C65’, indicating that two
triple bonds are slightly distorted. The bond lengths of C2–
C63, C63–C66’, C66’–C65’, and C65’–C64’ are 1.414, 1.205,
1.368, 1.202, and 1.417 Š, respectively, which reveal distinct
bond-length alternation.
The TPA cross-section values were measured at 800 nm by
an open-aperture Z-scan method[17] with wavelength-tunable
120-fs pulses and are listed as follows: 1H (5150 GM), 1Zn
(5220 GM), 1Ni (4890 GM), 2H (8830 GM), 2Zn (9620 GM),
and 2Ni (8540 GM) (Figure S1in the Supporting Informa-
tion) These values are certainly larger than those for their
corresponding porphyrin monomers (< 100 GM). Interest-
ingly, the double bridge leads to almost doubling of the TPA
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Figure 2. UV/Vis absorption (solid lines) and fluorescence (dotted
lines) spectra of 7aZn, 7bZn, 1Zn, and 2Zn in CHCl3. The absorption
spectrum of ZnII 5,10,15-tris(3,5-di-tert-butylphenyl) porphyrin is indi-
cated as a solid black line for comparison.
values. To explore the relationship between the photoexcited-
state dynamics and molecular structures, the fluorescence
lifetimes were determined by a time-correlated single-
photon-counting technique for 1H (8.34 ns), 2H (8.12 ns),
1Zn (1.97 ns), and 2Zn (1.78 ns). The lifetimes suggested no
particular difference in the fluorescence-decay process for
singly and doubly butadiyne-bridged diporphyrins. The tran-
sient absorption decays of NiII diporphyrins at 610 nm exhibit
a biexponential decay with time constants of 17 ps (71%) and
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behaviors of singly and doubly butadiyne-bridged diporphyr-
ins do not show any dependence on the excited-state
dynamics. This feature can be explained by the fact that
TPA involves instantaneous and simultaneous two-photon-
absorption processes through a virtual ladder state to reach
the two-photon allowed excited states. Instead, it may be
concluded that the double-butadiyne bridges force the overall
planar conformation, hence enhancing the TPA properties
through the effective p conjugation.
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In summary, b-to-b doubly 1,3-butadiyne-bridged dipor-
phyrins 2 were synthesized on the basis of the newly
developed direct borylation of porphyrins. The diporphyrins
2 show large TPA values as a result mainly of the fully p-
conjugated electronic structure that arises from the two 1,3-
butadiyne bridges.
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Received: February 7, 2007
Revised: March 23, 2007
Published online: June 5, 2007
[16] Crystallographic data for 2Zn:
C152H179N8O6Cl17Zn2, Mw =
¯
Keywords: conjugation · fluorescence · nonlinear optics ·
porphyrinoids · two-photon absorption
2947.42, triclinic, space group P1 (No. 2), a = 9.287(5), b =
20.381(5) c = 21.529(5) Š, a = 77.218(5), b = 80.967(5), g =
.
Angew. Chem. Int. Ed. 2007, 46, 5125 –5128
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5127

Communications
80.500(5)8, V= 3889(2) Š3, Z = 1, 1calcd = 1.259 gcmÀ3
,
T=
[17] M. Sheik-Bahae, A. A. Said, T.-H. Wei, D. G. Hagan, E. W.
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111, 6500.
À1838C, 37697 measured reflections, 13643 unique reflections
(Rint = 0.0420), R1 = 0.0990 (I > 2s(I)), wR2 = 0.2942 (all data),
GOF = 1.058. CCDC 635463 contains the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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