Synthesis and properties of acetylene-bridged N-confused porphyrin dimers

Article abstract of DOI:10.1246/cl.2011.1021

Acetylene-bridged N-confused porphyrin (NCP) dimers were synthesized by Stille coupling reactions. Moderate electronic interaction between the NCP chromophores was observed in the inner-3H tautomers while strong interaction was observed in the inner-2H tautomers.

Full text of DOI:10.1246/cl.2011.1021

doi:10.1246/cl.2011.1021
Published on the web September 5, 2011
1021
Synthesis and Properties of Acetylene-bridged N-Confused Porphyrin Dimers
Motoki Toganoh,1 Takumi Takayama,1 Nandy Ritesh,2 Nobuo Kimizuka,1,2 and Hiroyuki Furuta*1,2
1Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395
2International Research Center for Molecular Systems, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395
(Received June 30, 2011; CL-110550; E-mail: hfuruta@cstf.kyushu-u.ac.jp)
Acetylene-bridged N-confused porphyrin (NCP) dimers
were synthesized by Stille coupling reactions. Moderate
electronic interaction between the NCP chromophores was
observed in the inner-3H tautomers while strong interaction was
observed in the inner-2H tautomers.
Cross-coupling reactions are one of the most powerful
methods to construct large ³-conjugated molecules.1 Among
Figure 1. NH-Tautomerization of N-confused porphyrins.
such molecules, porphyrin dimers as well as oligomers are a
particularly important class of compounds, which often show
fascinating electronic properties and functions, such as two-
photon absorption, light-harvesting, and electron transport.2
N-Confused porphyrin (NCP)3 is a uniquely interesting
porphyrin analog, showing prominent functions in coordination
chemistry, photochemistry, and structural organic chemistry.4
One intrinsic characteristic of NCP is NH-tautomerization,
which allows change of the electronic state significantly
(Figure 1).5 Thus, the inner-3H tautomer (NCP3H) has an
[18]annulenic substructure and is strongly aromatic, while the
inner-2H tautomer (NCP2H) loses the [18]annulenic substruc-
ture and is moderately aromatic.6 Accordingly, NCP3H is
thermodynamically more stable than NCP2H and much dif-
ference is found in their photophysical properties.7 Previ-
ously, we have demonstrated that intramolecular energy
transfer could be controlled by NH-tautomerization in the
porphyrin-NCP dyad system.8 Additionally, substitution effect
at the 3-position would be much different between NCP3H
and NCP2H.9 Along the lines of these findings, we can
expect interesting electronic structures in 3,3¤-acetylene-bridged
NCP dimers. In this paper, the synthesis and properties of
3,3¤-acetylene-bridged NCP dimers by the Stille coupling are
reported.
Scheme 1. Synthesis of NCP-CC-Ph.
First, the reactions of 3-bromo-NCPs with tributyl(phenyl-
ethynyl)tin were studied to prepare 3-alkynyl-NCPs (Scheme 1).
The alkynylated NCPs were previously prepared by the
Sonogashira coupling9 or inversion of alkynylated N-fused
porphyrins.10 The reaction of N-confused 3-bromotetraphenyl-
porphyrinatosilver(III) complex 1 proceeded smoothly with
Pd(OAc)2/AsPh3 catalyst to give AgIII(NCP3H)-CC-Ph 2 in
63% yield. Transmetalated product, PdII(NCP2H)-CC-Ph 3,
was also obtained in 19% yield. This unique reductive trans-
metalation from Ag(III) to Pd(II) inside the NCP core was also
observed in the Sonogashira coupling reaction.9 The reaction of
N-confused 3-bromotetrakis[4-(trifluoromethyl)phenyl]porphy-
rinatocopper(II) complex 4 proceeded in a similar manner to
give CuII(NCP2H)-CC-Ph 5 in 48% yield. Transmetalated
product 6 was also isolated in 10% yield. The AsPh3 ligand so
far afforded the better yields than the phosphine ligands such as
PPh3 and diphenylphosphinoethane.
Scheme 2. Synthesis of NCP dimers.
Then, syntheses of acetylene-bridged NCP dimers were
examined by the reactions with bis(tributylstannyl)acetylene
(Scheme 2). The Pd-catalyzed reaction of 1 successfully gave
AgIII(NCP3H)-CC-AgIII(NCP3H) 7 in 20% yield together with
AgIII(NCP3H)-CC-PdII(NCP2H) 8 in 7% yield. In the 1H NMR
spectrum of 7 in CDCl3, the signals due to ¢-pyrrolic protons
(12H) were observed in the region of ¤ 8.5-8.9, suggesting its
strong aromatic character. Additionally, the number of proton
signals indicated the symmetric structure of 7. Similarly, the
reaction of 4 afforded CuII(NCP2H)-CC-CuII(NCP2H) 9 in 35%
yield and CuII(NCP2H)-CC-PdII(NCP2H) 10 in 9% yield. The
reactivity of NCP in the Stille coupling would be higher than
Chem. Lett. 2011, 40, 1021-1023
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1022
Scheme 3. [18]Annulenic ³ circuit and resonance structures
of ¢,¢¤-acetylene-bridged regular porphyrin dimer.
Figure 2. Absorption spectra of 7, 2, and 12 in CH2Cl2.
Table 1. Properties of acetylene-bridged NCP dimers and
regular porphyrin dimer
-
¾
Soret
Q
Compound State
FWHMSoret OSSoret
/nm /M¹1 cm
¹1
7
1211
9
13
11
14
OFF dimer
667 252000
637 177000
830 101000
2486
1329
6810
5694
2562
657
2.66
1
1.58
1.32
1.12
1.28
monomer
ON dimer
monomer
ON/OFF dimer 609 129000
monomer 585 539000
768
89100
described depending on the resonance structures. Properties of
7, 9, and the corresponding monomer complexes are listed
in Table 1. The data of ZnII(TPP)-CC-ZnII(TPP) (11) and
ZnII(TPP) (14) is also shown. Since the energy of -Q is a
measure of the HOMO-LUMO gap or the porphyrin-porphyrin
conjugative interactions in the ground state, the large red shift of
9 from 13 (62 nm) would indicate strong interaction between
NCP chromophores at the ON state. Similarly, the moderate red
shift of 7 from 12 (30 nm) suggested relatively weak interaction
between NCP chromophores at the OFF state. In the case of
regular porphyrin 11, the smaller red shift from 14 (24 nm) was
observed, indicating weak porphyrin-porphyrin interaction in
the ground state.
Figure 3. Absorption spectra of 9, 5, and 13 in CH2Cl2.
regular porphyrin: the reaction of ¢-bromotetraphenylporphyri-
natozinc (II) (¢-Br-ZnII(TPP)) with bis(tributylstannyl)acetylene
gave ZnII(TPP)-CC-ZnII(TPP) (11) in 8% yield.11
The absorption spectrum of 7 in CH2Cl2 was compared with
those of 2 and AgIII(NCP3H)12 (12) (Figure 2). In the parent
Ag(III) complex 12, the Soret band was observed at 447 nm
and the Q band of the longest wavelength (-Q) was observed
at 637 nm. Introduction of phenylethynyl group (2: 466 and
667 nm) caused a moderate red shifts (µ30 nm) from 12. The
absorption wavelengths of 7 (473 and 667 nm) were similar to
those of 2.
The full width at half maximum of the Soret band region
(FWHMSoret) can be regarded as an approximate measure of the
relative extent of porphyrin-porphyrin excitonic coupling in the
S2 state.11 The FWHMSoret of 7 (2486 cm¹1) is similar to that
of 11 (2562 cm¹1), while the FWHMSoret of 9 (6810 cm¹1) is
significantly larger than the others. This might be indicative of
stronger excitonic coupling in 9 at the ON state. However, the
intrinsically large FWHMSoret of NCP monomer complex 13
Absorption spectra of 9, 5, and CuII(NCP2H) (13) in
CH2Cl2 are shown in Figure 3. In the parent monomer complex
13, the Soret band appeared at 477 nm, and -Q appeared at
768 nm. In the case of the phenylethynyl derivative 5, split of the
Soret band (449 and 474 nm) as well as the red shift of 43 nm
from 13 in -Q (811 nm) was observed. Significant broadening
and a further red shift of the Q band were observed for 9
(µ830 nm, shoulder).
The uniqueness of 3,3¤-acetylene-bridged NCP dimers is
made explicit by comparison with ¢,¢¤-acetylene-bridged
regular porphyrin dimer (POR-CC-POR). In NCP3H-CC-
NCP3H, the acetylene moiety and the [18]annulenic ³ circuit
are not connected directly (“OFF” state), while they are
connected in NCP2H-CC-NCP2H (“ON” state) although the
³ circuit is not complete (Scheme 3). Potentially, ON/OFF can
be switched by NH-tautomerism or oxidation/reduction.8,13 In
the case of POR-CC-POR, both ON and OFF states can be
(5694 cm¹1) hampered further discussion.
11,14
Thus, the relative oscillator strengths (OSSoret
)
of the
Soret band region are examined, which would be related to the
porphyrin-porphyrin electronic interaction. The OSSoret of 7
(2.66) is much larger than that of 9 (1.58) and 11 (1.12),
suggesting weak electronic interaction between NCP chromo-
phores at the OFF state.
Finally, demetalation of the alkynyl-NCP metal complexes
was studied (Scheme 4). When 7 was subjected to similar
reaction conditions (excess NaBH4) for demetalation of 2
reported previously,9 progress of the reaction was confirmed
by mass analysis (m/z 1250). Unfortunately, isolation of the
target free-base compound was so far difficult possibly due to its
Chem. Lett. 2011, 40, 1021-1023
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1023
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2
3
4
Scheme 4. Demetalation of CuII(NCP2H).
instability. Then, acid-mediated copper demetalation was exam-
ined. When 5 was treated with CF3CO2H, the corresponding
free-base NCP 15 was obtained in 53% yield. Similarly,
treatment of 9 with CF3CO2H gave the target free-base
compound 16 in 14% yield. This time, 16 is stable enough to
be isolated with standard silica gel column chromatography
because of the electron-withdrawing effect imposed by trifluoro-
methyl groups. Slight modification of the product yield (22%)
was observed in reductive demetalation of 9 by SnCl2.15 In the
1H NMR spectrum of 16 in CDCl3, the sharp signal due to inner-
CH moieties appeared at ¤ ¹4.80 (2H), and the broad signal due
to inner-NH moieties appeared at ¤ ¹2.05 (4H), which indicated
successful isolation of the free-base NCP dimer.16
In summary, the 3,3¤-acetylene-bridged NCP dimers were
readily synthesized by one-step three-component Stille coupling
reactions. Considerable interactions between the NCP chromo-
phores were suggested by the absorption spectra, in which clear
difference was recognized between NCP3H and NCP2H dimers.
Strong interaction was suggested for the NCP2H-CC-NCP2H
at the ON state and relatively weak interaction was suggested
for the NCP3H-CC-NCP3H at the OFF state. Further study
including spectroelectrochemistry as well as ESR studies to
reveal electronic communication between NCP chromophores in
datail will be reported in the near future. Studies on control of
NCP2H/NCP3H or ON/OFF state in NCP dimer by external
stimuli are also underway.
5
6
7
8
9
M. Toganoh, H. Miyachi, H. Akimaru, F. Ito, T. Nagamura,
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11 Previously, Zn(TPP)-CC-Zn(TPP) was prepared by the
Sonogashira coupling reactions in a stepwise manner.
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14 Oscillator strengths were calculated over the following
wavelength domains: 7, 350-514 nm; 9, 360-540 nm; 11,
360-529 nm; 12, 350-490 nm; 13, 360-560 nm; 14, 360-
450 nm.
This work was supported by the Grant-in-Aid for Scientific
Research (Nos. 2175004, 21108518, and 22350020) and the
Global COE Program “Science for Future Molecular Systems”
from the Ministry of Education, Culture, Sports, Science and
Technology of Japan.
This paper is in celebration of the 2010 Nobel Prize
awarded to Professors Richard F. Heck, Akira Suzuki, and
Ei-ichi Negishi.
15 T. H. Ngo, W. V. Rossom, W. Dehaen, W. Maes, Org.
Biomol. Chem. 2009, 7, 439.
16 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
References and Notes
1
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Chem. Lett. 2011, 40, 1021-1023
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/