Synthesis of thieno-bridged porphyrins: Changing the antiaromatic contribution by the direction of the thiophene ring

Article abstract of DOI:10.1021/ja3082999

Two types of thieno-bridged porphyrins were synthesized by incorporating a thiophene group across their meso and β positions with different directions of the thiophene ring to investigate the aromaticity of these porphyrins with extended π-systems. The 2,3-thieno-bridged porphyrin showed a larger antiaromatic contribution than did the 3,4-thieno-bridged porphyrin. In the former, the antiaromatic contribution is based on a 20-π-electron conjugated circuit. The two thieno-bridged porphyrins were characterized by calculations of nucleus-independent chemical shift and anisotropy of the induced current density as well as by X-ray crystallography, NMR spectroscopy, UV-vis-NIR absorption spectroscopy, electrochemical studies, time-resolved excited-state analysis, and two-photon absorption cross section measurements. Chemical derivatization of the 2,3-thieno-bridged porphyrin was also demonstrated.

Full text of DOI:10.1021/ja3082999

Communication
pubs.acs.org/JACS
Synthesis of Thieno-Bridged Porphyrins: Changing the Antiaromatic
Contribution by the Direction of the Thiophene Ring
Yusuke Mitsushige,† Shigeru Yamaguchi,† Byung Sun Lee,‡ Young Mo Sung,‡ Susanne Kuhri,§
Christoph A. Schierl,§ Dirk M. Guldi,*,§ Dongho Kim,*,‡ and Yutaka Matsuo*,†
†Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-Ku, Tokyo 113-0033, Japan
‡Department of Chemistry, Yonsei University, Seoul 120-749, Korea
§Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-Universitat
̈
Erlangen-Nurnberg, Egerlandstraße 3, 91058 Erlangen, Germany
̈
S
* Supporting Information
Chart 1. π-Extended Porphyrins and Pyracylene with
Resonance Structures: (a) Porphyrins with a Double Bond
Extending the π Network; (b) Pyracylene; (c) 2,3-Thieno-
Bridged Porphyrin 3; (d) 3,4-Thieno-Bridged Porphyrin 4
ABSTRACT: Two types of thieno-bridged porphyrins
were synthesized by incorporating a thiophene group
across their meso and β positions with different directions
of the thiophene ring to investigate the aromaticity of
these porphyrins with extended π-systems. The 2,3-thieno-
bridged porphyrin showed a larger antiaromatic contribu-
tion than did the 3,4-thieno-bridged porphyrin. In the
former, the antiaromatic contribution is based on a 20-π-
electron conjugated circuit. The two thieno-bridged
porphyrins were characterized by calculations of nucleus-
independent chemical shift and anisotropy of the induced
current density as well as by X-ray crystallography, NMR
spectroscopy, UV−vis−NIR absorption spectroscopy,
electrochemical studies, time-resolved excited-state anal-
ysis, and two-photon absorption cross section measure-
ments. Chemical derivatization of the 2,3-thieno-bridged
porphyrin was also demonstrated.
orphyrin is a representative π-functional skeleton that has
Pbeen actively investigated in various fields of chemistry. To
improve upon the properties of this macrocycle, extension of
the π conjugation is a commonly used method.1 In some cases,
only a small extension of the porphyrin π system leads to
largely altered properties. For instance, the properties of the
macrocycle are substantially perturbed when an olefin2 or
phenyl3 moiety is incorporated across the meso and β positions
(Chart 1a). These peripherally functionalized porphyrins
bearing a fused five-membered ring have been known to have
unusual properties such as largely altered light absorption
extending to the far-red region.3 Although disrupted aromatic
character has been suggested in such compounds,3d the origin
of the unusual properties has yet to be fully explained.
Pyracylene, which is one compound that shows such unusual
properties, provides a clue for solving this problem. In the
resonance structures of pyracylene, both aromatic 10-π-electron
(10π) and antiaromatic 12π circuits can be drawn (Chart 1b).
The anomalous aromaticity of pyracylene has been a matter of
debate since it was first synthesized in 1967.4,5
bridged porphyrins (Chart 1c,d). The notable characteristic of
the thieno-bridged porphyrins is that the π network can be
manipulated by changing direction of the thiophene ring while
maintaining the same size of the π faces. In this way, we can
examine the effects of the additional π networks on the overall
electronic structures of porphyrins. In the compound shown in
Chart 1c, an antiaromatic 20π circuit would contribute to the
overall electronic structure, while in the compound shown in
Chart 1d, antiaromaticity would be weakened or disrupted by
the sulfur atom on the 24π conjugated pathway, leading to
normal or slightly perturbed aromatic character of the
porphyrin macrocycle. To test this working hypothesis, we
In this paper, we will rationalize such unusual properties and
aromaticity with the aid of rationally designed porphyrin
derivatives. To this end, we synthesized two types of thieno-
Received: August 21, 2012
Published: September 18, 2012
© 2012 American Chemical Society
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Communication
Scheme 1. Synthesis and Further Functionalization of
a
Thieno-Bridged Porphyrins
a
Reagents and conditions: (a) Pd2dba3, PPh3, Cs2CO3, toluene/DMF,
100 °C; (b) Pd(OAc)2, PCy3·HBF4, K2CO3, DMF, 155 °C; (c) N-
bromosuccinimide, CHCl3, pyridine, 0 °C. Ar = 3,5-di-tert-
butylphenyl.
Figure 1. ORTEP drawings of 3′ [(a) top view; (b) side view], 4′ [(c)
top view; (d) side view], and 7 [(e) top view; (f) side view]. The meso-
aryl substituents have been omitted for clarity. The thermal ellipsoids
are scaled to the 50% probability level.
synthesized these π-extended porphyrins and evaluated their
aromaticities.
Our synthetic strategy for the thieno-bridged porphyrins is
shown in Scheme 1. Suzuki−Miyaura cross-coupling of meso-
borylporphyrin 1 with 2-chloro-3-bromothiophene and 3,4-
dibromothiophene followed by intramolecular Heck reactions6
afforded 2,3- and 3,4-thieno-bridged porphyrins 3 and 4,
respectively, in yields of 68 and 47% over two steps. Crystal
structures of their analogues 3′ and 4′ are shown in Figure 1a−
d.7
According to calculations, pyracylene has a paratropic ring
current on its five-membered rings as a result of superposition
of 10π and 12π conjugated circuits.5d We expected that
superposition of 18π and 20π conjugated circuits in 3 would
lead to a similar phenomenon, meaning that a paratropic ring
current would be generated on the five-membered ring between
the porphyrin and thiophene moieties (Figure S18 in the
Supporting Information). To examine this hypothesis, we
calculated the anisotropy of the induced current density
(AICD), which represents the three-dimensional delocalized
electron density with a scalar field and illustrates the
paramagnetic term of the induced current density; aromatic
molecules show clockwise current density and antiaromatic
species show counterclockwise current density.8,9 AICD plots
for 3 and 4 are shown in Figure 2. On the porphyrin
macrocycles of both 3 and 4, clockwise current density was
observed, indicating that 3 and 4 are, on the whole, aromatic.
Remarkably, in line with our expectations, counterclockwise
current density was observed on the five-membered ring
between the porphyrin and thiophene moieties of 3, whereas in
the AICD plot of 4, clear current density was observed on only
the porphyrin skeleton.
Figure 2. AICD plots for (a) 3 and (b) 4 at an isosurface value of 0.05.
fully used as a measure of aromaticity.10 At the five-membered
ring between the porphyrin and thiophene moieties, the
NICS(0) values for both 3 and 4 are positive (+30.5 and +13.1
ppm, respectively). The large positive NICS(0) value for 3
indicates a large contribution from the antiaromatic 20π circuit.
A smaller contribution from the antiaromatic 24π circuit of 4 is
also possible, judging from the positive value at the five-
membered ring between the porphyrin and thiophene moieties
of 4. The NICS(0) values at the porphyrin macrocycle of 3 are
more positive (−10.1 to −13.6 ppm; points “a” in Figure 1a)
than those of 4 (−14.6 to −16.1 ppm; points “a” in Figure 1c),
suggesting that the antiaromatic network makes a larger
contribution to the overall electronic structure in 3 than in 4.
One of the most useful experimental methods for assessing
aromaticity is 1H NMR spectroscopy. A paratropic ring current
For 3 and 4, we also calculated values of the nucleus-
independent chemical shift (NICS), which has been success-
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Communication
DFT) calculations on 3 and 4 were performed at the B3LYP/6-
31G* level. The calculated transitions agreed fairly well with
the experimental absorption spectra in terms of both position
and relative intensity. Compared with 4, which maintained the
degeneracy of its frontier MOs as a typical porphyrinoid system
does, thieno-bridged porphyrin 3 showed a relatively broken
degeneracy between the lowest unoccupied MO (LUMO) and
the LUMO+1 as well as between the highest occupied MO
(HOMO) and the HOMO−1, which accounts for the red-
shifted absorption of 3.
The electrochemical properties of 2, 3, and 4 were examined
by cyclic voltammetry. The first oxidation waves (Eox1) of 2 and
4 were observed at 0.38 and 0.25 V vs ferrocene/ferrocenium
(Fc/Fc+), respectively. In comparison with the first oxidation
potentials of 2 and 4, that of 3 was found to be substantially
more negative (0.02 V vs Fc/Fc+).12 The first reduction waves
(Ered1) of 2, 3, and 4 were observed at −1.83, −1.57, and −1.68
V vs Fc/Fc+, respectively. ΔE (Eox1 − Ered1) decreased in the
order 2 (ΔE = 2.21 eV) > 4 (ΔE = 1.93 eV) > 3 (ΔE = 1.59
eV), consistent with the UV−vis absorption spectra.
1
Figure 3. H NMR spectra of (a) 2, (b) 3, and (c) 4 in CDCl3. Red
and blue dots indicate signals of the porphyrin β and meso protons,
respectively.
Femtosecond transient absorption (fs-TA) and time-resolved
fluorescence measurements were carried out in toluene at room
temperature to explore the photophysics of the aromaticity-
dependent excited states in the compounds. As expected from
the results described above, the temporal decay profiles of
thieno-bridged porphyrins 3 and 4 also indicated quite different
properties. The singlet excited state lifetime of 4 was evaluated
as 790 ps by monoexponential fits of the time-resolved
fluorescence and fs-TA decay profiles, but the fs-TA decay
profile of 3 had to be fit using a double-exponential decay
function, revealing two time constants with values of 850 fs and
7.5 ps. The femtosecond time constant of 3 corresponds to the
internal conversion time from the Q-like state to the lowest S1
state (700−1000 nm), while the picosecond time constant
corresponds to the lifetime of the lowest S1 state, as in the
profiles of antiaromatic porphyrinoids.11 In addition to these
fast decay profiles, we observed additional long time constants
from fs-TA measurements of both 3 and 4. These other time
constants are attributable to triplet excited state lifetimes. Thus,
nanosecond-transient absorption (ns-TA) measurements were
also carried out. Here, a triplet excited state lifetime of 1.9 μs
for 3 was found from its ns-TA decay profile; this lifetime is
slightly shorter than that of 4 (2.5 μs). The singlet and triplet
excited state lifetimes of the two thieno-bridged porphyrins 3
and 4 are shorter than those of Zn(II) tetraphenylporphyrin (2
ns and 1 ms, respectively). The shorter S1 lifetime for 3 than for
4 is consistent with the lack of fluorescence in 3. This optical
properties of 3 arise in part from the acceleration of
nonradiative internal conversion from the S1 state to the S0
state due to the reduction in the energy gap caused by breaking
of the frontier orbital degeneracy as a result of the strong
antiaromaticity of 3. The shorter lifetimes of the triplet excited
states in both cases essentially arise from the heavy-atom effect
of sulfur in the bridging thiophenes
Figure 4. UV−vis−NIR absorption spectra of 2 (red), 3 (black), and 4
(blue) in CH2Cl2.
shifts the signals of the peripheral protons on arenes upfield,
1
and a diamagnetic ring current shifts them downfield. The H
NMR spectra of 2, 3, and 4 in CDCl3 are shown in Figure 3.
The signals of the meso and β protons are shifted further
upfield in the order 2 < 4 < 3, corresponding to the increasing
contributions of antiaromaticity in resonance.
Recent studies of antiaromatic and aromatic porphyrinoids
have revealed that UV−vis−NIR absorption spectroscopy is
helpful in distinguishing between them.11 UV−vis−NIR
absorption spectra of 2, 3, and 4 are shown in Figure 4.
Compound 2 showed Soret and Q bands, which are typical of
aromatic porphyrins. Soret and Q bands were also observed in
the spectrum of 4, but both bands were weaker and broader in
comparison with those of 2. Compounds 2 and 4 exhibited
fluorescence maxima at 588 and 674 nm and fluorescence
quantum yields of 0.014 and 0.009, respectively. In contrast, the
optical properties of 3 were markedly different from those of 2
and 4. The Soret and Q bands of 3 were notably attenuated.
Moreover, weak NIR absorption appeared, with the absorption
edge extending to 1000 nm. No fluorescence was observed.
Interestingly, changing the direction of the thiophene moiety
induced major differences in the optical properties. It is also
noteworthy that the optical properties of 3 closely resemble
those of antiaromatic porphyrinoids, which exhibit no
fluorescence and have broad Soret bands and featureless,
smeared Q-like bands.11
Antiaromaticity of porphyrinoids leads to significant changes
in not only the linear optical properties but also the nonlinear
ones.11 Thus, the two-photon absorption (TPA) cross sections
of 3 and 4 were measured by the wavelength-scanning open-
aperture Z-scan method over wavelength ranges of 1100−1600
and 900−1350 nm, respectively, where the contribution of one-
photon absorption is negligible. Despite the similar structures
of these compounds, 3 exhibited a small TPA cross section of
200 GM (1 GM = 10−50 cm4 s photon−1) at the lowest-energy
To gain insight into the absorption spectra, molecular orbital
(MO) and time-dependent density functional theory (TD-
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Communication
TPA band maximum, which is only about one-third of the value
of the TPA cross section of 4 at the same position (550 GM),
suggesting that the electronic delocalization in 3 is quite unique
and different from that of 4. However, at 1300 nm,
corresponding to the Q-like band, the TPA cross sections of
3 and 4 are roughly the same (400 and 550 GM, respectively).
In other words, the TPA cross sections in the Q-like band
region are mainly affected by Zn(II) porphyrin moieties.
Overall, we propose that the unusual properties of
porphyrins that are peripherally functionalized with a fused
five-membered ring come from the 20π conjugated antiar-
omatic circuit, which was characterized by the theoretical and
experimental investigations described above. More importantly,
through a comparison with 3,4-thieno compound 4, the
antiaromatic contribution of the 20π circuit in 3 was clearly
elucidated.
Lastly, we examined the reactivity of thieno-bridged
porphyrin 3. As shown in Scheme 1a, bromination of 3 with
N-bromosuccinimide occurred at the α position of the
thiophene moiety, affording brominated thieno-bridged por-
phyrin 5 in 62% yield. Cross-coupling reactions of 5 would
enable the introduction of various substituents, thereby giving a
means of further understanding this unique aromaticity. In fact,
phenyl and (dimethylamino)phenyl substituents could be
introduced via Suzuki−Miyaura coupling to afford 6 (51%)
and 7 (35%). The X-ray crystal structure of 7 is shown in
Figure 1e,f. The anomalous aromaticity of 3 was retained in the
products 6 and 7, as confirmed by their 1H NMR and UV−vis−
NIR absorption spectra.
In conclusion, we have employed the concept of anti-
aromaticity to rationalize the unusual properties of peripherally
π-extended porphyrins. To examine the antiaromatic contribu-
tion in π-extended porphyrins, we synthesized 2,3- and 3,4-
thieno-bridged porphyrins, which have different directions of
the thiophene ring and the same size of the π faces. The former
compound showed a clear antiaromatic contribution due to its
20π conjugated circuit, whereas the latter exhibited a lesser
contribution of antiaromaticity because of the disruption of the
possible 24π antiaromatic circuit by the sulfur atom. These
results provide a useful guideline for controlling the properties
of porphyrins, an important class of π-functional molecules.
supported by the Midcareer Researcher Program (2010-
0029668) and the World Class University Program (R32-
2010-000-10217) of the Ministry of Education, Science, and
Technology (MEST) of Korea. The quantum calculations were
performed using the supercomputing resources of the Korea
Institute of Science and Technology Information (KISTI). This
work was also supported in part by the Deutsche
Forschungsgemeinschaft Cluster of Excellence “Engineering
of Advanced Materials” and NDRL, which is supported by the
Division of Chemical Sciences, Geosciences and Biosciences,
Basic Energy Sciences, Office of Science, United States
Department of Energy through grant number DE-FC02-
04ER15533.
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ASSOCIATED CONTENT
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General synthetic procedures, NMR and UV−vis−NIR spectra,
cyclic voltammograms, time-resolved photophysics data,
theoretical calculation data, and CIF files for 3′, 4′, and 7.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
guldi@chemie.uni-erlangen.de; dongho@yonsei.ac.kr;
matsuo@chem.s.u-tokyo.ac.jp
Notes
The authors declare no competing financial interest.
(12) The first oxidation wave of 3 was irreversible and was probably
due to the reaction at the α position of the thiophene moiety. The first
oxidation potential of 3 was thus determined by differential pulse
voltammetry.
ACKNOWLEDGMENTS
■
This work was supported by the Funding Program for Next-
Generation World-Leading Researchers. We thank Prof.
Hiroshi Shinokubo (Nagoya University) for conducting ESI-
TOF-MS measurements. The work at Yonsei University was
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